Guest Post by Willis Eschenbach
My theory about the climate is that the global temperature is regulated in large part by the timing and strength of the daily appearance of tropical clouds and thunderstorms. I hold that when the tropical temperatures are higher, that the cumulus cloud field and associated thunderstorms forms both earlier and more strongly. This cools the surface by reflecting sunshine back into space and thus capping the possible temperature increase.
In my last post, TAO Sea and Air Temperature Differences, I discussed an oddity. The oddity is that the daily cycle of sea-air temperature differences is virtually identical for some 7,000 miles (11,000 km) along the Equator all across the Pacific. Here’s the main graph from that post.
Figure 1. Daily cycle of anomalies of differences between sea temperatures and air temperatures at eight equatorial TAO buoys.
I found this quite surprising, and I said so in the post. In response, a commenter claimed that the reason for this Pacific-wide equality was obvious, viz (emphasis mine):
The answer is quite simple! It’s been well-known for many decades among geophysical professionals that the heat capacity of water is much greater than that of air, resulting in generally negative air-sea temperature differences. The shape of the diurnal cycle of those differences is governed by the rate of heat transfer between ocean and atmosphere, modulated In the tropics by a characteristic diurnal pattern of cloud cover. PhD theses require more than a rehash of known processes. … Since the diurnal cycle of solar radiance is uniform throughout the narrow tropical zone and since the specific heat of water and air is virtually constant, that the diurnal cycle of air-sea temperature differences varies uniformly with longitude is not particularly noteworthy scientifically.
I always get a laugh out of the passion and certitude with which some anonymous commenters express their unsupported opinions. In this case, I thought “Say what? The diurnal cycle of solar radiation in the tropics is NOT uniform, not even near to it.” How do I know this? Well, because I’ve invested lots and lots of hours studying things like the TAO buoy dataset, including the downwelling solar data … plus, of course, I lived for twenty years in the tropics. Nothing like having experience as a foundation to build on and improve with endless study …
In any case, I have to thank the commenter. He spurred me to look further at the question of the differences in the surface solar radiation at the various locations. And as always, natural data contains surprises.
To begin with, on a 24/7 average, the amount of solar energy striking this group of equatorial buoys varies from lows of about 248 W/m2 at the two lowest buoys, up to the highest buoy reading of about 284 W/m2. Thus, the buoy with the most sunlight has about 15% more sunlight than the two buoys getting the least sunlight. Not uniform at all.
Here’s the next oddity. The two buoys with the least sunlight striking the surface are at the opposite edges of the Pacific, and thus at opposite temperature extremes. One is the TAO buoy at 95°W off of Ecuador (Spanish for “Equator”), in the coolest Pacific equatorial water. The other is the TAO buoy at 165°E, near Papua New Guinea in the warmest Pacific water. One averages 248.4 W/m2 of sunshine at the surface; the other averages 248.8 W/m2 of sunshine … virtually identical. Hmmm … Figure 2 shows the situation:
Figure 2. Downwelling solar energy flux at the surface, in watts per square metre (W/m2).
The small red squares on the Equator at the east and west edges of the Pacific are the location of the buoys with the least surface sunlight. Figure 2 shows CERES data. Note the good agreement between the CERES derived surface dataset and the TAO buoy data, with the gray-colored lines showing the recorded value from the TAO buoys of 248 W/m2.
So where is the difference between the warm and cold buoy sunshine? Well, as I’ve said from my first explanation of tropical thermal regulation, it’s in the timing and strength of the emergence of the daily cloud cover. To illustrate this I’ve subtracted the downwelling solar energy flux at the coolest-water buoy from that at the warmest-water buoy. Figure 3 below shows that result. Positive values mean more sun at the warmer location.
Figure 3. Daily cycle of the differences between 24/7 average downwelling solar energy at the cool and warm edges of the equatorial Pacific. Both buoys are on the Equator. The buoy at 95°W is in the cool area off of South America. The buoy at 165°E is in the Pacific Warm Pool off of Papua New Guinea north of Australia. Positive values mean more sunshine at the warm buoy. Solar data taken at ten minute intervals.
Figure 3 reveals that in the warm area, there’s more sunshine in the early morning. But then around nine or ten in the morning, the clouds and the thunderstorms start to come on strong at the warmer buoy. This cuts down both the noon and the afternoon sunlight in the warmer regions.
Nor are these small swings. They are plus or minus about forty watts per square metre, which is a lot of energy.
There are several interesting aspects of this. The first is that although the total solar energy reaching the surface is the same in both locations, they are at very different temperatures. Curious.
The next is the large difference in the amount and timing of the clouds. At the warm buoy, there is much greater cloud coverage starting about 9 AM, as evidenced by the much smaller amount of sunshine being allowed in compared to surface sunshine at the cool buoy. This agrees with my theory regarding thermal regulation via cloud cover.
Next, think about how this relates to Figure 1 above. Figure 1 shows that all the buoys have nearly identical daily changes in sea-air temperature differences (∆T). Whatever physical processes might explain those identical cycles, clearly it is NOT from some imaginary “uniform diurnal cycle of solar radiance”. Solar radiance at the different buoys shows a large inter-buoy variation, both in the total amount of incident sunshine and in the timing of the sunshine.
Finally, remember that this is the average timing. On warmer days, the clouds form earlier and more densely, and the reduction in sunshine is greater. On cooler days, the clouds form later and more scattered, and more sunshine comes in. This process cools the warm days and warms the cool days … what’s not to like?
Figure 4. Average surface air temperature changes on warmer and colder days at the TAO buoy at 165°E on the Equator.
Always more to learn …
Regards to everyone,
THE USUAL REQUEST: When you comment, please QUOTE THE EXACT WORDS THAT YOU ARE DISCUSSING. I can defend my own statements. I can’t defend your memory of some statement you think I made at some unknown time and place. Please quote what you are talking about—it makes conversations possible and prevents misunderstandings.