Equatorial currents
Typically, the north-south component of the wind,  y , and its variation with
longitude,  x , are negligible compared to the zonal winds. In fact, the
very large x-scale compared to y-scale means that x-variations are generally
negligible in the equatorial region in almost all terms except the pressure
y
p
gradient, x
y
If we ignore meridional winds, then  x = 0 and
curl  
 x
y
Then the continuity equation
M X M y

0
x
y
can be used to calculate the zonal (west-east direction) transport Mx from the
Sverdrup relation:
M X
M y
 1

  ( curl )
x
y
y 

 1 
(
)
y  y
These terms depend only on latitude, so integrating with respect to x gives:
MX 
M X
 1 
 1 
dx  
(
)dx  x( (
))
x
y  y
y  y
Variations in the wind stress dominate over variations in  in this analysis.
The point here is that the zonal transport Mx is roughly linearly proportional
to longitude x, recognizing that Mx=0 at the eastern boundary.
Equatorial winds and Ekman divergence/convergence
The analysis of Svedrup Flow revealed that:
   1   
1  2 X
   x
M X  x 

 y 2
 y   y  
which emphasizes that quite subtle features of the latitudinal variation of
winds drive the major east-west current systems.
But we can obtain the essence of this equatorial Sverdrup balance
qualitatively by considering the pattern of equatorial winds and the
associated Ekman divergence and convergence, and then deduce the effect
of these on sea level.
This equatorial convergence occurs in a zone referred to the Intertropical
Convergence Zone (ITCZ)
The ITCZ shifts seasonally toward the summer hemisphere.
This is most pronounced in Indian Ocean. The northern summer is the time
of the southwest monsoon, with strong heating over the land causing low air
pressure and moist warm winds approach the Indian subcontinent from the
southwest.
In the Pacific Ocean this seasonal movement is much less pronounced
because there are no heating extremes over surrounding land. Nevertheless,
the imbalance in the distribution of the continents globally means that the
location of the ITCZ is biased toward the northern hemisphere.
The ITCZ is a region of low wind speeds – a quiet zone between the
relentless, steady, easterly flow of the Trade winds. These are the Doldrums.
In Moby Dick, Captain Ahab and crew of the Pequod languished for weeks
in the doldrums of the Pacific Ocean before they could continue Ahab’s
obsessive quest to find the white whale.
The doldrums do not fall on the equator, so there is a modest easterly Trade
wind along the equator in the Pacific. This has important consequences for
the oceanic circulation at the equator.
At the equator – upwelling and westward downwind flow
At the equator, there is no Coriolis force, and the easterly wind blows water
toward the west. The accumulation of warm equatorial water in the western
Pacific is the source of the West Pacific Warm Pool.
Off equator currents
Just off the equator, easterly flow generates Ekman currents in the two
hemispheres that diverge, causing upwelling and locally decreased sea level.
Low sea level at the equator leads to geostrophic westward currents off the
equator, i.e. flowing in the same direction as the directly wind driven surface
at the equator.
In the center of the doldrums the wind stress is a minimum, so  y = 0 and
if we ignore the variation of wind stress with longitude, then  y = 0
means curl   0 .
The meridional transport is therefore zero, and we expect this latitude to
form a natural boundary between current systems.
What are the zonal currents doing?
Think first of the Ekman transports:
South of the equator:
 sea level is lowered at the equator due to the Ekman divergence and
the upwelling of cold water
 from south to north the sea level falls, which in the southern
hemisphere directs geostrophic flow to the left and westward along
the equator
 this is the South Equatorial Current (SEC)
North of the equator but south of the doldrums:
 Northward Ekman transport is greater to the south, so there is a
convergence that locally elevates the sea level
 The convergence is greatest where the winds are changing most
rapidly.
 South of the locally high sea level, the geostrophic flow is westward,
and this is still considered part of the SEC
North of the locally high sea level, flow is eastward.
 This is the North Equatorial Countercurrent (NECC)
 It is termed a countercurrent because it flows directly opposite the
local winds, which are the easterly Trades.
North of the doldrums:
 the northeast Trades build in strength going northward
 the Ekman transport again diverges, causing locally lowered sea level
 North of this divergence the sea level rises heading toward the gyre
center, so we again have westward flow
 this is the North Equatorial Current (NEC).

At the latitude where the wind stress curl given by
   x

does not change with latitude, i.e. y  y
 x
y

  0

   1   
1  2 X
   x
M X  x 

 y 2
 y   y  
the zonal flow is zero because
and this is the latitude defining the boundary between westward NEC
and eastward NECC.
Remember this pattern of sea level because we will see it shortly in satellite
observations of equatorial sea level from the TOPEX altimeter.
The pattern of equatorial currents has a dramatic influence on biological
productivity. Notably in the east Pacific, where the thermocline is shallow,
the divergence of equatorial Ekman transport causes upwelling that brings
deep nutrients to the surface.
Upwelled water is rich in nutrients but not plankton.
Equatorial biological oceanography
Ocean primary productivity is kicked off by the new nutrient source, causing
a local maximum in phytoplankton biomass close to the equator (0.5 – 1.0o
latitude)
Herbivorous zooplankton increase with the available food supply, and
subsequently carnivorous zooplankton increase.
Predatory fish tend to congregate near the convergence at the boundary
between the SEC and NECC, because prey species in the plankton
accumulate there.
Zonal patterns
While all this is going on north-south, what east-west structure is brought
about by the east-west currents?
Right at the equator we forget about the whole Coriolis business.
Water flows downwind just like in ought to.
Steady easterly winds blow water westward, and these waters continually
warm the whole way. This leads to an accumulation of very warm water in
the western Pacific:
The West Pacific Warm Pool is a huge mass of very warm water much of it
greater than 28oC.
The accumulated warm pool depresses the thermocline in the west, and
accordingly sea level is elevated in the WPWP compared to the east Pacific.
So there is a pressure force directed from west to east. This is obviously not
sufficient to overcome the westward flow of warm water at the surface
(otherwise we’d never get the Warm Pool).
But below the upper ocean mixed layer, where the direct influence of the
winds is lost, the pressure gradient remains and a west to east undercurrent
forms at the bottom of the mixed layer.
This Equatorial Undercurrent (EUC) is a remarkable flow. It is effectively a
ribbon of intense flow with speeds approaching 1.5 m s-1 only 200 m deep
yet 300 km wide. It is deepest in the west and gradually shoals eastward
along with the thermocline itself.
A down pressure gradient flow like this can only occur at the equator
because otherwise geostrophic balance would require the current to flow
perpendicular to the pressure gradient.
However, Coriolis still plays a role in stabilizing the EUC.
Any strong current is prone to instabilities that cause it to meander. We’ve
seen this in the meandering path of the Gulf Stream and its tendency to
spawn cyclonic and anticyclonic eddies.
If the EUC strays from its zonal path, say into the Southern hemisphere, the
Coriolis force starts to come into play deflecting the current left and back
toward the equator. Similarly, northward meanders are deflected southward,
and the EUC is trapped.
Isotherms and other property distributions are deflected around the core of
the EUC.
The waters of the EUC originate from the southern hemisphere, at the
western boundary, from the New Guinea Coastal Current, though there is
some exchange with surrounding waters during the course of the EUC transit
across the basin.
The NECC is supplied primarily by northern hemisphere waters, also
originating in the west from the Mindinao Eddy.
ENSO
What has become apparent through research over the past decade is that the
phenomenon we call ENSO is an instability of the tightly coupled tropical
atmosphere-ocean system.
The term ENSO is a combination of
  El Nino: the episodic warming of waters along the Peruvian coast
  Southern Oscillation: pattern of sea level pressure variability that
is coherent over the Pacific and much of the Indian Ocean region
SOI
The Southern Oscillation index (SOI) is classically defined as the anomaly
in the difference in sea level pressure between Tahiti and Darwin.
  When SOI is negative, the air pressure gradient along the equator
is less than usual, and we would expect the Trade winds to be weaker
than usual
  Conversely, positive SOI would indicate stronger Trades.
But there is more to ENSO than just variability in the equatorial winds. It
has become clear that in an ENSO event the entire Pacific air-sea system …








rain bands
associated winds
wind-driven currents
and SST patterns
… all move eastward together.
It was only in the late 1960s that it was recognized that these two processes
(El Nino, and SO) were linked.
Now we know that just about any upset to a component of the coupled
ocean-atmosphere circulation in the tropics will cause a feedback that links
all the components. (Chicken and Egg)
Understanding an ENSO event begins with understanding the evolution of
the SST field.
Many factors influence SST
 
Change in wind speed
1. 1. evaporative (latent) and sensible heat loss
2. 2. Ekman transports
  advect heat laterally
  produce Ekman pumping which changes deeper
density field and affects temperature of water available
for upwelling
  alters geostrophic flow
3. 3. alter the depth of directly wind mixed layer
  SST itself alters cloud cover, and incoming solar radiation
This plethora of processes affecting SST has made it difficult to understand
the details of the ENSO cycle, and it is not clear that there is one single
mechanism that is the trigger for an ENSO warm event
Westerly wind bursts
However, there is general agreement, that a necessary ingredient in the
commencement of an ENSO event is a reversal of the Trade wind pattern in
the western Pacific.
Winds are typically light in the west Pacific, but occasionally an outbreak of
westerly winds occur that persist for perhaps a week or more, coherent over
many 1000s km (sometimes from Indonesia to the Dateline).
[Tomczak and Godfrey fig. 19.8 tropical cyclone pair from severe westerly
wind burst]
A wind burst such as this sets in train wave motions that are characteristic of
the equatorial region.
[Sketch schematic of Kelvin wave dynamics]
The westerly wind burst causes:
1.
2.
3.
4.
5.
1.
2.
3.
4.
5.
converging Ekman transports (off equator) that increase sea level
and depress the thermocline
eastward geostrophic flow converges to the east
and diverges to the west
the pattern moves eastward
Note that the same happens for an easterly wind burst: the equatorial
Kelvin waves only go east so the anomalous pattern cannot easily
reset itself even if the WWB is followed by easterly wind anomalies.
6. 6. the equatorial Kelvin wave speed sqrt(g’h) is about 2.5 m/s
The observed speed is about 10 – 20% faster than this due to advection by
the EUC
The cyclonic wind patterns off equator cause local doming of the
thermocline - wind stress curl generates a pair of Rossby waves.
At 5-7o latitude the simple 1½ layer model we used to consider Rossby wave
propagation speed doesn’t work all that well, and the propagation speed is
about 0.3 m/s.
These Rossby waves take a few months to reach the western boundary. At
the west, they reflect as Kelvin waves but with a thermocline elevation (that
partly resets the thermocline deepening of the original Kelvin wave).
Meantime, the Kelvin wave has reached the eastern boundary of the Pacific.
It lowers the thermocline …
… raises sea level
… and warms SST
At the coast, it generates elevated sea level (and depressed thermocline) that
propagates poleward on both coasts.
These in turn radiate Rossby waves westward back across the basin.
As the Kelvin wave propagates eastward it takes thermocline waters with it,
expanding the West Pacific Warm Pool toward the east.
In doing so, it moves the region of warm SST and strong atmospheric
convection eastward.
This movement in the convection effectively short-circuits the Trades. The
local winds become westerly (or at least their anomaly from the mean does)
so that the Kelvin wave generation mechanism continues.
By generating further westerly wind anomalies as it propagates, the Kelvin
wave generation mechanism remains active further fostering downwelling of
the thermocline
  more warm water is drawn eastward
  this further translates the convection center
  and further weakens the Trades
This positive feedback mechanism is the essence of the ENSO phenomenon.
The deepening of the thermocline in the east Pacific propagates along the
Peruvian coast, so that the water that is upwelled (along the equator, and the
coast) due to divergent Ekman flow, is no longer the cooler nutrient rich
water below the thermocline, but warmer already nutrient depleted near
surface water.
This leads to a collapse of the primary production that sustains the equatorial
and coastal fisheries, and is the reason that ENSO warm events are such a
catastrophe in this region.
In the west Pacific, the loss of thermocline waters causes a shrinking and
shallowing of the West Pacific Warm Pool.
Eastward displacement of the major convection center causes negative
rainfall anomalies in Australia and Indonesia. Drought has severe negative
impacts on the economy and biosphere in those regions.
The ENSO process actually seems to be more sensitive to the modest SST
anomalies in the west Pacific, than to the dramatic SST anomalies in the
east.
The reasons for this aren’t all that clear, but efforts to improve predictability
of ENSO focus more on understanding the coupled west Pacific atmosphereocean system, and especially the trigger mechanism related to the WWBs.
Observational networks in the equatorial ocean
Important advances in ENSO predictability have been achieved through the
establishment of an observational network in the Pacific (and Atlantic):
The TAO array gives real-time data on:








subsurface ocean temperature
velocity
surface winds
air temperature and humidity
Cold episodes: La Nina
The 1997 El Nino was followed by a dramatic cold episode, or La Nina.
However, a quick look at the SOI shows that warm episodes do not always
transition to cold episodes. Often the SOI stays near zero and then returns to
a negative anomaly.
Researchers haven’t actually been able to determine what causes the
transition to La Nina following El Nino, or not.
To reset the ocean back to “normal,” or more correctly “non-ENSO,”
conditions, requires transport of heat to recharge the Warm Pool.
Planetary wave dynamics (Kelvin and Rossby) mean that temperatures can’t
be reset just by running everything in reverse. The delay in resetting the SST
patterns that allows anomalous patterns to persist for many seasons, and
even years, is probably related to the slow westward speed of the Rossby
waves.
The strong dependence of Rossby wave speed on latitude only serves to
complicate this resetting process.
What is clear is that significant global scale changes in the oceanatmosphere heat budget result from ENSO.
During El Nino, heat is transported to higher latitudes:
  the warming of the eastern equatorial Pacific reduces the air-sea
heat exchange there that usually draws a lot of heat out of the
atmosphere
  east Pacific positive SST anomalies propagate poleward along the
coast
  additional heat goes to the atmosphere through evaporation
  global average air temperatures rise by as much as 0.3oC in the
months after a strong El Nino
After La Nina
  increased solar input occurs due to lower than normal cloud cover
in the West Pacific Warm Pool
The tropical Pacific loses heat during El Nino, and gains it during La Nina.