The Surface Currents OCEA 101
Why should you care? - the surface ocean circulation controls the major ocean biomes - variations in ocean circulation control the supply of nutrients for marine organisms - a combination of the ocean circulation and natural selection determines spawning cycles
Overview The Ekman spiral Ekman layers and transport Wind-driven gyres Western boundary currents Eddies Upwelling Circulation influences on marine organisms
The Wind-Driven Circulation Recall the Coriolis force: - fluid motion is deflected to the right of initial motion in the northern hemisphere - fluid motion is deflected to the left of initial motion in the southern hemisphere Consider what happens when a wind blows over the surface of the ocean. The small ripples on the ocean surface that result from surface tension, allow the surface winds to more effectively grip the ocean.
A Simple Thought Experiment Now let s do a simple thought experiment. Imagine that the ocean is made up of lots of thin layers. WIND Layer 1 Ocean surface Layer 2 Layer 3 Current Depth Layer 4 Friction Layer 5 etc What will happen? What about the Coriolis Force?
The Ekman Spiral Clockwise spiral in NH Counter clockwise spiral in SH Ekman layer depth depends on latitude and viscosity. Ekman Layer ~100m What is the AVERAGE direction of motion?
Ekman Transport Ekman transport is the net transport (velocity x depth) over the Ekman layer Ekman transport is perpendicular to the wind direction: - to the RIGHT of wind direction in NH Ekman - to the LEFT of wind direction in SH
Ekman Transport
Ocean Gyres Tropical trade winds drive Ekman transport AWAY from equator in both hemispheres. Midlatitude westerlies drive Ekman transport towards equator both hemispheres. The result is convergence and elevated sea level at subtropical latitudes.
~ 2m PGF PGF F g F g LOW P HIGH P LOW P Consider pressure at some reference depth
Geostrophic Flow F g = Pressure gradient force F c = Coriolis force V = surface current F g = F c GEOSTROPHIC BALANCE water flows along isobars
Wind Pattern Development
Wind-Driven Ocean Currents
The Major Wind-Driven Ocean Gyres N. Pacific Subpolar Gyre N. Atlantic Subpolar Gyre N. Atlantic Subtropical Gyre N. Pacific Subtropical Gyre S. Pacific Subtropical Gyre S. Atlantic Subtropical Gyre S. Indian Subtropical Gyre
Western Boundary Currents The currents in all ocean gyres are typically much stronger along the western margins (i.e. along eastern coastal boundaries) than in the interior parts of the ocean. This is due to a process called western intsensification. How does it work?
Western Intensification Western intensification is due to the combined effects of: - Latitudinal variations in the strength of the Coriolis effect (varies as sine of latitude) - Latitudinal variations of the wind direction - Friction along the coast
Vorticity Vorticity=angular momentum or spin.
Planetary Vorticity due to rotation of the earth
Relative Vorticity due to Shear - + - +
Absolute Vorticity Absolute Vorticity= Planetary vorticity + Relative vorticity Absolute vorticity is CONSERVED by fluid motion.
Gain of +ve vorticity from boundary by friction requires large velocity gradients Gain of -ve vorticity from wind
Rossby Waves Vorticity is carried by Rossby waves.
Rossby Waves Rossby waves have some very special properties: (1) Long wavelength Rossby waves propagate rapidly westwards (2) Short wavelength Rossby waves propagate very slowly eastward (3) At a western ocean boundary (continental east coast) long RWs reflect as short RWs which means that short RW energy accumulates along the western boundary. This leads to so-called western intensification and the formation of a western boundary current.
Western Boundary Reflection Long Rossby Wave (Fast) Western Boundary Eastern Boundary Short Rossby Wave (Very slow)
Eastern Boundary Reflection Short Rossby Wave (Very slow) Western Boundary Eastern Boundary Long Rossby Wave (Fast)
Rossby Waves The dynamics of the ocean circulation along the eastern and western continental margins is fundamentally different because of Rossby wave dynamics.
The North Atlantic Subtropical Gyre
Major Western Boundary Currents N. Atlantic Gulf Stream S. Atlantic Brazil Current N. Pacific Kuroshio Current S. Pacific East Australia Current N. Indian Somali Current S. Indian Agulhas Current
Ocean Eddies Eddies are ubiquitous in the ocean! They are very important because: - they can transport heat - nutrients - marine organisms - they can mix up the ocean
California Coast Temperature Chlorophyll
Satellite Measured Surface Irradiance (AVHRR)
GOM SSH and CHL Movies ~Andy/OCEA80B/SSH_1993to2006.mov
Convergence and Divergence Z Convergence Divergence Downwelling Upwelling Significantly influences marine organisms in open ocean Significantly influences marine organisms in some coastal and equatorial regions
Langmuir Circulations An example of small-scale convergence and divergence
Large-Scale Convergences and Divergences
Convergence Zones Although NADW and AABW are most important, convergence zones form intermediate water
Ventilation and Subduction Z Subtropical Convergence Westerly (Eastward) Wind ρ 1 Ekman Transport ρ 2 ρ 2 > ρ 1 Poleward
The Global Ocean Conveyor Belt Circulation
Intermediate Waters AAIW, NPIW and Nordic Sea Waters
Bottom Waters - AABW
Shallow Mode Waters Associated with Shallow Overturning Cells
Heat transport (PW) (Talley, 2003, JPO)
Most of the basin is filled by the gyres nutrient poor, warm waters deep nutricline due to Ekman convergence and downwelling
Thermocline and Nutricline Generally closely related Thermocline is a very effective barrier against vertical motion. Small vertical motions of sea surface are manifest as large displacements of the thermocline (and nutricline) in the opposite direction.
Pacific Distribution of Euphausia Brevis Note the remarkable correlation of distribution with subtropical gyre circulation. Biantitropical distribution.
Central Subtropical Gyres Satellite obs show low surface Chlorophyll. Surveys suggest significant zooplankton communities in central gyre regions. How come? Several possibilities
1. All phytoplankton is consumed by zooplankton immediately 2. Phytoplankton are below the surface and not visible from satellite 3. Phytoplankton and zooplankton obs are not coincidental (satellite obs from 1990 s, zooplankton net data from 1950s and 60s) perhaps there have been decadal changes? It appears that 1 and 2 are both true
Observations show that the daily production of phytoplankton stock is consumed by zooplankton every day
Subsurface Chlorophyll Maximum Vertical section of Chlorophyll from Alaska to Hawaii South of 45N maximum in Chlorophyll is ~100m deep.
Several factors: Nutricline close enough to surface that there is an intermittent supply of nutrients from below. Cyanobacteria are important for recycling DOM via the microbial loop Some phytoplankton species have evolved to utilize the blue light wavelengths at depth efficiently because of certain pigments.
Coastal upwelling and downwelling is promoted by Ekman transport associated with alongshore winds
High The California Current (CC)
California Upwelling Temperature Coastal upwelling is often enhanced by the presence of coastal capes and headlands eg Point Sur and Point Reyes. Chlorophyll
California Current Euphausid Distributions North Pacific distribution of Nyctiphanes simplex, a euphausid.
The California Current System
Eastern Boundary Currents Equatorward Eastern boundary current Geostrophic balance Equatorward Wind CF F c PGF F g Reference depth Eastern boundary currents: California Current (N Pac) Humboldt Current (S Pac) Canary Current (N Atl) Benguela Current (S Atl) Leeuwin Current (S Ind) High Pressure PGF Low Pressure Northern Hemisphere Example
Other Forms of Upwelling Topographic Upwelling Capes and Points Differential Currents Langmuir Cells
Equatorial Upwelling
Seasonal Variations in Upwelling and Downwelling
Map of the equatorial Pacific with isopleths of surface nitrate concentration.
Ocean Chlorophyll and Major Coastal Upwelling Regions
Distributions of tropical species of Copepods and Euphausids