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.

Renno et al. (94) Emanuel (91) convection Chou et al. (91), Chou (92) radiation No clouds; crude PBL Surface winds fixed free troposphere T imposed SST imposed PBL T calculated (above PBL) Single column model under the weak temperature gradient approximation (Sobel & Bretherton 2000, Zeng & Neelin 1999, Neelin & Held 1987…)

Sobel and Bretherton 2000 Precipitation in WTG simulations with Emanuel’s model. Dots are from observed January climatology of SST and Precip over oceans 20S-20N.

low SSThigh SST z stable, no rain unstable, rain free tropospheric T(z) same in both places CAPE>0 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 CAPE<0

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

westeast Central & E. Pacific Free tropo PBL “remote tropics” (atmospheric column) This and several following slides adapted from John Chiang (work from Chiang & Sobel, 2002) This suggests a single-column modeling strategy to understand tropical ENSO teleconnections

free troposphere T imposed slab ‘ocean’ SST calculated PBL T calculated (above PBL) still fixed (though now time-dep) tropospheric temperature, but now over an interactive slab ocean mixed layer.

79-99 observed tropical tropospheric temperature perturbation X FORCING Sea surface temperature precipitation OUTPUT In this case, we force the SCM with time-varying tropospheric temperature, representing that imposed on remote regions by SST in the Pacific.

T’ Planetary boundary (subcloud) layer Free troposphere ocean SST’ Convection Surface fluxes  e ’ ( equivalent potential temp) 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 the “upped ante” (Neelin et al. 2003)

Model precip Forcing mld=1m mld=40m mld=160m 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).

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).

Sahel Regional modeling of Sahel Drought Herceg, Sobel, and Sun. Clim. Dyn., in press.

The Sahel drought of the late 20 th 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 RCM Obs 1950s 1980s difference 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 1950s + or Regional model ECHAM 80s-50s  T NCEP/NCAR 80s-50s  T

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.