Diapositiva 1

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Clouds and circulation over the eastern South
Pacific René D. Garreaud Department of
Geophysics Universidad de Chile www.dgf.uchile.
cl/rene
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• Clouds and circulation over the eastern South
  Pacific
• Outline
• Climate context Mean features and annual cycle
• Sub-monthly scale variability
• Interannual variability

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1. Climate 1/16 2. Sub-monthly 3. Interannual
Key atmospheric features over the SEP
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1. Climate 2/16 2. Sub-monthly 3. Interannual
?(674 hPa)
?(887 hPa)
Full
Topo
Full Continental monsoon Hadley cell
Mountains Topo Hadley cell Mountains
The similarities between upper and lower panels
imply an important role for the interaction
between the zonal mean flow and the topography in
sustaining the subtropical anticyclones. The
differences emphasize the importance of the
zonally asymmetric heating. (Rodwell and Hoskins
2001)
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1. Climate 3/16 2. Sub-monthly 3. Interannual
(Rodwell and Hoskins 2001)
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1. Climate 4/16 2. Sub-monthly 3. Interannual
?
v
Ideal monsoon heating _at_ 25S-90E
Continental drag
Newtonian cooling
Ideal. mountain
(Rodwell and Hoskins 2001)
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1. Climate 5/16 2. Sub-monthly 3. Interannual
LTM annual mean SLP and surface wind
speed (NCEP-NCAR Reanalysis)
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1. Climate 6/16 2. Sub-monthly 3. Interannual
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1. Climate 7/16 2. Sub-monthly 3. Interannual
A more detailed, realistic description of the
coastal jet can be derived from QuikScat data,
1997-2003, 0.25º
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1. Climate 8/16 2. Sub-monthly 3. Interannual
Surface wind variability (SONDJF)
Diurnal amplitude
Standard deviation
Max. ?, but ?/mean 0.4
Min. PM-AM
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1. Climate 10/16 2. Sub-monthly 3. Interannual
Simulated (MM5) structure of the coastal jet
Garreaud and Muñoz 2005
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1. Climate 11/16 2. Sub-monthly 3. Interannual
SCu Low level, shallow cloud layer at the top of
the MBL
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1. Climate 12/16 2. Sub-monthly 3. Interannual
Net Cloud Radiative Forcing ?R ?LW ?SH,
where ?? ?clear - ? average For SCu
clouds ?LW 0 as Tc SST ?SH effect) ?R SEP SCu -50 W/m2
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1. Climate 13/16 2. Sub-monthly 3. Interannual
Understanding / modeling of SCu remains elusive
as its maintenance depends on microscale processes
Well mixed Coupled MBL
Not-well mixed Uncoupled MBL
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1. Climate 14/16 2. Sub-monthly 3. Interannual
Annual cycle of SCu an inter-regional differences
largely explained by low level stability ?700-
?sfc (Klein and Hartmann 1993)
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1. Climate 15/16 2. Sub-monthly 3. Interannual
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1. Climate 16/16 2. Sub-monthly 3. Interannual
Y. Wang et al. 2004
Cooling effect of the SCu deck over the SEP
further enhance the subsidence and hence the
southerly flow
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1. Climate 2. Sub-monthly 1/8 3. Interannual

• Jet-structure in mean field, but how often a jet
  occurs?
• Cluster analysis using ws individual fields
• Similarity measured by spatial correlation
• Ward method
• Two best separated clusters

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1. Climate 2. Sub-monthly 2/8 3. Interannual

• V 8 m/s off central Chile almost alwayd
  associated with a southerly jet (dark shaded)
• Jet events typically a week long (3-15 days)
• More frequent, stronger and longer in summer.

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1. Climate 2. Sub-monthly 3/8 3. Interannual
1-Point correlation map
V(33S/73W) against U,V elsewhere (vectors) WS
elsewhere (contour) Cloud elsewhere (colors) Jet
events associated with Stronger
anticyclone Anticyclone displaced south Reduced
Sc near the coast Increased Sc off the coast
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1. Climate 2. Sub-monthly 4/8 3. Interannual
Jet under clear skies evident in coastal data as
well
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1. Climate 2. Sub-monthly 5/8 3. Interannual
LTS-cloud frequency over SEP also holds for
submonthly variability
San Felix Island (28S-80W) data 2003
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1. Climate 2. Sub-monthly 3. Interannual
Sub-monthly variability
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1. Climate 2. Sub-monthly 6/8 3. Interannual
Steady-state Dynamics
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1. Climate 2. Sub-monthly 7/8 3. Interannual
Steady-state dynamic seems to hold for day-to-day
changes
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1. Climate 2. Sub-monthly 8/8 3. Interannual
Local R2 vsfc d?900/dy
Thus, day-to-day changes in the meridional
pressure gradient (alongshore) explain more than
70 of the changes in meridional wind over the
Jet region Meridional pressure gradient is in
turn modulated by the passage of mid-latitude
disturbances Indirect effect on zonal
(cross-shore) pressure gradient
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1. Climate 2. Sub-monthly 3. Interannual 1/5
Surface wind speed at the core of the coastal jet
(33S-75W), exhibits substantial interannual
variability not associated with ENSO
Interannual variability in wind speed in this
region is still largely controlled by meridional
SLP gradient
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1. Climate 2. Sub-monthly 3. Interannual 2/5
Association between Wspd and SLP at interannual
scale is similar than at sub-monthly scale .
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1. Climate 2. Sub-monthly 3. Interannual 3/5
Association between Wspd and Cloud at interannual
scale is similar than at sub-monthly scale .
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1. Climate 2. Sub-monthly 3. Interannual 4/5
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1. Climate 2. Sub-monthly 3. Interannual 5/5
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Conclusions I Strong feedback among mean features
over the SEP and beyond
SAM
ITCZ
Enhanced equatorward flow ?v ? f?w/ ?z Enhanced
LS subsidence to keep ?T/ ?t ? 0
Formation and maintenance of the SCu deck (moist
air confined to the MBL)
Net cooling at the top of the SCu (Radiative
effect condensational heating)
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• Conclusions II
• Coastal jet exhibits significant modulation in
  the synoptics time-scale, associated with the
  passage of extratropical disturbances farther
  south that modify the meridional pressure
  gradient.
• Enhancement of the coastal jet leads to a local
  reduction of SCu but also impacts the SCu over
  open ocean downstream of the jet, in a way that
  is consistent with the LTS-SCu relationship that
  is observed in the annual cycle (probably because
  cold advection below and warm advection aloft)
• Interannual variations of the wind speed over
  the coastal jet region are still related with
  changes in the meridional pressure gradient.
• Internnual variations of the cloud coverage over
  the SEP are small, particularly in SON, and not
  explained by changes in the LTS.