Central America: A conduit for

1

• Central America A conduit for
• Atlantic-to-Pacific influences
• Shang-Ping Xie
• Timmermann, Y. Okumura,
• S. de Szoeke, H. Xu, J. Small, T. Miyama, Y.
  Wang
• International Pacific Research Center, University
  of Hawaii
• UCAR FRCGC, JAMSTEC, Japan

• Water hosing experiments (GCMs)
• Regional model results
• AR4 CMIP
• Orographic effects

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Pan-Atlantic Pattern SST wind
NAO ? subtropical wind
Xie Tanimoto (1998, GRL)
WES feedback
Tropical meridional mode ? Shift in ITCZ
A model for interpreting paleo variability?
(Chiang 2004, Hadley circulation book)
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Close connection between the subpolar and
tropical North Atlantic
Cariaco Basin north of South America
Wet tropical South America
Warm Greenland
Peterson et al. (2000, Science)
Chiang (2004, Hadley circulation book)
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Annual mean response (Yr 81-100)
A. Timmermann, Y Okumura, et al. 2006
SST wind stress
GFDL_CM2.1
NCAR CCSM2
ECHAM5/MPI-OM
HadCM3

• Meridional (WES) mode in ATL
• Cross-Central American winds
• Reduced NS asymmetry in EP

5
Use Regional Models to resolve the narrow and
mountainous Central America
January Wind (QSCAT)
h (km)
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IPRC Regional Ocean-Atmosphere Model (iROAM)
on Earth Simulator
Atmosphere IPRC-RAM 0.50.5, L 28
GFDL Modular Ocean Model 2 0.50.5, L 35
Prescribed OISST Land surface model
Ocean forced by NCEP reanalysis
Interactive
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Annual-mean climatology SST, precipitation
surface wind
iROAM
Obs TRMM, OISST QSCAT
25
25
Northward-displaced ITCZ and equatorial cold
tongue
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ITCZs meridional migration
Precip (gray shade), surface wind SST
(contours) in 125-95W
TRMM obs
IROAM
20N
10N
EQ
10S
20N
Jan
Mar
May
Jul
Sep
Nov
Jan
Mar
May
Jul
Sep
Nov
Double ITCZ for a brief period of Mar-Apr
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Mesoscale Features
February Gap wind jets
lt 40 m
Sfc wind, Z20(m) orography
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Response to a 2oC cooling in the North Atlantic

• Large equatorial cooling in Jan-Apr ? Reduced
  annual cycle
• Bjerknes feedback, instead of WES, is triggered.

Implications for water-hosing experiments of
relevance to Younger Dryas
Xie et al. (2007, JC, in press)
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-2oC
-2oC
Dec
Jan

• Seasonality
• Cooling begins in the Gulf of Panama in response
  to an intensified Panama jet.
• Panama cooling coincides with the seasonal
  shoaling of the thermocline there.
• The cooling intensifies and extends to the
  equator along the thermocline ridge.

-2oC
Feb
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Response to a 2oC cooling in the North Atlantic
SST Wind difference
-2oC
-2oC
January - March
July - September
Atmospheric adjustment mostly through the Panama
gap. The thermocline ridge in the Gulf of Panama
helps spread the SST cooling to the equator,
triggering the Bjerknes feedback.
Wind anomalies are larger, but with a smaller SST
response. ? Deepened thermocline in the Gulf of
Panama.
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Seasonality of the Pacific Response 1 Mean state

• Why is the coupled response favor the warm than
  the cold season?
• Mean northeasterlies in Feb ? increased wind
  speed in response to Atl cooling ? increased
  evaporation (wind speed dry advection)
  mixing.
• Shoaling thermocline in Feb (by gt 20 m)
• ? Stronger air-sea coupling in Feb than in
  July.

Sfc wind, Z20(m) orography
February
August
gt 60 m
lt 40 m
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du q at 100W, Eq
Seasonality of the Pacific Response (2)
Wind, Pressure Precip (July)
Pressure
850 mb

• During the cool season (Jun-Oct), strong
  easterlies on the equator at 850 mb, but not at
  the surface ? seasonal SST cooling and stable
  ABL?
• Anomalies patterns similar to the observed
  mid-summer drought suppressed convection
  intensified cross-Central American wind (Small et
  al. 2006). cf. Rowans talk

1000 mb
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Feb

• Factors for strong equatorial response during the
  cool season (Jan-May)
• Pros
• Mean NE wind
• Shoaling thermocline
• Strong vertical coupling in wind
• Con
• Stronger atmospheric response in boreal summer

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• Common among GCMs
• Reduced annual cycle
• cooling north/on the equator during Jan-May
• LBM-POP (Axel et al.)
• FOAM (L. Wu et al.)

iROAM
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Why does the meridional mode develop subsequently
in some models? ? Prolonged southern/double ITCZ
in some GCMs ? stronger WES feedback involving
convective heating
Equator
Wind-Evaporation-SST (WES) Feedback
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Intercomparison of AR4 models cf. Mechoso et al.
(1995), Devay et al. (2002)
by Simon de Szoeke

• Meridional asymmetry
• Role of the eastern Pacific warm pool

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latitude
EP climate in AR4 models SST, rain eq wind
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NE trades over the EP warm pool vs. meridional
asymmetry
GCM c
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NE wind off Central America vs. Meridional
asymmetry
r-0.87
4
)
2
-1
b
0
(m s
e
1
0
f
2
6
9
a
EQ
4
3
8
5
-2
FMA v
c
-4
d
7
0
2
4
6
8
m
DJF CA wind speed (
s-1)
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Westward and disconnected equatorial cold tongue
bias
SST
GCM 5
Obs
Latitude
West Longitude
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Niño 12 meridional wind cools SST
Nino 12 80-90 W, 0-10S
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Orographic Effects
by Haiming Xu, Justin Small

• EP warm pool convection
• Moisture transport from the Atlantic to Pacific
• Basin-scale climate

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Winter SST wind
ITCZ displaced on the south edge of EP warm pool
TMI SST precip QuikSCAT wind (Jan-Feb)
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Cross-Central American moisture
transport Hypothesized to hold the key to the
preference for AMOC
Mexico (lt27N) Panama (Jan-Feb 02)
0.25o orog
T42 orog
Xu et al. (2005, JC)
0.323
0.309
January Wind (QSCAT)
h (km)
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Summary

• Tropical response to a MOC shutdown
• Robust response in the tropical Atlantic the WES
  feedback and a meridional dipole
• Weakened meridional asymmetry in the eastern
  Pacific in some models but the zonal mode
  triggered in other
• The equatorial annual cycle weakens, and cooling
  north/on the equator during Jan-May. Response
  during May-Dec varies among models.
• AR4 CMIP CGCM run without mountains NE wind
  through Panama during Dec-Feb influences the
  subsequent development of meridional asymmetry
  (ITCZ displacement)
• Tropical Pacific response may be sensitive to the
  treatment of narrow and mountainous Central
  America.
• How can paleo data help constrain models?

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Upon coupling, the model SST tracks observations
closely. ? Air-sea feedback
Nino3 SST
MOM2
OISST
iROAM
Ocean spin-up
Coupled
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Role of internal air-sea feedback
Suppressed cloud radiative effect south of the
equator
ITCZ moves back and forth across the equator
Double ITCZ
20N
10N
EQ
10S
20N
Jan
Mar
May
Jul
Sep
Nov
Precip (shade), SST sfc wind
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q (g/kg)
0.25o orog
Wind speed (m/s)
Panama
Mexico
T42 orog