Variability of nutrients and carbon dioxide in the Antarctic Intermediate Water between 1990 and 2014

Abstract

Antarctic Intermediate Water (AAIW) formation constitutes an important mechanism for the export of macronutrients out of the Southern Ocean that fuels primary production in low latitudes. We used quality-controlled gridded data from five hydrographic cruises between 1990 and 2014 to examine decadal variability in nutrients and dissolved inorganic carbon (DIC) in the AAIW (neutral density range 27 < γ n < 27.4) along the Prime Meridian. Significant positive trends were found in DIC (0.70 ± 0.4 μmol kg −1 year −1 ) and nitrate (0.08 ± 0.06 μmol kg −1 year −1 ) along with decreasing trends in temperature (−0.015±0.01 • C year −1 ) and salinity (−0.003±0.002 year −1 ) in the AAIW. Accompanying this is an increase in apparent oxygen utilization (AOU, 0.16 ± 0.07 μmol kg −1 year −1 ). We estimated that 75% of the DIC change has an anthropogenic origin. The remainder of the trends support a scenario of a strengthening of the upper-ocean overturning circulation in the Atlantic sector of the Southern Ocean in response to the positive trend in the Southern Annular Mode. A decrease in net primary productivity (more nutrients unutilized) in the source waters of the AAIW could have contributed as well but cannot fully explain all observed changes.

Intermediate water masses, a major supplier of oxygen for the eastern tropical Pacific ocean

“…In addition to an oxygen decrease in tropical regions, Schmidtko et al (2017) showed a decrease of oxygen levels by 2-5 mmol.m -3 in the regions of formations of AAIW. Based on repeated cruise observations, Panassa et al (2018) highlighted an increase of the apparent oxygen utilization in the core of the AAIW, related to a 5 % increase in nutrient concentrations from 1990 to 2014. The transport of this modified AAIW, poorer in oxygen and richer in nutrients, toward the low latitudes both by small scale processes (section 3) and at the equator by the EICS (section 4), may explain a significant part of the occuring deoxygenation in the equatorial ocean.…”

The Fate of Carbon and Nutrients Exported Out of the Southern Ocean

“…Recently, large interannual variability in high-latitude Southern Hemisphere surface climate (Jones, Gille, et al, 2016) and a large decadal variability of the SO carbon sink (Landschützer et al, 2015;Le Quéré et al, 2007) have been reported. Furthermore, nutrient concentrations show signals of variability or change in multiple regional studies from surface to depth (Ayers & Strutton, 2013;Hoppema et al, 2015;Iida et al, 2013;Panassa et al, 2018;Pardo et al, 2017). Given that mode and intermediate water masses reach the subtropical thermocline on a time scale of a few decades (Jones, Meijers, et al, 2016;Rodgers et al, 2003), it is very likely that these changes may reappear in the low latitudes on a time scale relevant to human anthropogenic CO 2 emissions and emission reduction endeavors (Palter et al, 2010;Pasquier & Holzer, 2016).…”

Influência das variações de baixa frequência da Circulação de Revolvimento Meridional na concentração de clorofila - a no Atlântico Sul

“…Esse mecanismo alimenta de 33% a 75% da produtividade primária tropical entre 30 o N a 30 o S (Palter et al 2010) com uma escala espacial de menos de 40 anos para todo o nutriente ser utilizado neste espaço. Palter et al (2010) ainda afirmam que os nutrientes da SAMW alimentam primeiro a produtividade primária ao sul de 20 o S. Este mecanismo conseguiu explicar o resultado em diversos estudos variando de tópicos desde a biogeoquímica do Atlântico norte a estudos paleoclimáticos (Marinov et al 2006;Moore et al 2009;Anderson et al 2009) e levando a formulação de outros estudos pelo desdobramento desse mecanismo (Ayers and Strutton 2013;Panassa et al 2018;Palter et al 2011).…”

“…O primeiro é a Oscilação Antártica (AAO). Sua escolha foi baseada pelo seu efeito nos ventos de oeste na região subpolar, que por sua vez alteram a ressurgência antártica, afetando assim diretamente a concentração nutrientes que adentram na Água Modal Subantártica (SAMW) e AIA (Água Intermediária Antártica) e que serão transportados pela MOC do Atlântico Sul (Thompson et al 2011;Marshall 2003;Panassa et al 2018). Além disso, a AAO pode afetar o vazamento das Agulhas, que traz águas do Oceano Índico no Atlântico Sul (Biastoch and Böning 2013;Beal et al 2011;Marini, Frankignoul, and Mignot 2011).…”

“…Dada a importância do oceano Austral diversos estudos tem estudado a biogeoquímica da SAMW e a AIA sob efeito de forçantes físicas (Iida, Odate, and Fukuchi 2013;Pardo et al 2017;Panassa et al 2018;Ayers and Strutton 2013). Ayers and Strutton (2013) mostraram que no setor do Pacífico do oceano Austral um aumento na ressurgência antártica e na MOC implicaria uma maior concentração de nutrientes na SAMW.…”