Publication Date

2018-04-25

Availability

Open access

Embargo Period

2018-04-25

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Meteorology and Physical Oceanography (Marine)

Date of Defense

2018-03-29

First Committee Member

Arthur J. Mariano

Second Committee Member

William E. Johns

Third Committee Member

Igor V. Kamenkovich

Fourth Committee Member

Rick Lumpkin

Fifth Committee Member

Tamay Özgökmen

Abstract

This dissertation uses near-surface current velocity observations from Global Drifter Program (GDP) drifters, combined with a suite of satellite products, to investigate the air-sea exchange of mechanical energy at global scales. In particular, this work tests the conclusions of recent numerical studies that air-sea coupling mechanisms operating at the ocean mesoscales, arising from the dependence of wind stress on surface currents and on mesoscale SST fluctuations, can create non-zero air-sea fluxes of mechanical energy associated with the quasi-geostrophic ocean variability. First, the slip bias of undrogued drifters is corrected, thus recovering about half of the GDP dataset; and a new approach for decomposing Lagrangian data into mean, seasonal and eddy components is developed to reduce the smoothing of spatial gradients inherent in data binning methods. The sensitivity of the results to method parameters, the method performance relative to other techniques, and the associated estimation errors, are evaluated using statistics calculated for a test dataset consisting of altimeter-derived geostrophic velocities subsampled at the drifter locations, and for the full altimeter-derived geostrophic velocity fields. It is demonstrated that (1) the correction of drifter slip bias produces statistically similar mean velocities for both drogued and undrogued drifter datasets at most latitudes and reduces differences between their variance estimates, (2) the proposed decomposition method produces pseudo-Eulerian mean fields with magnitudes and horizontal scales closer to time-averaged Eulerian observations than other methods, and (3) standard errors calculated for pseudo-Eulerian quantities underestimate the real errors by a factor of almost two. Next, the influence of mesoscale SST anomalies on the near-surface winds is analyzed, aiming to determine the intrinsic spatial-temporal scales where the effect takes place and its association with the mesoscale eddy field. Specifically, cross-spectral methods are used to examine the linear spectral relationship between SST and equivalent-neutral 10-m wind speed (w) fields from satellite products at scales between 10^2-10^4 km and 10^1-10^3 days. The transition from negative SST/w correlations at large-scales, to positive at oceanic mesoscales, is found to occur at wavelengths coinciding with the atmospheric first baroclinic Rossby radius of deformation; and that the dispersion of positively-correlated signals is compatible with tropical instability waves near the equator, and with Rossby waves and/or mesoscale eddies in the extratropics. Transfer functions for the spectral linear SST/w relationship are used to estimate the SST-driven w response in physical space, a signal that explains 5-40% of the mesoscale w variance in the equatorial cold tongues, and 2-25% at extratropical SST fronts. The signature of coherent ocean eddies is clearly visible in the SST-driven w, accounting for 20-60% of its variability in eddy-rich regions. The cross-spectral analysis is repeated in two climate model (CCSM) simulations based on ocean grid resolutions of 1^o (eddy-parameterized, LR) and 0.1^o (eddy-resolving, HR). A realistic relationship between both quantities is only obtained in HR, highlighting the importance of ocean phenomena with wavelengths between 20-250 km, typical of mesoscale ocean eddies, for conditioning the SST-driven coupling characteristics revealed by satellite observations. Finally, concurrent drifter and satellite observations are used to estimate the contribution of time-mean, seasonal, and eddy components of the wind stress and surface geostrophic velocity fields to the total power exchange. It is found that, of the ~1.22 terawatts (1 TW = 10^12 W) supplied by the winds to the general ocean circulation via the time-mean and seasonal components, about 0.23 TW is lost back to the atmosphere via the covariances between the eddy fluctuations in the winds stress and the quasi-geostrophic velocity fields. Estimates of the impact of the wind stress dependency on surface ocean currents to the mechanical energy fluxes, obtained (a) via theoretical expressions, and (b) by recomputing the energy fluxes using wind stress estimates with the current influence removed using drifter observations, indicate that the negative covariances can be largely explained by the current-driven air-sea coupling. The influence of SST-driven coupling mechanism is detectable and produces well-defined large-scale patterns, although its magnitudes are, on average, about 30 times smaller than those driven by the current effect. These results provide observational evidence that the current-driven coupling gives rise to a non-negligible sink of kinetic energy for the oceanic quasi-geostrophic variability, and may serve as a basis for evaluating the competing conclusions of recent numerical experiments on the impact of the SST-driven coupling to the ocean energetics.

Keywords

Air-sea interactions; ocean energetics; ocean drifters; satellite observations; climate modeling; mesoscale ocean eddies

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