Generation of low-frequency spiciness variability in the thermocline

The generation of variance by anomalous advection of a passive tracer in the thermocline is investigated using the example of spiciness, density-compensated temperature and salinity anomalies. A coupled Markov model in which wind stress curl forces the baroclinic pressure that in turn controls anomalous geostrophic advection of spiciness predicts that white noise atmospheric forcing results in a frequency (ω) spectrum of spiciness that decays proportional to ω^-4. This “double integration” of atmospheric forcing results in spiciness variability that is concentrated at low frequencies.

This mechanism is studied using an eddy-permitting regional model 1950–2007 hindcast of the Northeast Pacific. As predicted, large-scale spiciness variability is exceptionally smooth with a frequency spectrum proportional to ω^-4 for frequencies greater than 0.2 cpy, though the slope of the spectrum is wavenumber-dependent. At high wavenumbers the eddies whiten the spiciness spectrum, but for wavelengths greater than ∼500 km the high-frequency variability cancels and the -4 slope of anomalous geostrophic advection emerges. Large-scale and long-term measurements are needed to observe the variance of spiciness, or any other passive tracer subject to anomalous advection in the thermocline.