Coherent Fluctuations of Fxtratropical Geopotential Height and Tropical Convection in Intraseasonal Time Scales

Abstract

Coherent fluctuations between extratropical circulation and tropical conviction in the intraseasonal time scale are studied. Possible relationships between 500-mb height field and outgoing longwave radiation (OLR) are examined using correlation, complex EOF and composite techniques. Results show that in the 20-70 day bands data, there is a systematic evolution of extratropical wavetrains from Eurasia across the Pacific to North America and the North Atlantic in the time scale of 5 to 10 days. Over the tropics, the dominant mode of intraseasonal variation in convection is an east-west dipole-like feature which propagates from the western Indian Ocean eastward to the dateline with a quasi-period of 40-50 days. The space-time evolution of these extratropical wavetrains is found to he coherent with the tropical dipolar convection and with the strongest convection over Indonesia/central Pacific in approximate quadrature with the peak phase of the Eurasia and Pacific-North America wavetrains. While the extratropical anomalies are found to occur over fixed geographical locations along the entire latitude circle, tropical convection is restricted to the Indian Ocean/western Pacific region. Localized convection near the dateline and to the east appears to be either forced or decoupled from the extratropical circulation anomalies. The above results are discussed in light of present theories of low frequency variability of the extratropical and tropical atmosphere. It is suggested that they may be the manifestations of the presence of coupled normal modes between the tropics and midlatitudes.