Decadal variability in the sources of Pacific equatorial undercurrent water
Abstract
A global primitive equation ocean circulation model (OGCM) has been used to study changes in the equatorial thermocline structure and circulation associated with the 1976/77 shift in the Pacific. The model used (ORCA2, version 8.2 of OPA) has 0.5 ^o meridional resolution along the equator, and thus captures the critical scales of the equatorial current system. For the control run, the model has been forced with NCEP reanalysis buoyancy and momentum fluxes at the sea surface. The low frequency variability in the circulation has been analyzed for changes in the structure and the strength of the equatorial undercurrent (EUC) using Eulerian diagnostics at 151W, in conjunction with Lagrangian trajectory analysis. Our main finding is that there was a significant weakening and shoaling (in density space) of the EUC during the 1970s. We have used the model to show that these changes are driven by surface wind stresses rathern than surface buoyancy fluxes. This weakening is consistent with the slowdown of the subtropical cells (STCs) of the Pacific inferred from observations (McPhaden and Zhang, 2002). The shoaling of the EUC transport maximum in density space was an unexpected result of our analysis, and has potentially important implications for the temperature, TCO2 content, and nutrient load of the water entrained via upwelling into the mixed layer of the eastern equatorial Pacific.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA....11769R