In the California Current System (CCS), upwelling is one of the most important features that enrich the coastal ecosystem. It is highly dependent on both wind stress and wind stress curl, because they contribute to the upwelling system through Ekman transport away from the coast and Ekman pumping as a result of the surface divergence, respectively. Various wind stress products are known to contain sharply different patterns of wind stress, and well-resolved wind forcing products have been shown to drive stronger upwelling due to their better-resolved wind stress curl in previous studies. However, sensitivities of upwelling to changes in wind stress patterns, and each of their control to the source waters and paths of the upwelling cells, are not yet well known for the CCS. Here we study these effects using the Regional Ocean Modeling System (ROMS) and its adjoint model under idealized wind stress forcing patterns representing three widely-used products in addition to a constant wind stress field (no curl): the NCEP/NCAR Reanalysis, the QuikSCAT satellite observations, and the Regional Spectral Model (RSM) downscaling.Changes in currents and isopycnal patterns during the upwelling season are first studied in ROMS under the four different wind stress fields. The model simulations show that the locations of the core of the equatorward flow and the gradient of the cross-shore isopycnals are controlled by the wind stress curl field. The core of the equatorward flow is found under negative wind stress curl, and a deeper upwelling cell is found as the gradient from positive and negative wind stress curl increases. Source waters for the upwelling in each of the four wind stress patterns are investigated using the ROMS adjoint model. The simulations follow a passive tracer backward in time and track the source waters for upwelling in two key areas of interest: inshore and offshore of the Point Sur region of California. The upwelling source waters depend strongly on the depth of the upwelling cell and the alongshore current location. We further relate these results to recent studies of the observed trends in upwelling favorable winds and consequent wind stress curl changes in the CCS.
Bibliographical noteFunding Information:
This study formed a part of the Ph.D. dissertation of HS at Scripps Institution of Oceanography. Funding was provided by NSF (CCE-LTER: OCE-0417616 and OCE-1026607 ) and NOAA (IOOS: NA17RJ1231 ). BDC was also supported by NOAA grant NA17RJ1231 for the Consortium for the Ocean’s Role in Climate. The views expressed herein are those of the authors and do not necessarily reflect the views of NOAA or any of its subagencies. Supercomputing resources were provided by COMPAS at SIO. We thank Ketty Chhak for letting us use her model configurations. We also thank Andrew Moore for his advice. The authors would like also to thank two anonymous reviewers for valuable comments and suggestions.
All Science Journal Classification (ASJC) codes
- Computers in Earth Sciences
- Atmospheric Science