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

Abstract

The thermohaline stream function has previously been used to describe the ocean circulation in temperature and salinity space. In the present study, the Lagrangian thermohaline stream function is introduced and computed for northward flowing water masses in the Atlantic Ocean, using Lagrangian trajectories. The stream function shows the water-mass transformations in the Atlantic Ocean, where warm and saline water is converted to cold and fresh as it flows from 17 • S to 58 • N. By analysing the Lagrangian divergence of heat and salt flux, the conversion of temperature is found to take place in the Gulf Stream, the upper flank of the North Atlantic subtropical gyre and in the North Atlantic Drift, whereas the conversion of salinity rather occurs over a narrower band in the same regions. Thus, conversions of temperature and salinity as shown by the Lagrangian thermohaline stream function are confined to the same regions in the domain. The study of a specific, representative trajectory shows that, in the absence of air-sea interactions, a mixing process leads to the conversion of temperature and salinity from warm and saline to cold and fresh, and that this process is confined to the North Atlantic subtropical gyre. However, to define and to understand this process, further investigation is needed.

Lagrangian Views of the Pathways of the Atlantic Meridional Overturning Circulation

“…There is also the ability to track fluid particles back in time, which is particularly useful for determining the origin of waters passing through a specific region. Furthermore, Lagrangian model output can be used to examine water mass transformations over long distances and investigate the structure of heat and fresh water fluxes along the AMOC pathways (e.g., Berglund et al, ; Chenillat et al, ; Durgadoo et al, ; Lique et al, ; Rimaud et al, ; Rühs et al, ; Speich et al, ). Of note is that simulated trajectories are perhaps least reliable in the deep ocean, where observations (Eulerian or Lagrangian) are too sparse to provide adequate model verification.…”

Cold vs. warm water route – sources for the upper limb of the Atlantic Meridional Overturning Circulation revisited in a high-resolution ocean model

“…That is, we released particles in the NBC at 6 • S for years 1960-1969 and 2000-2009 (highAL) and then traced them backwards towards the predefined source sections by looping through the velocity data of each period for a maximum of 40 years (instead of making use of the whole simulation period 1958-2009). Even though this looping technique has already been employed by various authors (e.g., Döös et al, 2008;Rühs et al, 2013;Thomas et al, 2015;Berglund et al, 2017;Drake et al, 2018;Durgadoo et al, 2017), the obtained results have to be interpreted with caution. Looping may introduce unphysical jumps in the velocity and tracer fields and, consequently, also in the volume transport pathways and along-track tracer changes.…”

Atmospheric Freshwater Transport From the Atlantic to the Pacific Ocean: A Lagrangian Analysis

“…TRACMASS (Döös, 1995), a Lagrangian trajectory model, was adapted for the present study to track freshwater in the atmosphere. In contrast to most other Lagrangian trajectory models, TRACMASS uses mass fluxes instead of velocities (Vries & Döös, 2001), which has made it possible to trace water-transport pathways in the ocean (Berglund et al, 2017;Döös et al, 2008) and air masses in the atmosphere (Kjellsson & Döös, 2012). A more detailed description of TRACMASS can be found in Döös et al (2017).…”