• Significant effect on marine life -- Reduction in ...
  ... Nutrient supply
  ... Phytoplankton productivity
  ... Population of fish (anchovies) and fish-eating birds.

• 3) Global weather changes observed during an El Niño year include:
• Droughts (summer) -- monsoon conditions are blocked: SE Asia, India, Australia, Africa
• Heavy rainfall and storms:
  • Central Pacific, coastal South America (summer)
  • Gulf Coast and California (winter)
• Warm winters:
  Alaska, Western Canada, Northern USA

 

Example: 1997-98 El Niño

• Increase in surface water temperatures off S.Am.: 5 deg C
• Possibly the most powerful El Niño system in 150 years.
• Winter weather as predicted: wet and stormy in the southwest USA, Texas, and most of the Gulf Coast

 

4) La Niña -- opposite conditions

• Strong trade winds; cold water upwelling in East. Pacific
• Predicted weather:
  • More severe hurricanes
  • Severe winter, dry summer in Midwest
• Sea surface and thermocline flatten
  

DEEP CIRCULATION in the Oceans

Driving force: creation of dense water masses (cold, saline) at the surface

Happens only at high latitudes (particularly the Atlantic)

Modifying factors:

1. Coriolis effect
2. Continents and mid-ocean ridge systems

General vertical layering in the oceans

Well-mixed layer = Surface zone (to roughly 100 m depth)

Mixed by turbulence of wave action, winds, and currents
Seasonal variations in depth of this

Below that (100 - 1,000 m): Usually rapid changes in density (changes in T and S)

Names for this: Pycnocline = Thermocline = Halocline

Below the thermocline (> 1,000 m): Deep waters

T (< 5 deg C) and S are fairly uniform- cold and high salinity

Controls on density of seawater -- T, S, pressure (depth)

1) Temperature -- most important

Warm waters are at surface (at low and mid latitudes)
Cold waters formed during seasonal cooling at
high latitudes --> sink

2) Salinity

-Important at high latitudes in open ocean where waters
are uniformly cold

Winter cooling and sea-ice formation
--> Increases S and density --> sinking

-Important in coastal areas and marginal, semi-enclosed
seas where evaporation is high.

High surface S ---> Sinking and outflow at depth to
adjacent oceans

 

Pressure (depth)

Sea water is compressible (density increases with pressure)
But pressure effects do not cause sea water to sink (or rise)

 

Densest water masses of world ocean are formed at high latitudes in the Atlantic:

Highest latitude, coldest surface T
High salinity

Evaporation
Outflow of saline waters from Mediterranean and Caribbean

 

Video: "Ocean Currents and Winds" 2nd part

Evidence for bottom currents on the sea floor

Origin and flow of Antarctic Bottom Water (AABW)

Origin and flow of North Atlantic Deep Water (NADW)

[called "Arctic Water" and "N. Atl. Bottom Water" in video]

Mixing of surface currents
Southward flow in Atlantic
Upwells and mixes with AABW off Antarctica
Nutrient rich, productive waters
AABW-NADW flows into Indian and Pacific

Upwelling of nutrient-rich deep waters off the west coast of continents
Wind-driven, Ekman transport offshore
Highly productive regions

 

El Niño Events

Warm waters invade Eastern Equatorial Pacific
Frequency: about every 2-7 years
Effects of the l983 El Niño

---

(Detailed notes start here)

El Niño events are changes (periodic oscillations) in surface-ocean and atmospheric conditions in the Equatorial Pacific that occur about every 3 to 7 years. During an El Niño, warm surface water develops off the coasts of Peru and Ecuador, extending northward to Central America, Mexico, and even the USA. Trade winds decrease in the Eastern Pacific, sometimes to nothing! In addition, atmospheric pressure tends to decrease in the Eastern Pacific and increase in the Western Pacific. As a result, the area of heavy rainfall migrates eastward (from Indonesia, etc.) to the Central Pacific.

El Niño conditions in the Equatorial Pacific changes weather patterns for a number of months in wide-spread areas of the world. Summer droughts are severe in Southeast Asia, India, Australia, and Africa because the normal summer monsoon conditions are blocked. Heavy rainfall and storms occur in the central Pacific and the Pacific coast of South America during the summer, and in the USA Gulf Coast and California during the winter. Alaska, western Canada, and northern USA have unusually mild winters.

One of the most intense El Niños of the past 150 years was the 1997-98 event. This El Niño created predictably severe winter stroms and rain in the southwestern USA, California, Texas, and much of the Gulf Coast.

La Niña conditions are exactly the opposite of those of El Niño -- strong trade winds and relatively cool surface water in the Eastern Pacific. We are coming into a La Niña now. La Niña conditions are also felt world-wide. For example, they are typically associated with severe hurricanes (e.g., the recent Hurricanes Georges and Mitch) and with dry summers in the midwestern USA.

Normal conditions. In the absence of El Nino, cool surface water occur in the eastern Equatorial Pacific. Because of prevailing trade winds and Ekman transport, there is significant upwelling along the coast of western South America. In addition, winds and ocean currents transport warm waters to the Asian (western) side of the Pacific. As a consequence of the piling up of warm surface waters, sea level is higher and the thermocline (see below) is deeper in the Western Pacific.

El Nino conditions. The Trade Winds and Equatorial Currents diminsh, and hence there is less effective westward transport of warm surface water. As a result, the sea surface and thermocline tend to flatten out. In addition, warm waters that had piled-up in the Western Pacific migrate eastward, creating a "lid" of warm water that inhibits upwelling in the Eastern Pacific. As upwelling of nutrient-rich waters decreases, so does the production of phytoplankton (marine algae) that is the base of the food chain. Consequently, the populations of fish (e.g., anchovies) and fish-eating birds decrease dramatically and rapidly. El Ninos can be ecological disasters in the eastern Equatorial Pacific and the coastal waters of South America.

Feedback and Oscillations. The El Niño phenomenon oscillates, that is, it occurs, dies out, and eventually builds up again every few years. Why? Because it is really two processes, one oceanic, the other atmospheric, that depend on each other. An El Niño develops as follows:

1. Warm surface water develops off the coasts of Peru and Ecuador

2. This causes atmospheric pressure decreases in Eastern Pacific

3. This causes the trade winds to weaken, especially in the East Pacific

4. This causes less effective westward transport by Trade Winds

5. This causes greater migration of warm waters eastward (feedback loop connecting to 1. above)

 

This is a positive feedback situation; change in the ocean causes a change in the atmosphere which in turn increases the changes in the ocean, which in turn increases the changes in the atmosphere, and so on...Each change is amplified by the response or "feedback" from the other process. Eventually, the El Niño stops increasing and dissipates because of other processes (note that once it starts to die out, feedback is again important- decreases lead to more decreases) This leads to oscillations: conditions fluctuate back and forth. The El Niño is triggered somehow, grows because of the feedback/amplification, then dies out.

 

Many natural phenomena on the earth are like this; complex interrelationships make it possible for small changes to grow into much larger ones. These interrelationships make it difficult to predict or understand many natural phenomena on our earth, such as El Niño, other aspects of our weather, global warming (i.e., the greenhouse effect), and ice ages.

Deep Circulation of the Oceans. The driving force for deep circulation is the creation of dense (thus cold and saline) water masses at the surface of high-latitude oceans, particularly in the Atlantic. These dense water masses sink and spread laterally, creating deep current. Like surface currents, deep currents are modified (deflected) by the Coriolis effect and by continental barriers; in addition, deep currents are deflected by mid-ocean ridges in some cases.

In studying currents in the deep ocean, it is useful to desribe the general vertical structure of the ocean, i.e., the depth zones of the oceans.

Surface zone = Mixed layer. This zone extends to a depth of about 100 m; the depth can change seasonally. Water in the zone are mixed by the turbulence of wave actions, winds, and currents. In other words, temperature and salinity are fairly uniform in this zone.

Pycnocline = Thermocline = Halocline. In this "cline" zone, extending down to about 1,000 m , temperature, salinity, and hence density change rapidly. This is a transition zone between the surface, mixed layer and the ...

Deep ocean. Below about 1,000m the temperature and salinity are pretty uniform. Subtle variations in T and S in the deep ocean reflect and identify different water masses.

Controls on the density of sea water. We've discussed this in several previous lectures. But now the issue is more important because density dictates whether surface waters will sink and become part of the deep-ocean layer and deep circulation.

Temperature is the most important factor controlling density of surface waters in the open ocean. Note that warm waters remain at the surface -- in the mixed layer at low and mid-latitudes. Cold waters formed during seasonal cooling at high latitudes tend to sink into the deep ocean.

Salinity is an important influence on density at high latitudes in the open ocean where surface waters are uniformly cold. Small increases in salinity, such as during seasonal sea-ice formation, controls which cold surface waters will actually sink. Saline waters that overflow from marginal, semi-enclosed seas (Mediterranean, Caribbean, Red Sea) are dense enough to sink and become part of deep circulation in major ocean basins.

Pressure (depth) does, in principle, effect the density of sea water. Because water and sea water are slightly compressible, density increases with pressure. So, as sea water sinks, its density increases. But pressure effects do not cause sea water to sink, and therefore have nothing to do with how and where deep-water masses form at the surface.

Temperature and salinity control the density of surface sea water and thus its tendency to sink. Density-driven circulation is therefore called "thermohaline" circulation. The temperature and salinity of sea water are determined by processes occuring at the sea surface:
Exchange of heat with the atmosphere controls T
Exchange of water with the atmosphere controls S

Why do the densest water masses of the oceans form at high latitudes in the Atlantic Ocean? The Atlantic extends to higher latitudes in both the Northern and Southern Hemisphere than the Pacific. Thus, high-latitude surface waters are cold. In addition, the Atlantic is a little more saline than the Pacific. Two factors contribute to this. (1) Because of atmospheric circulation (Trade Winds), water vapor evaporating from the Equatorial Atlantic is transported to the Equatorial Pacific across Central America -- the Atlantic is "exporting" water to the Pacific. (2) Saline water flows out of the Mediterranean and Caribbean to the Atlantic. There is no similar situation in the Pacific.

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