Short‐term upwelling events at the western African coast related to synoptic atmospheric structures as derived from satellite observations

Abstract

[1] Satellite scatterometers provide continuously valuable surface wind speed and direction estimates over the global ocean on a regular grid both in space and time. The Level 3 data derived from the Advanced Scatterometer (ASCAT), available at 1/4 spatial resolution (hereafter AS25), and Quick Scatterometer (QuikSCAT), available on 1/2 and 1/4 horizontal grids (QS50 and QS25, respectively), are studied at regional scales in both the Benguela and Canary upwelling systems. They are compared to the European Center for Medium-Range Weather Forecast surface wind analysis, with insight into their intrinsic and actual spatial resolutions. In the coastal band, the finest spatial patterns are found in the QS25 winds and are O(75 km). This demonstrates the sensitivity of the high-resolution satellite-derived winds to coastal processes related to sea surface temperature (SST) perturbations and land-sea transition. Next, short-lived upwelling episodes (SUEs) calculated from SST anomalies are defined consistently with the QS25 actual resolution. These cold events refer to local, short-lived perturbations that add to seasonal upwelling variability. We characterize concomitant atmospheric synoptic conditions for SUEs identified at chosen latitudes and highlight two subregions in both upwelling systems, with contrasted patterns for the alongshore wind stress component and curl. The complexity of the latter patterns is closely linked to local, short-term SST variability. Closer to the shore, numerical sensitivity experiments show that the imbalance between Ekman transport and Ekman pumping has an impact on ocean dynamics: wind reduction in the coastal QS25 forcing, partially induced by orography, tends to reduce coastal SST cooling.

Western boundary upwelling dynamics off Oman

“…Such phase differences have been noticed in satellite observations ) but have so far been unexplained. The lag is in contrast with EBUs dynamics where wind forcing and SSH/SST are generally in phase (Wang 1997) or lag by no more than a few days (Desbiolles et al 2014). Following a suggestion by J. McCreary in his review of an initial version of this manuscript, we propose that the lag results from the onshore propagation of long-wavelength, upwelling-favorable, Rossby waves.…”

Challenges and Prospects for Reducing Coupled Climate Model SST Biases in the Eastern Tropical Atlantic and Pacific Oceans: The U.S. CLIVAR Eastern Tropical Oceans Synthesis Working Group

“…Plans for dedicated atmospheric observations at and slightly south of the oceanic Angola-Benguela Front are still lacking. Because the ocean upwelling responds quickly to changes in the surface wind structure (Desbiolles et al 2014), assessments of fast SST error growth can potentially readily identify the importance of wind errors for the upwelling regions for individual models. A search for the commonalities across models in the upwelling regions can help narrow down the root causes.…”

Response of the Southern Benguela upwelling system to fine-scale modifications of the coastal wind

“…The three wind stress products are mainly upwelling-favorable during the 2-month period of analysis. In a previous study, we showed that the QS50 winds are less statistically related to the SST field than the QS25 winds and that the QS50 product most often overestimates the wind velocity in regions where the SST front is sharp (Desbiolles et al, 2014b). This sensitivity to the SST front is detectable in the average wind stress structure resulting in a broader and nearer inshore atmospheric jet in the QS50 product (see Fig.…”

“…This generates small-scale features in the low-level wind (Chelton et al, 2004;O'Neill et al, 2005;Small et al, 2008). In a previous study, we showed that the coupling between wind and SST is the main driver of both the magnitude and the variability of the wind stress curl during the upwelling season in the Benguela and Canary systems (Desbiolles et al, 2014b). SST-driven modifications to the wind contribute to a wind reduction in a transition region that may extend over 100 km offshore (Boé et al, 2011).…”

“…Cross-shore variations of the wind have received considerable attention since they modify both the coastal and basin-scale circulation linked to upwelling dynamics (Albert et al, 2010;Capet et al, 2004;Marchesiello et al, 2003;Rykaczewski and Checkley, 2008). Various processes can produce fine spatial and temporal wind scales, which often results in a wind reduction in the nearshore region of EBUSs, the so-called wind drop-off (i.e., Capet et al, 2004;Perlin et al, 2007;Renault et al, 2015 for the California system; Renault et al (2012) for the Humboldt system; Desbiolles et al (2014b) for both the Benguela and Canary systems). As reported and quantified in Renault et al (2015), physical processes linked to coastal orography (Edwards et al, 2001), the presence of a cape (Perlin et al, 2011), SST-wind interactions (Chelton et al, 2007), and sharp changes in surface drag at the sea-land interface (Edwards et al, 2001) are involved in the modification of the coastal winds.…”