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 the Climate Prediction Center's Merged Analysis of Precipitation (CMAP) are calculated for Northern Hemisphere winter La Niña and El Niño months, and similar composites are found for MJO phase 1 and 5 days 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 mostly similar between La Niña and MJO phase 1 (El Niño 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 Niña and MJO phase 1 (El Niño 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. Rossby Wave Source (RWS) terms are calculated to investigate the dynamical processes for each region. It is found that for convective anomalies over some regions, such as over the Maritime Continent, there is cancellation between RWS terms, and the corresponding extratropical response is therefore less amplified, while for convective anomalies over other regions, such as the Indian Ocean and the eastern North Pacific's intertropical convergence zone, the total RWS is larger and the extratropical response is more amplified. Finally, 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.