Sea surface temperatures Sea water T varies with position in oceans Amount of insolation absorbed depends upon angle of incidence –With normal incidence, 98% of insolation enters water & 2% is reflected –With oblique incidence, more light is reflected –Warming concentrated at low latitudes Light penetrates no more than 500 m, so therefore only warms surface waters Have surface zone ( 18°C) & deep zone, the lower reaches of the oceans, where water temperatures are low (<3°C)

Vertical temperature gradient Between surface zone & deep zone, T changes rapidly with increasing depth Region with steep T gradient = thermocline zone –Thermoclines common in the tropics & the subtropics –Thermoclines occur seasonally in mid-latitudes –Thermoclines are rare at high latitudes Thermoclines usually create a region of rapid increase in sea water density, called a pycnocline In such cases, the ocean is stably stratified

Sea surface salinity Variation in precipitation & evaporation generate changes in sea water salinity –Near equator, little evaporation & high rainfall; salinity of surface water is usually low (<35 PPT) –In subtropics, much evaporation & little rainfall; salinity of surface water is high (>35 PPT) –In mid-latitudes, little evaporation & high rainfall; salinity of surface water is low (may be seasonal) –At poles, freezing of sea water freezes at surface; salinity of surface water is high

Vertical salinity gradient When salinity of surface waters differ from that that of underlying waters, usually have a zone in which salinity changes rapidly with depth Steep salinity gradient = the halocline zone (or halocline) –Haloclines common in the tropics, where low-salinity surface water sits on moderate salinity deep water –Haloclines common in subtropics, where high salinity surface waters sits on moderate salinity deep water –Haloclines occur seasonally in mid latitudes, when precipitation creates low salinity surface waters Haloclines may give rise to pycnocline

Density gradients in the oceans Distinguish surface zone, pycnocline zone, & deep zone Near equator, strong thermocline & strong halocline lead to well-developed pycnocline In subtropics, strong thermocline leads to well- developed pycnocline even though surface water has higher salinity than water beneath it In mid-latitudes, halocline & thermocline form in summer, leading to a seasonal pycnoclines Near poles, find cold, highly saline water at surface - pycnoclines are rare

Thermohaline circulation Temperature & salinity variations cause variations in sea water density Density variations drive vertical circulation called thermohaline circulation –At poles, dense water masses at surface sink –Surface waters move toward poles to replace sinking water –Draw water to surface at low latitudes –Deep waters flow from poles toward equator Thermohaline circulation is slow; a parcel of water may take 100’s to 1000’s of years to make a circuit

The second effect of differential heating & cooling at equator & poles is the system of surface currents in the oceans We call the surface currents drift currents because they are driven by the wind

Drift current circulation Surface currents move large volumes of water across the ocean basins Surface currents transport significant amounts of heat across the ocean basins Surface currents flow at the same time as the thermohaline circulation occur –In some ways, the two current patterns are independent of each other –In significant ways, the two current patterns interact with each other to create a complex, 3D ocean circulation system

Drift current circulation patterns Have similar surface current patterns in all major ocean basins At low latitudes, surface currents consist of large, essentially closed systems known as gyres –Current gyres are elongate east-west, & are dominated by east-west directed currents –Gyres are centered on the subtropics (30°N or S latitude) West-directed flow occurs in north & south equatorial currents East-directed flow return flow occurs > 45°N or S North & south directed flow occurs in eastern & western boundary currents

Drift current circulation patterns, II West-directed flow in north & south equatorial currents driven by trade winds Equatorial countercurrent, a narrow region of east- directed flow, separates N & S equatorial currents Western boundary currents are distinct, narrow ( 100 km/day), & deep (affect water to depth of 2 km) Eastern boundary currents are broad (~1000 km wide), weak (water velocities are ~ 10’s km/day), & shallow (affect water to depth = ~ 500 m) Smaller, less well developed current gyres occur in sub-polar & polar regions

Current directions are not parallel to prevailing wind directions If prevailing winds drive surface currents, why do current directions differ from prevailing wind directions?

Ekman flow Wind exerts frictional drag on water surface, setting a thin layer of water at surface in motion –Transfer of momentum is not efficient; speed of induced current = ~2% of wind speed –As water begins to move, Coriolis effect causes surface current is to veer the right (or left) of wind As a layer of water begins to move, it exerts a viscous drag on layer of water immediately below Each successive layer is deflected to right (or left) of layer immediately above Produces a pattern where water velocity decreases with increasing depth & the angle between flow direction and wind direction increases with depth

Ekman spiral Velocity vectors at different depths trace out a spiral about a line perpendicular to water surface –Wind-induced currents are 90°, 180° or more to wind direction –Where flow is 180° to wind, velocity is ~4% of surface velocity We say that the wind ‘penetrates’ to depth where flow is 180° to wind Water above this depth of penetration is Ekman layer Wind speed, water viscosity, & magnitude of Coriolis effect all affect depth to wind penetrates Wind Generally penetrates to greater depths at low latitudes (except right at equator)

Ekman spiral Vectors denoting the wind-induced velocity at different depths trace out a spiral about a line perpendicular to water surface –Steady wind induces flow in lower layers at 90°, 180° or more to wind direction –Current magnitude where flow is 180° to wind direction is usually ~4% of surface current Definitions: (1) wind ‘penetrates’ to depth where flow is 180° to wind direction; (2) water above this depth of penetration is Ekman layer Wind speed, water viscosity, & magnitude of Coriolis effect all affect depth to wind penetrates Generally wind penetrates to greater depths at low latitudes (except right at equator)

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