Simple barotropic representations of the extratropical circulation response to ENSO Sea Surface Temperature (SST) and related tropical heating anomalies rely 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, as the maximum level of divergence is assumed to be a constant level everywhere and at all times.

We will present results showing that variations in the RWS response can be traced to different vertical divergent structures and ultimately different profiles of tropical heating. The analysis is performed for the major reanalyses, versions of the Community Atmosphere Model (CAM) and large ensemble simulation sets. The aim is to demonstrate that the variations in the mean and anomalous divergent outflow characteristics can impact the nature of the ENSO-forced Rossby wave patterns and that they 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.
 

This analysis represents a significant 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.