This first installment in an occasional series on tropical cyclones over the winter months covers a topic that has recently seen some press on the message boards and in operational circles: trough interaction. Specifically, this feature covers trough interaction leading to tropical cyclone development (i.e. intensification) and not decay. There are two basic paradigms of trough interaction leading to intensification, trough-related and upper low-related, and while both share some similarities there are unique aspects to each leading to different types of development. While I am not explicitly covering the cases of upper level lows aiding in the formation of tropical cyclones, a process hinted at in some of the 2005 season-in-review articles, many of the characteristics of that development are seen in the trough-related development paradigm.
The trough-related paradigm is pretty much as the name states -- an upper-level trough constructively interacts with a tropical cyclone, leading to enhanced development. This cannot be any upper-level trough, however; instead, it needs to be a trough on an appropriate scale and strength so as not to rip the tropical cyclone apart through shearing processes. In general, a favorable trough interaction in this circumstance occurs when the horizontal scale (or width) of the trough matches that of the tropical cyclone. More often than not this is not the case, as the trough is usually much larger in scale than the tropical system. If there is a sufficiently strong upper-level ridge near the tropical cyclone, however, or some other blocking mechanism, the trough can gradually narrow (often accompanied by it weakening) until it may closely resemble the scale of the tropical system. At this point, enhancement occurs via two means: a region of weak diffluent flow aloft (i.e. the winds spreading apart from a single point) above the tropical system and what we call "superposition of cyclonic vorticity anomalies." This is a fancy way of saying that an upper-level and lower-level cyclone can constructively interact, leading to the former (in this instance, the tropical cyclone) spinning up/becoming stronger. Owing to all of these processes, it can be hard to identify and predict such an evolution beyond saying that the potential is there for it to occur.
This is generally viewed as "baroclinic enhancement" of a tropical cyclone, whereby middle latitude features play a direct role in enhancing the strength of the storm. Oftentimes, due to these impacts, the storm may not have a symmetric appearance; it may also not be located in a truly favorable environment for tropical cyclone development. Partially as a result of this and partially due to the locations (namely the subtropics) in which it tends to occur, this type of trough interaction is generally found with weaker systems, i.e. tropical storms and weak hurricanes, and can lead to modest intensification. Through this, it is rare to see development beyond category 2 or minimal category 3 intensity; there is generally a limit to how far a storm can develop given suboptimal conditions, whether in the environment or underlying oceanic surface. The most cited example of this enhancement comes from Elena in the Gulf of Mexico in 1985; John Molinari and others at SUNY-Albany have done quite a bit of work on this storm and the general paradigms behind this development. Anyone interested in more of the technical aspects can feel free to contact me and I will point them toward some other works.
The upper low-related paradigm is also pretty much as the name states -- an upper-level low constructively interacts with a tropical cyclone, leading to enhanced development. Central to this development is the distance of the upper-level low from the tropical cyclone -- too close and it will tend to shear it apart; too far away and it will have negligible impact upon the system. Also key, albeit not as crucial, is the intensity of the upper-low; generally a weak to moderate strength upper-level low is better than a deep, intense upper-level low. These sorts of interactions generally occur in the transition region from the tropics to the subtropics, unlike the trough-related paradigm and thus away from middle latitude troughs.
As we all know, developing tropical cyclones tend to have pretty good outflow at upper levels, usually beneath an upper-level ridge. As the outflow is drawn away from the center of the storm, the heating it provides aloft often enhances such an upper-level ridge. As this occurs, the height gradient between the upper-level ridge and the outside environment increases, leading to the development of a jet streak, often taking the form of an outflow jet. This is where the presence of an upper-level low can play a role; it provides a focusing mechanism for such a jet and also helps to enhance the gradient between the ridge and its environment, enhancing the jet itself. (Note that how the height gradient can lead to the development of a jet ties directly into concepts of potential vorticity as well as the governing equations for the atmosphere and is probably beyond the scope of this discussion.)
A tropical system can be viewed as a three (truly four) part circulation -- inflow at lower levels, upward motion in the center of the storm, and outward motion (outflow) at upper levels. As the outflow from the storm is drawn away from the storm at a faster rate, it allows for the upward branch of the storm's circulation to be enhanced. At this point, it draws more energy from the underlying surface in the form of heat and moisture, at least as long as such energy is available to be drawn from the surface. (Thus, it helps if the storm is in a region of warm SSTs and high oceanic heat content. Such circumstances can lead to a storm approaching its maximum potential intensity, another concept entirely.) This allows for enhanced inflow at lower levels, often leading to the spin-up of the storm's circulation as a whole. As the storm intensifies, outflow is often further enhanced; this gets fed into the vicinity of the upper-low, which often strengthens under such circumstances due to something we call "shear vorticity." This shear vorticity is a result of curvature on the side of a jet; if you have flow going from west to east, on the left side of this flow, the wind will tend to want to deflect to the left, resulting in cyclonic spin. At this point, the whole system loops back and repeats, oftentimes until something comes along to break it (or at least limit its effects, such as an eyewall replacement cycle) or all of the available energy has been extracted from the underlying surface.
It is unknown whether or not the upper-level low and the tropical cyclone interact mutually or if one leads to the other; it is somewhat akin to the chicken and the egg problem. From experience, it seems that they act in concert with one another; as the tropical cyclone develops, outflow is enhanced, feeding into the vicinity of the upper low, where shear vorticity leads to its development, allowing for the further enhancement of the outflow jet and the storm's outflow. An ideal case of this -- and one that also demonstrates the lack of predictability associated with such features -- is with in the 2005 season. As developed, a weak upper low that trailed it served as a focusing mechanism for its outflow. Coupled with strong easterly flow to the north of the upper low, it quickly gained strength; as it did so, the outflow pattern from continually became enhanced, leading to the ability for it to strengthen further. This upper-level low was located just to the northwest of Hurricane Phillipe; as the upper-low intensified, it strongly impacted the development of Phillipe, leading to many blown intensity forecasts and the ultimate demise of the storm. As you might expect, this paradigm is most often found with strong hurricanes and is likely a direct player in the intensification of many storms to category 4 & 5 intensities. As with the other case, I would be happy to provide more technical information if desired.
Suggestions for future topics are always welcome; please feel free to send me a PM on the boards here if you have any you'd like to see in particular. Feel free to discuss this here as well, whether on the main page (if appropriate) or in the Blogger Discussion forum. Resources used in the compilation of this document come from personal experience, teachings in classroom settings, and with concepts applied from papers by John Molinari and Kerry Emanuel (although no specific information was utilized from their works). A full list of citations is available upon request.
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