TOPIC 4: CIRCULATION IN THE EQUATORIAL PACIFIC AND EL NINO EVENTS
Readings: Ocean Circulation: Chap 5 (pages 143-153 and 170-176)
Introduction to Physical Oceanography (on-line text): Chap 14 (all)
Web Site: http://www.elnino.noaa.gov/
A. Importance of the Equatorial Ocean
1. Large geographic regions
 in the Pacific Ocean the equatorial area of the ocean is 160° wide (120ºE to 80ºW) and 20°
broad (10ºS to 10ºN).
 global equatorial ocean surface area is 90x1012 m2 (50x1012 m2 in Pacific; 20x1012 m2 each in
Atlantic and Indian) out of 360 x1012 m2 or ~25% of total ocean area
2. Importance of equatorial region to global climate
 region of highest solar insolation heat input (affects earth’s heat budget)
 region is a major source of atmospheric water vapor input (affects earth’s water budget)
 location of ITCZ, the boundary between northern and southern Trade Wind regimes
3. Biologically a very productive region of the ocean
- estimated that one-third of total ocean photosynthesis occurs in equatorial ocean
4. El Nino (and La Nina) events
 most important rapid climate change events on earth
 affect global temperature and precipitation patterns
B. Trade Winds control Equatorial Circulation
1. Energy source for equatorial ocean currents is the trade winds. (Fig 1)
 trade winds are strong and consistent, with strength of southern Trades exceeding the northern
Trades
 they impart a lot of energy into the equatorial ocean via wind stress force
2. Intertropical Convergence Zone (ITCZ) (Fig. 2)
a. ITCZ marks the location of the earth’s highest temperatures and determines the boundary
between NE Trade Winds and SE Trade Winds
b. ITCZ is located near the equator but its position varies significantly both seasonally and
longitudinally. The SE Trades can penetrate into the northern hemisphere and the NE Trades
can penetrate into the southern hemisphere depending on whether the ITCZ is located south or
north of the equator.
c. Seasonally, the latitude of the ITCZ migrates as the hemispheres heating rate changes
 thus the ITCZ is displaced north of the equator in the boreal (northern) summer and south
of the equator (slightly) in the austral (southern) summer
 the ITCZ excursions north of the equator exceed those south of the equator because there
is more land mass, which heats up rapidly, in the northern hemisphere
d. In the eastern Pacific the ITCZ is always located north of the equator
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 In January, northeast trades stop at about 5°N and southeast trades cross the equator
 In July, northeast trades stop at about 10°N and southeast trades cross the equator
e. The ITCZ identifies a region called the doldrums where calm periods occur ~30% of the time
which is in stark contrast to the trade wind regions where strong winds (especiallky SE
Trades) blow continuously
f. ITCZ also indicates the location of strongest upward flow of air into the upper troposphere,
which in turn causes high precipitation rates
3. At the equator, there is divergence of surface water because the net Ekman transport from the
Trade Winds is to the right north of the equator and to the left south of the equator (Fig. 3)
 thus Ekman transport caused by the Trade Winds is poleward and westward in both
hemispheres
 this contrasts the situation in the subtropical gyres which are a site of convergence because
of the Ekman transport caused by the Trades and Westerlies
4. The Trade Winds cause north-south variations in sea surface height as a result of Ekman transport.
(Fig. 4)
a) the change in direction of the Ekman Transport as the Trade Winds cross the equator
causes low sea surface height at the equator
b) the meridional (north-south) variations in the strength (speed) of the NE and SE Trade
Winds causes both convergence and divergence
c) the ITCZ location is a region of divergence
5. South of the equator, there is a poleward (southward) increase in sea surface height towards the
subtropical gyre (Figs. 4, 5)
 this is a result of the southward decrease in Trade Wind speed which cause a southward
decrease in Ekman transport
Note: since the wind stress exerted by winds is proportional to the square of wind speed
(tau = c*ρ*U2) and the Ekman transport depends on wind stress, then the Ekman transport
depends on the square of the wind speed. Thus changes in wind speed cause greater
changes in Ekman transport (e.g., double wind speed and Ekman transport quadruples).
 the southward decrease in Trade Wind strength causes water to pile up and increases the sea
surface height southward, that is, at these latitudes the Ekman transport from the north exceeds
the Ekman transport to the south and this causes the sea surface height to increase southward
(convergence)
6. North of the equator there is an initial increase in the sea surface height between 0° and ~5°N
because there is a northward decrease in the strength of the Trade winds in this region (Fig. 5)
 this occurs for similar reasons as described above, that is, the northward Ekman transport from
the south is higher than the northward Ekman transport away to the north because of the
northward decrease in wind speed (convergence) and this causes a northward increase in sea
surface height
 the peak in sea surface height at ~ 5ºN is the result of convergence and causes downwelling
(opposite to the situation at the equator where there is low sea surface height and upwelling)
7. The ITCZ is located at about ~5°N where there is a minimum in Trade Wind strength (Figs. 5)
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8. Between 5°N and 10°N the Trade Wind speeds increase and thus there is a northward decrease in
sea surface height (Fig. 5)
 the rapid increase in trade wind strength at about 8-10°N causes a minimum in sea surface
height in this region, that is, the Ekman transport to the north exceeds the Ekman transport
from the south in this region (divergence) and the sea surface height decreases
9. The sea surface height minimum at around ~10°N caused by Ekman divergence results in
upwelling (but not as strong as at the equator) (Figs. 4, 5)
10. North of 10°N, the NE Trade Wind strength decreases and produces a northward increase in sea
surface height to the middle of the subtropical gyre (~30°N) (Fig. 5)
- Ekman transport from the south exceeds the Ekman transport to the north in this region
(convergence) and causes the sea surface height to increase
11. In summary, the meridional (north-south) variations in the strength of the Trade Winds and the
change in the direction of Ekman transport at the equator yield the observed meridional trends in sea
surface height and the locations of upwelling and downwelling within the equatorial ocean.
C. Equatorial Surface Currents
1. The observed meridional trend in sea surface height causes meridional trends in the horizontal
pressure gradients (P/y, where y represents the north-south direction) (Fig. 5)
 this is important because HPGs produce geostrophic currents in the Equatorial Ocean
2. Because the strongest HPGs are oriented north-south in the equatorial ocean (Fig. 5), the
geostrophic current directions are east-west
- because the Coriolis Effect is weak at these low latitudes, the current speeds are faster for a
given HPG that at more poleward latitudes
 Remember: the Coriolis Effect approaches zero at the equator.
3.Geostrophic currents are faster when the Coriolis Effect is weaker.
-the geostrophic current results from a balance between Coriolis Force and HPG force and the
current speed depends inversely on f, where f = 2*Ω*sin (latitude)
- the geostrophic current velocity = g/f *ΔZ/Δy
- as the latitude approaches zero, the sin (latitude) approaches zero and f approaches zero
- thus for an equal HPG force (ΔZ/Δy), a current at 5ºN will be ~6x faster than a current at
30 ºN because the sin (5º) = 0.087 and sin (30º) = 0.5.
-Thus current speeds in the equatorial ocean are higher than other regions despite the sea
surface height gradients not being anomalously high
4. The horizontal pressure gradient force between 15ºS and 0º and between 0º and 5º N point towards
the equator (Fig. 5)
 This HPG orientation causes westward geostrophic flow between 5°N and 15°S
o Remember: the geostrophic current flows perpendicular to the HPG direction
 This westward flowing current is called the South Equatorial Current (SEC) crosses the
equator to ~ 5ºN
 the SEC has mean speed of ~20 cm/sec and transport of ~35 Sv
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5. The northward decrease in sea surface height between 5° and 10°N causes a northward pointing
HPG force, which causes eastward geostrophic current flow (Fig. 5)
 this eastward flowing current is called the North Equatorial Countercurrent (NECC)
 the NECC has an annual mean speed of ~40 cm/s and transport of 20 Sv
6. North of 10°N, the sea surface height increases northward and the horizontal pressure gradient
force points southward, which causes westward flow from about 10°N to 20°N (Fig. 5)
 this current is called North Equatorial Current (NEC)
 the NEC has mean speeds of 20 cm/s and transport of 20 Sv
 the NEC represents the southern boundary of the subtropical gyre and the northward boundary
of the equatorial currents
7. Notice that the meridional (N-S) trend in the depth of the thermocline mirrors the N-S trend in SSH
(Figs. 4, 5).
-this is a result of the baroclinic adjustment of the isopycnal slopes to the meridional trend in
sea surface slope
8. In summary, the equatorial ocean is dominated by zonal (east-west) geostrophic currents resulting
from meridional (north-south) horizontal pressure gradients that are a result of meridional
variations in sea surface height which, in turn, are a result of meridional trends in Trade Wind
strength and the change in direction of the Coriolis Force across the equator
D. The (Subsurface) Equatorial Undercurrent
1.The NE and SE Trade Winds cause an east-west horizontal pressure gradient in the ocean by piling
water up in the western portion of the basin due to Ekman transport in the surface layer (Fig. 6)
-both the Southeast and Northeast Trade Winds result in Ekman transport to the west (and
poleward) because the Coriolis Effect is oriented towards the left of the wind direction in the
Southern Hemisphere and to the right in the Northern Hemisphere
-this causes the sea surface height in the west to be about 60 cm higher than in the east
2. The westward increase in SSH causes an eastward pointing horizontal pressure gradient, which in
turn causes an eastward current flow along the equator
 Why does the current flow eastward?
- at all latitudes, except for the equator, an eastward HPG would cause either a southward
(in Northern Hemisphere) or northward (in Southern Hemisphere) flowing geostrophic
current
 However because the Coriolis Effect is zero at equator, the current that flows along the
equator is not a geostrophic current
o Remember: a Geostrophic Current requires a balance between HPG force and Coriolis
Force
3. The eastward flowing current follows the equator below the westward flowing surface currents
(Fig. 7)
 this subsurface current is called Equatorial UnderCurrent (EUC) or Cromwell Current
 it was discovered about 50 years ago when fisheries scientists noticed that nets set at ~100m
along the equator drifted eastward, opposite to the westward direction that their ship was
drifting due to the SEC
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4. The EUC is a major sub-surface ocean current that is present in all three equatorial oceans but is
strongest in the Pacific Ocean
5. The higher sea surface height in the western portion of the basin causes the thermocline to be
deeper in the west than in the east and therefore tilt upward (shoal) towards the east (Fig. 6)
-this tilting represents baroclinic adjustment of the isopycnals. Can you explain why?
-the core of the EUC is located in the thermocline and thus the depth of the EUC shallows
towards the east as the thermocline shallows (Fig 6c)
6. The Equatorial UnderCurrent is trapped along the equator
 any deviations of the EUC from following the equator are deflected back towards the equator
by the Coriolis Force
- if the EUC moves northward (northern hemisphere), the Coriolis Effect deflects it to the
right and back towards the equator
- if the EUC moves southward (southern hemisphere), the Coriolis Effect deflects it to the
left and back towards the equator
 this situation is a peculiarity of a current flowing along the equator
7. Although the EUC is not a geostrophic current, a force balance situation can exist
 Friction is the force that opposes the HPG force for the EUC, since there is no Coriolis Force
at the equator
 friction force is high because the South Equatorial Current located above the EUC is flowing
westward (opposite to the flow direction of the EUC)
 this reversal in current direction with depth causes high velocity shear (change in velocity
versus depth or Δvelocity/Δz )
 The greater the velocity shear, then the greater the frictional force opposing motion
 thus there is a force balance between the HPG Force (pointing eastward) and Frictional Force
(Remember: frictional force points in the opposite direction of current flow (in this situation
the eastward flowing EUC exerts a westward frictional force that opposes the eastward HPG
force) and is proportional to current speed
 the EUC reaches high current speeds in order to produce sufficient frictional force to balance
the HPG force
8. The EUC flows eastward following the equator (between 2°S and 2°N) at depths of 50 to 200m in
the Pacific (Figs. 6, 7)
 it is a fast subsurface current with speeds of 30 cm/sec (mean) to 150 cm/sec (max)
o these maximum speeds approach that of the Gulf Stream
o the depth of the EUC decreases eastward and follows the depth of the thermocline
where vertical velocity shear (and thus friction) are greatest
 the EUC’s volume transports is ~40 Sv (mean) and a maximum of ~70 Sv
E. Upwelling in the Equatorial Ocean
1. Upwelling at the equator results from the surface divergence caused by the Trade Winds (Fig. 3)
 the mean Ekman transport caused by the Trade Winds is poleward away from the equator on
both sides of the equator
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
Remember: Coriolis Effect acts to the right in the northern and to the left in the southern
hemisphere and the Ekman transport is perpendicular to the wind direction
2. Surface water diverging at the equator is replaced with upwelling water from below (Figs. 3, 4)
-the upwelled water is from shallow depths <200m
3. The strength of the upwelling depends on the strength of the trade winds and varies seasonally
 typical upwelling velocities on the equator are about 1-3 m/day (0.0012 - 0.0035 cm/s)
 highest upwelling rates at about 120-150°W, where the wind stress is the greatest (Fig. 6)
 the volume transport of upwelled water, when integrated over the equatorial Pacific, is large at
~40-60 Sv
o How does the upwelling transport compare to the surface current transports in the
equatorial Pacific?
4. Upwelling at the equator has a major impact on the surface ocean properties in the equatorial
Pacific
a. Lowers sea surface temperature (SST) by bringing colder water to surface (Fig. 8)
- most clearly seen in the eastern equatorial Pacific
b. Increases the nutrient concentrations along the equator (Fig. 9)
c. Increased nutrient causes higher photosynthesis rates and higher chlorophyll concentrations
along the equator (Fig. 10)
5. Cross section of T, S, O2, Phosphate and Velocity in the Equatorial Pacific (Fig. 11)
- “doming” on isotherms expected under region of Ekman divergence
- surface waters south of the equator are significantly saltier than north of the equator
- oxygen decreases and phosphate increases with depth as expected
- zonal (east-west) current velocities indicate the dominance of the SEC, the position of
the NECC sandwiched between the SEC and NEC and the high speed of the eastward
flowing subsurface EUC
F. El Nino (ENSO) Events
NOTE: Look at the El Nino Web Page: http://www.elnino.noaa.gov/ for current conditions in the
Equatorial Pacific and general information and images about El Nino events.
Background
1. El Nino originally is the name given to times when surface waters were much warmer than usual
off the coast of Ecuador and Peru
 this unusual warming often was greatest around Christmas (Nov-Jan), hence the name El Nino
(Spanish for the boy and referring to the Christ child)
 this warming results from a decrease in the upwelling rates of cold subsurface water off the
coast of Peru and a decrease in the strength of the cool northward (equatorward) flowing Peru
Current along the coast
 these events were well recorded back to the late 1800s
2. Warming of surface waters off coast of Peru caused a drastic decrease in the catch of anchovies, the
major fisheries in the region, which prefer colder water (Fig. 12)
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3. There are presently ~70 buoys (started in 1982) placed in the equatorial Pacific, called the Tropical
Atmosphere Ocean (TAO) buoy array, which records in real time atmospheric winds, SST and
current speeds (Fig. 13)
 the data collected from these buoys has shown oceanographers the time sequence of events
occurring during the development of an El Nino and the spatial extent of ocean changes
5. El Nino represents a phenomenon of very large scale in the Equatorial Pacific (Fig. 14)
 warmer surface waters, by up to several degrees, are observed especially in the eastern
equatorial Pacific
 decreases in chlorophyll concentrations are observed in the surface equatorial ocean
 El Nino conditions are not restricted only to the coast of Peru but are basin wide across the
entire equatorial Pacific
The Onset of an El Nino
1. El Nino occurs when Trade Wind strength decreases substantially (Fig. 15)
 this happens when there is a decrease in the gradient (or difference) in atmospheric pressure
between the high pressure cell in eastern tropical Pacific and low pressure cell in the western
equatorial Pacific
 this situation is caused by a weakening and eastward migration of the atmospheric low
pressure cell in the western Pacific and a weakening of the atmospheric high pressure cell in
the eastern South Pacific
 the processes that initiate this change in atmospheric pressure gradients are not well
understood
-this is why it is difficult to predict when a future El Nino will occur
2. The changes in atmospheric pressure gradient between normal (strong) and El Nino (weak) Trade
Wind conditions, which result from the weakening of the large low pressure cell which normally
lies over Australia, is called the Southern Oscillation (SO)
 the associated change in both the atmospheric and ocean circulation is referred to as an ENSO
(El Nino and Southern Oscillation) event
 during an ENSO event, the western (Indonesian) low has higher pressure than normal and
South Pacific high has lower pressure than normal
o this reduces the horizontal pressure gradient in the atmosphere during an ENSO event,
which in turn reduces the strength of the Trade Winds
 a measure of the strength of the atmospheric pressure gradients on the regions is called the
Southern Oscillation Index
Sequence of Events during ENSO
1. Changes in Atmospheric Conditions during an ENSO event (Figs. 16)
a. The southeast to northwest atmospheric pressure gradient decreases
b. Trade wind strength decreases, in particular the SE Trades
c. The ITCZ migrates equatorward (in part because of decreased strength of SE trades)
d. Rainfall increases in eastern tropical Pacific due to warmer SSTs which, in turn, cause an
increased atmospheric convection (e.g., flooding in Peru)
e. Rainfall decreases in western tropical Pacific due to cooler SSTs and decreased atmospheric
convection (e.g., drought in Indonesia)
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2. Change in Oceanic Conditions during an ENSO event (in response to weaker Trade Winds)
a. Decrease in meridional sea surface height gradients because of reduced Ekman transport.
b. Decrease in meridional and zonal Horizontal Pressure Gradients because of reduced sea
surface heights.
c. Decrease in strengths of westward surface currents and eastward Equatorial Undercurrent
because of weaker HPG forces.
d. Increase of sea surface temperatures in the eastern equatorial Pacific due to the migration of
the warm surface waters eastward as a result, in part, of reduced velocities of westward
flowing surface currents.
e. Decrease in east-west thermocline tilt because of reduced sea surface heights in the western
equatorial Pacific.
f. Decreased upwelling rate because reduced surface Ekman transport (divergence) at the
equator.
g. Increase in sea surface temperatures in the eastern equatorial Pacific because of reduced
upwelling rates and deeper thermocline.
h. Decreased nutrient supply because of reduced upwelling rates and deeper thermocline.
i. Decreased biological productivity because of reduced surface nutrient concentrations.
3. Observed Effects in Ocean under El Nino (versus Normal) Conditions (Figs. 14, 17)
a. Sea surface temperature (SST) warms in the eastern Pacific because the supply of cold
upwelled water is decreased and the pool of warm water in the western equatorial Pacific
migrates eastward.
b. Tongue of cooler water along the equator is reduced or absent due to decreased upwelling.
c. Sea surface heights in the western Pacific are reduced because of reduced Ekman transport to
the west by the weaker Trade Winds.
d. Reduction in chlorophyll concentrations (implying reduced photosynthesis rates) in response
to reduced nutrient concentrations caused by reduced upwelling rates
4. The time sequence of surface temperature changes during an El Nino (Fig. 18)
a. development of the warm SST along the coast of Peru
b. warming along the equator in the central and eastern Pacific
c. peak effects during fall and winter (Nov-Jan) of El Nino year (1997)
-There is lots of information about El Nino events at the NOAA web site
http://www.elnino.noaa.gov/
Frequency of El Nino Occurrence
1. El Nino events have occurred at an irregular interval of about 3 to 7 years (Fig. 19)
 The one that started in 1992 lingered on until 1995
 the El Nino in 1998 was one of the most intense on record
 generally, the recent El Nino’s have been the most intense over the last 50 years
- Not sure why. Some oceanographers are concerned that the global warming may be
causing ENSO events to be more intense and last longer than in the past.
 atmosphere-ocean circulation models do not do a very good job at long-term prediction of El
Ninos
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o difficult to accurately predict the trigger for the onset of an El Nino event
2. There is a strong correlation between occurrence of El Nino events and the Southern Oscillation
Index (SOI) (Fig. 19)
-the SOI is a measure of the atmospheric pressure difference (gradient) between Tahiti and
Darwin, Australia (on the northcentral Australian coast)
-this implies that atmospheric pressure changes in the equatorial Pacific Ocean is a key
component of El Nino events
Climatic Impacts of El Nino
1. During an El Nino event there is a reversal of the zonal atmospheric circulation cell (called the
Walker Cell) in the equatorial Pacific (Fig. 20)
-air rises in the east and sinks in the west (opposite of typical circulation)
-this reversal occurs because the ocean surface temperatures are anomalously warm in the east,
which cause warm moist air to rise
-cooler SSTs than normal in the western equatorial Pacific causes air to sink in west
-this reversal of the Walker Circulation cell results in high precipitation rates in the eastern
equatorial Pacific which often causes flooding in Peru and lower than normal precipitation rates in
the western equatorial Pacific and drought in Australia and Indonesia
-the increased precipitation in the eastern Equatorial Pacific occurs when the rising warm
moist air eventually cools and water vapor condenses into precipitation
-the decreased precipitation in the western Equatorial Pacific occurs when sinking cool air
warms which allows the air to hold more water vapor as it sinks and results in a condition that is
unlikely to yield precipitation
2. Importantly, El Nino and La Nina effects on Temperature and Precipitation are seen globally as
well as regionally in the equatorial Pacific (Fig. 21, 22)
a. El Nino- e.g., warm and dry conditions in SE Asia, warm in Canada, dry in Amazon bsain
b. La Nina- e.g., cool and wet conditions in SE Asia, cool in Canada, wet in Amazon basin
3. In the US, there are clear effects of El Nino on temperature and precipitation patterns (Fig. 23 and
24)
- Warm/dry conditions in the Pacific NW
- Warmer temperature in upper midwest
- Cool/wet conditions in southeast US
4. These El Nino patterns in the US are related to a change in the position of the jet stream (Fig. 25)
5. Current Conditions in the Equatorial Pacific (Nov 2009) (Fig. 26)
a. Web Page with Current El Nino conditions http://www.elnino.noaa.gov/
b. Looks like El Nino conditions currently exist in the Equatorial Pacific
c. This should mean slightly warmer and dryer wintertime conditions in Pacific NW
6. El Nino events also have an economic impact
- tropical storm and landslide damage
- change in crop production due to precipitation shifts
- change in hydroelectric power production due to precipitation and temperature shifts
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- changes in fisheries due to changes in SST and upwelling
H. KEY POINTS
1. Equatorial ocean circulation is driven by the Trade Winds
2. The meridional gradient in the strength of the Trade Winds causes meridional slope in sea surface
height that, in turn, causes a meridional trends in HPG force
3. Meridional HPG gradients cause east-west (zonal) geostrophic currents like SEC, NEC and NECC,
with the westward flowing currents (North and South Equatorial Currents) dominating.
4. Along the equator, the Trade Winds cause a west to east HPG that, in turn, causes an eastward
subsurface flow along the equator (Equatorial UnderCurrent) (not a geostrophic current)
5. The opposite direction of Coriolis Force in the Southern versus Northern Hemispheres causes
surface water divergence at the equator and, as a result, upwelling.
6. Upwelling cools SST and increases nutrients at equator with the latter effect contributing to an
increase in biological photosynthesis rates and chlorophyll concentrations along the equator.
7. El Nino describes a condition when SSTs in eastern equatorial Pacific and off the coast of Peru are
significantly warmer than usual.
8. El Nino is initiated by a decrease in the Trade Wind strength and occurs at approximately 3 to 7
year intervals.
9. The processes that initiate El Nino’s aren’t clearly understood and thus El Nino’s cannot yet be
accurately predicted.
10. However, the sequence of events that occurs after the El Nino starts is fairly consistent and well
documented since the TAO buoys have been installed.
11. El Nino and La Nina are global climate events and its impact is seen in precipitation and
temperature anomalies worldwide.
QUESTIONS
1.Why are equatorial geostrophic currents stronger than subtropical or polar geostrophic currents
responding to the same HPG?
2. How long does it take for the S. Equatorial Current to cross the equatorial Pacific?
3. Assume the mixed layer at the equator is ~50m deep and the upwelling rate is 2 m/day. Given a
westward flowing surface current at ~20 cm/s on the equator (like the SEC), how far westward would
a mixed layer parcel of water travel before the water in the parcel is replaced by upwelled water?
Hint: What is the residence time of the water in the mixed layer?
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4. What would be the poleward velocity of currents in a 50m deep surface layer in order to balance
the input of water upwelling at 3 m/d across a 1° latitude band (0.5°S to 0.5°N)? Assume that the
northward and southward flowing surface currents have the same speed and there is no net westward
volume transport.
5. (a) At what longitude would you expect the greatest equatorial upwelling rate based only on the
data presented in the top panel (A) in Fig. 6? Explain. (b) At what longitude would you expect the
lowest SST based only on the data presented in the top panel (A) in Fig. 6? Explain. (c) Explain how
the information in panel C of Fig. 6 would change your expectations of where the SST would be
lowest (compared to your response in part b)?
6. What change in the atmospheric conditions initiates El Nino events? How frequently do they
occur?
7. Explain why the thermocline depth decreases eastward at the equator (see Fig. 6). How does this
trend affect the eastward trend in the depth of the Equatorial Undercurrent?
8. Explain why the Equatorial Undercurrent is or is not a geostrophic current. What forces affect the
force balance for the EUC?
9. During El Nino conditions, typically how do weather conditions change in the Pacific NW and in
the equatorial Pacific?
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