Response of atmosphere-ocean system to latitudinal shifts of the North Pacific subarctic frontal zone: basin-scale two-way feedback

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Monday, 5 January 2015
Bunmei Taguchi, Japan Agency for Marine-Earth Science and Technology, Yokohama, Kanagawa, Japan; and M. Nonaka, N. Schneider, and H. Nakamura

In the extratropical North Pacific, the subarctic frontal zone (SAFZ) swings in latitude responding to basin-scale wind change and thus generates pronounced sea surface temperature variability there on interannual-to-decadal time-scale. Recent observational diagnostic studies suggest that the latitudinal shifts of the North Pacific SAFZ can in turn have significant impacts on basin-scale atmospheric circulation. Robustness of the large-scale atmospheric response remains to be confirmed by modeling studies and it is an open question whether the WBC-induced atmospheric circulation change may exert any dynamical forcing on the ocean possibly leading to two-way ocean-atmosphere feedback in the extratropical North Pacific.

To address such climatic implications of interactions between SAFZ and the atmosphere, we use an ocean-front resolving coupled GCM and conduct ensemble sensitivity experiments in which latitudinal shifts of simulated WBCs are deliberately induced by imposing idealized wind stress anomaly in the central North Pacific during the coupled integration. These partially constrained integrations are followed by further free integrations without wind stress anomaly, during which simulated ocean-atmosphere response is examined. Composite analysis across ensemble members reveals the existence of two regimes in the atmospheric circulation response and its feedback on the ocean. Namely, ensemble composite with warm (cold) SAFZ tends to be accompanied by weakened (enhanced) Aleutian Low acting as positive (negative) feedback. The positive (negative) feedback promotes persistency (phase transition) of latitudinal position and SST anomalies in SAFZ, a dynamical feedback that can be crucial for Pacific decadal variability.