Abstract
The north Atlantic subpolar gyre (SPG) has been widely implicated as the source of large-scale changes in the subpolar marine environment. However, inconsistencies between indices of SPG-strength have raised questions about the active role SPG-strength and size play in determining water properties in the eastern subpolar North Atlantic (ENA). Here, by analyzing various SPG indices derived from observations and a global coupled model, we show that the choice of the SPG index dictates the interpretation of SPG strength-salinity relationship in the ENA. Variability in geostrophic currents derived from observed hydrography and model based Lagrangian trajectories reveal zonal shifts of advective pathways in the ENA and meridional shifts in the western intergyre region. Such shifts in advective pathways are manifestations of variability in the size and strength of the SPG, and they impact salinity by modulating the proportion of subpolar and subtropical waters reaching the ENA. SPG indices based on subsurface density and principal component analysis of sea surface height variability capture these shifts in advective pathways, and are therefore best suited to describe SPG-salinity relationship in the ENA. Our results establish the dynamical constraints on the choice of the SPG index and emphasize that SPG indices should be cautiously interpreted.
Highlights
The northward transport of heat and salt in the North Atlantic is an important aspect of climate variability as it determines the exchanges of heat and salt with the Arctic Mediterranean
For the altimeter period (1993–2016), sea surface height (SSH) data is often considered as a proxy for the observed variability in the subpolar gyre (SPG) circulation, and can be used to assess the relationship between SPG strength and salinity in the eastern subpolar North Atlantic (ENA)
Instead our results suggest that the initiation of the intergyre gyre circulation anomaly or the northern recirculation gyre in the western intergyre region, both of which relate to latitudinal shifts in the North Atlantic Current (NAC), is due to wind stress curl driven changes in the SPG circulation
Summary
Variability in the size and strength of the SPG may not always be attributable to the NAO5,6, it has repeatedly been linked to salinity changes in the ENA7,8, leaving a minor role for local air-sea fluxes[9] and the Atlantic Meridional Overturning Circulation (AMOC)[10,11] In contrast to this abundant evidence, recent investigations[12,13] do not find any relationship between SPG size/strength and salinity in the ENA. A mechanistic understanding of the SPG-associated variations in the ENA is necessary in order to ascertain whether different SPG indices, which determine causal relationship of SPG-strength with hydrographic variability in the ENA, emphasize regional or basin scale circulation variability These disagreements raise the following questions: Is there a robust relationship between SPG circulation and salinity in the ENA? Idealized Lagrangian experiments with MPI-ESM-LR allow us to clarify the SPG strength-salinity relationship and illuminate inconsistencies among SPG indices
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