Variability of the ocean carbon cycle in response to the North Atlantic Oscillation

Abstract

A B S T R A C T Climate modes such as the North Atlantic Oscillation (NAO), representing internal variability of the climate system, influence the ocean carbon cycle and may mask trends in the sink of anthropogenic carbon. Here, utilising control runs of six fully coupled Earth System Models, the response of the ocean carbon cycle to the NAO is quantified. The dominating response, a seesaw pattern between the subtropical gyre and the subpolar Northern Atlantic, is instantaneous ( B3 months) and dynamically consistent over all models and with observations for a range of physical and biogeochemical variables. All models show asymmetric responses to NAO ' and NAO ( forcing, implying non-linearity in the connection between NAO and the ocean carbon cycle. However, model differences in regional expression and magnitude and conflicting results with regard to airÁsea flux and CO 2 partial pressure remain. Typical NAO-driven variations are 910 mmol/m 3 in the surface concentration of dissolved inorganic carbon and alkalinity and 98 ppm in the airÁsea partial pressure difference. The effect on the basin-wide airÁsea CO 2 flux is small due to compensating fluxes on the sub-basin scale. Two models show a reduced carbon sink in the north-eastern North Atlantic during negative NAO phases, qualitatively in accordance with the observed decline during a phase of predominantly negative NAO.The results indicate that wind-driven dynamics are the main driver of the response to the NAO, which Á via vertical mixing, upwelling and the associated entrainment of dissolved inorganic carbon and nutrients Á leave an imprint on surface pCO 2 and the airÁsea CO 2 flux as well as on biological export production, pH and the calcium carbonate saturation state. The biogeochemical response to the NAO is predominantly governed by vertical exchange between the surface and the thermocline; large-scale horizontal transport mechanisms are of minor importance.

Time of emergence of trends in ocean biogeochemistry

“…We find clear indications of ENSO in the SD patterns of SST, DIC and pCO 2 , and a weak signal for pH. Another area of high natural variability is the North Atlantic, which is influenced by modes like the North Atlantic Oscillation (NAO; Hurrell and Deser, 2009) or changes in the Atlantic Meridional Overturning Circulation (AMOC; Carton and Häkkinen, 2011), both known for affecting the ocean carbon cycle (e.g., Keller et al, 2012;Perez et al, 2013). A further region with high variability in the ocean carbon system is the Southern Ocean (see e.g., Bacastow, 1976;Marinov et al, 2006;Lovenduski et al, 2007;Le Quéré et al, 2007;Resplandy et al, 2013b), where we find a corresponding signal in the ensemble SD pCO 2 pattern.…”

“…Possible reasons for the model spread in the Southern Ocean include the inadequate representation of bottom-water formation processes in many CMIP5 models (Heuzé et al, 2013) and a systematic wind bias inherent to many models, which impacts physical processes like Antarctic Circumpolar Current and Southern Ocean water mass formation and, consequently, the ocean carbon cycle (Swart and Fyfe, 2012). In areas with high natural variability, such as the equatorial Pacific or the North Atlantic, IMS might arise from differences in time and space of the representation of climate modes such as ENSO or NAO (e.g., Keller et al, 2012). Another possible factor is the model resolution, especially in areas dominated by local processes such as coastal upwelling.…”

“…Still, it remains a challenge to identify clear external forcing signals. An important issue is the presence of internal variability, which has the potential to enhance or mask forced trends in the atmosphere, land, or ocean (e.g., Latif et al, 1997;Raible et al, 2005;Frölicher et al, 2009;Dolman et al, 2010;Keller et al, 2012). For instance, McKinley et al (2011) stated that carbon dioxide trends in the North Atlantic require 25 yr to exceed the range of decadal-scale variability.…”

The ocean carbon sink – impacts, vulnerabilities and challenges

“…These variations are at least partially attributed to oceanic variability in the North Atlantic associated with a surface pressure pattern change known as North Atlantic Oscillation (Wetzel et al, 2005;Thomas et al, 2008;Tjiputra et al, 2012). In a model study with six coupled Earth system models, Keller et al (2012) identified a see-saw pattern of variations in sea surface pCO 2 between the North Atlantic subtropical gyre and the subpolar Northern Atlantic with an amplitude of ±8 ppm. Such variations make identification of long-term trends in oceanic carbon uptake more difficult.…”

Influence des régimes de temps atmosphériques sur la circulation océanique de l'Atlantique Nord

“…Une reconstruction des projections atmosphériques futures, combinant les projections d'occurrence des régimes aux caractéristiques observées, peut également être envisagée pour forcer un modèle d'océan (de plus haute résolution horizontale que la composante océanique des modèles de climat, par exemple), afin de s'affranchir de l'effet négatif des biais moyens de l'atmosphère dans les modèles de climat. Au-delà de ces problématiques, l'approche en régi-mes de temps semble aussi prometteuse pour mieux comprendre l'influence de la variabilité atmosphérique sur la biogéo-chimie marine, et plus particulièrement sur les concentrations en carbone (Keller et al, 2012) et en chlorophylle (Patara et al, 2011) dans l'océan.…”