Climate Change and Society

1. Lecture 8
El Nino/La Nina, and their connection with
Hong Kong’s climate
LSGI1B02 Climate Change and Society
LEUNG Wing-mo

2. Climate variability
Variability 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.

3. Climate Variability and Climate Change
Normals
Climate Change
Climate Oscillations
Climate Variability
Normals
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)

4. Drought in Queensland,
Australia, 2014
https://www.youtube.com/watch?v=dzat1
6LMtQk
Understanding El Nino, Bureau of
Meteorology, Australia

5. El Nino
(Boy Child):
Sea Surface
temperature
(SST) changes
in the Pacific
Jan
Dec
Equatorial “cold
tongue”

6. Major ocean currents -
result of winds, continental land masses, and Coriolis effect
Gulf
stream
Calif.
current
Kuroshio
current

7. Walker Circulation

8. 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

9. Relationship between El Nino (SST) and Southern
Oscillation (pressure)
El Nino,
SST
changes
Southern
Oscillation,
surface
pressure
changes

10. 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)

11. 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.

12. 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 stronger
than average
during La
Niña,
resulting in
negative SST
anomalies

13. Effects on fishery (upwelling affects phytoplankton
concentration)
(Weak
upwelling)

14. “El Niño”
Warm
Cold
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.

15. “La Niña”
ColdWarm
Warm
Cold
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.

16. Thermocline

21. Global El Niño Impacts

22. Global La Niña Impacts
Mid-latitude impacts
generally occur during
the winter season (NH –
DJF; SH- JJA).

23. Atlantic
Hurricanes

24. Specific Impacts
Historical:
• 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

25. Spring rainfall (Mar – May) in HK

26. 30 June 1997

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

28. Cyclonic vortex near HK in strong El Nino summers

29. Wind anomalies following La Nina onset – anomalous anticyclone

30. Number of tropical cyclones within 500 km of Hong
Kong
Aug - Oct

31. Tropical Cyclone
Track
Density

32. 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

33. Composite
circulation, late
season:
(i) Genesis
near Asia
in La Nina
(ii) Stronger
Pacific
ridge steers
typhoons
towards
land

34. Winter temperature in HK

35. La Nino, 2008 China snowstorm

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

37. 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/

38. 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

39. ENSO links
IRI: http://iri.columbia.edu/climate/ENSO/background/basics.html
NOAA:
http://www.pmel.noaa.gov/tao/elnino/faq.html
http://www.oar.noaa.gov/k12/html/elnino2.html
http://www.cpc.noaa.gov/products/precip/CWlink/MJO/enso.shtml
http://www.pmel.noaa.gov/tao/elnino/nino-home.html
http://www.cdc.noaa.gov/map/clim/sst_olr/el_nino_anim.shtml
http://faculty.washington.edu/kessler/occasionally-asked-questions.html#q1
Australian Government, Bureau of Meteorology:
http://www.bom.gov.au/climate/enso/
http://www.bom.gov.au/climate/current/soi2.shtml
http://www.bom.gov.au/climate/enso/australia_detail.shtml
http://www.longpaddock.qld.gov.au/
China
http://bcc.cma.gov.cn/en/
International Research Institute for Climate and Society:
http://iri.columbia.edu/climate/ENSO/index.html
Met Office, UK:
http://www.metoffice.gov.uk/research/hadleycentre/obsdata/climateindicators.html
http://www.metoffice.gov.uk/research/seasonal/elnino/index.html
http://www.metoffice.gov.uk/research/seasonal/regional/index.html
HKO:
http://www.hko.gov.hk/lrf/enso/enso-front.htm
http://www.weather.gov.hk/education/edu01met/wxphe/article/46-nclau.pdf