Seasonal and Spatial Variability of Near-Inertial Kinetic Energy from Historical Moored Velocity Records

Abstract

Temporal and spatial patterns of near-inertial kinetic energy (KE moor ) are investigated in a database of 2480 globally distributed, moored current-meter records (deployed on 690 separate moorings) and compared with the distribution of wind-forced mixed-layer energy flux F ML . By computing KE moor using short (30 day) multitaper spectral windows, the seasonal cycle is resolved. Clear winter enhancement by a factor of 4-5 is seen in the Northern Hemisphere for latitudes 25°-45°at all depths Ͻ4500 m, in close agreement with the magnitude, phase, and latitudinal dependence of the seasonal cycle of F ML . In the Southern Hemisphere, data coverage is poorer, but a weaker seasonal cycle (a factor of 2) in both KE moor and F ML is still resolvable between 35°and 50°. When Wentzel-Kramers-Brillouin (WKB) scaled using climatological buoyancy-frequency profiles, summer KE moor is approximately constant in depth while showing a clear decrease by a factor of 4-5 from 500 to 3500 m in winter. Spatial coverage is sufficient in the Northern Hemisphere to resolve broad KE moor maxima in the western portion of each ocean basin in winter, generally collocated with F ML maxima associated with storm forcing. The ratio of depth-integrated KE moor to F ML gives a replenishment time scale, which is about 10 days in midlatitudes, consistent with 1) previous estimates of the dissipation time scale of the internal wave continuum and 2) the presence of a seasonal cycle. Its increase to Ϸ70-80 days at lower latitudes is a possible signature of equatorward propagation of near-inertial waves. The seasonal modulation of the magnitude of KE moor , its similarity to that in F ML , and the depth decay and western intensification in winter but not in summer are consistent with a primarily wind-forced near-inertial field for latitudes poleward of Ϸ25°.

Near-inertial and thermal upper ocean response to atmospheric forcing in the North Atlantic Ocean

“…The patterns of surface intensification and winter enhancement are in agreement with the global patterns of near-inertial energy variability presented by Alford and Whitmont (2007). To first order, the depth-integrated model shows that the nearinertial energy imparted by the wind is balanced by localized dissipation.…”

“…The surface intensification of the near-inertial kinetic energy, along with the dominance of downward propagating near-inertial energy at large vertical scales, endorse the hypothesis that near-inertial internal waves in this region are chiefly surface forced. Furthermore, the seasonal cycle in the observed near-inertial kinetic energy supports the idea that the near-inertial motions are predominantly forced by the passage of winter storms, as was concluded by Alford and Whitmont (2007).…”

“…autocorrelation functions.) Alford and Whitmont (2007) observed surface-intensified enhancement of wintertime WKB-scaled near-inertial kinetic energy with a decay in energy from 500 meters to 3500 meters depth by a factor of 3-4; here we observe a decay of approximately a factor of 2 from 500 meters depth to 3200 meters depth.…”

“…Global analysis of mooring records indicates that near-inertial energy is surface intensified and enhanced during winter months, both of which are indicative of a surface source (Alford and Whitmont, 2007). Any wind stress which is variable in time can force near-inertial motions; winds which rotate at the inertial frequency produce a resonant forcing with the maximum possible energy input.…”

“…Accordingly, there is commonly a winter maximum in the wind energy input into near-inertial motions because of the prevalence of storms in this season (Alford, 2001). Recent work by Alford and Whitmont (2007) examined temporal and spatial patterns of near-inertial kinetic energy observed at current meter moorings across the globe. They concluded that there is a surface-intensified, seasonal cycle in nearinertial kinetic energy that is correlated with the estimated wind forcing.…”