Mechanisms of Northward-Propagating Intraseasonal Oscillation—A Comparison between the Indian Ocean and the Western North Pacific

Abstract

Mechanisms of northward-propagating intraseasonal oscillations (ISOs) over the Indian Ocean (IO) and the western North Pacific (WNP) are examined for the possibility of their existence in observations. They include the following: 1) the vorticity advection effect, which is associated with the advection of anomalous baroclinic vorticity by mean baroclinic meridional winds; 2) the vertical wind shear effect, which is the vertical advection associated with the meridional gradient of baroclinic divergence and mean easterly vertical wind shear; 3) the moisture advection effect induced by mean flow; and 4) the air–sea interaction via surface latent heat flux. Because of differences in mean state, the influence of each mechanism on the northward-propagating ISOs is different between the IO and the WNP. The vorticity advection effect is consistently found over both the IO and the WNP, while the air–sea interaction has different impacts on the northward-propagating ISOs over the IO and the WNP. The vertical wind shear effect and the moisture advection effect are relatively important over the IO but not over the WNP. Processes to determine changes in SST are also different between the IO and the WNP. Over the IO, SST is mainly associated with surface solar radiation. Wind-stirring effects, surface latent heat flux, and subsurface water entrainment are secondary. Over the WNP, wind-stirring effects become important, but surface solar radiation is secondary.