Sirpa Hakkinen (NASA Goddard Space Flight Center, United States)

Surface ocean-current and SSH variability, made visible with altimetry, reflects the dominant circulations beneath. Transports of mass, heat and fresh water and adjustment of water-column stratification are the most important oceanographic products for climate and ecosystems. The complementary combination of profiling ARGO floats and altimetry has yielded impressive reconstruction of time-variable AMOC (Atlantic meridional overturning circulation) and time-mean 1000m-depth lateral circulation, as well as subsurface material transports by mesoscale eddies and their origins in boundary currents. Core potential vorticity and water-mass potential temperature/salinity (θ/S) tracers are carried long distances in virtually a new mode of circulation. It demonstrates a ‘mean circulation’ defined by key water masses (~conditioned Lagrangian average) which may circulate and mix differently than traditional time-averages of velocity (~Eulerian averaging).

We review connections between subtropical and subpolar surface gyre circulations and meridional circulations beneath. Ultimately oceanic overturning, transformation of water-masses and global warming are best described on the θ/S plane. Shallow-deep interaction and θ/S dynamics are seen in these striking events of altimetry-era Atlantic circulation:
o spin-down of the subpolar gyre and massive θ/S increase of the subpolar Atlantic and Nordic Seas
o the 45N transition zone and water-mass transformation site for Gulf Stream/North Atlantic Current connection between subtropics and high latitude
o global ocean warming studies applying circulation and eddy observations, incorporating the ‘thermohaline (θ/S-) vector’ of atmosphere/ocean interaction, and exploiting the altimetric measure of oceanic heat-content variability.
o deep convective mixing and interleaving at the Atlantic polar front (and AMOC ‘choke-point’) near Iceland, revealed by pairing Seaglider sections with altimetric surface velocity and satellite SST
(see figure left: SST (14 May 2009) and Seaglider tracks crossing Iceland-Faroe Ridge at polar front, right: AVISO surface velocity vectors/streamlines and (black) Norrona upper 150m adcp subsurface mean velocity, with ridge topography; below: glider-observed salinity anomaly section, due to mixing across the sloping polar front. Beaird, Rhines & Eriksen, J.Geophys. Res 2016)

Future observations of the subsurface water column will gain considerably by coordinating with satellite altimetry orbits: populating the world ocean with controllable, deep gliders along these lines, in efforts complementing the randomly drifting ARGO floats already in place.