Boundary Currents - combine knowledge of global winds and Ekman flow - surface transport can be determined from wind direction/velocity - surface transport alters ocean surface dynamic height - dynamic height differences => geostrophic flow in ocean - the existence of a coastline leads to upwelling or downwelling Geography “Physical Geography of the World’s Oceans”

equatorward winds along a coastline lead to offshore Ekman transport mass conservation requires these waters replaced by cold, denser waters from below subsurface water brings nutrients to surface, thus enhancing biological production offshore Ekman transport creates sea level slope, thus equatorward geostrophic current Coastal Upwelling

global atmospheric pressure and winds equatorward wind off California coast in summer (anti-cyclonic winds around atmospheric high pressure)

Equatorward winds drive offshore Ekman transport

offshore Ekman transport sets up sea level slope Upwelling must occur to replace water transported offshore

sea level slope drives equatorward geostrophic current Coriolis pressure indicates flow out of the page

offshore Ekman flow & equator-ward geostrophic current

coastal upwelling – northern hemisphere level surface

coastal upwelling – northern hemisphere level surface - sea surface tilts up offshore - pressure force on shore - geostrophic current downcoast - geostrophic current downwind current pressure Coriolis

CalCOFI Data April 2000 temperature sea level

CalCOFI Data April 2000 sigma-t phytoplankton increased phytoplankton => enhanced fisheries

CalCOFI Data April 2000 more phytoplankton (previous slide) because of more nutrients (this slide)

phytoplankton temperature upwelling drives cool and biologically productive waters near the coast

upwelling: Arabian Sea upwelling can occur on east coasts with poleward winds (NH) wind upwelling

equatorward winds along a coastline lead to offshore Ekman transport mass conservation requires these waters replaced by cold, denser waters from below subsurface waters brings nutrients into surface waters creating biological production offshore Ekman transport creates sea level gradient, thus equatorward geostrophic current typical vertical velocity ~1 m per day Coastal Upwelling

poleward winds along a coastline lead to onshore Ekman transport surface waters reach shore and move downward nutrient rich waters near shore are pushed downward, thus no biological productivity onshore Ekman transport creates sea level gradient and poleward geostrophic current Coastal Downwelling

coastal downwelling – northern hemisphere level surface Wind current pressure Coriolis - sea surface tilts up onshore - pressure force offshore - geostrophic current downwind - geostrophic current poleward

equatorial upwelling equator Trade winds

cyclonic & anti-cyclonic wind upwelling Ekman transport convergence, downwelling Ekman transport divergence, upwelling

do convergence – divergence Ekman flows examples on board

need to know 4 things for exam - given coastal wind direction, coastline orientation, and hemisphere determine upwelling or downwelling system - know upwelling systems are cool and biologically productive - know direction of geostrophic flow in upwelling system - know California current is an upwelling system

10 m s -1 per 100 km

Eastern and Western Boundary Currents - western boundary currents are narrow and fast moving - eastern boundary currents are wider and slower moving - must know Kuroshio and Gulf Stream are western boundary currents - must know California Current is eastern boundary current

global wind-driven circulation know Gulf Stream, Kuroshio and California Current locations

North Atlantic currents and transport Gulf Stream

Gulf Stream from space

Benjamin Franklin ( ) Wondered why sailing from America to Europe took less time than sailing from Europe to America

Franklin’s chart of Gulf Stream

Henry M. Stommel ( )

Gulf Stream, Stommel (1947) - constant rotation rate (i.e. Coriolis force fixed) - no western intensification results stream lines sea surface elevation

Gulf Stream, Stommel (1947) - rotation rate (Coriolis force) increases with latitude - western intensification results stream lines sea surface elevation

ring formation in the Gulf Stream - warm core eddies are anti-cyclonic - cold core eddies are cyclonic

Gulf Stream rings warm core => anti-cyclonic circulation small gyre circulation features called eddies

Readings (Surface and Deep Circulation): Text Chapter 9 (pgs ) Reader pgs. 105 – 129 (Thermohaline Circulation)