Prioritize...
By the end of this section, you should be able to describe the difference between the terms baroclinic and barotropic, and associate the proper term with the tropical atmosphere. Furthermore, you should also be able to explain what outgoing longwave radiation (OLR) is, how weather conditions determine its intensity, and how meteorologists use plots of OLR to analyze patterns of clouds and rainfall.
Read...
In order to contrast temperature patterns in the tropics with those in the middle latitudes, allow me to briefly employ an analogy. It might sound a little bizarre, but I'm going to liken temperatures in the tropics and middle latitudes to human personality types. One theory of human personality defines two types -- Type A and Type B. In a nutshell, people with a "Type A personality are "high-strung," obsessed with details and organization, and somewhat rigid. "Type B" personalities on the other hand, are more laid back, "go-with-the-flow" types. They're less stressed out about organization and details.
If I could label the middle latitudes with a human personality, I would probably rate them "Type A". Recall that the middle latitudes mark the region where advancing warm and cold air masses invariably collide. Like a typical "Type A" personality, the middle latitudes seem to be obsessed with organization, dutifully structuring the lower troposphere into narrow zones of relatively large temperature gradients (cold, warm, and stationary fronts). The middle latitudes are constantly trying to manage their temperature gradients in an attempt to be as "organized" as possible.
In contrast, the tropics have a "Type B" personality. As a general rule, horizontal temperature gradients are weak and much more "laid back". To understand why, let's start with the short background video (2:21) below. In case you're wondering, the values of absorbed solar and emitted infrared radiation plotted in the video represent latitudinal (sometimes called "zonal") averages.
Let’s apply the concept of energy budgets to better understand the tropics and how they relate to higher latitudes. This graph is a plot of average absorbed solar and emitted infrared radiation versus latitude, assuming that we treat the earth and atmosphere as one system. The equator is in the middle and the poles are at the sides of the graph. Overall, there’s a net energy gain in the tropics and a net energy loss in the middle and high latitudes. So, let’s see why that’s the case.
The amount of energy per unit area received by the earth depends on the angle at which the sun’s rays strike the earth. Therefore, solar heating is a maximum over the tropics because the intensity of solar radiation is greatest over low latitudes, and over the course of a year, the tropics receive much more incoming radiation than the poles.
On the loss side of the energy ledger, the amount of energy per unit area emitted by the earth depends on surface temperature. The tropics emit a bit more infrared radiation to space because they’re warmer than higher latitudes. But, the amount of infrared radiation emitted in the tropics still pales in comparison to incoming solar radiation.
So, if we construct an energy budget, we’ll see that the tropics are constantly gaining energy because more energy comes in during the course of the year than goes out. Higher latitudes, on the other hand, are constantly losing energy because more energy goes out over the course of the year than comes in.
By itself, this set-up would cause the tropics to get warmer and warmer every year because they always have this surplus of radiation. On the flip side, higher latitudes would get colder and colder every year because they always run a radiation deficit over the course of a year.
But, obviously that doesn’t happen and the reason why is that energy gets transferred throughout the earth system. Energy from the tropics gets transported from low latitudes toward the poles by the atmosphere and ocean to help keep the system balanced, and prevent runaway temperature increases in the tropics and decreases at higher latitudes.
We can confirm the great emission of infrared radiation from the tropics discussed in the video by viewing plots of outgoing long-wave radiation (OLR). For the record, OLR is most intense where surface temperatures are the greatest, such as hot subtropical deserts (the Sahara, for example) during summer. In contrast, OLR is the least intense where it's colder, either because the ground is cold or because deep convection is present. That's because cloud tops in areas of deep convection are high and cold and thus weakly emit long-wave (infrared) radiation.
If we look at the long-term average of OLR across the globe, we can see the general pattern described in the video. The blazing hot Sahara Desert in northern Africa is clearly an area of high OLR values (some of the highest on Earth, denoted by dark purples), while other tropical areas frequently characterized by deep convection (like the Amazon River Basin in northern South America) have lower values. OLR charts have lots of other practical applications for studying trends in cloudiness and rainfall over the tropics (if you're interested in checking out the variety of OLR products available, check out the Earth System Research Laboratory page of OLR plots).
The relatively large losses of infrared energy to space over the tropics only partially offset major-league solar heating, resulting in a broad surplus of energy (shaded in red in the interactive graph above) that varies little with latitude between 30 degrees north and south. This relatively even distribution of surplus energy across the tropics accounts, in part, for the general lack of moderate to strong horizontal temperature gradients in the tropical troposphere.
One other reason for the generally weak temperature gradients at low latitudes is that the water covers approximately 75% of the tropics. That means that the uniform surplus of energy in the tropics gets distributed over large expanses of water, thus further limiting opportunities for strong temperature gradients to form (cold air traveling over relatively warm ocean waters gets rapidly modified).
The figure above represents the long-term average of annual surface air temperatures across the globe. I point out that there are indeed temperature gradients between tropical land masses and surrounding oceans, but the overall pattern of temperature gradients in the tropics is weak compared to those at higher latitudes.
Now I readily admit that any annual average in temperature tends to "wash out" strong signals of gradients in winter, so perhaps a look at temperatures for a single day would more effectively drive home my point. Check out daily global surface temperatures for January 23, 2013, when sharp temperature gradients existed over eastern North America, for example, on the fringe of a continental Arctic air mass. Now, compare them to the flabby gradients over the tropics. No contest, wouldn't you agree? Notice that there are some sharper gradients along the outer fringes of the tropics near 30 degrees north. These larger gradients near 30 degrees are not unusual, given that Arctic air masses drive farther south in winter (occasionally into the fringes of the tropics). In the heart of the tropics, however, gradients are weak by almost any standard.
The lack of large temperature gradients does not stop at the surface, of course. At 500 mb, for example, the lack of strong temperature gradients over the tropics is striking compared to the middle latitudes (check out the annual mean 500-mb temperatures across the globe). So, with regard to temperature gradients, the tropical troposphere has a completely different personality than the middle latitudes.
I hope the analogy to personality types helps you to understand the different nature of temperature patterns in the tropics and middle latitudes, but now it's time to get a bit more formal. How do we formally describe these different "personalities" of the middle latitudes and the tropics? Meteorologists formally refer to the "Type A" middle latitudes as baroclinic and the "Type B" tropics as barotropic. In the broadest terms, a baroclinic atmosphere is one where horizontal temperature gradients prevail. The middle latitudes, for example, are highly baroclinic during winter, when large horizontal temperature gradients often set the stage for strong temperature advection. A barotropic atmosphere, on the other hand, is one in which temperature advection is pathetically weak. In the presence of wind, that means that horizontal temperature gradients must all but vanish. For all practical purposes, the tropics are bereft of horizontal temperature gradients, so "barotropic" best describes the tropical atmosphere.
Recall from the video discussing absorbed solar and emitted infrared radiation versus latitude that, while the tropics run a surplus in energy, the middle and polar latitudes run a deficit. Thus, to balance the ledger of the earth-atmosphere system, it is pretty obvious that there must be a transfer of heat energy poleward from the tropics. This transfer is accomplished by the meridional transport of heat energy by the atmosphere and the oceans. You may already be familiar with some mechanisms for this transport, such as the Gulf Stream (an ocean current that conveys heat energy northward from low latitudes).
As far as atmospheric transport of heat energy goes, there are several mechanisms working to export heat energy out of the tropics, which we'll explore in later lessons. For now, though, recall that large mid-latitude cyclones are very effective at transporting warm air northward and cold air southward with their broad circulations. Given the large north-south temperature gradients that prevail in the middle latitudes during the cold season, the large impacts on regional temperatures from strong advection qualify mid-latitude cyclones as "big business" in the world of heat transport. Is the same true for tropical cyclones? Not really. Tropical cyclones transport some heat energy and moisture from the tropics to higher latitudes, but their overall contribution pales in comparison to other transport mechanisms. If you're interested, check out the "Explore Further" section below for more on this topic and another peculiarity that arises from the barotropic nature of the tropics. Otherwise, get ready to explore another aspect of the "Type B" behavior of the tropics on the next page.
Explore Further...
Tropical Cyclones and Meridional Heat Transport
Although hurricanes (intense low-pressure systems that develop over warm tropical seas and attain maximum sustained winds of 64 knots (74 mph) or more) usually grab top billing on the evening news, they are "small-potatoes" when it comes to exporting tropical heat energy (and moisture). Granted, these "heat engines" sometimes venture far northward (check out animation of visible and infrared satellite images of Hurricane Irene between August 19 and August 29, 2011, for example), but even fairly large hurricanes like Irene are small in the grand scheme of weather systems. For another perspective on Irene's size, check out this full-disk infrared satellite image from GOES-East taken at 15Z on August 26, 2011. Irene (which again, was large by hurricane standards) doesn't look very big, does it? Not surprisingly, then, in the final analysis, the storm didn't transport much heat energy or moisture poleward. Also keep in mind that hurricanes form during the warm season, so the impact of their heat energy on the already warm middle latitudes is limited.
Now, for comparison, check out this sequence of GFS forecasts of 850-mb temperature, wind, and surface highs and lows from the 12Z run on February 22, 2019. Using 850-mb temperatures to track warm and cold air, watch how the temperature field evolves as a low-pressure system develops in the central Plains and then deepens substantially on its trek through the Great Lakes in to eastern Canada. Clearly, colder air plunges southward on the western side of the developing low thanks to cold advection (note that the 0 degree Celsius isotherm dips to the Georgia / Tennessee border by the end of the loop). Meanwhile, east of the low, warmer air surges northward thanks to warm advection (the 0 degree Celsius isotherm advances as far north as Quebec). Without reservation, this mid-latitude low is a big-business meridional transporter of heat energy.
Seasonal Variations in Tropical Temperatures
Unlike the middle latitudes, there are places in the tropics that have two annual peaks in temperature during the warm season (instead of one). For example, compare the plot of the annual variation in average temperatures at St. Louis, Missouri, with a similar plot at Bhopal, India. Note the single peak in average temperatures at St. Louis around the middle of July. In contrast, the trace of average temperature at Bhopal shows a much smaller annual variation, and shows two peaks -- one in early May and another just before the start of October.
The relatively small annual variation at Bhopal occurs in large part because of the relatively direct solar radiation that occurs year-round at Bophal's latitude (around 23 degrees North). Seasonal changes in clouds and rainfall, however, make substantial differences in Bhopal's temperatures from one season to another. The "dip" in temperatures that occurs at Bhopal from May through September, for example, coincides with the rainy season in Bhopal (advance to the second slide to view average monthly precipitation at Bhopal). We'll explore the reasons behind these seasonal changes in clouds and rainfall in a later lesson.