A Case Study of an Outbreak of Twin Tropical Cyclones Carl J. Schreck, III Department of Earth and Atmospheric Sciences University at Albany, SUNY

Tropical cyclogenesis within 15° latitude of equator –First fix in JTWC best track Northern Hemisphere storms are red- filled symbols Southern Hemisphere storms are blue- filled symbols Black box shows region to be focused on later 24 h 2.8° 1.5° 42 h 8.6° 6.75 days t

Vertical lines show months with One, Two, or Three sets of twins Oct 97

Mean SST (OI.v2) for October 1997 Hurricane symbols indicate genesis locations of twin tropical cyclones

Averaged 20 September – 9 November hPa Wind (vectors) and Relative Vorticity (shading) from ECMWF operation analyses

Synoptic Maps of CLAUS T b and 700-hPa winds every 6 days Hurricane symbols indicate genesis of twin storms within 3 days of plot

Unfiltered T b –Averaged 10°S–10°N –Lower values (convection) in warm colors Unfiltered u –Averaged 4.5°S–4.5°N –Westerlies in warm colors –Easterlies in cool colors t

Filtered T b (shading) and u (contours) 50-day low-pass ER-band t

Vertical lines indicate tropical cyclogenesis Unfiltered data in gray 50-day low- pass in green ER-band in red Sum of filters in black u TbTb

Summary & Conclusions Three sets of twin tropical cyclones form in the Central Pacific during October 1997 Twins formed in favorable background conditions Two modes modulate active convection and equatorial westerlies –Low-frequency eastward moving mode –Higher-frequency westward moving mode: ER waves Both modes appear to be important for the formation of the twins