Posts Tagged ‘el niño’

Report 1 on El Niño presented a few features of the coastal upwelling along the west coast of the Americas, a gentle introduction to a complex global shift in atmospheric and oceanic circulations and to the local effect called El Niño. In this report we will continue that discussion by describing the large circulation shift called the Southern Oscillation and how that relates to El Niño so that the entire ocean-wide phenomenon is called “El Niño/Southern Oscillation” or ENSO.

Imagine a bathtub

Before we start this discussion let us try a thought exercise to help vision the features of ENSO. Imagine the entire Pacific Ocean is in your bathtub (assuming you are one of the lucky people in the world who has a bathtub). At one end of the tub we place a powerful fan, like the one used to simulate wind in the movie “The Perfect Storm.” That had to be a big fan, right? Now run the fan at full force so that, at the far end of the tub, water is piled up, and at the upwind, fan end the water level is down. The friction of the wind on the surface pushes the water down wind so there is a slope. As long as the fan roars away the water level has the upward slope and the system is in equilibrium, another word for steady.

Do you have that vision of the bathtub with water set up at the far end? Now, shut off the fan. What happens? The surface water runs down hill and the surface becomes level. This, to the first order of simplicity, describes El Niño-Southern Oscillation, or for short, ENSO. Of course reality is much more complex than water in a bathtub.

Now, think of the bathtub as the equatorial Pacific Ocean, including Peru, Central America, New Guinea, and Northern Australia. The wind machine becomes the trade winds which force the surface waters toward the equator and, more importantly, pushes them toward the west. The result is that, under normal conditions, the western side of the Pacific is around 4 meters higher than the coast of South America.

As we have said many times, El Niño is not an isolated weather event, local to Peru and South America. It is not even localized to the North and South American continents. Rather, it is a re-adjustment to the global ocean and atmospheric circulations whereby weather patterns are disrupted, often in extreme ways. In this report I want to describe a few of the major global patterns in an attempt to give a clue, just a clue, to the magical complexity of this regular event.

Reader, stay with me now. A linked chain defines the most important features in oceanic circulation: The figure above shows the Pacific Ocean circulation features for normal conditions. (a) Solar heating: Solar radiation at the equator is intense and heats the sea surface. (b) Convection: The sea surface heats the atmosphere. Hot air rises and therefore, along the equator, the air rises high into the atmosphere. (c) Equatorial convergence: Low level air flows toward the equator to replace the rising air. (d) Trade winds: The equatorward flow is turned westward by the rotation of the Earth. (e) Mid-ocean high cell: Moist air rises from the equator, rains out all of its moisture, and sinks as dry air at around 30 degrees north and south of the equator. Large oceanic cells of high pressure with circular air flow are formed. (Clockwise flow in the northern hemisphere and counter-clockwise in the southern hemisphere.) (f) Upwelling regions: The northern hemisphere high cell drives northerly coastal winds along the west coast of North America. Ocean currents move to the right of the wind (see the previous report) which creates coastal upwelling. Along Peru, the trade winds cause southerly coastal winds (from the south) which drive offshore currents and create a trade wind driven upwelling. (g) Warm Pool: Intense solar heating, the trade winds, and resulting ocean circulations form a massive region of very warm sea surface temperature (SST) in the tropical western Pacific Ocean. This is one of the most important features in global atmospheric dynamics; we will discuss it further in the next section.

Figure 1. A NASA derived map of the global sea surface temperature (SST) for July of 2001. This was a normal, e.g. non-El Niño/Niña year. Several features, which are discussed in this report, are shown. The trade winds deliver a broad westward force across the equatorial Pacific. The trades and associated ocean circulations contribute to the massive pool of warm water in the western Pacific, the “warm pool,” and the coastal winds along North and South America lead to the upwelling regions, notably on the California and Peruvian coasts. (Click on the map to bring up a high resolution version.) Image credit: NASA

The Warm Pool, the firebox for the atmospheric engine

Figure 1 shows the features we discussed above on a real satellite-derived map of global sea surface temperature. The yellow color denotes very warm surface water and the tropical western Pacific warm pool region stands out as a huge region. The official definition of the warm pool is water that is above 28 degrees Centigrade (about 82 deg F). Typically the size of the warm pool is greater than the United States.

Inside the warm pool convective clouds, cumulo-nimbus towers, extend to the stratosphere where their tops are sheared off by the strong westerly (from the west) winds above. The rising air, having come across thousands of miles of ocean, is full of water which falls from the clouds. The rainfall in the warm pool is typically 3-5 meters (9-15 feet) per year making this one of the rainiest places on the Earth.

Exactly how and why the warm pool forms as intensely as it does is not well understood. The physics of the warm pool is still a subject of research and debate. But scientists all agree that if one thinks of the atmosphere as a steam heat engine (hot, moist air rising and cooler, dry air falling) then the warm pool would be the “hot box” for the entire world’s atmosphere.

Now, turn off the trade winds

Think back to the bath tub analogy. When the fan stops blowing, the water level in the tub relaxes back toward level. This is what happens in the Pacific in an event called the Southern Oscillation. The Southern Oscillation has a period of from 3-6 years. It is called an oscillation because the winds cycle in a back-and-forth way. On one part of the oscillation, the trade winds drop considerably. Sometimes the wind direction changes from easterly (from the east) to westerly in what is called a westerly wind burst because it tends to come on suddenly.

When the trade winds drop, the ocean surface across the entire ocean begins a slow process of adjustment, whereby the sea level changes (down in the west, up in the east) as vast amounts of hot equatorial water flow eastward, toward Peru. The process is very slow because the surface layer is thin, and the distances are thousands of miles. Gyroscopic forces from the rotating Earth focus the flow onto the equator.

The warm pool is part of this adjustment, and it is carried eastward into the center of the Pacific Ocean. It carries its rainfall with it, and desert islands in the central pacific, such as the Galapagos, experience huge rainfall increases.

Figure 2. A map of the SST anomaly (difference of current SST from a 30-year average) during the peak of hurricane season, August-September-October, for 1987. When discussing a quantity such as rainfall or sea surface temperature as they relate to climate, it is much easier and more forceful to consider the anomaly rather than the actual quantity. We say rainfall is 6 cm above “normal,” or that daily maximum temperature is above the long-term average temperature. In discussions on ENSO the anomaly of a quantity is the difference between quantity and the average over the last thirty years (or some such reasonable long length of time). In the above figure for SST, the white areas are not statistically different than the long term average, e.g. no change. Yellow and red areas are warmer water and blue regions are colder. The scale shows the amount of change from the long term average. The Niño 3.4 region, part of the official definition of El Niño is shown. Image credit: NOAA/ESRL.

A Definition of El Niño

A question one might ask is how do we know that an El Niño is here? What temperature, sea level, or other indicator is needed to say, officially, that the ENSO is happening? Or how do we know that it has gone away? The official definition of El Niño was proposed by Kevin E. Trenberth (one of our leading researchers in climate change) of the National Center for Atmospheric Research as follows:

If a five-month running mean of the surface temperature anomaly in the Niño 3.4 region (5N-5S, 120-170W) exceeds 0.4 degC for 6 months or more.

Kevin E. Trenberth, “The definition of El Niño”, Bulletin of the American Meteorological Society, Vol 78 (12), pages 2771-2777, December 1997

The Niño 3.4 region is shown in figure 2. The anomaly at this time was well above the threshold. At the other end of the Southern Oscillation, when the trades are strong, the situation is called La Niña, or the baby girl.

Figure 3. A recent global map of the SST anomaly for the month of March 2010. By the definition above, El Niño is still in progress though diminishing. The most warming at this time is in the central Pacific. Image credit: NOAA/NCEP.

Hurricanes, El Niño, and Modiki El Niño

Not all El Niño events are created equal when it comes to their impact on Atlantic hurricane activity. Differences in El Niño events are described by Dr. Jeff Masters. The global effect of an El Niño depends on how far the warming travels across the Pacific Ocean. If the strong warming travels all the way across the Pacific to Peru, the pattern is called an Eastern Pacific Warming (EPW) pattern. EPW conditions occurred, most recently, during the El Niño years of 1997, 1987, and 1982.

In contrast, more warming occurred in the Central Pacific during the El Niño years of 2004, 2002, 1994, and 1991. The warm pool migration seems to stall in the central Pacific. This pattern is called Central Pacific Warming (CPW).

A recent paper published in the journal “Science” attempts to explain why some El Niño years see high Atlantic hurricane activity. “Impact of Shifting Patterns of Pacific Ocean Warming on North Atlantic Tropical Cyclones”, by Georgia Tech researchers Hye-Mi Kim, Peter Webster, and Judith Curry, theorizes that Atlantic hurricane activity is sensitive to exactly where in the Pacific Ocean El Niño warming occurs.

Over the past 150 years, hurricane damage has averaged $800 million/year in El Niño years and double that during La Niña years. The abnormal warming of the equatorial Eastern Pacific ocean waters in most El Niño events creates an atmospheric circulation pattern that brings strong upper-level winds over the Atlantic, creating high wind shear conditions unfavorable for hurricanes. Yet some El Niño years, like 2004, don’t fit this pattern. Residents of Florida and the Gulf Coast will not soon forget the four major hurricanes that pounded them in 2004–Ivan, Frances, Jeanne, and Charley. Overall, the 15 named storms, 9 hurricanes, and 6 intense hurricanes of the hyperactive hurricane season of 2004 killed over 3000 people–mostly in Haiti, thanks to Hurricane Jeanne–and cost $40 billion in damages.

During EPW years, when the warming occurs primarily in the Eastern Pacific, near the coast of South America, the resulting atmospheric circulation pattern creates very high wind shear over the tropical Atlantic, resulting in fewer hurricanes.

On the other hand, CPW years had lower wind shear over the Atlantic, and thus featured higher hurricane activity than is typical for an El Niño year. One of the paper’s authors, Professor Peter J. Webster, said the variant Central Pacific Warming (CPW) El Niño pattern was discovered in the 1980s by Japanese and Korean researchers, who dubbed it modiki El Niño. Modiki is the Japanese word for “similar, but different.”

Michael Reynolds, Ph.D., RMR Company

OCEAN WATCH departs from Los Sueños, Costa Rica today. We will be sailing into what is left of the 2009-2010 El Niño event which is considered to be one of the most intense of the past decade. We have new instruments to help us define and record the structure of the surface waters through which we will pass. The coastal waters off of the west coast of the Americas, and the atmospheric boundary above, are extremely important for local economies and are crucial for our understanding of climate changes. In this and following reports we will endeavor to explain some of the complexity of ocean and atmospheric circulations as they relate to our observations.

Peruvian Fisheries

One of the world’s richest fisheries is off the coast of Peru. In most years winds from the southeast, in a process called “upwelling,” push warm surface water away from the coast. In its place, upwelling brings cold water rich in nutrients to the surface. (See the left map of sea surface temperature below.) The nutrients provide nourishment for the microscopic plants know as plankton. Plankton normally provide food for a vast community of anchovies and other fish. The fish in turn supply food for seabirds. Not only is the fish catch economically important, the harvesting of bird excrement (guano) provides a supply of valuable fertilizer.

Every few years, typically five years, the pattern of air circulation over the equatorial Pacific changes in a way that shuts down the coastal upwelling. When the upwelling stops, surface waters warm considerably from less than 60 to over 80 degrees F (15-35 degC). Everything that depends on the overturning suffers catastrophic decline: nutrient concentration declines which reduces plankton productivity followed by the collapse of the fishery. Birds die by the thousands. The natives of Peru knew this regular event and the named it “El Niño” for the Christ child.

Normal Years	El Niño 2010
This map shows the sea surface temperature (SST) over the world’s oceans for a normal year. The dashed boxes show regions of upwelling where overturning of the surface water brings up nutrients. The deeper water is colder and thus one sees that the coastal water SST is much cooler than that offshore. The fertile waters drive huge local fisheries.	This map shows global SST for February 2010, an El Niño year. Notice the waters off Peru are now warm and the upwelling region has moved south and offshore. The waters off of Costa Rica are quite warm too. The upwelling area off of North America is still active if not broader. This SST map is for February 2010 and the El Niño event is coming to an end.

Some biologists fear that the overfishing of the anchoveta by humans, plus the eating of anchovies by large fish and seabirds, combined with the injurious effects of an intense El Niño episode, like the one in 1997-98, could reduce the anchoveta stock to such critically low numbers that recovery could be difficult. The 1972-73 El Niño caused a serious drop in the fish catch which took years to recover. Since then, the Peruvian government has worked hard to regulate fishing in their territorial waters. Fortunately they have been successful, and the fishery has recovered from even severe El Niños like the one in 1988-1989.

El Niño: a local effect of a global event

If we would please society we must be prepared to be taught many things we already know by people who do not know them!
Nicolas Chamfort

I want to explain this very interesting physical process, but how? The subject is so well researched and so well explained in thousands of articles, books, and web pages that I cannot imagine how I could contribute to that infinitude of knowledge available to people of every degree of expertise. Nevertheless, with Chamfort’s advice in mind I shall forge ahead.

By now most people have heard of El Niño, if only to know the name refers to some kinds of abnormal weather. The definition of “abnormal” varies widely with geography, though. For people who live in Indonesia, Australia, or southeastern Africa, El Niño can mean severe droughts and deadly forest fires. Ecuadorians, Peruvians, or Californians, on the other hand, associate it with lashing rainstorms that can trigger devastating floods and mudslides. Severe El Niño events have resulted in a few thousand deaths worldwide, left thousands of people homeless, and caused billions of dollars in damage. Before the winter of 2010 residents on the northeastern seaboard of the United States credited El Niño with milder-than-normal winters (and lower heating bills) and relatively benign hurricane seasons. After the past severe winter in the Northeast US, all bets are off and one can say simply that weather is chaotic and extreme.

If you ever wanted to have an appreciation of how beautifully interconnected Nature can be, taking into consideration all the workings of weather and climate, as well as the responses of the biological sphere and concomitant chemistry, take a moment to consider the intricacies of El Niño. Back in the 70’s, before we had global warming to blame, just about any extreme weather event, drought in California, floods in the midwest, heavy rain in California, intense hurricanes, or anything else you might mention was blamed on El Niño.

On 10 April, 2010 the headlines were different: “Landslides and floods kill 200 people as Rio de Janeiro drowns under ELEVEN inches of rain in just 24 hours.” The death toll following the heaviest rains in Rio de Janeiro’s history was set to soar above 200 after a new mudslide hit neighbouring slums. The latest landslide surged into a rain-sodden hillside shanty town in Rio’s neighbouring city of Niteroi, engulfing at least 40 homes in a cascade of mud. The ground gave way in steep hillside slums, cutting red-brown paths of destruction through shantytowns. Heavy flooding in some places and terrible droughts in others result from a readjustment in circulation patterns caused by the El Niño and associated events.

Note that the disruptions from the El Niño occur every 4-6 years and last 1-2 years. These events are superimposed on the global climate change which is on a scale of centuries.

But El Niño is a local manifestation of a global change in weather patterns. As described above, the name was coined in the late 1800s by fishermen along the coast of Peru. Today, the term no longer refers to the local seasonal current shift but to part of a phenomenon known as El Niño-Southern Oscillation (ENSO), a continual but irregular cycle of shifts in ocean and atmospheric conditions that affects the entire world. El Niño has come to refer to the more pronounced weather effects associated with anomalously warm sea surface temperatures interacting with the air above it in the eastern and central Pacific Ocean. Its counterpart–effects associated with colder-than-usual sea surface temperatures in the region–was labeled “La Niña” (or “little girl”) as recently as 1985.

The OCEAN WATCH crew certainly noticed the very warm water as they sailed along the Peruvian coast to the Galápagos and on to Costa Rica. We expected the cooler waters and upwelling, but water temperatures were well above 28 degC (82 degF) which made cabin conditions very unpleasant. The dormant fishing fleets lined the docks and the ports were quiet.

The shift from El Niño conditions to La Niña and back again takes about four years. Understanding this irregular oscillation and its consequences for global climate has become possible only in recent decades as scientists began to unravel the intricate relationship between ocean and atmosphere. Although meteorologists have long been forecasting daily weather based on atmospheric measurements taken around the world, they had relatively little information about conditions in many parts of the world’s oceans until the advent of arrays of fixed, unmanned midocean buoys in the Pacific Ocean and orbiting satellites.

But technological advances were not the only key to understanding. Atmospheric and oceanographic researchers, after years of independent inquiry into the basic workings of air and sea, have at last joined forces. An elegant synthesis of these two fields of research now enables climatologists and oceanographers to construct theoretical models to simulate and predict the broad climate changes associated with ENSO. For example, scientists can now warn vulnerable populations of an impending El Niño event several months in advance, providing precious time in which to take steps to mitigate its worst effects. Invaluable as this prediction of El Niño is, it is just the first step toward the much longer-term goal of providing the climatic counterpart to the daily weather prediction that we have come to take for granted.

Future reports

Today we sail from Los Sueños and head north towards Puerto Vallarta. We have two new pieces of oceanographic instrumentation with us which we will discuss in future reports. The premier manufacturer of oceanographic instrumentation, Sea-Bird Electronics, in Bellevue Washington, has provided us with a thermosalinograph (TSG) which will provide a continuous record of surface temperature and salinity. The TSG is run with the SeaKeeper instrument and will provide validation and cross reference to the SeaKeeper measurements. Secondly we have a Sea-Bird conductivity-temperature-depth (CTD) instrument which we will lower from the boat to get a profile of the temperature, salinity, and pH through the surface waters. We will be able to report on our findings as we progress. As a scientist, I must say how excited I am to have these instruments in this very important El Niño event.

Michael Reynolds, Ph.D.
April 10, 2010