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 1997• 700-hPa Wind (vectors) and Relative Vorticity

(shading) from ECMWF operation analyses

• Synoptic Maps of CLAUS Tb and 700-hPa winds every 6 days

• Hurricane symbols indicate genesis of twin storms within 3 days of plot

• 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

Filtered Tb (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

Tb

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