Coriolis force

Moving objects, such as air and water are affected by the Earth's rotation. Objects moving in the northern hemisphere are deflected to the right and in the southern hemisphere to the left. The French mathematician, Gaspard Gustav de Coriolis described the force in 1835. The Coriolis force is defined by the following equation:

Coriolis force per unit mass = 2 (omega)V sin(phi)

where:

omega is the angular velocity of the Earth (360 degrees, or 2pi radians per 24 hours)
V is the velocity of the object relative to the earth
phi is the latitude

The Coriolis force is zero on an object moving over the equator (theta = 0).

Ocean Circulation

Definitions

gyre - a large, closed system of surface current circulation

eutrophic - designating a body of water in which the increase of mineral and organic nutrients has reduced the dissolved oxygen producing an environment that favors plants over animals.

El Nino

Along the equator, the western Pacific has some of the world's warmest ocean water, while in the eastern Pacific, cool water wells up, carrying nutrients that support large fish populations.
Every two to seven years, strong westward-blowing trade winds subside, and warm water slowly moves back eastward across the Pacific, like water shifting in a giant bathtub.
The warm water and shifting winds interrupt the upwelling of cool, nutrient-rich water. Fish die; climatic changes affect many parts of the world.
Peruvians named this phenomenon El Niño, for the Christ child, because it first appears around Christmas.
The most severe El Niño of the century occurred in the winter of 1982 and 1983. Disastrous effects and meteorological changes occurred around the world. Total damages were estimated at over $8 billion.

Australia-Drought and bush fires
Indonesia, Philippines-Crops fail, starvation follows
India, Sri Lanka-Drought,fresh water shortages
Tahiti-6 tropical cyclones
South America-Fish industry devastated
Across the Pacific-Coral reefs die
Colorado River basin-Flooding, mud slides
Gulf states-Downpours cause death, property damage
Peru, Ecuador-Floods, landslides
Southern Africa-Drought, disease, malnutrition

Normal and El Nino conditions in the tropical Pacific Ocean

Schematic Diagram of Normal and El Nino conditions

Winds and Temperature Isotherms in Normal and El Nino conditions

El Nino is a disruption of the ocean-atmosphere system in the tropical Pacific having important consequences for weather around the globe.
- Among these consequences are increased rainfall across the southern tier of the US and in Peru, which has caused destructive flooding, and drought in the west Pacific.
- It is sometimes associated with devastating brush fires in Australia.
In normal, non-El Nino conditions, the trade winds blow towards the west across the tropical Pacific. These winds pile up warm surface water in the west Pacific, so that the sea surface is about 1/2 meter higher at Indonesia than at Ecuador.
The sea surface temperature is about 8 degrees C higher in the west, with cool temperatures off South America, due to an upwelling of cold water from deeper levels.
This cold water is nutrient-rich, supporting high levels of primary productivity, diverse marine ecosystems, and major fisheries.
Rainfall is found in rising air over the warmest water, and the east Pacific is relatively dry.
During El Nino, the trade winds relax in the central and western Pacific leading to a depression of the thermocline in the eastern Pacific, and an elevation of the thermocline in the west.
The observations at 110W show, for example, that during 1982-1983, the 17-degree isotherm dropped to about 150m depth.
This reduced the efficiency of upwelling to cool the suface and cut off the supply of nutrient rich thermocline water to the euphotic zone.
The result was a rise in sea surface temperature and a drastic decline in primary productivity, the latter of which adversely affected higher trophic levels of the food chain, including commercial fisheries in this region.
Rainfall follows the warm water eastward, with associated flooding in Peru and drought in Indonesia and Australia.
The eastward displacement of the atmospheric heat source overlaying the warmest water results in large changes in the global atmospheric circulation, which in turn force changes in weather in regions far removed from the tropical Pacific.

El Nino can be seen in Sea Surface Temperature in the Equatorial Pacific Ocean

SST in a normal year

SST in an El Nino year

El Nino can be seen in measurements of the sea surface temperature. In January 1991, the sea surface temperatures and the winds were near normal, with warm water in the Western Pacfic Ocean, and cool water, called the "cold tongue" in the Eastern Pacific Ocean. The winds in the Western Pacific are very weak, and the winds in the Eastern Pacific are blowing towards the west (towards Indonesia). The bottom panel of the January 1991 plot shows anomalies, the way the sea surface temperature and wind differs from a normal January. In this plot, the anomalies are very small (yellow/green), indicating a normal January.

January 1992 was the peak of an El Nino year. In January 1992, the warm water has spread from the western Pacific Ocean towards the east (in the direction of South America), the "cold tongue" has weakened, and the winds in the western Pacific, usually weak, are blowing strongly towards the east, pushing the warm water eastward. The anomalies show clearly that the water in the center of Pacific Ocean is much warmer (red) than in a normal January.

Mean and anomalies of sea surface temperature from 1986 to the present, showing El Nino's in 1986-1987, 1991-1992, 1993 and 1994.

Sea Surface Temperature in Equatorial Pacific Ocean

El Nino years are easier to see in the anomalies on the right hand panel. The anomalies show how much the sea surface temperature is different from the usual value for each month. Water temperatures significantly warmer than the norm are shown in red, and water temperatures cooler than the norm are shown in blue. In the right-hand plot of sea surface temperature anomalies, it is very easy to see El Nino's, with water warmer than usual (red) in the eastern Pacific, during in 1986-1987, 1991-1992, 1993 and 1994. Notice the very cool water (blue), in the Eastern Pacific, in 1988-1989. This is a La Nina, which occurs after some (but not all) El Nino years. It is unusual for El Ninos to occur in such rapid succession, as has been the case during 1990-1994.

ENSO Forcasting as a Weapon Against Infectious Disease

Climate variability has profound effects on the Earth's biosphere.
Because climate affects temperature, humidity, and precipitation, it influences human health via three interconnected pathways:
1. distribution and quality of surface water;
2. life cycle of disease vectors and host/vector relationships;
3. ecosystem dynamics of predator/prey relationships which control populations of disease vectors.
Changes in temperature, precipitation, humidity, and storm patterns, often related to the El Niño-Southern Oscillation (ENSO) phenomenon, are associated with upsurges of water-borne diseases such as hepatitis, shigella dysentery, typhoid, and cholera; of vector-borne pathogens such as malaria, dengue, yellow fever, encephalitis, schistosomiasis, plague and hantavirus; and of agricultural pests such as rodents, insects, fungi, bacterium, and viruses.
And because climate variability can be forecasted, the potential exists to predict the likelihood of outbreaks of infectious disease.

Dr. Paul Epstein, M.D., M.P.H., Harvard University, School of Medicine
Consider India's autumn of disease last year. For much of the summer, temperatures had soared from their normal 80-90 degrees Fahrenheit and hovered around 124 degrees. Three-month monsoons led to conditions conducive to breeding malaria, dengue fever, and pneumonic plague. By the time the epidemics ran their course, the three diseases had afflicted thousands of people and killed as many as 4,000.

Climate variability has profound effects on the Earth's biota; and alterations in the incidence, duration, onset and intensity of storms, hurricanes, floods, and droughts greatly impact societies, productivity and development through changes in the diseases of human, animals, and plants. Because climate affects temperature, humidity, and precipitation, it influences human health via three interconnected pathways: 1) distribution and quality of surface water; 2) life cycle of disease vectors and host/vector relationships; and 3) ecosystem dynamics of predator/prey relationships which control populations of disease vectors. Changes in temperature, precipitation, humidity, and storm patterns, often related to the El Niño-Southern Oscillation (ENSO) phenomenon, are associated with upsurges of water-borne diseases such as hepatitis, shigella dysentery, typhoid, and cholera; of vector-borne pathogens such as malaria, dengue, yellow fever, encephalitis, schistosomiasis, plague and hantavirus; and of agricultural pests such as rodents, insects, fungi, bacterium, and viruses. And because climate variability can be forecasted, the potential exists to predict the likelihood of outbreaks of infectious disease.

ENSO Disrupts Ecosystem Dynamics

Climatic extremes effect animal, plant and human health by affording opportunistic species fresh terrain and generating new bursts of activity. Droughts encourage locusts and rodents, while floods foster fungi and mosquitoes. Fluctuations in climate which alter the structure or function of ecosystems can change the population dynamics of opportunistic pests and disease vectors, and of the predatory species which normally check their population growth. Owls, for example, help control rodent populations involved in Lyme disease and Hantaviruses. In the U.S., deforestation in the Pacific Northwest and prolonged drought in the Southwest, both encourage the proliferation of rodents by damaging owl refuges.

Rodents are involved in the life-cycle of many groups of diseases around the world. In the U.S. a new disease, the Hantavirus (with a 60% mortality rate), emerged in the "Four Corners" area near the borders of Colorado, Nevada, New Mexico, and Utah, following an explosion of the deer mouse population. Six-year droughts in the southwestern U.S. that devastated populations of owls, snakes, and other rodent predators, were followed by heavy rains in 1993 that increased food sources for rodents; in the absence predators the well nourished rodents flourished. The story is similar in southern Africa, where heavy rodent infestations closely followed the El Niño years of 1976, 1983, and 1993.

Since the 1960's researchers in southern Asia have observed an association between algal blooms and upsurges of cholera. It is becoming increasingly clear that ENSO warm events are also associated with upsurges of cholera, perhaps via the marine reservoir and/or the contamination of ground water accompanying ENSO-related flooding. Recent increases in coastal algal blooms and related cholera epidemics may be linked to climatic perturbations of ecosystems already stressed by pollution, habitat destruction, or the introduction of non-indigenous species.

ENSO Forecasting as a Weapon Against Infectious Disease

New developments in climate forecasting can provide the basis for a proactive approach to the spread of human diseases and agricultural pests--mitigating or preventing outbreaks before they occur, saving scarce public health resources and saving lives. Integrating health surveillance with environmental and climatological monitoring, disease early warning systems for conditions conducive to outbreaks can aid in the design of timely, ecologically sound and environmentally benign public health interventions. Climate forecasting can also be extremely useful in targeting scarce funding for surveillance and response, research and training, and emergency production of vaccines, drugs, and diagnostics, in the U.S. and abroad.

While climate forecasting cannot predict exactly where, when, and to what extent outbreaks will occur, current forecast capabilities combined with the state of understanding regarding the links between climate and health can be used in assessing the vulnerability of populations to outbreaks of infectious disease, and in determining the likelihood or risk of epidemics. Even at this early stage, probabilistic climate forecasting can arm public health practitioners with a powerful tool for reducing the morbidity and mortality caused by outbreaks of infectious disease. In this case, as in others, chance favors the prepared mind.

An animation of El Nino

If you have an MPEG animation viewer, and sufficient memory, you can view an animation of El Nino which shows the changes in monthly sea surface temperature in the tropical Pacific Ocean. The animation is about 1 Megabyte in size. As you view this animation, you will see the warm water spreading from the western Pacific to the eastern Pacific during 1991. The bottom panel in the animation, labeled anomalies, shows how much the sea surface temperature for each month is different from the long term average for that month. The red color in the anomalies plot indicates that the temperature of the water is much warmer than is normal for that month. Blue color indicates that the water is much cooler than is normal for that month.

Selected References

Philander, S.G.H., 1990: El Nino, La Nina and the Southern Oscillation. Academic Press, San Diego, CA, 289 pp.

Hayes, S.P., L.J. Mangum, J. Picaut, A. Sumi, and K. Takeuchi, 1991: TOGA-TAO: A moored array for real-time measurements in the tropical Pacific Ocean. Bull. Am. Meteorol. Soc., 72, 339-347. (abstract available)

McPhaden, M.J., 1993: TOGA-TAO and the 1991-93 El Niño-Southern Oscillation Event. Oceanography, 6, 36-44. (entire paper available)

El Nino references: TAO refereed journal articles and other TAO papers.

Reports to the Nation - El Nino and Climate Prediction

El Nino Theme Page - Central access to widely distributed El Nino data and information.

TAO Project Office - People to Contact - What's new in TAO?

Credits and acknowledgements