Global map of near surface currents anomaly

'''DEFINITION '''

The surface currents anomaly map for 2017 is obtained using GLORYS2V4 (GLOBAL_REANALYSIS_PHY_001_025). The currents near 15m (we use the depth of the velocity observations obtained from drifting buoys in order to ease the comparisons) are averaged over the year 2017 at each horizontal grid point, and the same quantity averaged over the period 1993-2014 (climatology) is substracted in order to obtain the anomaly map. Only the anomalies larger than the average interannual variability are shown. The standard deviation of the 15m currents over the years 1993-2014 is used to estimate this variability, but the years 1997 and 1998 are withdrawn from the computation, so that the average interannual variability is not too much drawn towards the extreme ENSO event of 1997/1998.

'''CONTEXT ''''

Ocean currents (Section 1.3) transport heat, fresh- water, plankton, fish, heat, momentum, oxygen and carbon dioxide and are thus a significant component of the global biogeochemical, energy and hydrological cycles. Knowledge of ocean currents is also important for marine operations involving navigation, search and rescue at sea, and the dispersal of pollutants. The ocean has an interconnected current, or circulation, system powered by winds, solar energy and water density differences, and steered by the Earth’s rotation and by tides, waves and bathymetry. Surface currents experience intrinsic oceanic interannual variability (Penduff et al. 2011; Sér- azin et al.2015); they may respond to air–sea large-scale variability patterns at interannual scale such as the North Atlantic Oscillation (Frankignoul et al.2001), ENSO (see Section 2.6) and may undergo changes due to global warming (e.g. Yang et al.2016; Armour et al.2016). Deep ocean currents are density-driven and contribute to the Meridional Overturning Circulation (see Section 2.8).

'''CMEMS KEY FINDINGS '''

At the global scale, the tropical currents are the strongest signal in the mean state for 1993–2014 (not shown) as well as in the 2017 anomaly. The signature of the decaying 2016/2017 la Nina in the Tropical Pacific is the strongest feature of the global 2017 current anomaly. An annual mean positive anomaly (acceleration) of the trade winds (not shown) generates anomalies in the westward flowing south equatorial currents in the Tropical Indian, Atlantic and Pacific basins. At mid-latitudes eastward flowing currents (especially the North Atlantic drift current) are stronger in 2017 than on the average 1993-2014 period