Recorded presentation9.3
Generation of low-frequency spiciness variability in the thermocline
Thomas J. Kilpatrick, University of Hawaii, Honolulu, HI; and N. Schneider and E. Di Lorenzo
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.
Session 9, Role of Air-Sea Interactions in Low-Frequency Climate Variability
Thursday, 30 September 2010, 9:00 AM-10:00 AM, Capitol AB
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