Introduction

The Arctic and sub-polar ocean is home to several distinct processes and phenomena of importance to the global climate system. We have developed a regional eddy-permitting Arctic/subpolar gyre State Estimate (ASTE) using gradient-based optimization methods that are well developed in the computational science community, and that have been successfully applied for global-scale ocean climate state estimation by the Estimating the Circulation and Climate of the Ocean (ECCO) consortium. The approach is, fundamentally, a least-squares fit of allavailable observations to a coupled ocean/sea-ice model by means of the adjoint method (Heimbach and Wunsch 2012; Wunsch and Heimbach 2013b). The underlying model used is the MIT general circulation model (MITgcm). The fit of the model to the observations is achieved through variation of a number of control variables, such as initial conditions; time-mean parameter fields; and atmospheric forcing fields.

Accomplishments

1. The project has contributed to the production of a global 1992-2011 state estimate on a new-generation global LLC90 grid. The grid includes the Arctic and provides a natural 3:1 nesting with the higher-resolution LLC270 grid. By way of example, Fig. 1 depicts the time-mean misfit between in-situ potential temperature observations and the state estimate at three depth levels for a non-optimized (a) and optimized (b) solution.

2. Fenty et al. (2013a,b) conducted studies of mass and enthalpy budgets and associated ocean-ice-atmosphere fluxes during wintertime sea ice quasi-equilibrium state in the Labrador Sea and Baffin Bay, a critical region of the North Atlantic subpolar gyre,, and analyzed what sets the maximum sea ice quasi-equilibrium extent (Fig. 2). 

3. Nguyen et al. (2011,2012) inferred sources, pathways, and seasonal development of Upper Halocline Waters in the Western Arctic ocean from Pacific water intrusions via Bering Strait, and studied the implications for heat advection through the Chukchi Sea and Canada Basin.

4.. The potential use of first-year (FY) versus multi-year (MY) ice area retrievals from QuikSCAT as data constraints in ASTE requires an approach to simulate sea ice age and its classification according to FY and MY ice (Fig. 3, top panels). Such an algorithm has recently been incorporated into the MITgcm (Rampal et al., unpublished). Fig. 3 (bottom) shows apparent trends in satellite-retrieved (black line) and simulated (blue line)  MY sea ice area fraction changes between 2000 and 2009.

5. A related study investigated the cause of IPCC AR4 models  to underestimate thinning rates of sea ice thickness over the last three decades. To this end kinematic properties were intercompared, and compared with to SCICEX and ICESat thickness data as well as IABP buoy drift data. The causes of the model deficiency were identified to be a too weak feedback between the sea ice state and its kinematic properties.

6. With the provision of open boundary conditions to ASTE from the global solution, an understanding of basic transport properties becomes a priority. An initial such analysis was performed by Wunsch and Heimbach (2013a) for the Atlantic meridional overturning circulation (AMOC) covering the period. At 26N, where the RAPID/MOCHA array exists, the AMOC exhibits large month-to-month variability (Fig. 4, black curves), as pointed out by Wunsch and Heimbach (2006). The transport estimates have been contributed to the NOAA State of the Climate Report 2012 (Baringer et al., 2013). Note that in contrast to RAPID, the ASTE estimates enable a wide range of Atlantic basin scale analyses (Heimbach et al., 2011). They reach back to 1992, showing no discernible tre...

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