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

Mechanisms of northward-propagating intraseasonal oscillations (ISOs) over the Indian Ocean (10) 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 10 and the WNP. The vorticity advection effect is consistently found over both the 10 and the WNP, while the air sea interaction has different impacts on the northward-propagating ISOs over the 10 and the WNP. The vertical wind shear effect and the moisture advection effect are relatively important over the 10 but not over the WNP. Processes to determine changes in SST are also different between the 10 and the WNP. Over the 10, 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.