Fortnightly tidal mixing signatures in sea surface temperature: A global perspective

A long time series of sea surface temperature (SST) measurements can sometimes reveal the presence of significant tide-driven ocean mixing. The signature is a fortnightly peak in the SST spectrum which is associated with the spring-neap tidal cycle. The physical mechanism is as follows: During times of spring tides, stronger currents act to lower SST as colder subsurface water is mixed upwards. The SST spectral peaks are surprisingly sharp. For semidiurnal tidal regimes the period equals 14.77 days (also the period of the long-period MSf constituent) and reflects the beating of the M2 and S2 constituents. For diurnal tidal regimes the period is 13.66 days (also the period of Mf) and reflects beating between K1 and O1. The spectra also tend to have sidelines at ±1 cpy from modulation by the seasonal cycle. Estimated MSf (or Mf) phases generally lag the astronomical forcing, depending on the local age of the tide and the diffusivity of the ocean. We have previously reported these effects at and around Lombok Strait and along the southern sill of the Sulu Sea. We have now extended the analysis to the global ocean, using both microwave and microwave+infrared SST products, spanning almost 20 years. We find a number of fortnightly “hot spots,” most notably in the Andaman Sea, the Alboran Sea, the northern Gulf of California, along the Kuril Island chain, and several other places. All these spots are locations where strong internal tides are generated, so the presence of tidal mixing is not unexpected. Near Luzon Strait both MSf and Mf peaks are pronounced, reflecting the presence of strong internal tides in both the diurnal and semidiurnal bands.