The Climatological Distribution of Potential Vorticity over the Abyssal Ocean

Climatological maps of the large-scale potential vorticity field Q along isopycnals are diagnosed for the abyssal waters over the global ocean. The inferred patterns of Q vary with density, the basin, and hemisphere. At middepths, the distribution of Q is controlled by the background planetary vorticity gradient. On deeper isopycnals, there are regions where Q contours deviate from latitude circles and even regions of nearly uniform Q. These different regimes appear to be robust features over the interior of ocean basins, as the standard error is found to be relatively small there. The nearly uniform Q occurs in the deep waters of the North Pacific and, possibly, in the bottom waters of the western North Atlantic and North Pacific. The nearly uniform Q has a low magnitude in each case, as well as a relatively low variability for the deep waters of the North Pacific. This nearly uniform Q signal appears to be formed when a single water mass enters the basin from a low latitude source.