Guest Post by Willis Eschenbach
For some time now I’ve been wondering what kind of new evidence I could come up with to add support to my Thunderstorm Thermostat hypothesis (q.v.). This is the idea that cumulus clouds and thunderstorms combine to cap the rise of tropical temperatures. In particular, thunderstorms are able to drive local temperatures to a level below that necessary for thunderstorm initiation. This allows them to act as a true governing mechanism.
Recently I came across a fascinating study by Zeng et al called “A Multiyear Hourly Sea Surface Temperature Data Set“(PDF). Figure 5 of their paper contains a host of information in three panels, from which I have extracted the average hourly changes in surface temperature in three areas of the Pacific shown below. The information is from moored buoys.
Figure 1. Multi-year hour by hour long-term average surface air temperatures for three areas in the tropical Pacific (2°N 95°W, 0°N 155°W, and 0°N 156°E, shown by colored circles). Two full days are shown, in order to see the complete temperature change during the night.
Note the strange shape of the hour-by-hour temperature record, and yet how similar it is between the various sites. Of course the question arises … where in Figure 1 is the evidence for a homeostatic system regulating the temperature? To investigate that, I looked at the data in a slightly different way.
Figure 2 shows exactly the same data as in figure 1, but shown as anomalies about the average, and only covering one day rather than two. There is information in the similarities as well as in the differences between the three locations.
Figure 2. Anomalies in hourly Pacific temperatures. Locations and colors are the same as in Figure 1. Intensity of the theoretical clear-sky solar energy at the surface (surface insolation) is shown by the line running through the sun (right scale). Blue oval shows general cumulus initiation time, green oval shows thunderstorm initiation time, and the brown oval shows the cloud dissipation time.
Let me give the timeline of the tropical day that is shown in Figure 2, pointing out the important time periods.
At dawn the day is clear and the air is calm. As a result, the air temperature starts rising quite quickly after sunrise.
As the day warms, at some point before noon a threshold is passed. Cumulus clouds (the small puffy white summer clouds) start to form, and the lower tropospheric circulation switches to a new pattern. The cumulus clouds can be thought of as flags that mark an area of rising air. Around each of them, there is a ring of descending air. This change has a couple of immediate effects.
The first is an immediate reduction in the incoming solar energy because of reflections from the cloud tops. This is accompanied by a rise in the surface wind. Since the evaporation varies linearly with the wind, if the wind doubles from 1 metre/second (2 knots) to 2 metres/second, evaporation doubles. This also cools the surface.
As a result of the changes in the lower troposphere, from calm with no clouds to increasing numbers of circulation cells with cumulus clouds, in the central and western Pacific the temperature actually starts dropping. This is happening despite the continuing increase in the amount of incoming solar energy (negative climate sensitivity, go figure). In the eastern Pacific, the air (and the ocean) is much cooler. As a result the cumulus coverage is less complete and develops more slowly, and the temperature rise is not reversed, but the temperature rise is significantly slowed.
However, even in the western and central Pacific the steady input of solar energy overwhelms the cumulus, and the temperature starts to rise again. At some point, usually early afternoon (and earlier in the day if the temperature is warmer), another threshold is passed and thunderstorms start to form. The temperatures continue to rise, but at a much slower rate than earlier in the day.
At some point in the late afternoon, the amount of incoming energy is less than the amount of outgoing energy, and the air starts to cool. It is aided in this cooling by the thunderstorms, which in warmer seas often continue until several hours after dark.
After the sun goes down, of course, clouds have a warming effect because they greatly increase the amount of downwelling “greenhouse” radiation. This can be seen in the kink in the curve between about 19:00 and 21:00 hours.
Once the clouds dissipate, however, the air is much freer to radiate out to space, and rapid cooling sets in which lasts until dawn. By that time, there are no clouds and the atmosphere is generally calm … and the cycle starts over again.
There are two main effects to having this daily cycle. The first is the rapid warming in the morning. The second is the abrupt slowing, and in warmer waters the reversing, of the rapidly rising temperatures by clouds and thunderstorms. These effects work together to keep the daily temperatures within a fairly narrow band.
The differences between the three locations are revealing. The onset of the cumulus is nowhere near as effective at preventing the temperature rise in the warmer waters of the western Pacific “hot pool”. In this warmer area the temperature starts rising again around noon, where in the central Pacific this doesn’t happen until about 2PM (14:00). In the much cooler waters of the eastern Pacific, on the other hand, it appears that not many clouds form, and the result is to push the peak daily excursion higher than that of the warmer waters. And because there are fewer thunderstorms in the cooler eastern Pacific, the temperature stays warmer after dark and does not show as pronounced a kink after dark.
Also, in the hottest region the peak temperature occurs earlier in the day than in the two cooler areas. Presumably this is because of the more complete development of the thunderstorm regime in the warmer waters, leading to more rapid cooling.
One unexpected curiosity in this data involves the fact that it takes energy to warm the air. If we start at the coldest point in the day and sum the temperature anomalies around that point, it will give us a relative measure of how much energy it took to warm the air during the 24 hours.
Here’s the odd part. Despite the differences in the local average air temperature, all three areas warm the local air the same amount, within about ± 5%. I have no idea why this might be so. Whatever the combination of clouds and thunderstorms is doing, it leads to about the same amount of warming of the air in all three areas. I have no idea why.
The hourly air temperatures over the tropical Pacific show a clear pattern that is due to the specific timing, formation rate, and number of cumulus and cumulonimbus (thunderstorm) clouds. This pattern is completely congruent with the idea that cumulus and cumulonimbus clouds act as a homeostatic mechanism to prevent the oceanic air temperatures from getting too warm or too cold.
I have not yet taken a look at the underlying dataset, I’m sure that there’s much more to learn. So to make this an official science paper, let me say that additional studies are needed …
All ideas welcome, be nice, attack the ideas all you want but please don’t attack individuals or cast aspersions on motives. That angrifies my blood mightily and I tend to do things I later regret. I’d prefer to avoid that.
Best regards to all,