Abstract
AbstractThe upwelling of deep waters in the Southern Ocean is a critical component of the climate system. The time and zonal mean dynamics of this circulation describe the upwelling of Circumpolar Deep Water and the downwelling of Antarctic Intermediate Water. The thermodynamic drivers of the circulation and their seasonal cycle play a potentially key regulatory role. Here an observationally constrained ocean model and an observation‐based seasonal climatology are analyzed from a thermodynamic perspective, to assess the diabatic processes controlling overturning in the Southern Ocean. This reveals a seasonal two‐stage cold transit in the formation of intermediate water from upwelled deep water. First, relatively warm and saline deep water is transformed into colder and fresher near‐surface winter water via wintertime mixing. Second, winter water warms to form intermediate water through summertime surface heat fluxes. The mixing‐driven pathway from deep water to winter water follows mixing lines in thermohaline coordinates indicative of nonlinear processes.
Highlights
The upwelling, transformation, and downwelling of water masses in the Southern Ocean exerts a profound influence on the heat, freshwater, and carbon budgets of the world ocean (Iudicone et al, 2011; Purkey & Johnson, 2010; Rintoul & Naveira-Garabato, 2013; Sabine et al, 2004; Talley, 2013)
A recent analysis of Lagrangian upwelling pathways in the Southern Ocean indicates that this upwelling deep water may initially become colder and fresher upon reaching the surface mixed layer before forming intermediate water (Tamsitt et al, 2018). This hints at the importance of considering seasonally varying air-ice-sea buoyancy fluxes in setting the strength of the Southern Ocean meridional overturning circulation (MOC) (Evans et al, 2014). Using both models and observations, we investigate the seasonal variation in the transformations of water masses in the Southern Ocean, to assess the role of air-ice-sea fluxes and diapycnal mixing in setting the seasonal formation and destruction of deep water, intermediate water, and Antarctic winter water which forms the winter mixed layer south of the Polar Front
Having shown that the transformation of saline deep water into fresh intermediate water occurs via a Southern Ocean cold transit of the thermohaline circulation, we demonstrate that this cold transit is linked to the seasonal cycle of air-ice-sea buoyancy fluxes and subsurface mixing
Summary
The upwelling, transformation, and downwelling of water masses in the Southern Ocean exerts a profound influence on the heat, freshwater, and carbon budgets of the world ocean (Iudicone et al, 2011; Purkey & Johnson, 2010; Rintoul & Naveira-Garabato, 2013; Sabine et al, 2004; Talley, 2013) This residual meridional overturning circulation (MOC; Marshall & Speer, 2012) is thought to be governed by a balance between wind-driven upwelling/downwelling, eddy-induced advection and stirring, diapycnal mixing, and air-ice-sea buoyancy fluxes (Abernathey et al, 2011; Naveira Garabato et al, 2007; Speer et al, 2000; Zika et al, 2009). A recent analysis of Lagrangian upwelling pathways in the Southern Ocean indicates that this upwelling deep water may initially become colder and fresher upon reaching the surface mixed layer before forming intermediate water (Tamsitt et al, 2018) This hints at the importance of considering seasonally varying air-ice-sea buoyancy fluxes in setting the strength of the Southern Ocean MOC (Evans et al, 2014). We suggest that cabbeling may be of critical importance to the strength of the Southern Ocean overturning but in relation to the upwelling and transformation of deep waters into intermediate waters
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