Global transient ocean surface wind anomaly

'''DEFINITION''' The 2019 global ocean surface transient wind anomaly map is obtained from the reprocessed winds of the ASCAT scatterometer on board Metop-A at 25 km resolution (WIND_GLO_WIND_L3_REP_OBSERVATIONS_012_005). The Level 3 scatterometer winds from ascending and descending tracks are combined into monthly averages for every surface grid cell. Monthly transient wind or wind variability is calculated as the standard deviation of the wind around its monthly mean value (Belmonte Rivas et al., 2019). These monthly averages form the basis for the climatology (2007-2014) and annual anomalies, 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''' In monitoring surface winds, one is often limited to keeping track of the mean wind statistics. Much of the wind energy is however carried by eddy transients in the form of turbulent kinetic energy. The transient wind or wind variability affects the ocean circulation through vertical mixing via wave motion, inertial currents and transient upwelling/downwelling events. Notable annual anomalies in the transient winds are mostly spatially correlated with anomalies in the mean wind field. In the (sub)tropics, the mean and transient wind anomalies typically have the opposite sign and transient eddies draw energy from the mean flow. At higher latitudes, transient and mean wind anomalies generally have the same sign, indicating an overall regional increase or decrease in wind energy.

'''CMEMS KEY FINDINGS''' The 2019 transient wind anomaly map shows a large region with enhanced wind speed variability over the equatorial and subtropical North Pacific Ocean. This anomaly coincides with reduced easterly mean winds, associated with weak El Niño conditions during most of 2019. The pattern of positive and negative transient wind anomalies in the Indian Ocean matches with mean wind anomalies induced by a strongly positive Indian Ocean Dipole phase during the second half of 2019.