Tropical Thermostats and Global Warming

A recent post at Watts Up With That by Willis Eschenbach, and then embraced by Roger Pielke Sr., goes into detail concerning a purported "tropical thermostat" that sets an upper limit on the ocean temperature.  This thermostat could presumably help regulate the response to radiative forcing in a higher CO2 world, constraining ocean temperatures to be no greater than the threshold value.

The underlying hypothesis is actually not of WUWT-origin and has some roots that were discussed in the literature dating back a couple decades.  WUWT presents a histogram of observed ocean temperatures, which shows a sharp cut off at ~31 °C (diagram reproduced below).  The figure indicates that no measurements in the worlds oceans show temperatures much higher than that value.  Based on this data, WUWT (as well as several older scientific papers based on similar data) suggested this is a theoretical 'maximum ocean temperature' and is independent of solar or greenhouse forcing. Kleypas et al 2008, for example, based a study on corals on the premise that such data support a maximum ocean temperature.

Figure 1. A “histogram” shows how many data points fall in each of the 1°C intervals shown along the bottom axis. The maximum is in the interval 28°-29°C. Figure and Caption reproduced from WUWT article.

In another example, in a paper on mass extinctions, Veron, 2008 mentioned that:

"...the surface temperature of the largest oceans would have been limited by the Thermal Cap of ~31C, widely believed to be the highest temperature large oceans can reach.’"

Based on the premise of a maximum ocean temperature, there have been several proposed 'thermostat' mechanisms to explain why tropical sea surface temperatures (SSTs) don't get much greater than ~31°C.  Proposals involve negative feedback cloud responses (ex. Ramanathan and Collins 1991) or enhanced evaporation that keeps the SSTs down (ex. Newell, 1979 or Hartmann and Michelsen, 1993). 

But is this actually "widely believed" as Veron, 2008 state? It turns out the answer is no. 

Several older papers rebutted the cloud thermostat hypothesis of Ramanathan and Collins (ex. Fu et al., 1992 and later observational papers) and related thermostat arguments have also been refuted a number of times (Wallace, 1992). Pierrehumbert, 1995  discussed the regulation of tropical SSTs and showed that there is no physical basis for an upper temperature bound.  More recent papers (Sud et al., 2008; Williams et al., 2009) came to similar conclusions. 

Before I explain the discrepancies,  it is worth reviewing some basic tropical meteorology:

As mentioned before, several papers find no evidence of a 'maximum SST', so how do we reconcile that with the observed data?

What about the observed Histogram of SSTs?

Answering this question essentially boils down to the question of what SST is required for the onset of deep convection (and thus deep cloud formation).  In the modern climate, this value occurs around 28 °C.  In general, it depends on when air near the surface can become buoyant relative to air in the upper atmosphere and thus have enough energy to rise freely. Because atmospheric temperature gradients in the tropics are small, the threshold temperature for convection depends primarily on the local SST. There is a consequence to this: If the troposphere warms, then the SST threshold to kick in convection (and related cloud feedbacks) must also increase.  In other words, it is the threshold for deep convection that promotes the skewed plot seen in the first figure, not any universal demand for a maximum ocean temperature. 

This concept is shown below.  The figure shows a model result for the longwave and shortwave fluxes as clouds form.  The threshold for deep convection is readily seen in figure 2a (the longwave flux) and occurs when SST's exceed 25°C or so. When convection starts, lots of clouds form and the absorbed longwave radiation spikes upwards due to the cloud greenhouse effect. As the climate warms (going from the blue to red line), the SST temperature threshold also rises (i.e., moves to the right).

Figure 2. a) TOA cloud LW flux as a function of SST, b) TOA cloud SW flux as a function of SST; Solid blue and dashed red lines correspond to the ensemble median over years 0–20 and 60–80, respectively, from 15 IPCC AR4 coupled ocean-atmosphere models for the 1% per year scenario. Vertical lines indicate the interquartile range. From Willaims et al (2009)

Faulty Mechanisms

In order to understand why neither clouds nor evaporation act as an inherent buffer in the modern tropical climate, it's worth considering a few more bits of physics:

The Observed and Past Climate Record 

In order to cross-validate whether these results are correct, it would be useful to consult the paleoclimate record, in order to establish whether tropical SSTs were warmer than the modern alleged 'threshold' value. There is abundant palaeoclimate evidence that tropical sea temperatures can rise well above present values.  Improved understanding of oxygen proxies and the development of new proxies ('thermometers of the past') such as TEX86 and Mg/Ca have shown that Eocene SST's in the tropics could have been even hotter than 35 °C (see ex, Huber, 2008).

For modern observations, Johnson and Xie, 2010 find an upward trend in the threshold of deep convection associatd with rising temperatures.  This also means that the threshold SST for when hurricanes form will change in a new climate.


There have been a number of "false thermostats" and incorrect assumptions about a universal and unchanging "convective threshold" that kicks in heavy cloud formation. There's been a long history of refuting thermostats of this sort, but apparently this isn't universally appreciated.  Note I have said little of the cloud feedback issue relevant for climate sensitivity; rather, there is no compelling physical jusitification to suggest that the tropical sea surface temperatures must be pegged at some maximum value independent of the forcing, or that clouds/evaporation must act as some sort of tropical regulation mechanism.  As a further example, the figure below shows a simulation by Sud et al (2008) showing 10N-10S latitude tropical SSTs for present day heating versus the extra heating that would happen if CO2 were doubled:

Figure 3. Solid lines show the 10 N- 10 S SST for a present-day (green) and 2xCO2.

Posted by Chris Colose on Thursday, 16 February, 2012

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