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Dynamics of Double Inter-Tropical Convergence Zones over the Western Tropical Indian Ocean and their Relation to the Indian Summer Monsoon
  • Andrew Geiss,
  • Gad Levy,
  • Ramesh Kumar
Andrew Geiss

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Gad Levy
NorthWest Research Associates Inc
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Ramesh Kumar
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The Inter-Tropical Convergence Zone (ITCZ) is a persistent band of organized convection in the tropics that arises due to the surface convergence of the Hadley cells. The location and intensity of the ITCZ is heavily influenced by sea surface temperature and low-level latent heat transport. The ITCZ undergoes an annual march across the equator, and during the summer moves north over India and the Bay of Bengal, affecting the Indian summer monsoon. Occasionally a second parallel band of convection forms to the south, referred to as a double-ITCZ. Double-ITCZs in the tropical east Pacific have been heavily studied, and their development is understood to be linked to seasonal changes in sea-surface temperature. The existence of double ITCZs over the tropical Indian Ocean is well documented, but the underlying mechanism is poorly understood. We develop an algorithm to identify this phenomenon in NOAA outgoing longwave radiation data, and create a thirty-year record of double-ITCZ occurrence. We then use this record to investigate linkages between summer-time double-ITCZ occurrence and intra-seasonal variability in the Indian summer monsoon, and discuss possible physical mechanisms.