Trends in the central tropical Pacific

Well, since we've been thinking about the tropical Pacific, here's a bit more to ponder. There is an ongoing discussion in the literature about whether global warming, particularly across the tropical Pacific, will look more like El Nino or La Nina. One way this has tended to shake out is that the atmospheric scientists seem to favor El Nino conditions as the world warms, but oceanographers tend to lean toward La Nina [Eos]. The truth of the matter is that in the past couple of years, this has been shown to be a false analogy; it seems like there is evidence that the atmospheric circulation is changing to look somewhat more like El Nino, but changes in the ocean act against some of these atmospheric effects, looking more like La Nina. In fact, as these arguments mature, it seems like the dynamics involved are not really related to the dynamics that control El Nino and La Nina cycles [Vecchi].

The apparent controversy, though, is too good not to glom onto, and many authors have used it as a construct to present results. This is fine except that it clouds the emerging picture of climate change in the tropical Pacific.

A new paper by Nurhati et al (2009) includes this El Nino/La Nina kind of argument to highlight new isotopic measurements of coral reefs in the central Pacific. The geochemical techniques are applied to coral at three central Pacific islands, and show monthly-resolved temperature and salinity records over the 20th Century. The bottom line seems to be that there are statistically significant linear trends toward fresher, warmer water around these islands. The authors say that these trends are more consistent with more El Nino-like conditions in the central Pacific, are similar to other estimates of temperature changes, and is in line with modeling studies showing decreased upwelling of deep water in a warming world. Only in the final paragraph do the authors finally reveal that this El Nino stuff shouldn't be taken too seriously,

... this analogy likely over-simplifies the complexity of tropical Pacific anthropogenic climate change. Indeed, any of a number of large-scale climate changes that are likely to occur in a greenhouse world might overwhelm or at the very least fundamentally reshape the expected impacts of an “El Niño-like” trend. ... In this regard, the prominent warming and freshening trends uncovered in the coral reconstructions undoubtedly represent a combination of dynamics that are fundamentally different than those associated with the ENSO phenomenon.

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 Niño 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.

El Nino 2009/2010... waiting and watching

I am finally giving in and starting what I'm sure will become a series of updates on the emerging warm-phase of the ENSO phenomenon, namely El Nino. It's becoming pretty apparent that the tropical eastern Pacific is anomalously warm, and is likely to stay that way for the next year or so [LINK]. At this point, what we can say is that the indices that are used operationally to define and track El Nino point toward a moderate to strong El Nino, but their nature is difficult to predict. You can see the development of warm anomalies of 1-2 degrees Celsius along the equator and stretching from South America into the central Pacific Ocean at the CPC page. This is the major symptom of El Nino. The impacts are not completely understood, but the slackening of the trade-winds and shift in northern hemisphere jetstream are normal. These changes tend to increase wind shear over the tropical Atlantic, which reduces the number of Atlantic hurricanes (note there haven't been any yet this year). It's also expected to make the winter of 2009/2010 mild across much of the northern hemisphere, which might lead to 2010 being a record warm year in the global average.

I'm sure we'll revisit the topic frequently in the coming months, including some review of important aspects of ENSO, and maybe some debunking of ENSO myths (e.g., increased rainfall in Southern California?).