On the Role of Indian Ocean SST in Influencing the Differences in Atmospheric Variability Between 2020-2021 and 2021-2022 La Niña Boreal Winters

The difference in observed atmospheric anomalies over the Northern Hemisphere winter between 2021-22 and 2020-21 La Nina years indicated a tripole pattern consisting of a Japan cyclone, a Bering Sea anticyclone, and a cyclone over the North American continent. This feature, however, was not replicated in the North American Multi-Model Ensemble (NMME) forecasts. A set of model sensitivity experiments was performed to better understand the cause of this discrepancy. The results revealed the possible role of the influence of sea surface temperature (SST) anomalies, particularly over the Indian Ocean, on the observed circulation differences that was further modulated by internal atmospheric variability. The failure in predicting circulation changes in NMME was next attributed to the errors in SST predictions over the Indian Ocean and highlights the need for improvements in SST forecasts over this region. The tropical SST anomalies associated with the El Nino-Southern Oscillation (ENSO) are known to influence the global atmospheric circulation and are the major source of skill in U.S. seasonal predictions. As the cold phase of ENSO, La Nina features below-normal SST anomalies and suppressed convection over the equatorial central and eastern Pacific. Such a tropical heating distribution favors the formation of the atmospheric circulation pattern that has a roughly opposite effect on U.S. surface climate compared to El Nino, the warm phase of ENSO, although the effect is not strictly symmetric. For the recent two La Nina boreal winters of 2020-21 and 2021-22, the observed circulation patterns differed, but dynamical seasonal prediction failed to replicate this feature. Understanding the cause for the discrepancy of circulation changes between prediction and observations is of fundamental importance for the improvement of seasonal forecasts. Toward this, we designed numerical experiments that are forced with observed and predicted SST anomalies over different ocean basins. The results show that it is the errors in SST prediction over the Indian Ocean that contributed to the failure in the prediction of circulation changes, highlighting the importance of skillful prediction of SST over this region. The difference in observed atmospheric anomalies for 2022 versus 2021 La Nina boreal winters featured a Northern Hemisphere tripole pattern Indian Ocean SST contributed to the formation of observed tripole pattern, with internal atmospheric variability modulating its magnitude Errors in SST predictions over the Indian Ocean led to the failure in predictions of the circulation changes in NMME forecasts