Flow in Ekman layer Surface current typically 20°-40° to wind direction By definition, current at base of the Ekman layer makes a 180° angle to wind direction Average or net flow of water in Ekman layer is 90° to wind Average or net flow in Ekman layer is the drift current

Changing wind patterns & monsoon currents Consider Indian Ocean north of equator From Sept. to May, winds blow off Asia & over ocean –Coriolis effect creates a typical NE trade wind –Ekman flow induces in a typical northern equatorial current From May to Sept., winds blow from Indian Ocean over Asia –Coriolis effect creates westerly winds –Ekman flow induces a southwest monsoon current –Creates a seasonal east-flowing or clockwise current gyre in northern Indian Ocean

Significance of surface circulation Surface currents move heat toward poles In tropics, westward currents cause water to ‘pile up’ along the W sides of ocean basins –Have dynamic topography - currents control the elevation of water surface –Dynamic topographic high on W side of basins depresses the pycnocline –Dynamic topographic low on E side of basin elevates the pycnocline, & may draw deep zone waters to surface (creating upwellings along eastern sides of ocean basins) –Topography leads to subtle ocean currents, e.g. the equatorial countercurrent & undercurrent

Significance of surface circulation In subtropics, Ekman flow drives surface waters toward the centers of current gyres Surface zone waters there are very warm, & so have lower density (greater volume) than surrounding waters Two effects combine to create dynamic topographic highs in centers of low-latitude current gyres Low-latitude current gyres are one of several locations where we observe water flowing in geostrophic balance

Geostrophic flow in current gyres Consider the dynamic topographic high in a low- latitude current gyre –Surface water tends to move downhill, radially away from topographic high in center of gyre –Surface water feels the Coriolis effect, & is deflected to right (or left) –For water in gyre, Coriolis effect tends to move water toward center of gyre –Eventually, the tendency to flow toward the center of gyre (due to Coriolis effect) equals the tendency to flow toward periphery (due to gravity), so water flows around gyre along a line of constant sea surface elevation At that time, have geostrophic flow or say that flow is in geostrophic balance

Geostrophic flow? Who cares? In subtropics, see little precipitation & much evaporation, so surface waters have high salinity Geostrophic flow causes surface waters to follow paths about centers of gyres –Do not refresh surface waters by mixing low salinity water from continents Strong pycnocline in subtropics results from well- developed thermocline (despite strong halocline) –Do not refresh surface waters by upwelling lower salinity water from deep zone Sea water in center of gyres is isolated from rest of ocean water - C 14, tritium, Pb, GEOSECS, etc.

North Atlantic circulation system Ekman transport causes water below N & S equatorial currents to flow away from equator –Diverging flow contributes to upwelling at equator & creates dynamic topography –N & S equatorial currents flow parallel to lines of constant sea surface elevation (have geostrophic flow) Gulf Stream is archetypal western boundary current –Narrow (75 km wide), swift (1 to 3 m/s), & deep (> 2 km) –Separates coastal waters from Sargasso Sea –It meanders, forming warm core & cold core rings Upwelling along east side draws high salinity water from Mediterranean Sea into circulation system –Contributes to vertical circulation when it freezes at high latitudes

Circulation systems elsewhere In southern Atlantic, southern Pacific Ocean, & southern Indian Ocean, ‘returen flow’ at ~45-50°S latitude combine to form a circumpolar current, the west wind drift –Separates surface waters adjacent to Antarctica from those nearer to the equator –Slows the oceanic transport of heat toward the South Pole

Vertical movements of water Existence of pycnocline implies stable stratification Where oceans are stably stratified, vertical mixing is suppressed Ekman flow can, under some circumstances, overcome stable stratification & generate local vertical movements of water, called upwellings or downwellings Upwellings mix nutrient-rich subsurface waters & oxygen-rich surface waters, thereby encouraging the growth of phytoplankton

Coastal upwelling Wind blows parallel to coast Ekman transport away from coast generates a dynamic topographic low along coast Expect water to move in response to local changes in surface elevation, but land prevents lateral flow The only source for water to replenish the low sea surface elevation is from below Low sea surface draws subsurface water to surface, i.e. causes an upwelling

Coastal downwelling Wind blows parallel to coast Ekman transport toward coast generates a dynamic topographic high along coast Expect water to move in response to local changes in surface elevation, but land prevents lateral flow The only way for water to flow out of the bulge in sea surface elevation is downward Pile of water that is the local high sea surface drives downwelling

Open-ocean upwellings Movement of air in cyclonic wind pattern generated by air rising in region of low barometric pressure Ekman flow moves water radially away from center of the cyclonic system Radial flow of water away from point = divergence Divergence causes surface depression, raising the pycnocline & driving upwelling

Open-ocean downwellings Movement of air in anticyclonic wind pattern generated by air sinking in region of high barometric pressure Ekman flow moves water toward center of anticyclonic system Radial flow of water toward point = convergence Convergence causes surface bulge, depressing the pycnocline & driving downwelling

Deep ocean circulation Oceans consist of three distinct volumes –2% of total volume in surface zone –18% of total volume in pycnocline zone –80% of total volume in deep zone Oceanographers recognize on the basis of their distinctive temperatures & salinities several distinct subsurface water masses called intermediate or bottom waters These water masses move and mix very slowly Thermohaline circulation is much more intricate & subtle than can be captured in the three volume idealization

Bottom water Sea water must have very high density in order to sink and become bottom water Bottom water usually is very cold & highly saline Few places on earth create such cold, saline water

Bottom water formation, I In Northern Atlantic, saline water from Mediterranean Sea carried to high latitude by drift currents At high latitudes, water at surface looses heat to cold air by conduction Dry air blows off Greenland, Spitzbergen, & Norway; evaporation further cools the water Surface waters freeze, leaving behind sea water that is both colder & more saline than the water that began freezing Cold, saline water sinks to become North Atlantic Bottom Water

Bottom water formation, II In southern ocean, West Wind Drift isolates water at high latitude Surface waters loose heat by conduction; isolation means waters left to cool at high latitude for long time Dry air blows off Antarctica; evaporation further cools the surface waters Surface waters freeze, leaving behind sea water that is both colder & more saline than the water that began freezing Cold, saline water sinks to become Antarctic Bottom Water

Actual deep circulation Trace bottom water flow by geochemical tracers –AOU = saturation oxygen content - measured oxygen content; larger numbers mean less oxygen –Delta 14 C = ratio of 14 C/ 12 C; more negative numbers mean less 14 C relative to 12 C North Atlantic Bottom Water –Distinct bottom water masses form in Labrador Sea, Greenland Sea, & Norwegian Sea –Eastern water mass flows south, crosses mid-Atlantic ridge at Gibbs fracture zone to join other two masses –Flow south along west side of Atlantic, cross equator, and turn east joining deep water masses formed off Antarctica Antarctic Bottom Water –Forms off Antarctica, particularly in the Weddell Sea

Actual deep circulation NABW & AABW join, & circle Antarctica Spurs of this deep current separate from waters circling Antarctica & flow into Indian Ocean or into the Pacific Ocean Deep water masses in Pacific have been out of contact with surface for almost 1000 years When combined with surface circulation pattern, we get the oceanic conveyor belt

Intermediate waters Other important water masses form at persistent convergences, where currents associated with subtropical gyres meet currents associated with high-latitude gyres At Antarctic convergence, form Antarctic Intermediate Water –Not as dense as bottom waters, so flows northward below surface waters but above bottom waters –Extends to 20°N latitude At Arctic convergence in Pacific, form North Pacific Intermediate Water –Flows south to meet and mix with AAIW Red Sea Water is a mass of warm, highly saline water with low dissolved oxygen that flows out of the Red Sea –RSW flows southward, and is deflected above AAIW