Kosaka and Xie, take 2

This is my take 2. See here for my incautious take 1. Take 2 is not as interesting as take 1 - I no longer have an overall theme, and I don't feel inclined to contradict the take-home message. That reduces me to quibbling and a slight feeling of unease, though that may quite possibly be because I now feel biased against this paper for giving me a hard time.

So, take their "We present a novel method of uncovering mechanisms for global temperature change by prescribing, in addition to radiative forcing, the observed history of sea surface temperature over the central to eastern tropical Pacific in a climate model". But this isn't desperately novel, nor is that quite accurate - what they actually do is restore SSTs towards the observed pattern in this region, by modifying the surface heat fluxes (and if you compare figure 2 and b closely, you'll see that the restoring isn't perfect; in that 2b, within the inner box, clearly isn't the same as 2a in the same region. In fact its rather more different than you'd expect, which is odd. Ditto 3 a, b). Which isn't new; here for example is a random example from 2006. In fact, later on they say The POGA experimental design has been used to study the global teleconnections of the interannual El Nino/Southern Oscillation (ENSO)11,12. Here we present a novel application of POGA... so I think they've mis-spoken in their abstract: the method isn't new, only the application. You'd have thought that Nature-quality reviewing would have caught that. But I'm quibbling.

Although the surface temperature prescription is limited to only 8.2% of the global surface, our model reproduces the annual-mean global temperature remarkably well with correlation coefficient r = 0.97 for 1970–2012

This is the bit I reacted somewhat badly to in my first go, claiming that it wasn't surprising because the ENSO region is so important. Just to make sure that Captain Cockup doesn't come to visit again, here are their experiments:

* (HIST) is forced with observed atmospheric composition changes and the solar cycle.
* (POGA-H) Pacific Ocean–Global Atmosphere (POGA) experiments, SST anomalies in the equatorial eastern Pacific are forced to the observed evolution, and the radiative forcing is identical to HIST
* (POGA-C) is like POGA-H, except radiative forcing is fixed at the 1990 value. In both cases, outside the equatorial eastern Pacific, the atmosphere and ocean are fully coupled and free to evolve.

So if you look at the lower figure, and compare the wiggles to the upper, they match pretty well - especially if you ignore the volcano years. Which does indeed suggest that ENSO is driving much of the interannual variability, but now I'm obliged to admit that comes with the trend coming from the radiative forcing (as, in retrospect, you'd expect). It also appears to imply that the volcanoes mainly affected extratropical temperatures for some reason, but that's another matter.

A comment (which isn't original to me): even attributing the change to ENSO doesn't tell you if its forced or not: whilst ENSO is a natural mode of the system, its perfectly possible for Anthro forcing to act not to change the long-term-mean-state, but to push it further into a warmer regime; there has long been speculation that warming could manifest as "more El Nino and less La Nina". OTOH, in this case we appear to be seeing more La Nina, and its somewhat hard to see how that could be forced; K+X address this, a little bit, in their "Whether the La-Nin˜a-like decadal trend is internal or forced is still unclear..." para.

Errm well there you have it: it looks OK to me.