General Ocean Circulation

75% of the Earth’s surface Couples atmospheric processes with tectonic processes Important in regulating atmospheric CO 2 Important in global heat transport

Effect of differential heating and cooling of the earth’s surface Temperature and salinity (density) gradients (thermohaline circulation) Atmospheric circulation, wind cells and surface ocean currents (drift currents)

Oceans 75% of Earth’s surface Important for heat transport Cycling in the ocean important for elements –Couples shorter atm-ocean cycles with longer tectonic cycles –Ultimately involve burial in marine sediments Regulation of atm CO2

Ocean layers Thin surface mixed layer (~50 – 100 m) box –Sunlight penetrates –Net primary prod/ps drives biogeochem cycles –OM produced here sinks out and is remineralized below –Important for surface heat transport Large –dark, cold and deep water box –Most of the ocean volume (~90%) –Isolated from surface for long periods of time (~500 – 1000 yr) = average mixing time for bottom waters –Important for understanding oceanic CO2 uptake Processes within and between boxes Ocean zones defined by density differences

Surface currents Move large volumes of water at basin- scale Transport heat Work simultaneously with thermohaline circulation –Some independence –Some interaction (complex 3D circulation

Wind driven circulation About 10% of the water is moved by surface currents Surface currents are primarily driven by the wind and wind friction Move fast relative to thermohaline circulation (1 to 2 m/s) Most water moved is above the pycnocline Reflect global wind patterns and Coriolis effect!

Surface circulation But, surface flow is NOT parallel to wind Why? –Coriolis force –Ekman flow

Coriolis Effect Earth rotates on its axis 1x/day (CCW with N at top) Radius of earth = 6371 km Circumference of earth = 2  r = ~40,000 km Speed of object at equator is ~40,000 km/d (~1668 km/h) At 60 o, radius is smaller by factor of 2 (cos 60 = 0.5) So speed is half that at equator (834 km/h) At 30 o speed is 1442 km/h (cos 30 = 0.866)

Coriolis force Apparent deflection of things moving long distances due to rotation of the earth Variations in rotational speed.

Coriolis force Apparent deflection of things moving long distances due to rotation of the earth Consequences of Coriolis deflection.

Relative speeds of objects at different radii moving at the same angular speed

Inertia keeps objects moving at their original speed We could see this if viewed from space but we’re moving as well Has an effect when things move between latitudes

Result for wind cells is you go from 2 cell model to 6-cell model

Surface Ocean Circulation Nansen first connected wind with currents Showed his measurements to Ekman who formulated a mathematical explanation of surface currents

Ekman spiral Wind flows over surface and creates drag on water Wind driven flow is deflected to right in N hemisphere by Coriolis effect Water flows at only about 3% of the speed of the driving wind. Current flows at 45 o to the right of the wind direction in the northern hemisphere But, only the surface feels the wind Each layer down only feels the layer above so is deflected based on the layer above Each layer down moves more slowly than the layer above

Wind creates a drag on surface waters and successive layers exert drag on each successive layer below. Each layer is subject to Coriolis deflection

Ekman flow Wind exerts frictional drag on water causing a thin layer of water to move –Transfer of momentum is not efficient; induced current is about 2% of wind speed –Coriolis force causes water to veer right or left of wind As the surface layer of water begins to move, it exerts frictional drag on the layer below And so on, each layer moving slower and deflected relative to the layer above Produces a pattern of decreased speed with depth and increased angle between flow and wind direction with depth (Ekman spiral)

Ekman spiral Velocity vectors at different depths trace out a spiral around a line perpendicular to the surface –Steady wind induces flow at depth at 90 o and 180 o or more to wind direction We say the wind “penetrates” to a depth where flow is 180 o to the wind (flow at this depth is about ~4% of flow at surface) Water above this depth is the Ekman layer Wind speed, water viscosity and the Coriolis effect all affect the depth of wind penetration Winds penetrate deeper at low latitudes except right at the equator

Flow in Ekman layer Surface current typically o to wind direction By definition, current at base of Ekman layer is 180 o to wind direction Average or net flow of water in Ekman layer is 90 o to wind Average or net flow in Ekman layer is the drift current Wind direction Surface current direction Direction of net Transport within the Ekman layer

Ekman flow Water doesn’t really spiral downward At some depth water flow will be opposite surface flow and at this depth friction dissipates horizontal flow Effects of surface wind felt to approximately 100m The net motion of the water movement, after the sum of the effects of the Ekman spiral is the Ekman transport or flow In theory, Ekman transport is 90 o to the right of the wind in the N hemisphere In nature, it barely reaches 45 o because of the interaction between the Coriolis effect and pressure gradient

Surface currents - wind First order control by predominant wind pattern – friction between atm and surface ocean More complex in the real world –Position of continents Ekman transport –Friction plus Coriolis –Pushes water to center of gyres –Regions of convergence and divergence Geostrophic flow – interaction between pressure gradient associated with Ekman transport/convergence and Coriolis effect Broad general subtropical gyres

Fig. 5-1

Moving water “piles up” in the direction the wind is blowing Continents and land masses also deflect flow in E-W direction Water pressure increases where its piled up so tries to slide back along a pressure gradient Coriolis effect intervenes deflecting currents to the right of wind direction (in N hemisphere) Surface currents

Ocean gyres Circular flow around the periphery of an ocean basin This flow is often broken down into interconnected currents (e.g., North Atlantic gyre) Why doesn’t flow spiral toward center because of Coriolis force?

Pressure gradients develop in the ocean because the sea surface is warped into broad mounds and depressions with a relief of about one meter. Mounds on the ocean’s surface are caused by converging currents, places where water sinks. Depressions on the ocean;s surface are caused by diverging currents, places from where water rises. Water flowing down pressure gradients on the ocean’s irregular surface are deflected by the Coriolis effect. The amount of deflection is a function of latitude and current speed.

Fig. 5-4 Downwelling of water Creation of geostrophic currents as a result of the pressure gradient Upwelling of deep water to replace surface water in areas of divergence - e.g., along the equator

Fig. 5-3 (a) Ekman spiral upper ~100 m Fig. 5-3 (b) Ekman transport In the center of gyres water piles up (converges)

Water piles up in the direction of flow so piles up in middle of gyres due to Ekman transport and creates a pressure gradient in the opposite direction.

Pressure gradient

Ocean gyres Circular flow around the periphery of an ocean basin n Westerly-driven ocean currents in the trade winds, easterly- driven ocean currents in the Westerlies and deflection of the ocean currents by the continents result in a circular current, called a gyre. This flow is often broken down into interconnected currents (e.g., North Atlantic gyre)

Fig. 5-2

Gyre circulation To deflect further than 45 o, water would have to move uphill against a pressure gradient To deflect away from the pressure gradient would defy the Coriolis effect So water circulates clockwise around the gyre balanced between the pressure gradient in the center of the gyre and the Coriolis deflection - Coriolis deflection versus gravity Higher sea surface height at the center of gyres and maintained by wind energy

Gyres in balance between pressure gradient and Coriolis effect Their currents are geostrophic currents Because of wind patterns and positions of continents, major gyres are largely independent of each other in each hemisphere. Six great surface current circuits in the world, one is technically not a geostrophic gyre The Antarctic circumpolar current (west wind drift) moves eastward around Antarctica driven by westerly winds and is never deflected by a continent Geostrophic gyres/flow

Sea surface height Hill is offset to the western side of basins because of western intensification

Western intensification Earth turns CCW Water piles up against land on west side of basins

Less (or no) Coriolis force at equator so water doesn’t turn until it hits and obstacle (land) More Coriolis at mid and high latitudes so current turns sooner Geostrophic Flow Around the North Atlantic Ocean

Sea surface height

Effect on gyres The geostrophic mound is deflected to the western part of the ocean basin because of the eastward rotation of the Earth on its axis. The Sargasso Sea is a large lens of warm water encircled by the North Atlantic gyre and separated from cold waters below and laterally by a strong thermocline. Western boundary currents, such as the Gulf Stream, form a meandering boundary separating coastal waters from warmer waters in the gyre’s center. Meanders can be cut off to form warm- core and cold-core rings.

The current flow pattern in gyres is asymmetrical with narrow, deep and swift currents along the basin’s western edge and broad, shallow slower currents along the basin’s eastern edge. Geostrophic Flow Around the North Atlantic Ocean

Narrow, fast, deep currents at the western margins of gyres Bring warm water poleward Gulf-stream is the largest –2 m/s (5mph), 450 m deep, ~70 km wide Large volume of water transported –Expressed as a Sverdrup (1 Sv = 1 mil m 3 /s) Moves like a river (hose analogy) but moves a lot more water Water within the current is warm, clear and blue (not much by way of nutrients or life compared to surrounding water) Western boundary currents

Surface circulation of the N Atlantic with flow in Sverdrups (1 Sv = 1 million cubic meters/s) – western boundary currents very important for heat transport

Wide, shallow, slow currents on the east side of ocean basins (off the west coasts) –1000 km wide, 2 km/h (1.2 mph) Bring cold water equatorward Lack defined boundaries and lack eddies Eastern boundary currents

Sea surface temperatures Think about gyres and heat transport

Other notable currents Equatorial countercurrent – no Coriolis and not much wind and so some water moves back east –Pacific is wider so more pronounced Undercurrents – again, if water doesn’t turn, it piles up, sinks as far as it can (density) and then tries to return on a density surface High latitude currents – continental collisions (tendency of water to flow around obstacles) more important than Coriolis force at high N latitudes; can also get polar easterly influence West wind drift or Antarctic circumpolar current – unimpeded westerlies

Equatorial upwelling – water on either side of equator moving westward is deflected slightly poleward and is replaced by deeper water Some upwelling and downwelling induced by gyre circulation –Depression of thermocline on western side of basin –Shallow current on eastern side of basin Along coastal areas Ekman transport can induce downwelling or upwelling by driving water towards or away from the coast, respectively. –Wind blowing parallel to the shore or offshore Upwelling and downwelling

East versus west coast climates

Northern hemisphere coastal upwelling – eastern side of basin

Surface chlorophyll High from nutrient-rich deep water upwelling along CA Cold-water upwelling also chills the air

Northern hemisphere coastal downwelling – eastern side of basin

Areas of upwelling – coastal upwelling has seasonality (as do winds)

Surface currents - patterns Similar in all basins At low latitudes, have large, “closed” gyres –Gyres elongated in the E-W direction –Gyres centered on the subtropics (~30 o N or S) West-directed flow at N and S equatorial currents East-directed flow ~ 45 o N and S N-S directed flow at eastern and western boundary currents

Surface currents - patterns West-directed flow driven by tradewinds (coming from N or S-east East-moving equatorial countercurrents Western boundary currents are distinct, narrow ( 100 km/day) and deep (2 km) Eastern boundary currents are broad (>1000 km), weak (~10s km/day) and shallow (~500 m) Have smaller, less developed polar gyres in N Have circumpolar “gyre” in the S

Transverse currents E-W currents driven by the trade winds (easterlies) and mid-latitude westerlies Link the boundary currents Equatorial currents –Moderately shallow and broad –Pile up water on west side of basin (W Atl is 12 cm [8”] higher than Pac; W Pac is 1 m higher than E Pac) Eastward flowing currents at mid-latitudes are weaker (wider and slower) than equatorial currents Differences in land mass distribution in N and S hemispheres affects flow

Fig. 8-9 Why is this important? Processes in surface, wind-driven layers are different but connected to processes in deep waters.