Pressure in the Tropics: More "Type-B" Behavior
Pressure in the Tropics: More "Type-B" BehaviorPrioritize...
Upon completion of this section, you should be able to compare typical pressure gradients in the tropics with those in the middle latitudes, and be able to interpret frequency of wind directions and speeds from wind rose diagrams.
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Just as the tropics display a "Type B" personality with respect to temperatures, they generally maintain that same personality when it comes to pressure. In the tropics, pressure gradients tend to be much more relaxed than they do in the "Type-A" middle latitudes (resuming our analogy from the previous page).
Indeed, the relatively small temperature gradients over the tropics go hand in hand with relatively small pressure gradients, just as the larger temperature gradients in the mid-latitudes go along with larger pressure gradients there. To get a feel for the relaxed pressure gradients and other key aspects of pressure patterns in the tropics compared to the mid-latitudes, check out the short video below.
Tropical Pressure Patterns (2:32)
Transcript: Tropical Pressure Patterns (2:32)
To get a feel for patterns of pressure in the tropics versus the mid-latitudes we have here a global view of mean sea-level pressure. This happens to be a model analysis, and the shadings represent the anomaly, or departure from the long-term average pressure in that region. A couple of things should jump out at us right away. First, the mid-latitudes in both hemispheres have fairly large contrasts in pressures between high and low pressure systems. Those differences create large pressure gradients. The contour interval on this map is 2 mb, so that’s why the packing of the isobars in the mid-latitudes is really tight in the areas with the large gradients.
Now, I’ve grayed out the middle and high latitudes so that we can focus on the tropics, which are a very different story from the mid-latitudes. Note the relative lack of large, strong high- and low-pressure systems for starters. Furthermore, notice that except for a few exceptions near the edges of the tropics around 30 degrees north and south latitude, the shaded anomalies are rather faint in the tropics. That means that the pressure values are quite close to the long-term averages, in contrast to the mid-latitudes where there are many strong highs and lows with large anomalies. Overall, the pressure gradients in the tropics are far more lax than those in the mid-latitudes.
Of course, there are some exceptions to this pattern of relaxed pressure gradients in the tropics. There were actually 3 tropical cyclones present in the western Pacific at the time of this analysis, which I’ve pointed out with arrows. They appear as little bulls-eyes of much lower pressure – they have darker blue shadings, indicating that pressures were substantially below normal for the area. They also have much larger pressure gradients around them since their pressures were so much lower than the surrounding areas in the tropics. But, again, tropical cyclones are the exception to the rule. Certainly in the deep tropics, the packing of isobars overall is quite loose, indicating weak pressure gradients, and pressures weren’t varying very much from climatology. I’d even go as far to say that prominent centers of high and low pressure can be hard to find in the tropics, except for tropical cyclones, of course.
If we track how this pressure pattern was predicted to evolve over a 5-day period, we can see that the overall message of weak pressure gradients in the tropics away from tropical cyclones didn’t change much. A consequence of that is that, away from tropical cyclones, pressures in the tropics tend not to change much in time. That’s in contrast to the mid-latitudes, where pressures are much more changeable in time as the parade of larger high- and low-pressure systems marches around the globe.
The overwhelming message from the video is that pressure gradients are typically weak across the tropics, while the main "flies in the ointment" seem to be tropical cyclones. Indeed, tropical cyclones do something that is unheard of in the middle latitudes: They form in an environment bereft of large temperature gradients yet somehow develop very large pressure gradients around their center. We'll explore this conundrum a little later in the lesson.
Of course, the small pressure gradients throughout the tropics also mean that changes in surface pressure with time at any given location are usually puny compared to the larger increases and decreases that regularly accompany the approach and passage of mid-latitude high- and low-pressure systems. In the equable tropics, pressure patterns can persist for very long periods (weeks and even months). Yet, almost mysteriously, there is a regular daily rhythm of changes in surface pressure that meteorologists detect in the tropics. If you're intrigued, check out the "Explore Further" section at the end of the page.
The relaxed gradients in the tropics don't stop at the surface. The height patterns on constant pressure surfaces over the tropics are similarly relaxed. Consistent with the general lack of temperature gradients at 500 mb over the tropics that we covered previously, note the absence of strong gradients between 30 degrees latitude (north and south) on the chart of long-term mean 500-mb heights below. Height contours on the other mandatory pressure levels in the tropical troposphere show a similarly relaxed pattern.
Since the pressure gradient force is a primary driver of wind speed, you might think that the winds are almost always weak in the tropics (outside of tropical cyclones, that is), with the weak pressure gradients at the surface and aloft. But, that's far from the truth! To help you visualize the fact that many places in the tropics are quite breezy, despite weak surface pressure gradients, I'm going to introduce a new type of plot -- the wind rose. Wind roses display the observed frequency of wind directions (and sometimes speeds) at a particular location. On the left below is a histogram displaying frequencies of observed wind speeds (in meters per second) at an ocean buoy moored at 8 degrees South, 95 degrees West (opens in a new window) during a single year. On the right is the corresponding wind rose for the buoy, which shows the frequency of observed wind directions during the same year.
From these two images, we can quickly get two important messages. First, wind speeds at the buoy were between five and nine meters per second (roughly 10 to 20 mph) the vast majority of the time, which hardly constitutes "weak" winds. Second, the direction from which the wind blew during the year was remarkably consistent. To get your bearings with the wind rose, note that each concentric ring represents a ten-percentage point increase in the relative frequency of the observed wind direction. Thus, the daily mean wind direction of 130 degrees (from the southeast) occurred on nearly 45% of the days, and the daily mean wind direction of 140 degrees occurred on about 28% of the days! The wind rose clearly demonstrates that winds retained their overall southeasterly direction for almost the entire year (and didn't deviate much from 130 degrees). Small variations in wind direction and breezy conditions are fairly typical in tropical locations because of the famous belt of "trade winds," (opens in a new window) which we'll cover formally in a later lesson.
Many wind roses that you'll encounter also include wind-speed data right on the wind rose plot. The short video below walks through an example of how to interpret such wind roses (a Key Skill from this section). The wind rose in the video is from a mid-latitude location; note how much more variable wind directions are compared to our example from the tropics above.
Wind Roses (3:08)
Transcript: Wind Roses (3:09)
This is a wind rose for the month of March at Grand Rapids, Michigan. Wind directions during this month are pretty variable, which is typical of mid-latitude locations that don’t have any overwhelming terrain influences or other localized factors that control wind direction. On this wind rose, each concentric ring represents a two-percent increase in the relative frequency of the observed wind direction, so if we want to know the most common wind direction at Grand Rapids during March, we would just look for the longest spoke. It’s a close call, but the spoke representing winds from the east, or from 90 degrees, is the longest, extending out to just shy of 10%. The various colors along each "spoke" represent wind speed ranges according to the color key at the bottom of the image.
Let’s zoom in a bit and take a closer look at that easterly spoke so that we can analyze it. Along the 90-degree spoke, winds between 1.80 meters per second and 3.34 meters per second , which is roughly 3.5 - 6.5 knots, are marked by the yellow shaded area. That yellow area extends out to the ring marked 2%, so easterly winds in that range of speeds occur about 2% of the time.
Winds between 3.34 meters per second and 5.40 meters per second, or roughly 6.5 knots - 10.5 knots, are marked by the red shaded area. We can tell how often winds in that speed range occur simply by subtracting the percentage at the inner edge of the red area, which is 2%, from the percentage at the outer edge of the red area, which is 5.5%. So, if we do the subtraction, wind speeds in that range occur about 3.5% of the time during March.
Our next speed range is 5.40 meters per second to 8.49 meters per second, which is roughly 10.5 to 16.5 knots, and is marked by the blue shaded area. Winds in that speed range also occur around 3.5% of the time, which we can tell by subtracting the percentage at the inner edge of the blue shaded area from the percentage at the outer edge of the blue shaded area. 9% minus 5.5% = 3.5%.
To summarize what we’ve done so far, the range of wind speeds marked by the yellow area occurs about 2% of the time, wind speeds marked by the red area occur about 3.5% of the time, and wind speeds marked by the blue area also occur about 3.5% of the time. So, our total frequency of all those wind speed ranges combined would be about 9% of the time, just summing those individual percentages. And our blue area ends about halfway between the 8% and 10% rings, so that makes sense. Along any given spoke, the individual percentages for each range of wind speeds should sum to the total percentage associated with the entire spoke.
So, we know we were around 9% by the end of the blue area, which means that these last two speed ranges, marked by green and cyan make up the difference that would get us close to 10% total for the spoke, which means that the fastest two speed ranges must occur a little less than 1% of the time combined.
I strongly recommend taking some time to practice extracting information from wind roses (you can start with the "Key Skill" section below). Wind roses can provide lots of practical information. For example, consulting meteorologists use wind roses when they work on the design of airports (runways should be built to avoid strong crosswinds), and skilled forecasters regularly use wind roses when studying the climatology of a particular location. After you're comfortable with interpreting wind roses (and check out the "Explore Further" section, if you wish), you'll be ready to examine another difference between the tropics and the middle latitudes -- the structure of mid-latitude cyclones versus the structure of tropical cyclones.
Key Skill...
You'll need to interpret wind roses not only in this course, but future courses, so it's a good idea to spend a little time making sure you're comfortable with gathering basic information from them. Consider the March wind rose plot (opens in a new window) from Grand Rapids, Michigan and answer the following questions. If you do not understand the answers to these questions, be sure to review the guidelines for interpreting wind roses above and / or ask your instructor for clarification.
Question #1
During the month of March at Grand Rapids, which wind direction is observed the least frequently on average? What percentage of the time is this wind direction observed?
Answer: North-northeasterly winds are observed least frequently at Grand Rapids during March. Winds from the north-northeast are only observed slightly less than 3% of the time.
Question #2
Which wind direction most frequently produces wind speeds greater than 11.06 meters per second (roughly 21.5 knots)?
Answer: West-southwesterly winds most frequently produce speeds greater than 21.5 knots (almost 1% of the time), followed closely by southwesterly winds. The cyan shaded area corresponding to these speeds is largest along the west-southwesterly and southwesterly spokes.
Question #3
What percentage of the time do winds blow from the west-southwest between 3.34 meters per second and 8.49 meters per second (roughly 6.5 - 16.5 knots)?
Answer: Winds blow from the west-southwest between 6.5 knots and 16.5 knots slightly more than 5% of the time. We have to add the percentages that correspond to the red shading (slightly less than 3%) and blue shading (more than 2%).
Explore Further...
As you learned on this page, pressure gradients in the tropics tend to be very relaxed, and changes in surface pressure with time at any given location are usually puny compared to the larger variations that regularly accompany the approach and passage of high and low-pressure in the middle latitudes. In the equable tropics, pressure patterns can persist for very long periods (weeks and even months). Yet, almost mysteriously, there is a regular daily rhythm of changes in surface pressure that meteorologists detect in the tropics.
To see what I mean, focus your attention on the time-trace of barometric pressure at Nauru, a tropical island in the western Pacific just a tad south of the equator. The trace in barometric pressure spans a nine-day period. Although the fluctuations in pressure are relatively small in the grand scheme of weather (only a few millibars), there is an undeniable rhythm to the ebb and flow of the barometer. Indeed, much like the tides of the oceans, there are two high and two low "tides" in pressure that occur each day. In other words, there is a persistent oscillation in barometric pressure at Nauru that has a period of half a day (one high tide and one low tide in 12 hours). To better see this "semi-diurnal" oscillation in pressure at Nauru, check out this annotated version of the barograph trace (opens in a new window). This semi-diurnal oscillation in barometric pressure is a staple of the tropics.
As it turns out, the amplitude of the pressure tides is largest in the tropics, where pressure variations generated by passing weather systems are routinely small. So, it's no wonder that these pressure tides stand out on barograph traces. In contrast, the amplitude of pressure tides is much smaller over the middle latitudes (the amplitude of the semi-diurnal pressure tide falls off dramatically with increasing latitude), so they are usually dwarfed by much larger pressure variations produced by passing weather systems (making them difficult or impossible to detect on barograph traces).
For the record, the greatest amplitude (opens in a new window) of the semi-diurnal pressure tide, which is a approximately one or two millibars, occurs at the equator. So, why do they exist? In a nutshell, the atmosphere absorbs only about 10 percent of the incoming solar energy. Ozone in the stratosphere (opens in a new window) accounts for a large fraction of the atmosphere's absorption, while, to a lesser degree, tropospheric water vapor accounts for most of the rest of the atmosphere's absorption of solar energy. At any rate, the resulting warming of the atmosphere after sunrise (and cooling on the other side of the earth) creates sufficient changes in air density that internal gravity waves form and propagate both vertically and horizontally. As these density-driven waves reach the earth's surface, they induce noticeable changes in pressure over the equable tropics. At higher latitudes, these gravity waves become "vertically trapped" and their affects on surface pressure become increasingly unimportant (the proof of the vertical trapping of internal gravity waves at higher latitudes involves very sophisticated mathematics and is beyond the scope of this course).