1. Ocean Currents & Global Climates
6/11/2018

2. Global ocean currents

3. Ocean Currents Surface Currents Density Currents In the upper 1000 m
Driven by temperature
Also know as the global ocean currents
Density Currents
Driven by Density
More dense water sinks and where they circulate for 500 to 2000 years

4. Surface Currents Warm Currents Cold Currents Counter Currents
Driven by Winds (temperature)
Flow away from the equator towards the poles
Cold Currents
Flow away from the poles towards the equator
Counter Currents
Flow the opposite of the “wind” currents
They “Fill” in the gaps

5. Global surface currents
Surface currents mainly driven by surface winds
North/ South Equatorial Currents pile water westward, create the Equatorial
Countercurrent
western ocean basins –warm poleward moving currents (example: Gulf Stream)
eastern basins –cold currents, directed equatorward

6. What Are the El Nino and La Nina?

7. The Walker Circulation
Mean ascent (rising air), and low surface pressure, over warmest SST associated with deep convection (T-storms)
Subsidence (sinking air), and high surface pressure, in non-convection regions (clear skies)
Equatorial trades blow from high to low pressure, thus the easterly trade winds blow from east to west
Low slp
High slp

8. El Nino During El Nino trade winds slacken
E-W tilt of thermocline & upwelling of cold water are reduced.
SST rises in central/eastern equatorial Pacific
Changes Walker Circulation

9. Tropical mean state: Sea surface temperature (SST)
Indo-Pacific warm pool
Eastern Pacific cold tongue
2 basic regions

10. Ocean upwelling & the Thermocline
Ocean Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water.
The thermocline is the transition layer between the mixed layer at the surface and the deep water layer. The definitions of these layers are based on temperature.
The mixed layer is near the surface where the temperature is roughly that of surface water. In the thermocline, the temperature decreases rapidly from the mixed layer temperature to the much colder deep water temperature.
The mixed layer and the deep water layer are relatively uniform in temperature, while the thermocline represents the transition zone between the two.

11. Thermocline, Upwelling, & El Nino
Thermocline and wind patterns during the normal Walker Circulation.
Cooler, nutrient rich water moves upward from below along the South American coast.
Thermocline and wind patterns during the normal El Nino – Southern Oscillation conditions
Cooler, nutrient rich water does not “upwell” to the ocean surface.
Warm surface water piles up along the South American Coast.

12. El Nino
Mean climate
El Niño ’82/83

13. SST anomalies during El Nino
Dec 1982
Sept 1987

14. ENSO Comparison Animations

15. The 1997/98 El Nino
Jan 1997
Trade Winds

16. The 1997/98 El Nino
Jun 1997
Nov 1997
Trade Winds

17. The 1997/98 El Nino
Nov 1997
Trade Winds

18. The 1997/98 El Nino
Mar 1998
Trade Winds

19. The 1997/98 El Nino
Jan 1997
Mar 1998

20. The 1997/98 El Nino
Jun 1997

21. The 1997/98 El Nino
Nov 1997

22. The 1997/98 El Nino
Mar 1998

23. What about La Nina?
In La Nina conditions SST in the central and eastern equatorial Pacific is unusually cold & easterly trade winds are unusually strong
Dec 1982
Nov 1988
La Nina conditions sometimes occur in the year following an El Nino event (e.g followed 1987 El Nino)

24. Typical ENSO period is 3-7 years, but with significant irregularity

25. The dramatic impacts of ENSO around the globe
Flood in Lakeport, California as a result of the 1998 El Nino event
Bushfire in Australia as a result of the 1998 El Nino event

26. The dramatic impacts of ENSO around the globe
Flood in Lakeport, California as a result of the 1998 El Nino event
Bushfire in Australia as a result of the 1998 El Nino event

27. Disastrous effects of 1982-1983 El Nino:
1.Australia-Drought and devastating brush fires
2.Indonesia, Philippines-Crops fail, starvation follows
3.India, Sri Lanka-Drought,fresh water shortages
4.Tahiti-6 tropical cyclones
5.South America-Fish industry devastated - decrease in nutrients off Peru- fewer fish (anchovy)
6.Across the Pacific-Coral reefs die
7.Colorado River basin-Flooding, mud slides
8.Gulf states-Downpours cause death, property damage
9.Peru, Ecuador-Floods, landslides
10.Southern Africa-Drought, disease, malnutrition

28. The Seasonal “Monsoon”
A seasonal reversal of wind due to seasonal thermal differences between landmasses and large water bodies
Orographic lifting often enhances precipitation totals