Anomalous Hawaiian Sea Level Pressure is Key to Initiate the North Pacific Meridional Mode

Abstract The spring North Pacific Meridional Mode (NPMM) has a significant impact on the development of the following El Niño-Southern Oscillation (ENSO). Thus, understanding the source of the NPMM is of great importance for predicting the subsequent ENSO. The prevailing explanation for the formation of the spring NPMM is associated with the North Pacific Oscillation (NPO) during the preceding winter. Using observational data and Coupled Model Intercomparison Project Phase 6 (CMIP6) outputs, this study clarifies that the anomalous sea level pressure (SLP) over the Hawaiian region from January to March (JFM), rather than the NPO, is key to initiate the spring NPMM. Analysis of the JFM SLP anomalies related to the spring NPMM reveals that the greater standard deviation of SLP anomalies at higher latitudes amplifies the impact of extratropical atmospheric variability on the formation of the spring NPMM. The key role of the Hawaiian SLP anomalies is further supported by their ability to initiate the spring NPMM independently of the NPO, whereas the NPO cannot. The Hawaiian SLP anomalies can also initiate the spring NPMM independently of the wintertime central Pacific ENSO which is NPMM’s another possible source. The results of the CMIP6 outputs demonstrate that the impact of the JFM Hawaiian anomalies on the spring NPMM is consistent with the observation, whereas the impact of the JFM NPO is inconsistent. Not all the JFM SLP anomalies associated with the spring NPMM exhibit the NPO-like pattern in these models.