The South Pacific Meridional Mode: A Mechanism for ENSO-like Variability and a Precursor for ENSO

Monday, May 12, 2014 - 07:00
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Literature has shown that the North Pacific atmospheric variability can trigger ENSO events via the North Pacific Meridional Mode (NPMM, Chiang and Vimont, 2004). Here in a multi-model ensemble of AGCMs coupled to a slab ocean, we identify a similar Meridional Mode in the South Pacific (SPMM), with its physics including the off-equatorial southeast trade wind variability associated with extra-tropical atmospheric variability and the subsequent Wind-Evaporation-SST (WES) feedback that propagates signals into the tropics. A positive cloud feedback also contributes to its development, but this effect is model-dependent.

 

While physically analogous to the NPMM, the SPMM has a larger equatorial signature, directly leading to ENSO-like variability despite the lack of ocean-atmosphere dynamical coupling. This interhemispheric asymmetry, based on both numerical experiments and analytical solutions, is largely attributed to the asymmetric mean trade winds in the tropical Pacific that determine the propagation of the WES feedback.

 

Further analysis shows the SPMM is active in fully coupled models and observations, and can also trigger ENSO events. In addition, our results suggest that the equatorial signals associated with the NPMM and the SPMM resemble the central and eastern Pacific El Nino, respectively, implying that the two Pacific Meridional Modes may be related to different ENSO flavors. 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.