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A Case Study of an Outbreak of Twin Tropical Cyclones

A Case Study of an Outbreak of Twin Tropical Cyclones. Carl J. Schreck, III Department of Earth and Atmospheric Sciences University at Albany, SUNY. t. Tropical cyclogenesis within 15 ° latitude of equator First fix in JTWC best track Northern Hemisphere storms are red-filled symbols

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A Case Study of an Outbreak of Twin Tropical Cyclones

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

  2. t • 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 8.6° 6.75 days 1.5° 42 h 24 h 2.8°

  3. Oct 97 Vertical lines show months withOne, Two, orThree sets of twins

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

  5. Averaged 20 September – 9 November 1997 • 700-hPa Wind (vectors) and Relative Vorticity (shading) from ECMWF operation analyses

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

  7. Unfiltered Tb • 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

  8. Filtered Tb (shading) and u (contours) 50-day low-pass ER-band t

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

  10. 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

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