Relative roles of El Niño-induced extratropical and tropical forcing in generating Tropical North Atlantic (TNA) SST anomaly

Abstract

During El Nino decaying spring, Tropical North Atlantic (TNA) region displays a significant warm SST anomaly (SSTA). The relative role of El Nino-induced extratropical and tropical forcing in generating TNA SSTA is investigated through both an observational analysis and idealized numerical model simulation. The maximum warming tendency in TNA occurred in El Nino mature winter and was mainly caused by enhanced surface evaporation, which was primarily affected by reduced trade wind. A partial regression analysis was employed to reveal the relative roles of El Nino induced extratropical forcing through Pacific North American pattern (PNA) and tropical forcing via remote Gill response. It is found that the extratropical forcing contributes around 62–66% in inducing a southwesterly anomaly over TNA region while the tropical forcing contributes approximately 34–38%. Idealized numerical model (ECHAM4) experiments were further carried out. This model could successfully reproduce PNA pattern and tropical Gill response by adding an El Nino forcing. Then two sensitivity experiments were designed to assess the relative contribution of the extratropical and tropical forcing. (1) An experiment was designed in which a zone (15°–26°N, 90°E–85°W) was given a strong Newtonian-type damping to prevent the Rossby wave propagating to the mid-latitudes, which greatly suppresses PNA induced mid-latitude forcing effect. (2) Tropical forcing is attempted to be suppressed in the model by adding a positive diabatic heating rate anomaly over the tropical Atlantic region where the El Nino forcing tends to induce negative heating anomaly. The numerical model results show that the southwesterly anomaly over TNA region is attributed to extratropical forcing and tropical forcing with 58–68% and 32–42% contribution respectively, which is consistent with the observational partial regression analysis.