Mapping abyssal current and pressure fields under the meandering Gulf Stream

Abstract

Using bottom pressure measurements and current meter measurements at 3500-m depth at 12 sites under the Gulf Stream near 68°W daily optimally interpolated (OI) maps have been constructed for June 1988 to August 1990. Prior to mapping, the pressure records were leveled (referenced to the same absolute geopotential) using the current measurements under the assumption that their mean fields are geostrophic. The leveled pressures were subsequently used together with the current measurements in a multivariate, nondivergent OI mapping procedure. This procedure significantly reduced the mapping errors, because both pressure and its (geostrophic) gradient are specified as inputs. The mapped bottom pressure and current fields have typical mean-square errors of only 0.0005 db2 and 4 cm2 s-2, compared to typical signal variances of 0.0035 db2 and 80 cm2 s-2. The daily maps of abyssal pressure are used to identify the characteristic space-time structures of dynamical processes. Examples are shown in which deep-level cyclones spin up jointly with steep stationary troughs in the Gulf Stream and deep anticyclones with steep propagating crests. Another sequence of maps illustrates the propagation of topographic Rossby waves and ring-stream interactions.