Tropical Upper Tropospheric Troughs
Lesson 9. Wind Fields in and around Tropical Cyclones
Introduction to TUTTs
During the summer months, the pattern of 200-mb heights over the North Atlantic, South Atlantic, North Pacific and South Pacific Ocean basins reveals semipermanent features called Tropical Upper Tropospheric Troughs. The analysis shown below indicates the average summertime position of the TUTT over the Pacific basin (from June through August). Keep in mind that TUTTs can shift positions from day to day.
At any rate, the TUTT over the Atlantic basin first appears in June, strengthens in July and August, and then weakens in September and October. During August (the start of prime time for Atlantic tropical cyclones), the TUTT tilts from southwest to northeast, spanning from Cuba to roughly latitude 35 degrees North. For confirmation, check out the average positions of the Atlantic TUTT during July and August. I note here that the standard contour interval on a 200-mb map is 120 meters, but any monthly average over the Tropics, where 200-mb height gradients are typically weak, demands a smaller contour interval in order to adequately represent a TUTT (15 meters, 30 meters, etc., depending on the type of analysis). TUTTs over the Pacific basin reach peak intensity from July to September and can persist into late fall and early winter.
Why should we study the topic of Tropical Upper Tropospheric Troughs? Fundamentally, TUTTs are unique and rather quirky features (more to come in just a moment). More importantly, TUTTs act like double agents...they can contribute to the development of tropical cyclones, and they can also discourage tropical cyclogenesis. Intrigued? Then let's get going!
I wasn't kidding when I said that TUTTs are a bit quirky. For starters, TUTTs are three-dimensional features (below, the cross-sectional view of the horizontal temperature anomalies associated with a TUTT confirms this notion). As you can see, TUTTs are essentially cold core below 200 mb (shadings of blue represent negative temperature anomalies). If you look closely at the cross section, you'll see that the greatest negative temperature anomaly occurs around 300 mb (on the order of -4oC). Below 300 mb, horizontal temperature anomalies fade with increasing pressure (decreasing altitude), essentially vanishing below 850 mb. Such a vertical profile of negative temperature anomalies provides the basis for describing a TUTT as a "floating iceberg." A neat metaphor, yes, but such a description doesn't tell the whole story. That's because a TUTT typically becomes warm core near and above 200 mb! Let's investigate.
First, let's look at a specific example of the temperature structure of a tropical upper troposphere trough. On August 17, 2007, a TUTT set-up just to the east of Florida close to its climatological position. To confirm its presence, I offer this GFS model analysis of 200-mb streamlines at 00Z on August 17 (annotated version) and this loop of water vapor images (again, focus your attention on the developing cyclonic circulation just to the east of Florida; the loop spans from 00Z on August 14 to 18Z on August 17). As you recall from Meteo 101, water vapor loops indicate the moisture patterns in the upper troposphere, so the cyclonic circulation associated with a TUTT should indeed show up on loops of water vapor imagery. By the way, the dashed, dotted line on the images in the loop that's in the vicinity of the TUTT is an artifact of splicing together images from two different geostationary satellites (yes, it's annoying).
Looking for further proof? Check out the GFS model analysis of 200-mb heights and 200-mb absolute vorticity at 00Z on the 17th. On this image, the lime-green contours indicate 200-mb heights, and the dashed, purple contours are isopleths of 200-mb absolute vorticity. Additionally, the highest values of absolute vorticity are color-filled (units are 10-5sec-1). I thought it was important to show you this analysis because TUTTs, like any other trough that has a relatively short wavelength, should indeed be associated with a vort max. I'll have more to say about 200-mb heights and absolute vorticity a bit later on this page.
But first, let's polish off our discussion about the vertical temperature structure of tropical upper tropospheric troughs (glance back to the vertical cross-sectional view of a TUTT). Check out, below, the GFS model analysis of 300-mb temperatures, and note the pocket of cold air to the east of Florida (larger image). This cold pocket at 300 mb is consistent with the cross-sectional view of temperature anomalies above (note the pocket of negative temperature anomalies near 300 mb).
Okay, note that the temperature anomalies on the cross-sectional view decrease with increasing pressure (decreasing altitude) below 300 mb. Do the model data for the TUTT east of Florida on August 17, 2007, support the temperature structure shown on the cross-sectional view? Absolutely. Check out the GFS model anayses of 500-mb isotherms and 700-mb isotherms at 00Z on August 17, 2007, and note that the cold pocket east of Florida, although present at both pressure levels, weakens with increasing pressure (decreasing altitude). So, yes, model data support the temperature structure of our prototypical TUTT at 300 mb and below (shown on the cross-sectional view). In other words, the metaphor that a TUTT is like a floating iceberg is a plausible description.
What about our observation that a TUTT is warm core near and above 200 mb (revisit the cross-sectional view)? Well, the GFS model analysis of 200-mb isotherms at 00Z on August 17, 2007, supports the cross-sectional view. Indeed, the GFS model analysis shows a warm pocket at 200 mb to the east of Florida. This warm pocket is consistent with the positive temperature anomalies beginning around 200 mb on the cross-sectional view. The GFS model analysis of 150-mb isotherms (below; larger image) seals the deal on my claim that a TUTT is warm core near and above 200 mb. Indeed, there's no doubt that the conspicuous warm pocket east of Florida (below) fits in perfectly with a TUTT's temperature structure on the cross-sectional view.
More on the High-Altitude Warm Core of TUTTs
What causes the warm temperature anomalies above 200 mb in the core of a TUTT? Not surprisingly, the positive temperature anomalies at these rarefied altitudes are the signature of subsidence (compressional warming) in the upper troposphere and lower stratosphere. Let's explore the connection between TUTTs and subsidence.
Let's start with the observation that there's a lack of convective clouds and precipitation over the mid-oceans during the summer months. To confirm this observation, check out, below, the analysis of the long-term mean of outgoing longwave radiation (OLR) for June, July and August (units expressed in Watts per square meter; larger image). By way of review, the purplish areas, which indicate areas of weaker OLR, correspond to the cold, high tops of convective clouds (remember that the intensity of emitted radiation is a function of the fourth power of absolute temperature; Stefan-Boltzmann's Law).
More importantly, note the ribbon relatively high values of OLR that stretches over the mid-Pacific. These relatively high values indicate a lack of tall convective clouds and precipitation (convection occurs over the warming continents, which favors subsidence over the mid-ocean regions). In the mean, the high values of OLR over the central Pacific during summer essentially correspond to long-wave radiation emitted by the relatively warm ocean and the relatively warm tops of shallow, generally piecemeal maritime clouds that can develop from time to time. By the way, the largest values over the eastern Pacific are the footprints of the Pacific high-pressure system.
Any way you slice it, the overall lack of tall convective clouds and associated convective precipitation is the footprint of persistent subsidence in the upper troposphere over the middle of the ocean. In turn, this subsidence sets the stage for a TUTT to form (compare the mean position of the TUTT shown at the top of this page with the long-term mean of OLR below). See the connection? So what's the scientific basis for this connection?
The Role of Radiational Cooling
For starters, compressional warming in the upper troposphere and lower stratosphere increases the thickness between pressure levels in this region of the atmosphere (remember from Meteo 101 that the thickness between pressure levels is directly proportional to an air column's mean temperature). As this idealized cross section shows, compressional warming between, say, 100 mb and 200 mb, causes 200-mb heights to fall (and 100-mb heights to rise). Falling 200-mb heights are consistent, of course, with a trough (TUTT) near these pressure altitudes.
Subsidence and the associated drying also tightens the vertical moisture gradient in the upper troposphere and lower stratosphere (keep in mind that Pacific moisture still remains at lower altitudes, where upper-level subsidence does not reach). As it turns out, a tightening vertical gradient in moisture paves the way for enhanced radiational cooling.
At any rate, this enhanced radiational cooling sets the stage for a TUTT to be self-sustaining. That's because the enhanced radiational cooling drives subsidence (cooling creates negative buoyancy). In turn, more subsidence further enhances radiational cooling and keeps the vertical moisture gradient tight (and so on, and so forth). This positive feedback loop breaks down (and TUTTs weaken and eventually dissipate) during the fall as the continents cool relative to the ocean (and upper-level subsidence is no longer favored). Like clockwork, TUTTs return the following summer.
TUTTs and Tropical Cyclogenesis
Based on my own forecasting experience, I'd have to say that most TUTTs shear tropical systems (rather than aid tropical cyclogenesis). To gain some insight, check out (below) the GFS model analysis of 250-mb heights and 250-mb isotachs at 00Z on August 17, 2007 (recall the TUTT east of Florida that I've referenced several times on this page; larger image). Southerly winds as high as 40 and 50 knots on the eastern flank of the TUTT (and northerly winds as high as 60 and 70 knots on the western flank) contributed to relatively large vertical wind shear that would have been unfavorable for tropical cyclogenesis. Indeed, check out the pattern of vertical wind shear between 850 mb and 200 mb in the vicinity of the TUTT at this time (blue wind barbs represent the vertical wind shear, and lime-green contours correspond to 200-mb heights). Recall from an earlier lesson that wind shear is a vector...it has both direction and magnitude (in this case, the magnitude of vertical wind shear is designated in knots). Also remember that the wind barbs indicate the bulk vertical wind shear between 200 mb and 850 mb.
The bottom line here is that tropical forecasters want to know where TUTTs are during hurricane season primarily because they generate relatively large wind shear that precludes or disrupts the development of tropical cyclones.
Having said this, I point out that TUTTs can sometimes promote cyclogenesis by enhancing the high-altitude outflow in the vicinity of of a tropical cyclone (I laid the groundwork for this observation at the beginning of Lesson 9). By way of review, the enhancement of the high-altitude outflow jets by a TUTT is depicted on the rightmost panel of this schematic, which displays favorable upper-level wind patterns for tropical cyclogenesis (the dotted-yellow line on two of the schematic's panels represents the equator). Note that the upper-level winds associated with the TUTT (red streamlines on the schematic's rightmost panel) enhance the outflow jet in the tropical cyclone's northeast quadrant. In turn, this enhanced high-level outflow promotes greater divergence atop the tropical cyclone, paving the way for the system to intensify.
TUTTs and Thunderstorms
When a TUTT moves over land, widespread thunderstorms erupt in response to steeper low-level lapse rates (and, of course, relatively steep lapse rates associated with the TUTT's cold-core anomalies aloft). A TUTT moving over land happens every now and then over the Gulf Coast States (this observation is based primarily on my own forecasting experience, but a paper co-authored by Steve Lyons in the early 1990s supports my position). Steve Lyons was a student of James Sadler, who pioneered early work on TUTTs over the Pacific Ocean (paper on early-season impacts of TUTTs on typhoons; paper on mid-season impacts of TUTTs on typhoons). I should point out that the life expectancy of TUTTs over land is not very long because latent heating eventually eliminates the cold-core aloft.
Over the oceans, TUTTs tend to be relatively cloud free near their cores, but they can stimulate deep, moist convection around their edges. On August 29, 2002, a TUTT floated like an iceberg to the northwest of Hawaii (see the 12Z analysis of 200-mb heights below). The corresponding water-vapor image shows that the "TUTT core" lacked any high cloud tops, while the twin infrared image suggests that most of the cloudiness occurred around the periphery of the TUTT's core. The corresponding visible image closes the deal on a basically clear TUTT core.
Deep convection sometimes erupts east and south of the center of a TUTT, where divergence associated with the corresponding 200-mb vorticity maximum encourages ascent. The water vapor image below (18Z on August 31, 2002) shows convection flaring up in the southeast quadrant of a TUTT north and west of Hawaii (the remnants of Hurricane Fausto flared up on August 31 when the system moved under the TUTT; Fausto re-intensified into a tropical storm the next day).
By way of background, Hurricane Fausto became a Category 4 hurricane on August 24, 2002, packing maximum sustained winds of 125 knots (visible satellite image; high resolution, resize your window). Soon thereafter, Fausto weakened as it took a northwestward track toward the central Pacific. The demise of Fausto came, in part, from relatively large vertical wind shear associated with 200-mb west-southwesterly winds east of the nearly stationary TUTT near Hawaii. For confirmation, check out the August 29 daily composites of 200-mb heights, 200-mb wind vectors and 850-200-mb wind shear. As you've already learned, a TUTT generates vertical wind shear, thereby acting as an impediment to tropical cyclogenesis. But once a tropical cyclone moves out of this zone of wind shear and underneath a TUTT (in the case of Fausto, a TUTT cell), all bets are off and the tropical cyclone can develop. Yes, TUTTs are like double agents!
In contrast to these "wet" TUTTs, there are "dry" TUTTs that lack extensive cloudiness. Research has shown that when the cold pool of air associated with a TUTT fails to extend below 300 mb, TUTTs tend to be virtually cloud free. In such cases, water vapor imagery becomes indispensable for detecting these "cloaked" TUTTs.
Sometimes, closed lows will develop from a TUTT. In the Pacific, these closed lows are known as TUTT cells. The analysis of 200-mb heights (12Z on August 30, 2002) shows a TUTT cell that closed off north and west of Hawaii. This TUTT cell was the closed low under which the remnants of Hurricane Fausto eventually moved (discussed earlier), sparking the "rebirth" of the storm on September 1 (the TUTT that initially sheared Fausto eventually closed off and became a TUTT cell).
To give you another example of how a TUTT cell can enhance tropical development, check out the discussion issued by the Joint Typhoon Warning Center at 15Z on December 7, 2006. The statement addresses the status of Tropical Depression 25W over the Northwest Pacific Basin (centered about 145 miles north of the Palau Islands). Note the reference to a TUTT cell and its expected impact on the depression's high-level outflow and subsequent intensification. You can see the TUTT cell in the 12Z 250-mb analysis. Note that the contour interval for this analysis is only ten meters so that you can better see the TUTT's structure and position (the standard contour interval at 250 mb is 120 meters) Also notice the flare-up of convection on the 15Z water-vapor image. Eventually, Tropical Depression 25W developed into Typhoon Utor (MODIS sateliite image; track), which impacted the Philippine Islands.
Although tropical forecasters must be aware of the position of TUTTs, they pay closer attention to the currents of air that steer tropical cyclones. Let's explore this topic in greater detail.