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Subtropical Cyclones

If you search the World Wide Web under the topic of subtropical cyclones, the most common description you'll find goes something like this: A storm that has both tropical and extratropical characteristics. Well, that leaves the door pretty wide open for interpretation. Thus, you should not be surprised that there are different kinds of storms falling under the one umbrella of subtropical cyclones. For a subtropical cyclone to have "tropical characteristics", there must be some semblance of organized convection that feeds back positively on the larger scale of the low-pressure system. The reference to "extratropical" means, of course, that temperature gradients played a role in the genesis and intensification of the system (baroclinic cyclogenesis).

When these hybrid storms form over the North Atlantic Ocean, the National Hurricane Center tags them with a name from the current season's list of tropical cyclones. Like tropical cyclones, subtropical cyclones go through stages of development -- subtropical depression and subtropical storm. For example, check out Subtropical Storm Nicole, which formed near Burmuda in early October, 2004.

A multi-channel satellite image of Subtropical Storm Nicole over the Atlantic Ocean at 18Z on October 10, 2004 (the lime-green splotch northeast of the storm's center is Bermuda). The yellowish swirl of clouds marks the low-level circulation of the storm. Nicole's center passed approximately 50 nautical miles to the northwest of Bermuda at about 00Z on October 11. Courtesy of the Applied Physics Laboratory at Johns Hopkins University. (large image, resize your window)

Storms that develop by processes similar to North Atlantic subtropical cyclones also occur over the Great Lakes (check out "Hurricane Huron" on September 14, 1996; read about it), the South Atlantic Ocean off the coast of South America (the hurricane in March, 2004, was likely a subtropical cyclone at one point during its development), the Mediterranean Sea (check out this image of a Mediterraean cyclone on January 15, 1995; read more about it), the North Central Pacific Ocean (Kona lows near Hawaii) and the Indian Ocean near Madagascar (for example, Subtropical Cyclone Luma in April, 2003).

By and large, the common bond shared by North Atlantic subtropical cyclones and their "first cousins" is that they develop over relatively warm water beneath a cold, upper-level low that has cut-off equatorward of the main westerly current of air. There can be subtle differences, of course, and I'll highlight the salient disparities between a Kona low and a classic North Atlantic subtropical cyclone (of course, both can produce gales and torrential rains, so their impacts are similar).

A more distant cousin of North Atlantic subtropical cyclones also forms over the Arabian Sea. Indeed, these cyclones are a slightly different beast because they form in concert with the monsoon trough over eastern Pakistan and northwest India. I'll address them briefly after I offer more classic examples of subtropical cyclones in the North Atlantic basin (Subtropical Storm Ana in April, 2003) and Kona lows in the North Pacific Ocean.

One of the curious things that sets some subtropical cyclones apart from pedigree tropical cyclones is that relatively large vertical shear often accompanies the genesis stage. In April, 2003, Subtropical Storm Ana formed in an environment where the magnitude of the wind shear between 850 mb and 200 mb approached 20 meters per second. When subtropical cyclones make the transition from a subtropical cyclone to a tropical storm (as Ana did), vertical wind shear must decrease, of course (a topic beyond the scope of Meteo 241, although, if the spirit moves you, feel free to research the topic and add it to your e-portfolio).

To kick off the discussion, let's shift our focus from the North Atlantic to the North Pacific Ocean.

North Central Pacific Ocean

From the earliest research, Kona lows near Hawaii have stood as classic examples of subtropical cyclones (see the enhanced water vapor image below). The Polynesian translation of Kona is "leeward". Indeed, surface winds associated with Kona storms are southerly or southwesterly - roughly the opposite direction to the prevailing trades - shifting the focus of heavy rains from the windward slopes (relative to the trades) to the leeward slopes. Historically, Kona lows can barrage the islands with hail, high winds, pounding surf and lightning. Heavy rains heighten the risk of flash flooding and landslides. Waterspouts sometimes form over offshore waters. Needless to say, Kona lows are nothing to sneeze at.

A Kona low lashed the Hawaiian Islands on December 29, 2003. The bluish blob east of the system's center represents the tops of convective clouds. Courtesy of the Cooperative Institute of Research in the Atmosphere at Colorado State University.

During the cold season (November-March), upper-level troughs traveling in the subtropical westerlies sometimes cut-off in the vicinity of Hawaii. On December 27, 2003, a 500-mb trough in the subtropical westerlies approached Hawaii (see image below). By the 29th, the trough cut-off from the subtropical westerlies, forming a cold, closed low in the middle troposphere (check out this animation of 500-mb charts to observe the cut-off process; small version).
The 03Z analysis of 500-mb heights over the Pacific Ocean on December 27, 2003, revealed a trough to the west-northwest of Hawaii. The trough later closed-off and then cut-off from the main branch of the 500-mb westerlies, setting the stage for a Kona low to develop. The system lashed Hawaii with heavy rain and strong winds.

The whole story is not quite as simple as an upper-level trough cutting off from the subtropical westerlies, however. The image below shows the surface analysis at 12Z on December 27, 2003. Note the cold front and trailing high-pressure system associated with the 500-mb trough in the subtropical westerlies that would eventually cut-off and pave the way for a Kona low to form. As the northerly winds associated with the transient high encountered southeasterly winds associated with a sprawling high-pressure system over the eastern Pacific, they sharpened the surface front, setting the stage for a low-pressure system to rapidly develop and occlude. Meanwhile, the transient high continued to press eastward, merging with the eastern Pacific high and trapping the occluded cyclone. Check out the animation of sea-surface pressures (small version). Pay close attention to how the occluded cyclone gets trapped. In this way, a Kona low is born, with southerly winds and pronounced convection on its eastern flank (as indicated by the cold cloud tops on the water-vapor image above).
The mean sea-level pressure pattern over the Pacific on December 27, 2003. Northerly winds on the eastern flank of a high-pressure system (which earlier crossed the International Date Line) worked in tandem with southeasterly winds on the western flank of the subtropical high over the eastern Pacific to sharpen a cold front wast of Hawaii, setting the stage fora low-pressure system to rapidly develop and occlude.

For the most part, North Pacific Kona lows occur during winter (unlike North Atlantic subtropical cyclones, which mostly form during the warm season). Apparently, cold, baroclinic processes dominate the development of Kona lows (more so than North Atlantic subtropical storms -- details in just a moment). During winter, sea-surface temperatures in the North Pacific are generally too low to promote large surface fluxes of heat energy and moisture needed to drive a warm-core, lower-tropospheric cyclone (like we observe in Atlantic subtropical storms). So I believe that, while the two types of storms are related, the vertical structures of Kona lows and North Atlantic subtropical cyclones are essentially different. Indeed, Kona lows have a more intense upper-level cold low and a cold, near-surface cyclonic reflection. North Atlantic subtropical cyclones, on the other hand, are more of a true hybrid because they have a warm cyclonic circulation in the lower troposphere and a cold low in the upper troposphere. Let's investigate.

North Atlantic Ocean

Most subtropical cyclones that form over the North Atlantic Ocean occur mostly during the active period of Atlantic hurricane season, although, as you are about to learn, a subtropical storm named Ana went way outside the lines in April, 2003. The latitude band between 25 and 33 degrees North encompasses the primary breeding grounds for North Atlantic subtropical cyclones, so their label of "subtropical" is well deserved. Favorable longitudes for genesis roughly span from 35 degees to 85 degress West.

After the National Hurricane Center christened Ana a subtropical cyclone on April 20, 2003 (read the NHC discussion), Ana subsequently made the tropical transition, becoming the first documented tropical storm ever to form in the Atlantic basin during the month of April (on April 21, 1992, a subtropical storm formed between Puerto Rico and Bermuda - that's the only other Atlantic subtropical or tropical storm on record in April). The early stages of the system were undoubtedly baroclinic about 200 miles south of Bermuda, but subsequent sporadic bursts of convection around its center alerted forecasters of the possibility that the storm would be classified "subtropical" and perhaps make the transition to a tropical storm (thunderstorms around the center of a low indicate "tropical characteristics"). Check out this 24-hour loop of infrared satellite images from 00Z on the 19th to 00Z on the 20th (small version). Notice the burst of convection as Ana starts to intensify.

A multi-channel, AVHRR satellite image of Tropical Storm Ana at approximately 20Z on April 21, 2003. Courtesy of the Applied Physics Laboratory at Johns Hopkins University.

On the 18th, a 500-mb trough swung over a frontal zone and initiated an area of low pressure at the surface. While over relatively warm waters, convection around the low's center started to flare up on the 19th (revisit the infrared satellite loop above). By 06Z the 20th, forecasters at the National Hurricane Center christened the system Subtropical Storm Ana, following through on guidelines that a subtropical storm must have both tropical and subtropical characteristics and meet the minimum requirement for sustained winds (at least 33 knots). For the record, NHC forecasters classify a low as a subtropical depression when its maximum sustained winds are less than 33 knots. I should also point out that, before 2002, NHC forecasters did not name subtropical storms (they still issued forecasts and warnings similar to those for tropical cyclones). By convention, names for subtropical storms come from the year's official list of names for tropical cyclones.

By 00Z on the 21st, with thunderstorms and their associated convective heating blossoming over relatively warm water, Ana made the transition to Tropical Storm Ana with maximum sustained winds of 50 knots (this was the peak intensity of the storm). Check out this 48-hour loop of satellite images from 18Z on the 20th to 18Z on the 22nd. (small version). Jogging generally eastward, Ana weakened and, on the 23rd, merged with a cold front about 80 miles east of Bermuda.

In the final analysis, the development of Ana was reminiscent of the Kona low near Hawaii, with an upper-level low playing an integral role in both cases. So, in a way, subtropical storms in the Atlantic are, in my opinion, Kona-type storms.

Arabian Sea

I'll end this discussion with another type of subtropical cyclone that forms along the west coast of India during the summer monsoon.

From the mid May to mid July (see image below), a heat low forms near the border of eastern Pakistan and northwestern India. Despite intense solar heating, surface pressure falls associated with the stationary heat low typically do not exceed six millibars during the period. The heat low generally tilts equatorward with height (toward the Arabian Sea and relatively warm air columns) before vanishing above 400 mb, where upper-tropospheric easterlies dominate.

The long-term mean in sea-level pressure between May 15 and July 15 shows the footprint of a heat low along the border between eastern Pakistan and northwestern India.

Between 700 mb and 500 mb over the northeast Arabian Sea, cyclonic circulations associated with the heat low typically develop in early June. In 2004, for example, a cyclonic circulation developed off the northwest coast of India over the northern Arabia Sea around June 10 (see image below). This mid-tropospheric subtropical cyclone drew moist air carried in the Low-level Somali jet northward, setting the stage for the monsoon rains to commence at Bombay. For this reason, this subtropical cyclone served as the onset vortex (Lesson 5) for the 2004 monsoon season in west-central India.

In a truly synergetic interaction, subsidence (and its associated compressional warming) on the northern flank of the subtropical cyclone serves to intensify the heat low, which, in turn, transfers heat energy back over the Arabian Sea, which paves the way for more subtropical cyclones to develop during the summer monsoon season.