Tropical-Midlatitude Interaction on the Time Scale of 30 to 60 Days during the Northern Summer of 1979

Abstract

The relationship between the low-frequency oscillation of convective activity in the tropics and the intensity of planetary-scale upper-tropospheric (200 mb) systems in the subtropics during the Northern Hemisphere (NH) summer is studied using data obtained in the First GARP Global Experiment (FGGE). The mid-Pacific trough and the South Asian (Tibetan) and Mexican anticyclones undergo cycles of amplification and decay with the 30?60 day fluctuation of convective activities in the Indonesia-western Pacific (IWP) region and in the intertropical covergence zone (ITCZ) over Central America. The thermal contrast between the Asian continent and the North Pacific and the resulting east-west circulation show similar time variations. This circulation regulates the intensity of the South Asian anticyclone and the mid-Pacific trough. Divergent circulation associated with convection over Central America maintains the Mexican anticyclone. The low-frequency transients in the upper troposphere around the mid-Pacific trough play a dominant role in the northward transport of westerly momentum. Their effect is recognizable in the variation of zonally averaged relative angular momentum, which originates near 10°N and then propagates northward. At the same time, the westerlies that develop southeast of the trough form an equatorial westerly ?duct,? through which wave energy can propagate into the tropics from midlatitudes. Thus, the mid-Pacific trough acts as a two-way link between the tropics and midlatitudes.