El Niño/ Low Southern Oscillation PhaseVS.
La Niña/ High Southern Oscillation Phase
Signals in Tropical Pacific:• Sea surface temperatures (SSTs)• Precipitation• Sea Level Pressure• The Southern Oscillation (High vs. Low Phases) • Low-level Winds and Thermocline Depth
Low-Level Winds & Thermocline Depth
La Niña: stronger-than-average easterlies lead to a deeper (shallower)-than-average thermocline in the western (eastern) eq. Pacific.
El Niño: weaker-than-average easterlies lead to a deeper (shallower)-than-average thermocline in the eastern (western) eq. Pacific.
ENSO: A Coupled Ocean-Atmosphere Cycle
ENSO is a “coupled” phenomenon: atmosphere drives the ocean and the ocean drives the atmosphere.
“Positive Feedback” between ocean and atmosphere. Example:
Weaker equatorial trade winds cold water upwelling in the east will decrease surface warming of the ocean reduced east-west temperature gradient Weaker equatorial trade winds
What is “Average?”
WarmCold
WarmCold
Winds and Sea Surface Temperature are COUPLED. The SSTs influence the winds and vice versa.
(1) Easterly trade-winds help push warm water to the western Pacific and upwell cold water along the equator in the eastern Pacific Ocean.
(2) Warm water heats the atmosphere, the air rises, and low-level trade winds converge toward the warm water. Subsiding air occurs in the eastern Pacific basin.
December-February Average Conditions
“El Niño”
WarmCold
Warm
ColdWarm
• Convection shifts eastward over the central and/or eastern Pacific Ocean. Convection becomes suppressed over the far western Pacific/ Indonesia.
• Easterly trade winds weaken
• Thermocline deepens and the cold water upwelling decreases in the eastern Pacific.
“La Niña”
ColdWarm
Warm Cold
Stronger Stronger
Upwelling
Enhanced
More Convection
becomes more shallow
• Convection becomes stronger over the far western Pacific Ocean/ Indonesia and more suppressed in the central Pacific.
• Easterly trade winds strengthen
• Thermocline becomes more shallow and the cold water upwelling increases in the eastern Pacific.
Cold
Typical Evolution of the ENSO Cycle
• Irregular cycle with alternating periods of warm (El Niño) and cold (La Niña) conditions
• El Niño tends to occur every 3-7 years and generally lasts 12-18 months
• La Niña episodes may last from 1 to 3 years• Transitions from El Niño to La Niña are more rapid
than transitions from La Niña to El Niño.
Normal Condition & La Nina
El Nino
El Nino
Madden Julian Oscillation• The Madden–Julian oscillation (MJO) - Each cycle lasts approximately 30–60
days. Because of this pattern, The MJO is also known as the 30–60 day oscillation, 30–60 day wave, or intraseasonal oscillation. Rather than being a standing pattern (like ENSO) it is a traveling pattern, propagating eastwards at 4 to 8 m/s, through the atmosphere above the warm parts of the Indian and Pacific oceans. This overall circulation pattern manifests itself in various ways, most clearly as anomalous rainfall. This was discovered by Roland Madden and Paul Julian(again the comparison with ENSO is instructive, since their local effects on Peruvian fisheries were discovered long before the global structure of the pattern was recognized).
• The MJO is characterized by an eastward progression of large regions of both enhanced and suppressed tropical rainfall, observed mainly over theIndian Ocean and Pacific Ocean. The anomalous rainfall is usually first evident over the western Indian Ocean, and remains evident as it propagates over the very warm ocean waters of the western and central tropical Pacific.
Madden Julian Oscillation
Madden Julian Oscillation
