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Drake Passage

Sustained Observation Summary

For nearly 20 years, Drake Passage has been the most comprehensively observed part of the Southern Ocean: it provides the link between the Pacific and Atlantic Oceans, and it constricts the Antarctic Circumpolar Current to a narrow geographical region, which makes this the best place to measure its properties. The continent-to-continent measurements described below are unique: no other ocean basin has such good resolution over such a time period. The volume of water carried by the Antarctic Circumpolar Current dwarfs the Gulf Stream, so even small future changes in its properties could have a profound impact on the rest of the world's oceans: but the longer the record grows, the better will be our ability to detect gradual long-term changes within the natural ocean variability. The measurements have been made as part of successive NERC core and strategic research programs, and the present funding is allocated as National Capability under Sustained Observation Work Package 6, in Theme 10 of Oceans 2025.

The two figures above show the variation of volume transport since the present series of measurements began, and the average temperature of the water being carried through Drake Passage. The temperature panel shows both the raw observations, and the data after adjustment for seasonal variation. Although these two properties have remained stable, other more subtle changes have been identified. Further explanation of the calculations can be found lower down the page.

Drake Passage and the Southern Ocean

The Southern Ocean is unlike any other on the planet, due to its uninterrupted circumnavigation of the Antarctic continent. Subsequently it provides a link for the Atlantic, Pacific and Indian Oceans. This therefore makes the Southern ocean extremely important in terms of climate forcing, because it is the conduit through which heat and freshwater, as well as other biogeochemical properties, are transported between the major ocean basins. Not only is the Southern Ocean important in the transport of water between the major oceans, but it also plays an important role in the ventilation of ocean interiors via the meridional overturning of watermasses. Although the Southern Ocean forms a continuous band around the high Southern latitudes, it is not of uniform width. Instead it is marked by several “chokepoints”, defined by WOCE (World Ocean Circulation Experiment). One such chokepoint is the focal interest of this section; Drake Passage.

Drake Passage is the narrowest stretch of water in the Southern Ocean, spanning approximately 500miles between the Southern tip of South America and the West Antarctic Peninsula. A hydrographic section was designated as SR1 by WOCE in Drake Passage. In a recent cruise, JC031 sections SR1 and SR1B were occupied which added to occupations in previous years. SR1b has been occupied annually since 1993 except for two years (1995 and 1998). SR1 has previously been covered twice, once by METEOR in 1990 and once by the RRS James Clark Ross in 1999.

In addition to the SR1 and SR1b hydrographic sections occupied in Drake Passage, DIMES is another important project taking place in the Southern Ocean, with the aim of measuring the diapycnal and isopycnal mixing occuring here.

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Sustained observation in Drake Passage

A sustained measurement program has been carried out in Drake Passage since November 1993, led by NERC scientists. A variety of programs over the years have enabled this work to continue, most recently under NERC's Oceans 2025 program. The work was initially led by scientists from the Institute of Oceanographic Sciences at Wormley, and more recently by teams from the National Oceanography Centre at Southampton. The main partners have been from NERC's British Antarctic Survey; many students from UK Universities have also taken part, most notably from the University of East Anglia. A large number of scientists have led or participated in the 16 cruises to date, including principal contributions from B. King, S. Alderson, S. Cunningham, M. Brandon, M. Meredith S. Bacon, A. Williams, M. Sparrow, K. Stansfield, G. Quartly H. Venables, M. Yelland and E. McDonagh.

The Drake Passage cruises are presently supported by NERC's National Capability funding. Each cruise is relatively short, being at sea for typically three weeks rather than the six weeks that is more commonly required for a major hydrographic expedition. In addition to establishing a unique time series of ocean measurements, the cruises have fostered collaboration between NERC institutes and HEI groups, have provided many PhD students with an opportunity to experience deep-ocean physical oceanography fieldwork and have been used to test or develop new measurement techniques.

Oceans 2025 - Theme 10

In addition to the work that is done under Theme 1 of Oceans 2025, Theme 10 also plays an important role in ensuring that this work continues. Theme 10 has set up 14 "sustained observations" (SO) which cover many different regions including the open oceans and coastal seas. Drake Passage falls into sustained observation 6 (SO6) which focuses on the Antarctic Circumpolar Current.

The objectives highlighted within SO6 are:

  • maintain an annual hydrographic section across the ACC at Drake Passage based on physical and biogeochemical measurements. (i.e. section SR1b)
  • provide datasets to test whether models produce the correct level of interannual variability
  • maintain the ocean bottom pressure and temperature time series from Drake Passage
  • adapt the bottom pressure monitoring system to maximise the value of other observational programmes in the region

For access to the entire Oceans 2025 Theme 10 document click here

Below is a table of all the Drake Passage repeat sections that have been occupied by the UK since 1993.
Links for the respective cruise reports and data are clickable.

The table also includes summaries of the volume transport and mean temperature of water carried through Drake Passage by the Antarctic Circumpolar Current. The transport calculation is based on geostrophic velocity shear between pairs of stations. For each station pair, the geostrophic shear is calculated and referenced to zero at the deepest common level shared by adjacent stations. The resulting velocity is profile is multiplied by the distance between stations to give the volume transport between those stations, and the velocity field is then accumulated across the section to give an estimate of the total transport for each year that the section was occupied. Total volume transport is measured in units of Sverdrups (abbreviation Sv). One Sv is one million cubic metres per second. The average of all sections is 136.6 Sv with a standard deviation of 6.7 Sv. There is no statistically significant trend over the 17 years spanned by these measurements. Long-term trends are therfore either too small to detect from this length of time series, or masked by the inetrannual variability

One of the reasons we have sustained measurements in Drake Passage is because we are interested in heat exchanged between the Pacific and Atlantic basins. The average temperature of water carried across the section is shown in the next column of the table. The average temperature in any year is determined by multiplying each element of volume transport by the temperature of that element, and dividing by the total volume transport. This is referred to as the transport-weighted mean temperature. Since we have made measurements in months between November and February, we are able to detect a seasonal variation. The four months difference between the beginning of November (early southern spring) and the end of February (southern summer) account for 0.4°C in the average temperature of the water. It is important therefore to compensate for this effect before looking for trends in the data. The mean temperatures in the table allow for the seasonal variation. The mean temperature of all the years is 2.18°C, with a standard deviation of 0.07°C. The two seasons 2002/03 and 2004/05 are unusually cold, at 1.94°C and 2.03°C respectively. The reasons for this are being investigated. The standard deviation of the remaining temperatures in the table is 0.03°C, without a statistically significant trend in the data between 1993/4 and 2008/9.

Although the total volume transport and mean temperature have remained stable during this series of observations, other properties have not. The paper listed below by Naveira Garabato et al. analyses decadal changes in several water properties in Drake Passage.

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Drake Passage Cruises

Season Start Date* End Date* Cruise Code Report Data Baroclinic Transport (Sv)** Mean Potential Temperature (degC) (ITS-90)***
1993/94 21/11/1993 26/11/1993 JR0a/JR00_1 Click here Click here 135 2.21
1994/95 15/11/1994 21/11/1994 JR0b/JR00_2 Click here Click here 142 2.14
1995/96 no cruise
1996/97 15/11/1996 20/11/1996 JR16 Click here Click here 126 2.18
1997/98 29/12/1997 07/01/1998 JR27 Click here Click here 147 2.13
1998/99 no cruise
1999/00 12/02/2000 17/02/2000 JR47 Click here Click here 144 2.17
2000/01 22/11/2000 28/11/2000 JR55 Click here Click here 143 2.20
2001/02 20/11/2001 26/11/2001 JR67 Click here Click here 141 2.15
2002/03 27/12/2002 01/01/2003 JR81 Click here Click here 136 1.94
2003/04 11/12/2003 15/12/2003 JR94 Click here Click here 146 2.21
2004/05 02/12/2004 08/12/2004 JR115 Click here Click here 128 2.03
2005/06 07/12/2005 12/12/2005 JR139 Click here Click here 133 2.20
2006/07 08/12/2006 12/12/2006 JR163 Click here Click here 127 2.15
2007/08 30/11/2007 05/12/2007 JR193 Click here Click here 136 2.14
2008/09 13/12/2008 18/12/2008 JR194 Click here Click here 132 2.24
2008/09 20/02/2009 26/02/2009 JC031 Click here Click here 133 2.18
2009/10 19/11/2009 26/11/2009 JR195 Click here Click here

(*start and end date of measurements)

(**relative to deepest common level)

(***seasonally adjusted and transport weighted)

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Upcoming Cruises in Drake Passage

JR242 Nov 2010, Principal Scientist: Margaret Yelland
Start/End Date - Early December/21st Dec 2010
Ports - Starting in Rothera completing a Northward section across Drake Passage, finishing in Stanley in the Falklands.

UK DIMES 2, Principal Scientist: Alberto Naveira Garabato
December 2010/January 2011
RRS James Clark Ross
Scotia Sea Survey (boundaries)

UK DIMES 3, Principal Scientist: Andy Watson
February 2012
RRS James Cook
Scotia Sea Survey (full tracer)

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Follow the link to view a Drake Passage bibliography

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Site created by David Hamersley
Site last updated 09:02, 17 September 2010