1. Systems • Open System: Open System: Energy and Matter can be exchanged between systems • Closed System: Closed System: Exchange of Matter greatly restricted, but may allow exchange of energy • Isolated System: Isolated System: No Energy or Matter can be transferred in or out of the system • Stable System: Stable System: resists change and reverts back to this state when disturbed • Unstable System: Unstable System: Once disturbed the system cannot return to the original state • Metastable Metastable System: System: Can have several stable states.
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1. Systems
•• Open System:Open System: Energy and Matter can be exchanged
between systems
•• Closed System:Closed System: Exchange of Matter greatly
restricted, but may allow exchange of energy
•• Isolated System:Isolated System: No Energy or Matter can be
transferred in or out of the systemtransferred in or out of the system
•• Stable System:Stable System: resists change and reverts back to
this state when disturbed
•• Unstable System:Unstable System: Once disturbed the system
cannot return to the original state
•• MetastableMetastable System:System: Can have several stable states.
2. Feedback
• Processes in one system influences processes in
another interconnected system by exchange of
matter and energy. The exchange is called feedback.
•• Positive Feedback:Positive Feedback: Change in one system causes
similar change in the other system. Can cause
runaway instabilityrunaway instability
•• Negative FeedbackNegative Feedback means a positive change in one
system causes a negative change in the other
Changing CO2 induces positive water vapor feedback
Changing CO2 induces positive albedo feedback
3. Low frequency climate variability:
sub-seasonal variation, seasonal variation,
annual variation, and interannual variation.
4. Walker circulation
The Walker Circulation refers to an east-west circulation of the atmosphere above
the tropical ocean in the zonal and vertical directions, with air rising above
warmer ocean regions (normally in the west), and descending over the cooler
ocean areas (normally in the east). Its strength fluctuates with the change in sea
surface temperature.
5. El Niño and La Niña
El Niño is characterized by
unusually warm ocean
temperatures in the
Equatorial Pacific, as
opposed to La Niña, which
characterized by unusually
cold ocean temperatures in
the Equatorial Pacific. El
La Niña condition
the Equatorial Pacific. El
Niño is an oscillation of the
ocean-atmosphere system in
the tropical Pacific that is
closely related to the change
in the Walker circulation and
has important consequences
for weather and climate
around the globe.
El Niño condition
TahitiDarwin
6. Southern Oscillation
1958-1998
The Southern Oscillation is the atmospheric
component of El Niño/ La Nina. This component is an
oscillation in surface air pressure between the tropical
eastern Pacific and the western Pacific Ocean waters.
El Niño/La Niña-Southern Oscillation (ENSO)
Teleconnections via atmospheric Rossby waves
7. Impact of ENSO on Global Climate
8. ENSO and hurricane
• Less hurricane days during El nino years
mainly due to stronger vertical wind shear
• More hurricane days during La nina years
mainly due to weaker vertical wind shear.
9. Pacific Decadal Oscillation (PDO)
PDO is a long-lived ENSO-like pattern of Pacific climate variability
usually persisting for a long time period about 20-to-30 years.
ENSO and PDO are not the independent anomalies
but are somehow linked phenomena.
10. Some extreme climate anomalies
(a) A decade of western North American drought
could be related to both human activities and natural
climate anomalies, such as ENSO.
(b) A possible cause for the 2003 European heat wave is
the polarwaord migration of polar jets in a warm climate.
(c) The vanishing snow of Kilimanjaro may be due to the (c) The vanishing snow of Kilimanjaro may be due to the
fact that the maximum warming occurs in the mid
troposphere over the Equator.
11. Challenges of numerical simulation of climate
� Insufficient observations – leading to
inaccurate initial conditions;
� Chaotic nature of the atmospheric and
oceanic system;
� Inherent deficiency of numerical models
with limited resolution that fails to resolve with limited resolution that fails to resolve
sub-grid physical processes.
�Data assimilation;
�Ensemble forecast;
�Parameterization.
Our answers to face the challenges:
12. Cloud radiative effect
Cooling effect: reflecting solar radiation
Warming effect: absorbing and emitting longwave radiation
Shortwave cloud forcing:
-50 W/m2 (cooling)
Longwave cloud forcing:
30 W/m2 (warming)
Net cloud forcing ∆CRF: -20 W/m2 (cooling)
Current climate:
13. Cloud-climate feedback
feedback cloud negative 0 CRF
feedback cloud zero 0 CRF
feedback cloud positive 0 CRF
→<∆
→=∆
→>∆
The impact of clouds on global warming depends on how
the net cloud forcing changes as climate changes.
14. Cloud radiative effects depend on height.
gT
cT
cg TT ≈
cT aT
ac TT <<
Low cloud High cloud
SW cloud forcing dominates,
cooling effect
LW cloud forcing dominates,
warming effect
15. In general circulation models (GCMs), clouds
are the sub-grid scale processes and are not
resolved. They are represented parametrically in
models. The cloud-climate feedback is one of the
largest uncertainties in climate simulations.
16. Cloud formation
Two processes, acting together or individually, can lead to
air becoming saturated: cooling the air or adding water
vapor to the air. But without cloud nuclei, clouds would not
form.
17. Precipitation
Cloud droplets need to grow up to a certain size in order to
fall to the surface due to gravity
18. Aerosol feedback
Direct aerosol effect: scattering, reflecting, and absorbing