The Surface Wind At Near Surface (SWANS) OSTST project (2017-2020)

Frédéric Frappart (LEGOS, France)


Orlando Astudillo (CEAZA, Chile); Boris Dewitte (LEGOS, France); Abderrahim Bentamy (IFREMER, France); Katerina Goubanova (CEAZA, Chile); Marc Mallet (CNRM, France)

Event: 2020 Ocean Surface Topography Science Team Meeting (virtual)

Session: Salient results from the 2017-2020 OSTST PIs

Presentation type: Type Forum

The Surface Winds from Altimetry at Near Shore (SWANS) OSTST project (2017-2020) was dedicated to the analysis altimetry-based wind speed along the coasts of the continents with a focus on Eastern Boundary Upwelling systems (EBUS) where the on-shore decrease of the wind amplitude (so-called wind drop-off) can produce significant impact on the regional oceanic circulation. Most of the work has focused on the Peru/Chile EBUS that has the widest latitudinal range and a strong heterogeneity in coastal orography. Along-track altimetry-derived surface wind speed data from ENVISAT, Jason-1, Jason-2, and SARAL satellites have been used to document the spatial variability and seasonality of the mean wind drop-off near the coast. The data were first calibrated from the scatterometer observations of previous and current satellite missions (QuikSCAT, ASCAT) over the on-shore region. The calibrated data were then analyzed near the coast and estimate of the wind drop-off scale were derived. The analyses indicate that the wind drop-off takes place all along the coast, though with a significant alongshore variability in its magnitude that is associated to details in orographic features (e.g. embayments, caps, mountains). Differences between products are shown to be related both to the differences in repeat cycle between the different altimetry missions and to the peculiarities of the coastline shape at the coastal latitudes of the incident tracks. Despite the aliasing effect associated with the weak repetitivity of the satellite orbit and the high frequency variability of the winds in this region, the analysis suggests that the seasonal cycle of the costal surface winds near the coast could be resolved at least off Peru where high-frequency winds are weaker than off Central Chile. Mean altimetric coastal winds were compared to high-resolution atmospheric model (WRF) simulations at various resolutions (36km, 12km, 4km) indicating consistencies, and thereby confirming that the wind drop-off not seen by the scatterometers (e.g. blind zone) is a strong limitations for regional oceanic modeling in EBUS. In particular regional ocean model (CROCO) simulations in these regions exhibit a cool SST bias that is associated to an overestimation of coastal wind forcing resulting from the extrapolation of scatterometer products in the coastal fringe. Such an extrapolation procedure is not currently constrained from coastal meteorological observations that remain sparse in the South Hemisphere. It was also shown that the coastal winds also influence the regional circulation through the non-linear coastal upwelling dynamics. In particular the consideration of a more realistic wind drop-off in the atmospheric forcing of regional oceanic model results in a more realistic simulation of SST along the coast (reducing the usually observed cool bias), which is associated to the heat flux-induced reduced cooling mediated by the reduction in the mixed‐layer depth, rather than to the reduction in Ekman transport. Our work emphasizes the need to better constrain the coastal winds in atmospheric Reanalysis products, and the value of altimetry-based winds for documenting and understanding long-term trends in EBUS, an area of current active research in the frame of international programs (SCOR Working group on EBUS, CLIVAR Research Focus on EBUS) and projects (FutureMares, CE2COAST)
Frédéric Frappart