Local and remote forcing of coastal upwelling near Valparaiso, Chile (33S) in late spring of 1996 (La Niña) and 1997 (El Niño)

Important upwelling foci along the chilean coast include Punta Curaumilla (PC), south of the Vaparaiso bay (VB) (33 S). As in other similar areas, physical, chemical and biological processes present large time and space variabilities within an extensive area around the upwelling focus, as usually described through "in situ" and satellite observations.

Oceanographic measurements in the outer edge of the VB and meteorological nearby observations in the late austral springs of 1996 and 1997 have been compared with SST and ocean color (OC) images (1997 only) between 31.6 °S and 33.4°S. As a general result, a sensitive and fast response (less than 24 hours) of the outer portion of the VB to the upwelling events generated at PC was apparent in the OC data. Those events were generated by a combination of local (wind driven) and remote (coastal trapped wave : CTW) forcing factors.

In both field campaigns alternating cold and warm periods have been associated with an intraseasonal (30-60 days) oscillation in the low-pass filtered sea level data, that propagated southwards along the coast as a CTW. The cold periods occurred while the sea level was rising, indicating a phase lag of about 90 degrees with respect to the minima in the sea level. Active upwelling episodes during the cold periods showed cold water tongues extending around 100 km off PC in a NW direction.

Associated patterns of satellite-derived SST and chlorophyll were consistent and always connected with the PC upwelling activity for each warm/cold period .

Local wind forcing was associated with evenly distributed 6 and 8 southerly wind events during the 1996 and 1997 campaigns, respectively. Those events were primarily associated with atmospheric coastal-trapped disturbances (coastal lows) with a periodicity of the order of 5 days. The lack of effectivenes of the local forcing during the warm phases was due not only to a depressed thermocline buy also to a larger static stability in the upper water column.

Interannual variability related to the strong El Niño conditions in 1997 as compared with La Niña large-scale conditions in 1996 was only noticed in terms of a 2 deg. C higher average SST during the warm periods in 1997 over the study area. No significant differences among the 1996 and 1997 observation periods in terms of local and remote forcing were apparent.