Interaction between Intraseasonal Oscillations of the Midlatitude Flow and Tropical Convection during 1979 Northern Summer: The Pacific Ocean

Abstract

It has been inferred and illustrated by many studies in terms of outgoing longwave radiation (OLR) and velocity potential (chi) that energy is input into the intraseasonal oscillation through tropical cumulus convection, especially over the monsoon region. The existence of this low-frequency oscillation of regional extratropical oscillation has been observed by other studies. Previous findings lead us to speculate upon a hemispheric interaction between tropical cumulus convection and extratropical circulation in terms of an intraseasonal time scale. This study demonstrates that during the 1979 summer a coherent intraseasonal oscillation existed between cumulus convection and divergent circulation over the equatorial western Pacific and mid-Pacific troughs. Moreover, the intraseasonal oscillations of streamfunction (psi) associated with these troughs, the tropical OLR, and the tropical psi field exhibit coherent eastward propagations. Based upon these two coherent relations, it is hypothesized that a tropical-midlatitude interaction with the intraseasonal time scale can be established through the chain relation between tropical diabatic heating, global-scale divergent circulation, and extratropical rotational flow depicting the mid-Pacific troughs. A diagnostic scheme employing a chi-maintenance equation and a psi-budget equation is adopted to substantiate the aforementioned hypothesis. The former equation relates the global-scale intraseasonal chi-oscillation to tropical diabatic heating, whereas the latter illustrates how the eastward propagation of global-scale intraseasonal chi-oscillation induces the intraseasonal psi-tendency associated with the mid-Pacific troughs. The hypothesized tropical-midlatitude interaction through the eastward propagation of the intraseasonal chi-oscillation emerges clearly from our diagnostic results.