SST is an ocean variable that is readily measured by satellites and in situ sensors, and it is needed as a key input to forecasting systems to constrain the modeled upperocean circulation and thermal structure, and for the exchange of energy • AMERICAN METEOROLOGICAL SOCIETY

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Dense arrays of surface drifters are used to quantify the flow field on time and space scales over which high-frequency (HF) radar observations are measured. Up to 13 drifters were repetitively deployed off the Santa Barbara and San Diego coasts on 7 days during 18 months. Each day a regularly spaced grid overlaid on a 1-km 2 (San Diego) or 4-km 2 (Santa Barbara) square, located where HF radar radial data are nearly orthogonal, was seeded with drifters. As drifters moved from the square, they were retrieved and replaced to maintain a spatially uniform distribution of observations within the sampling area during the day. This sampling scheme resulted in up to 56 velocity observations distributed over the time (1 h) and space (1 and 4 km 2 ) scales implicit in typical surface current maps from HF radar. Root-mean-square (RMS) differences between HF radar radial velocities obtained using measured antenna patterns, and average drifter velocities, are mostly 3-5 cm s Ϫ1 . Smaller RMS differences compared with past validation studies that employ current meters are due to drifter resolution of subgrid-scale velocity variance included in time and space average HF radar fields. Roughly 5 cm s Ϫ1 can be attributed to sampling on disparate time and space scales. Despite generally good agreement, differences can change dramatically with time. In one instance, the difference increases from near zero to more than 20 cm s Ϫ1 within 2 h. The RMS difference and bias (mean absolute difference) for that day exceed 7 and 12 cm s Ϫ1 , respectively.

We characterize stratification patterns over diel, seasonal, and annual time scales in inshore and offshore regions of Lake Victoria, East Africa; determine conditions leading to horizontal exchanges; and, using surface energy budgets derived from local meteorological stations and two reanalysis products, address whether stratification depends on advective as opposed to local processes. The largest change in the surface energy budget occurred when winds intensified at the end of the long rains, with the wind's intensification, duration, and spatial extent dependent on El Niñ o-Southern Oscillation cycles. These winds flush inshore waters and cause cross-basin upwelling similar to that observed in the deep African Great Lakes. Wedderburn numbers indicated mixing and cross-basin within-thermocline transport. The internal wave-induced mixing and enhanced latent heat fluxes of 2300 to 2400 W m 22 contributed to the loss of seasonal stratification. Advection of cool water was required to balance the heat budget of northern offshore waters in the latter half of the southeast monsoon except in an El Niñ o year. Northern waters became weakly stratified after the southeast monsoon, with nocturnal winds contributing to heat transport and ventilation of the lower water column. Following the rainy season, downwelling by sustained southerly albeit low winds is a likely cause of the seasonal thermocline. Inshore waters are 0.2-1.5uC warmer than those offshore, conditions conducive to horizontal convective circulation except during onshore winds. The seasonal cycle of stratification and inshore-offshore and cross-basin exchanges are moderated by differential heating, cooling, and basin-scale thermocline tilting.The diel, seasonal, and annual variations in stratification within lakes influence the magnitude of vertical mixing, inshore-offshore exchanges, persistence of hypoxia, fluxes of greenhouse gases and nutrients, and the timing and duration of phytoplankton growth, with consequences for higher trophic levels. The variations in temperature and stratification depend on the surface energy budget, that is, the net shortwave and longwave radiation and sensible and latent heat fluxes combined with advection and the input of momentum from winds. Surface energy budgets are powerful tools for understanding controls on latitudinal variations in lake thermal structure and for quantifying how the energy inputs and water losses from lakes vary with changes in climate (Lenters et al.

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