The thermohaline circulation (THC) and the oceanic heat and freshwater
transports are essential for understanding the global climate system.
Streamfunctions are widely used in oceanography to represent the THC and
estimate the transport of heat and freshwater. In the present study, the
regional and global changes of the THC, the transports of heat and freshwater
and the timescale of the circulation between the Last Glacial Maximum (LGM,
The thermohaline circulation (THC) is the large-scale ocean circulation
associated with the transports of heat and salt
Our understanding of the past THC relies on reconstructions based on
paleo-proxies and climate model experiments. The reconstructions of the Last
Glacial Maximum (LGM,
Streamfunctions are widely used to investigate and represent the ocean
circulation. They show the averaged circulation in a two-dimensional
framework and capture the wind-driven and the thermohaline contributions. The
latitude–depth coordinates (see e.g. Fig.
The ocean meridional heat and freshwater transports are important for
understanding the global climate system in terms of energy and water budgets
(e.g. sea level change, ocean hydrological cycle). The meridional heat
transport of the different overturning cells can be evaluated using the
overturning streamfunction calculated in latitude–temperature coordinates
In the present study, we investigate the regional and global change of the THC between the LGM and the present day from a numerical experiment. We use a combination of streamfunctions computed in various coordinate systems to examine the thermohaline transformation, the transport of heat and freshwater and the timescale of the circulation between the glacial and interglacial period. The ocean model is integrated long enough to investigate the deep ocean circulation. After introducing the experimental design in Sect. 2, the streamfunctions in geographical coordinates and the thermohaline streamfunction are presented and discussed with regard to other climate simulations and proxy-reconstructions.
Streamfunctions were computed from the three-dimensional temperature,
salinity and velocity fields originating from integrations carried out with
the ocean general circulation model NEMO
The two following experiments were designed:
A present-day ocean-only hindcast simulation forced by an ERA40-based
atmospheric forcing covering 1958 to 2006 A LGM ocean-only simulation forced by a 49 yr long atmospheric
forcing and an initial state extracted from a coupled experiment by
A comparison between the ERA-40 and the LGM atmospheric forcing is provided
in the Supplement (Fig. S1) as well as in
Maximum volume transport in the LGM
NEMO was run for a period of 1000 years by periodically repeating the surface
atmospheric forcing. Our analysis is based on the last 50 years of each
experiment. The simulations have a weak drift after 1000 years (see Fig. S2,
in the Supplement). The globally averaged temperature trends in the upper
1000 m were less than 0.05
In this section, the THCs in LGM
The barotropic streamfunction gives the vertically averaged circulation in
longitude–latitude coordinates. In this coordinate system, the circulation
consists of basin-scale gyres (Fig.
Barotropic streamfunctions for
The THC is commonly investigated in the latitude–depth coordinates. In this
coordinate system (see Fig.
LGM and PD meridional overturning circulation in latitude–depth
coordinates superimposed on the temporally and zonally averaged salinity in
the Global Ocean
The tropical cells in LGM
The maximum transport of the AMOC in the Paleoclimate Modelling
Intercomparison Project Phase 2 models ranged between 9 and 12 Sv, while in
the AABW it ranged between 5 and 10 Sv. The reduced overturning at high
latitudes and the boundary between the NADW and the AABW near 1500 m as
simulated in LGM
Paleo-proxy reconstructions agree on a shallower NADW and a larger intrusion
of the AABW in the North Atlantic during the LGM
Most cells identified in latitude–depth coordinates are recovered in the
latitude–density, the latitude–temperature and the latitude–salinity
coordinates. The tropical cells in LGM
LGM and PD meridional overturning circulation in latitude–neutral
density coordinates for
Important changes in the volume, heat and freshwater transports take place in
the Atlantic Ocean and the Southern Ocean. The maximum of the AMOC varies
between the coordinate systems. It is larger in PD
LGM and PD meridional overturning circulation in
latitude–temperature coordinates for
LGM and PD meridional overturning circulation in latitude–salinity
coordinates for
Compared to the averaging in latitude–depth coordinates, the circulation
associated with the Deacon Cell is reduced by about 40 % in
latitude–density (Fig.
This section presents the THC in thermohaline coordinates, the volumetric
distribution in the
The ocean circulation in thermohaline coordinates consists of three main
cells (Fig.
Thermohaline streamfunction computed for
The tropical cell in PD
The large-scale transport in the Conveyor Belt cell (e.g. the transport
between the the Indo-Pacific surface-waters (16 the waters cool near 36.5–37 PSU, become fresher (between they sink to the deep ocean; finally they upwell in the North Pacific basin as cold and fresh waters.
Smaller-scale transformations exist in the Indo-Pacific and Southern Oceans.
For instance, the maximum volume transport in LGM
Schematic illustration of the water cycle in the various thermohaline cells.
The AABW cell is particularly strong in LGM
The different thermohaline regimes between LGM
For each simulation, the shortest turnover times (see Appendix A3.2) are
found in the tropical cell (Fig.
Sea-water volume density distribution projected in the
temperature–salinity diagram for
The morphology of the thermohaline circulation during the LGM and the present day is herein presented from numerical experiments and streamfunctions projected in various coordinate systems. We found that important changes between the LGM and the present-day THC take place in the Atlantic basin, the Southern Ocean and in the abyss which are consistent with paleo-proxy reconstructions. In comparison to the present day, the mean thermocline depth is shallower during the LGM. Below this thermocline, the ocean is filled with the most saline waters originating from the Southern Ocean. Near the surface, the volume transports are about 10 % larger in the tropical cells due to the larger surface wind stress. Consequently, the maximum transport of heat in the tropics is between 15 to 25 % larger during the LGM. The Gulf Stream has a more zonal propagation, reducing the heat transport at high latitudes by almost 50 %. The AMOC is shallower but its strength is similar to the present day. The circulation in the AABW cell is more vigorous in the Southern Ocean and occupies more volume than under present-day conditions. In the North Pacific and North Atlantic basins, the deep circulation is almost sluggish due to the weak meridional density gradients.
Turnover times (in years) in each stream layer of the thermohaline
streamfunction computed for the
The circulations in latitude–salinity and thermohaline coordinates illustrate
the different haline regimes between the glacial and the interglacial
periods. They also highlight the Atlantic and Southern oceans as regions of
important rearrangement. The thermohaline structure in LGM
The present study shows that the maximum transport of volume, heat and
freshwater by the main ocean overturning cells depend strongly on the choice
of coordinate system. For instance, the maximum of the AMOC varies between 9
and 16 Sv during the LGM, and between 12 to 19 Sv for the present day. The
new thermohaline streamfunction is a powerful tool of analysis to investigate
and summarise the thermohaline structure between different model
integrations. It also allows us to estimate the timescales (turnover times)
of the ocean water cycles. The changes in the ocean thermohaline regime
between the LGM and the present day raise some interesting questions about
the ocean's role for controlling the atmospheric CO
The barotropic streamfunction is the vertically integrated volume transport
in Sv (
The barotropic streamfunctions in LGM
The streamfunction in latitude–depth coordinates is, at a given latitude, the
volume transports in Sv (
The MOC in latitude–depth coordinates in LGM
The streamfunction in latitude–density coordinates is, at a given latitude,
the volume transport in Sv (
The Meridional overturning circulation (MOC) in latitude–density coordinates
in LGM
The streamfunction in latitude–temperature coordinates is, at a given
latitude, the volume transport in Sv (
The integral of the transports along each isotherm at a given latitude is an
estimate of the advective meridional heat transport in PetaWatt
(PW)
The MOC in latitude–temperature coordinates in LGM
The streamfunction in latitude–salinity coordinates is, at a given latitude,
the volume transport in Sv (
The integral of the transports along each isohaline at a given latitude is an
estimation of the meridional freshwater transport in Sv (Eq.
The MOC in latitude–salinity coordinates and the associated transports of
freshwater in LGM
The thermohaline streamfunction (Eq.
Using this representation allows us to evaluate the transport of heat in PW
within a specific isohaline range (Eq.
The thermohaline streamfunction and the associated heat and freshwater
transports in LGM
The thermohaline streamfunction and the volumetric distribution in the
temperature-salinity diagram makes possible an estimate of the turnover time
This work has been financially supported by the Bert Bolin Centre for Climate Research and by the Swedish Research Council. The Swedish National Infrastructure for Computing (SNIC) is gratefully acknowledged for providing the computer resources on the Vagn and Ekman facilities funded by the Knut and Alice Wallenberg Foundation. We would like to thank three anonymous reviewers for their constructive comments on the paper. Edited by: M. Hecht