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Wave Trains Excited by Cross-Equatorial Passage of the Monsoon Annual Cycle

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  • 1 Department of Meteorology, The Florida State University, Tallahassee, Florida
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Abstract

In this paper the authors illustrate wave trains that are excited during the equatorial passage of the annual cycle of monsoonal convection. Twice a year, during roughly the months of December–January and March–April, the annual cycle of monsoonal convection crosses the equator. A principle axis of annual cycle monsoon precipitation extends from the Java Sea to the eastern Himalayas. Monsoonal convection makes a north–south seesaw roughly along this axis each year. Near-equatorial convection provides a tropospheric heat source somewhat akin to that of El Niño over the equatorial Pacific Ocean. This equatorial passage of the monsoonal heat source excites a wave train, somewhat similar to the familiar Pacific–North American pattern. Monsoonal wave trains were extracted from a 9-yr dataset, and a composite geometry was constructed. This note also illustrates excitation of short-period wet and dry spells associated with excitation of this wave train. This is illustrated for several trough and ridge locations of the wave train by examining rainfall for a sequence of days some 10 days prior to and 10 days subsequent to passage of this wave train. There is a strong suggestion that equational passage of monsoon convection does influence short-term dry and wet spells along the wave train; that is, beneath upper troughs (ridges), wet (dry) weather prevails.

Corresponding author address: Dr. T. N. Krishnamurti, Dept. of Meteorology, B-161, The Florida State University, 430 Love Building, Tallahassee, FL 32306-3034.

Email: tnk@cloud1.met.fsu.edu

Abstract

In this paper the authors illustrate wave trains that are excited during the equatorial passage of the annual cycle of monsoonal convection. Twice a year, during roughly the months of December–January and March–April, the annual cycle of monsoonal convection crosses the equator. A principle axis of annual cycle monsoon precipitation extends from the Java Sea to the eastern Himalayas. Monsoonal convection makes a north–south seesaw roughly along this axis each year. Near-equatorial convection provides a tropospheric heat source somewhat akin to that of El Niño over the equatorial Pacific Ocean. This equatorial passage of the monsoonal heat source excites a wave train, somewhat similar to the familiar Pacific–North American pattern. Monsoonal wave trains were extracted from a 9-yr dataset, and a composite geometry was constructed. This note also illustrates excitation of short-period wet and dry spells associated with excitation of this wave train. This is illustrated for several trough and ridge locations of the wave train by examining rainfall for a sequence of days some 10 days prior to and 10 days subsequent to passage of this wave train. There is a strong suggestion that equational passage of monsoon convection does influence short-term dry and wet spells along the wave train; that is, beneath upper troughs (ridges), wet (dry) weather prevails.

Corresponding author address: Dr. T. N. Krishnamurti, Dept. of Meteorology, B-161, The Florida State University, 430 Love Building, Tallahassee, FL 32306-3034.

Email: tnk@cloud1.met.fsu.edu

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