Nonlocal Component of Radiative Flux Perturbation

Radiative flux perturbation (RFP) is defined as the top-of-the-atmosphere (TOA) radiative imbalance after the atmosphere-land system adjusts fully to an external perturbation, and serves as a useful metric for quantifying climate forcing. This paper presents an effort to address the issue whether a forcing imposed initially over a specific region may alter the radiative balance elsewhere through atmospheric circulation, thus giving rise to a non-local component of RFP. We perturb an atmospheric general circulation model (AGCM) by increasing the cloud droplet number concentration exclusively over land, and observe widespread positive (warming) RFP over ocean, along with the expected negative (cooling) RFP over land. A detailed analysis suggests that the oceanic (or non-local) RFP is closely associated with a reduction in low cloud amount, which can be attributed primarily to the horizontal advection of drier land boundary layer air and to the oceanic boundary layer top entrainment of drier free troposphere air. By examining how the land surface and atmospheric energy balances are re-established, we are able to identify the physical mechanisms behind the strong hydrological impact of a tropical land shortwave (SW) forcing (the multiplier effect). In contrast, the hydrological cycle is relatively insensitive to an extratropical forcing.