Seasonal overturning variability in the eastern North Atlantic subpolar gyre: A Lagrangian perspective

• Main Authors: Tooth, Oliver John, Johnson, Helen Louise, Wilson, Chris, Evans, Dafydd Gwyn
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2022
• Subjects: article; Verlagsveröffentlichung; Greenland; Irminger Sea; East Greenland; east greenland current; Iceland; north atlantic current; North Atlantic; Sea ice
• Online Access: https://doi.org/10.5194/egusphere-2022-1334; https://noa.gwlb.de/receive/cop_mods_00063771; https://egusphere.copernicus.org/preprints/egusphere-2022-1334/egusphere-2022-1334.pdf

Changes in the high-latitude Atlantic Meridional Overturning Circulation (MOC) are dominated by water mass transformation in the eastern North Atlantic Subpolar Gyre (SPG). Both observations and ocean reanalyses show a pronounced seasonality of the MOC within this region. Here, we investigate the nature of this seasonal overturning variability within the eastern SPG using Lagrangian water parcel trajectories evaluated within an eddy-permitting ocean sea-ice hindcast simulation. Our analysis highlights the critical role of water parcel recirculation times in determining the seasonality of overturning measured in both the traditional Eulerian and complimentary Lagrangian frames of reference. From an Eulerian perspective, we show that the minimum of the MOC seasonal cycle in autumn results from a combination of enhanced stratification and increased southward transport within the upper East Greenland Current. This convergence of southward transport within the MOC upper limb is explained by decreasing water parcel recirculation times in the upper Irminger Sea, consistent with a gyre-scale response to seasonal wind forcing. From a Lagrangian perspective, we find that upper limb water parcels flowing northwards into the eastern SPG participate in a recirculation race against time to avoid wintertime diapycnal transformation into the lower limb of the MOC. The majority of water parcels, sourced from the central and southern branches of the North Atlantic Current, are unsuccessful and thus determine the mean strength of overturning within the eastern SPG (8.9 ± 2.2 Sv). The seasonality of Lagrangian overturning is explained by a small collection of upper limb water parcels, recirculating rapidly (≤ 8.5 months) in the upper Irminger and Central Iceland Basins, whose along-stream transformation is dependent on their time of arrival in the eastern SPG.