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Lecture 08
Short-term Climate Variability
短期氣候變異
Climate: 30 years interval （WMO: 1951-1980）
Climate variability: fluctuations within 30 years
Climate change: differences between intervals
Time scales:
Climate change: a hundred- to a thousand year
Climate variability: interannual to decadal scales
UNFCCC makes a distinction between
“climate change” attributable to human
activities altering the atmospheric
composition,
and
“climate variability” attributable to natural
causes (both internal and external).
United Nations Framework Convention on Climate Change
Natural
Climate
Variability
An act of God
Both Internal and external
Internal climate processes and feedbacks may
cause variations in the radiative balance by their
impact on the reflected solar radiation or emitted
infrared radiation, but such variations are not
considered part of radiative forcing.
External forcings, such as the solar radiation or
the large amounts of aerosols ejected by volcanic
eruption into the atmosphere, may vary on widely
different time-scales, causing natural variations in the
radiative forcing.
El Nino(聖嬰) and La Nina(辣妞)
What is El Niño?
A natural mode of the coupled oceanatmosphere system
ENSO: EN and SO together:
Refers to whole cycle of warming and cooling.
ENSO events have been going on for centuries
(records in corals, and in ice layers in glaciers
in South America)
ENSO arises from air-sea interactions in the
tropical Pacific
ENSO is a natural phenomenon. It simply
just happens.
Normally, the trade winds and equatorial currents flow toward the west. At
the same time, an intense Peruvian current causes upwelling of cold water
along the west coast of South America.
The Walker Cells
The East-West SST gradient in ocean is tied to the pressure
fluctuation in air.
Annual march of Sea Surface Temperature
Originally, the term
“El Nino” was used to
describe the episodic
appearance of warm
water by the end of
year (Christmas) in the
far east tropical Pacific.
Why studying El
Nino ?
Increased SSTs cause
Enhanced convection
Upper troposphere:
outflow, divergence
Anticyclonic pair
Wave-train
Tele-connection Rossby
wave train (Trenberth et al. 1998)
Storm tracks:
southward, enhanced
northward, diminished
Typical Jan-March weather anomalies and
atmospheric circulation during El Nino and La Nina.
Each El Nino
(or La Nina)
event is
unique.
El Nino and La Nina or the El-Nino Southern Oscillation (ENSO) is a
quasi-periodic climate pattern occurring across the Pacific Ocean. It is
a pattern that affects weather around the world. In this pattern there are
two oscillating phases: El Nino, which is characterized by warmer
weather, and La Nina, which is characterized by cooler weather. ENSO
generally occurs alternatively every 3 to 5 years, however it has
historically varied at the outskirts 2 to 7 years.
Various Nino indices
El Nino conditions (warm
ENSO phase)
Weakened annual cycle
La Nina conditions (cold ENSO
phase)
Enhanced annual cycle
Bjerknes Feedback loop
El Nino
trade winds
weaken
Eastern Pacific
warms
upwelling
slows
La Nina
trade winds
strengthen
Eastern Pacific
cools
upwelling
intensifies
El Niño theories
• Previous
El Niño sows the seeds for the next
• Leads to quasi-cyclic behavior
• Bit like a pendulum
El Niños occur 31% of time, La Niñas 23%
• Ocean is “primed” for an event to occur
• But the trigger may be “random weather”
• “Delayed Oscillator”
Ocean waves are a key:
Rossby (off equatorial, westward traveling)
Kelvin (equatorial, eastward traveling)
Waves reflect off boundaries and return altered
“random weather”
as a triggering
mechanism
Q’L>0
Q’SW<0
Warm SST’
Cooling SST tendency
West
Q’L<0
Warming SST tendency
East
Westerly winds
Subsurface ocean temperature change along
equator during the development of 1997-98 El Niño
1996. 12
1997. 8
1997. 4
1997. 12
There is an embedded precursor in the equatorial
western Pacific warm pool one year ago !
Is global warming contributing to changes in El
Niño? Likely, yes, to some extent.
But, which part is natural/anthropogenic
variability?
How will El Niño events change with global
warming?
El Niño involves a build up and depletion of heat as well as major
redistribution of heat in the ocean and the atmosphere during the
course of events.
• Because GHGs trap heat, they interfere.
• Possibly expand the Pacific Warm Pool.
• Enhance rate of recharge of heat losses.
• More warming at surface: enhanced thermocline  enhanced swings
More frequent El Niños?
• Some models more El Niño-like with increased GHGs.
• But models do not simulate El Niño well
• Nor do they agree
The hydrological cycle is expected to speed up with increased GHGs.
Increased evaporation enhances the moisture content of the
atmosphere which makes more moisture available for rainfall.
ENSO-related droughts are apt to be more severe and last longer, while
floods are likely to be enhanced.