Here’s the release from the USGS:

Forests across the country have been impacted by increasingly prevalent drought conditions in the last two decades, according to a new study by scientists from 14 research institutions.

Drought has the potential to impact the distribution of species, biodiversity, and the goods and services that forests provide. These and other responses have already been observed in the West, a region which has experienced its fair share of drought and resulting forest diebacks (when a community of trees has above average mortality rates), changes in the composition and structure of forests, and wildfires. Droughts are also now affecting forests in the eastern U.S., in particular in the South and Upper Midwest—regions where there is comparatively less information on how drought affects forests, particularly at the larger ‘stand’ (forest community) scale.

Understanding how drought has already affected forests will help managers anticipate what the broader scale impacts of drought will look like in the future as the climate continues to change. To address this need, researchers – including Southwest Climate Science Center director Stephen Jackson – produced a synthesis of the current state of knowledge of the effects of drought on forests across the continental U.S. The results were recently published in the journal Global Change Biology.

Researchers synthesized studies on the effects of drought on tree growth and mortality, tree recruitment, range shifts, and community composition. They also examined the role of forest management practices in mitigating and exacerbating the effects of drought, and summarized various modeling approaches used to estimate forest change.

Key findings include:

• In the West, stand- and regional-level consequences of drought are well-documented. Extensive drought has coincided with reduced tree growth and an increased risk of mortality. In the East, there is very little information available at the stand-scale, though numerous studies have looked at individual tree response.
• Human fire suppression in the West has resulted in higher density stands comprised of small-diameter trees. High density forests face increased moisture stress during drought, and fires that occur in these forests have more fuel and are more severe than those that occur under low-density forest conditions.
• In the eastern U.S., where there are large areas of low relief, even a modest change in climate can result in large shifts in the location of suitable habitat. Meanwhile, in the more mountainous West, moisture and temperature gradients are more compact and migration has the potential to be more effective.
• Models suggest that suitable habitats of many tree species are shifting faster than populations are adjusting by adaptation or migration.

Interestingly, the researchers note that forest managers already engage in several practices that can be harnessed to mitigate the effects of drought. For example, thinning can be used to selectively remove moisture-demanding species, allowing drought tolerant species to take their place and improving moisture access to remaining trees. Artificial regeneration (i.e. plantings) can also be implemented during drought to help forests recover.

Anticipating the future of forests is a challenging task that relies heavily on observational data, which is difficult to extrapolate to larger scales and is unavailable in some parts of the country. Furthermore, other variables that affect forests are changing simultaneously, such as rising CO2 levels, nitrogen deposition, and invasive species, making it difficult to attribute observed changes in forests to drought.

The results of this study show that we have some understanding of how drought affects the individual trees that make up a forest. However, we do not yet have sufficient information to predict the future of forest ecosystems in the face of a changing climate. The authors emphasize that future research should prioritize identifying how drought effects forests at the larger community scale, so that managers can take appropriate action to protect forests.

From NOAA (Anthony Barnston):

Neutral ENSO conditions reigned during June and so far in July, as the sea surface temperature in the Niño3.4 region, often used to measure the El Niño/La Niña (ENSO) state, was between -0.5°C and 0.5°C away from average. ENSO-neutral means forecasters can relax a bit, and phones are no longer ringing off the hook now that this whale of an El Niño finally bit the dust.

However, we can’t rest too much. During the last 60 or so years, strong El Niño years have often been followed by La Niña years. There are known physical reasons for this, which we have covered (here and here). Although the record is short, it appears the stronger the El Niño, the greater the chance for a La Niña the next year. And we just had a pretty strong El Niño!

Sure enough, forecasters currently believe there is an approximately 50% chance that sea surface temperatures for the Niño3.4 region will drop to at least -0.5°C below average (qualifying as a La Niña condition) during the August–October season, and a 55-60% chance for La Niña for the fall. Although 55-60% is far from a sure thing, it means that getting some grade of La Niña, even if just a weak one, is more likely than not.

At the moment, there is a good supply of cooler-than-average water below the surface in the central and eastern tropical Pacific. This cold water has been making contact with the surface along portions of this vast stretch of ocean right along the equator, and the Niño-3.4 index dropped into weakly negative territory by late June as a result.

Weakened Probabilities for La Niña
However, the 55-60% chance for La Niña represents a noticeable drop in probability from last month, which had peak probabilities of about 75%. Why the decrease? A typical cop-out answer would be “The ENSO forecast models now show a decrease in the likelihood, and also in the peak La Niña strength, and we trust them.”

But why are they showing the decrease? What are the models seeing that the sea gull of average intelligence flying over one of the TAO or Argo buoys in the eastern tropical Pacific might not see? Or that some egghead scientist sitting on the 13th floor of an ivory tower at a high-powered university overlooking a green hill might miss in eyeballing a dozen busy-looking charts and graphs?

First, let’s probe the obvious, and look at the recent monthly rate of decrease of the ocean temperature in the Niño3.4 region since the peak of the 2015-16 El Niño, in comparison with what happened following other past El Niño events—in particular, the very strong 1997-98 and 1982-83 events. The figure below shows the result.

We can see that between May and June this year, the temperature decrease lost pace with that of 1998, after more closely paralleling it up through May. Note that a significant La Niña developed by late summer 1998. However, this year’s rate of decrease has been steeper, so far, than the one in 1983, and a weak La Niña condition even emerged in late 1983 (but it didn’t last long enough to make in on NOAA/CPC’s official list). So far, we’re on a trajectory this year that is somewhere in between those two cases.

But looking carefully at the behavior of 1998, 1983 and other significant post-El Niño cases like 1973, 1992, 1958 and 1988, we see that monthly fluctuations in behavior may not mean much in the grander scheme of things. Remember that ENSO usually acts more on a seasonal time scale than a monthly one, although ENSO-relevant events can occur within a single month from time to time, such as when the MJO interacts with ENSO. We may not have cooled at a very fast rate between May and June, but could easily make up for it in the coming months. So then, why are the models weakening their forecasts at this point in time?

One likely answer is the delay in the expected onset of La Niña-like ocean-atmosphere coupling in the eastern tropical Pacific Ocean. What is coupling? To set the stage, cooler than average water currently sits beneath the surface, and is now impacting the surface across the eastern Pacific Ocean, as seen in the figure below.

With below-average water temperature along the equator in the eastern tropical Pacific, we would expect some atmospheric coupling, such as enhanced trade winds that blow from the colder-than-average water in the east toward the warmer-than-average waters of the central and western Tropical Pacific. Sinking motion (higher surface pressure) tends to occur over anomalously cool water, with rising motion (lower pressure) over warm water. And surface winds blow from high toward low pressure. If the trade winds were to increase, they would cause upwelling of the even more anomalously cold waters from below the surface, which would then cause even stronger trade winds, and so on—the typical La Niña feedback loop. This is nothing more than a strengthening of the Walker circulation that typifies La Niña.

Enhanced Trades—Where Are You??
But the first nod toward above-average trade winds has not yet been observed, as we can see in the figure below. Trades are very close to average across most of the tropical Pacific.

To get a La Niña started, someone—the ocean or the atmosphere—has to make the first move. The ocean has done that—the ocean surface has cooled somewhat, and appears to be “waiting” for some stronger surface winds to upwell still colder water. To get the La Niña to grow, we need more upwelling. It is the atmosphere’s turn to play ball. Is it going to kick in or isn’t it?

Climate forecast models, which look at the latest observations, are beginning to wonder the same thing. Therefore, they recently have become less bullish on the La Niña—both for the rate of onset and the peak strength later in 2016. But they are not forecasting a cancellation of the event, so we need to keep our eyes peeled over the coming weeks and months to see if we get some action.