ENSO Teleconnections and the Internal Variability of Tropical Pacific Diabatic Forcing in the CESM2 and E3SMv2 Large Ensembles

The extratropical circulation response to ENSO Sea Surface Temperature (SST) and related tropical heating anomalies relies on a ‘typical’ maximum divergent outflow level for the calculation of the Rossby Wave Source (RWS) terms. However, the actual level of the maximum divergence and its spatial structure can vary significantly both in models and even among different reanalysis products. These variations can impact the interpretation of the divergent source terms in the RWS, and impact the strength and nature of extratropical teleconnections.

We present results from the CESM2 and E3SMv2 large ensemble sets showing that variations in the RWS response can be traced to different vertical divergent structures and ultimately different profiles of tropical heating. These variations in the mean and anomalous divergent outflow characteristics can impact the nature of the ENSO-forced Rossby wave patterns and can be linked to variations in the vertical heating response to SST, which is ultimately associated with physics formulations in models. Among the findings are that even though the anomalies in divergence may be well represented, the level of maximum anomalous divergence may not be the same as the mean divergent level. This separation can be significant, both regionally and for different ENSO phases. In addition, the variation in diabatic processes within each large ensemble can impact the extratropical variability in distinctly different ways, depending on the specific large ensemble.

This analysis also represents a significant skill test for models as the region of divergence in the upper troposphere is the most sensitive to model physics parameterizations along with where the spread in large-ensembles is greatest. These diagnostics are being developed as part of the NOAA Model Diagnostics Task Force (MDTF) effort.