Author: Pam Knox

NOAA recently announced that La Niña is favored to continue through summer and fall this year and could last through next spring. This forecast is bound to strike fear in gardeners in the western United States, since La Niña is associated with drought in the western parts of the country which sorely needs more rain. Los Angeles has announced some stringent watering restrictions due to impending water shortages, and that means gardeners will have to be especially careful there to use the water they have wisely.

What is La Niña?

Many people have heard the terms La Niña and El Niño but for those who don’t, let me take a few minutes to describe them. You can also read more in my blog post from last fall when this winter’s La Niña was just getting going. El Niño and La Niña are two opposite phases of an oscillation in the atmosphere and ocean in the Eastern Pacific, with neutral conditions in between the two phases as the oscillation swings back and forth like a seesaw. When that region’s sea surface temperature is warmer than usual near the equator, rising air above the warm water creates thunderstorms which act like a rock in a river diverting the flow of air along the southern US, especially in winter when El Niño and La Niña are usually strongest.

In El Niño winters, the Southeast is usually wetter and cooler than usual due to the presence of the subtropical jet stream overhead. It pushes storms with their associated rain and cloudy conditions through the region, recharging soil moisture for the next growing season. In La Niña winters, the jet stream is shifted to the north over the Ohio River Valley, leaving the Southeast warm, dry, and sunny. That means conditions for severe weather are more favorable in the Southeast than in other phases; we have certainly seen plenty of that this year so far. The lack of a strong jet stream also means that tropical activity in the Atlantic Ocean is more frequent and stronger than in El Niño years. In northern parts of the country La Niña winters are usually cold and snowy with a late start to spring, as we have seen this year. The Pacific Northwest is often wet, which also matches what has occurred in their coastal areas this year.

How often does a third year of La Niña occur?

The atmosphere usually swings back and forth between El Niño and La Niña roughly every 3-5 years. Right now we are ending a second consecutive winter of La Niña; with its predicted continuation, that would make it three years in a row. This is not unprecedented, but it is certainly unusual, since 1950 we have only had two “triple-dip” La Niñas. Since there are so few direct comparisons it can be hard to determine exactly what to expect this growing season and on into fall and winter. Our best bet is to assume that typical La Niña conditions will occur. The 3-month composites of the expected anomalies (differences from average; MAM means March-April-May, etc.) show the seasonal variability of El Niño and La Niña for temperature and precipitation across the US. La Niña and El Niño’s effects stretch far beyond the US and affect global weather patterns.

What does this mean for the growing season across the United States?

Typically effects from a La Niña are weakest in the summer because sea surface temperature anomalies are not strong and is often switching from La Niña through neutral conditions to an El Niño the next year. However this year the La Niña is still going strong, so this seems less likely. That means the pattern of warm and dry southern states are likely to continue, which is trouble for the already drought-ridden Southwest including California (where severe water restrictions are now in place). With the high temperatures, low rainfall, and low humidity, that means water stress on gardens will be higher than normal, and drought and wildfires could dominate that part of the country for the next few months.

In the Southeast, the active spring severe weather season will likely give way to an active tropical season in summer and fall. Rainfall in the Southeast in summer is dominated by tropical systems and small-scale convective rain events that provide only hit-or-miss rain. If you are in the path of a tropical storm, you can experience several inches of rain while areas a few counties away can see none, resulting in a feast or famine of rain. In the Pacific Northwest wet conditions in coastal areas will give way to drier conditions in the summer but may return again in the fall, while inland areas may continue to see very dry conditions that will lead to increasing drought and water shortages. The Northeast could see wetter than normal conditions so a drought there this year seems unlikely. The central part of the United States could be the hardest hit by drought conditions and the drought that is already present across a large part of the central and western US is likely to get worse over the next few months with little rain expected. That will affect not only gardeners but the farmers of the main grain-growing area of the US, at a time when Ukraine, normally a big grain producer, is not likely to be able to produce a regular crop this year because of the ongoing war.

Managing your garden in La Niña

Gardeners in the Southwestern US will have the most difficult conditions to manage this year due to the water restrictions and ongoing drought there. Proper use of irrigation and conserving soil moisture through mulch and appropriate choice of plants are good ways to keep water use lower. This may also be true of gardeners in the central US, where the drought could also be severe this summer. In the Southeast, the summer rain you get will depend on tropical activity and where the storms go so you could see either wet or dry conditions. Managing your garden for both dry periods and potentially heavy rains is a challenge that you may need to deal with this year. In the Northeast, the climate may be easier to contend with this year but even short-term dryness can be a problem for plants that need regular infusions of water. In the Pacific Northwest, predictions for a warmer and drier than usual summer mean you should pay careful attention to water-conserving measures, especially in inland areas where drought is already a problem. If you are outside the US, then make sure you understand how La Niña is likely to affect your region and manage your garden accordingly.

I started writing for The Garden Professors a little over a year ago. My very first posting was on “The weather where you are.” In that article, I described some simple ways to measure the microclimates around your yard using some simple hand instruments. But many of you are already well past that and have your own weather stations. For those of you who don’t, here are some considerations for adding a weather station to your garden and a shameless plug for CoCoRaHS (Community Collaborative Rain Hail and Snow Network), a citizen science network of rainfall (and snowfall!) observers around the United States and Canada as well as a few additional stations in Mexico and the Bahamas. I am the current state CoCoRaHS coordinator for Georgia and we are in the last week of the March Madness competition to sign up new observers that they have every year. Even though this year’s competition ends on March 31 you can sign up and contribute to the precipitation record for your state any time. They have links to purchase their required rain gauge on their website on the bottom right side. They also have a very useful guide for Master Gardeners. If you are not in the United States or neighboring countries, you may be able to find rainfall observing networks in your country that you can join as well.

Equipment that is used to measure the weather at a location can vary from a very simple thermometer and rain gauge that you can buy at a hardware store to a sophisticated piece of equipment holding multiple sensors that costs thousands of dollars. The research-grade Campbell Scientific stations that we use in the University of Georgia Weather Network cost about $12,000 each, which is well out of reach of most homeowners, but there are plenty of options for weather enthusiasts that are much more reasonable in price.

A basic weather station may just measure a few variables like temperature and pressure but most people like to add additional sensors like humidity, precipitation, and wind speed and direction. If you are even more ambitious, you might add solar radiation, soil temperature and moisture, and more specialized sensors like leaf wetness. Weather Underground has a useful list of personal weather stations with some details about what sensors each one has, although you will have to click through the links to get pricing. Weather Underground also provides information on how to hook up some of these stations to the web so that you can share your weather information with others and contribute to their own citizen science network.

The single most important factor in getting useful information from your weather station is putting it in a good location. The weather station should be sited where there is good air flow so that you get a representative temperature and humidity for the area. The temperature sensor should also be shaded so that it does not warm up due to direct sunlight. Many stations include an enclosure to shield the thermometer from the sun’s energy. The enclosures are usually white to reflect sunlight and have louvers to let air flow through the enclosure. Some use fans to increase the ventilation of the temperature sensor, especially when winds are light.

Rainfall measurements also require good siting. Precipitation gauges should be placed where they will not feel the effects of any nearby trees or buildings. Usually you need a cone of 45 degrees wide above the top of your rain gauge that does not have any blockage from trees or buildings. Even that may not be enough in all conditions. My own rain gauge is located to the west of my house because that is the only open spot in my tree-filled yard and I notice that in storm systems with wind from the east, the rainfall is lower than other nearby stations because the building is blocking the wind and keeps some of the rain from falling into the gauge. Obviously, you don’t want any moisture from trees, wires, eaves, or fenceposts dripping into the gauge, so look around before you settle on a spot. Dr. Peggy LeMone from the National Center for Atmospheric Research in Colorado described her struggles with making accurate rainfall measurements and why siting is important after a big rain event in 2013.

Rain gauges come in a variety of types. The simplest is a can or tube with vertical walls that you can use to catch rain and measure it at regular intervals (usually once a day at the same time each day for consistency). The CoCoRaHS gauge is a 4-inch diameter plastic tube with a funnel and an inner and outer cylinder that can be easily read to 0.01 inches. It holds up to 11 inches in all, and in some big rain events, it might need to be emptied several times in a day! Many personal weather stations use a tipping bucket rain gauge that has an opening with a funnel that drips the water into a bucket that has two sides on a pivot point. The National Weather Service uses weighing rain gauges to calculate the depth of precipitation based on the weight of the water inside the gauge. The Weather Makers has a good description of how these three types of gauges work as well as illustrations about what they look like. Other newer types of rain gauges include optical gauges that use a photoelectric eye to count water droplets as they pass through a funnel past a light source and haptic gauges that use the sound of raindrops hitting a surface to estimate how much rain has fallen based on the raindrop impacts.

Wind sensors should also be placed in an open area with no blockages from trees or buildings nearby. Putting them on top of a roof might seem like a good idea, but the wind flow over the roof can divert the air and speed it up, so that is generally not a good place to put them, although they are certainly very decorative. Some wind sensors have separate instruments for measuring the speed and direction of the wind while others use a combined sensor that can do both at once.

If you love the weather and want to know more about what is happening in your yard or garden, adding a weather station can provide you with entertainment as well as information that can be helpful to track the climate conditions in your garden such as when frost occurs and how much rain you got so you can water appropriately. It also provides a great place to compare conditions with the other gardeners in your area—you might be surprised at how measurements change from one neighborhood to the next!

Is it spring yet where you are? How can you tell? Here in the Southeast, we are well along the path to spring, even though the calendar says we are still in winter. I can tell by the daffodils, spring peepers, and migrating birds I see overhead. I know those of you farther north may not be seeing any signs of spring yet, with winter storms still moving through your states and lots of snow on the ground as well as frigid temperatures, but trust me, it is coming!

What is phenology?

I first heard the description of the onset of spring as the “green wave” in “The American Seasons”, a book by naturalist Edwin Way Teale. It refers to the northward movement of the appearance of the first green leaf on bushes and trees as warmer temperatures move north and the days get longer. I find it to be a very imaginative and effective way of visualizing how spring moves from south to north (in the Northern Hemisphere) over the course of the season. Phenology is the study of when specific biological and natural events occur, such as seeing the first green leaf of the year, watching your forsythia bloom, seeing your local lake freeze over, seeing sandhill cranes fly north on their annual migration, or watching your favorite tree reach peak color in fall. Many of you probably keep track of these occurrences in your own gardens and use them to compare the climate from one year to the next. But did you know that there is a whole group of dedicated observers who have done this over long time periods and recorded their data for others to see and use?

The National Phenological Network (NPN) is a group of dedicated citizen scientists and others who keep track of the yearly occurrence of when different indicators occur and report them to the NPN. Maybe some of you are part of this network!  They have an excellent database on their website with information for many different species of plants and birds as well as other interesting phenomena. You can explore it in a number of different ways, including through time series and maps. It helps to know the Latin names for the species you are interested in because different species respond differently to the weather! I even used it a couple of weeks ago to help a film director determine how long he had to shoot a Christmas movie before the trees leafed out in Georgia (response: do it soon!).

Where is the green wave now?

One section of the NPN site shows the 2022 movement of the green wave north with time and how it compares to the long-term average conditions. This week’s map is shown below, with areas later than average highlighted in blue and areas that are earlier than average in red. You can see that while southern Florida was ahead of normal, the green wave slowed up quite a bit later in January and early February as colder temperatures covered a lot of the region. That has switched more recently, with warm temperatures across the southern Plains showing the green wave reaching there about four days earlier than usual. Spring is also early coming to large parts of the West Coast, which is currently experiencing much warmer than normal conditions in most areas. If your area is not colored yet, you are still in the depths of winter, but keep watching and spring will (I hope!) be coming soon. I don’t know of a similar product in other parts of the world, but if any of you know, please share the information in the comments.

What do phenological records tell us about climate change?

While our local records in the United States are only a couple of hundred years old at most, other parts of the world have much longer records. Last year, Japanese scientists released a graph showing the change in the peak bloom date of cherry trees in Kyoto, Japan, for the year 800 to the present. While there are a lot of ups and downs over time, the trend towards an earlier peak bloom in more recent years is unmistakable. Since 1912, the average peak bloom date for the cherry trees in Washington, DC, has also shifted forward from April 5 to March 31. Other records showing the warming of the world include migration patterns of birds, pollen counts from trees, and ice-off dates on lakes in colder areas. Glacial ice and sediment cores from lakes and the ocean can provide timelines of how local biological systems have changed over time periods going back thousands of years. Many scientists are worried about the long-term consequences of these changes since not all species are migrating at the same rate and so some animals, birds, and insects may outrun their main sources of food if they move north faster than the plants that feed them.

Phenological records are important for monitoring long-term climate change because the records go back in time much farther than instrumental weather records do. Even though blooms and leaves on plants respond to temperature and sunlight in a non-linear way because they integrate all of the influences into one observed piece of data, they can still provide very useful information about how the environment is changing over time. A really interesting related use of this information was described recently in a story showing that the meteor that ended the Cretaceous period 66 million years ago probably occurred in spring due to the remains of fish that died in the devastating massive waves in the Gulf of Mexico that occurred after the meteor hit. Scientists assumed that the fish died immediately following the impact, and used their bones to determine that the fish were early in their annual growing cycle. Similar work has used buried vegetation to trace past tsunamis in coastal areas that may have been linked to other asteroid impacts or earthquakes that occurred before history was written down.

Wherever you are, I hope you enjoy watching the change in the seasons and in the world around you as much as I do. In spring, every day is a new adventure in seeing what is changing and hoping for the summer to come. I encourage you to keep a diary or other record of what changes are occurring in your garden so that you can see for yourself how the climate is changing from year to year.

If you have watched the news at all in the last two weeks, you know that there was a huge underwater volcanic eruption near Tonga in the South Pacific Ocean on January 15, 2022, that spewed ash and gases into the atmosphere. It blew with such force that the sound of the eruption was heard in Alaska thousands of miles away and the atmospheric pressure wave it set off has traveled around the earth as many as ten times according to satellite and ground-based sensors. With such a large signal, you might wonder what impact the eruption could have on our weather and climate for the next year. In this post, we will explore how volcanoes in general can affect the climate around the world and whether the Tonga eruption is likely to change our gardens’ climate this year.

What do volcanic eruptions emit into the atmosphere?

When volcanoes erupt they put out both ash and gases. The ash is made of tiny particles of rocky material from solidified lava and sometimes pieces of the volcano destroyed by the eruption. These particles are carried downwind in a direction determined by the winds at the heights to which the ash can rise. In a long eruption, the plume of ash can blow in a different direction each day, covering the ground when it falls back to earth. Usually ash does not rise very high in the atmosphere because it is quite heavy and so most of it falls out in just a few days.

Volcanoes also emit gases as they erupt. About 99 percent of all emissions are water vapor, carbon dioxide, and sulfur dioxide, with some trace amounts of hydrogen sulfide, carbon monoxide, and other minor gases. The gases are lighter than ash and so they can get lofted much higher up into the atmosphere than ash can. Because water vapor and carbon dioxide are greenhouse gases volcanic eruptions are often blamed for the recent rise in carbon dioxide in the atmosphere instead of human causes. A careful analysis of the relative amounts of carbon dioxide from the two sources easily shows that volcanic activity only contributes in a small way to greenhouse warming compared to fossil fuel burning and land-use changes. But the gases emitted do have a short-term effect on climate that can last several years in the largest tropical eruptions.

What causes volcanic cooling?

Volcanic cooling of the climate is due mainly to the effects of sulfur dioxide and water vapor. As the gases rise, the water vapor condenses and joins with the sulfur dioxide to form tiny droplets of sulfuric acid that can rise to 50,000 feet or more, higher than most commercial jets fly. Those droplets are as shiny as the glass beads they use in stop-sign paint to make the signs reflective, and the droplets have the same effect on incoming sunlight. When they reflect sunlight back to space before it can reach the earth’s surface it reduces the energy we receive at the ground, and the earth gets cooler until those droplets fall out of the atmosphere. Because of their height and small size, that can take several years.

How does the cooling affect global climate?

We know that when you have a large volcanic eruption emitting a lot of sulfur dioxide, especially if it happens in the tropics where the sunlight is most direct, you can see cooling around the globe for the next 2-7 years depending on how much gas the eruption puts out. In the worst case, an eruption like Mount Tambora in 1815 in present-day Indonesia (along with some other eruptions around the same time) resulted in the “Year Without a Summer” in 1816. In the United States, frost was observed every month of the year in New England and eastern Canada, resulting in the loss of many crops. Even the crops that survived had low yields and poor quality that resulted in dramatic increases in food prices. Europe also saw very cold temperatures that resulted in food shortages there.

Other more recent eruptions have also had some impact on global climate, although none was as severe as the Tambora eruption. The most recent large eruption that affected global climate occurred in 1991 with the eruption of Philippine volcano Mount Pinatubo. As the volcanic emissions spread around the globe, the earth’s annual temperature dropped by almost 1 degree F in the years 1991–1993. Sunsets were also spectacular with the scattering of sunlight from the aerosols high in the atmosphere. Some scientists think painters like J. M. W. Turner were inspired by the spectacular sunsets that occurred after volcanic eruptions in the 1800’s.

Will the Tonga eruption affect the climate in our gardens in the next few years?

Since this is a blog for gardeners, you might want to know if the recent eruption will affect the climate in the same way that other eruptions like Tambora and Pinatubo did. If it is going to be much colder than average, then that could affect what you plant in your garden, especially if the plants you want to use are sensitive to frost. Or it could tell you that you might want to hold off on planting those tomato seedlings a little later than usual in spring. In this case, the amount of sulfur put out by the Tonga volcano was only about 60 kilotons compared to 20,000 for Pinatubo, so any cooling effects from the most recent eruption are so small that we will not be able to observe them. Gardeners can breathe a sign of relief this time! But when the next big eruption occurs, the climate may temporarily cool for a few years before it starts to warm again under the impacts of the “human volcano” emitting many more gases and pollutants than natural volcanoes into the atmosphere.

This week is the end of 2021 and the start of the new year. What a year 2021 has been! Without even talking about politics, COVID-19, sports, or the economy, it was certainly one to remember from the standpoint of weather and climate. No matter where you live, you probably saw some extreme weather during the past 12 months.

Exploding flower bed fireworks, Eric Kilby via Commons Wikimedia.

Extreme weather in 2021

In the United States, the map below shows just the 2021 billion-dollar disasters through October 8. That does not include the tornadoes that ravaged the Midwest, including Mayfield KY, in early December or the fires that burned through the suburbs between Boulder and Denver CO, earlier this week, since those losses have not yet been tabulated. This also does not include the terrible disasters that happened in other parts of the world, such as the devastating spring frost in France’s wine country or the awful flooding in parts of Germany and Belgium last summer. While there is no doubt that a warming climate is partially to blame for many of these disasters, we are also putting ourselves in harm’s way by building in areas that are prone to flooding, wildfires, and other natural hazards that can lead to human disasters. Even if the climate were not changing, we are making matters worse by putting ourselves at higher risk in the way we build and develop land.

Looking back over last year’s climate

Climatologists are generally very busy this time of year, since everyone (especially the media) wants to know how the year that just ended compared to previous years. While it usually takes a few days for the preliminary data to be complete, and a few months before the final quality-controlled data are available, we can take a quick look at the past year using online tools like the High Plains Regional Climate Center’s ACIS Maps that compile climate information into simple displays. The map below shows the percentage of normal rainfall for 2021 across the contiguous United States. As usual, there are areas with very wet conditions and areas that received less than a quarter of their expected precipitation. Sometimes those areas are not very far apart—just compare southern California with Arizona right across the border. Does the map agree with what you experienced?

If we look at the temperature map below, it shows that very few areas in the United States were colder than normal temperature. What makes this particularly concerning is that our normals were just updated this year, as I discussed in an earlier blog post. Because the temperature trend across the United States is upward, not flat, we can expect to see more years above than below normal in the future. This is leading to concerns about increased water and heat stress on gardens and gardeners who are working outside as well as damage to natural ecosystems not adapted to the warmer temperatures.

Reviewing your garden in 2021

New Year’s is a great time to evaluate the past year and plan for the next twelve months in your garden, too. If you have the chance, take a walk through your garden and see how it looks (of course, that assumes it is not covered by snow). How did your plants, shrubs, and trees do this year? Are new plantings well established, or do they need to be moved or replaced? Are your new trees correctly planted and growing well? Are you maintaining soil health and moisture with appropriate surface cover? If not, there is plenty of advice on how to correct problems in this blog—just do a search to find information that is based on current science, not hype. It’s also a great time to think about what you will be doing with your garden in the coming year. Perhaps the warmer temperatures will allow you to try new plants that you have not been able to grow before. I know many of you are already looking at the new seed and garden catalogs for next year, so dream away!

Don’t forget to prepare for bad weather, too

In addition to your planning for next year’s garden, don’t forget to prepare for extreme weather, too. Have a plan for where to go when severe weather threatens, and how to contact each other if you are away from home when it strikes. Keep an eye on weather forecasts so you know when conditions are likely to threaten. Have multiple ways to get severe weather warnings, including a NOAA weather radio and a cell phone that is charged and ON with the volume turned up and an appropriate warning app or two loaded. Don’t count on an outdoor siren to wake you up—they are not designed to warn people indoors. Get helmets to protect your heads from falling or flying debris if you can. Make an inventory of your household goods and store it somewhere safe (a good idea for any disaster, not just a weather event).

See you in 2022!

I’ve enjoyed sharing some of my weather and climate knowledge with you over the past few months and look forward to continuing this in 2022. I’ve also learned a tremendous amount from my co-authors and know that you have, too. Enjoy the rest of your winter holidays! I hope that your 2022 is beautiful, safe, and productive for all of you and your gardens too.

Linda’s post last week about “drought-resistant” plants made me think about drought and how different types of drought affect gardeners in different ways. In her article, she defined drought as “an unusual lack of rainfall”. This is one of four different kinds of drought that climatologists talk about, and I thought it might be interesting for you to hear about how the four (or maybe five) types of drought differ and how they affect gardeners in diverse ways. A great source of drought information across the U.S. is https://www.drought.gov/.

Meteorological drought

The first type of drought, the one Linda described last week, is what climatologists consider a meteorological drought. A meteorological drought is related to how much rain you get compared to usual conditions at your location. I like to think of it as “too many days of nice weather in a row”, since in these dry conditions, the sun is shining and it is a great time to garden, play golf, or do construction. Of course, if you don’t get rain for a long time, you start to see impacts on plants, water bodies, and wells, but meteorological drought is usually not identified in terms of impacts, just on the amount of precipitation measured over weeks, months or years. Meteorological droughts look different depending on where you are. It is possible to have drought even in a desert if rain does not fall over an unusually long time. Droughts in the Pacific Northwest might look quite different since the frequency and amount of rain looks a lot different there. In the Southeast, drought can be hard to identify by looks since even when rain does not fall for a long time, things tend to stay relatively green because in our worst droughts we still get 35 inches of rain a year. Most gardeners can cope with meteorological drought by watering their plants at appropriate intervals and reducing impacts of the dry conditions by mulching to help keep moisture in the soil.

Agricultural drought

I spend a lot of time talking about agricultural drought to the farmers and extension agents I work with, because agricultural drought is always on their mind. Agricultural drought is defined by a negative water balance that can be related to both lack of rainfall and/or high temperatures that increase evaporative water stress on growing plants. It occurs mainly in the growing season because that is when the crops are actively growing and impacts are most noticeable. A 3-week dry spell may not be a problem for most gardeners that water their plots, but if you are a dryland farmer without irrigation, you can lose an entire crop of corn for the year if the dry spell occurs when the corn is pollinating and the silk dries out before the pollen can stick to it. Often agricultural drought can occur even when there are no other impacts to us because it is subtle; most people don’t see the impacts until months later during harvest. If you have limited access to water for irrigation or very sandy soil in your garden, then you are more likely to be affected by agricultural drought since it will be harder to maintain plant health when the soil is dry.

Agricultural droughts are often related to flash droughts. Flash droughts are characterized by very rapid development or intensification over a short time period, and crops are often the first things affected because of their need for frequent watering. Flash droughts are often characterized by a lengthy dry spell coupled with very high temperatures, something that is common when you have a persistent area of high pressure right over your location.

Hydrological drought

Where agricultural drought is related to a shortage of water over time periods as short as a week to a month, hydrological drought is related to a shortage of water over months or years. Climatologists measure hydrological drought as precipitation deficits over periods that range from three months to multiple years. You can see hydrological drought in dropping stream, lake, and reservoir levels and in dropping groundwater levels if the deficit lasts long enough. A hydrological drought can occur even if no agricultural drought is observed when you get rain at frequent intervals but it is less than normal over a long time period, as long as the rainfall is enough to sustain the crops (or if it is winter, when there are not many crops growing).

Hydrological drought tends to affect gardeners’ access to water for irrigation because the long-term water deficits lead communities to enact water conservation measures to protect drinking water supplies. Most local and state governments have tiered conservation measures that get more strict as the water supplies get lower and lower. They may start by merely providing educational materials on water conservation and then progress to even-odd watering by dates or watering during overnight hours only (since there is less loss of water due to evaporation in cooler night-time temperatures). In the worst droughts, they may cut off the use of water for establishing new lawns and gardens (often with an exception for gardens that are used for food production). If a drought lasts for many years or even decades, then it is considered a megadrought, such as the one that is occurring now in the Southwest U.S. Megadroughts are related to long-period shifts in global atmospheric patterns and can lead to the abandonment of cities because of the loss of water to keep their citizens alive over time.

Socio-economic drought

Socio-economic drought is a little different than the other kinds of drought mentioned above. It is drought caused by a lack of water due to overuse, hoarding, or war. An example of a socio-economic drought might be one caused by one country damming a major river in their country to create a reservoir, keeping the river water from flowing downstream to other countries that depend on the water for agriculture or water supply. In the United States, disagreements between who is allowed to use available water often end up in court as cases like the Georgia-Florida “water war” that was recently adjudicated in the U.S. Supreme Court. Locally, disagreements about who is allowed to use the water sometimes result in tiered water pricing, where the more water you use, the higher the price. This affects gardeners who have plots that use a lot of irrigation because of the use of water features, plants with significant water needs, or lack of mulching or other methods of protecting soil moisture.

Recently, a fifth type of drought called ecological drought has been identified, since a lack of rainfall can affect natural ecosystems in ways that are distinct from gardens, farms, or watersheds. I won’t address it further here, but if you are interested in how natural ecosystems are affected by dry conditions, you will no doubt read about ecological drought in publications in the future.

Drought is a naturally occurring part of the climate across the world, and gardeners must understand the nature of drought in their area to recognize how it affects the weather and climate where they live. Linda’s article last week gives some good guidelines for how to make your garden work in your climate.

I got a Facebook message early this week from a friend in Sacramento CA that said after over 200 days with no rain, she got 4.83 inches in a 24-hour period from the latest extreme rainfall that occurred over northern California. Others have reported up to a foot of rain in three days. If you follow the news, you may have heard the term “atmospheric river” used to describe the torrential rains and flooding that have occurred this week in San Francisco and other parts of Northern California. In this post, I want to explain what atmospheric rivers are and how they affect rain climatology in the Western U.S. as well as other parts of the United States and the world.

Tahquamenon Falls–Autumn. Source: Wfgc, Commons Wikimedia.

What is an “atmospheric river”?

The term “atmospheric river” first appeared in the modern scientific literature in the early 1990s. Since it was first used, there has been a lot of discussion about what the term actually means. Commonly, it is seen as a band of very moist air flowing into a coastal area, bringing the potential for a lot of rain to the region that is at the downwind end of the flow. In some respects, it is like being on the receiving end of a firehose streaming high-intensity water right towards you! After a lot of discussion by meteorologists (described in this Bulletin of the American Meteorological Society article) the official definition in the Glossary of Meteorology became:

Atmospheric river – A long, narrow, and transient corridor of strong horizontal water vapor transport that is typically associated with a low-level jet stream ahead of the cold front of an extratropical cyclone. The water vapor in atmospheric rivers is supplied by tropical and/or extratropical moisture sources. Atmospheric rivers frequently lead to heavy precipitation where they are forced upward—for example, by mountains or by ascent in the warm conveyor belt. Horizontal water vapor transport in the mid-latitudes occurs primarily in atmospheric rivers and is focused in the lower troposphere. Atmospheric rivers are the largest “rivers” of fresh water on Earth, transporting on average more than double the flow of the Amazon River.

Source: NASA Earth Observatory

Why do atmospheric rivers produce so much rain?

The strong flow of moisture into a region provides an excellent source of water vapor for the development of heavy rain, especially if it is moving into an area with flow up mountain slopes that can help storms develop vertically. That enhances the rain-producing process. The West Coast of the United States provides a perfect location for the occurrence of atmospheric rivers since there is a broad expanse of ocean to provide the water vapor, dynamic storms that concentrate the flow into bands that can stretch all the way from the Hawaiian Islands to California (which explains an alternate name, “Pineapple Express”), and mountains near the coast to provide lifting for the moist air once it comes onshore. Cliff Mass of the University of Washington often discusses them in his blog on the weather of the Pacific Northwest.

Do atmospheric rivers occur in other places?

The short answer is Yes! While historically they are discussed most often when talking about weather on the West Coast, atmospheric rivers (ARs) can and do occur in other places as well. Anywhere that has a good source of moisture plus dynamic storms with strong airflow can experience ARs. In the Southeastern U.S., we get them when strong flow from the Gulf of Mexico or the Atlantic Ocean feeds into our region, usually ahead of a strong low pressure center that provides the necessary dynamics to create a narrow band of moisture feeding into the region. According to research by University of Georgia researchers, they occur most often in the cold months of November through March but can occur in any month of the year. In the Southeast, we get about 40 events per year that are classified as ARs. I was surprised to read that there are slightly more events on the East Coast than along the Gulf of Mexico, but anywhere along the Southeast coast can be affected. No trend towards more or fewer events was seen in the 1979-2014 period.

NOAA’s Physical Sciences Laboratory’s page describing ARs says that on average, about 30-50% of annual precipitation in the west coast states occurs in just a few AR events, thus contributing to water supply. ARs move with the weather and are present somewhere on the Earth at any given time. This site has some great resources for tracking and forecasting ARs around the world.

Of course, atmospheric rivers are not the only source of heavy rain events, but they are one of the primary sources for the West Coast. In other areas, tropical systems like slow-moving hurricanes or stalled fronts can also drop a lot of rain. You can also get very heavy rains from small local systems of thunderstorms if conditions are right, especially if the storms “train” or move one after another over the same area like cars on a freight train. We saw this in the Nashville area a few weeks ago, where the heavy rains resulted in significant flooding over a few counties.

Rain garden in the Allen Centennial Gardens on the campus of the University of Wisconsin-Madison. Source: James Steakley, Commons Wikimedia.

How do atmospheric rivers and other heavy rain events affect gardeners?

If you are a gardener in the Western United States, you are already well aware of the long dry season over the summer followed by bouts of rain that can occur over the winter months. The timing of the switchover from dry to wet conditions depends on how far north you are on the coast, with the summer dry spell coming earliest in southern California and moving northward with the position of the jet stream as the summer progresses. Dealing with the effects of an AR is like any other attempt to protect your garden against heavy rainfall, and can mean proactive action to make sure that water-sensitive plants and trees are not located in low-lying areas where rain collects. This 2013 article from the Garden Professors blog on-site assessment is still good advice for planning ahead for soggy conditions by walking through your property in the rain. Designing for erosion control, such as rain gardens, can also help divert water in high-intensity rainfall.

In spite of the heavy rain that fell in this last atmospheric river event, the rainfall barely made a dent in the long-term drought that is present across a lot of the Western U.S. Drought will continue to be a part of the hydrologic cycle that affects gardeners, farmers, and water managers across that region and across the world.

Do you wish you had a crystal ball that could tell you what the climate will be next year when you plan your garden? So do many other gardeners (and climatologists). But while there is no magic answer, we do know that in many parts of the United States and other countries, year-to-year climate variability is strongly dominated by what is going on in the eastern tropical Pacific Ocean. This is through a phenomenon called “El Niño Southern Oscillation” or ENSO for short.

What is ENSO and how does it affect climate?

ENSO has three phases—a cold phase with unusually cold water in the equatorial Eastern Pacific Ocean (EPO) called “La Niña”, a warm phase with unusually warm water in the EPO, and the neutral phase that occurs between the two extreme phases. The ocean see-saws back and forth between the two opposite phases on a semi-regular pattern that usually lasts between two and five years from one El Niño to the next. Sometimes you can have two La Niña years (or even three) back-to-back (the end of 2021 is expected to be a second La Niña in a row), but you almost never have two consecutive years of El Niño.

In many parts of the world, the phase of the ENSO is highly correlated with the climate. Scientists can use that relationship to predict what the climate might be like in the coming months. That is helpful for gardeners who need to know what to expect both next season and next year for planning purposes. Not all parts of the world have a climate that is well correlated with ENSO, however, and so folks in those areas will have to depend on other methods to look ahead to next growing season. Winter has the best correlation between ENSO phase and climate, while summer is much less predictable. And every El Niño and La Niña is distinct, leading to variations from the statistical pattern we expect.

How does the temperature of the tropical Pacific Ocean affect climate in other parts of the world?

You might think that unusually warm or cold water in the equatorial Pacific Ocean would not have much impact in other parts of the world because of the distances involved, but it does. Since the atmosphere flows like a river, putting unusually warm water (El Niño) into the EPO acts like putting a rock into a stream. The flow of water (or air) shifts around the rock, changing the pattern of atmospheric winds that blow weather systems around. When we are in a warm El Niño phase, the storm track shifts south and covers the southern US, leaving the northern US warmer and drier than usual. When I lived in Wisconsin, we noted that lake ice cover in El Niño winters did not last as long as other years, which made ice fishermen like my dad unhappy. La Niña shifts the storm track in the opposite direction. Because of that, La Niña winters are colder and wetter than average in the northern US since the storm track shifts north into the Ohio River Valley and sometimes even farther. This leads to cold, damp winters in the northern US. Similar correlations, called teleconnections, are seen statistically in climate records at many places on earth.

If we know what the phase of ENSO is likely to be, that tells us what climate conditions are expected in areas where there is a teleconnection between the EPO and that region. While every El Niño and La Niña is unique, statistically they do provide guidance on what to expect in that region, and most years they are correct, although once in a while a wildcard like a Sudden Stratospheric Warming will occur and give us an occasional busted forecast, as it did in February 2021.

What do we expect this year?

Right now, we are in neutral conditions following last winter’s La Niña, but we are headed back towards another La Niña in the next couple of months (almost an 80% chance in the November through January period). That phase should last for most of the winter but is expected to return to neutral by spring.  After that, it is too far out to make a believable prediction. The Global ENSO Temperature and Precipitation Linear Regressions website provides global correlations between the ENSO phase and what kind of temperature and precipitation anomalies to expect. In it, each three-month period shows the relationship between the temperature anomaly of the EPO and other parts of the world (regression) and how strong that relationship is (correlation).

In the map below for December-February (DJF) temperature, it shows that if the EPO is unusually warm (+) in an El Niño, then the northern part of the US will also be unusually warm (+) while the southern states are cooler than normal (-). The storm track over the southern US in an El Niño year brings rain and clouds to that region, keeping conditions wet and cool due to lack of sunshine. A La Niña year is just the opposite. The strong correlation in both southern and northern states shows that it happens most of the time, but in areas with little correlation, you can’t use ENSO reliably to predict seasonal conditions. If you have a hard time interpreting these maps, the website has a tab that explains it in more detail.

The bottom line

For this coming winter, I expect warmer and drier conditions than usual in the southern tier of US states as the storm track shifts north. That means more overwintering of insect pests and diseases; an early start to the growing season is also likely. The northern US is expected to see colder and wetter conditions than usual, which means a later start to the 2022 growing season but less chance of drought next year, although fungal diseases could be bad if the damp conditions continue into spring and summer. Western Europe could see warmer conditions than usual but the correlation is weak so that is not a strong forecast. Australia is likely to be colder than normal, with a fairly high probability because the correlations are high, at least near the coasts. This should last until spring, when the La Niña ends, and we swing back into neutral conditions when other climate factors become more important. In the Southeast, the summer after a La Niña ends is also a hot and dry summer due to the lack of recharging rain over the winter, so I think we have the potential for drought in the Southeast next summer.

In the past week, two new major climate reports have been released. One is the latest (6^th) report from the Intergovernmental Panel on Climate Change (IPCC) and the other is the State of the Climate 2020 report. Of the two, the IPCC report has garnered a lot more press, but both are compilations of work by hundreds of scientists looking at recent weather and climate patterns and how they are affecting us here on earth. The IPCC report also provides projections of what the future climate might be like, using a number of assumptions about how the earth behaves, which can be difficult, and how humans respond, which is arguably even tougher to determine. In this post, there is no way that I can cover both sets of reports in meaningful detail and I won’t address how we need to address the rapidly changing climate here, but I do want to try to pull out some things that you can use as gardeners now. {Note, the pictures are ones I have taken myself on recent trips to use as eye candy!}

What do the new reports tell us?

The State of the Climate 2020 report, published jointly by NOAA and the American Meteorological Society, focuses on global climate events that happened in 2020. You can read some of the notable findings from the report at my blog. The report also discusses many of the “big” climate events of 2020 and puts them into historical context, including how frequently these extreme events occur and how the changing climate is making them more likely.

The United Nations’ IPCC 6^th Assessment Report presents similar information but also makes more explicit the cause of the warming, which scientists have known for well over 100 years has been caused primarily by human emissions of greenhouse gases in the atmosphere that trap heat near the earth’s surface. The IPCC report makes it clear that the rapid pace of the warming will cause severe changes to the earth’s climate that will be difficult for humans and ecosystems to adapt to.

What do the conclusions of these reports mean for gardeners?

Here are some of the changes that we will have to adapt to in the future:

• Rising temperatures across the globe—Temperatures are rising across nearly all the globe, both on land and in the oceans. Warmer temperatures mean warmer winters, hotter summers, and longer growing seasons. They also mean more increases in both evaporation from water surfaces and more evapotranspiration from plants, resulting in increases in water stress. That means you may need to water more often or use other techniques like mulch to preserve soil moisture. You may also need to switch to more heat-tolerant species as the USDA plant hardiness zones shift north (in the Northern Hemisphere). It may become harder to work in the middle of the day when it is the hottest.
• Rising temperature leads to rising humidity levels, at least where there is a source of water vapor nearby. The higher humidity is contributing to higher night-time temperatures, which puts stress on animals living outdoors (pets, livestock, and wildlife) and also stresses some plant species. It can also lead to more clouds, which reduce direct sunlight and cool the air but also reduce solar radiation available for plants, slowing their development. You may have to manage your gardens for more diseases that are related to the high humidity levels.
• Some areas like the northern US may see more rain, while others like the Southwest become increasingly dry. Year-to-year variability in precipitation is also likely to increase, with both more floods and more droughts. In both cases, water management of your gardens will become increasingly important, with the heavy rain events causing more erosion and the potential for loss of plants and trees from too much water and not enough air in the soil, and the longer dry spells making gardens more dependent on either drought-tolerant species or more frequent irrigation. You may have to put in rain gardens to help slow the movement of water through your gardens in heavy rain.
• With the rising temperatures, frost and snow will become less likely but will still occur (there will still be winter!). This will allow you plant earlier than in previous decades but will still make the plants vulnerable to late-season frosts.
• Increases in carbon dioxide may provide some fertilization of some plants, but only if there is enough water available for growth. Since some weedy species are more efficient at using carbon dioxide than other plants, you may need to deal with more weeds and invasive species in the future than you do now.
• Strong storms like hurricanes and derechos may occur more often and be more damaging than the ones we are already seeing now. The research in this area is less definitive than that for rising temperatures because there are many different factors that go into storm development, but scientists generally agree that the number of hurricanes seems to be climbing upward and that the seasons are getting longer. In addition, the storms appear to be moving slower, and that is likely to lead to more rain from the storms over a specific area and more likelihood of rapid storm development. If you live in an area that is prone to strong thunderstorms or tropical cyclones, you may see them more often and the season may start earlier in the year. Rains and winds are likely to increase, leading to more tree damage and flattened plants.

Will we be able to see these changes over the next few years?

Year-to-year variations in climate will continue to plague gardeners, since whatever happened last year is unlikely to occur again this year. The climate naturally varies over time and space as well as exhibits these long-term changes. That means it can be hard to see the creeping trends in temperature and precipitation in the noise of yearly climate swings. If you are only worried about next year’s garden, what is happening in 50 years may not be of much interest. But if you care about your children’s gardens and their future on a warmer earth, than it is something these two reports make clear we have to think about and do something about.

Personal note: This week I was also invited to participate as an author on another upcoming large climate report, this one the 5^th National Climate Assessment (NCA) that focuses on changing climate in the United States. I will be one of a number of authors contributing to the chapter on the Southeast US. If you are interested in what the content of that report includes, you can view the 4^th National Climate Assessment, released in November 2018. There are chapters for each section of the country, but also chapters that deal with economic sectors like water and agriculture. The 5^th NCA will update the information in the previous version as well as add additional information based on scientific studies completed since then.

References:

The State of the Climate report in a peer-reviewed series published annually as a special supplement to the Bulletin of the American Meteorological Society. The journal makes the full report openly available online, here. NCEI’s high-level overview report is also available online, here.

Sixth Assessment Report, Climate Change 2021: The Physical Science Basis is now out.  The report addresses the most up-to-date physical understanding of the climate system and climate change, bringing together the latest advances in climate science, and combining multiple lines of evidence from paleoclimate, observations, process understanding, and global and regional climate simulations. Get more information including links to the press release and some videos here.

Do you have a favorite kind of weather that you love to experience? For me, it’s the first warm evening of spring, when the air is just warm enough and the wind just strong enough for the air to feel as though it is dripping off my fingers. On nights like that, I can tell that the air really is a fluid, the topic of this week’s blog.

What causes atmospheric weather patterns?

You may have noticed this year that the weather patterns across both the United States and Europe have been very persistent. That has led to the occurrence of record-setting high temperatures in some locations like the Pacific Northwest and the central US and southern Europe as well as day after day of rain in the Southeastern US. Both of these weather patterns have caused no end of grief for gardeners and farmers, since weather is seldom stuck on day after day of “perfect” conditions (even if you could define what those are).

To understand how weather patterns get stuck, it helps to know how the air moves through the atmosphere. Wind is driven by differences in heating between two areas. That contrast leads to differences in density and pressure between the areas, and the air flows from higher pressure to lower pressure to try to equalize the amount of air between those areas. The large-scale weather patterns across the globe are caused by differences in the sun heating the spherical earth at the equator and at the poles at different angles due to latitude. There are also smaller-scale wind circulations due to differences in heating between land and water (oceans or lakes) that cause the same movement of air molecules. The earth is not uniform, with continents in some areas and oceans in others and has mountain ranges that also divert the flow of air. The earth is also rotating, and that makes the wind appear to be turning towards the right in the Northern Hemisphere (left in the Southern Hemisphere), which we call the “Coriolis force.” Friction can also slow down the wind and cause it to change direction near the earth’s surface, which adds to the calculation.

Atmospheric waves

The net result of all these forces acting on the air are a series of atmospheric waves of high and low pressure around the earth that control where the weather goes (these are known as Rossby waves after the great meteorologist Carl-Gustav Rossby). A great website to view these waves is at https://earth.nullschool.net/, with a dynamic view of the air flowing around the earth. You can move the earth around with your mouse by clicking and dragging it, and you can change the size by pinching and dragging on your touchscreen if you don’t have a mouse with a wheel. If you click on “earth” on the bottom left of their map, it pulls up a menu that allows you to pick different heights in the atmosphere (1000 mb is closest to the surface, 500 mb is about halfway up in the atmosphere, and 250 mb (shown in this figure from July 29, 2021) is roughly the height that jets fly.

The image shows areas of strong winds (in red) and weak winds (in blue). The strong winds at 250 mb are called “jet streams”, and they push weather systems around. The strong jet stream over the upper Great Lakes in this image helps explain why the severe weather that moved through Wisconsin on July 29 moved from northwest to southeast. The jet streams outline the atmospheric waves that control the large-scale weather patterns. In the Northern Hemisphere, winds blow clockwise around areas of high pressure (“ridges”) and counterclockwise around low pressure (“troughs”). It is much easier to see on the website than on this static image, although traditional weather maps usually show the wind direction using arrows to help. As the image shows, often a ridge in one half of the continent is accompanied by a trough in the other half just like an ocean wave.

The large-scale weather patterns that are connected to atmospheric waves are constantly shifting as the air in the atmosphere tries to balance out all the forces that are pushing it around. (This 30-second video shows how the waves typically evolve over time.) Often the waves move, usually from west to east in the mid-latitudes, and so the weather at those latitudes also tends to move from west to east. But sometimes the weather patterns get stuck in one spot, and cause day after day of the same weather. We call these “blocking” patterns because they block the natural movement of the waves, locking the weather pattern in place for long periods. That is what we have seen this summer, with a very strong high-pressure center locked over the western U. S. and a trough of low pressure draped over the eastern U. S. for much of the last few months. In summer, high pressure usually causes sinking air, lack of clouds, warm temperatures, light winds, and no rain. In contrast, low pressure leads to rising air that forms clouds and rain and cooler temperatures due to the clouds.

What is happening this summer?

The blocking this summer has been more persistent than usual, with strong high pressure in the west causing a very hot and dry area to form (sometimes called a “heat dome”). Once you get a strong high like that to form, it can get anchored in place by the dry conditions and high temperatures at the surface, making it very hard to move. The length of time that this summer’s high has lasted has contributed to the string of record-setting temperatures and lack of rain and subsequent drought that region has experienced this year. By contrast, in the Southeast, low pressure has led to a stubborn low-pressure trough that has brought rain to the region almost every day, preventing farmers from doing field work or drying hay and increasing the occurrence of fungal diseases on their crops. If you know you are in a blocking pattern, that means you should prepare to experience the same weather for protracted periods of time and adjust your gardening schedule to accommodate the weather you are likely to see for the next week.

How will atmospheric patterns change in the future?

Some but not all atmospheric scientists think that as the earth gets warmer, the temperature difference between the equator and the poles will decrease and that this will lead to atmospheric wave patterns that are loopier, like winding rivers in an almost flat coastal plain. This could lead to both more extreme weather and potentially, more frequent blocking patterns. Gardeners need to prepare by designing their gardens to handle both more extremes of weather, including heat waves and floods, and periods of more persistent weather, which could lead to more frequent and longer droughts. That means making sure that you need to have good drainage for high-intensity rainfall events but also need to use methods like mulching with arborist chips to preserve the moisture in the soil for those times when no rain is in the forecast for extended periods.