Effects of variable wind stress on ocean heat content

Abstract

Ocean heat content change (ocean heat uptake) has an important role in variability of the Earth's heat balance. The understanding of which methods and physical processes control ocean heat uptake needs improvement in order to better understand variability in the Earth's heat balance, improve the simulation of present-day climate, and improve the understanding and projection of future climate. Wind stress can play a strong role in ocean heat uptake on all timescales, and short timescale wind stress effects have not been well studied in the literature. This study for the first time examines short timescale spatial and temporal patterns of global variable wind stress datasets in a coupled atmosphere-ocean climate model. NCEP wind stress dataset was characterized for years 1978 to 2007. NCEP monthly means and monthly standard deviations are of the same magnitude, and strong wind stress events (tropical cyclones) are observed. A variety of metrics cannot reliably identify significant timescales or spatial patterns of the variable wind stress. Model behavior with and without variable wind stress is studied. This study uses the MIT IGSM, a 4°x 11 vertical level zonal atmospheric model coupled at the four hour timestep to a 20x2.50x22 vertical level ocean model with the K profile parameterization. Ocean properties in a no forcing scenario are sensitive to variable wind stress. In a weak forcing scenario (observed forcing over the last century), ocean properties are sensitive to variable wind stress, and internal modes of variability (such as an equatorial Pacific oscillation) are observed. In a global warming scenario (1% CO2 rise per year or a business as usual emissions scenario), the strong forcing overwhelms the more subtle responses due to the differences in variable wind stress forcing. Regardless of forcing, the high frequency variable wind stress (monthly or less) variable wind stresses can force a low frequency response. Hence the major source of annual variability of the MOC in this coarse resolution model is surface wind variability.