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A Case Study of an Outbreak of Twin Tropical Cyclones
Carl J. Schreck, III
Department of Earth and
Atmospheric Sciences
University at Albany, SUNY
Introduction
• Twin tropical cyclones typically occur in the Pacific Ocean about 2.4 times per year (Keen 1982)
• Three sets of twins form in the Central Pacific during a 1-month period in October 1997
• This study investigates the background and synoptic conditions that led to this outbreak of twin tropical cyclones
Data
• JTWC Best Track– Cyclogenesis is defined as the first fix for a
given storm
• Reynolds OI SSTs
• CLAUS brightness temperature
• ECMWF operational analyses
• Tropical cyclogenesis within 15° latitude of equator
• Northern Hemisphere storms indicated by red-filled symbols
• Southern Hemisphere storms indicated by blue-filled symbols
• Three sets of twin tropical cyclones form during October 1997
24 h
2.8°
1.5°
42 h
8.6°
6.75 days
• Averaged 20 September – 9 November 1997• 1000-hPa Wind (vectors) and Height (contours)
• Brightness Temperature (Tb, shading)
• Averaged 20 September – 9 November 1997• Mean 850–200-hPa Vertical Shear
– Vector-mean (vectors)– Scalar-mean (shading)
Summary of Background Conditions
• All three sets of twin tropical cyclones form in a region where conditions are favorable for cyclogenesis within 10° latitude of the equator in both hemispheres– Warm water – Lower 1000-hPa heights– Active convection– Lower-tropospheric cyclonic vorticity– Relatively low or easterly vertical shear
• But these favorable conditions are broad and long-lasting. Why did the twins form where & when they did?
• Synoptic Maps of 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
Summary of Synoptic Conditions
• Unfiltered Tb and u show evidence of two primary modes:– Low-frequency eastward propagation
• Appears most clearly in the 30-day low-pass filtered Tb
• Phase speed ≈ +1.8 m s−1
• Possibly associated with a slowly moving MJO event– Higher-frequency westward propagation
• Appears most clearly in 10–30 band-pass u• Phase speed ≈ −6.7 m s−1
• Period ≈ 18.5 days• Fits n = 1 ER wave dispersion for H ≈ 65 m
• Twins form during periods of equatorial westerlies and active convection
• Vertical lines indicate tropical cyclogenesis
• Unfiltered data in gray
• 30-day low-pass filtered data in green
• 10–30-day band-pass filtered data in red
• Sum of 10-30 day band-pass and 30-day low-pass filtered data in black
u
Tb
Summary of Time–Amplitude Plots
• The sum of 10-30 day band-pass and 30-day low-pass filters qualitatively captures much of the unfiltered evolution in u and Tb
• Neither mode alone can explain all of the twin tropical cyclones
• The twin tropical cyclones form when the sum of the modes is – u > 5 m s−1
– Tb < 275 K
Summary & Conclusions
• Three sets of twin tropical cyclones form in the Central Pacific during a 1-month period (October 1997)
• These twins form in a broad region of favorable conditions near the equator in both hemispheres– Warm water – Lower 1000-hPa height– Active convection– Lower-tropospheric cyclonic vorticity– Relatively low vertical shear
Summary & Conclusions
• The active convection and the equatorial westerlies/relative vorticity seem to be modulated by two modes– Low-frequency eastward moving mode (MJO?)– Higher-frequency westward moving mode (ER
waves?)
• No storms form during the anticyclonic (convectively suppressed) phase of the ER waves
• Both modes appear to be important for the formation of the twins