fnctId=thesis,fnctNo=367
Quantifying Freshwater-Climate Feedbacks in Earth System Models
- 작성자
- 기후시스템전공
- 저자
- Hyuna Kim
- 발행사항
- 발행일
- 2024-08
- 저널명
- 국문초록
- 영문초록
- Greenhouse warming influences the pattern and magnitude of global freshwater fluxes to the oceans by way of precipitation, river run-off, sea-ice, and land-ice. These fluxes in turn affect salinity and corresponding horizontal and vertical density gradients, which can further influence ocean circulation, shaping climate via coupled processes. This thesis examines the role of freshwater-climate feedbacks across a variety of timescales and from past to future. More specifically, my thesis addresses (1) how glacial ice-sheet growth may have influenced ocean circulation and stratification during ice ages, (2) the fidelity of oxygen isotopes as a proxy for Antarctic ice-sheet melt for present and future climate, and (3) the role of projected increases in equatorial rainfall in shaping the Pacific climate response to greenhouse warming.First, growing ice-sheets during glacial periods relocate water from the ocean to land, causing a sea level drop of ~125 m during the Last Glacial Maximum (LGM) and a corresponding global mean ocean salinity increase of 1 psu. Furthermore, the growth of ice-sheets increases the ocean’s oxygen isotope value δ18O by about 1 to 1.2 permil (‰) per 100 m sea level drop. To a first-order approximation, salinity and δ18O of seawater (δ18Osw) both respond linearly to the change in ice volume, and the δ18Osw pattern of the glacial ocean is therefore closely linked to the salinity pattern. In paleoceanographic studies, it is often assumed that the anomalies in ocean salinity and δ18Osw due to ice volume changes are distributed homogenously across the ocean. However, pore-fluid chlorinity data from deep ocean sediment cores suggests otherwise (Adkins et al., 2002). They show that deep ocean salinity during glacial periods increased by up to 3 psu. This raises the general question of which processes were involved in creating the 1 psu global mean salinity increase during glacial periods. Furthermore, to better interpret paleoceanographic proxies, it is important to understand the spatial pattern of the resulting salinity changes. These questions are addressed in Chapter 2 of my thesis, where I present results from an idealized freshwater perturbation experiment conducted with the Community Earth System Model (CESM). The experiment mimics the fact that during the buildup-phase of the Eurasian and Laurentide ice-sheets, the main river systems (i.e., catchment regions and deltas), which under present-day conditions transport ~0.16 Sv of freshwater into the Arctic Ocean, were blocked by land-ice. Given computational constraints, the “freshwater withholding” by Northern Hemisphere ice-sheets is only simulated for 1,000 model years, which resulted in an increase in the global mean salinity of 0.1 psu (10% of the full glacial signal). The model simulation helps in tracking the salinity dynamics that emerge in response to the blocked Arctic rivers. A positive subsurface salinity anomaly spreads along the deep western boundary current, eventually reaching the Indian and Pacific Oceans after 700 model years. The vertical distribution of salinity anomalies (exceeding the global mean) in the Atlantic Ocean shows a maximum of about 1000-2000 m, increasing the upper ocean stratification. This process, which highlights the role of the Arctic and North Atlantic, is different from previously proposed deep ocean stratification mechanisms that invoke increased Southern Ocean brine rejection and enhanced Southern Ocean Bottom water formation during glacial periods. Moreover, the increased salinity due to the Arctic river blockage increases the Atlantic Meridional Overturning Circulation (AMOC) by about 30%, which in turn leads to sea-ice retreat and surface warming in both hemispheres, thereby providing a negative climate-freshwater feedback for ice-sheet growth and freshwater withholding.In Chapter 3, I address how future freshwater discharges from the disintegrating Antarctic ice-sheet (AIS) impact ocean salinity and δ18Osw. More specifically, using the isotopologue-enabled Community Earth System model (iCESM) in combination with freshwater estimates from a fully coupled climate-ice-sheet model, I study whether δ18Osw can be used as a proxy to monitor the Antarctic ice-sheet freshwater discharge (IFD) induced by greenhouse warming. The fidelity of δ18Osw is evaluated using freshwater and isotope perturbation experiments with the iCESM. To this end, I conducted simulations using three 21st-century warming scenarios (Shared Socioeconomic Pathways, SSP1-1.9, 2-4.5, 5-5.8) with transient IFD projections from the coupled climate-ice-sheet model LOVECLIP. In these scenarios, the IFD signal in δ18Osw usually emerges in the Southern Ocean (in the Ross Sea in particular) several decades before the anthropogenic salinity signal becomes detectable. A more in-depth analysis reveals that the advantage of using water isotopes over salinity stems from a combination of effects: i) larger relative endmember differences between the ocean and ice-sheet; ii) fewer dilution effects; and iii) reduced natural variability. The final topic of my thesis investigates whether the commonly projected tropical rainfall enhancement plays a role in shaping future Pacific warming patterns. Several hypotheses have been put forward to explain why the majority of coupled general circulation models simulate intensified eastern equatorial Pacific warming relative to the western Pacific warming pool. The explanations usually invoke thermodynamic arguments, while ignoring the role of salinity. However, consistently across the majority of climate models, the equatorial Pacific is also the area exhibiting the most dramatic increase in rainfall (and also in precipitation minus evaporation). This can lead to substantial changes in upper ocean salinity, with potential effects on ocean currents and stratification. To address this issue, I conducted idealized tropical freshwater perturbation experiments using the Community Earth System Model. The freshening of the upper tropical Pacific Ocean caused by the increased rainfall leads to a flattening of the thermocline and a shoaling of the equatorial undercurrent. This contributes to accelerated warming in the eastern equatorial Pacific and a slowdown of the Pacific Walker circulation. This mechanism has the potential to accelerate the warming of the eastern Pacific Ocean and thereby constitutes a positive climate-freshwater feedback.In conclusion, my thesis explores previously overlooked climate-freshwater feedbacks (Chapters 2, 4), which highlight the role of salinity in long-term climate change and identify seawater isotopes as a means to detect anthropogenic trends of the Antarctic ice-sheet (Chapter 3). Freshwater perturbation experiments conducted with the Community Earth System model are the common methodological thread of my research, and wherever possible, my model results are compared with observational datasets to strengthen the main conclusions.
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