ENSO-like variability arising from a South Pacific Meridional Mode

The impacts of atmospheric variability in the mid- to high- latitude South Pacific on the climate of the tropical Pacific are investigated in models of different degrees of coupling to the ocean. A robust thermodynamically coupled mode of variability, defined as the South Pacific Meridional Mode (SPMM), has been identified in multiple Atmospheric General Circulation Models coupled to a slab ocean. The physical interpretation of the SPMM is nearly identical to the north Pacific Meridional Mode (NPMM, Chiang and Vimont, 2004), and consists of the off-equatorial southeast trade wind variability originating from south Pacific mid-high latitudes, its effects on latent heat flux and sea surface temperature (SST), and the wind-evaporation-SST (WES) feedback. We also show that a positive cloud feedback plays a role in the development of this mode, but this effect is model-dependent. While physically analogous to the NPMM, a key difference for the SPMM is that it has a stronger expression in the equatorial Pacific and directly perturbs the zonal gradients of SST and sea level pressure, thus acting as a more effective conduit through which mid-high latitude intrinsic variability affects the tropical Pacific variability. Preliminary analysis shows that the SPMM exists in fully coupled climate models as well as observations. This study highlights the important role of the southern hemisphere in the tropical climate variability, and suggests that including observations from the data-poor south Pacific could improve the ENSO predictability.;