Regional And Mesoscale Meteorology Team
Daily Satellite Discussion
Friday December 5, 1997
Occasionally low-level westerly winds are observed along the Equator, and when those winds are particularly strong with a well defined speed maximum that propagates to the east, the phenomenon has been called a "westerly wind burst." This often results in the formation of tropical cyclone pairs or twins located at approximately the same longitude and the same distance on opposite sides of the Equator. The formation of twin tropical cyclones typically occurs in the western Pacific and Indian Ocean during the months of December through May. During the first few days of December, 1997, this type weather pattern is located unusually far to the east in the central Pacific. This is likely due to the influence of the ongoing strong El Nino.
The associated cloud pattern is seen in a series of daily GOES-9 IR images remapped to Mercator projection. One can see that a large region of the tropical Pacific is influenced by the active deep convection and cirrus. This can be quite spectacular with a somewhat symmetric "mirror image" cloud pattern aligned along the Equator. The formation of twin tropical cyclones is also seen in the IR images. Streamlines, isotachs, and wind barbs from the 850 hPa analysis of the global AVN Model at 12 UTC, 2 Dec 97, depict the low-level winds. At 1500 UTC, 3 Dec 97, Tropical Storm Paka was centered at 8.5N 167.8W, with an intensity of 45 kt, and drifting to the WNW. The Southern Hemisphere twin cyclonic circulation near 10S 169W had not yet intensified to a tropical storm.
Click here for an IR loop.