Intermediate and deep ocean circulation Flashcards
Average depth of the ocean
3.5-3.8km
Low latitudes vs high latitudes
Net warmingin low latitudes and net cooling in high latitudes
Mid- latitiude waters
Warmer and more saline than interior waters
Weak thermocline
High intrinsic density= cooled waters sink
–> Common in high northern latitudes
Halocline
Vertical zone in water where there is a rapid change in salinity
Ocean surface waters mixing
Wind-driven waves and Ekman spiral cause mixing meaning T and S change little with depth
Flow in surface waters
Dominated by wind stress
Equatorial upper water- equatorial winds
Trade winds blow west in the tropics–> warm surface water piles up on W of ocean basins–. creates strong eastward currents –> Coriolis deflects moving water
Thermocline- equatorial regions
DEEP thermocline - WESTERN side
SHALLOW thermocline- EASTERN side
Central Open waters
While surface water sinks in subtropical gyre centers, Ekman transport driven by the gyre’s wind system piles up saltier water in mid-latitudes due to convergence.
Seasonal effects on mixing
In mid-latitudes, warming produces a warm and saline layer of surface water which is thickened and mixed but remains buoyant. In winter, water cools and sinks–> causes vigorous mixing
Central Ocean Water (COW) boundaries
COW= warmer and saltier therefore stays distinct from surrounding water masses down to 1,000 meters. Can be separated by underwater features like island chains. The Coriolis effect and wind patterns create separate bodies of COW in the N and S Hemispheres.
Western NACW
–> Forms by sinking and mixing of surface water in the Sargasso Sea gyre; salinity 35.5-36.5 PSU; temp 7-20C
–> SACW; 5-18C, 34.3-35.8 PSU
Arctic and Antarctic Upper Water
Direct wind drag causes convergence and piling up surface waters around polar front
Temperature contrast between surface and deep water
= small in high latitudes so the distinction between Surface, Upper and Intermediate
–> Arctic and Antarctic Upper Waters = surface occurrence of the corresponding Intermediate Waters
Intermediate water masses
Denser than Upper and Surface Water but less dense than Deep Water
–> Intermediate density due to reduced salinity or high temp
Antarctic Intermediate Water (AAIW)- formation
Dilution of cool deep water by icebergs just to the north of the south polar front
Features of AAIW
Low temperature 3-5C makes water contract ad sink but low salinity (34.3%) makes it float above dense saline water
Depths of AAIW
500-900m/ 700-1200m
Features Mediterranean Water
- High salinity 35-36.2 offset by high temp (8-10C)–> Intermediate density
Another name for Pacific Subarctic intermediate water=
North Pacific Intermediate Water (NIPW)
North Pacific vs North Atlantic
Less evaporation and more precipitation therefore less saline- drops below 32%
Features of NPIW
Cool- 3-8C and low salinities 34-34.5
NADW - North Atlantic Deep Water (NADW)
Generated by the sinking of surface waters in high latitudes generating cold, highly saline water
2 principle original deep water masses
North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW)
Formation of NADW
Formed by winter cooling of high salinity surface water in N Atlantic
Features of NADW
Moderately cold:2-5C
Highly Saline (34.9%)
Formation of AABW
Formed by sea ice freezing and is the most widespread ocean water type
Features of AABW
Very cold :-2 to 1 C
Moderately high salinity (34.6-34.7%)
Density AABW vs NADW
AABW is slightly denser than NADW
NADW is a mixture of water masses from 3 main areas:
- Norwegian and Greenland Seas
- Central Arctic Ocean
North Atlantic Drift
Warm, salty water from Gulf stream and N Atlantic Gyre reach Norwegian sea as the North Atlantic Drift= key source of salinity in NADW
Winter cooling NADW
Strong cold winder winds in Norwegian Sea cause intense cooling and mixing of the surface water with underlying colder and less saline waters
LNADW- lower North Atlantic Deep Water
= v.dense- formed in Norwegian sea. Temp=higher than AABW
UNADW- Upper North Atlantic Deep Water
- Formed in the Labrador Sea through winter cooling - lower density than LNADW
Sea ice
Forms from salt water but the ice crystal are of pure water, leaving the salt behind in the un-frozen residual sea water.
Why doesn’t salt in ocean freeze?
Salt ions interfere with the ability of water molecules to form ice crystal lattice structure
Origin of ice at sea:
- Continental ice sheet that has slip off but flows coherently
- Icebergs- float independently
- Salt water freezing
Polynya
Continuing open water in a region of sea ice e.g. Weddell Sea
Where do submarine waterfalls occur?
Middle of Atlantic Ocean
Order of Atlantic deep ocean circulation N-S
Lowest density –> Highest density=
1. Mediterranean overflow water (MOW)
2. Antarctic Intermediate water (AAIW)
3. North Atlantic Deep Water (NADW)
4. Antarctic Bottom Water (AABW)
Journey of NADW
NADW forms in two basins in the North Atlantic, separated by ridges. It spills southward over sills, with some denser Norwegian Sea water traveling through a specific zone. It travels south along the west side of the Atlantic, then overrides colder bottom water around 20°N. Continuing south, NADW becomes a layer between deeper and shallower water masses. Eventually, it mixes with them to form a new water type that feeds into the formation of another important layer and gets carried around the globe by a major current.
AABW - journey
AABW (cold)- cannot cross the MOR in the S Atlantic
Bottom Water in Nordic seas cannoAAt cross the Shetlands-Iceland Ridge
AABW flow patterns
AABW takes a complex path through the South Atlantic. The Weddell Sea funnels AABW into the Argentine Basin. Some “Common Water” from the ACC also enters. Underwater ridges block direct flow from the ACC into another basin (Angolan). AABW mainly travels north, exiting the Argentine Basin through a gap to reach western North Atlantic basins. A smaller portion goes east, warming slightly as it crosses a ridge to reach the Guinea Basin. Northward flow from there is limited, but some AABW spills south. Minor amounts enter eastern North Atlantic basins through fracture zones.
Pacific- Indian Ocean Common Water (PIOCW)
Mixture of AABW and NADW
Features of PIOCW
salinity- 34.7%
temperature : 1.5C
