SIO 210: Introduction to Physical Oceanography

Southern Ocean - circulation and water properties

Lynne Talley, Fall, 2016



Watch powerpoint
Download powerpoint
Download pdf

Reading

DPO: Chapter 13. (Skim, pay greater attention to sections dealt with in the powerpoint and board presentations.)

Additional resources:

Tomczak and Godfrey, chapter 6

WOCE Hydrographic Programme Southern Ocean Atlas (Orsi and Whitworth, 2005), for maps of properties at many depths and isopycnals

WOCE Hydrographic Programme Atlantic, Pacific, Indian Ocean Atlases for maps of properties at many depths and isopycnals

National Snow and Ice Data Center : nice resource for snow and ice images, data


Notes

The following notes are abbreviated. Please see powerpoint and DPO chapter.

The circumpolar region around Antarctica is open, with no boundaries at the latitude of Drake Passage; strong effect on the circulation, dynamics, water mass formation, and connectivity of the Atlantic/Pacific/Indian Oceans.

Strong westerly winds dominate wind forcing; force the Antarctic Circumpolar Current. Ekman upwelling throughout most of region, affecting circulation and water mass formation. Sverdrup dynamics north and south of Drake Passage latitudes.

Seasonal sea ice covers a large region each winter and melts far back. Ice shelves closer to the continent. Dense water formation through brine rejection, creates Antarctic Bottom Water and also significantly modifies deep waters that are less dense than this.

Circulation
The Antarctic Circumpolar Current is one of the major currents of the world and flows eastward completely around Antarctica in this open region. It is driven by the westerly winds, whose maximum strength is also in this region.
The circulation can be considered a combination of Sverdrup dynamics (north and south of Drake Passage) and zonal jet dynamics (at D.P. latitudes). North of D.P., Svedrup balance due to Ekman upwelling would create a very long west-east cyclonic gyre. The ACC accomodates this through a western boundary current along South America (Malvinas or Falkland Current) that shoots the ACC northward to about 40S, and then a long shift southwards through the Atlantic, Indian and Pacific until reaching the Chilean coast of South America.

South of Drake Passage, circulation is also cyclonic, broken into two separate cyclonic gyres in the Weddell and Ross Seas.

At Drake Passage latitudes, ACC dynamics is that of zonal jets; the momentum balance is between eastward wind stress and westward bottom pressure gradient force. The dynamics are not yet well understood.

The ACC is not a broad eastward flow but is composed of several fronts with strong currents along the fronts. Most of the ACC's eastward transport is in the fronts. These are: the Subantarctic Front, the Polar Front, the Southern ACC Front, and the Continental Water Boundary. The Subantarctic Front is marked by the existence of the salinity minimum Antarctic Intermediate Water to the north of the front and also by very thick mixed layer north of the front, called "Subantarctic Mode Water". The Polar Front lies south of the Subantarctic Front and is marked by a near-surface temperature minimum to the south of the front, resulting from strong winter cooling/ice formation.

Circulation is in essentially the same direction (nearly zonal or cyclonic) from top to near-bottom. Close to the bottom, circulation is dominated by the complex topography. Deep Western Boundary Currents carry newly-modified/formed Antarctic waters northward.

Water masses
Some upper ocean water masses:
Antarctic Surface Water: cold, fresh surface water south of the Polar Front, associated with sea ice (near-freezing temperature, relatively fresh).
Subantarctic Mode Water (SAMW): the thick mixed layers north of the Subantarctic Front. These mixed layers are thickest in the eastern Indian and across the Pacific sectors of the ACC. Their temperatures decline from 14C in the western South Atlantic (just north of the SAF which is also just east of the Falkland Loop) to 4C in the eastern South Pacific, just west of Chile where the ACC enters Drake Passage. Part of the SAMW of the southeast Indian Ocean circulates northward into the Indian Ocean subtropical gyre and is the source of the oxygen maximum layer throughout the Indian Ocean. Part of the SAMW of the southeast Pacific circulates northward into the Pacific Ocean subtropical gyre and is the source of the low salinity Antarctic Intermediate Water there. Part of the latter (SE Pacific) SAMW flows eastward through Drake Passage and is the source of the low salinity AAIW for the combined Atlantic and Indian subtropical regions.

Intermediate water: Antarctic Intermediate Water. Low salinity in the vertical, emerging from the Drake Passage region to fill the southern hemisphere subtropics and tropics. Located north of the Subantarctic Front. Sources are the SAMW and surface waters south of the Subantarctic Front.

Deep waters:
Inflows from the north:
North Atlantic Deep Water: high salinity water entering from the Atlantic Ocean
Indian and Pacific Deep Waters: low oxygen, high nutrient deep waters entering from the Indian and Pacific. PDW is relatively fresh and helps to dilute the NADW, reducing the salinity of the Circumpolar Deep Water compared with the NADW saline input.

Deep waters modified or created within the ACC region and to the south:
Upper Circumpolar Deep Water (deep water of the circumpolar current) upwells south of the ACC and is the source water for dense water formation around the Antarctic continent. Marked by low oxygen in the vertical; source waters are the IDW and PDW. Significant modification in the ACC region, towards higher oxygen,lower salinity, etc.
Dense water formation (Lower Circumpolar Deep Water and/or Antarctic Bottom Water) occurs mainly as a result of ice formation (brine rejection) over the continental shelves, mainly in active coastal polynyas. Primary sites are in the Weddell and Ross Seas, although there are also shelf water sources in the region south of Australia. The cold, dense shelf water flows down the shelves, mixing, and then out towards the east. Deep waters throughout the region are created this way, as well as the dense bottom water. Its pathways are strongly affected by bottom topography.

Selected figures

Atlantic Ocean meridional section at about 25W
Potential Temperature,
Salinity,
Potential Density (0 dbar),
Potential Density (4000 dbar)
Neutral Density (Jackett and McDougall)
Oxygen (umol/kg)
Silicate (umol/kg)

The data were described in previous lectures.

Pacific Ocean meridional section at about 150W (From the Pacific WHP Atlas) Look at:
Potential Temperature,
Salinity,
Potential Density (relative to 0 dbar in upper panel, to 0/2000/4000 dbar in lower panel)
Neutral Density (Jackett and McDougall)
Oxygen (umol/kg),
Silicate (umol/kg).

South Pacific Ocean zonal section at about 17S (from the Pacific WHP Atlas) Look at:
Potential Temperature,
Salinity,
Potential Density (relative to 0 dbar in upper panel, to 0/2000/4000 dbar in lower panel)
Neutral Density (Jackett and McDougall),
Oxygen (umol/kg),
Silicate (umol/kg).

Indian Ocean meridional sections at about 80E and 95E (from the Indian WHP Atlas) Look at
Potential Temperature,
Salinity,
Potential Density (0 dbar),
Potential Density (4000 dbar)
Oxygen (umol/kg),
Silicate (umol/kg)


Study questions


1. Are there subpolar (cyclonic) wind-driven "gyres" in the southern hemisphere?

2. Why are the properties in the three southern hemisphere oceans fairly similar compared with those in the northern hemisphere?

3. What distinguishes the Antarctic Circumpolar Current from all of the other strong (non-equatorial) currents?

4. What are the major fronts of the Antarctic Circumpolar Current? How do you distinguish them from each other on a north-south transect across them?

8. How and where does the Antarctic Circumpolar Current deviate from being exactly zonal (following a single latitude)? Where is it farthest north? Where is it farthest south?


SIO 210 HOME Last modified: Nov. 28 2016