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 medium-resolution data-constrained
ocean-sea ice model-data synthesis spanning the period 2002-2017 is
presented. 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, 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.