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?

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 ( ) 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 El Nino Southern Oscillation (ENSO) (i) Observations (ii) Theory for ENSO (iii) Impacts Interannual variability in Atlantic SSTs 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:

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 Ni ñ o ’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 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 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

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: Live TAO data:

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 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 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 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 Several nations launch the Tropical Ocean- Global Atmosphere (TOGA) program, a 10- year study of tropical oceans and the global atmosphere 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 The array of instruments monitoring the Pacific, plus coupled ocean-atmosphere models, enable scientists to warn the public of an impending El Niño event.

Section 2.3 Interannual Variability El Nino Southern Oscillation (ENSO) (i) Observations (ii) Theory for ENSO (iii) Impacts Interannual variability in Atlantic SSTs The North Atlantic Oscillation (NAO)

Section (ii) Theory of ENSO The “Bjerknes Feedback”: A mechanism for growth of El Nino or La Nina

Fig.1 Schematic of Ekman Spiral in the Ocean

Spring 2013 Equatorial Upwelling

Spring 2013 Coastal Upwelling Motion of surface waters away from coast requires upwelling of water from below to satisfy continuity of mass. Andes Mts. S. Pacific Ocean

Observed SST Distribution Equatorial Upwelling Coastal Upwelling Winds have a major influence on tropical SST pattern.

Spring 2013 La Niña conditions: Strong cold tongue El Niño conditions: Cold tongue absent

Fig.2 SSTs in Pacific in Dec

Upwelling linked to hurricanes

Fig.3 From S.-P. Xie (U Hawaii)

ENSO mechanism: the Bjerknes feedback Fig.4

Bjerknes feedback - equatorial sections ocean longitude atmos longitude Fig.5

Bjerknes feedback - equatorial sections El Niño ocean longitude La Niña atmos longitude Fig.5

Section (ii) Theory of ENSO The Delayed Oscillator

How does the phase of ENSO reverse? What triggers an El Nino event?

Power Spectrum What determines the 4-year periodicity?

Recent History of El Niño and La Niña

Temporal Evolution of El Niño/La Niña

Equatorial Waves Equatorial waves in the ocean are believed to play an important role in the onset and variability of ENSO Two types: –Kelvin waves (propagate eastward along the equator and also along coasts) –Rossby waves (long waves propagate westward) The relevant waves are baroclinic: internal to the ocean, propagating along the density contrast of the thermocline Equatorial Kelvin waves travel 3 times faster than the fastest equatorial Rossby waves

How does the phase of ENSO reverse? Battisti and Hirst, 1989; Suarez and Schopf, 1988 Thermocline depth Delayed Oscillator Theory Westerly winds force downwelling on Equator and upwelling to North and South => Excites Kelvin and Rossby waves Figures from IRI:

Delayed Oscillator Theory 2 25 days 100 days50 days 75 days Upwelling Rossby waves Downwelling Kelvin wave

Delayed Oscillator Theory days 275 days175 days 225 days Phase has reversed

Delayed Oscillator Theory 4 Equatorial ocean waves offer a mechanism to reverse the phase of perturbations to the thermocline depth Without further wind forcing waves eventually decay Thermocline depth perturbations influence SST in the upwelling regions of central / eastern equatorial Pacific => coupling to atmosphere Bjerknes feedback + equatorial waves can generate a self-sustaining oscillation

discharging discharged recharged Recharge/Discharge theory (Jin, 1997) Prior to El Nino heat content in equatorial region builds up During El Nino heat is “discharged” eastward and polewards Source: A. Timmermann

Equatorial temperature sections

What triggers El Nino? Still not fully understood High heat content (deep thermocline) in the equatorial region necessary but not sufficient condition “Westerly wind bursts” (few days duration) associated with the Madden Julian Oscillation may act as one trigger. G. Vecchi

Onset of the 1997/98 El Nino Downwelling equatorial Kelvin waves triggered by westerly wind bursts

Summary of El Nino onset Need high heat content in equatorial Pacific Triggering by wind fluctuations (e.g. WWBs) over central/western Pacific Growth through Bjerknes positive feedback mechanism

Theories for ENSO Oscillations Delayed Oscillator (e.g. Battisti and Hirst, 1989; Suarez and Schopf, 1988 Recharge/discharge theory (Jin, 1997) Western Pacific Oscillator (e.g. Weisberg, R. H., and C. Wang, 1997) Advective-Reflective Oscillator (e.g. Picaut et al, 1997) Unified Oscillator (Wang, 2001 J Clim)

The 1997/98 ENSO event

Multivariate ENSO Index (upto October 3 rd 2014) Index is based on 6 parameters relevant to phase of ENSO: sea level pressurezonal and meridional wind at surface sea surface temperatureair temperature cloudiness

A weakening and reversal of trade winds in Western and central Pacific led to anomalous warm SSTs east of date line in early 1997

Warm waters (> 29C migrated eastwards, collapse of cold tongue in east, which failed to develop.

A weakening and reversal of trade winds in Western and central Pacific led to anomalous warm SSTs east of date line in early 1997 Warm waters (> 29C migrated eastwards, collapse of cold tongue in east, which failed to develop.

El Nino developed so rapidly that each month between June-Dec 1997 a new monthly record high was set for SSTs in eastern equatorial Pacific.

Development of El Nino in equatorial Pacific was significantly modulated by higher frequency variability. Weakening and reversal of trades was punctuated by a number of westerly wind bursts of increasing intensity along the equator (linked to several MJOs) Ocean currents forced by these WWBs, advected warm water eastwards. Also, WWBs forced downwelling K-waves (thermocline depth decreased by 90m in eastern Pacific by end of 97) – opposite anomaly in western Pacific.

Rossby waves excited by the initial weakening of the trades propagated westwards towards the west. Net result was a flattening of the thermocline and disapperance of “normal” east-west gradient across the equator, resulting in a weakening of the trades etc etc etc (Bjerknes Feedback).

Note anomalous SSTs in early 1998, SST anomalies superimposed on seasonal warming. Cooling did not take place until winds returned close to normal (easterly) in the middle of Very abrupt change associated with upwelling due to “normal” easterly wind strength. Change to La Nina.

Note build up of heat content in Western Pacific at least a year before onset of the 97/98 El Nino – due to stronger than normal trade winds (linked to weak La Nina). Although these conditions helped to set the stage for the development of El Nino – the onset did not occur until the intensification of the MJO in late Observed heat content anomalies (from 0 to 400m) averaged between 2N and 2S from the TAO array

The delayed oscillator theory and the 1997/98 El Nino Evolution of equatorial Pacific on interannual timescales is governed by interplay between large-scale equatorial wave processes and atmosphere- ocean feedbacks. The only downwelling Kelvin waves seen in 1997/98 were associated with a succession of MJOs. DO theory predicts demise of El Nino to be associated with reflected upwelling Rossby waves at the western boundary – which was seen to occur after the onset of the El Nino. In addition there was direct wind-forcing of upwelling Kelvin waves associated with appearance of easterly wind anomalies in the West Pacific in late 1997 early 1998; so the demise was likely associated with upwelling Kelvin waves due to two different processes.

Section 2.2 Interannual Variability El Nino Southern Oscillation (ENSO) (i) Observations (ii) Theory for ENSO (iii) Impacts Interannual variability in Atlantic SSTs The North Atlantic Oscillation (NAO)

2.2.1 El Nino Southern Oscillation (ENSO) (iii) Impacts

Questions What are the impacts? What are the causes of the impacts? How does ENSO variability impact remote regions and why?

Different Ways of viewing Impacts 1.Schematic Overviews 2.Station Data Statistics 3.Gridded Data Statistics 4.Specific Events Important to consider these as a function of season, location and variable. Most common variables considered are T and PPN

Schematic Overviews

IRI impacts website: NOAA impacts website: Composite ENSO plots: Risk of extremes: Statistics

Events

Mechanisms for Local Impacts

Teleconnections Mechanisms for Remote Impacts

Mechanisms for remote impacts Latent heating associated with tropical precipitation anomalies excites waves in the atmosphere: –Atmospheric equatorial Kelvin and Rossby waves propagate zonally. Associated subsidence warms troposphere and suppresses precipitation –Rossby waves can also propagate into extratropics (especially in winter) causing large- scale circulation anomalies which impact weather. (Note that propagation requires a westerly mean flow and the relevant Rossby waves have an eastward group velocity)

Source: D. Neelin

Current ENSO Advisory: _advisory/