Global Impacts of Subseasonal (<60 Day) Wind Variability on Ocean Surface Stress, Buoyancy Flux, and Mixed Layer Depth

Abstract

AbstractSubseasonal surface wind variability strongly impacts the annual mean and subseasonal turbulent atmospheric surface fluxes. However, the impacts of subseasonal wind variability on the ocean are not fully understood. Here, we quantify the sensitivity of the ocean surface stress (𝛕), buoyancy flux (B), and mixed layer depth (MLD) to subseasonal wind variability in both a one‐dimensional (1‐D) vertical column model and a three‐dimensional (3‐D) global mesoscale‐resolving ocean/sea ice model. The winds are smoothed by time filtering the pseudo‐stresses, so the mean stress is approximately unchanged, and some important surface flux feedbacks are retained. The 1‐D results quantify the sensitivities to wind variability at different time scales from 120 days to 1 day at a few sites. The 3‐D results quantify the sensitivities to wind variability shorter than 60 days at all locations, and comparisons between 1‐D and 3‐D results highlight the importance of 3‐D ocean dynamics. Globally, subseasonal winds explain virtually all of subseasonal 𝛕 variance, about half of subseasonal B variance but only a quarter of subseasonal MLD variance. Subseasonal winds also explain about a fifth of the annual mean MLD and a similar and spatially correlated fraction of the mean friction velocity, where ρsw is the density of seawater. Hence, the subseasonal MLD variance is relatively insensitive to subseasonal winds despite their strong impact on local B and 𝛕 variability, but the mean MLD is relatively sensitive to subseasonal winds to the extent that they modify the mean u*, and both of these sensitivities are modified by 3‐D ocean dynamics.