Guest Post by Willis Eschenbach

I got to thinking about how I could gain more understanding of the daily air temperature cycles in the tropics. I decided to look at what happens when the early morning (midnight to 5:00 AM) of a given day is cooler than usual, versus what happens when the early morning is warmer than usual. So what was I expecting to find?

Well, my hypothesis is that due to the emergence of clouds and thunderstorms, when the morning is cooler than usual, there will be less clouds and thunderstorms. As a result the day will tend to warm up, and by the following midnight it will end up warmer than where it started. And when the morning is warmer than usual, increased clouds and thunderstorms will cool the day down, and by the following midnight it will end up cooler than when it started. In other words, the emergent thermoregulatory phenomena will cause the temperature to tend to revert to some mean, not over months or years, but on a daily basis.

Now, this is the third post in a series discussing the effects of albedo and thunderstorms on the tropical temperatures. In order they were Albedic Meanderings, An Inherently Stable System, and The Daily Albedo Cycle. This post will make more sense if you’ve read those three first and seen the Figures.

So to investigate warm and cold days what I did was to take the air temperature data from some sixty-seven TAO buoys. I sorted them by average temperature, and I started to look at them. Figure 1 shows the temperature data from one of the coolest TAO buoys, where the mean temperature is 24°C. I split the data into “warm” and “cool” days, based on the average early morning temperature from midnight to 5 AM, and then took an hourly average of the warm and cool datasets individually.

Figure 1.Cool TAO buoy, averages of the days with warmer early mornings (Midnight-5AM) and the days with cooler early mornings. Straight lines connect the temperature at midnight at the start of the day with the midnight temperature 24 hours later. “Mean” is the mean temperature of all days. “Recovery” is how much the following midnight averages have moved towards the mean compared to the opening midnight averages. “Recovery Percentage” is the same as “Recovery”, expressed as a percentage of the distance from the beginning temperature to the mean.“Warm Recovery” is how much the warm temperatures have moved towards the mean, and “Cool Recovery” is how much the cool temperatures have moved to the mean. Horizontal black line shows the mean (average) temperature of all midnights. Red and blue straight lines connect the starting and ending midnight temperatures.

My hypothesis says that the temperatures should move towards the mean. That is to say, the temperatures at midnight of the end of the day (hour twenty-four in Figure 1) should be closer to each other than the temperatures at midnight at the start of the day (hour zero in Figure 1). So I have measured the difference between the opening distance (warm-to-cool temperature difference at opening midnight), and the closing distance (warm-to-cool temperature difference at closing midnight ). This I have called the “recovery” in Figure 1. This movement towards the mean is reported both in °C and as a percentage of the opening warm-to-cool difference. I’ve also noted how much the ending midnight temperatures of the warm and cool days separately have moved towards the mean midnight temperature.

However, there’s not a lot happening in Figure 1. The temperatures are barely moving towards the mean. When the day starts out cold it seems that it stays cold, and when it starts out warm, it stays warm. There is very little difference over the 25 hour period shown (0-24). Looking at other buoys I found that at the coolest end of the TAO buoy locations, there is little indication of my hypothesized thermoregulatory mechanisms. None of the TAO buoys in the cooler locations show any significant thermoregulated recovery to the mean.

But then I looked at the records from a TAO buoy at one of the warmest locations, where the mean temperature is over 28°C. There, the situation is totally different.

Figure 2. Warm TAO buoy, averages of the days with warmer early mornings (Midnight-5AM) and the days with cooler early mornings. Straight lines connect midnight at the start of the day with midnight 24 hours later. “Mean” is the mean temperature of all days. “Recovery” is how much the following midnight averages have moved towards the mean. “Warm Recovery” is how much the warm temperatures have moved towards the mean, and the same for “Cool Recovery”.

Now, this is quite different. At the warm end of the TAO buoy locations, the warm days end up cooler, and the cool days end up warmer, exactly as my hypothesis predicts.

One of the most interesting things about Figure 2 is how rapidly the restorative forces are able to move the temperature back towards the mean. In only one day, on average the temperature at midnight moves sixty percent of the way back to the mean midnight temperature … that’s a very rapid and rigid temperature control compared to what is happening in the cooler TAO buoy locations.

To close out this part, here’s a typical record from an intermediate temperature TAO buoy, with average temperatures of 27°C:

Figure 3. Intermediate TAO buoy, averages of the days with warmer early mornings (Midnight-5AM) and the days with cooler early mornings. Straight lines connect midnight at the start of the day with midnight 24 hours later. “Mean” is the mean temperature of all days. “Recovery” is how much the following midnight averages have moved towards the mean. “Warm Recovery” is how much the warm temperatures have moved towards the mean, and the same for “Cool Recovery”.

As you can see, the recovery towards the mean in this medium-temperature TAO buoy is somewhere in between the coolest and warmest buoys. In a single day the midnight temperature moves about a quarter of the way back to the mean.

One oddity that I noted was that although in absolute terms (°C) the recovery was different between the cold and warm days, in percentage terms (for the buoys shown above at least) the recovery is about the same.

This led me to ask, what is the overall dependence of the restorative thermoregulatory forces on the temperature? To see this, I took a scatterplot. Since I wanted to also see if the warm/cold recovery percentages were different, I used a scatterplot of the warm recovery percentages and the cool recovery percentages separately as a function of temperature. Figure 4 shows how the recovery percentage is related to temperature. I have again used the average temperature from midnight to 5 AM as the dividing factor for warm and cool days.

Figure 4. Scatterplot, daily thermoregulatory response to warmer (red) and cooler cooler (blue) days versus annual mean temperature. “Recovery Percentage” is how much closer to the mean the temperature of the midnight at the end of the day is, compared to midnight at the start of the day. If it moved all the way back to the mean it would be 100%.

First, it’s clear that the strength of the thermoregulatory response is a function of temperature. There is almost no thermoregulation at the low end of the temperature scale, while at the high end the midnight temperature moves halfway back to the mean or more in the course of a single day.

Next, it’s kind of hard to see the red and the blue because there is so little difference between them. I’ve printed them transparent so when they overlap they make purple … but in no case is there any significant difference between the warm and cold recoveries when expressed as percentages. This is despite the fact that often they are different in absolute terms (°C), as is shown in Figure 5 below. I have no explanation of why this should be so. Always more puzzles …

Figure 5. Scatterplot, absolute daily thermoregulatory response to warmer (red) and cooler (blue) days versus annual mean temperature in degrees C. “Recovery Amount” is how much closer to the mean the midnight temperature at the end of the day is, in degrees C, compared to the midnight temperature at the start of the day.

Here, we see that the thermal regulatory mechanisms at the upper end of the ocean temperature range can warm or cool a single day by a third to half of a degree C, midnight to midnight …

CONCLUSIONS: Well, I can say that this result is certainly consistent with my hypothesis that there are emergent thermoregulatory mechanisms that warm up the cool days and cool down the warm days in the wet tropics.

Now, scientists have previously proposed temperature mechanisms which they thought were involved in holding the temperature down in the Pacific Warm Pool (PWP), where we find the warmest of the TAO buoys. Sea temperatures in that area are the warmest in the open ocean … but despite that, the sea temperatures rarely exceed 30°C. Ramanathan proposed a “Super-greenhouse” effect to explain this temperature limit, and Lindzen proposed an “Iris Effect”, in order to explain the strong downward pressure on the temperature in the PWP. And those proposed mechanisms may indeed exist, they are not in opposition to my hypothesis.

What I have not seen mentioned previously, however, is that in addition to there being the strong downward pressure on the temperature of the warm days in the PWP noted by previous researchers, there is also a strong upward pressure on the cool days in the PWP … and as far as I know, mine is the only one of those three hypotheses that predicts such an effect.

However, it’s a big world out there, and I certainly could have either missed or misinterpreted previous art …

Finally, my hypothesis is that the temperatures displayed above are regulated by the emergence of cumulus, thunderstorms, and organized squall lines. HOWEVER, this analysis can say little about whether my hypothesis is the actual reason for the remarkably strong daily recovery towards the mean of warm tropical ocean temperatures. All it can say is that such a powerful thermoregulative effect certainly exists, that it operates on both the cool and the warm days, and it is consistent with my hypothesis.

It does not provide evidence that the mechanism is cloud-based. That’s hard to establish with the TAO buoys because they don’t contain information on the cloud coverage. But I think there’s a way to do it, which will be the subject of an upcoming post.

w.

You May Have Heard This Before: If you disagree with someone’s words, please have the courtesy to quote the exact words you disagree with. That way we can all understand your objection.

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