The Ocean Circulation in Thermohaline Coordinates

Abstract

The thermohaline streamfunction is presented. The thermohaline streamfunction is the integral of transport in temperature-salinity space and represents the net pathway of oceanic water parcels in that space. The thermohaline streamfunction is proposed as a diagnostic to understand the global oceanic circulation and its role in the global movement of heat and freshwater. The coordinate system used filters out adiabatic fluctuations. Physical pathways and ventilation time scales are naturally diagnosed, as are the roles of the mean flow and turbulent fluctuations. Because potential density is a function of temperature and salinity, the framework is naturally isopycnal and is ideal for the diagnosis of water-mass transformations and advective diapycnal heat and freshwater transports. Crucially, the thermohaline streamfunction is computationally and practically trivial to implement as a diagnostic for ocean models. Here, the thermohaline streamfunction is computed using the output of an equilibrated intermediate complexity climate model. It describes a global cell, a warm tropical cell, and a bottom water cell. The streamfunction computed from eddy-induced advection is equivalent in magnitude to that from the total advection, demonstrating the leading-order importance of parameterized eddy fluxes in oceanic heat and freshwater transports. The global cell, being clockwise in thermohaline space, tends to advect both heat and salt toward denser (poleward) water masses in symmetry with the atmosphere's poleward transport of moisture. A reprojection of the global cell from thermohaline to geographical coordinates reveals a thermohaline circulation reminiscent of the schematized ''global conveyor.''

Lagrangian tracing of the water–mass transformations in the Atlantic Ocean

“…As noted above, Döös et al (2012) and Zika et al (2012) introduced the thermohaline stream function. This describes the volume flow in TS-coordinates, and is computed as the volume transport across an isotherm (isohaline) where the salinity (temperature) is less than S (T) : …”

“…The definition of water masses makes it possible to trace water in the zonal, meridional and vertical directions, and from their specific characteristics it is possible to identify them in TS-space (Tomczak, 1999), thus connecting the ocean circulation with the temperature and salinity distribution in the ocean. Döös et al (2012) and Zika et al (2012) introduced the thermohaline stream function, which describes the entire ocean circulation, including both the wind-driven circulation and the thermohaline circulation, in TS-space. This stream function shows the conversion of temperature and salinity through the whole ocean interior, including both deep and surface waters.…”

“…However, it should be noted that the cells might be overlapping, and could thus individually span larger ranges of temperature and salinity than shown here. Groeskamp et al (2014a) constructed a diathermohaline stream function, which is the stream function introduced by Döös et al (2013) and Zika et al (2012) together with a local stream function. This local stream function arises from seasonal movement of isohalines and isotherms, in the absence of geographical displacements.…”

“…To identify this, both Döös et al (2012) and Zika et al (2012) undertook a latitudelongitude reprojection of a specific stream layer in the Conveyor Belt cell and the Tropical cell; this to define the TScirculation in the ocean interior. They suggested that the Trop- Fig.…”

“…In this study, they allow for separating water-mass transformations in the Atlantic Ocean from the rest of the world ocean in TS-space. Döös et al (2012) and Zika et al (2012) suggested that the right-hand side of the Conveyor Belt cell (grey-marked area in Fig. 1) reflects a cooling and freshening of the northward transport in the Atlantic Ocean.…”