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Low-frequency isopycnal variability in the Alaska Gyre from Argo

Cummins, Patrick F., Masson, Diane
Progress in oceanography 2018 v.168 pp. 310-324
advection, climate models, heat, prediction, wind, Alaska, Pacific Ocean
The deployment of a large number of Argo floats in the northeast Pacific Ocean since 2001 has led to a great increase in the volume of data available to study the regional variability on interannual to decadal time scales. In this paper, gridded fields of hydrographic data from Argo are used to examine the low-frequency variability of representative isopycnal surfaces over the Alaska Gyre in the eastern subpolar North Pacific. Anomalies in the depth of isopycnal surfaces and spice anomalies on these surfaces are considered over the 14 year period, 2004–2017.An EOF analysis is used to characterize the variability occurring at large scales over the region. The leading mode of isopycnal depth anomalies through the upper 1000 m of the water column is found to be related to the Pacific Decadal Oscillation. The response of a stochastic climate model driven by local Ekman pumping is shown to give a reasonable account of variability in the depth of the shallow main pycnocline over the Alaska Gyre, although ocean dynamics must play a role in shaping the spatial response to the wind forcing.Evidence is presented for counter-propagating isopycnal depth anomalies in the Subarctic Current. At the depth of the main pycnocline, propagation is dominated by disturbances that are advected eastward by the mean flow. At greater depths, isopycnal depth anomalies propagate westward at approximately twice the speed of first mode, long baroclinic Rossby waves, consistent with the propagation of Rossby waves seen in altimetric observations of sea surface height variability.The eastward advection of spice anomalies is also evident in the upper ocean. However, the dominant signal in the record is a large-scale positive spice anomaly that forms locally on subsurface isopycnals. Evidence is presented that this anomaly arises mainly from the downward diffusion across the pycnocline of heat associated with the marine heatwave of 2013–14 over the northeast Pacific. The extent to which anomalies are generated locally may limit the potential for decadal prediction of upper ocean conditions over the region.