Impacts of North American forest cover changes on the North Atlantic ocean circulation

Abstract. Atmosphere-ocean heat fluxes in the North Atlantic Labrador Sea region are a key driver of deep water formation and the Atlantic Meridional Overturning Circulation (AMOC). Previous research has shown that anthropogenic warming leads to reduced ocean heat loss and thereby reduced deep mixing in the North Atlantic. This results in AMOC decline and causes regional cooling of sea surface temperatures (SSTs) which has been referred to as the North Atlantic warming hole (NAWH). Similar responses of the AMOC and the formation of a NAWH have been found for changes in wind stress and fresh water forcing in the North Atlantic. Moreover, recent research has also revealed such an AMOC and North Atlantic SST response in global-scale forestation experiments and a reversed response in deforestation experiments. Here, we test the hypothesis that forest cover changes in particular over North America are an important driver of this response in the downstream North Atlantic ocean. To this end, we perform simulations using the fully coupled Earth system model CESM2 where pre-industrial vegetation-sustaining areas over North America are either completely forested (forestNA) or turned into grasslands (grassNA), and compare it to the control scenario without any forest cover changes. Our results show that North American forestation and deforestation induce a North Atlantic warming and cooling hole, respectively. Furthermore, the response is qualitatively similar to previously published results based on global extreme land cover change scenarios. Forest cover changes mainly impact the ocean through modulating land surface albedo and, subsequently, air temperatures. Around 80 % of the ocean heat loss in the Labrador Sea occurs within comparably short-lived cold air outbreaks (CAOs) during which the atmosphere is colder than the underlying ocean. A warmer atmosphere in forestNA compared to the control scenario results in fewer CAOs over the ocean and thereby reduced ocean heat loss, with the opposite being true for grassNA. The induced SST responses further decrease CAO frequency in forestNA and increase it in grassNA. Lagrangian backward trajectories starting from CAOs over the Labrador Sea confirm that their source regions include (de-)forested areas. A closer inspection of the ocean circulation reveals that the subpolar gyre circulation is more sensitive to ocean density changes driven by heat fluxes than to changes in wind forcing modulated by land surface roughness. In forestNA, sea ice growth and the corresponding further reduction of ocean-to-atmosphere heat fluxes forms an additional positive feedback loop. Conversely, a buoyancy flux decomposition shows that freshwater forcing only plays a minor role for the ocean density response in both scenarios. Overall, this study shows that forest cover changes over North America alter the frequency of CAOs over the North Atlantic and, as a consequence, the circulation of the North Atlantic. This highlights the relevance of CAOs for the formation of North Atlantic SST anomalies.