02 July 2019

Cross-Spectral Analysis of the SST/10-m Wind Speed Coupling Resolved by Satellite Products and Climate Model Simulations

Ocean eddies drive coupled air-sea interactions important for climatic space and time scales.

Science

Although the occurrence of positive correlations between SST and near-surface wind speed over oceanic mesoscale ranges is well-known, the intrinsic spatial and temporal scales over which this air–sea coupling regime takes place are not well established. The contribution of the near-ubiquitous mesoscale ocean eddies in driving the observed coupling characteristics, relative to that of larger-scale ocean phenomena such as extratropical SST fronts and Rossby waves, also remains unclear. This work addresses these gaps using cross-spectral statistics calculated between SST and equivalent-neutral 10-m wind speed from satellite products, and from two CCSM4 simulations based on identical atmospheric components but with contrasting horizontal ocean resolutions of 0.1º, capable of resolving mesoscale eddies (HR), and of 1º, whose eddy effects are parameterized (LR).

Impact

This study provides further evidence of the importance of resolved mesoscale ocean phenomena, in particular of coherent eddies, for driving the air-sea coupling characteristics revealed by satellite observations and identifies potential limitations of the CCSM4 in resolving this interaction. It also argues in favor of the use of cross-spectral methods over simple linear regressions for objectively defining the characteristic spatial-temporal scales of different air-sea coupling regimes, and the variation of the coupling properties over different scales.

Summary

This study aims to determine the spatial-temporal scales where the SST forcing of the near-surface winds takes places and its relationship with the action of coherent ocean eddies. Here, cross-spectral statistics are used to examine the relationship between satellite-based SST and 10-m wind speed fields at scales between 100-1000 km and 10-100 days. It is shown that the transition from negative SST/w correlations at large-scales to positive at oceanic mesoscales occurs at wavelengths coinciding with the atmospheric first baroclinic Rossby radius of deformation; and that the dispersion of positively-correlated signals resembles tropical instability waves near the equator and Rossby waves in the extratropics. To provide further insight on the role of ocean eddies in the SST-driven coupling, the analysis is repeated for two climate model (CCSM) simulations using ocean grid resolutions of 1º (eddy-parameterized, LR) and 0.1º (eddy-resolving, HR). The lack of resolved eddies in LR leads to a significantly underestimated mesoscale wind speed variance relative to HR.

Contact
Ben Kirtman
University of Miami
Publications
Laurindo, LC, L Siqueira, AJ Mariano, and BP Kirtman.  2019.  "Cross-Spectral Analysis of the SST/10-m Wind Speed Coupling Resolved by Satellite Products and Climate Model Simulations."  Climate Dynamics 52(9-10): 5071-5098.  https://doi.org/10.1007/s00382-018-4434-6.