Deep Sea Research Part II: Topical Studies in Oceanography
Warming of deep and abyssal water masses along the Greenwich meridian on decadal time scales: The Weddell gyre as a heat buffer
Introduction
The overturning circulation of the world oceans is a powerful process to store heat and gases in the deep ocean and as such an important component of the climate system. As part of this, the Southern Ocean is pivotal, connecting all major oceans basins (e.g., Rintoul et al., 2001, Schmitz, 1996). In the Southern Ocean, shoaling Circumpolar Deep Water (CDW) feeds into a shallow meridional overturning cell leading to the formation of intermediate waters, and a deep cell transforming Circumpolar Deep Water and forming Antarctic Bottom Water (AABW). A major contribution to the deep and bottom water formation is made by the Atlantic sector, i.e., the Weddell Sea (Carmack, 1977, Orsi et al., 1999, Rintoul, 1998). This water mass transformation is controlled by the transport of source waters into the Weddell Sea, processes within the Weddell Sea and the transport of modified water out of it (Foster et al., 1987, Gill, 1973, Mosby, 1934). In the Weddell Sea, CDW enters from the east and the north and circulates as Warm Deep Water (WDW) in intermediate depths within the large-scale cyclonic gyre (Fig. 1). This flow regime is confirmed on the basis of data from vertically profiling floats deployed on the Greenwich meridian and in the Weddell Sea (Fig. 2) and moorings in the northern and southern limb of the gyre (Fig. 3). WDW is the main source water of the other major water masses of the Weddell gyre, namely the Weddell Sea Bottom Water (WSBW) and Weddell Sea Deep Water (WSDW).
Observations have demonstrated that the global ocean has been warming (Gouretski and Koltermann, 2007, Levitus et al., 2005). In the Southern Ocean a warming trend over several decades is identified in the upper parts of the Circumpolar Deep Water (Böning et al., 2008, Gille, 2008, Sprintall, 2008). There is evidence that the Antarctic Bottom Water is subject to change, albeit becoming colder and fresher (Aoki et al., 2005, Johnson et al., 2008, Rintoul, 2007) in the Indian and Pacific sectors of the Southern Ocean. Warming of AABW was observed in the Atlantic Ocean in the Vema Channel (Zenk and Morozov, 2007), near the equator (Andrié et al., 2003), and basin wide (Purkey and Johnson, 2010). We explore the processes that determine the way in which global warming affects water mass formation in the Weddell Sea, the latter being a major source of AABW.
It has been long known that WDW shows significant variations from year to year (e.g., Gordon, 1982). With long term observations based on repeat hydrographic data, decadal variations may be examined. Warming and a salinity increase of the WDW observed during the 1990s were followed by cooling during the 2000s (Fahrbach et al., 2004, Robertson et al., 2002). The variations are most likely due to changes in the inflow from the circumpolar water belt, which are induced by changes in atmospheric forcing conditions. Various modes of climate variability in the Southern Ocean are known, of which the Southern Annular Mode (Hall and Visbeck, 2002, Thompson and Solomon, 2002) is the most important, and these may affect the Weddell Sea. Kerr et al. (2009) indeed suggested a correlation between the Southern Annular Mode and the Weddell Sea Bottom Water. Sallée et al. (2010) show that a recent trend in the SAM leads to a more southern path of the westerly winds and changes in mixed layer depths. Whereas the variations of properties of the Weddell Sea Deep Water have long time remained within the uncertainty of our observations, the observed changes in the Weddell Sea Bottom Water were found to be significant (Fahrbach et al., 2004). Since the WDW is one of the source components of bottom water, the variations of the two water masses are possibly related through the formation process; using our unique repeat section time series we explore also other explanations.
We use the term Weddell gyre for the circulation system reaching from the Weddell Sea up to 30°E into the Enderby basin in the Indian sector and the term Weddell system for the combination of advective and transformation processes which in the Weddell gyre acts to produce from the inflowing source water the deep water masses contributing to the global overturning. In this study we show that even when the properties of WDW in the Weddell gyre are subject to decadal fluctuation, the excess of heat and salt transported during such events is stored in the deep and abyssal water masses inducing an increase of temperature and salinity. It follows that the processes in the Weddell system, despite of producing cold and low salinity Weddell Sea Bottom Water, transfer heat and salt from the CDW directly into the Weddell Sea Deep Water without necessarily affecting the bottom water production.
Section snippets
The data
Almost all data were collected during cruises with the ice breaker RV Polarstern between 1992 and 2008 (except for the AJAX cruise in 1984). This warrants a high consistency of the data set. The data set obtained in 2008 (Fahrbach and De Baar, 2010) was part of an International Polar Year project called Climate of Antarctica and the Southern Ocean (CASO; http://classic.ipy.org/development/eoi/proposal-details.php?id=132). Some sections were occupied as part of the WOCE Repeat Sections Program (
The mean water mass properties
The section of potential temperature along the Greenwich meridian obtained during cruise ANT-XXIV/3 in 2008 and the first section occupied by Polarstern during cruise ANT-X/4 in 1992 serve as a background for the hydrographic situation (Fig. 4). Detailed accounts on the hydrography of the region have been published by Deacon (1979), Whitworth and Nowlin (1987), Orsi et al. (1993), Schröder and Fahrbach (1999) and Fahrbach et al. (2004). The northern boundary of the Weddell gyre at this
Potential causes of the variations
There are two processes that are potentially responsible for the variations of the deep water mass properties in the Weddell gyre:
- •
Variations of the inflow of CDW from the ACC into the Weddell gyre.
- •
Variation of the circulation within the gyre.
Variations in the inflow of CDW during the warming phase of the 1990s were discussed in Fahrbach et al. (2004), where especially instabilities of the Weddell gyre boundary were found to have an impact. Smedsrud (2005) emphasized ocean–ice–atmosphere
Conclusions
Pronounced variations on a multi-annual time scale occurred in the properties of all water masses on the Greenwich meridian over a time period of 24 years. The mean WDW temperature increased until the mid-1990s and decreased until 2005. A connection between WDW trends in the Weddell Sea and the adjacent, downstream Scotia Sea may exist (Jullion et al., 2010, Meredith et al., 2008). The mean temperature of the WSDW and the WSBW on the Greenwich meridian has generally risen from the 1980s till
Acknowledgments
This work is based on seven cruises of FS Polarstern. We are grateful to the masters and crews for their ongoing and most dedicated support. We are not able to cite all those by name who contributed by their continuous efforts on land to keep Polarstern in operation. We want to thank the reviewers who provided us a great deal of very helpful suggestions. In particular they pushed us towards using neutral density surfaces as water mass boundaries, which increased clearly the visibility of some
References (80)
- et al.
On the flow of water out of the Weddell Sea
Deep-Sea Res.
(1975) - et al.
Circulation and transport of water masses in the Lazarev Sea, Antarctica, during summer and winter 2006
Deep-Sea Res. I
(2011) The Weddell Gyre
Deep-Sea Res
(1979)Mooring Design & Dynamics—a Matlab® package for designing and analyzing oceanographic moorings
Mar. Models
(1999)- et al.
Causes of deep-water variation: comment on the paper by L.H. Smedsrud ‘‘Warming of the deep water in the Weddell Sea along the Greenwich meridian: 1977–2001’’
Deep-Sea Res. I
(2006) - et al.
Frontal zone mixing and Antarctic Bottom Water formation in the southern Weddell Sea
Deep-Sea Res.
(1976) - et al.
Mixing and bottom water formation in the shelf break region of the southern Weddell Sea
Deep-Sea Res
(1987) Circulation and bottom water production in the Weddell Sea
Deep-Sea Res.
(1973)- et al.
The wind-driven circulation in the Weddell–Enderby basin
Deep-Sea Res
(1981) - et al.
Weddell Gyre: structure of the eastern boundary
Deep-Sea Res. I
(1993)
Frontal structure and Antarctic Bottom Water flow through the Princess Elizabeth Trough, Antarctica
Deep-Sea Res. I
The transport of the Weddell Gyre across the Prime Meridian
Deep-Sea Res. II
Modification and pathways of Southern Ocean Deep Waters in the Scotia Sea
Deep-Sea Res. I
On the export of Antarctic Bottom Water from the Weddell Sea
Deep-Sea Res. II
Seasonal variability of the Antarctic Coastal Current and its driving mechanisms in the Weddell Sea
Deep-Sea Res. I
Circulation, mixing, and production of Antarctic Bottom Water
Progr. Oceanogr
On the circulation and stratification of the Weddell Gyre
Deep-Sea Res. I
An improved calibration method for the drift of the conductivity sensor on autonomous CTD profiling floats by θ–S climatology
Deep-Sea Res. I
Long-term temperature trends in the deep waters of the Weddell Sea
Deep-Sea Res. II
Studies of deep water formation and circulation in the Weddel Sea using natural and anthropogenic tracers
Mar. Chem
On the structure and the transport of the eastern Weddell Gyre
Deep-Sea Res. II
Warming of the deep water in the Weddell Sea along the Greenwich meridian: 1977–2001
Deep-Sea Res. I
Long-term trends and interannual variability of temperature in Drake Passage
Progr. Oceanogr
Variability of AABW properties in the equatorial channel at 35°W
Geophys. Res. Lett.
Freshening of the Adélie Land Bottom Water near 140°E
Geophys. Res. Lett.
Weddell Gyre: temperature maximum stratum
J. Geophys. Res.
The response of the Antarctic Circumpolar Current to recent climate change
Nat. Geosci
Water characteristics of the Southern Ocean south of the Polar Front
Carbon Chemistry of the South Atlantic Ocean and the Weddell Sea: The Results of the Atlantic Long Lines (AJAX) Expeditions, October, 1983–February, 1985
Stability of IAPSO standard seawater
J. Atmos. Ocean. Technol
Decadal-scale variations of water mass properties in the deep Weddell Sea
Ocean Dyn.
Formation and discharge of deep and bottom water in the northwestern Weddell Sea
J. Mar. Res.
Transport and structure of the Weddell Gyre
Ann. Geophys.
Ice shelf water overflow and bottom water formation in the southern Weddell Sea
J. Geophys. Res.
Southern Ocean warming due to human influence
Geophys. Res. Lett.
Warming of the Southern Ocean since the 1950s
Science
Decadal-scale temperature trends in the southern hemisphere ocean
J. Climate
Cited by (86)
Recent changes and distribution of the newly-formed Cape Darnley Bottom Water, East Antarctica
2022, Deep-Sea Research Part II: Topical Studies in OceanographyOngoing ocean warming threatens the rich and diverse macrobenthic communities of the Antarctic continental shelf
2019, Progress in OceanographySeasonal and Interannual Variability Between Upper Ocean Processes and the Slope Current in the Region Around the Cosmonauts Sea Off East Antarctica
2024, Journal of Geophysical Research: OceansThe Role of Bottom Friction in Mediating the Response of the Weddell Gyre Circulation to Changes in Surface Stress and Buoyancy Fluxes
2024, Journal of Physical OceanographyA Southern Ocean supergyre as a unifying dynamical framework identified by physics-informed machine learning
2023, Communications Earth and Environment