State estimation of the Labrador Sea with a coupled sea ice-ocean adjoint model

Abstract

Sea ice (SI) and ocean variability in marginal polar and subpolar seas are closely coupled. SI variability in the Labrador Sea is of climatic interest because of its relationship to deep convection/mode water formation, carbon sequestration, and Northern Hemisphere atmospheric patterns. Historically, quantifying the link between the region's observed SI and oceanic variability has been limited due to in situ observation paucity and technical challenges associated with synthesizing ocean and SI observations with a three-dimensional time-evolving dynamically consistent numerical model. To elaborate upon the relationship between SI and ocean variability, a one year (1996- 1997) state estimate of the ocean and sea ice state in Labrador Sea and Baffin Bay is constructed and analyzed. The estimate is a synthesis of a regional coupled 32 km ocean and sea ice model with a suite of contemporary in situ and satellite hydrographic and SI data. The synthesis of SI data is made possible with the (novel) adjoint of a thermodynamic SI model. Model and data are made consistent, in a least-squares sense, by iteratively adjusting several control variables, such as ocean initial and lateral open boundary conditions and the atmospheric state, to minimize an uncertainty-weighted model-data misfit cost function. It is shown that the SI pack attains a state of quasi-equilibrium in mid-March during which net SI growth/melt approaches zero; newly-formed SI diverges from coastal areas and converges, via wind/ocean forcing, in the marginal ice zone (MIZ). It is further shown that SI converging in the MIZ is primarily ablated by turbulent ocean-SI enthalpy fluxes. The primary source of energy required for sustained MIZ ice ablation is revealed to be the sensible heat reservoir of the subtropical-origin subsurface waters. Enthalpy from the heat reservoir is entrained into the mixed layer via buoyancy loss-driven convective deepening and brought to the SI via vertical mixing. An analysis of ocean surface buoyancy fluxes reveals a critical role of low-salinity upper ocean anomalies for the advancement of SI seaward of the Arctic Water/Irminger Water thermohaline front. Anomalous low-salinity waters slow the rate of buoyancy loss-driven mixed layer deepening, shielding an advancing SI pack from the subsurface heat reservoir, and are conducive to a positive surface stratification enhancement feedback from SI meltwater release, both of which extend SI lifetimes. Preliminary analysis of two additional one-year state estimates (1992-1993, 2003-2004) suggests that interannual hydrographic variability provides a first-order explanation for SI maximum extent anomalies. Additional research on the mechanisms controlling the origin and distribution of upper ocean salinity anomalies is required to further understand observed SI variability in the northwest North Atlantic.