Ocean Structure and Circulation

Regions of sinking water masses: deep water and intermediate water Intermediate water masses form at ________ convergence zones or ______ ______ front.

Arctic Antarctica Polar

Regions of sinking water masses: deep water and intermediate water They are as dense as deep water and spread at shallower depths t or f

ARE NOT AS DENSE AS DEEP SEA WATER

How do we measure current speed in the ocean?

Aanderaa current meters, with propeller and current and rudder Doppler shift current profiler. Recording Doppler

Gyres (continued) 2. Subpolar gyres are ____ than subtropical gyres

Smaller and fewer than subtropical gyres E.G. Subpolar gyre (North Atlantic, Weddell Sea), Alaskan Gyre

Deep water currents Form when surface water _______ and becomes ________ than surrounding water masses and sink

cools denser

Net water transport

in Ekman layer is at a right angle of the wind direction (to the right in the Northern Hemisphere, left in the S. Hem.)

Downwelling

is caused by along shore winds that push water against the coast

Upwelling

is caused by along shore winds, that initiate Ekman transport away from the coast

Gyres

large circular-moving loops of water within an ocean basin

1. Subtropical gyres Five main gyres (one in each ocean basin):

- North Pacific - South Pacific - North Atlantic - South Atlantic - Indian

four major coastal upwelling areas

1. Off California/Oregon (California Current) 2. Off Peru (Peru Current) 3. Off NW Africa (Canary Current) 4. Off SW Africa (Benguela Current)

Generally __ currents in each subpolar gyre

2

Subtropical gyres are centered at about ___º north or south latitude

30

Generally __ currents in each subtropical gyre

4

Arrows are current vectors, depicting direction and speed of current layers. Each layer is ___ degrees offset to the layer above

45

subpolar gyres are centered at about ___º north or south latitude

60

What is the main cause of surface water sinking to cause the deep, thermohaline ocean currents? A. Density increase caused by increased precipitation B. Density increase caused by cooling by the westerlies and relatively high salinity of the source water C. Surface currents being warmed by westerlies, driving them downward D. Density decrease caused by greater salinity

B. Density increase caused by cooling by westerlies and relatively high salinity of the source water See animation that shows Westerlies cool the surface waters of the North Atlantic while picking up heat that ends up warming middle/northern Europe. The source water originates from Gulf Stream that is warm water that has experienced high evaporation and hence is fairly saline compared to higher latitude waters.

Which water masses would you encounter doing a CTD cast at 40 N in the Atlantic, from the sea surface down? A. Surface water/ Central water, North Atlantic Deep Water, Antarctic Deep Water, Mediterranean Outflow Water B. Surface water/central water, North Atlantic Deep Water, Mediterranean Outflow Water C. Surface water/central water, Mediterranean Outflow Water, North Atlantic Deep Water D. Surface water/central water, North Atlantic Deep Water

C. Surface water/central water, Mediterranean Outflow Water, North Atlantic Deep Water A is incorrect because NADW (North Atlantic Deep Water) is denser than Mediterranean Outflow Water (MOW), and Antarctic Bottom Water (ABW) does not go that far north; B is incorrect again because NADW is above MOW and surface water is missing, D is incorrect because MOW is missing

Which wind belt drives the major upwelling zones? A. Westerlies B. Easterlies C. Trade winds D.West wind drift

C. Trade winds

Thermohaline currents are density driven currents that __________. A. Are part of the conveyor belt circulation B. bring oxygen-rich water to the deep ocean C. bring nutrient-rich water to the surface D. All of the above.

D. All of the above.

The Ekman spiral describes the direction of near surface water movement in response to wind forcing. If you lowered a current-measuring device over the side of a ship in the northern hemisphere, what would you observe? A. Subsurface water would move as one layer to the right of the wind direction. B. Subsurface water would move as one layer to the left of the wind direction. C. As the instrument descended, the current direction would move progressively to the left of the wind until it actually was going in the opposite direction of the wind. D. As the instrument descended, the current direction would move progressively to the right of the wind until it actually was going in the opposite direction of the wind.

D. As the instrument descended, the current direction would move progressively to the right of the wind until it actually was going in the opposite direction of the wind.

How does water move with respect to wind?

Fritjof Nansen made the observation that ice did not move in wind direction but 20-30 degrees to the right of the prevailing wind. • When returning home, he communicated this to Walfrid Ekman, a Swedish oceanographer • Ekamn deduced that wind stress, friction and Coriolis effect cause the movement of currents.

why do gyres turn

Geostrophic flow

North Atlantic Ocean gyre: Example of western intensification

Gulf Stream is a typical western boundary current and is the fastest current in the world (2-2.5m/s)).

Deep water currents Cold, heavy water masses form mainly in the subpolar ______ _______ and the _______ ______ (Antarctica)

North Atlantic and the Weddell sea

Satellite view (TOPEX/POSEIDON):

Radar altimetry shows 'hills' in red in each gyre. Along the slopes of the hills, the strongest current speeds are observed.

Regions of sinking water masses: deep water and intermediate water Mediterranean outflow water forms at the

Strait of Gibraltar

Western intensification

Stronger Coriolis effect in higher latitudes causes broad flow in the gyre towards equator, which is compensated by a strong return current in the west. Western intensification Ø "Hill" is steeper on western side ØWestern currents are fast, narrow, deep

Deep water currents Factors affecting density of surface water:

Temperature (most important factor) Salinity

Geostrophic flow

Wind induced Ekman transport piles up "hill" of seawater at about 30o N and 30o S • Water flows downhill under gravity and veers right (Northern hemisphere) due to Coriolis effect • Balance of Coriolis and gravity results in the geostrophic flow moving around center

Before the opening of the Atlantic Ocean, Pangaea was surrounded by the Panthalassic Ocean, which covered most of the globe. What would the current pattern have looked like? A. Four gyres B. Two gyres C. Three gyres D. No gyres

a) Four gyres b) Two gyres c) Three gyres Explanation: Several answers can be right for this question. Probably there would be two very large gyres in the Panthalassic Ocean with maybe one small one in the Tethys Sea because that basin is partly enclosed. D is incorrect because there would certainly be gyres.

Upwelling and downwelling are caused by

along shore winds, that initiate Ekman transport either away from the coast or that push water against the coast

why do gyres turn? The balance of Coriolis and gravity results in geostrophic current moving around ______

center

why do gyres turn? Ekman transport pushes water towards __________ to form ' a hill'.

center of the gyre

trade winds

create ocean surface currents that move water from east to west

El Niño Southern Oscillation (ENSO) is responsible for worldwide climate fluctuations. Why is the fishing economy off Peru greatly affected by these fluctuations? A. Cold, nutrient-poor water has covered the warm, nutrientrich waters near Peru's coast. B. The La Niña is a cool phase that decreases fisheries. C. Monsoons in the North Indian Ocean cause a decrease in Peru fisheries. D. Cold, nutrient-rich upwelling waters that facilitate the fisheries are replaced by the warm, nutrient-poor waters of El Niño.

d) Cold, nutrient-rich upwelling waters that facilitate the fisheries are replaced by the warm, nutrient-poor waters of El Niño. Explanation: During the El Niño phase of ENSO, coastal upwelling at the Peru coast ceases as warm water moves into the area. The warm water causes downwelling and the result is a decrease in productivity along the Peru coast. During the La Niña phase, productivity is enhanced as upwelling brings cool, nutrient-rich water to the surface.

Importance of thermohaline circulation (=Conveyor Belt Circulation) Ventilation of the _______ ocean with oxygen rich water

deep

Affect deep water below pycnocline (90% of ocean water) Driven mainly by density differences (buoyancy differences), but are also influenced by surface winds Larger and slower than surface currents

deep currents

Importance of thermohaline circulation (=Conveyor Belt Circulation) Uptake of anthropogenic CO2 and transport to ______ water

deeper

Importance of thermohaline circulation (=Conveyor Belt Circulation) Transport of nutrients from ______ to _______

depth to surface

Ekman spiral:

describes the speed and direction of flow of surface waters at various depths, together forming the Ekman layer DE

Equatorial upwelling. * Because the meteorological equator (ITCZ) is north of the geographical equator most of the year, the SE trade winds straddle the _______.

equator

A 'switching off' of the ______ _______ may have been responsible for this massive cooling event. (The Younger Dryas)

global conveyor

Importance of thermohaline circulation (=Conveyor Belt Circulation) Conveyor belt is linked to _____ ______ climate variability (cooling events during interglacial times, for example)

large scale

Importance of thermohaline circulation (=Conveyor Belt Circulation) Heat transport from ______ to _____ latitudes

low to high

westerlies

move water from west to east, creating a giant loop of water

subpolar gyres rotate in the _______ direction of adjoining subtropical gyres

opposite

why do gyres turn? Gravity counteracts this effect, as water flows downhill, it is deflected by the Coriolis effect to the _________ (N Hemisphere).

right

Equatorial upwelling. SE trade winds cause Ekman transport to the _______ of the wind direction north of the equator (= in the northern hemisphere), south of the equator to the _______ (=southern hemisphere). This causes divergence of surface water and nutrient rich deep water to rise to the surface where it supports rich plankton communities.

right left

Equatorial upwelling. * Remember the _______ _______ that underlies these areas of high production.

siliceous ooze

Affect surface water within and above the permanent pycnocline (10% of ocean water) Driven by major wind belts of the world

surface currents

Wind is always the source of energy for currents, and Coriolis effect causes the deflection. Friction causes the reduction of speed with increasing depth. t or f

t

Deep water currents Deep currents are also known as

thermohaline circulation

why do gyres turn? The center of the gyres are shifted to the ______. ________ boundary currents are intensified.

west western

Spillage of Lake Agassiz into the North Atlantic could have caused a shutdown of the thermohaline circulation because of:

Ø Large inflow of freshwater into subpolar North Atlantic causes a reduction in critical density Ø deep water formation ceases Ø flow of warm Gulfstream water north is stopped or reduced Ø extreme temperature drop in northern hemisphere

What is a water mass?

• Parcels of water that exhibit a certain combination of temperature and salinity (i.e., density), determined by the surface water properties in the regions where they are created (and some mixing at depth) • They appear as a segment in a T-S diagram