Teleconnections and Oscillations Teleconnection climate anomalies being related to each other over a large scale Oscillations: Macroscale movement of atmospheric systems that can influence weather, climate, and ocean properties on a global or regional scale Coupled systems Change occur simultaneously to ocean and atmospheric systems El Nino Southern Oscillation ENSO Coupled Ocean/Atmosphere Process 3 Phases: Neutral, normal, average, non-el Ni El Niño - warm phase La Niña - cold phase El Niño and La Niña influence the prevailing circulation of the atmosphere in middle latitudes, especially in winter History Currents Originally, El Niño was the name given by fisherman to the seasonal occurrence of an unusually warm southward flowing ocean current and poor fishing off the coast of Peru and Ecuador during the Christmas season In 1982 climate scientists from around the world met to devise a plan to observe El Niño events. The scientists were not aware the strongest El Niño of the century was occurring as they met. Now, scientists reserve the term El Niño for long-lasting atmosphere/ocean anomalies Occurs every 3-7 years Persists for 12-18 months or longer Significant changes to the atmosphere and ocean circulation Pressure Cells Neutral Conditions Strong Trade Winds Southerly winds along S. America 1
Typically, southerly winds blowing along the west coast of S., America drive warm surface waters to the left (Ekman transport) away from the coast Abundance of nutrients brought to sunlit surface waters spurs an explosive growth of phytoplankton, which supports a highly productive fishery Upwelling Non-El Nino Neutral Neutral Conditions Strong Trade Winds Southerly winds along S. America Wet in the west, dry in the east Clockwise Walker cell Fast currents Upwelling along S. America Warm surface in west, cold surface in east Southern Oscillation An El Niño episode begins when the air pressure gradient across the tropical Pacific begins to weaken, causing a slackening of the trade winds Eventually the Indonesia Low migrates from the west over to the Eastern Pacific Trades slow, stop or reverse. No offshore Ekman Transport, thus no upwelling El Nino EL Niño Conditions Weak or reversed Trade Winds Northerly winds along S. America Dry in the west, wet in the east CCW Walker cell Slow currents No upwelling along S. America Warm surface everywhere El Niño 2
La Niña Very strong Trade Winds Strong southerly winds along S. America Very wet in the west, Very dry in the east La Niña Conditions ENSO - How Observed 1) Southern Oscillation Index Computed by subtracting the East Pacific surface pressure (mb) minus the West Pacific pressure (mb) Strong positive values indicate La Niña conditions Strong negative values indicate El Niño conditions Fast currents Much upwelling across Pacific Cold surface waters in eastern& mid-pacific 2) SST Anomaly 3) ENSO Observing System Movie ENSO s effect on Weather ENSO has a ripple effect on the weather and climate throughout the world. A linkage between atmospheric circulation changes in widely separated regions of the globe is known as a teleconnection. S. Hemispere, the weather changes with the migration of the pressure cells 3
El Niño s effect on Weather N. Hemisphere, generally both Jet Streams migrate north. El Niño Winter Northern U.S. - warm Southern U.S. - wet Subtropical Jet Stream now affects U.S. Weather La Nina Winter Brings opposite weather extremes than El Niño La Niña Winter Northern U.S. - cold, stormy Southern U.S. - dry, mild ENSO During El Niño events Atlantic tropical storms are significantly reduced; La Nina events enhances activity. Reason: Tropospheric vertical shear in the Atlantic is increased during El Niño. What is the North Atlantic Oscillation? Changes in the intensity of the Azores High and Icelandic Low over the North Atlantic. This will affect the strength and pathway of winter storm systems crossing the Atlantic from the US east coast to Europe. Accounts for much of the variability in winter temperature & precipitation 4
What is the North Atlantic Oscillation? Mainly affects the atmospheric circulation over the Atlantic from December to March. It is a measure of the pressure difference between the Azores High and the Icelandic Low. This pressure difference determines the strength of the Westerlies. is an index that measures the strength of the westerly winds blowing across the North Atlantic Ocean in the 40-60 N latitude belt. There are 2 phases (+ and -) to the NAO index n Stronger than usual subtropical high pressure center n Greater pressure gradient causing stronger winds. n Winds advect cold polar air over the eastern U.S., so winters there are colder. Positive Phase n More frequent and stronger storms moving in a more northerly track so winters are warm and very wet in Europe n Mediterranean region is dry Positive Phase n Weak subtropical high and weak Icelandic low n Fewer and weaker storm systems due to a reduction in pressure gradient. Storms move in a more west-east direction n Europe is dry, Mediterranean region is wet Negative Phase Negative Phase n Eastern U.S. has mild winters but higher chance of big snow events. n In the Southeast winters are colder than usual 5