Altimeter Detection of Coastal-Trapped Wave Responses to Winds on the US West Coast
Event: 2020 Ocean Surface Topography Science Team Meeting (virtual)
Session: Coastal Altimetry
Presentation type: Type Forum
Along the US west coast (USWC), a primary source of sea level anomalies and along-coast geostrophic velocity variations is poleward-propagating coastal-trapped waves (CTWs). CTWs are generated both to the south of the USWC and by wind fluctuations along the USWC itself. Phase velocities of the first baroclinic modes of freely-propagating CTWs along the USWC are ~2–3 m s-1 poleward, with daily phase displacements of ~200-300 km or ~2-3° of latitude. Thus, during even the most rapid (10-day) repeats of the reference missions (TOPEX/Jason), the sea-level anomaly (SLA) signature of a CTW can travel from northern Mexico (30°N) to southern Canada (50°N) during one repeat cycle, making it impossible for the altimeter to resolve the time variability of individual CTWs at a single ground track location. The CTW signals can be tracked by six-hour tide gauge data from multiple latitudes along the coast, providing the ground truth as to the propagating changes in sea level due to CTWs, as reported in previous studies; however, the tide gauges provide no data on the profile of sea-surface height away from the coast. We are testing the ability of both RADS and ALES alongtrack altimeter data to resolve these CTW signals by averaging over multiple events, defined using an index of wind reversals along the US west coast, which force the CTW response. We define “day 0” of a wind-relaxation event to occur when the observed winds at 35°N switch from strongly equatorward (upwelling-favorable) to calm or poleward. We collect the alongtrack altimeter data on each ground track in daily bins stretching from -5 to +5 days around the event for all ground tracks intersecting the USWC between 34-48°N. The altimeter overpass at each track occurs at a different time lag relative to the start time of each wind relaxation and CTW event. By compositing along-track data from wind events during the March-August periods of the years 2002-2018, we form composites from over 200 Jason altimeter cycles. Since the CTWs are a combination of waves forced by winds along the USWC (our signal) and freely-propagating waves from farther south (our noise), the assumption is that remotely-generated CTWs arriving from south of 35°N are not coherent with the local winds and thus are averaged out in the composite over many wind events, isolating the CTW response to wind events within the USWC system. We focus on two aspects of the along-shore propagation of the CTW: 1) the change in alongtrack sea level as close to the coast as possible at latitudes poleward of 34°N during the several days before and after the event (approximating the tide gauge data); and 2) the change in the alongtrack profiles of SLA next to the coast at the same times and latitudes. The results show that both RADS and ALES alongtrack data can resolve a noisier version of the tide gauge response (sea level fall and rise) from points within 10-20 km of the coast (on average), with more data points provided by the ALES data than the RADS data. Detection of the alongtrack (approximately cross-shore) profiles corresponding to the offshore shape of the sea level fluctuations presents a greater challenge.