Sustained observations in Drake Passage

High-quality hydrographic measurements have been made on the SR1b line across Drake Passage in almost every summer since 1993/1994, enabling scientists to monitor changes and study the dynamics of this important section of the Southern Ocean.  

People and Institutions

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 (NOC) at Southampton.  Many scientists and students from the NOC and other institutions, including NERC's British Antarctic Survey and the University of East Anglia, have taken part in the 24 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, E. McDonagh, A. Watson, A. Meijers, and J.-B. Sallée. 

Yvonne Firing is the current principal scientist for these measurements.  

Cruises and Data

A list of cruises that have included occupations of SR1b is here, along with links to cruise reports and data.  Each cruise is relatively short, typically around three weeks at sea rather than the six weeks more commonly required for a major hydrographic expedition.  The SR1b section has often been combined with other work in the area, such as ACCLAIM and DIMES.  

In addition to establishing a unique time series of ocean measurements leading to interesting science results, the cruises have fostered collaboration between NERC institutes and Higher Education Institute 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.

Results

The twenty-one complete occupations of SR1b to date have enabled the scientific community to monitor interannual variability and trends in ACC transports of water and heat (below) as well as the properties and distribution of different water masses found in Drake Passage, and to study a range of oceanic processes and ocean-atmosphere interactions, from the role of eddies in turbulence and mixing, to modulation of the meridional overturning circulation.  Data from SR1b have been used directly in numerous journal publications and theses, as well as being incorporated into ocean atlases.   

Annual occupations of SR1b enable us to monitor the transport of the ACC, detecting everything from meandering of the ACC fronts to potential trends associated with changing Southern Hemisphere winds.  Another reason we have sustained measurements in Drake Passage is that we are interested in heat exchanged between the Pacific and Atlantic Basins.  


The transport calculation is based on geostrophic velocity shear between pairs of stations. For a single occupation, geostrophic shear can be referenced to zero at the deepest common level shared by adjacent stations on that occupation (this method was used for the values in the cruise table). For comparing multiple different occupations, we use a "time-series deepest common level", the shallowest of all occupations' deepest common levels at that point along the section (gray stepped line). The resulting velocity 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 million cubic metres per second, or Sverdrups (Sv).

For the purposes of studying heat exchanged through Drake Passage, 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.

 

This latitude-depth plot compares temperature and salinity from the cruise in January 2016, JR15003, with the average temperature and salinity over all 21 occupations. In-situ temperature and calibrated salinity from stations sampled on JR15003 are plotted as coloured dots. The time mean temperature and salinity are contoured with the same colour scale (every 1oC and every 0.2 psu), while mean neutral density contours (isopycnals) are plotted in black (every 0.2, as labelled).

The surface water temperature minimum in the southern part of the passage is visible in the mean contours, as is the Circumpolar Deep Water salinity maximum at mid-depths.  The time average smears out the Polar and Subantarctic Fronts, so that they appear as a single region of sloping isopycnals between 58 S and the northern boundary.  In the JR15003 section, however, the Polar Front is well to the north of its mean position, with the result that temperatures along the section are colder than average.  

 

Since we have made measurements in months between November and April, 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.

There is no statistically significant trend in either volume or temperature transport between 1993-1994 and 2015-2016, indicating that any long-term trend that is present is either too small to detect in the 22 years spanned by these measurements, or masked by the interannual and shorter-time-scale variability.  

Although the total volume transport and mean temperature have remained stable during this series of observations, other properties have not. Some of the works listed in the publications section of this website analyse interannual to decadal changes in water properties in Drake Passage.