Abstract
Abstract. Observations and model runs indicate trends in dissolved oxygen (DO) associated with current and ongoing global warming. However, a large-scale observation-to-model comparison has been missing and is presented here. This study presents a first global compilation of DO measurements covering the last 50 yr. It shows declining upper-ocean DO levels in many regions, especially the tropical oceans, whereas areas with increasing trends are found in the subtropics and in some subpolar regions. For the Atlantic Ocean south of 20° N, the DO history could even be extended back to about 70 yr, showing decreasing DO in the subtropical South Atlantic. The global mean DO trend between 50° S and 50° N at 300 dbar for the period 1960 to 2010 is –0.066 μmol kg−1 yr−1. Results of a numerical biogeochemical Earth system model reveal that the magnitude of the observed change is consistent with CO2-induced climate change. However, the pattern correlation between simulated and observed patterns of past DO change is negative, indicating that the model does not correctly reproduce the processes responsible for observed regional oxygen changes in the past 50 yr. A negative pattern correlation is also obtained for model configurations with particularly low and particularly high diapycnal mixing, for a configuration that assumes a CO2-induced enhancement of the C : N ratios of exported organic matter and irrespective of whether climatological or realistic winds from reanalysis products are used to force the model. Depending on the model configuration the 300 dbar DO trend between 50° S and 50° N is −0.027 to –0.047 μmol kg−1 yr−1 for climatological wind forcing, with a much larger range of –0.083 to +0.027 μmol kg−1 yr−1 for different initializations of sensitivity runs with reanalysis wind forcing. Although numerical models reproduce the overall sign and, to some extent, magnitude of observed ocean deoxygenation, this degree of realism does not necessarily apply to simulated regional patterns and the representation of processes involved in their generation. Further analysis of the processes that can explain the discrepancies between observed and modeled DO trends is required to better understand the climate sensitivity of oceanic oxygen fields and predict potential DO changes in the future.
Highlights
Anthropogenic carbon dioxide emissions alter our oceans in numerous ways, some of which have reached considerable attention, like ocean acidification and sea level rise
It shows declining upperocean dissolved oxygen (DO) levels in many regions, especially the tropical oceans, whereas areas with increasing trends are found in the subtropics and in some subpolar regions
Depending on the model configuration the 300 dbar DO trend between 50◦ S and 50◦ N is −0.027 to −0.047 μmol kg−1 yr−1 for climatological wind forcing, with a much larger range of −0.083 to +0.027 μmol kg−1 yr−1 for different initializations of sensitivity runs with reanalysis wind forcing
Summary
Anthropogenic carbon dioxide emissions alter our oceans in numerous ways, some of which have reached considerable attention, like ocean acidification and sea level rise. Models have, so far and in contrast to the observations over the past decades, mostly shown an increase in oxygen concentrations in the tropical thermocline (Bopp et al, 2002; Matear and Hirst, 2003) These models show an associated reduction in the extent of suboxic areas under projected 21st century global warming, unless CO2-induced changes in C : N ratios (Oschlies et al, 2008) or CO2-induced ballast effects were assumed (Hofmann and Schellnhuber, 2009). The aim of the present work is a confrontation of biogeochemical model results with observed large-scale DO changes for which a new global compilation of oxygen measurements taken over the past decades is constructed This allows, for the first time, a global overview of the DO changes over the last 50 yr, which is not confined to the difference of two temporal periods, and covers more than just the tropical and subtropical regions. We investigate whether specific assumptions about the strength of diapycnal mixing, about direct CO2 effects on the stoichiometry of the biological pump, and about historical wind patterns yield good or bad representations of the observed changes in dissolved oxygen
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