Linking Southern Oscillation with El Niño

• What Normally Happens
• The Composite El Niño Event

Son of the world renowned meteorologist Vilhelm Bjerkness, Jacob Bjerkness is primarily recognized for his work in developing the theoretical cyclone model as part of the "Bergen School" during the 1920's. In addition to this work, it was Bjerkness who made the first link between El Niño and the Southern Oscillation in the 1960's.

Bjerkness made heavy use of data gathered during the 1957 International Geophysical Year, which happened to be a time with a strong warm (El Niño) event in order to determine the link between El Niño and the Southern Oscillation.

What Normally Happens

Bjerkness noticed that the default state of sea surface temperatures (SSTs) at the eastern end of the Pacific are remarkably cold for such low latitudes. Since the western Pacific is very warm, a large SST gradient exists along the equatorial Pacific. As a result, there is a direct thermal circulation in the atmosphere along the Pacific. The cool dry air above the cold eastern equatorial Pacific waters flows westward along the surface toward the warm west Pacific. There, the air is heated and supplied with moisture from the warm water. This systematic equatorial circulation associated with the zonal pressure gradient was named the "Walker Circulation" by Bjerkness. Bjerkness thought that fluctuations in this circulation initiated pulses in Walker's Southern Oscillation.

While the surface winds are being driven westward along the equator by the zonal SST gradient, they act to create the cold upwelling ocean water in the east. The cause of the cold eastern equatorial Pacific waters are explained by the horizontal advection of westward currents along the equatorial Pacific, upwelling along the equator, and upward thermocline displacement.

Bjerkness associated the feedback loop of the oceanic and atmospheric circulation over the tropical Pacific as a "chain reaction", noting that "an intensifying Walker Circulation also provides for an increase of east-west temperature contrast that is the cause of the Walker Circulation in the first place." Bjerkness also found that the interaction could operate in the opposite: a decrease in the equatorial easterlies diminishes the supply of upwelling cold water and the lessened east-west temperature gradient causes the Walker Circulation to slow down. He thus provided an explanation for the association of the low phase of the Southern Oscillation with El Niño as well as the association of the high phase with normal cold state of the eastern Pacific.

The Composite El Niño Event

Prelude

Typically, there are stronger than average easterlies in the western equatorial Pacific preceeding an El Niño event, especially a strong event. These winds move water westward, and consequently sea level is higher than normal in the west and lower in the east. Equatorial Sea Surface Temperature (SST) is slightly warm in the west and somewhat cold in the east.

Onset

In the fall preceeding El Niño, the warm anomaly in the South Pacific develops a northward extension across the equator in the vicinity of the date line. This is associated with a northeast shift of the South Pacific Convergence Zone, which brings it closer to the equator than normal. The easterlies west of the dateline have started to diminish, and the sea-level slope along the equator has begun to relax. There are positive precipitation anomalies west of the date line, but no discernable pattern to the convergence anomalies.

Event

The anomalies warming off the coast of South America begins in January or February and increases until June. For the first several months it is difficult to distinguish it from the normal warming that occurs every winter. At the same time, the sea level rises in a narrow region along the South American coast and the thermocline in the Eastern Pacific deepens. There is a strong southward flow at the coast, as well as evidence for a sea-level rise north of the equator. The SST anomaly at the equator in the vicinity of the date line persists and can potentially expand throughout this period. At this time, there are westerly wind anomalies along the equator, with maximum shear near the date line. The Intertropical Convergence Zone has shifted equatorward in the east, so there is enhanced convergence and precipitation all along the equator, including many sections of Peru.

During the next half year, the warm anomaly spreads northwestward and then westward along the equator at a speed of about 1 m/s. By late fall the eastern anomaly has merged with the one in the central Pacific: Warm water now girdles a quarter of the Earth. At this time, SST at the coast is only slightly anomalous, although the thermocline is still substantially deeper than normal there. 

Wyrtki links Walker Circulation and Ocean Movement

Bjerkness had pointed out that during El Niño the ocean had to be responding dynamically rather than to changes in the surface heat flux. This concept was first developed into a specific theory by Klaus Wyrtki, an oceanographer at the University of Hawaii. Wyrtki discovered the changes in the Pacific Equatorial Countercurrent and its relationship to ENSO in the 1970's.

Wyrtki had a network of tide gauges in the tropical Pacific which gave records of sea level. In the tropics, monthly average sea level is an excellent substitute for the monthly average depth of the thermocline -- that is, for the thickness of the upper ocean warm layer. Wyrtki showed that an El Niño event is associated (preceded in fact) by a transfer of warm water from west to east. The figure below shows a sequence of sea level (thermocline depth) maps for 1975-6 (an ENSO year). Initially sea level is low in the east and high in the west, but by April 1976 sea level in the east is already high. This precedes the warmest SST anomalies there. It is this transfer of warm water to the east that triggers the warm phase of ENSO.

 

How it Works

It is the transfer of warm water to the east that triggers a warm event But what triggers the movement of waters to the east? Think of the tropical Pacific as a huge tub, with the waters sloshing back and forth. In the cold phase the warm waters are low in the east, so they must be high somewhere else. This is because water is conserved and because warm water is very nearly conserved: there is some heat exchange with the atmosphere, but from the ocean's point of view it doesn't amount to much. (From the atmosphere's point of view its quite a lot -- it is just this rearrangement of the atmospheric heating that sets off the worldwide climate anomalies associated with El Niño.) The "somewhere else" that the water level is high is primarily the western tropical Pacific. Eventually this water will return to the east and set off the next warm event. Most immediately, it pushes down the thermocline and raises the temperature of the upwelled waters. The Bjerkness positive feedback takes over: the winds weaken and still more water flows east and SSTs warm. The main center of atmospheric convection shifts eastward, disrupting the world's "normal" weather patterns. The eastward sloshing overshoots any equilibrium. Since there is now more warm water in the east, there is less in the west. Eventually this message (the raised thermocline signal) is transmitted back to the east and the warm event starts to weaken, to be replaced in turn by a normal to cold phase. And so on, forever (or at least thousands of years, judging from the observational record). There is one more wrinkle in the story to point out: part of what makes the oscillation possible is an asymmetry between eastward and westward motions in the ocean. Along the equator there is a relatively fast eastward (and only eastward) motion called an equatorial Kelvin wave. Peaking somewhat off the equator are westward motions called Rossby waves. These carry the message of the high (say) thermocline in the west westward to the boundary of the ocean (Philippines, New Guinea, Australia) where they are reflected eastward in the equatorial Kelvin wave. This delay is needed for the oscillation -- without it one would have the amplification in place that Bjerkness contemplated.

Comparison of Normal vs. ENSO Conditions

Past & Present ENSO Events

1982-83

1997

Impacts & Tele connections

Floods & Droughts

The quintessential ENSO -- rainfall correlation is the connection between India rainfall and ENSO. The heights of the bars give the relative strength of the rainfall in the India monsoon: up means greater than normal, down is a drought. The colors give the strength of ENSO for that year: red is a hot (El Niño) event, blue is a cold (La Niña) event, and off-white indicates normal temperatures. The bars indicate that most major droughts in India occur during El Niño events.

The shifting precipitation patterns causes much of southeast Asia and parts of Australia to experience dry conditions during an ENSO event, leading to drought conditions in many areas. This is especially devastating to countries who depend on rainfall for their crops. The figure on the right shows the correlation between India rainfall and ENSO.

While some regions experience dry conditions, other areas from California to Argentina are inundated with above normal precipitation. In some cases, these areas are normally dry, so even relatively weak ENSO events can have a significant impact. Also significant is the rise in hurricane activity which usually occurs during an ENSO event. These hurricanes can bring strong winds, large amounts of precipitation, and storm surges which can cause beach erosion.

Biological

Any biological phenomenon that is climatically based can be affected by the ENSO cycle. In some areas, this may be a positive notion. For example, malaria is one of the diseases that is closely correlated with climate. While it is principally temperature that determines survival rates for the mosquito, precipitation directly influences the abundance of breeding sites. If ENSO conditions result in above average rainfall for a particular region, the mosquito infection rate may increase.

Information provided by: http://hjs.geol.uib.no