Ernesto Rodriguez (Jet Propulson Laboratory, United States)

Simultaneous observations of ocean vertical velocity from two independent platforms—the NASA airborne DopplerScatt Ka-band pencil-beam Doppler scatterometer and an upward-looking Doppler sonar mounted on a neutrally buoyant float—demonstrate that surface horizontal velocity divergence fields can be used to estimate subsurface vertical velocities when appropriately averaged. These measurements were collected in a region of active mesoscale and submesoscale variability off the coast of California during the Sub-Mesoscale Ocean Dynamics Experiment (SMODE). The float data reveal that the largest vertical velocities are due to surface waves and to internal waves with buoyancy frequencies near the buoyancy period (5–20 minutes). Lower-frequency components become apparent only after multi-hour averaging. Vertical velocities estimated from DopplerScatt divergence fields multiplied by a suitable vertical scale, show significant correlation with float-derived velocities when averaged over several DopplerScatt passes. Typical time-averaged vertical velocities are on the order of 1 mm/s.

These results support theoretical expectations that vertical velocity fields are dominated by small spatial and temporal scales, making instantaneous measurements poor indicators of lower-frequency motions. Estimates of vertical velocity from SWOT data will therefore need to carefully define the spatial and temporal averaging scales assumed in the analysis. The magnitude of the computed velocity is likely to strongly depend on these scales.