2. Natural Climate Variability  

2.1 Introduction

2.2 Interannual Variability

2.3 Decadal Variability

2.4 Climate Prediction

2.5 Variability of High Impact Weather

Time series of SSTs in the East Pacific (region Nino3.4)

2.1 Introduction: What is Interannual and Decadal Variability?

2.1 Introduction: What is Interannual and Decadal Variability?

Time series of SSTs in the North Atlantic highlighting the Atlantic Multidecadal Oscillation (AMO) – source Knight et al (2005).

2.1 Introduction: What is Interannual and Decadal Variability?

The NAO index is defined as the anomalous difference between the polar low and the subtropical high during the winter season (December through March).

The positive phase is associated with more storminess in the Atlantic storm track and the negative phase with reduced storminess.

2.1 Introduction: Why is Climate Variability important?

Interannual-to-Decadal Variability impacts Societies:

•Food resources

•Water security

•Health

•Demographics

• Energy

Time series (1941-2001) of average normalized April-October rainfall departure for 20 stations in the West African Soudano-Sahel zone (11-18N and West of 10E); following methodology of Lamb and Peppler, 1992).

2.1 Introduction: Can we predict SI Variability?

Section 2.2 Interannual Variability 

2.2.1 El Nino Southern Oscillation (ENSO) 

(i) Observations(ii) Theory for ENSO(iii) Impacts

2.2.2 Interannual variability in Atlantic SSTs

2.2.3 The North Atlantic Oscillation (NAO)

 

2.2.1 ENSO: Observations

• Philander, S.G.H, 1990: El Nino, La Nina and the Southern Oscillation

• Useful El Nino pages:

http://www.pmel.noaa.gov/tao/elnino/nino-home.html

http://www.gfdl.noaa.gov/~atw/enso

Mean State of the tropical Pacific coupled Ocean-Atmosphere System

• Warmest SSTs in west, “cold tongue” in east• Precipitation associated with warmest SSTs• Easterly trade winds advect equatorial surface

waters westward

The tropical Pacific thermocline

The Walker Circulation

• Mean ascent, and low surface pressure, over warmest SST associated with deep convection

• Subsidence, and high surface pressure, in non-convecting regions

• Equatorial trades blow from high to low pressure (balanced by friction since Coriolis force =>0)

Low slp

High slp

El Nino

• During El Nino trade winds slacken

E-W tilt of thermocline & upwelling of cold water are reduced.

SST rises in central/eastern equatorial Pacific

Changes Walker Circulation

Mean climate El Niño ’82/83

El Nino

SST anomalies during El Nino

Dec 1982

Sept 1987

The 1997/98 El Nino

Jan 1997

The 1997/98 El Nino

Nov 1997Jun 1997

The 1997/98 El Nino

Nov 1997

The 1997/98 El Nino

Mar 1998

The 1997/98 El Nino

Mar 1998Jan 1997

The 1997/98 El Nino

Jun 1997

The 1997/98 El Nino

Nov 1997

The 1997/98 El Nino

Mar 1998

What about La Nina?

Dec 1982

Nov 1988

La Nina conditions sometimes occur in the year following an El Nino event (e.g. 1988 followed 1987 El Nino)

In La Nina conditions SST in the central and eastern equatorial Pacific is unusually cold & easterly trade winds are unusually strong

Walker Circulation• British mathematician, director

general of observations for India (formed after monsoon failure of 1877- worst famine in Indian history)

• Arrived in 1904, shortly after huge famine caused by drought

• Goal to predict Indian Monsoon• Found that many global climate

variations, including Monsoon rains in India, were correlated with the Southern Oscillation

Tahiti

Darwin

• Mean pressure is lower at Darwin than Tahiti• The term “Southern Oscillation” was also coined by Gilbert Walker• The SOI measures the strength of the Pacific Walker circulation

The Southern Oscillation Index

x

Bjerknes recognised that the “El Nino” warming of the ocean was related to variations in the SOI.

During El Nino:

• SOI is low

• Trades are weak

• Precipitation is enhanced over central equatorial Pacific (indicated by low OLR)

ENSO

The “Nino3” SST index

Note that El Nino events do not occur regularly

Why is El Nino important?

• Major climate and economic impacts on countries around the tropical Pacific, and further afield.

• Droughts in some regions, floods in others• Collapse of coastal

fishery in Peru(largest average

annual catch of marine fish in

the world)

Impacts on global climate, ecosystems and society

Observing El Nino

TAO Data

TAO on Youtube:

http://www.youtube.com/noaapmel#p/u/0/nzBAWirHMvA

Live TAO data:

http://www.pmel.noaa.gov/tao/jsdisplay/

Satellite Observations

of Sea Surface Height

Chronology of Events in the History of Understanding El Niño and La Niña

late 1800s Fishermen coin the name El Niño to refer to the periodic warm waters that appear off the coasts of Peru and Ecuador around Christmas.

1928 Gilbert Walker describes the Southern Oscillation, the seesaw pattern of atmospheric pressure readings on the eastern and western sides of the Pacific Ocean.

1957 Large El Niño occurs and is tracked by scientists participating in the International Geophysical Year. Results reveal that El Niño affects not just the coasts of Peru and Ecuador but the entire Pacific Ocean.

1969 Jacob Bjerknes, of the University of California, Los Angeles, publishes a seminal paper that links the Southern Oscillation to El Niño.

1975 Klaus Wyrtki, of the University of Hawaii, tracks sea levels across the Pacific and establishes that an eastward flow of warm surface waters from the western Pacific causes sea surface temperatures to rise in the eastern Pacific.

1976 Researchers use an idealized computer model of the ocean to demonstrate that winds over the far western equatorial Pacific can cause sea surface temperature changes off Peru.

1982 A severe El Niño develops in an unexpected manner, but its evolution is recorded in detail with newly developed ocean buoys.

1985 Several nations launch the Tropical Ocean-Global Atmosphere (TOGA) program, a 10-year study of tropical oceans and the global atmosphere.

1986 Researchers design the first coupled model of ocean and atmosphere that accurately predicts an El Niño event in 1986.

1988 Researchers explain how the "memory" of the ocean--the lag between a change in the winds and the response of the ocean--influences terminations of El Niño and the onset of La Niña.

1996-1997 The array of instruments monitoring the Pacific, plus coupled ocean-atmosphere models, enable scientists to warn the public of an impending El Niño event.