Adjustment to tropospheric warming over
ocean and land surfaces
Adam Sobel, John Chiang, Deborah Herceg,
Liqiang Sun, Michela Biasutti
+ many discussions with Isaac Held, Alessandra Giannini,
Jian Lu, David Neelin…
Sahel Workshop, March 20, 2007
Free tropospheric temperature is homogeneous, compared to SST
and precipitation.
Single column model under the weak temperature
gradient approximation (Sobel & Bretherton 2000, Zeng &
Neelin 1999, Neelin & Held 1987…)
(above PBL)
free
troposphere
T imposed
PBL
T calculated
SST imposed
Renno et al. (94)
Emanuel (91) convection
Chou et al. (91), Chou (92) radiation
No clouds; crude PBL
Surface winds fixed
Precipitation in WTG simulations with Emanuel’s model. Dots are from
observed January climatology of SST and Precip over oceans 20S-20N.
Sobel and Bretherton 2000
Deep convection is controlled by stability of the sounding (e.g.,
Arakawa and Schubert 1974). The stability here is determined by
the free tropospheric temperature and the SST.
moist adiabat rising from sfc
stable, no rain
unstable, rain
z
free tropospheric T(z)
same in both places
low SST
high SST
Typical El Nino precip anomalies are negative throughout the tropics,
except the eastern/central Pacific. This suggests a global-scale explanation.
Ropelewski and Halpert 1987
Interannual tropospheric temperature anomalies are very homogeneous
within the tropics.
Yulaeva and Wallace 1994
This suggests a single-column modeling strategy to understand tropical
ENSO teleconnections
Free
tropo
PBL
Central & E. Pacific
west
east
“remote tropics”
(atmospheric column)
This and several following slides adapted from John Chiang
(work from Chiang & Sobel, 2002)
still fixed (though now time-dep) tropospheric temperature,
but now over an interactive slab ocean mixed layer.
(above PBL)
free
troposphere
T imposed
PBL
T calculated
slab ‘ocean’
SST calculated
In this case, we force the SCM with time-varying tropospheric temperature,
representing that imposed on remote regions by SST in the Pacific.
Sea surface temperature
79-99 observed tropical
tropospheric temperature
perturbation
FORCING
X
precipitation
OUTPUT
The ocean surface warming is impressed from above by the free troposphere,
via the convective adjustment of the PBL to that, and then surface fluxes
Free troposphere
Planetary boundary
(subcloud) layer
ocean
T’
the “upped ante”
(Neelin et al. 2003)
e’ (equivalent
potential temp)
SST’
Convection
Surface fluxes
The precipitation response results from a disequilibrium: the upper ocean
has not yet had a chance to adjust to the tropospheric warming. Adjustment
is slower for a deeper mixed layer. Precip response requires ocean “memory”
(unless TOA radiation changes).
Forcing
mld=1m
Model
precip
mld=40m
mld=160m
We view the tropospheric warming as being the ultimate driver of
Tropical ENSO teleconnections, since the sign of precip anomalies
is so spatially uniform.
However, a range of processes, incl. 3D ones (advection) make things
more complex than the single-column picture (many papers by Neelin
and colleagues)
How similar is global warming to ENSO? Not just temperature changes,
but also radiation does, in a partly temperature-independent way
In some models (GFDL) global warming looks somewhat ENSO
like in the Sahel. Not in others (NCAR).
Regional modeling of Sahel
Drought
Sahel
Herceg, Sobel, and Sun.
Clim. Dyn., in press.
The Sahel drought of the late 20th century is well simulated by GCMs run
in AMIP mode, which shows that it was a response to SST.
Held et al. 2005
Some of the Sahel signal appears to be induced by sulfate aerosols, which
induce a north-south SST gradient. Some may also be due to warming,
a la ENSO.
We take a GCM (ECHAM) run in AMIP mode, which gets the drought, and
use it to force a regional climate model (NCEP Regional Spectral Model).
GCM
1950s
1980s
difference
RCM
Obs
JAS Precipitation
Then, we take the boundary conditions from a wet year, but raise the
tropospheric temperature by a horizontally uniform vertical profile
(with appropriate structure). This represents global warming, or ENSO.
ECHAM
80s-50s T
ECHAM
1950s
+
or
NCEP/NCAR
80s-50s T
Regional
model
The Sahel dries in response to a uniform warming of the troposphere.
1980s
minus
1950’s
precip
T from GCM
T from NCEP reanalysis
These results show that at least some of the Sahel drought (or its
El Nino response) may be due to tropospheric stabilization by SST
increases elsewhere, as in our ocean ENSO SCM simulations.
However, we don’t understand the land-atmosphere interactions
very well yet.
The nature and duration of the surface “memory” (needed for
precip response) is the first question.