Ming and Ramaswamy: Nonlocal Component of Radiative Flux Perturbationx

Radiative flux perturbation (RFP) is defined as the top-of-2 the-atmosphere (TOA) radiative imbalance after the atmosphere-land sys-3 tem adjusts fully to an external perturbation, and serves as a useful metric 4 for quantifying climate forcing. This paper presents an effort to address the 5 issue whether a forcing imposed persistently over a specific region may al-6 ter the radiative balance elsewhere through atmospheric circulation, thus giv-7 ing rise to a nonlocal component of RFP. We perturb the climate simulated 8 with an atmospheric general circulation model (AGCM) solely by increas-9 ing the cloud droplet number concentration over the land, and observe widespread 10 positive (warming) RFP over the ocean, along with the expected negative 11 (cooling) RFP over the land. A detailed analysis suggests that the oceanic 12 (or nonlocal) RFP is closely associated with a reduction in low cloud amount, 13 which can be attributed primarily to the horizontal advection of drier land 14 boundary layer air and to the oceanic boundary layer top entrainment of drier 15 free troposphere air. By examining how the land surface and atmospheric 16 energy balances are re-established, we are able to identify the physical mech-17 anisms behind the strong hydrological impact of a tropical land shortwave 18 (SW) forcing (the multiplier effect). In contrast, the hydrological cycle is rel-19 atively insensitive to an extratropical forcing.