Climate Modeling MEA 719 Lecture Set 10 African

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Climate Modeling MEA 719Lecture Set 10
African European Climate Variability
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KEY QUESTIONS

• What are the main characteristics of the African
  Climate mean conditions?
• What are the primary modes of variability of the
  African Climate System?
• How well do current dynamical models simulate
  African Climate?
• Which deficiencies do dynamical models have that
  account for inadequacies in the simulation of
  African Climate?

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KEY QUESTIONS

• What are the main characteristics of the European
  Climate mean conditions?
• What are the primary modes of variability of the
  European Climate System?

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Rainfall Annual Cycle (NCAR-CCM3 model)
JAN
JUL
APR
OCT
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The mean conditions of the African climate
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Role of Africa in the General Circulation

• The 2nd/3rd largest heat source and has
  significant influence on global teleconnections
• Source of trans-Atlantic tropical cyclones
• Influencing on adjacent Ocean circulation
• Latent energy released is transported polewards
  through mechanisms involving tropical-extratropica
  l interactions (Lorenz, 1955 1967 Dutton and
  Johnson, 1967)
• Equatorial Rossby waves from Africa capable of
  propagating vast distances interacting with
  remote regions in the tropics and extratropics
  example of equatorially forced Rossby wave
  trains

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Significance of the Tropical Heat Source

• Significance of the African heat sources may be
  investigated by replacing the tropical rain
  forest with reduced vegetation

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Influence of the Tropical Heat Source

• Rainfall response to forced trapped Rossby wave
  train (Semazzi Song, 2001)

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Equatorial Rossby waves from Africa capable of
propagating vast distances interacting with
remote regions in the tropics and extratropics
example equatorially forced Rossby wave train
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Role of Tropical Wave Systems in Modulating
Intraseasonal Climate Variability over Africa
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Westerly Kelvin Waves African Easterly Waves
Makonnen et al 2007 ( opportunity for improving
intra-seasonal prediction)
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Westerly Kelvin Waves African Easterly Waves
Makonnen et al 2007 ( opportunity for improving
intra-seasonal prediction)
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Westerly Kelvin Waves African Easterly Waves
Makonnen et al 2007 ( opportunity for improving
intra-seasonal prediction)
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Primary modes of variability of the African
Climate System

• El-Nino SO (ENSO)
• TAV (Tropical Atlantic Variability)
• IOV (Indian Ocean Variability)
• NAO (North Atlantic Oscillation)

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ENSO DIPOLE PATTERN
ENSO DIPOLE PATTERN
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Western Africa Eastern Africa Southern Africa
82/83
82/83
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INDIANOCEANDIPOLE
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Eastern Africa (1962-not ENSO But IOD)
82/83
82/83
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Construction of E0F Timeseries
Correlation Matrix
Data
data map at tkth Column
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E0Fi , ampp Var?p
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Stong SST-Rainfall relationship up to 3 months
time lag
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Semazzi Sud (1986)
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Atlantic SST EOFs (regular)
Largest loading Over Northern tropical Atlantic
Largest loading Over Southern tropical Atlantic
ENSO signal (largest loading over the Pacific)
Semazzi Sud (1986)
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Western Africa Eastern Africa Southern Africa
82/83
82/83
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High Phase
NAO Index is based on Lisbon minus
Stykkisholmur SLP
and drier conditions over N. Africa
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Locations of Stykkisholmur Lisbon
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Location of Stykkisholmur in Iceland
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Location of Lisbon in Portugal
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Low Phase
and wetter conditions over N. Africa
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Performance of dynamical models in simulating the
African Climate

• GCM Modeling
• Regional Climate Modeling
• Watershed-scale Climate Modeling

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GCM SIMULATION OF 97/98 ANOMALIES (Latif et al,
1998)
GCM
Observations
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IRI RCM-DOMAIN(Liqiang Sun, personal
communication)
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OND ACTUAL RAINFALL (MM/DAY) 1970-95 AVERAGE

• PERFORMANCE IN AN SIMULATING CLIMATOLOGY (IRI)
• 1970-1995 AVERAGE
• Observations
• ENCHAM GCM
• RCM-Low Resolution Model
• RCM-High Resolution Model

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Summary

• The climatology of the African climate is
  primarily dictated by the passage of the ITCZ
  mainly determined by the overhead position of the
  sun. As a result there are two rain seasons in
  the equatorial regions and one season further
  polar ward.
• The rainfall ranges from up to about 16mm over
  the tropical rain forest in central Africa to
  nearly zero over the desert regions
• The extreme wet conditions over the tropical rain
  forests are associated with the rising branch of
  the Walker circulation over Africa. The extreme
  dry conditions over the Sahara and Kalahari
  deserts are associated with the sinking motion
  associated with the Hadley circulation.
• The behaviour of the ITCZ is modified by
  secondary factors including, land-ocean
  contrasts, vegetation contrasts, and asymmetry
  about the equator plays an important role in
  modulating the General Circulation through
• - latent heat production
• - generation of easterly waves responsible for
  trans-Atlantic tropical cyclones
• - influencing on adjacent Ocean circulation
• - export of momentum to higher latitudes
• - generation of Rossby waves capable of
  propagating vast distances interacting with
  remote regions in the tropics and extratropics
• ENSO, TAV, NAO, and IODP all play significant
  role in modulating interannual variability
• In Europe NAO is the primary source of
  interannual variability but also affects the
  northern rim of Africa

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Summary

• NAO Index is based on Lisbon minus Stykkisholmur
  SLP
• NAO varies on decadal interannual timescales
• NAO-High Index
• - High SLP anomalies over North Tropical
  Atlantic
• - Low SLP anomalies over Iceland
• - Enhanced easterlies over North Atlantic
• - Milder (warmer) Wetter over Europe
• - Drier over Mediterranean North Africa

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Summary

• NAO-Low Index
• - Relatively lower SLP anomalies over North
  Tropical Atlantic
• - Relatively higher SLP anomalies over Iceland
• - weaker easterlies over North Atlantic
• - Colder drier over Europe
• - Wetter over Mediterranean North Africa

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Summary

• How well do GCM and regional models capture
  African climate?
• Results are presently mixed - models not entirely
  satisfactory over Africa
• Annual cycle reasonably portrayed
• Higher harmonics more deficient than first
  harmonic
• Interannual variability is reasonably simulated
  but coarse resolution of GCMs is a major
  constraint
• Added value of regional models has been
  demonstrated further work needed to investigate
  failure modes regional dependency
• Future improvement are expected from use of
  variable resolution AGCMs OGCMs
• How much can one really infer from model results?
• Mechanistic experiments are useful for
  determining relative role of forcing
• There is evidence of skillful GCM prediction at
  the seasonal timescales