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The role of Southern Ocean stratification in future ocean CO2 and heat uptake

Applicant Haumann Alexander
Number 186681
Funding scheme Postdoc.Mobility
Research institution Atmospheric and Oceanic Sciences Princeton University
Institution of higher education Institution abroad - IACH
Main discipline Oceanography
Start/End 01.07.2019 - 30.09.2020
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All Disciplines (4)

Discipline
Oceanography
Meteorology
Climatology. Atmospherical Chemistry, Aeronomy
Hydrology, Limnology, Glaciology

Keywords (11)

Stratification; Freshwater; Heat; Ocean; Southern Ocean; Global climate model; Climate change; CO2; Future; Carbon-dioxide; Sea ice

Lay Summary (German)

Lead
Der Austausch von Wassermassen zwischen dem oberflächennahen und tiefen Ozean hat einen grossen Einfluss auf das Klima der Erde, da durch diesen Prozess auch grosse Mengen des klimawirksamen Treibhausgases Kohlenstoffdioxid und Wärme zwischen dem Ozean und der Atmosphäre ausgetauscht werden. Dieser Austausch findet vorwiegend im Südpolarmeer statt, wo tiefes Wasser an die Oberfläche gelangt und wieder absinkt. Bei diesem Prozess spielt die vertikale Dichteschichtung der Wassersäule eine wesentliche Rolle und mögliche zukünftige Veränderungen dieser Dichteschichtung könnten zu einer Verstärkung oder Abschwächung der globalen Klimaerwärmung führen. Bis heute haben Klimamodelle jedoch Schwierigkeiten diese Dichteschichtung im Südpolarmeer realistisch abzubilden, was zu Unsicherheiten in Abschätzungen des zukünftigen Klimawandels führt.
Lay summary

Inhalt und Ziel des Forschungsprojekts

Das Ziel unseres Projektes ist es den Einfluss von Veränderungen der Dichteschichtung im Südpolarmeer auf die zukünftige Aufnahme von Kohlenstoffdioxid und Wärme besser zu verstehen und dadurch Unsicherheiten in Modellabschätzungen der zukünftigen globalen Klimaerwärmung einzuschränken. Dafür verwenden wir neue Messdaten und Analysemethoden um einerseits bestehende Modellsimulationen zu evaluieren und um andererseits die Dichteschichtung in Modellsimulationen des Südpolarmeers zu verbessern.

Wissenschaftlicher und gesellschaftlicher Kontext des Forschungsprojekts

Unser Projekt wird neue Erkenntnisse über den Einfluss von Veränderungen im Südpolarmeer auf die zukünftige Klimaerwärmung liefern und helfen diese Prozesse in Klimamodellen zu verbessern. Unsere Arbeit zielt darauf ab die bestehenden Unsicherheiten in den zulässigen Kohlenstoffdioxidemissionen und dem Meerespiegelanstieg, welche mit dem politischen 1.5°C Ziel verbunden sind, zu verringern und somit eine bessere Planung von Anpassungsstrategien an den Klimawandel zu ermöglichen.

Direct link to Lay Summary Last update: 25.06.2019

Responsible applicant and co-applicants

Associated projects

Number Title Start Funding scheme
175162 The role of Southern Ocean stratification in future ocean CO2 and heat uptake 01.10.2017 Early Postdoc.Mobility

Abstract

Throughout most of the global ocean, biological production and sinking of organic matter remove carbon and nutrients from the surface ocean [Sarmiento and Gruber, 2006]. In a natural system, this sinking flux is balanced by an upwelling of carbon and nutrients through ocean circulation and mixing [Marinov et al., 2006; Sarmiento et al., 2004]. The majority, i.e. about 80%, of the upwelling deep waters return to the surface in the Southern Ocean [e.g. Morrison et al., 2015]. This upwelling plays a vital role in the global carbon cycle and the global climate system. Sarmiento and Toggweiler [1984] were among the first to suggest that this connection between the deep and the surface ocean is the key controlling factor of natural long-term variations in the atmospheric carbon-dioxide (CO2) concentration. However, the underlying processes that alter the vertical exchange of carbon-rich waters have not been fully understood so far and, therefore, also the future response of the system to human-induced climate change remains poorly constrained. As a consequence, large uncertainties arise in future climate projections from feedbacks between climatic changes and changes in Southern Ocean vertical water mass exchange that could significantly diminish or amplify global warming.Future changes in the marginally stable vertical density gradient (here referred to as stratification) of the Southern Ocean could critically alter the vertical exchange of water masses and therefore the exchange of CO2 and heat with the atmosphere [Manabe and Stouffer, 1993; Sarmiento et al., 1998; Sigman et al., 2004]. Yet, current global climate models reveal large biases in this region [e.g. Downes et al., 2011; Sallée et al., 2013]. Consequently, it is critical to better understand the role of Southern Ocean stratification to project the magnitude of the expected future global warming accurately.In this project, we propose to investigate the effect of stratification on the future CO2 and heat uptake and release by the Southern Ocean. We will assess the related uncertainty in projected future global climate change by comparing the sensitivity of global climate model projections to changes in Southern Ocean stratification to new observational data and to a newly developed simplified model that simulates the interaction between Southern Ocean stratification and the global carbon cycle. The first part of the project (Early Postdoc.Mobility fellowship) aimed at revealing the influence of surface freshwater fluxes on Southern Ocean water masses and stratification by using the oxygen isotopic composition in water samples and analyzing profiling biogeochemical float data to determine the sensitivity of the vertical exchange to stratification changes. The here proposed continuation of this project now contrasts these new findings with global climate simulations and assesses their future response. This analysis will provide insights into the processes responsible for the large discrepancies between models. In a second step, we will develop a simplified multi-column model of the ocean to analyze the vertical exchange of water masses and their effect on changes in ocean CO2 and heat uptake and release. We will then impose freshwater flux scenarios on this simple model by perturbing the surface fluxes according to expected future changes to study the potential future response of the system and use present-day and past glacial conditions as a benchmark. This latter analysis might also reveal potential surprises in the effect of Southern Ocean stratification changes on global warming that might not be represented in current global climate models due to their biases.
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