Dominant intraseasonal variability modes over the eastern Pacific ITCZ and their representation in climate models

Our recent analysis of TRMM rainfall revealed two dominant intraseasonal variability (ISV) modes over the eastern Pacific (EPAC) ITCZ during boreal summer, i.e., a 40-day mode and a 20-day mode. While the possible linkage between the 40-day mode and the MJO has been previously suggested, we found this mode to also exhibit northward propagation, an aspect that had only previously documented for ISV over the Asian monsoon region. Moreover, it is further illustrated that this northward propagating ISV over the EPAC also exhibits meridional asymmetric structures that are very similar to the Asian monsoon counterpart. As both the EPAC and Asian monsoon region are characterized by the easterly vertical wind shear associated with monsoonal circulation, these results may provide additional independent evidence for the essential role that easterly vertical wind shear may be playing in the meridional propagation of ISV as previously proposed.

The second ISV mode (i.e., 20-day mode) associated with the EPAC ITCZ rainfall, to the best of our knowledge, is documented for the first time. This 20-day ISV mode is largely characterized by northward propagation. While its strongest signals associated with this mode are present over the EPAC, the impacts of this are discerned over the North American Monsoon, the Gulf of Mexico, and Caribbean Sea. Further analysis of the low-frequency variability of these two ISV modes over the EPAC shows that they are anti-correlated on the interannual time scales.

Representation of these two dominant ISV modes over the EPAC ITCZ in six atmospheric general circulation models (GCMs) and three coupled GCMs are also examined, including one super-parameterized GCM (SPCAM) which utilizes embedded 2-D cloud resolving mode, and one recently developed high-resolution GCM (GFDL HIRAM) with horizontal resolution of about 50km. Note that eight of these nine models are the same as those recently analyzed by the CLIVAR MJO Working Group to assess MJO simulation fidelity.