Observability of the Irminger Sea circulation using variational data assimilation

Abstract

SUMMARYThis paper explores the prospects for constraining circulation and stratification estimates in the Irminger Sea and Denmark Strait using variational data assimilation. A regional circulation model with 9 km horizontal grid spacing, seven vertical levels and open boundaries is used. The circulation it simulates is realistic in several important respects and includes a chaotic mesoscale eddy field. Pulsation of the outflow of dense water through the Denmark Strait is also reasonably realistic, although entrainment and mixing of this water downstream is crudely represented. Identical twin assimilation experiments are performed to address the observability of the system with the existing in situ and satellite observing network in the absence of systematic errors. That is, the constraints exerted on the circulation and stratification by the data assimilation system are quantified. Synthetic data come from a reasonably realistic observing network that includes in situ hydrographic and current measurements, and remotely sensed surface temperature and sea level. Over a 15-day assimilation period in summer, the system fits all the observations to the relevant noise level, largely by correcting the initial conditions. Only modest changes in the time-dependent surface forcing and negligible changes to the open boundary conditions are needed. The system can accurately estimate sea surface height and sea surface temperature, correcting errors in the locations of surface fronts and eddies. The mid-depth salinity is almost completely unconstrained, however, whereas the velocity fields are somewhat constrained by the assimilation, especially by the remote-sensing data. Assimilation of satellite data enables us to accurately track the phase and amplitude of the overflow variability at the exit of Denmark Strait in the identical twin experiments. At other model outflow transects the assimilated solution is contaminated with spurious inertial oscillations. This is true even at a section offshore of Angmagssalik where a current-meter mooring array exists (in our experiments and in the real ocean). Nevertheless, improvement of the model dense outflow near the moorings is significant and propagates several hundred kilometres upstream and downstream.

On the Nature and Variability of the East Greenland Spill Jet: A Case Study in Summer 2003*

“…Third, the MITgcm has useful features that will be exploited in the future: namely, its nonhydrostatic capability and the packages to simulate snow and sea ice dynamics (Zhang et al 1999;Zhang and Rothrock 2000). Finally, previous configurations have been already set up in the same area (Lea et al 2006;Haine et al 2009;Haine 2010).…”

Upper ocean state estimation in the Southern Ocean Gas Exchange Experiment region using the four-dimensional variational technique

“…Applying 4D‐Var to a regional, high‐resolution ocean circulation model with realistic physics is a good strategy to capture aspects of the real flow in energetic regions. There have been only a few studies in this direction [ Gebbie , 2004; Gebbie et al , 2006; Lea et al , 2006; Losch et al , 2008; Mazloff , 2008], however, mainly because of the technical challenge, and further experience is needed. Performing high‐resolution regional data assimilation in the SO using 4D‐Var is the subject of this paper.…”

A Low-Order Model Investigation of the Analysis of Gravity Waves in the Ensemble Kalman Filter

“…In regimes in which modeled flows are expected to be balanced, it has been found that 4D data assimilation can cause the excitation of spurious unbalanced motion, essentially because of the development of unphysical correlations (Polavarapu et al 2000;Lea et al 2002;Houtekamer and Mitchell 2005;Neef et al 2006, hereafter NPS06) A general solution to this problem involves imposing balance constraints on the analysis (Courtier and Talagrand 1990;Dee 1991;Todling and Cohn 1994;Polavarapu et al 2000;Kepert 2004) or simply filtering out fast waves. In the upper stratosphere and mesosphere, excessive filtering can eliminate important gravity waves and either remove or amplify the tides (Sankey et al 2007).…”