A recent post at RealClimate by Matthew England discusses the results of his (and others’) recent paper (England et al. 2014, Recent intensification of wind-driven circulation in the Pacific and the ongoing warming hiatus, Nature Climate Change, doi:10.1038/nclimate2106) about changes in wind patterns in the tropical Pacific, their impact on ocean circulation, and the resulting impact on global temperature.
England gives an excellent 1-paragraph summary thus:
A consistent picture has now emerged to explain the slowdown in global average SAT since 2001 compared to the rapid warming of the 1980s and 1990s: this includes the link between hiatus decades and the Interdecadal Pacific Oscillation, the enhanced ocean heat uptake in the Pacific (see previous posts) and the role of East Pacific cooling. All of these factors are consistent with a picture of strengthened trade winds, enhanced heat uptake in the western Pacific thermocline, and cooling in the east – as you can see in this schematic:
All of this serves to emphasize that many of the proposed explanations for the putative “pause” in global surface temperature are far from mutually exclusive. For instance, England notes “there are obvious parallels to Kosaka and Xie’s study assessing the impact of a cooler East Pacific,” and the cooling effect of the el Nino southern oscillation is also implicated in Foster & Rahmstorf (2011).
What struck me most was their clear statement of the results of their study:
The slowdown in warming occurs as a combined result of both increased heat uptake in the Western Pacific Ocean, and increased cooling of the east and central Pacific (the latter leads to atmospheric teleconnections of reduced warming in other locations). We find that the heat content change within the ocean accounts for about half of the slowdown, the remaining half comes from the atmospheric teleconnections from the east Pacific.
This, of course, made me curious about ocean heat content in the tropical Pacific, how it has changed, and especially what geographic pattern it shows. So, I retrieved ocean heat content data for the upper 700m of the ocean (from NODC, via Climate Explorer).
Here’s the data for the tropical Pacific, which I defined as latitudes from 20S to 20N, longitudes from 120E to 280E:
There’s a clear upward trend overall, but just as clearly the increase has been highly irregular. What this doesn’t tell us, is anything about the contrast between ocean heat content in the western and eastern tropical Pacific regions.
I decided to look at OHC data in small longitude bands, 10 degrees wide each (and extending from 20S to 20N latitude), from longitude 120 to 280 (so the first band is 120-130E, the next 130-140E, etc.). This amounts to 16 longitude bands covering the tropical Pacific. In order to find which patterns dominate these time series, I applied PCA (principal component analysis) [technical note: because all the time series are the same variable on the same scale, the time series were centered but not normalized before computing PCA].
The first PC (principal component) dominates, accounting for 67% of the variance of the data. Usually when such is the case, the dominant PC is close to the overall average of the data (which would well approximate the overall trend). But in this case, the dominant PC turns out to be the contrast between west and east Pacific. Here are the “loadings” (or what I call the loadings, the weight assigned to each longitude band) as well as the resultant time series:
The graph on the left clearly shows that this is the difference between western and eastern Pacific data. When that on the right (the time series) is positive, the west has more heat content than the east, which roughly corresponds to la Nina-like conditions, but when it’s negative it roughly corresponds to el Nino-like conditions. Note particularly the big dip around 1998 when we experienced the very strong el Nino. Also obvious is that the trend over the most recent decade or so has been decidedly upward (more heat in the west, less in the east) which confirms that this pattern anti-correlates with surface air temperature time series.
England et al. also demonstrate the very recent trend in surface winds over the tropical Pacific which is the cause of much of the fluctuations in ocean heat content, especially the contrast between east and west. This lends considerable credence to their hypothesis, and the fact that it is in accord with the ideas of others makes it yet more credible.
Perhaps the million-dollar question is: what will happen when the enhanced winds subside (essentially, when we see another el Nino event)? The simulations by England et al. suggest that surface warming will resume, and that for the most part little or no trace will be left of the so-called “pause.” If that’s the case, we should expect record-high global average temperatures and an end to talk about the “pause” except from the most hard-core deniers.
But some (e.g. Mike Mann) suspect that the dominance of la Nina-like conditions recently may actually be a result of climate change. If so, it may be quite a while before we see anything like a strong (or even moderate) el Nino. We can, however, expect warming to continue even in a persistently non-el Nino world, a la this post by John Nielsen-Gammon.