North Pacific temperature and precipitation response to El Niño-like equatorial heating: sensitivity to forcing location

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This study investigates the sensitivity of oceanic and atmospheric response in the extra-tropics, especially in the North Pacific, to the position of equatorial El Nino-like heating within a slab-ocean climate model. In a suite of numerical experiments, we impose an idealized equatorial sea surface temperature (SST) anomaly in the Pacific and systematically vary its longitudinal position along the equator to mimic different “flavors” of El Nino. We find that regardless of the forcing location, the induced SST pattern closely resembles a positive phase of the Pacific Decadal Oscillation with a characteristic warming along the North American coast as part of an arc-shaped pattern, accompanied by wind anomalies around the Aleutian low. However, the extent and magnitude of the coastal warming vary nonmonotonically when the forcing shifts westward along the equator. The strongest response is found when the equatorial forcing is located in the central Pacific close to the Dateline. In contrast, precipitation response over Southern California is strongest for an eastern Pacific warming centered at 150°W, even though its magnitude is highly uncertain since the boundary between dry and wet precipitation anomalies passes through this region. We repeat the experiments for cold (i.e. La Nina-like) anomalies and observe a significant asymmetry in the SST and atmospheric response between the warm and cold cases. Finally, our experiments suggest that tropical heating (or cooling) over the Western Pacific warm pool generates the largest tropical rainfall response and hence the largest global-mean SST anomaly.