(Pinet)

Major ocean current systems 4

Surface patterns extend as deep as 1000 m 5

Convergence Ekman Transport The red box... Ekman Transport Westerlies Southward transport Northward transport Convergence Trades At the convergence, water piles up and sinks, thus depressing the thermocline and deepening the nutricline! Ekman transport is proportional to wind stress  greater transport for greater wind stress

Convergence (sea surface pile-up)

Pressure gradient force Sea surface Pressure gradient force Coriolis force Velocity into page Geostrophic Balance Pressure gradient force balanced by Coriolis force requires that the velocity be into the page, along the pressure lines, not across them (opposite to our daily experience of a “ball rolling downhill”).

Geostrophy – a frictionless balance between the pressure gradient and the Coriolis acceleration – generates currents that move around the ‘hill’

Snapshot of ocean temperature – shows lots of eddies… Warm-core ring Cold-core ring 10

Vertical Temperature Section along 36o North PERMANENT THERMOCLINE km 1 2 3 4 5 6 Vertical Temperature Section along 36o North

Wind-Driven Circulation Section along 36o North Gulf Stream STRONG NORTHWARD FLOW Open Gyre SLOW SOUTHWARD FLOW km 1 2 3 Med Outflow Part of Thermohaline Circulation 4 5 6 Wind-Driven Circulation Section along 36o North (Fuglister, 1960)

Vertical Salinity Section along 36o North km 1 2 3 4 5 6 Vertical Salinity Section along 36o North (Fuglister, 1960)

Atlantic Ocean Permanent Thermocline NORTH SOUTH NORTH-SOUTH VERTICAL SALINITY SECTION Atlantic Ocean

How does equatorial upwelling occur? ITCZ

Generalized picture of equatorial currents

ECC EUC NEC SEC ECC and EUC: geostrophic currents; ECC: runs along doldrums w/ little wind opposition

Surface water transport from E to W wind W E Mixed surface layer (EUC: flows below wind driven layer) thermocline

Current velocity (cm/s) of EC

ECC NEC SEC S EUC N

Pacific – ‘normal’ equatorial flow

Trade winds

El Niño sets in Decrease in trade winds in central, W Pacific Accompanying decrease in horizontal pressure gradient thermocline sinks from 20 to 100 m, upwelling continues, but upwelled water is warm (29C, not 16-18 C) Change propagates W to E in 2-3 months SST increases, region of ascending moist air over W Pacific expands E

End of El Niño Begins in W Pacific: N, S of Equator, thermocline rises, cooler water to surface--> cooler SST Lower T--> lower atm convection, trade winds switch to normal pattern

Review of global wind and current systems (idealized)