One of the most well-known climate patterns that we have come to recognize and better understand is the El Niño. Every three to seven years during the months of December and January, the balance between, wind, ocean currents, oceanic and atmospheric temperature and bioshpere breaks down, resulting in a severe impact on global weather.
In a normal year the trade winds blow westward and push warm surface water near Australia and New Guinea. When this warm water builds up in the western Pacific-Ocean, nutrient-rich cold waters are forced to rise up from the deeper ocean just off of the west coast of South America. This colder nutrient-rich water fosters the growth of the fish population.
During an El Niño event the trade winds weaken. Warm, nutrient-poor water is not pushed westward and comes to occupy the entire tropical Pacific Ocean. The cold water is not forced to the surface and the coastal waters of Peru and Ecuador are unusually warm. This warmer water has a devastating impact on their fishing crops which rely on cool waters to thrive. The region also experiences an extremely higher than average amounts of rainfall.
While the impact of an El Niño is most dramatic off of the coast of Western South America its impact is felt in weather around the world. A severe El Niño will enhance the jet stream over the western Pacific and shift it eastward, leading to stronger winter storms over California and southern United States, with accompanying floods and landslides. In contrast, El Niño may also cause severe droughts over Australia, Indonesia, and parts of southern Asia. Further, while El Niño is known to lower the probability of hurricanes in the Atlantic, it increases the chances of cyclones and typhoons in the Pacific.
Oceanography from Space
El Niño is one example of how observing the ocean from space leads to significant insights. Researchers use data from NASA Earth observing satellites to create telling images of how El Niño events form in the ocean, and the factors that may impact its strength and duration in a given climate cycle.
NOAA's AVHRR and NASA's Aqua satellite have provided scientists over 25 years of sea surface temperature data. The vast tropical Pacific Ocean receives more sunlight that any other region on Earth. Much of this sunlight is stored in the ocean in the form of heat. During an El Niño water temperatures in the Pacific Ocean may rise on average 3 - 5 degrees above average. This happens as the water in seas around Indonesia, referred to as the Pacific Warm Pool are not forced westward due to weakened east to west trade winds. This pool of warmer water expands westward toward North and South America. Scientists have observed that the size and the frequency of an El Niño is impacted by the fluctuations in the Pacific Warm Pool.
Currents and tides influence topography, as does temperature. Water expands as it gets warmer, and the lack of cold water dependent nutrients make it less dense. This expanded, less dense water results in a rise in sea level, observable from space. Ocean surface height may rise as much as 6 to 13 inches above normal in some ocean regions during an El Niño.
The QuikSCAT satellite tracks vector winds, and the Jason and Topex/Poseidon missions track their effects. Typically, the Pacific trade winds blow from east to west, dragging the sunlit warm waters westward where they accumulate in Pacific Warm Pool mentioned above. Weakened east to west trade winds during an El Niño event result in the Pacific Warm Pool waters expanding westward. NASA's QuikSCAT satellite data have shown how trade wind irregularities of less than a few months in duration leading up to an El Niño may have dramatic results on the weather events to follow.
Currents, or circulation within ocean is influenced primarily by two physical factors, the sinking and rising of warming and cooling water, and movement due to the forcing of the surface waters due to wind. The interactions of ocean water temperature and the strength and direction of winds create currents within the ocean that define the strength and duration of the El Niño. NASA satellite data in combination with data collected from buoy systems and ships provide scientists an expanding understanding of the relationship between ocean currents, weather and climate.
Ocean water salt content or salinity is a key variable in understanding the ocean's capacity to store and transport heat. Salinity and temperature combine to dictate the oceans' density. Greater salinity, like colder temperatures, results in an increase in ocean density with a corresponding depression of the sea surface height. In warmer, fresher waters, the density is lower resulting in an elevation of the sea surface. These ocean height differences are related to the circulation of the ocean. Beginning in 2009 the Aquarius mission will take regular measurements of the changes in ocean surface salinity. By knowing the how changes in salinity impact the physical processes in the ocean scientists will be able to create computer models that may more accurately predict El Niño episodes.
Sea ice modulates planetary heat transport by insulating the ocean from the cold polar atmosphere, and also by modulating the thermohaline circulation of the world ocean. Moreover, the high albedo of snow-covered ice further insulates the polar oceans from solar radiation and introduces another positive feedback in the climate system. The El Niño and its related Southern Oscillation appear to affect regional ice distributions around Antarctica. Understanding this connection between the Southern Oscillation and southern ocean climate and the sea ice cover will substantially improve our understanding of global climate. El Niño episodes affect the Weddell and Ross Seas, areas that are regarded as key sources of cold and dense bottom water that influences global ocean circulation. The strongest links were observed to be in the Amundsen, Bellingshausen and Weddell Seas of the west Antarctic. Within these sectors, higher sea level pressure, warmer air temperature and warmer sea surface temperature are generally associated with the El Niño phase.
By combining data sets from various satellite and in situ measurements we have been able to learn a great deal about how the physical properties of the ocean interact to create climate patterns. Scientists will continue to create models that combine data allowing them to better understand and predict complex ocean processes.
By using data collected from discrete measurements of physical properties of the ocean scientists have learned a great deal about El Niño and how it is impacted by the ocean system. Scientists are just beginning to have data of a time span sufficient to allow them to begin to predict climate patterns longer than the 3 to 7 year El Niño pattern. The ability to study the El Niño climate pattern and create models to simulate conditions, has helped us better predict its impact on our climate and weather.
El Niño is only one of the climate anomalies that scientists have observed;others include the North Atlantic Oscillation, the Atlantic Intertropical Convergence Zone oscillation, the Pacific Decadal Oscillation. Together with El Niño these systems are believed to be responsible for well over fifty percent of the climate variability on Earth. As models are created to simulate the individual patterns they will be combined to create a global model of climate change. If scientists get to the point to where they understand all of these climate cycles they may be able to predict major weather patters months in advance.