15. Arctic and Southern Oceans Flashcards
The southern ocean is ___ surrounded by ___, while the arctic ocean is ___ surrounded by ____
southern: land surrounded by ocean
arctic: ocean surrounded by land
The contribution of arctic water masses for the world ocean differs substantially from the southern ocean water masses. Why is this?
topography!
Arctic is separated from the major ocean basins by sills, which limit the exchange of deeper waters
The Arctic ocean is divided into 2 basins:
What are these basins separated by?
Canadian basin
Eurasian basin
Lomonosov ridge
How does the arctic ocean ‘communicate’ with the atlantic ocean?
through the canadian archipelago and nordic seas
How does the arctic ocean communicate with the pacific ocean?
through the Bering straight
There is ___ pressure near the north pole, and the arctic ocean is under the influence of which winds?
high
polar easterlies
What’s the effect of the polar easterlies in the arctic ocean? What about in the southern ocean?
Arctic: force anticyclonic sfc circulation
Southern: only noticeable in a weak westward current along the Antarctic continent
In the summertime, the winds are much _____(stronger/weaker) and the atmospheric low is centered over:
weaker
the north pole
The arctic surface circulation is divided roughly into 2:
- cyclonic circulation in the north sea & eurasian basin
- around center of atmospheric low, not a defined/ closed gyre - anticyclonic circulation in the canadian basin (Beaufort gyre)
- around center of atmospheric high
T/F
The anticyclonic circulation of the beaufort gyre is not just on the surface, but also at depth
false
it’s superficially anticyclonic, but intermediate and deep circulations are cyclonic
What divides the 2 parts of the arctic circulation system?
Transpolar drift (TPD)/ transpolar current= major cross-polar circulation b/w the 2 systems
What inhibits the generation of strong currents in the artic?
permanent ice coverage
avg speeds are ~0.02m/s
T/F
The Arctic is a Mediterranean sea
Why/why not?
true
B/c it has only limited circulation with the major ocean basins and its circulation greatly influenced by thermohaline forcing
Mediterranean= enclosed with land (or mostly)
What happens when evaporation exceeds precipitation in the arctic?
increases the density of the sfc waters, resulting in deep vertical convection
- inflow of oceanic water in the upper layer and outflow of Mediterranean waters at depth
What does Mediterranean water outflowing into ocean water do if evap > precip?
it sinks until it reaches isopycnal that matches its own
so it can be traced through the ocean basin by its high salinity
Why are mediterranean seas “concentration basins”?
b/c the salinity of the oceanic water is increased as it passes through the mediterranean sea= concentration basin
- this happens if evap > precip
What happens if precipitation exceeds evaporation in the arctic?
freshwater gain drives an outflow into the ocean basin through the upper layer
- strong pycnocline develops, so renewal of the deeper waters is inhibited
- sfc density decreases, & the resulting density diff at the sill causes inflow of oceanic water in deep layers
If precip > evap, in a mediterranean sea, it becomes a ___ basin
dilution
Does the Arctic act as a dilution basin or a concentration basin for the atlantic ocean?
dilution
b/c precip over the arctic > evap
The reality of thermohaline forcing is more complex than the basic model.
- In the sfc layer, the outflow into the Atlantic Ocean is restricted to the ____ side of the sill
- The water below sill depth is what temp/ density?
- Is outflow of water from the arctic seas really restricted to the sfc layer?
- western
- colder than the oceanic water, so more dense. This means the inflowing water doesn’t sink but spreads out through an intermediate layer
- no
Anywhere in the Arctic (except Norwegian Sea) has 3 layers of water masses:
- Arctic surface water
- Atlantic water
- Arctic deep/ bottom water
Describe the Arctic surface water:
- depth range
- temp
- salinity
- surface to ~200m
- temp is close to freezing point
- salinity varies strongly:
Top sfc layer= ~50m thick
- low salinity due to runoff/ ice melt
Subsurface layer:
~50-200m
- strong salinity gradient but uniform temp (cold halocline)
What is the source of subsurface water?
the cold and saline shelf water produced during winter (sea ice freezing)
The seasonal sfc mixed layer is due to brine rejection during sea ice formation –> convective mixing
The means by which the arctic ocean halocline is maintained are very similar to those responsible for the formation of Antarctic bottom water. What’s the one big difference?
The Arctic halocline water is insufficiently dense to reach the bottom
T/F
The subsurface layer of sunken shelf water acts as a heat shield for the surface layer
true!
With its very low temp, this layer prevents the atlantic water from melting the ice layer above
What’s the source of subsurface water in the arctic?
relatively fresh inflow from the pacific ocean and the cold + relatively saline shelf water produced during winter sea ice freezing
Which layer is below the arctic surface water?
Atlantic water
Describe the atlantic water layer of the arctic
- depth range
- how does it enter the arctic?
- salinity/ temp compared to other layers
- 150-900m
- enters from the north atlantic
- same salinity as bottom water, but is warmer
- warmer than sfc water, but high salinity makes it denser
T/F
Arctic bottom water is a mix of different dense waters
true
- dense waters produced through deep convection
- dense water from arctic shelf
Arctic bottom water is the ____ water of the world ocean. Is it found anywhere other than the Arctic region?
densest
no
Explain how dense waters of arctic bottom water are produced through deep convection
sinking of water to the bottom occurs in events (usually storms- less than 1 week in a small area). Ice formation= concentration of salt= initiates sinking
- dense water sinks, warmer water upwells, which melts the ice and ends the cycle
Explain how dense water from the arctic shelf becomes arctic bottom water
shelf water has low salinity–> initiates ice formation. Brine rejection= salinity in water below the ice increases (& is very cold)
Give some freshwater inputs to the arctic
- numerous large rivers empty into the arctic shelf
- pacific water through bering straight –> also relatively fresh
Give some freshwater exports from the Arctic
- net freshwater transport when sea ice forms in the arctic and melts somewhere else
Mainly in the form of sea ice: goes to atlantic/ Beaufort Gyre. Can go into North Atlantic via Fram Straight and Canadian Arctic Archipelago
Freshwater=
the amount of zero-salinity water contained in a volume of water with a given salinity relative to a reference salinity
T/F
Beaufort Gyre has been getting more saline
false
the opposite; it’s been getting fresher
Why has the Beaufort Gyre been getting fresher?
changes due to shift in pathway of Eurasian runoff forced by strengthening of low pressure on the pole
What are the implications of freshening of the Beaufort Gyre?
- on arctic surface water
- on halocline
- on atlantic water layer
- sea ice
saltier arctic surface waters –> halocline weakens –> Atlantic water gets in contact with sea ice –> sea ice melts more
T/F
The southern ocean communicates with all other oceans
true
Where is the only region where the flow of water can continue all around the globe?
The southern ocean
ocean circulation comes closest to the situation observed in the atmosphere
Describe the temp differences observed b/w surface vs bottom of southern ocean
Temp diff doesn’t exceed ~5C
This is 20% of the diff seen in the tropics
The currents in the southern ocean are not restricted to the upper few hundred meters of the ocean, but can extend to great depths. Why is this?
Density variations with depth are small and the pressure gradient force is more evenly distributed over the water column
Which current has the largest mass transport of all ocean currents?
Antarctic circumpolar current (ACC)
- Moves a slab of water more than 2000m thick!
What are the geographical limits of the southern ocean (in each direction)?
- No east- west boundary
- southern limit= antarctic coastline
- definition of the northern limit is controversial
What is the northern boundary of the southern ocean, defined by:
1. International hydrographic organization
2. Oceanographic point of view
- Defined is at 60 degrees S (in year 2000)
- Subtropical Front is the northern boundary: this is the line where the tropical/ temperate dynamics break down (permanent thermocline reaches the surface)
Why does the topography of the ocean floor have a large impact on southern ocean circulation?
because the Antarctic circumpolar current reaches great depths
The Drake Passage and the Scotia Ridge have a dramatic combined effect on the circumpolar current. What happens?
The current accelerates to squeeze through the gap and hits the obstacle at increases speed. It emerges highly turbulent and shifts sharply northward
The coastline of Antarctica includes 2 major indentations:
1
2
- The Weddel Sea
- The Ross Sea
Do the Weddel and Ross Seas have western boundaries? What circulation is observed in them?
No
Wind-driven gyres with western boundary currents
Which winds drive the Antarctic circumpolar current?
westerlies (from west)
- drive the ACC –> Called the West Wind Drift
Which winds drive the Antarctic Coastal current?
easterlies
- called the East Wind Drift
Which 2 currents are the southern hemisphere equivalent of the subpolar gyres?
Antarctic circumpolar current (ACC) and Antarctic Coastal Current (ACoC)
Are the gyres in the Weddell and Ross Seas cyclonic or anticyclonic? Clockwise/ anti?
Cyclonic gyres
- clockwise (SH)
Are the following currents driven clockwise or counterclockwise?
- Antarctic circumpolar current
- Antarctic Coastal current
ACC= clockwise
ACoC= counterclockwise
Why are there such high waves in the southern ocean?
infinite fetch + large wind speeds + little variation of wind direction
Isopleths of all properties in the southern ocean are nearly _____(east-west) to great depths
zonal
The isopleths in the ACC are organized into __ major fronts separating __ broad zones in which isopleths are more widely spaced.
Within the fronts, the currents are strong and __ward
3
4
eastward
Describe the flow patterns in the zones between fronts in the southern ocean
the flow is dominated by eddies and can be in any direction
Subtropical Front (SFT)=
a narrow band around Antarctica where the salinity changes rapidly from N to S and temps drop rapidly as well
Subantarctic Front is found where?
the northern edge of the Antarctic circumpolar current
The Polar Front is identified by:
the northernmost location of the 2 deg C isotherm (shallow temp min)
Antarctic circumpolar current transport is concentrated in:
narrow bands (jets) which are focused along fronts
T/F
In regions of the ACC, countercurrents exist
true
Where does Antarctic convergence happen?
This results in the subduction of :
between subantarctic front and polar front (water coming from N meet waters coming from S)
= subduction of Antarctic intermediate water
Where does Antarctic divergence occur?
The results in the upwelling of”
b/w westerlies and easterlies (Ekman transport in opposite directions)
= upwelling of north atlantic deep water
Where does Antarctic Bottom Water begin its formation? Driven by what?
- Why are polynyas important?
through deep convection at the continental shelf driven by the freezing of sea ice
- Polynyas are important b/c give constant supply of dense water
How is Antarctic bottom water formation finalized?
intense mixing with the water of the ACC while sinking to the bottom
T/F
The formation process for Antarctic bottom water is a combination of deep convection and subsurface mixing
true
Explain how localized formation of Antarctic bottom waters occurs in the Weddell/ Ross Seas
brine rejection during sea ice formation creates very cold, saline, dense water
- sinks along shelf and spreads out along the bottom of ocean basins
Antarctic bottom water is exported to the bottom of all ocean basins. The pathway is dependent on what?
deep ocean bathymetry
