Global ocean Ekman upwelling velocity anomaly

'''DEFINITION''' The 2019 global ocean surface Ekman upwelling velocity anomaly map is calculated from the reprocessed wind product of the ASCAT scatterometer on board Metop-A at 25 km resolution (WIND_GLO_WIND_L3_REP_OBSERVATIONS_012_005). The Ekman upwelling velocity is defined as curl(τ/(ρ0f)), where τ is the wind stress vector, f the Coriolis parameter and ρ0 the seawater density, approximated constant at 1025 kg m-3 (Belmonte Rivas et al., 2019). The curl of the wind stress vector is calculated for the Level 2 product and interpolated to the Level 3 grid. The Ekman upwelling velocities are calculated for the ascending and descending tracks and combined into monthly averages. These form the basis for the climatology (2007-2014) and annual anomalies. The anomalies are calculated by subtracting the climatology from the annual averages. Grid cells with a limited number of observations (coastal areas) or a large seasonal variability in the number of observations (seasonal sea ice cover) over the climatology period are excluded from the analysis.

'''CONTEXT''' Ekman upwelling and downwelling are an important mechanism for vertically transporting heat, salt and nutrients in the upper ocean. Upwelling water will generally be cold and rich in dissolved nutrients, stimulating phytoplankton growth and a productive food chain. In downwelling areas, warm, nutrient-deficient surface water is transported to greater depths and biological productivity is reduced. Anomalies in the Ekman upwelling velocity are connected to (anti)cyclonic anomalies in the annual mean wind. As a rule of thumb, cyclonic winds will induce a net divergence in Ekman transport and upwelling, or Ekman suction, while anti-cyclonic winds will induce a net convergence and downwelling, or Ekman pumping.

'''CMEMS KEY FINDINGS''' The 2019 Ekman upwelling velocity anomaly map shows a strong negative anomaly in the equatorial Indian Ocean. This feature is associated with an anticyclonic anomaly in the annual mean wind, associated with a strongly positive phase of the Indian Ocean Dipole. The strong negative anomaly in the western equatorial Pacific Ocean is connected to a cyclonic mean wind anomaly in the South Pacific subtropical gyre, induced by weakly positive El Niño-Southern Oscillation conditions in 2019.