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.

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.

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

3-D: Normal Pacific ocean-atmosphere conditions

White arrows: currents Black arrows: winds

D20: depth of 20C isotherm Walker circulation

Normal Pacific EQ currents

EUC: 200km wide, and 100m deep

Pacific Annual Cycle of SST & winds

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?

2. The El Niño – Southern Oscillation (ENSO) Index regions Southern Oscillation Index (SOI) Sea Level Pressure (SLP) difference between Tahiti & Darwin

ENSO Phases Cold phase Warm phase

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

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;

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”

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

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

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?

Critical thinking Which season does El Nino obtain its peak? Why?

El Nino events: evolution of SST & SLP

SSTA (color) SLPA, contour Normally lasts for a year, but there are prolonged El Nino events

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)

Dec-Feb

Hovmoller Diagram of Anomalous SST and Zonal (east-west) winds 1997-1998 from the TAO array

Thermocline

Ocean Temperature Anomalies

Sea level height

thermocline

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

El Nino Sea LevelChanges

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?