Physical Mechanisms of Tropical Climate Feedbacks Revealed by Temperature and Moisture Trends

Abstract:

Although climate models' water vapour and lapse rate feedbacks are anti-correlated on a global scale, no such relationship is seen on regional scales. What determines the regional structure of these feedbacks?

Here we compare the modelled behaviour of tropical climate feedback processes with satellite observations over the period 1979-2010. We use data from the High-Resolution Infrared Sounder (HIRS) Channel 12 (~6.3 microns) to quantify changes in upper-tropospheric relative humidity. Investigating the regional structure of trends in tropospheric temperature and humidity, we discover consistent signatures of processes driving lapse rate and water vapour feedbacks across climate models and observational datasets.

Upper-tropospheric warming trends are relatively constant over the Tropics because the tropical atmosphere is unable to maintain strong temperature gradients. The regional structures of upper-tropospheric warming are similar between models and observations. Therefore, the majority of the regional variation in tropical lapse rate feedback actually comes from regional variation in surface temperature changes, not tropospheric temperature changes.

Upper-tropospheric moistening is mainly sensitive to regional precipitation trends. The degree to which moistening in the convective zones is transmitted to other parts of the tropics is central to determining the regional pattern of the water vapour feedback. These relationships could provide process-based metrics of climate models’ ability to simulate the physical processes driving tropical water vapour and lapse rate feedbacks.