North Pacific decadal variability: insights from a biennial ENSO environment

This study will present an overview of the major modes of decadal variability in the Pacific Ocean as simulated by the NASA GEOS-5 atmosphere-ocean model. The model simulates a realistic Pacific decadal oscillation (PDO) pattern that is resolved as the first empirical orthogonal function (EOF) of winter sea surface temperature (SST) anomalies. Similar to several state of the art GCMs, the ENSO has a strong biennial periodicity in the GEOS-5. While this is something to be corrected, it nevertheless provides a unique environment in which the influence of ENSO can be practically ruled out in the North Pacific decadal climate variability. We found that in GEOS-5, the simulated PDO is primarily forced by the Aleutian low through Ekman transport and surface fluxes, and shows a red spectrum without any preferred periodicity. The geostrophic transport in response to the Aleutian low is limited to the Kuroshio-Oyashio Extension, and is unlikely the main controlling factor in this model, although it reinforces the Ekman-induced SST anomalies. The delay between the Aleutian low and the PDO is relatively short (1 year) suggesting that the fast Ekman response (rather than Rossby wave propagation) sets the SST pattern immediately following an Aleutian low fluctuation.

The model shows a weak although plausible connection between the PDO and the North Pacific Gyre Oscillation (NPGO), the former leading the latter, prompting us to rethink whether the NPGO is in fact independent of the PDO.

The results, in general, emphasize the role of atmospheric variability in the North Pacific SST modes, thereby bringing questions about the predictability of the PDO. The results will be discussed in comparison with observations and paying attention to common model biases.