The relative impact of remote tropical forcing and extratropical air-sea coupling on the daily evolution of extratropical SST and atmospheric anomalies

We explore the relative effects of remote tropical forcing and extratropical air-sea coupling on daily atmospheric and SST variability in the North Pacific. The model used is an empirical coupled linear inverse model (LIM), derived from the simultaneous and 1-day lag covariances of observed OAFlux and ERA-Interim daily averaged anomaly maps over winter (November-March) for the past thirty years. The model can capture both the growth and propagation of typical synoptic events and can be used to make forecasts of individual events as well. More generally, the model can predict lag covariances of the daily data for lags well over a month. The predicted and observed lag covariances are generally found to agree quite well, justifying further diagnosis of air-sea coupling in the system.

The results show that while extratropical air-sea coupling generally has a small effect on the variance of daily atmospheric anomalies, it has a very large impact on their persistence. The picture of the effect of coupling on atmospheric circulation anomalies (e.g., sea level pressure) is more mixed: tropical forcing has a more pronounced impact in the Northeast Pacific while extratropical SST anomalies affect persistence farther north. Air-sea coupling also drives almost all the extratropical SST variability except in the Sea of Japan and in a small region associated with the Kuroshio current just east of Japan, where about half of the variance can be attributed to strictly internal oceanic processes. The impact of variations of SST in these regions on North Pacific atmospheric variability is further diagnosed, as is the relative importance of local vs. remote coupling effects entirely within the extratropics.