Lecture 8

El Nino/La Nina, and their connection with

Hong Kong’s climate

LSGI1B02 Climate Change and Society

LEUNG Wing-mo

Climate variabilityVariability ranges over many time and space scales such as localized thunderstorms and tornadoes, to larger-scale storms, to droughts, to multi-year, multi-decade and even multi-century time scales.

Some examples of this longer time-scale variability might include a series of abnormally mild or exceptionally severe winters, and even a mild winter followed by a severe winter.

Such year-to-year variations in the weather patterns are often associated with changes in the wind, air pressure, storm tracks, and jet streams that encompass areas far larger than that of your particular region.

At times, the year-to-year changes in weather patterns are linked to specific weather, temperature and rainfall patterns occurring throughout the world due to the naturally occurring phenomena known as El Niño and La Niña – a teleconnection phenomenon –changes occurring in places far away affect the weather/climate in other parts of the world.

Climate Variability and Climate Change

Normals

Climate Change

Climate Oscillations

Climate VariabilityNormals

Short term: (years to decadal) rises and falls about the trend line (El Nino Southern Oscillation, ENSO)

Long Term Trends or major shifts in climate: (centuries, significant and persistent change from mean state)

Multi-decadal oscillations in regional climate: (e.g. Pacific Decadal Oscillation, PDO, North Atlantic Oscillation, NAO)

Drought in Queensland, Australia, 2014

https://www.youtube.com/watch?v=dzat16LMtQk

Understanding El Nino, Bureau of Meteorology, Australia

El Nino

(Boy Child):

Sea Surface

temperature

(SST) changes

in the Pacific

Jan

DecEquatorial “cold tongue”

Major ocean currents -

result of winds, continental land masses, and Coriolis effect

Gulf stream

Calif. current

Kuroshio current

Walker Circulation

Walker Circulation & Southern Oscillation

Tahiti

Darwin

Southern Oscillation: fluctuations in the surface air pressure between Tahiti (east) and Darwin (west) – an indication of

the strength of Walker Circulation

Walker Circulation

Relationship between El Nino (SST) and Southern

Oscillation (pressure)

El Nino, SSTchanges

Southern Oscillation, surface pressure changes

Discovery of the “El Niño- Southern Oscillation (ENSO)”

• In 1960’s , Bjerknes found a link between the two phenomena, and that the anomalous warming of the waters during El Niño extended over a large portion of the equatorial Pacific.

• SOI (Southern Oscillation index) = Tahiti pressure anomaly –Darwin pressure anomaly

• Mean cycle of 4 years (2 – 7)

ONI - 3-month running means of SST anomalies in the Niño 3.4 region [5N-5S, 120-170W]. The anomalies are derived from the 1971-2000 SST climatology.

Equatorial

cold

tongue is

weaker

than

average or

absent

during El

Niño,

resulting

in positive

SST

anomalies

Equatorial

cold

tongue is

stronger

than

average

during La

Niña,

resulting

in negative

SST

anomalies

Equatorial cold tongue is weaker than average or absent during El Niño, resulting in positive SST anomalies

Equatorial cold tongue is strongerthan average during La Niña, resulting in negative SST anomalies

Effects on fishery (upwelling affects phytoplankton concentration)

(Weak upwelling)

“El Niño”

WarmCold

Warm

Cold

Warm

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

WarmCold

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.

Thermocline

Global El Niño Impacts

Global La Niña Impacts

Mid-latitude impacts generally occur during the winter season (NH –DJF; SH- JJA).

Atlantic

Hurricanes

Specific ImpactsHistorical:• Disease outbreaks (1918-1919 influenza

pandemic /Spanish flu)?• Loss of civilizations, civil wars?

Current:• Damages from floods and landslides in Peru

and southern California; • Forest fires in Indonesia - serious air pollution

problems • Crop failures - famine from droughts in

southern Africa • Collapse of the Peruvian anchoveta fisheries

because of warmer coastal waters. • Low and high agricultural yields - price

fluctuations;• Water resources;• Energy demand - disruption to hydropower;

1997 southeast Asian haze

Nazca civilization: 100BC-800AD

Spring rainfall (Mar – May) in HK

30 June 1997

Stronger SW winds at 850 hPa (around 1000m) following El Nino onset

Cyclonic vortex near HK in strong El Nino summers

Wind anomalies following La Nina onset – anomalous anticyclone

Number of tropical cyclones within 500 km of Hong

Kong

Aug - Oct

Tropical Cyclone

Track

Density

Relationship between the mean Nino-3.4 SSTA and the mean no. of landfalling TCs during the late season for(a) area 2 – China; (b) area 3 –Indochina and (c) area 4 - The Philippines

La Nina years

El Nino years

Composite

circulation, late

season:

(i) Genesis

near Asia

in La Nina

(ii) Stronger

Pacific

ridge steers

typhoons

towards

land

Winter temperature in HK

La Nino, 2008 China snowstorm

The current situation - SST anomalies 25 Feb 2015 ( in degree C), NOAA

What Next?

Forecast of Sea Surface Temperature (SST) anomalies in Nino 3.4 International Research Institute for Climate and Society:

http://iri.columbia.edu/our-expertise/climate/forecasts/enso/current/

Climate Change and ENSO

• IPCC-AR4: “No consistent indication at this time of discernible changes in projected ENSO amplitude or frequency in the 21st

century.”

• A weak shift towards average background conditions which may be described as ‘El Niño-like’, with sea surface temperatures in the central and east equatorial Pacific warming more than those in the west.

• Weakened tropical circulations, shifting rainfall eastward.

• ENSO projections differ from model to model.

• The sign of the sensitivity of ENSO amplitude and frequency to increased greenhouse gases remains highly uncertain.

• Continued ENSO variability in the future even with changes to the background state

ENSO linksIRI: http://iri.columbia.edu/climate/ENSO/background/basics.htmlNOAA: http://www.pmel.noaa.gov/tao/elnino/faq.htmlhttp://www.oar.noaa.gov/k12/html/elnino2.htmlhttp://www.cpc.noaa.gov/products/precip/CWlink/MJO/enso.shtmlhttp://www.pmel.noaa.gov/tao/elnino/nino-home.htmlhttp://www.cdc.noaa.gov/map/clim/sst_olr/el_nino_anim.shtmlhttp://faculty.washington.edu/kessler/occasionally-asked-questions.html#q1Australian Government, Bureau of Meteorology: http://www.bom.gov.au/climate/enso/http://www.bom.gov.au/climate/current/soi2.shtmlhttp://www.bom.gov.au/climate/enso/australia_detail.shtmlhttp://www.longpaddock.qld.gov.au/Chinahttp://bcc.cma.gov.cn/en/International Research Institute for Climate and Society:http://iri.columbia.edu/climate/ENSO/index.htmlMet Office, UK:http://www.metoffice.gov.uk/research/hadleycentre/obsdata/climateindicators.htmlhttp://www.metoffice.gov.uk/research/seasonal/elnino/index.htmlhttp://www.metoffice.gov.uk/research/seasonal/regional/index.htmlHKO:http://www.hko.gov.hk/lrf/enso/enso-front.htmhttp://www.weather.gov.hk/education/edu01met/wxphe/article/46-nclau.pdf