Notes

Outline

Lecture 4: OCEANS (Outline)  Basic Structures and Dynamics       Ekman transport       Geostrophic currents  Surface Ocean Circulation       Subtropicl gyre       Boundary current  Deep Ocean Circulation       Thermohaline conveyor belt

Basic Ocean Structures  Upper Ocean (the first few hundred m)     Shallow, warm upper layer where light is abundant and where most marine life can be found.  Deep Ocean    Cold, dark, deep ocean where plenty supplies of nutrients and carbon exist.

Basic Ocean Current Systems

The State of Oceans  Temperature     warm on the upper ocean, cold in the deeper ocean.  Salinity    variations caused by determined by evaporation, precipitation, sea-ice formation and melt, and river runoff.  Density     small in the upper ocean, large in the deeper ocean.

Potential Temperature

Salinity

Density

Density and Temperature and Salinity

Vertical Structure of Ocean

Mixed Layer Processes

Seasonal Variation of Mixed Layer

Two Circulation Systems

Global Surface Currents

Six Great Current Circuits in the World Ocean

Characteristics of the Gyres Currents are in geostropic balance Each gyre includes 4 current components:       two boundary currents: western and eastern       two transverse currents: easteward and westward Western boundary current (jet stream of ocean)        the fast, deep, and narrow current moves warm water polarward  (transport ~50 Sv or greater) Eastern boundary current       the slow, shallow, and broad current moves cold water equatorward (transport ~ 10-15 Sv) Trade wind-driven current        the moderately shallow and broad westward current (transport ~ 30 Sv) Westerly-driven current        the wider and slower (than the trade wind-driven current) eastward current

Major Current Names Western Boundary Current      Gulf Stream (in the North Atlantic)        Kuroshio Current (in the North Pacific)        Brazil Current (in the South Atlantic)        Eastern Australian Current (in the South Pacific)        Agulhas Current (in the Indian Ocean) Eastern Boundary Current       Canary Current (in the North Atlantic)        California Current (in the North Pacific)        Benguela Current (in the South Atlantic)        Peru Current (in the South Pacific)       Western Australian Current (in the Indian Ocean)

Gulf Stream

Surface Current – Geostrophic Gyre Mixed Layer       Currents controlled by frictional force + Coriolis force       à wind-driven circulation       à Ekman transport (horizontal direction)       à convergence/divergence       à downwelling/upwelling at the bottom of mixed layer  Thermocline       downwelling/upwelling in the mixed layer       à pressure gradient force + Coriolis force       à geostrophic current       à Sverdrup transport (horizontal)

Step 1: Surface Winds

Winds and Surface Currents

Step 2: Ekman Layer (frictional force + Coriolis Force)

Ekman Spiral – A Result of Coriolis Force

Formula for Ekman Transport

How Deep is the Ekman Layer?

Ekman Transport

Step 3: Geostrophic Current (Pressure Gradient Force + Corioils Foce)

Ekman Transport à Convergence/Divergence

Geostrophic Current

Step 4: Boundary Currents

Boundary Currents

Eastern Boundary Current

Costal Upwelling/Downwelling

Equatorial Current System  The Equatorial Counter Current, which flows towards the east, is a partial return of water carried westward by the North and South Equatorial currents.

Equatorial Under Current

Deep Ocean Circulation: Density-Driven

Thermohaline Circulation

Two Regions of Deep Water Formation

Two Processes to Increase Salinity in High Latitudes  Evaporation: Extremely cold, dry winter air enhances evaporation from the relatively warm ocean è increase salinity in the ocean.  Formation of Sea Ice: When sea ice forms, salts are left in the ocean è increase salinity

Ocean Water Mass Surface Water       to a depth of about 200 meters Central Water       to the bottom of the main thermocline Intermediate Water       to about 1500 meters Deep Water       below intermediate water but not in       contact with the bottom Bottom Water       in contact with sea floor

Thermohaline Conveyor Belt

Thermohaline Circulation  Thermo    è temperature    Haline    è salinity

Thermohaline Conveyor Belt

It Takes ~1000 Years for Deep Ocean Waters to Travel Around…  If we date a water parcel from the time that it leaves the surface and sink into the deep ocean è Then the youngest water is in the deep north Atlantic, and the oldest water is in the deep northern Pacific, where its age is estimated to be 1000 year.

The Most Unpolluted Waters are..    the waters in the deep northern Pacific.  The man-released CFC and the chemical tritium and C14, which were released through atmospheric atomic bomb test in the 1950s and 1960s, entered the deep ocean in the northern Atlantic and are still moving southward slowly.  Those pollutions just cross the equator in the Atlantic è They have not reached the deep northern Pacific yet!!

Global Warming and Thermohaline Circulation  If the warming is slow     The salinity is high enough to still  produce a thermohaline circulation The circulation will transfer the heat to deep ocean The warming in the atmosphere will be deferred.  If the warming is fast      Surface ocean becomes so warm (low water density) No more thermohalione circulation The rate of global warming in the atmosphere will increase.