The tropical ocean: more than just hurricanes

The Climate Prediction Center recently released a new El Nino Advisor [link]. The advisory says essentially that all indicators suggest that the current El Nino is still strengthening, and is expected to last through the winter and maybe into spring. It's likely, based on past El Nino events, that the largest anomalies of the tropical sea-surface temperature will happen some time in the next couple of months.

Recall that we knew this El Nino was forming last spring and summer, and that's why the Atlantic hurricane season was forecast to be relatively inactive. As we've seen, that forecast was pretty successful; we've only gotten up to "Ida" in the tropical storm names. The presence of El Nino conditions in the tropical Pacific ocean has effects that reach beyond hurricanes though, as this paragraph from the advisory lists:
Expected El NiƱo impacts during November 2009-January 2010 include enhanced precipitation over the central tropical Pacific Ocean and a continuation of drier-than-average conditions over Indonesia. For the contiguous United States, potential impacts include above-average precipitation for Florida, central and eastern Texas, and California, with below-average precipitation for parts of the Pacific Northwest. Above-average temperatures and below-average snowfall is most likely for the Northern Rockies, Northern Plains, and Upper Midwest, while below-average temperatures are expected for the southeastern states.

So those of you up north and in Seattle can probably expect relatively mild winters, which might not be bad news! Meanwhile, California is expected to have a wetter than normal year, which so far looks to be true. Some of these correlations aren't very robust, so you can't really count on them, but so far they seem to be holding.

It is also worth noting that the effects of the tropical oceans are not limited to this kind of El Nino action. There's a flip-side to the story, too, which has come to be called La Nina. This is the cold phase of the oscillation, when the eastern tropical pacific is a bit cooler than normal. An interesting side effect of La Nina conditions in the tropical Pacific ocean is that precipitation tends to decrease over the central part of the USA, especially Texas, but extending north into the upper midwest and also west through the southwest and California. The Pacific Northwest and much of the southeast experience extra precipitation. The crazy thing is that it isn't just a seasonal effect, but can be clearly seen at longer time scales. A new paper by McCrary and Randall (2009, link) examines this relationship in observations and climate models, confirming what I've just said on timescales of 6 years and longer. Much of the paper deals with comparing three leading climate models with the observed 20th Century droughts in the USA. While they find that the models do capture some aspects/statistics of long-term drought in the central USA, none of the models seems to convincingly capture the relationships between tropical ocean variability and precipitation seen in the observations.

Of course, the fact that the models struggle to establish these connections between the tropics and the extratropics does not come as a great surprise. A key challenge for these comprehensive climate models is to produce realistic patterns and cycles of El Nino and La Nina. One of the models in McCrary & Randall (2009) is the Community Climate System Model (v3.0), which is known to have an overly regular cycle of El Ninos, with a period of about 2 years. Along with this regular cycle, the observed connections with remote regions is underrepresented. (link) So when looking for longer-term variations, it's unlikely for CCSM to have realistic patterns. In this case, the CCSM's long-term droughts don't seem to be very connected to the tropical oceans at all. The other models have different problems, but do notably better at establishing at least some relationship between cool tropical Pacific surface temperatures and increased likelihood of drought conditions in the central USA.

A key point to emerge from this analysis is that the climate models only marginally represent long-term droughts, and without very convincing physical processes compared to the observations. This means that these models are not necessarily proper tools for studying the frequency of droughts in the future. This hasn't stopped people from doing just that, as the authors note. So if you come across stories about changes in drought, pay close attention to the methods used, and keep a skeptical view of the findings. In the meantime, climate models are now being developed that have much improved representations of El Nino and La Nina (see link above, e.g.), so the next generation of climate models may have more credible (and interesting) droughts. And if you're an optimist, they might even teach us something about how the future of the USA's grain belt will look, and if you are very optimistic, maybe they won't point toward perpetual Dust Bowl conditions in the future.

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