The Association between Intraseasonal Oscillations and Tropical Storms in the Atlantic Basin

Abstract

Tropical intraseasonal variations in the Pacific are related to the tropical storm activity in the Atlantic basin using outgoing longwave radiation anomalies (OLRAs) and circulation anomalies from the NCEP?NCAR reanalysis. Tropical storms are most likely to develop and maintain in the Atlantic, when enhanced convection associated with the tropical intraseasonal oscillations (TIOs) is located over the Indian Ocean and convection in the Pacific is suppressed. Tropical storm activity decreases when the TIO shifts to the opposite phase. The dominant signal associated with the TIO is the Madden?Julian oscillation. The atmospheric response in the Tropics is a dipole pattern in the 200-hPa streamfunction anomalies just north of the equator. Positive OLRA propagates eastward from the Indian Ocean to the central Pacific. The dipole moves eastward in concert with OLRAs. When enhanced convection is located in the Indian Ocean and convection in the Pacific is suppressed, positive 200-hPa streamfunction anomalies as a part of the dipole extend from Central America to the central Atlantic. There are more upper-tropospheric easterly wind anomalies over the Caribbeans and the tropical Atlantic. The vertical wind shear decreases. These conditions are favorable for tropical storms to development and enhance. When the TIO shifts to the opposite phase with enhanced convection in the Pacific, the wind shear in the tropical Atlantic increases and the occurrence of tropical storms decreases.