Planetary Wave Propagation off California and Its Effect on Zooplankton

Abstract

Lagged correlation of dynamic height from the gappy California Cooperative Oceanic Fisheries Investigation (CalCOFI) with monthly San Diego sea level for the period 1949?2001 shows that the dynamic height propagates westward at 4.10 cm s?1, about double the speed of the large-scale low-frequency Rossby wave (2.2 cm s?1). Ocean Topography Experiment (TOPEX)/Poseidon/Jason-1?s along-track sea level height estimates since January 1993, filtered interannually, propagate westward at 4.3 cm s?1, verifying that observed westward propagation is about double that expected. Including the effect of the mean California Current on the Rossby wave propagation does not explain the discrepancy but rather slightly increases it. If variations in the ocean depth in the CalCOFI region are also taken into account, the westward propagation is still only about one-half that observed. Standard theory therefore does not explain the observations. Because of the westward propagation, interannual variations in alongshore geostrophic surface current are proportional to the time derivative of sea level. This means that such large-scale interannual current variability can be monitored with appropriate lag by the time derivative of coastal sea level. The anomalous alongshore flow advects particles, the anomalous alongshore particle displacement being proportional to sea level. Since nutrient concentration is lower in the south, the anomalous alongshore displacement results in a lower nutrient concentration when sea level is anomalously high and a higher nutrient concentration when the sea level is anomalously low. Vertical displacement also results in a similar relationship between nutrients and sea level, so it is not surprising that sea level anomalies are strongly related to fluctuations in zooplankton population. In fact, consistent with the westward Rossby wave propagation, the logarithm of the zooplankton population averaged over the CalCOFI region is well correlated with coastal sea level anomalies and lags it by about 2 months. By this result monthly anomalous San Diego sea level can be used to monitor and predict interannual changes in the zooplankton population.