Role of the California Undercurrent in the export of denitrified waters from the eastern tropical North Pacific

Abstract

A transect off Point Sur, California (36°20'N), was occupied bimonthly from April 1988 to May 1990 to resolve spatial and temporal patterns in the physics and chemistry of the California Current system. Alongshore velocity fields and the chemical parameter N* [Gruber and Sarmiento, 1997] show coherence. Subarctic waters, conveyed by the California Current (CC), were characterized by high N* values, while in the region of poleward flow of the California Undercurrent (CU) a minimum of N* (N*< −5 μmol kg−1) was observed. Low N* is a signature of denitrification, and its possible sources are considered. The distribution of N* on isopycnal surfaces, using data from the World Ocean Circulation Experiment Hydrographic Programme, suggests that northward advection from the eastern tropical North Pacific by the CU is the primary source of low N*. Local benthic and water column denitrification rates cannot account for the observed values. From nitrate deficits and velocity fields an annual nitrate transport deficit off Point Sur of 4.6 Tg N yr−1 was estimated for the study period. Maximum offshore extent of the low N* signal was observed during the upwelling period, probably associated with an expansion of the CU and mixing due to mesoscale eddies detached from it. This mixing propagates the denitrification signal into the CC and the subtropical gyre. When the loss from the poleward flow due to horizontal mixing is calculated, an export out of the eastern tropical North Pacific of 8.3 Tg N yr−1 by the CU is estimated. The bulk of the denitrified waters off Point Sur were below 400 m and did not affect primary production in the central California upwelling ecosystem. However, the export of nitrate‐deficient waters into the subtropical gyre may play a role in the biogeochemical cycling of this region.