The Arctic Subpolar gyre sTate Estimate (ASTE): Description and
assessment of a data-constrained, dynamically consistent ocean-sea ice
estimate for 2002-2017
Abstract
A description and assessment of the first release of the Arctic Subpolar
gyre sTate Estimate (ASTE_R1), a data-constrained ocean-sea ice
model-data synthesis is presented. ASTE_R1 has a nominal resolution of
1/3o and spans the period 2002-2017. The fit of the model to an
extensive (O(10^9)) set of satellite and in situ observations was
achieved through adjoint-based nonlinear least-squares optimization. The
improvement of the solution compared to an unconstrained simulation is
reflected in misfit reductions of 77% for Argo, 50% for satellite sea
surface height, 58% for the Fram Strait mooring, 65% for Ice Tethered
Profilers, and 83% for sea ice extent. Exact dynamical and kinematic
consistency is a key advantage of ASTE_R1, distinguishing the state
estimate from existing ocean reanalyses. Through strict adherence to
conservation laws, all sources and sinks within ASTE_R1 can be
accounted for, permitting meaningful analysis of closed budgets at the
grid-scale, such as contributions of horizontal and vertical convergence
to the tendencies of heat and salt. ASTE_R1 thus serves as the biggest
effort undertaken to date of producing a specialized Arctic ocean-ice
estimate over the 21st century. Transports of volume, heat, and
freshwater are consistent with published observation-based estimates
across important Arctic Mediterranean gateways. Interannual variability
and low frequency trends of freshwater and heat content are well
represented in the Barents Sea, western Arctic halocline, and east
subpolar North Atlantic. Systematic biases remain in ASTE_R1, including
a warm bias in the Atlantic Water layer in the Arctic and deficient
freshwater inputs from rivers and Greenland discharge.