Tropical Temperatures: A "Type B" Personality
Tropical Temperatures: A "Type B" PersonalityPrioritize...
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.
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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 (opens in a new window). 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 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.
The Tropics and Earth's Energy Budget (2:18)
Transcript: The Tropics and Earth's Energy Budget (2:18)
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 longwave 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 longwave (infrared) radiation.
If we look at the long-term average of OLR across the globe (below), 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 (opens in a new window)).
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 graph in the video) 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 percent 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 image below 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 if you toggle the image slider below, you can see global surface temperatures for a single day in late January.
On this winter day, 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 climatology of 500-mb temperatures across the globe below). 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 (opens in a new window). 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 be very small. 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 (opens in a new window) 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 (opens in a new window) (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, check your knowledge of the basics of tropical temperatures in the Quiz Yourself section below before you begin exploring another aspect of the "Type B" behavior of the tropics on the next page.
Explore Further...
Tropical Cyclones and Meridional Heat Transport
You may sometimes hear folks say that the primary role of hurricanes in the grand scheme of the Earth system is to transport tropical heat energy to higher latitudes. But, in reality, even though these storms make big headlines for the havoc and destruction they can cause, they are relatively small players in the export of heat energy (and moisture) out of the tropics. The short video below explains.
Tropical Cyclones and Meridional Heat Transport (3:25)
Transcript: Tropical Cyclones and Meridional Heat Transport (3:25)
Although hurricanes, which are intense low-pressure systems that develop over warm tropical seas and attain maximum sustained winds of at least 64 knots or 74 mph, always make big headlines, they’re relatively small players when it comes to exporting tropical heat energy and moisture to higher latitudes. Granted, these “heat engines” sometimes venture far northward as we can tell from this track map of an Atlantic hurricane season. Note how many of the storms during this season ended up traveling out of the tropics and into the middle latitudes, and even to high-latitudes as non-tropical remnants. So, why are these impactful storms such small players in exporting tropical heat energy and moisture?
Well, for starters, their size is a factor. This visible satellite image shows a Category 5 hurricane Melissa – one of the most intense Atlantic hurricanes on record, just south of Jamaica.
While Melissa was an incredibly intense storm, it’s small in the grand scheme of weather systems – it’s downright tiny within the realm of the entire hemisphere.
As Melissa moved northward, it did get a bit larger – here a few days later it was a Category 1 storm with a somewhat larger cloud pattern and circulation, but still relatively small in the scheme of things.
Now, for comparison, check out this enhanced infrared satellite image, which shows a strong mid-latitude cyclone over eastern North America. You may recognize the familiar comma shape.
Its scope and circulation encompasses much of eastern North America and the western Atlantic Ocean. It’s far larger than Hurricane Melissa’s was.
To further the point, here’s a re-analysis of 850-mb temperatures during the time of this mid-latitude cyclone. Note the very large area of very cold air plunging southward in the eastern U.S. thanks to strong cold advection – 850-mb temperatures below -20ºC had plunged into the Southeast, while on the eastern flank of the storm, much milder air had surged northward into New England and southeastern Canada thanks to warm advection. The mid-latitude cyclone was drawing air into its circulation over a very large area.
Now compare to the reanalysis of 850-mb temperatures from a landfalling hurricane. This map shows hurricane Ida making landfall in Louisiana.
And I’ve added an arrow to pinpoint its much smaller circulation. But, size isn’t the only factor at work here. Hurricanes form in the warm season, when hemispheric temperature gradients are smaller, so they tend to form and travel in environments that are already warm, without large gradients, which means minimal advection.
Even as Ida moved northward over the next couple of days it did finally become embedded in more noticeable temperature gradients, along the Northeast Coast, and it did send some warm air northward and some cooler air southward, but the gradients here just aren’t in the same league as those associated with a mid-latitude cyclone in winter time. Ida was making its northward trek in early September – a time of year when temperature gradients still tend to be on the smaller side in the Northern Hemisphere. So, due to size and smaller gradients leading to smaller advection, hurricanes tend to be smaller players in meridional heat transport.
Now compare again to our mid-latitude cyclone case from December – the gradients are clearly much larger, and the cyclone’s circulation is much larger, so the mid-latitude cyclone was a much bigger player in meridional heat transport.
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.
Quiz Yourself...
Check your knowledge of tropical temperatures and OLR basics in the short quiz below: