Sensitivity of ocean heat uptake to changing atmospheric forcing in the context of climate variability
Final Report Abstract
During the period 1998-2013, a period of slow increase in air surface temperature despite continued emissions of greenhouse gases was observed, known as global warming hiatus. Changes in atmospheric warming rates necessarily imply changes of the heat fluxes at the upper (the Earth energy imbalance, EEI) or lower boundary of the atmosphere, or both. Understanding the contributions from these two factors to this event is however hindered by the fact that observations at the top of the atmosphere are too imprecise, while the necessary anomalous exchange of heat with the ocean in context of climate variability is considered to be an event with low likelihood. Several attempts exist to connect anomalous heat uptake by the ocean with anomalous storage of heat in the deep ocean and this with the particular state of climate variability in different basins during these years. Results suggest many different contributing factors, which however lack a quantitative assessment. In this project, we aimed at a decomposition of anomalous heat uptake from different processed in different oceans and their relation to individual climate modes. To this end, we utilized the adjoint model of the MIT general circulation ocean model to calculate atmospheric forcing patterns that most efficiently change ocean heat uptake. The sensitivities of heat uptake across various depth levels to the atmospheric surface state provide mechanisms influencing heat uptake, including a partitioning into processes for each basin. Intensified trade winds in the tropical Pacific and Atlantic, stronger westerlies in subtropical regions and the Southern Ocean, and enhanced meridional winds along major coastlines can contribute to uptake changes. Surface warming in higher latitudes and cooling in the tropics are effective for heat transfer increase. Zonal wind anomalies relative to what is expected from slow changes due to climate change, emerged as the dominant contributor. The Atlantic Multidecadal Oscillation (AMO) was found to be the primary driver of changes in anomalous heat uptake among the climate modes analyzed, followed by the El Niño and the related Interdecadal Pacific Oscillation (IPO). The main climate modes explain different amounts of the changes in each basin up to 55% and contribute, except for the Indian Ocean, to an enhanced heat transfer.
Publications
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Sensitivity of ocean heat content to atmospheric forcing in the period of global warming hiatus. Copernicus GmbH.
Albert Fernández, Chavely; Köhl, Armin & Stammer, Detlef
