Ocean Property Transformation as a Climate Evaluator
In collaboration with Princeton and NOAA-GFDL colleagues, Lawrence Livermore Lab scientists developed a new framework for evaluating ocean water mass changes. This framework elucidated the sensitivity of NOAA-GFDL ocean models to atmosphere and cryosphere surface-forced changes. It discovered a marked difference between the Antarctic Bottom Water (AABW) formation rate in the Southern Ocean (>30°S), dependent on the ocean model resolution, the higher (0.25°) resolution simulation showing four-times the AABW reduction when contrasted to the lower (0.5°) resolution ocean model. This work shows considerable uncertainties remain around the ocean’s role in heat and carbon storage due to climate change.
This latest work suggests that CMIP-class climate models may underestimate ocean changes driven by global warming and consequent atmosphere and cryosphere responses. Global ocean heat and carbon sink operations are strongly moderated by Southern Ocean processes and water mass transformations in this region. The large AABW reduction differences, as identified by the two NOAA-GFDL ocean configurations, have considerable implications for future climate projections and the resolved rate of climate change.
The global ocean plays an outsized role in ongoing climate change. It is responsible for 91% of excess heat and 26% of excess CO2 storage over the most recent decade, with a similar rate recorded over earlier periods. Consequently, any modulation of the role of the ocean reservoir in Earth’s energy and carbon budgets will have a large impact on future climate changes. While considerable research focus has been applied to the atmospheric responses to climate change, comparatively less attention has been paid to oceanic responses, and the realism of ocean models in their forced response. The lack of attention, in part, is due to limited historical observations that provide an imperfect view of long-term ocean change. Since the early 2000s, observational networks have markedly improved, providing new opportunities for ocean model evaluation, and the investigation of persistent uncertainties associated with ocean responses. The uncertainty of the Southern Ocean response to climate change is the focus of this work.
Using a water mass transformation framework, this study uncovered a marked difference between low (0.5°) and high (0.25°) horizontal resolution ocean model configurations, with a reduction in Antarctic Bottom Water (AABW) a known response to climate change, but four-times larger in the higher resolution configuration. As most CMIP-class ocean models have similarly low-resolution configurations, this raises the question about modeled realism, and whether current-generation CMIP models underestimate the rate of change. Follow on work is required to ascertain the robustness of resolution-moderated ocean responses across the CMIP-class model suite, and whether a reassessment of the role and rate of ongoing climate change will be required. To facilitate ongoing and future progress, this work package generated the xWMT (Water Mass Transformation) package, which has been made available as a CMEC analysis module (see https://github.com/cmecmetrics/cmec_xwmt).