New Evidence Regarding Tropical Water Vapor Feedback, Lindzens Iris Effect, and the Missing Hotspot
New Evidence Regarding Tropical Water Vapor Feedback, Lindzen’s Iris Effect, and the Missing Hotspot
As part of a DOE grant we are testing climate models against satellite observations, particularly regarding the missing “hotspot” in the tropics, that is, the expected region of enhanced warming in the tropical mid- and upper troposphere as the surface warms. Since 1979 (the satellite period of record), it appears that warming in those upper layers has been almost non-existent, despite some surface warming and increases in low-level humidity. For years I have claimed that the missing hotspot could be evidence of neutral or even negative water vapor feedback, which would also help explain weaker than expected surface warming. Climate modelers are all but certain that water vapor feedback is positive. I have discussed elsewhere (e.g. here) how that might not be the case, even as lower atmospheric water vapor increases, and it’s related to how precipitation efficiency might change with warming leading to drying of the troposphere above the boundary layer. This is also part of Lindzen’s “Iris Effect”. While water vapor at the lowest altitudes over the ocean is strongly tied to surface temperature, free-tropospheric humidity is controlled by precipitation microphysics, and we little information on how that changes with warming. So, I’ll get right to the subject of this post. We have analyzed 11 years of water vapor channel (183.3 GHz) data from the AMSU-B instrument on the NOAA-18 satellite, and compared it to the mid-tropospheric temperature data from AMSU channel 5 (the “MT” channel). Specifically, we computed monthly gridpoint anomalies in all channels over the 11 year period, and regressed the 183.3 GHz brightness temperature (Tb) anomalies against the channel 5 Tb anomalies. This should give information on how much the free troposphere moistens or dries when it changes temperature. The following image shows the gridpoint regression coefficients for the monthly anomalies during June 2005 through May 2015: Fig. 1. Gridpoint regression coefficients between the NOAA-18 AMSU-b 183.3 GHz channels Tb and AMSU-A channel 5 Tb during June 2005 through May 2015. Ch. 18 is 183.3+/-1 GHz, generally peaking in the upper troposphere; ch. 19 is 183.3+/-3 GHz peaking in the upper-mid troposphere, and ch. 20 is 183.3 +/-7 GHz peaking in the lower mid-troposphere. Yellow to red colors are where absolute humidity decreases with warming; green is humidity increasing to roughly maintain constant RH, and blue is where humidity increases …