Chapter 14 - Equatorial Processes
14.1 Equatorial Processes
Equatorial processes are important for understanding the influence of the
ocean on the atmosphere and the interannual fluctuations in global weather
The sun warms the vast expanses of the tropical Pacific and Indian Oceans, evaporating
water. When the water condenses as rain it releases so much heat
that these areas are the primary engine driving the atmospheric circulation (Figure
14.1). Rainfall over
extensive areas exceeds three meters per year
5.5), and some oceanic regions receive more than five meters of rain per
year. To put the numbers in perspective, five meters of rain per year releases
400 W/m2 of heat to the atmosphere. Equatorial currents modulate the
especially through the phenomenon known as El Niño, with global
consequences. We describe here first the basic equatorial processes, then the
year-to-year variability of the processes and the influence of the variability
|Figure 14.1 Average diabatic heating due to rain, absorbed
solar and infrared radiation between the surface and the top of
the atmosphere averaged over the period 1974–2004. Most of
the heating is due to the release of latent heat by rain. From the Japanese
25-year Reanalysis of atmospheric data.
14.1 Equatorial Processes
The tropical regions are characterized by a thin, permanent, shallow layer
of warm water over deeper, colder water. In this respect, the vertical stratfication
is similar to the summer stratification at higher latitudes. Surface waters are
hottest in the west (Figure
6.3) in the great Pacific-warm pool. The mixed layer
is deep in the west and very shallow in the east (Figure 14.2).
|Figure 14.2 The mean, upper-ocean, thermal structure
along the equator in the Pacific from north of New Guinea to Ecuador calculated
from data in the World Ocean Atlas 1998 (Image from NOAA Pacific Marine
The shallow thermocline has important consequences. The southeast trade-winds
blow along the equator (Figure
4.4) although they tend to be strongest in the east. North of the
equator, Ekman transport is
northward. South of the equator it is southward. The divergence of the Ekman
flow causes upwelling on
the equator. In the west, the upwelled water is warm. But in the east the
upwelled water is cold because the thermocline is so shallow. This leads to a
cold tongue of water at the sea surface extending from South America to near
the dateline (Figure
6.3). Surface temperature in the east is a balance among four processes:
- The strength of the upwelling, which is determined by the westward component
of the wind.
- The speed of westward currents which carry cold water from the coast of
Peru and Ecuador.
- North-south mixing with warmer waters on either side of the equator.
- Heat fluxes through the sea surface along the equator.
The east-west temperature gradient on the equator drives a zonal circulation
in the atmosphere, the Walker circulation. Thunderstorms over the warm pool
carry air upward, and sinking air in the east feeds the return flow at the
surface. Variations in the temperature gradient influences the Walker circulation,
which, in turn, influences the gradient. The feedback can lead to an instability,
the El Niño-Southern
Oscillation (ENSO) discussed in the next section.
||Figure 14.3 Average currents at 10m calculated
from the Modular Ocean Model driven by observed winds and mean heat fluxes
1981 to 1994. The model, operated by the NOAA
National Centers for Environmental Prediction, assimilates observed
surface and subsurface temperatures. From Behringer, Ji, and Leetmaa
The strong stratification confines the wind-driven circulation to the mixed
layer and upper thermocline. Sverdrup's theory and Munk's extension, described in
§11.1 and §11.3, explain the surface currents in the tropical Atlantic, Pacific, and Indian Oceans.
The currents include (Figure 14.3):
- The North Equatorial Countercurrent between 3°N and 10°N, which
flows eastward with a typical surface speed of 50 cm/s. The current is centered
on the band of weak winds
around 5-10°N where the wind stress curl changes sign. The band of weak
winds is called the doldrums.
Here where the north and south trade-winds converge, the tropical
- The North and South Equatorial Currents which flow westward in the zonal
band on either side of the countercurrent. The currents are shallow,
less than 200 m deep. The northern current is weak, with a speed less than
roughly 20cm/s. The southern current has a maximum speed of
around 100 cm/s, in the band between 3°N and the equator.
The currents in the Atlantic are similar to those in the Pacific because the trade-winds in that
ocean also converge near 5°-10°N. The South Equatorial Current in the Atlantic continues
northwest along the coast of Brazil, where it
is known as the North Brazil Current. In the Indian Ocean, the doldrums occur in the southern
hemisphere and only during the northern-hemisphere winter. In
the northern hemisphere, the currents reverse with the monsoon winds.
There is, however, much more to the story of equatorial currents.
Equatorial Undercurrent: Observations
Just a few meters below the surface on the equator
is a strong eastward flowing current, the Equatorial Undercurrent,
the last major oceanic current to be discovered. Here's the story:
In September 1951, aboard the U. S. Fish and Wildlife Service research
vessel long-line fishing on the equator south of Hawaii, it was noticed that
the subsurface gear drifted steadily to the east. The next year Cromwell,
in company with Montgomery and Stroup, led an expedition to investigate the vertical
distribution of horizontal velocity at the equator. Using
floating drogues at the surface and at various depths, they were able to
establish the presence, near the equator in the central Pacific, of a strong,
narrow eastward current in the lower part of the surface layer and the
upper part of the thermocline (Cromwell, et al., 1954).
A few years later the Scripps Eastropic Expedition, under
Cromwell's leadership, found the
current extended toward the east nearly to the Galapagos Islands but was
not present between those islands and the South American continent.
The current is remarkable in that, even though
comparable in transport to the Florida Current, its presence was unsuspected
ten years ago. Even now, neither the source nor the ultimate fate of its
waters has been established. No theory of oceanic circulation predicted
its existence, and only now are such theories being modified to account
for the important features of its flow.
— Warren S. Wooster (1960).
Evidence for an Equatorial Undercurrent had been noted by Buchanan, Krümmel, Puls, and others
in the Atlantic (Neumann, 1960).
However, no attention was paid to them. Other earlier
hints regarding this undercurrent were mentioned by Matthäus (1969). Thus the old
experience becomes even more obvious which says that discoveries not
attracting the attention of contemporaries simply do not exist.
Bob Arthur (1960) summarized the major aspects of the flow:
- Surface flow may be directed westward at speeds of 25-75 cm/s;
- Current reverses at a depth of from 20 m to 40 m;
- Eastward undercurrent extends to a depth of 400m with a transport
of as much as 30 Sv = 30 × 106 m3/s;
- Core of maximum eastward velocity (0.50 m/s - 1.50 m/s) rises from a depth
of 100 m at 140°W to 40 m at 98°W, then dips down;
- Undercurrent appears to be symmetrical about the equator and becomes much
thinner and weaker at 2°N and 2°S.
In essence, the Pacific Equatorial Undercurrent is a ribbon with dimensions
of 0.2 km × 300 km × 13,000 km (Figure 14.4).
||Figure 14.4 Cross section of the Equatorial
Undercurrent in the Pacific calculated from Modular Ocean Model with assimilated
surface data (See §14.5). The section
an average from 160°E to 170°E from January 1965 to December 1999.
Stippled areas are westward flowing. From Nevin Fuckar.
Equatorial Undercurrent: Theory
Although we do not yet have a complete
theory for the undercurrent, we do have a clear understanding of some of the more important processes at
work in the equatorial regions. Pedlosky (1996), in his excellent chapter on Equatorial Dynamics of the
Thermocline: The Equatorial Undercurrent, points out that the basic dynamical balances we have used
in mid-latitudes break down near or on the equator.
Near the equator:
- The Coriolis parameter becomes very small, going to zero at the equator:
here φ is latitude, β =
≈ 2Ω/R near
the equator, and y =
- Planetary vorticity f is also small, and
the advection of relative vorticity cannot be neglected. Thus the Sverdrup
balance (11.7) must be modified.
- The geostrophic and vorticity balances fail when the meridional distance L to
the equator is ,
where β = ∂f/∂y.
If U = 1 m/s, then L =
200 km or 2° of latitude. Lagerloe et al.,
(1999), using measured currents, show that currents near the equator can
be described by
the geostrophic balance for |φ| >
2.2°. They also show that flow closer to the equator can be described
using a β-plane approximation f = βy.
- The geostrophic balance for zonal currents works so well near the equator
because f and ∂ζ/∂y → 0
→ 0, where ζ is
sea surface topography.
The upwelled water along the equator produced by Ekman pumping is not
part of a two-dimensional flow in a north-south, meridional plane. Instead, the
flow is three-dimensional. The water tends to flow along the contours of
constant density (isopycnal surfaces), which are close to the lines of constant
temperature in Figure 14.2. Cold water enters the undercurrent in the far
west Pacific, it moves eastward along the equator, and as it does it moves
closer to the surface. Note, for example, that the 25° isotherm enters
the undercurrent at a depth near 125 m in the western Pacific at 170°E
and eventually reaches the surface at 125°W in the eastern Pacific.
The meridional geostrophic balance near the equator gives the speed of the
zonal currents, but it does not explain what drives the undercurrent. A very
simplified theory for the undercurrent is based on a balance of zonal pressure
gradients along the equator. Wind stress pushes water westward, producing
the deep thermocline and warm pool in the west. The deepening of the thermocline
causes the sea-surface topography ζ to
be higher in the west, assuming that flow below the thermocline is weak. Thus
there is an eastward pressure gradient along the equator in the surface layers
to a depth of a few hundred meters. The eastward pressure gradient at the
surface is balanced by the wind stress
Tx, (layer A in Figure 14.5), so
ρ Tx =
|Figure 14.5 Left: Cross-sectional
sketch of the thermocline and sea-surface topography along the equator.
Right: Eastward pressure gradient in the central Pacific caused
by the density structure at left.
Below a few tens of meters in layer B, the influence of the wind stress is small, and the pressure
gradient is unbalanced, leading to an accelerated flow toward
the east, the equatorial undercurrent. Within this layer, the flow accelerates until the pressure
gradient is balanced by frictional forces which tend to slow
the current. At depths below a few hundred meters in layer C, the eastward pressure gradient is too
weak to produce a current, ∂p/∂x ≈ 0.
Coriolis forces keep the equatorial undercurrent centered on the equator. If the flow strays northward,
the Coriolis force deflects the current southward. The opposite occurs if the flow strays southward.