Ocean Gyres

- 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

Geography 104 - “Physical Geography of the World’s Oceans”

Ekman transport is 90° to right (NH) of the wind

global atmospheric circulation

tropical

Global Atmospheric Pressure and Winds

July

January

cyclonic & anti-cyclonic wind effects

L H

cyclonic & anti-cyclonic wind effects

Ekman transport

convergence, downwelling

Ekman transport

divergence, upwelling

generally cooler water from upwelling

generally warmer surface waters

cyclonic & anti-cyclonic wind effects

Ekman transport

convergence, downwelling

Ekman transport

divergence, upwelling

Global Winds and Atmospheric Pressure

July

January

TradeWinds

Westerlies

Westerlies

= High Pressure

Geostrophic Currents

Ekman transport alters sea level (dynamic topography)

Sloped sea surface => pressure gradient

Geostrophic Flow in Ocean – Pressure and Coriolis force balance

Subtropical Gyre Circulation from Figure in Text

Subtropical Gyre Circulation from Figure in Text

Southern Hemisphere Subtropical Gyre going

wrong direction!!!!

geostrophic subtropical gyres

major ocean gyres and currents

global chlorophyll & primary productivity - Aug. 2004

lowest chlorophyll concentration in center of sub-tropical gyres

measuring dynamic height by satellite

deviation due to ocean currents

geoid

global dynamic height – satellite measurements

global dynamic height

Global Surface Currents from Altimetry

http://argo.colorado.edu/~realtime/gifs_tmp/global_ssh/gmt.13083.gif

dynamic height in the Gulf of Mexico

Mapped SSH in California Current

geostrophic flow direction is along lines of constant SSH

CTD deployment for dynamic height measurement

global dynamic height – shipboard measurements units: meters

5.3

4.1

5.6

5.0

6.35.6

4.4

5.5

Differences in dynamic height are important, not absolute values

Ocean Currents Associated with Dynamic Height Contours

major ocean gyres and currents

trash in the North Pacific subtropical gyre

Curtis C. Ebbesmeyer

~ 6 years to orbit N Pacific subtropical gyre

Summary

- cyclonic winds => surface divergence, upwelling

- anticyclonic winds => surface convergence, downwelling

- direct wind driven Ekman transport sets up sea level variations

- sea level variations (dynamic height) => geostrophic circulation

- for geostrophic flow currents are along lines of constant sea level (dynamic height)

- subtropical gyres are primary characteristic of ocean circulation

Readings (Surface and Deep Circulation):Text Chapter 9 (pgs 170 – 172)Reader pgs. 131 - 132