About that tropical "hot spot"

A couple of days ago I wrote about the new paper by Steven Sherwood and Nidhi Nishant, which was reporting the release of an updated radiosonde dataset, and findings from an analysis of those data.

Data sources: Scripps and GISS/NASA

One of the things the authors reported was the "tropospheric hot spot" - though they didn't use that term in the paper itself (only the press release). I've never previously thoroughly looked into this myself. I knew about it in general terms, because some deniers point to what they regard as lack of evidence as evidence that global warming isn't happening. Which is really, really weird when you think about it. Or when you look at surface temperature data as in the chart on the right.

Anyway, I decided to go and learn more about this so-called "tropospheric hot spot".

That hot "spot" is not really a spot and it's not very hot

Although it's in the troposphere, it's not really a "spot. It's more like a band in the upper troposphere above the tropics. Neither is it hot. The air is quite cold when you get a few kilometres up in the air. Also it's not a temperature at a particular time, it's the trend in the temperature over time. The diagram from the Sherwood and Nishant paper demonstrates this:

Figure 1. Temperature trend 1960–2012 versus latitude and pressure. The value for each latitude and pressure is the medians of the trends at individual stations in that (10°) latitude bin. Units are °C per decade. Source: Sherwood15

What the diagram above shows is the temperature trend at different levels in the troposphere, going from south to north (left to right) above the earth. The red colour is the greatest trend. You can see the blobs that show fastest warming at height, between around 30 south and 20 north. The y axis is pressure. Pressure drops with altitude. The blobs are evident from around 500 hPa upwards to near the interface between the troposphere and the stratosphere.

Terminology

Some terms used when talking about the temperature rise in the atmosphere above the tropics are:

• Lapse rate. This is the rate of decrease of temperature with increasing height above the surface.
• Adiabatic. This refers to a process in which there is no heat gained or lost.  
• Moist adiabatic lapse rate (MALR).  The dry adiabatic lapse rate is the rate of decrease of temperature in dry air, and the moist, as you can guess, is that in moist air. That is, the rate of decrease in temperature with altitude, of a parcel of air, as it rises in the atmosphere under adiabatic conditions.  The moist adiabatic lapse rate has to be less than the dry adiabatic lapse rate. This is because as water vapour rises it gets colder and condenses into water (or freezes into ice) . This condensation releases latent heat - the air at that height gets warmer than it would if there was no condensation (or freezing). 
• Tropical lapse rate. This is the lapse rate in the atmosphere above the tropics.
• Troposphere. This is the layer of the atmosphere immediately above the surface. The layer above that is the stratosphere. (Then there is the mesosphere, the thermosphere and the outer layer is the exosphere - see here.)

Expectations of the tropical lapse rate

Atmospheric physics indicates that above the tropics, the temperature decrease in the troposphere above the tropics should increase faster as the world warms. That is, there is a region where the rate of warming should be faster than the rate of warming at the surface. Now this isn't just from greenhouse warming. It should happen regardless of the cause. It could be from El Nino or from solar warming. In a discussion at Climate Dialogue, Steve Sherwood wrote:

Based on theoretical considerations and simulations with General Circulation Models (GCMs), it is expected that any warming at the surface will be amplified in the upper troposphere. The reason for this is quite simple. More warming at the surface means more evaporation and more convection. Higher in the troposphere the (extra) water vapour condenses and heat is released. Calculations with GCMs show that the lower troposphere warms about 1.2 times faster than the surface. For the tropics, where most of the moist is, the amplification is larger, about 1.4.

Carl Mears explained this in more detail:

In the deep tropics, in the troposphere, the lapse rate (the rate of decrease of temperature with increasing height above the surface) is largely controlled by the moist adiabatic lapse rate (MALR). This is true both in complicated simulations performed by General Circulation Models, and in simple, back of the envelope calculations (Santer et al, 2005). The reasoning behind this is simple.

• If the lapse rate were larger than MALR, then the atmosphere would be unstable to convection. Convection (a thunderstorm) would then occur, and heat the upper troposphere via the release of latent heat as water vapor condenses into clouds, and cool the surface via evaporation and the presence of cold rain/hail.
• If the lapse rate were smaller than MALR, then convection would be suppressed, allowing the surface to heat up without triggering a convective event.

On average, these processes cause the lapse rate to be very close to the MALR. Note that this argument does not apply outside the tropics, because the dynamics become more complex due to the Coriolis force and the presence of large north/south temperature gradients, or in regions with very low relative humidity, such as deserts, where the atmosphere may be far from saturated near the surface and thus the MALR does not apply. 

Because the MALR decreases with temperature, this means [that] any temperature increase at the surface becomes even larger high in the troposphere. This causes the so called hot spot, a region high in the troposphere that shows more warming (or cooling) than the surface. Note that at this point, I haven’t said a thing about greenhouse gases. In fact, this effect has nothing to do with the source of the warming, as long as it arises near the surface. Surface warming due to any cause would show a tropospheric hotspot in the absence of other changes to the heating and cooling of the atmosphere. Never the less, the tropospheric hotspot is often presented as some sort of lynchpin of global warming theory. It is not. It is just a feature of a close-to-unstable moist atmosphere. 

Some background to the research

Back in 2005, Steve Sherwood wrote an article for realclimate.org, based on a paper in Science mag. He explained about what is expected to happen in the troposphere and what was observed to that time (ten years ago). Here is some of what he wrote back then:

The troposphere—that part of the atmosphere that is involved in weather, about 85% by mass—is expected to warm at roughly the same rate as the surface. In the tropics, simple thermodynamics (as covered in many undergraduate meteorology courses) dictates that it should actually warm faster, up to about 1.8 times faster by the time you get to 12 km or so; at higher latitudes this ratio is affected by other factors and decreases, but does not fall very far below 1. These theoretical expectations are echoed by all numerical climate models regardless of whether the surface temperature changes as part of a natural fluctuation, increased solar heating, or increased opacity of greenhouse gases.

To reiterate, when people talk about the "tropospheric hot spot" they are referring to a region of the troposphere above the tropics, not the air over the mid-latitudes or the poles. And to it warming at a faster rate than warming at the surface.

In that article, Steve also points out that although observations (in 2005), did not show the expected rate of temperature increase in the tropical troposphere, this didn't necessarily mean that it wasn't happening. Only that the instruments didn't detect it. Some of the reasons he gave why that might be the case were:

• Satellites are subject to drift. At the time (and maybe even today - does anyone know?), satellites are adjusted every week to match radiosondes. 
• Radiosondes weren't designed to detect small climate changes. 

The biggest problem with radiosonde raw data is that the sensors get overheated in the daytime. As stated in the Science paper: "For many radiosonde designs this can elevate the temperature several °C above ambient during daylight, an effect that must be removed via an estimated correction". Other problems with using raw radiosonde data include the need to correct for:

• Changes from one instrument model to another and from one manufacturer to another 
• Changes in balloon technology
• Changes in software, and bug fixes in software.

Since 2005

There have been a number of papers looking at this subject since the paper by Steve Sherwood et al in 2005. Some argued there was no increase in tropical lapse rate, and some argued there was. In around 2008, Robert Allen, a student of Steve Sherwood, measured the effect by looking not at temperature data from radiosondes, but by analysing wind data from radiosondes. He found "Warming patterns [that] are consistent with model predictions except for small discrepancies close to the tropopause."

The recent paper was based on an analysis of a revised radiosonde data set, and also found warming broadly consistent with expectations. This is shown in the diagram up top and in Figure 2 from the paper.

Figure 2. Tropical (30S–30N) temperature trend versus height and its sensitivity to method changes since S08. Colors show two time periods, and line types distinguish standard calculation, that with linear basis as in S08, that with wind shear as in S08, and that with a narrower definition of the tropics. Values are first obtained by latitude bin as in figure 1, then averaged across latitudes. Error bars are two-sigma, based on the spread among contributing stations. Blue curve shows pseudoadiabatic warming given a fixed 70% relative humidity at 1000 hPa (including the additional latent heat of freezing for  C). Source: Sherwood15

There was another paper published recently, by Stephen Po-Chedley, Tyler J. Thorsen, and Qiang Fu, which I almost forgot to mention. They looked at satellite data for the middle troposphere (not the lower troposphere, which is often compared to surface temperature). They found that, when corrected for diurnal drift, these data also showed that the tropospheric warming over the tropics was consistent with that expected.

What if the world were cooling

If the world were cooling instead of warming, my understanding is that this too would be magnified in the tropical troposphere. That is, instead of a "hot spot" there would be a "cold spot". The air higher up in the troposphere above the tropics would cool more quickly than the surface. The surface would be cooling, less water would be evaporating, the air as a whole would be cooler, so water would condense (releasing latent heat) at an ever lower altitude in the troposphere above the tropics. Therefore the air higher up would cool more quickly.

(I can't find anything to support what I've just written, so feel free to correct me. I'm adding it because some comments yesterday suggested the opposite.)

Update: This section is correct and has been confirmed by Peter Thorne in the comments and Steven Sherwood (via email) - both of whom have published on this subject, and Victor Venema (in the comments). (The rest of the article has been certified correct, too :D)

References and further reading

Nine Denier 101 Techniques: Anthony Watts gets into a hot spot in the tropical troposphere - HotWhopper article about the new paper by Steve Sherwood and Nidhi Nishant - with a video and more references

Science Friday: The Tropical "Hot Spot" and Global Warming - an article by Michael Mann on his facebook page, with an update/correction added here

Steven C Sherwood, Nidhi Nishant. "Atmospheric changes through 2012 as shown by iteratively homogenized radiosonde temperature and wind data (IUKv2)." Environmental Research Letters, 2015; 10 (5): 054007 DOI: 10.1088/1748-9326/10/5/054007 (open access)

Stephen Po-Chedley, Tyler J. Thorsen, and Qiang Fu, 2015: "Removing Diurnal Cycle Contamination in Satellite-Derived Tropospheric Temperatures: Understanding Tropical Tropospheric Trend Discrepancies."  J. Climate, 28, 2274–2290. doi: http://dx.doi.org/10.1175/JCLI-D-13-00767.1 (subs req'd)

Sherwood, Steven C., John R. Lanzante, and Cathryn L. Meyer. "Radiosonde daytime biases and late-20th century warming." Science 309, no. 5740 (2005): 1556-1559. DOI: 10.1126/science.1115640 (pdf here)

The tropical lapse rate quandary - by Steve Sherwood at realclimate.org, 2005

Allen, Robert J., and Steven C. Sherwood. "Warming maximum in the tropical upper troposphere deduced from thermal winds." Nature Geoscience 1, no. 6 (2008): 399-403.  doi:10.1038/ngeo208 (pdf here)

Mitchell, D. M., P. W. Thorne, P. A. Stott, and L. J. Gray. "Revisiting the controversial issue of tropical tropospheric temperature trends." Geophysical Research Letters 40, no. 11 (2013): 2801-2806. doi:10.1002/grl.50465, 2013 (pdf here)

The (missing) tropical hot spot - discussion at Climate Dialogue - July 2013