Towed thermistor chain observations of fronts in the subtropical North Pacific

A thermistor chain was towed 1400 km through the eastern North Pacific subtropical frontal zone in January 1980. The observations resolve surface layer temperature features with horizontal wavelengths of 0.2–200 km and vertical scales of 10–70 m. The dominant features, which have horizontal wavelengths of 10–100 km, amplitudes of 0.2°-1.0°C, and random orientation, likely arise from baroclinic instability. Associated with them is a plateau below 0.1 cpkm in the horizontal temperature gradient spectrum. Strong temperature fronts O(1°–2°C/3–10 km) are observed near 33°N, 31°N, and 27°N. Temperature variability is partially density compensated by salinity, with the fraction of compensation increasing northward. There is evidence of vertical mixing during high winds. Temperature at 15-m depth is roughly normally distributed around the climatological surface mean, with a standard deviation of approximately 0.5°C. The standard deviation would correspond to an adiabatic meridional displacement of 80–100 km in the mean gradient. Horizontal temperature gradient at 15-m depth has maximum values in excess of 0.25°C/100 m and kurtosis near 80. In the band 0.10–1 cpkm, the 15-m gradient spectrum is inversely proportional to wave number, consistent with predictions from geostrophic turbulence theory, while the spectrum at 70-m depth has additional variance that is consistent with Garrett-Munk internal wave displacements.