Trends in Tropical Water Vapor (1980-2004):Satellite and GCM Comparison

Satellite Observed ----Model Simulated __

Held and Soden 2006: Robust Responses of the hydrological cycle to global warming, J Climate, 19, 3354-3360.

Total column water vapor over tropical oceans has increased over the past 20 years

Models accurately reproduce the observed variability and moistening of the tropical atmosphere.

Impact of Convectively Detrained Cloud Ice On Upper Tropospheric Water Vapor

Satellite Observations (AIRS/MLS) GCM Simulations (IPCC AR4)

John and Soden, 2006: Does convectively-detrained cloud ice enhance water vapor feedback?, Geophys. Res Lett., 33, L20701.

Spatial distribution of upper tropospheric water vapor is strongly correlated with amount of cloud ice

Models capture observed correlation between cirrus ice and upper tropospheric water vapor.

Water vapor feedback is insensitive to strength of relationship between cloud ice and water vapor

A Comparison of AIRS Retrievals with IPCC AR4 GCM Simulations

John and Soden, 2007: Temperature and humidity biases in global climate models andtheir impact on climate feedbacks, submitted GRL.

On average:

• Models are too cold in the free troposphere, especially in the NH.

• Models specific humidity is too low in the boundary layer and too high in the free troposphere.

• Models have a moist bias in RH, (the are also issues with the AIRS retrievals).

• Although these biases in the mean climate are large, they are shown to have no direct impact on the magnitude of the water vapor and temperature feedbacks in the models.

Weakening of the Walker Circulation due to Anthropogenically-Forced Changes in the Hydrological Cycle

Vecchi et al. 2006: Weakening of the tropical Pacific atmospheric circulation ... Nature, 441, 73-76.

Change in Sea Level Pressure (1861-1992)

Observed Model Simulated

Climate models predict a weakening of the Walker circulation in response to hydrological cycle changes in a warmer climate.

Observations from 20th Century reveal changes in SLP consistent with model projected weakening of Walker Cell.