Abstract
Abstract. Ocean deoxygenation has been observed in all major ocean basins over the past 50 yr. Although this signal is largely consistent with oxygen changes expected from anthropogenic climate change, the contribution of external forcing to recent deoxygenation trends relative to natural internal variability is yet to be established. Here we conduct a formal optimal fingerprinting analysis to investigate if external forcing has had a detectable influence on observed dissolved oxygen concentration ([O2]) changes between ∼1970 and ∼1992 using simulations from two Earth System Models (MPI-ESM-LR and HadGEM2-ES). We detect a response to external forcing at a 90% confidence level and find that observed [O2] changes are inconsistent with internal variability as simulated by models. This result is robust in the global ocean for depth-averaged (1-D) zonal mean patterns of [O2] change in both models. Further analysis with the MPI-ESM-LR model shows similar positive detection results for depth-resolved (2-D) zonal mean [O2] changes globally and for the Pacific Ocean individually. Observed oxygen changes in the Atlantic Ocean are indistinguishable from natural internal variability. Simulations from both models consistently underestimate the amplitude of historical [O2] changes in response to external forcing, suggesting that model projections for future ocean deoxygenation may also be underestimated.
Highlights
Hydrology andThe oceanic oxygen inveEntoaryrtihs cSouyplsedtetomthe climate system via making a number oxygen a oufsepfhuyl stircaaclearnSfdorcbidioeegtneecoctcihenegsmcihcaanl gpersocinessthees state of the earth system (Joos et al, 2003; Brennan et al., 2008)
Estimate of natural variability in [O2] data (Helm et al, 2010, 2011; Bindoff and Wunsch, 1992). This technique accounts for mesoscale processes and to some extent longer period internal variability such as the dominant climate modes. This dataset is used in an optimal detection analysis along with the biogeochemical output from two state-of-the-art Earth System Models (ESMs) (MPI-ESM-LR and HadGEM2-ES) participating in the CMIP5 experiments (Taylor et al, 2009, 2012). [O2] fields from MPI-ESM-LR and HadGEM2-ES have been selected here because they display a higher level of realism in simulating both climatological [O2] distribution and historical oxygen changes compared to the other available CMIP5 ESMs
We focus our single fingerprint analysis on the “all forcings” historical scenario of MPI-ESM-LR and HadGEM2ES, with model output being bi-linearly interpolated onto a 1◦ × 1◦ grid and masked to emulate the pattern of missing values found in the observations of Helm et al (2011) (Fig. 1). [O2] changes between ∼ 1970 and ∼ 1992 are calculated for these experiments and provided as model response patterns in the Total Least Squares (TLS) regression against corresponding observed [O2] changes (y) in order to estimate scaling factors (β)
Summary
Hydrology andThe oceanic oxygen inveEntoaryrtihs cSouyplsedtetomthe climate system via making a number oxygen a oufsepfhuyl stircaaclearnSfdorcbidioeegtneecoctcihenegsmcihcaanl gpersocinessthees state of the earth system (Joos et al, 2003; Brennan et al., 2008). CliHmiastteorcihcaanl gdee,ocxhyigeeflTnyahtdieouneCthoarseynboheaesnnpcaehsdseooccrieeaatendstwraitthifigclaotiboanl in a warming climate which increases the ventilation age of downwelling water masses, and augmented by the reduced solubility of dissolved oxygen at higher temperatures (Keeling et al, 2010). These findings are supported by prognostic simulations from a suite of Atmosphere-Ocean General Circulation Models (AOGCMs), which show ventilation-driven reductions in global mean [O2] between 3 and 12 μmol kg−1. Ocean deoxygenation and expansion of the OMZs has been projected to persist on millennial timescales for EMIC simulations with high greenhouse gas emissions or high climate sensitivity (Shaffer et al, 2009)
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