Search Results

You are looking at 1 - 5 of 5 items for :

  • Author or Editor: Alberto C. Naveira Garabato x
  • Journal of Climate x
  • Refine by Access: All Content x
Clear All Modify Search
Michael P. Meredith
,
Alberto C. Naveira Garabato
,
Arnold L. Gordon
, and
Gregory C. Johnson

Abstract

The Southern Ocean hosts the formation of the densest layers of the oceanic overturning circulation and provides a climatically sensitive element of deep ocean ventilation. An oceanographic section across the eastern Scotia Sea occupied in 1995, 1999, and 2005 reveals significant variability in the deep and bottom waters of Southern Ocean origin. Warming (∼0.1°C) of the warm midlayer waters in the Scotia Sea between 1995 and 1999 reversed through to 2005, reflecting changes seen earlier upstream in the Weddell Sea. The volume of deep waters with potential temperature less than 0°C decreased during 1995–2005, though such a reduction was only clear between 1995 and 1999 at the southern end of the section. The abyssal waters of the eastern Scotia Sea changed circulation between 1995 and 1999, with the dominant point of their entry to the basin shifting from the south to the northeast; by 2005, the former route had regained dominance. These changes are best explained by interannual variations in the deep waters exiting the Weddell Sea, superimposed on a longer-term (decadal) warming trend. The interannual variations are related to changes in the strength of the Weddell Gyre, reflecting large-scale atmospheric variability that may include the El Niño–Southern Oscillation phenomenon. The Scotia Sea is the most direct pathway for dense waters of the overturning circulation emanating from the Weddell Sea to fill much of the World Ocean abyss. The regional changes reported here have the potential to affect the climatically significant ventilation of the global ocean abyss.

Full access
Alberto C. Naveira Garabato
,
Loïc Jullion
,
David P. Stevens
,
Karen J. Heywood
, and
Brian A. King

Abstract

A time series of the physical and biogeochemical properties of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the Drake Passage between 1969 and 2005 is constructed using 24 transects of measurements across the passage. Both water masses have experienced substantial variability on interannual to interdecadal time scales. SAMW is formed by winter overturning on the equatorward flank of the Antarctic Circumpolar Current (ACC) in and to the west of the Drake Passage. Its interannual variability is primarily driven by variations in wintertime air–sea turbulent heat fluxes and net evaporation modulated by the El Niño–Southern Oscillation (ENSO). Despite their spatial proximity, the AAIW in the Drake Passage has a very different source than that of the SAMW because it is ventilated by the northward subduction of Winter Water originating in the Bellingshausen Sea. Changes in AAIW are mainly forced by variability in Winter Water properties resulting from fluctuations in wintertime air–sea turbulent heat fluxes and spring sea ice melting, both of which are linked to predominantly ENSO-driven variations in the intensity of meridional winds to the west of the Antarctic Peninsula. A prominent exception to the prevalent modes of SAMW and AAIW formation occurred in 1998, when strong wind forcing associated with constructive interference between ENSO and the southern annular mode (SAM) triggered a transitory shift to an Ekman-dominated mode of SAMW ventilation and a 1–2-yr shutdown of AAIW production.

The interdecadal evolutions of SAMW and AAIW in the Drake Passage are distinct and driven by different processes. SAMW warmed (by ∼0.3°C) and salinified (by ∼0.04) during the 1970s and experienced the reverse trends between 1990 and 2005, when the coldest and freshest SAMW on record was observed. In contrast, AAIW underwent a net freshening (by ∼0.05) between the 1970s and the twenty-first century. Although the reversing changes in SAMW were chiefly forced by a ∼30-yr oscillation in regional air–sea turbulent heat fluxes and precipitation associated with the interdecadal Pacific oscillation, with a SAM-driven intensification of the Ekman supply of Antarctic surface waters from the south contributing significantly too, the freshening of AAIW was linked to the extreme climate change that occurred to the west of the Antarctic Peninsula in recent decades. There, a freshening of the Winter Water ventilating AAIW was brought about by increased precipitation and a retreat of the winter sea ice edge, which were seemingly forced by an interdecadal trend in the SAM and regional positive feedbacks in the air–sea ice coupled climate system. All in all, these findings highlight the role of the major modes of Southern Hemisphere climate variability in driving the evolution of SAMW and AAIW in the Drake Passage region and the wider South Atlantic and suggest that these modes may have contributed significantly to the hemispheric-scale changes undergone by those waters in recent decades.

Full access
Michael P. Meredith
,
Alberto C. Naveira Garabato
,
Andrew McC. Hogg
, and
Riccardo Farneti

Abstract

The sensitivity of the overturning circulation in the Southern Ocean to the recent decadal strengthening of the overlying winds is being discussed intensely, with some works attributing an inferred saturation of the Southern Ocean CO2 sink to an intensification of the overturning circulation, while others have argued that this circulation is insensitive to changes in winds. Fundamental to reconciling these diverse views is to understand properly the role of eddies in counteracting the directly wind-forced changes in overturning. Here, the authors use novel theoretical considerations and fine-resolution ocean models to develop a new scaling for the sensitivity of eddy-induced mixing to changes in winds, and they demonstrate that changes in Southern Ocean overturning in response to recent and future changes in wind stress forcing are likely to be substantial, even in the presence of a decadally varying eddy field. This result has significant implications for the ocean’s role in the carbon cycle, and hence global climate.

Full access
Loïc Jullion
,
Alberto C. Naveira Garabato
,
Michael P. Meredith
,
Paul R. Holland
,
Peggy Courtois
, and
Brian A. King

Abstract

Recent decadal changes in Southern Hemisphere climate have driven strong responses from the cryosphere. Concurrently, there has been a marked freshening of the shelf and bottom waters across a wide sector of the Southern Ocean, hypothesized to be caused by accelerated glacial melt in response to a greater flux of warm waters from the Antarctic Circumpolar Current onto the shelves of West Antarctica. However, the circumpolar pattern of changes has been incomplete: no decadal freshening in the deep layers of the Atlantic sector has been observed. In this study, the authors document a significant freshening of the Antarctic Bottom Water exported from the Weddell Sea, which is the source for the abyssal layer of the Atlantic overturning circulation, and trace its possible origin to atmospheric-forced changes in the ice shelves and sea ice on the eastern flank of the Antarctic Peninsula that include an anthropogenic component. These findings suggest that the expansive and relatively cool Weddell gyre does not insulate the bottom water formation regions in the Atlantic sector from the ongoing changes in climatic forcing over the Antarctic region.

Full access
Sally E. Close
,
Alberto C. Naveira Garabato
,
Elaine L. McDonagh
,
Brian A. King
,
Martin Biuw
, and
Lars Boehme

Abstract

The evolution of the physical properties of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the Drake Passage region is examined on time scales down to intraseasonal, within the 1969–2009 period. Both SAMW and AAIW experience substantial interannual to interdecadal variability, significantly linked to the action of the Amundsen Sea low (ASL) in their formation areas. Observations suggest that the interdecadal freshening tendency evident in SAMW over the past three decades has recently abated, while AAIW has warmed significantly since the early 2000s. The two water masses have also experienced a substantial lightening since the start of the record. Examination of the mechanisms underpinning water mass property variability shows that SAMW characteristics are controlled predominantly by a combination of air–sea turbulent heat fluxes, cross-frontal Ekman transport of Antarctic surface waters, and the evaporation–precipitation balance in the Subantarctic zone of the southeast Pacific and Drake Passage, while AAIW properties reflect air–sea turbulent heat fluxes and sea ice formation in the Bellingshausen Sea. The recent interdecadal evolution of the ASL is consistent with both the dominance of the processes described here and the response of SAMW and AAIW on that time scale.

Full access