A ten-year time series from Monterey Bay, California: Seasonal, interannual and long-term patterns

I N T E R A N N U A L    V A R I A B I L I T Y

Fig. 4. Anomalies from the mean annual cycle for upwelling index, winds and currents, SST, temperature at 60 m, sea surface salinity, sigma-t, surface nitrate, nitrate at 60 m, surface chlorophyll, integrated primary production, centric diatoms, pennate diatoms, dinoflagellates, and small phytoplankton (less than 5 µm).

Winds and Currents

The El Niño signal is not clearly represented in the wind and current records. In general there are weaker upwelling indices and weaker northwesterly winds during the 1992-93 and 1997-98 El Niño but there are non-El Niño years, like 1995, that display similar, if not stronger anomalies (Fig 4). During the 1992-93 El Niño there is an indication of increased equatorward flow throughout the water but that same signal is not as clearly represented during 1997-98, if at all (Fig 4).

Temperature and salinity

The El Niño signal in SST is evident (Fig 4). The magnitude of the anomalies is about a °C warmer for the 1997-98 El Niño than the 1992-93 El Niño. The positive anomalies lasted longer for the 1992-93 El Niño but the 1997-98 El Niño had a broader peak of anomalies >0.5°C that lasted from mid-1997 to mid-1998. The magnitude of the subsurface anomalies was much larger for 1997-98 when compared to 1992-93 (Fig 4). The interannual signal in salinity is not as clear. There is a clear freshening during 1992-93; during 1997 but was strong during 1998. 1995 was also a fresh year (Fig 4).

Nutrients

The interannual variability in nitrate is very similar to temperature. There are substantial negative nitrate anomalies during El Niño and the strongest anomalies are observed during 1997-98 (Fig 4).

Chlorophyll and primary production

Primary production and chlorophyll are reduced during the El Niño years of 1992-93 and 1997-98. There are differences between the onset of the effects on chlorophyll and primary production (Fig 4). The SeaWiFS imagery collected during 1998 shows that a narrow near-coastal band of high chlorophyll water is maintained during the El Niño months of January-May. As El Niño waned the area of high chlorophyll broadened in the fall (Fig 6).

Phytoplankton taxonomic groups

Centric diatoms are the phytoplankton that seem most affected by El Niño. None of the groups presented seems favored during El Niño. Dinoflagellates were favored during 1995 when winds were light and waters stratified but nitrate levels were not as low as during El Niño (Fig 4).

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