Decadal-Scale Barotropic Sea Level Changes in the North Pacific

We have analyzed barotropic sea level inferred from the Gravity Recovery and Climate Experiment (GRACE) data between January 2003 and December 2009, along with satellite vector wind data, in order to understand the mechanisms behind a previously observed long-term trend in barotropic sea level in the sub-polar gyre of the North Pacific. The trend is close to 1 cm/year over the last 7 years and is a significant fraction of the total sea level change in the region observed by altimetry. We find that there is a significant reduction in the wind stress curl at 30°N, driven by a slowing of the westerly and easterly zonal winds over most of the Pacific connected with the Pacific Decadal Oscillation. The reduced curl led to decreased Ekman pumping at 30°N, which in turn led to a redistribution of mass from the subtropical gyre to the subpolar gyre. Using a simplified relationship between Ekman pumping and ocean bottom pressure gradients, we show that the observed trend can be almost completely explained by the change in Ekman pumping. These results indicate that in regions with large interannual changes in wind-curl, mass redistribution may be as important to regional patterns of sea level change as steric fluctuations.