Date of Award
Doctor of Philosophy in Oceanography
Peter C. Cornillon
Geostrophic turbulence predicts zonal jets in our oceans and atmospheres. Under this theory, energy cascades from small to large scales, while enstrophy - i.e., squared vorticity - cascades to increasingly smaller scales. The combined effects of the latitudinal variation in the Coriolis parameter and the inverse (forward) and cascade of energy (enstrophy) are expected to result in zonal jets [Rhines, 1975]. Other frameworks for the existence of zonal jet-like structures in the ocean include secondary instability theory [Pedlosky 1975; Berloff et al. 2009, 2011] and instabilities and/or β-plumes radiating from an eastern boundary current [Hristova et al. 2008; Wang et al. 2012; Centurioni et al. 2008; Melnichenko et al. 2010; Afanasyev et al. 2012; DiLorenzo et al. 2012].
Recent studies suggest such features exist in the ocean [Maximenko et al. 2005, 2008; Huang et al. 2007; Scott et al. 2008; Centurioni et al. 2008; Ivanov et al. 2009; van Sebille et al. 2011; Cravatte et al. 2012]. These studies make use of various combinations of altimeter measurements of sea surface height (SSH), wind reanalysis, upper ocean profiles and drift trajectories from floats to estimate ocean circulation patterns. These data are temporally averaged and generally spatially filtered to reveal jet-like structures in the world's oceans. Given an uncertainty regarding the physics of the jet-like features, they were referred to as striations [Maximenko et al. 2008]. Striations are zonally-elongated mesoscale (100 - 400 km) features observed in time-averaged zonal geostrophic velocity, u. Characterized by zonal scales > 1000 km, meridional scales ~100 km and speeds O(1 cm s-1), they alternate in direction in meridional cross-sections of u and are nominally separated by 200 km. Zonal patterns with similar appearance have also been observed in front probability derived from microwave sea surface temperature (SST) [Buckingham and Cornillon 2010] and measurements of surface winds [Maximenko et al. 2010; Divakaran and Brassington 2011].
In this study, we investigate the existence of stationary jet-like structures in subtropical oceans observed from space. Motivated by a kinematic explanation for the structures [Schlax and Chelton 2008], we explore the relationship between (1) mesoscale eddies and striations in SSH and (2) mesoscale eddies and zonal bands in microwave SST. Some of the more salient results of our study of striations include (i) propagating eddies explain a large fraction of the variance in striations, (ii) eddies having a broad range of amplitudes and scales are most correlated with and contribute most to the observed striations and (iii) the standard deviation of u does not decay inversely with averaging period. In our study of zonal bands, we find that large equatorward gradients explain a considerable fraction of the variance in mean equatorward SST gradient. High gradient events (HGEs) propagate westward with speed comparable to mesoscale eddies and appear to arise due to the combined effects of (i) contrasting temperatures of neighboring eddies and (ii) advection of surface waters by eddies embedded in a background temperature gradient. Results of both studies indicate mesoscale eddies are intimately related to the existence of multiple zonal jet-like structures, as postulated by Schlax and Chelton (2008). Furthermore, the persistence of patterns with averaging period suggest the existence of a secondary, latent signal and/or preferred eddy paths.
Buckingham, Christian E., "Ubiquitous Zonal Bands in Subtropical Oceans Observed From Space" (2013). Open Access Dissertations. Paper 53.