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 phase 6 of the Coupled Model Intercomparison Project (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.
Significance Statement
The North Pacific meridional mode (NPMM) is the leading mode of ocean–atmosphere variability over the tropical and subtropical North Pacific after removing the concurrent ENSO, which is most significant in boreal spring. Previous studies mainly assume that the spring NPMM formation is associated with the North Pacific Oscillation (NPO) during the preceding winter. This study elucidates that the anomalous sea level pressure (SLP) over the Hawaiian region, rather than the NPO, is key to initiate the spring NPMM. The greater standard deviation of SLP anomalies at higher latitudes is responsible for the emergence of the NPO-like pattern. These results can enhance our understanding of the extent to which the NPMM is affected by extratropical atmospheric variability.
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