Abstract
AbstractA method using a linear shallow water model is presented for decomposing the temporal variability of the barotropic stream function in a high‐resolution ocean model. The method is based on the vertically averaged momentum equations and is applied to the time series of annual mean stream function from the model configuration VIKING20 for the northern North Atlantic. An important result is the role played by the nonlinear advection terms in VIKING20 for driving transport. The method is illustrated by examining how the Gulf Stream transport in the recirculation region responds to the winter North Atlantic Oscillation (NAO). While no statistically significant response is found in the year overlapping with the winter NAO index, there is a tendency for the Gulf Stream transport to increase as the NAO becomes more positive. This becomes significant in lead years 1 and 2 when the mean flow advection and eddy momentum flux contributions, associated with nonlinear momentum advection, dominate. Only after 2 years, does the potential energy term, associated with the density field, start to play a role and it is only after 5 years that the transport dependence on the NAO ceases to be significant. It is also shown that the potential energy contribution to the transport stream function has significant memory of up to 5 years in the Labrador and Irminger Seas. However, it is only around the northern rim of these seas that VIKING20 and the transport reconstruction exhibit similar memory. This is due to masking by the mean flow advection and eddy momentum flux contributions.
Highlights
There is evidence of significant interannual to decadal variability in the circulation of the North Atlantic Ocean (e.g., DiNezio et al, 2009; Eden & Jung, 2001; Eden & Willebrand, 2001; Greatbatch et al, 1991; Joyce et al, 2000; McCarthy et al, 2018; Rossby et al, 2010; Smeed et al, 2018)
Over most of the domain, the standard deviation is less than 10 Sv, while somewhat larger (>20 Sv) variability is seen along the pathway of the Gulf Stream and the North Atlantic Current extending into the northwest corner region (Lazier, 1994)
The decomposition is based on the vertically averaged momentum equations and is carried out by running a linear shallow water model (SWM) to steady state with the forcing terms diagnosed from VIKING20 output
Summary
There is evidence of significant interannual to decadal variability in the circulation of the North Atlantic Ocean (e.g., DiNezio et al, 2009; Eden & Jung, 2001; Eden & Willebrand, 2001; Greatbatch et al, 1991; Joyce et al, 2000; McCarthy et al, 2018; Rossby et al, 2010; Smeed et al, 2018). To what extent the spreading of newly formed dense water influences the barotropic transport variability is not clear, Gerdes and Köberle (1995) offer a glimpse of what can happen in a model that does not resolve eddies Another important point to note when interpreting our results is that because our focus is on interannual time scales, classical (flat‐bottomed) Sverdrup balance is unlikely to be valid because of the much longer time scales required for baroclinic adjustment of the ocean (e.g., Anderson and Killworth, 1977; Anderson et al, 1979; DiNezio et al, 2009; see Anderson and Corry (1985) for an illustration on seasonal time scales).
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