Sensitivity of ocean heat uptake to changing atmospheric forcing in the context of climate variability

• Applicant: Dr. Armin Koehl
• Subject Area: Oceanography
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 453121308

Global warming is a consequence of a disturbed energy balance of the earth of about 1 W/m2 caused by the reduced ability of the earth to emit long wave radiation predominantly due to the presence of an increasing amount of greenhouse gases in the atmosphere. The oceans have a key role in the Earth’s Energy Imbalance (EEI) as about 93% of the excess heat is absorbed by the oceans. Monitoring of EEI is a key challenge to describe the state of the climate and monitoring the ocean heat content (OHC) is the most suitable way to estimate its changes on long time scales. Understanding the EEI variability became a focus, partly driven by the so-called “climate change hiatus”, a period between 1998 and 2013 with little change in global mean air temperatures with this the need to understand to what extent enhanced ocean storage may explain it.The topic of this proposal will address the question which mechanisms lead to decadal changes in the effectivity of the ocean to take up heat and where this heat is stored. Following questions will be addressed:• What are the patterns of atmospheric forcing that lead to a most efficient uptake of heat by the ocean?• How do these patterns project on climate modes?• How much of the variability in heat content and heat uptake can be associated with these optimal forcing patterns?• What are the mechanisms associated with anomalous large or small heat up take?• Does the heat taken up during a phase of anomalous high uptake persist or does it cause a phase of anomalous low uptake and accelerated global warming thereafter? The approach in this project builds on the unique tool of adjoint sensitivities, which allow to unravel the processes by which a single scalar quantity of a dynamical system can most efficiently by changed. Applied to the problem heat uptake, the plan is to first compute adjoint sensitivities of heat content within different depth ranges and different regions of the ocean with respect to the complete atmospheric forcing over decadal time scales. In an alternative formulation the heat flux though the upper boundary of a depth range will be considered. The resulting forcing patterns will be analyzed with respect to the physical mechanisms and their relation to climate variability. Complementary, the effect of forcing anomalies will also be studied in the forward model. The framework for this kind of study exists with the method of optimal observation, which are essentially the response functions to the forcing perturbations related to the adjoint sensitivities. With the optimal observations the physical response to the optimal forcing patterns and the evolution of this heat content anomaly after the generating forces become zero are studied.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr. Detlef Stammer