Wind-Driven Basin-Scale Internal Waves and Upwellings at Lake Tahoe Sveinn O. Palmarsson, Francisco J. Rueda, S. Geoffrey Schladow, Kelley L. Thompson, Simon J. Hook and Fred J. Prata Department of Civil and Environmental Engineering, University of California, Davis, CA 95616 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 CSIRO, Atmospheric Research, PB 1, Aspendale, Victoria 3195, Australia      Intense internal wave climate has been observed at Lake Tahoe in response to the wind. The EarthÕs rotation influences the wave field, leading to internal Kelvin and PoincarŽ waves. Kelvin waves propagate anti-clockwise around the lake. Their displacement is at maximum at the lakeÕs boundaries and decreases exponentially away from shore. Such waves have been found to travel around Lake TahoeÕs perimeter in approximately 2.5 and 5 days during winter. In contrast, PoincarŽ waves propagate clockwise around the lake, with maximum displacement and velocity at the center of the basin. At Lake Tahoe, PoincarŽ waves have been observed to have about 18 hour period when travelling around the lake in winter. The observations of these wave propagations were made during 5 winters, from 1996 to 2000, and furthermore they were verified with a three-dimensional numerical model for the 1999-2000 season. These basin-scale oscillations may lead to enhanced rates of particle transfer within the lake. The long hydraulic residence time of Lake Tahoe, approximately 650 years, underlines the importance of internal processes like these to the lakeÕs water transparency decline.      When the wind forces the fluid against the down-wind boundary, isopycnals (surfaces of equal density) are tilted and longitudinal pressure gradients are generated. This leads to a large vertical displacement at the sharp density gradient of the thermocline. If the wind is sufficiently strong, the excursion can be so great that the thermocline is brought to the water surface at the upwind end. This is referred to as upwelling. The heavier upwelled fluid flows over the lighter surface water, resulting in density inversions and eventual mixing.      Upwelling events are common in the weakly stratified lake during winter. A few total upwellings (when bottom water reaches the surface) that lasted for approximately one day have been observed in response to a strong wind event at a site near the western shore. Furthermore, a very large amplitude upwelling was observed in January of 2000, during winter cooling. A 6-day wind event of southwesterly winds commenced on January 9, with peak winds generally ranging from 8 to 15 m/s, leading to upwelling of bottom water at the western shore. All the thermistors present at the Index station mooring line recorded bottom lake temperatures for more than three days. A full relaxation of the upwelling was not obtained until about 4.5 days after the onset of the event (see plot).      Surface temperature records from the four rafts present in the central basin (TR1 through TR4) show that the cold-water front reached the two western rafts, signifying the great amplitude of the thermocline excursion. Images of satellite sensed lake skin temperatures stress the extent of the upwelling of cold bottom water along the entire western shore (see images from 3 satellite overpasses). The center image, from January 13, shows the skin surface temperature during the upwelling event, shortly after the cold spikes were observed at the two western rafts. Comparing this image to the other two, that are from relatively quiescent periods, one can see that approximately 1/3 of the basin has upwelled bottom water present at the surface. The extremely low temperatures measured by the satellite on the western side of the lake are most likely due to two factors. First, the skin temperature that is sensed by the satellite is typically about 0.5 ûC lower than the surface temperature immediately below the skin at this time of night. This has been determined for the case of Lake Tahoe specifically, with radiometers and temperature sensors placed on the four rafts (TR1 Ð TR4). Secondly, the lake experienced a great heat loss to the atmosphere during this period. This can point to extremely low water temperatures in the small water masses of the near-shore regions, in response to the cold atmospheric conditions, even temperatures near the freezing point. This cold shoreline water was potentially transported off shore with the active southwesterly winds.      Complete vertical mixing is obtained in the lake every few years. It has been shown by the Tahoe Research Group that this results in nutrient fluxes from nutrient rich bottom waters into the euphotic zone. It is possible that total upwelling events contribute significantly to the nutrient dynamics of the lake, by bringing bottom water to the surface and thus facilitating a pathway for the nutrients to the primary consumers, a process that has previously been uncounted for at Lake Tahoe.