Mismatch between observed and modeled trends in dissolved upper-ocean oxygen over the last 50 yr

[摘要] Observations and model runs indicate trends in dissolved oxygen (DO)associated with current and ongoing global warming. However, a large-scaleobservation-to-model comparison has been missing and is presented here. Thisstudy presents a first global compilation of DO measurements covering thelast 50 yr. It shows declining upper-ocean DO levels in many regions,especially the tropical oceans, whereas areas with increasing trends arefound in the subtropics and in some subpolar regions. For the Atlantic Oceansouth of 20° N, the DO history could even be extended back to about70 yr, showing decreasing DO in the subtropical South Atlantic. The globalmean DO trend between 50° S and 50° N at 300 dbar for theperiod 1960 to 2010 is –0.066 μmol kg⁻¹ yr⁻¹. Resultsof a numerical biogeochemical Earth system model reveal that the magnitude ofthe observed change is consistent with CO₂-induced climate change.However, the pattern correlation between simulated and observed patterns ofpast DO change is negative, indicating that the model does not correctlyreproduce the processes responsible for observed regional oxygen changes inthe past 50 yr. A negative pattern correlation is also obtained for modelconfigurations with particularly low and particularly high diapycnal mixing,for a configuration that assumes a CO₂-induced enhancement of theC : N ratios of exported organic matter and irrespective of whetherclimatological or realistic winds from reanalysis products are used to forcethe model. Depending on the model configuration the 300 dbar DO trendbetween 50° S and 50° N is −0.027 to–0.047 μmol kg⁻¹ yr⁻¹ for climatological windforcing, with a much larger range of –0.083 to+0.027 μmol kg⁻¹ yr⁻¹ for different initializations ofsensitivity runs with reanalysis wind forcing. Although numerical modelsreproduce the overall sign and, to some extent, magnitude of observed oceandeoxygenation, this degree of realism does not necessarily apply to simulatedregional patterns and the representation of processes involved in theirgeneration. Further analysis of the processes that can explain thediscrepancies between observed and modeled DO trends is required to betterunderstand the climate sensitivity of oceanic oxygen fields and predictpotential DO changes in the future.

[学科分类] 地球化学与岩石