Tidal mixing modulation of sea-surface temperature and diatom abundance in Southern California

In the Southern California Bight a clear seasonal cycle in temperature and diatom abundance is observed, with maximum temperatures in summer and maximum diatom abundance in spring, decreasing in summer. Within this seasonal cycle of temperature and diatom abundance, there is a weak fortnightly temperature variability. Here, we show that diatom abundance has lunar as well as seasonal variability, with the highest abundance corresponding to the coldest days within the lunar cycle. This suggests that at least part of the temperature and diatom abundance variability may be due to bottom tidal mixing. To explore the effect of tidal mixing and tidal straining on the control of the thermal structure of the water column in the Southern California region, a one-dimensional, hydrodynamic numerical model is used. The model is successful in explaining the seasonal cycle in temperature and partially explains the fortnightly and monthly variability in temperature. The observed temperature minimum shows a lag of about 2–3 days, when compared with model results. This lag is probably due to the fact that the model does not include the effect of internal waves, which will be an extra source of mixing and may have an advective effect that will modify the water-column structure and the diatom distribution.