Impact of Arctic greening on the seasonality of local and remote climate

Abstract

With global warming, it is expected that vegetation type may also change, particularly in the northern high latitudes over the tundra region. Under a global warming scenario, grass and shrub type of vegetation are expected to shift to boreal forests over the Arctic. This study examines the impact of potential Arctic vegetation change with global warming on the seasonality of local Arctic and remote tropical climates, and compares its impact with the response to CO2 doubling. Three experiments are performed with the NCAR Community Atmosphere Model 3 (CAM3) coupled to a slab ocean, which are perturbed by 1) Arctic vegetation change from grass and shrub to boreal forest with present day CO2 concentrations (335ppmv), 2) present day vegetation type with doubling of CO2, and 3) Arctic vegetation change to darker species with doubling of CO2. With darker vegetation over the Arctic, the Arctic region becomes warmer throughout the year due to the surface albedo reduction. However, because little insolation reaches the Arctic in boreal winter, the Arctic warming in boreal summer is four times greater than in boreal winter. Evapotranspiration changes also act to maximize the warming in boreal summer. In contrast, the Arctic warming peaks in November in response to a doubling of CO2. As a result, as Arctic vegetation change is taken into consideration in addition to the doubling of CO2, the peak of Arctic warming shifts to August, resulting in a weaker seasonality in the Arctic amplification. Arctic warming driven by vegetation change results in imbalance of atmospheric energy budget between the hemispheres. Larger incoming insolation in the northern extra-tropics is balanced by increased outgoing longwave radiation (OLR) via warming the atmosphere, and some fraction reaches the subtropics by quasi-diffusive atmospheric eddy energy fluxes. As the northern subtropics become warmer relative to the southern subtropics, the Hadley circulation responds to transport energy southward across the equator because large horizontal temperature gradients cannot be sustained within the tropics due to the smallness of Coriolis parameter. Since the Hadley circulation transports energy in the direction of its upper branch, and moisture is transported northward across the equator, the time-mean inter-tropical convergence zone (ITCZ) is shifted northward. However, atmosphere cooling from June to September in local area can shift ITCZ to the south when Arctic incoming solar radiation is low.