Testing the Sensitivity of the Extratropical Response to the Convective Source Region

Using a simple dynamical model, the sensitivity of the extratropical response to realistic tropical heating anomalies in specific regions is tested. Composite precipitation anomalies from Climate Prediction Center's Merged Analysis of Precipitation (CMAP) data are calculated for Northern Hemisphere winter La Niņa and El Niņo months, and similar composites are found for MJO phase 1 and phase 5 during ENSO-neutral months. First, the model is run with a spatial heating pattern over the tropics consistent with the anomalies found in the four precipitation composites listed above. Although the precipitation composites are most similar between La Niņa and MJO phase 1 (El Niņo and MJO phase 5), the model successfully reproduces the significant differences in the extratropical response over the North Pacific and North America as seen in composites calculated from ERA-Interim reanalysis data.

Next, tropical regions over which the precipitation anomalies are similar between the La Niņa and MJO phase 1 (El Niņo and MJO phase 5) composites are delineated, as are regions where the same composite pairs differ substantially. The model is run again separately with heating anomalies confined to each small region, for each of the four precipitation composites. It is found that the response in the model is nearly linear, such that a sum of the extratropical response to convection over each region is comparable to the response to convection over all regions. A guided analog method is developed to test the response to the same convective anomalies in the reanalysis, and it is found that the observed response is generally similar to the model response. These results suggest that the extratropical response to both ENSO and MJO convective heating anomalies can be understood as arising from the competing influences of Warm Pool and tropical central Pacific convection. For the MJO, the extratropical response arises primarily from the Warm Pool heating, whereas for ENSO, the extratropical response is determined mostly by tropical central Pacific convection.