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Effect Of The 1997-98 El Niņo On Chlorophyll And Primary Production Across The Central California Upwelling Zone: Temporal Evolution, Spatial Pattern, And Comparison To Climatology

Francisco P. Chavez, J. Timothy Pennington, and Reiko P. Michisaki


fig2.jpg (168238 bytes)Figure 2.  Chlorophyll contour comparisons

The spatio-temporal evolution of the 1997-98 El Niņo's effect on chlorophyll and primary production values is shown in Figures 2, chlorophyll contours,  and 3 , primary production contours, (middle rows), and compared to a 1988-91 'climatology' (top row) and to 1999 values (bottom row). In the climatology, chlorophyll and primary production values are low in winter with weak increases nearshore. High values develop nearshore during the spring early upwelling season, and extend further offshore during the summer late upwelling period as the upwelling system broadens. During the Fall oceanic season, high values recede towards the coast as upwelling wanes.

fig3.jpg (125908 bytes)Figure 3.  Primary Productivity contour comparisons

In early 1997, chlorophyll and primary production values were distributed normally during the early upwelling season. However at the onset of El Niņo during the late upwelling season, high values of chlorophyll and production began to collapse towards shore, and by the oceanic season values were very low. Values during the 1997-98 Davidson season were also very low, and the upwelling system essentially failed to develop during the 1998 early and late upwelling seasons --- values were extremely low except in a narrow band nearshore (~50 km). Chlorophyll and productivity levels recovered to normal or above-normal levels during the oceanic season of 1998.

A comparison of surface 1997-1998 data with climatology shows that El Nino depressed chlorophyll and productivity values 80-90% in some seasons, with somewhat more protracted effects offshore (Table 1). These anomalies were associated with increased temperature and severe depressions in nitrate levels.

table1.jpg (88303 bytes)Table 1 Seasonal Comparisons

In Figures 4, surface chlorophyll, and 5, surface primary production, the top panels show the 1989-99 time series of chlorophyll and primary production for 2 stations occupied during the 0-55 km cruises, with characteristic high variability daily averaged data but strong seasonal cycles (bottom panels).  Map of study area.

fig4.jpg (133740 bytes) fig5.jpg (165551 bytes)

Figures 4 and 5.  Time series graphs of daily averaged values, anomalies, and 1997, 1998, and climatology comparisons.

The middle panels show interannual variability in the chlorophyll and primary production anomalies. At coastal station 'C1' the 1992-1993 and 1997-1998 negative anomalies are strongly defined in both chlorophyll and primary production, while at mid-bay station 'M1', the anomalies are not as great and less well defined, particularly during the 1992-1993 El Niņo.

The bottom panels compare chlorophyll and primary production values by month for 'climatology' and El Niņo years. The 1992-93 El Niņo strongly depressed values in the late upwelling season of 1992 at station C1, but was hardly apparent in 1993 or at station M1 in either year. The 1997-98 El Niņo strongly depressed values during the early and late upwelling seasons of 1998 at station C1, and had a more variable but still apparent effect on values at station M1 (see also Table 2).

table2.jpg (192738 bytes)Table 2. Seasonal analysis of the nearshore stations.                                                                                       

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Last Updated: 07 June, 2002