Answers to Questions at end of Topic 4
1.Why are equatorial geostrophic currents stronger than polar geostrophic currents for the same HPG?
Geostrophic current velocities (V) = g/f *ΔZ/Δx. Since V is inversely proportional to “f”,
where f= 2 Ω sin (lat), at low latitudes near the equator the current speeds are higher for a given HPG
2. How long does it take for the SEC current to cross the equatorial Pacific?
If the SEC velocity is 30 cm/s, which = 26 km/d, and the Pacific is about 140 º longitude wide, then
the time is takes to travel across the equatorial Pacific is;
T= length/ velocity = (140 º *110km / º) / (26 km/d) = 592 days or 1.6 yrs
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?
Residence time = mixed layer depth (m) / velocity (m/s) = 50m/ 2 m/d = 25 days
Over that 25 day interval, the SEC will travel 0.2 m/s * 86400s/day * 25 days = 432 km
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.
Assuming a balanced water budget, the volume of water upwelled into the surface layer equals the
volume of water transported horizontally out of the surface layer. Assume an east-west length scale
of “L” (which will cancel). So the poleward transport in the surface layer and upward transport can
be expressed as follows:
Aup * Vup = Ay *Vy
1 º * 110km/ º *1000m/km *L(m) * Vup = 50m * L(m) * Vy
Thus the east-west length scale ‘L’ cancels.
Vy = 1 º * 110km/ º *1000m/km *L(m) * Vup / L(m)
Vy = 1 º * 110km/ º *1000m/km * 3 m/d / 50m
Vy = 6,600 m/d * 1d/86400sec = 7.6 cm/sec
If there were equal velocities at 0.5 ºN and 0.5 ºS, then the velocities would be 3.8 cm/s.
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)?
a. The greatest upwelling rate would occur where the wind stress is greatest, because this would
cause the Ekman divergence to be greatest. The greatest wind stress occurs between 150ºW
and 120ºW in Fig. 6.
b. Since SST would be lowest where upwelling is greatest, which occurs where the Ekman
divergence is greatest. Thus the lowest SST would occur between 150ºW and 120ºW based
only on top panel of in Fig. 6.
c. Since the depth of the thermocline also influences the temperature of the upwelled water and
the thermocline is shallowest in the east, which means the upwelling water would be coldest
in the east, the lowest SST would likely occur east of the region of maximum wind stress.
6. What change in the atmospheric conditions initiates El Nino events? How frequently do they
occur?
There is a decrease in the atmospheric air pressure gradient between the South Pacific High and
Indonesian Low. This reduces the strength of the Trade Winds, which triggers an El Nino. El Nino
events occur at about 3-7 year (irregular) intervals.
7. Explain why the thermocline depth decreases eastward at the equator (see Fig. 6). How does this
trend affect the depth of the Equatorial Undercurrent?
The thermocline depth decreases eastward because the sea surface height decreases eastward.
The isopycnal depth mirrors the sea surface height under baroclinic conditions.
8. Explain why the Equatorial Undercurrent is or is not a geostrophic current. What forces are in
balance for the EUC?
The EUC is not a geostrophic current because it represents a force balance between horizontal
pressure gradient force and friction force.
9. During El Nino conditions, typically how do weather conditions change in the Pacific NW and in
the equatorial Pacific?
In the Pacific NW, the temperatures are warmer and the precipitation rate decreases during an El
Nino event. In the western equatorial Pacific the air temperatures are cooler and the precipitation
rate decreases. In the eastern equatorial Pacific the air temperatures are warmer and the precipitation
rate increases.
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