Chapter 5: Other Major Current Systems

Key Points: Summary of Chapter 5

Components of the equatorial current system include:

• westward flowing N,S equatorial currents (driven by trades and geostrophy)

• eastward flowing counter current (surface/subsurface), and undercurrent

Equatorial Current System best defined in the Pacific (basin size, ITCZ)

ITCZ is north of the equator, as SE Trades cross hemispheres create divergence just south of the equator and convergence around 4o N

Prevailing easterly winds slope the sea surface up to the western end of the basin, creates an eastward directed pressure gradient, counter currents flow eastward in regions of low wind stress (doldrums)

This pressure gradient force also drives the Equatorial Undercurrent (wind stress at the surface around the equator is strong but at depth, baroclinic conditions are sufficiently strong to drive the fast flowing current to the east) Coriolis constrains the flow to the equator (meanders can exist)

Key Points Continued: Summary of Chapter 5

In the Pacific and Atlantic surface divergence south of the equator associated with the South Equatorial Current (SEC) produces regions of upwelling.

Upwelling is also produced by Trade Winds blowing along the eastern side of the basins. This can be seasonal in regions where the ITCZ migrates

The Asian Monsoons influence the circulation of the Indian Ocean. The Equatorial Undercurrent is seasonal, and the surface Somali Current reverses direction becoming an intense western boundary current during the Southwest Monsoon. The North Equatorial Current also reverses direction and become the South-West Monsoon Current

Disturbances in the ocean (such as Monsoons, ENSO, etc..) in part, propagate as Kelvin and Rossby waves both at the surface (barotropic waves) and along density boundaries (baroclinic waves)

Kelvin waves travel eastward along the equator or along the coast (NH coast on right side, SH coast on left side)

Rossby waves (Planetary waves, conservation of potential vorticity) travel westward along lines of latitude (slower)

Keep In Mind:

Coriolis Force on the Equator is zero

Coriolis Force by 0.5o influences flow of water

Currents To Know:

North Equatorial Current (NEC)

South Equatorial Current (SEC)

Equatorial Counter Current (ECC)

Equatorial Under Current (EUC)

Migration of the ITCZ

•NE and SE Trade Winds cross the equator

•Seasonal

•Variations in the magnitude and presence of equatorial currents, in each basin

•Equatorial current system best defined in the Pacific Ocean (trade winds blow over longer distances, less migration of the ITCZ)

Refined Picture of the Equatorial Current System: Improved Measuring Techniques

NEC, SEC are driven by wind and geostrophy

NECC, pressure gradient, Coriolis adds to the convergence at 4o N associated with the SEC

NECC, SECC flow in regions of weak winds ‘doldrums’

Subtropical counter-currents -------- horse latitudes?

Variability in magnitude

Equatorial Undercurrent (EUC) & Eastward Directed Pressure Gradient

Wind driven water from the surface mixed layer piled on the western side of the basin

Wind stress balances the pressure gradient (Coriolis Force ~= 0 at equator)

Adjustment (depression) of the thermocline on the western end

Baroclinic conditions at depth drive a jet-like current eastward eventually balance by friction (eddy viscosity)

Equatorial Undercurrent (EUC) & Eastward Directed Pressure Gradient

Core of EUC around 200m water depth

Velocity can approach 1.5 m/s

westward

EUCECC

North Subsurface Counter Current

Think About Recirculation in the Equatorial Current System

Cromwell current (EUC) water originates primarily from the SEC

Once reaching the eastern basin EUC water feeds the NEC and the SEC

Remember:

sea surface slope – pressure gradient in the surface mixed layer is opposite the slope of the thermocline

Thermocline: 17o C

Thermostads - pycnostads

‘doming’ isotherms

Upwelling In Low Latitudes: Eastern Tropical Atlantic

Seasonal variation, migration of ITCZ, strength of SE Trades

More permanent upwelling regions associated with westward directed wind stress

Monsoonal Circulation: winds that change seasonally

Northeast Monsoon in winter

• surface circulation of the northern Indian Ocean most closely resembles that of the Atlantic and Pacific

Southwest Monsoon in summer

• stronger of the two

Indian Ocean Surface Circulation

Somali Current 180o reversal becomes an intense western boundary current during southwest monsoon.

Chlorophyll concentrations (orange/yellow): illustrating the effects of upwelling associated with the southwest monsoon

Low level jet drives water away from the coast producing regions of more intense upwelling as well as the fast flowing Somali Current.

Note the Somali Current is an intense western boundary current but is not derived from the same balance of forces as the Gulf Stream.

Agulhas Current

Agulhas – Benguela system

Agulhas Current: is the next most powerful western boundary current, second only to the Gulf Stream. Its retroflection off the tip of southern Africa is a source of eddies, many of which are carried into the Atlantic.

Think about: basin land boundaries, strength of coriolis etc.

Waves: the ocean can respond to the winds in distant places by means of large-scale disturbances that travel as waves.

Barotropic: surface waves

Baroclinic: density surface (thermocline)

Rossby (Planetary Waves)

Kelvin

Examples of barotropic and baroclinic waves propagating through the ocean

Most tides are barotropic ‘Kelvin’ waves

Think about what would happen if the wind stress was dramatically reduced or changed directions in the case of the Asian Monsoon

Kelvin Waves

Travel eastward along the equator as a double wave ‘equatorial wave guide’

Travel along coasts (coast on right in the NH and on the left in the SH)

Balance between pressure gradient force and coriolis force.

Kelvin Waves

Surface equatorial kelvin waves travel ~200 m/s

Rossby radius of deformation L = c/f

•High latitudes smaller eddies closely trapped to the coast (increase in planetary vorticity)

•Low latitudes larger radius

Kelvin waves in the thermocline can have dramatic effects, particularly in low latitudes where the mixed surface layer is thin.

Northward migration of ITCZ in western Atlantic generates disturbance that propagates eastward

Splits into two coastal Kelvin waves when hits the eastern boundary

The region of the disturbance where the thermocline bulges upward cold nutrient rich sub-thermocline water can reach the surface

4-6 week travel time

Rossby Waves:

Propagate from east to west across basin

Travel along lines of latitude

Move slower than Kelvin waves

Conservation of Potential Vorticity

Example: Waves in the jet-stream

Modeled Propagation of Equatorial Kelvin Wave

At mid latitudes - western sides of ocean basins are more connected to mid ocean disturbances because Rossby Waves can communicate the information

At the equator the ocean can respond quicker to disturbances because both Kelvin and Rossby waves can propagate

ENSO: El Nino – Southern Oscillation

“Interest in the phenomenon of El Nino goes back to the mid-19th century but it was the El Nino event of 1972-73 that stimulated large-scale research into climatic fluctuations, which began to be seen as a result of the interaction between atmosphere and ocean.”

El Nino events are perturbations of the ocean-atmosphere system

Disturbance – a depression in the thermocline accompanied by a slight rise in sea-level propagates eastwards along the Equator as a pulse or series of pulses (Kelvin Waves)

SO Index and Multivariate ENSO Index

Atmospheric pressure at sea level, zonal and meridional winds, sea-surface temperature, surface air temperature, and the overall cloudiness

Satellite altimetry data for the 1997-98 ENSO event

At the eastern boundary, the equatorial Kelvin waves split into northward – and – southward traveling coastal Kelvin waves

Some energy is reflected back as a Rossby wave/s

Circulation At High Latitudes

The Arctic Sea:

Relatively enclosed basin (connection to the Pacific through the Bering Strait and to the Atlantic through the Greenland and Norwegian Seas)

Enclosed nature influences ice cover

Circulation: was originally deduced from ice flows and drifting ships, supplemented with direct current measurements and geostrophic calculations

Seasonal change in ice cover for northern and southern high latitudes

~10% of Arctic ice leaves annually through the Fram Straits between Greenland and Spitsbergen

The Great Salinity Anomaly

1973 – 1981

“The Day After Tomorrow” omg

Low salinity plume of water circulates on the surface and disrupts deepwater formation

Antarctic Circumpolar Wave (from Wikipedia)

The Antarctic Circumpolar Wave is a coupled ocean/atmosphere wave that circles the Southern Ocean in approximately eight years. Since it is a wave-2 phenomenon (there are two ridges and two troughs in a latitude circle) at each fixed point in space a signal with a period of four years is seen. The wave moves eastward with the prevailing currents.Note that although the "wave" is seen in temperature, atmospheric pressure, sea ice and ocean height, the variations are hard to see in the raw data and need to be filtered to become apparent. Because the reliable record for the Southern Ocean is short (since the early 1980s) and signal processing is needed to reveal its existence, some climatologists doubt the existence of the wave. Others accept its existence but say that it varies in strength over decades.The wave was discovered simultaneously by Warren White and R G Peterson; and Jacobs and Mitchell; in 1996. Since then, ideas about the wave structure and maintenance mechanisms have changed and grown: by some accounts it is now to be considered as part of a global ENSO wave.

References: White, W.B. and R.G. Peterson (1996): An Antarctic circumpolar wave in surface pressure, temperature and sea-ice extent. Nature 380:699–702.Jacobs, G. A., and J. L. Mitchell (1996): Ocean circulation variations associated with the Antarctic Circumpolar Wave. Geophysical Research Letters 23(21):2947–50