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Volume 1 Issue 1 Jan 2022
Subtropical Cyclone Occurs in the Eastern South Pacific
Brandon Capasso, Director of Maritime Operations, Senior Meteorologist

WRI’s team of meteorologists are continuously observing the world’s oceans for weather hazards. On January 12th, 2022, our forecasters monitoring the South Pacific observed a low-pressure system exhibiting sub-tropical characteristics on satellite imagery over the eastern portion of the South Pacific basin.

Figure #1 – Visible satellite imagery showing the subject low with convective bands wrapping inward toward a symmetric low level circulation center, with an eye feature present (courtesy CIRA/RAMMB).

Furthermore, measurements from the Advanced Scatterometer satellite instrument (ASCAT) indicated that winds were concentrated near the center of the low, with wind speeds up to 45-50kts measured, further indicating that the low had acquired tropical characteristics.

Figure #2 – Satellite measured ocean surface wind speed from ASCAT indicating winds 40-45kts around the center of the low.

This was reason for concern, not only because of the potential for dangerous weather conditions that can result from such cyclones, but also due to the fact that there has never been a recorded occurrence of a tropical cyclone this far east in the region.

Figure #3 – Map showing all recorded tropical cyclones in the South Pacific east of 180 degrees longitude since 1950. The location of the subject low is shown with a red “L” symbol.

Further analysis of the low indicated that while the system did display some important tropical characteristics, it also lacked some key features that would be found in a “typical” tropical cyclone:

1) While the system did feature a low-level warm core, it did not have a deep “warm core”, and was instead assisted by instability/lift associated with a pool of cold air-aloft (warm air rises into the colder environment aloft). Tropical cyclones traditionally have a recognizable warm core even in the upper levels.

Figure #4 – Plots of 925mb and 500mb temperature showing the low level warm core and upper level cold pool associated with the subject low.

2) Sea surface temperatures in the area were actually below normal, between 21°-24°C - far below the typical 26°-27°C threshold considered to be sufficient for tropical cyclone development.

Figure #5 – Plot of sea surface temperatures (°C) over the region on Jan 12th, 2022.

We suspect that the pool of cold air present aloft essentially “lowered the bar” for the sea surface temperature requirement to sustain tropical development. The small and symmetric core of the low tracked through an area with favorable upper level winds, and a calm eye feature developed. Post analysis indicates that the eye extended vertically to the top of the atmosphere (troposphere).

Figure #6 – Vertical cross-section plot along 33S latitude of wind speed (kts, color shaded) and relative humidity (%, white contours) showing that the eye extended vertically to at least 33000 feet (~10000m) altitude.

Considering the features discussed above, WRI would consider this low to be a sub-tropical storm. In fact, this system was better organized than Arlene in the North Atlantic in April 2017, a system which was classified by the National Hurricane Center (NHC) as a Tropical Storm. Arlene formed at a similar distance poleward from the equator (35N latitude), and featured a similar minimum pressure and maximum wind speed.

Figure #7 – Satellite image of Tropical Storm Arlene in the North Atlantic on April 20, 2017 (NOAA).

Considering the marginal environment, such a system can only sustain briefly, perhaps 24-36 hours,  before weakening and dissipating. For this reason, they are often not recognized or classified by world government agencies. Regardless, these systems can produce dangerous winds and heavy confused sea states just as any tropical cyclone.

Rest assured, WRI is continuously monitoring all hazardous weather conditions, including potential tropical development, worldwide at all times, even if this development occurs in “unusual” areas such as the eastern South Pacific, the western South Atlantic, or Mediterranean Sea, and we are always prepared to alert our clients of these hazards when they occur via our Tropical Alert and Heavy Weather Monitoring services.

Contact WRI today to discuss what products and services we can offer you to maintain safe and efficient operations at sea.

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