Geostrophic Closure of the Zonally Averaged Atlantic Meridional Overturning Circulation

• Published in: Journal of Physical Oceanography
• Main Authors: Sevellec, Florian, Huck, Thierry
• Format: Article in Journal/Newspaper
• Language: English
• Published: Amer Meteorological Soc 2016
• Subjects: Meridional overturning circulation; Thermohaline circulation; Atm/Ocean Structure/ Phenomena; Ocean circulation; Baroclinic flows; Geographic location/entity; Ekman pumping; Circulation/ Dynamics; Atlantic Ocean; Pacific; North Atlantic
• Online Access: https://archimer.ifremer.fr/doc/00323/43456/42843.pdf; https://doi.org/10.1175/JPO-D-14-0148.1; https://archimer.ifremer.fr/doc/00323/43456/

It is typically assumed that the meridional density gradient in the North Atlantic is well and positively correlated with the Atlantic meridional overturning circulation (AMOC). In numerical "water-hosing" experiments, for example, imposing an anomalous freshwater flux in the Northern Hemisphere leads to a slowdown of the AMOC. However, on planetary scale, the first-order dynamics are linked to the geostrophic balance, relating the north-south pressure gradient to the zonal circulation. In this study, these two approaches are reconciled. At steady state and under geostrophic dynamics, an analytical expression is derived to relate the zonal and meridional pressure gradient. This solution is only valid where the meridional density gradient length scale is shorter than Earth's curvature length scale, that is, north of 35 degrees N. This theoretical expression links the north-south density gradient to the AMOC and can be used as a closure for zonally averaged ocean models. Assumptions and shortcomings of the approach are presented. Implications of these results for paleoclimate problems such as AMOC collapse and asymmetry in the meridional overturning circulation of the Atlantic and of the Pacific are discussed.