Optimal Precursors of different types of ENSO events

Antonietta Capotondi (1), Prashant Sardeshmukh (2)

Abstract
Not all ENSO events are the same. They can have substantially different global impacts depending on whether the associated sea surface temperature (SST) anomalies are relatively stronger in the central Pacific (CP) or the eastern Pacific (EP). The predictability of such differences in both routine seasonal forecasting and climate change contexts has important implications for society. In this study, the optimal two-season precursors of different ENSO types are investigated in a Linear Inverse Modeling (LIM) framework using observational SST and thermocline depth data for 1958 to 2007. The optimal SST precursor pattern for generic ENSO events (that is, without regard to specific ENSO type) is identified first. Different portions of this pattern, including a portion similar to the so-called Pacific Meridional Mode pattern, are shown to generate very similar contributions to the mature ENSO structure. They therefore cannot help distinguish between different ENSO types. The optimal precursors of specifically CP versus EP types of ENSO events are determined next. These are found to be similar in their SST structures, but very distinct in their thermocline structures. In particular, a reduced initial zonal thermocline gradient strongly favors an EP event, whereas a steeper zonal thermocline gradient slightly favors a CP event. These results highlight the importance of using accurate initial subsurface information for predicting different types of ENSO events.