Eddy‐Resolving Simulation of the Atlantic Water Circulation in the Fram Strait With Focus on the Seasonal Cycle

Abstract

Eddy driven recirculation of Atlantic Water (AW) in the Fram Strait modifies the amount of heat that reaches the Arctic Ocean, but is difficult to constrain in ocean models due to very small Rossby radius there. In this study, we explore the effect of resolved eddies on the AW circulation in a locally eddy‐resolving simulation of the global Finite‐Element‐Sea ice‐Ocean Model (FESOM) integrated for the years 2000–2009, by focusing on the seasonal cycle. An eddy‐permitting simulation serves as a control run. Our results suggest that resolving local eddy dynamics is critical to realistically simulate ocean dynamics in the Fram Strait. Strong eddy activity simulated by the eddy‐resolving model, with peak in winter and lower values in summer, is comparable in magnitude and seasonal cycle to observations from a long‐term mooring array, whereas the eddy‐permitting simulation underestimates the observed magnitude. Furthermore, a strong cold bias in the central Fram Strait present in the eddy‐permitting simulation is reduced due to resolved eddy dynamics, and AW transport into the Arctic Ocean is increased with possible implications for the Arctic Ocean heat budget. Given the good agreement between the eddy‐resolving model and measurements, it can help filling gaps that point‐wise observations inevitably leave. For example, the path of the West Spitsbergen Current offshore branch, measured in the winter months by the mooring array, is shown to continue cyclonically around the Molloy Deep in the model, representing the major AW recirculation branch in this season.

Does the East Greenland Current exist in northern Fram Strait?

“…subduction under sea-ice and PSW. As described in Wekerle et al (2017), a correct eddy heat transport is also able to reduce a cold bias (modelled temperatures are lower than in observations) found in the central Fram Strait.…”

“…Some observational studies locate the northern (Rudels et al, 2005). There is evidence from drifter data (Gascard et al, 1995), hydrographic surveys (Marnela et al, 2013), an inverse modelling study (Schlichtholz and Houssais, 1999) and a numerical ocean model (Kawasaki and Hasumi, 2016) evidence from model studies in Fram Strait is at present inconclusive as the northern limit of the recirculation, the strength of individual recirculation pathways and of the boundary currents varies between models (Maslowski et al, 2004;Aksenov 5 et al, 2010;Hattermann et al, 2016;Ilicak et al, 2016;Wekerle et al, 2017). This appears to be related to the resolution of the models and the bathymetry (Fieg et al, 2010).…”

“…204) and has variously been 20 described as a topographically steered branch of the Greenland Sea Gyre in the southern part of Fram Strait (following the Knipovitch Ridge and the Greenland-Spitsbergen Sill), as a cyclonic circulation around the Molloy Hole seen in models (Aksenov et al, 2010;Kawasaki and Hasumi, 2016;Hattermann et al, 2016;Wekerle et al, 2017) and observations (Johannessen et al, 1987;Quadfasel et al, 1987) and as a field of topographically steered eddies, shed by the WSC, which merge with the EGC (Gascard et al, 1988(Gascard et al, , 1995. These eddies are also thought to be one mechanism that allows the AW to subduct under- 25 neath the sea-ice and PSW advected from the Arctic Ocean southwards in the EGC (Hattermann et al, 2016).…”

“…The wintertime peak in eddy kinetic energy (EKE) observed in Fram Strait (von Appen et al, 2016) can be explained by the greater baroclinic and barotropic instability of the WSC in winter compared to summer (Teigen et al, 2010(Teigen et al, , 2011 due to decreased stratification of the upper watercolumn (von Appen et al, 2016). An eddy resolving grid is required in numerical models to reproduce the observed EKE levels (Hattermann et al, 2016;Wekerle et al, 2017). This increases AW transport into central Fram Strait and 30 vertical transport of AW i.e.…”

“…4.2. Throughout, we utilize an eddy resolving numerical model (Wekerle et al, 2017) to put the synoptic observations in a larger temporal and spatial context and to assess in which state of the highly variable flow regime the observations were taken. We close with conclusions from our findings in Sect.…”