atoc 5051 introduction to physical...
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ATOC 5051 INTRODUCTION TO PHYSICAL OCEANOGRAPHY
Lecture 24: Climate variability mode: El Niño and the Southern Oscillation (ENSO) Learning objective: Understand the observed ocean-atmosphere anomalies associated with ENSO & ENSO’s climatic impacts. Specifics: 1. The normal tropical Pacific & annual cycle; 2. ENSO phenomenon.
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Ocean basin of the tropical Pacific & index regions
Index regions Southern Oscillation Index (SOI) Sea Level Pressure (SLP) difference between Tahiti & Darwin
From Ecuador in the east (80w) to Maritime continent (a collection of Islands of Indonesia-Australia) in the west; spans about 151 degrees.
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1. The observed normal tropical Pacific: surface
ITCZ SPCZ
ITCZ SPCZ
Annal mean SST & surface wind Surface wind, ITCZ,etc
Mean Precipitation
Low
Mean Sea Level Pressure
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3-D: Normal Pacific ocean-atmosphere conditions
White arrows: currents Black arrows: winds
D20: depth of 20C isotherm Walker circulation
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Normal Pacific EQ currents
EUC: 200km wide, and 100m deep
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Pacific Annual Cycle of SST & winds
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Summary: normal ocean-atmosphere circulation
1) Normal condition: Walker circulation; 2) SST, zonal surface currents, sea level and thermocline depth; 3) Seasonal cycle of surface wind, SST; 4) The cold tough is the strongest, and the east-west SST gradients are the largest during which season? 5) What about the seasonal variation of the ITCZ?
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2. The El Niño – Southern Oscillation (ENSO) Index regions Southern Oscillation Index (SOI) Sea Level Pressure (SLP) difference between Tahiti & Darwin
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ENSO Phases Cold phase Warm phase
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La Nina (cold phase): Enhanced Normal Condition: 1. Enhanced Walker circulation & SOI; 2. Oceanic circulation, SST, nutrients, fishery;
Low SLP
Figure 2
Fish
High SLP
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The El Nino condition: Warm phase of ENSO;
Figure 3
Higher SLP lower SLP
1. Weakened Walker Circulation and negative SOI; 2. Ocean: warm SST shift to east, bad fishery;
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What is El Niño and ENSO? Interaction between the atmosphere and ocean across the tropical Pacific
• Causes big changes in – Ocean temperatures (warm events – El Nino) – Winds – Thermocline depth, ocean currents and upwelling
• Involves Rossby and Kelvin waves
– Precipitation (Convection) – Sea Level Pressure (SLP)
• East-west SLP dipole called “Southern Oscillation” • El Niño + Southern Oscillation: “ENSO”
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Why is it called El Nino? • Originally named by Peruvian fisherman • For very warm water in the Pacific Ocean,
occurring around Christmas.
• El Niño means The Little One in Spanish. (Christ Child).
• El Niño means a warm event that occurs
about every 2-7 years across the tropical Pacific Ocean
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ENSO Evolution: SST Variability in Nino3.4 region NINO3.4 SST Anomaly Time series
Spec
tra
Standard deviation by month
2-7 years periods Phase-lock: fall-winter maximum
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Summary: El Nino Relative to the normal condition, during El Nino: 1) How does the Walker Cell change? 2) How does SST and thermocline depth change? 3) How does fishery change? 4) What about La Nina?
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Critical thinking Which season does El Nino obtain its peak? Why?
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El Nino events: evolution of SST & SLP
SSTA (color) SLPA, contour Normally lasts for a year, but there are prolonged El Nino events
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ENSO evolution: (July-Nov) regression Onto EQ SSTA index a) Surface wind+SLP; b) Surface wind+SST; c) Surface wind+OLR Convection (outgoing longwave radiation)
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Dec-Feb
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Hovmoller Diagram of Anomalous SST and Zonal (east-west) winds 1997-1998 from the TAO array
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Thermocline
Ocean Temperature Anomalies
Sea level height
thermocline
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1997/98 El Nino: High SLA in E. Pacific
Satellite TOPEX/Poseidon Sea Level Anomaly (SLA): Red: high SLA; Blue: low SLA
DEC
SLA in averaged Over the boxed region
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El Nino Sea LevelChanges
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Critical thinking
1) How does El Nino evolve from summer-fall to winter? (surface wind, SLP, convection & SST) 2) Based on the above observations, what are the most striking features that inspire you regarding El Nino evolution? What do these features indicate regarding ENSO mechanisms?