Levang Samuel J.

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Levang
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Samuel J.
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  • Article
    Centennial changes of the global water cycle in CMIP5 models
    (American Meteorological Society, 2015-08-15) Levang, Samuel J. ; Schmitt, Raymond W.
    The global water cycle is predicted to intensify under various greenhouse gas emissions scenarios. Here the nature and strength of the expected changes for the ocean in the coming century are assessed by examining the output of several CMIP5 model runs for the periods 1990–2000 and 2090–2100 and comparing them to a dataset built from modern observations. Key elements of the water cycle, such as the atmospheric vapor transport, the evaporation minus precipitation over the ocean, and the surface salinity, show significant changes over the coming century. The intensification of the water cycle leads to increased salinity contrasts in the ocean, both within and between basins. Regional projections for several areas important to large-scale ocean circulation are presented, including the export of atmospheric moisture across the tropical Americas from Atlantic to Pacific Ocean, the freshwater gain of high-latitude deep water formation sites, and the basin averaged evaporation minus precipitation with implications for interbasin mass transports.
  • Article
    Intergyre salt transport in the climate warming response
    (American Meteorological Society, 2020-01-20) Levang, Samuel J. ; Schmitt, Raymond W.
    Regional connectivity is important to the global climate salinity response, particularly because salinity anomalies do not have a damping feedback with atmospheric freshwater fluxes and may therefore be advected over long distances by ocean circulation, resulting in nonlocal influences. Climate model intercomparison experiments such as CMIP5 exhibit large uncertainty in some aspects of the salinity response, hypothesized here to be a result of ocean dynamics. We use two types of Lagrangian particle tracking experiments to investigate pathways of exchange for salinity anomalies. The first uses forward trajectories to estimate average transport time scales between water cycle regimes. The second uses reverse trajectories and a freshwater accumulation method to quantitatively identify remote influences in the salinity response. Additionally, we compare velocity fields with both resolved and parameterized eddies to understand the impact of eddy stirring on intergyre exchange. These experiments show that surface anomalies are readily exchanged within the ocean gyres by the mean circulation, but intergyre exchange is slower and largely eddy driven. These dynamics are used to analyze the North Atlantic salinity response to climate warming and water cycle intensification, where the system is broadly forced with fresh surface anomalies in the subpolar gyre and salty surface anomalies in the subtropical gyres. Under these competing forcings, strong intergyre eddy fluxes carry anomalously salty subtropical water into the subpolar gyre which balances out much of the local freshwater input.
  • Thesis
    The response of ocean salinity patterns to climate change: implications for circulation
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2019-06) Levang, Samuel J.
    Global patterns of ocean salinity arise from the exchange of freshwater between the sea surface and the atmosphere. For a quasi-steady state system, these surface fluxes are balanced by compensating transports of salt in the ocean interior. In a warming climate, the atmosphere holds additional water vapor which acts to intensify the global water cycle. Amplified freshwater fluxes are then absorbed at the surface and propagate along ocean circulation pathways. Here, we use coupled model results from the CMIP5 experiment to identify coherent responses in the atmospheric water cycle and in ocean salinity patterns. Some aspects of the response are consistent across models, while other regions show large inter-model spread. In particular, the salinity response in the North Atlantic subpolar gyre, where the mean salinity plays a role in maintaining high surface density for deep-water formation, has low confidence in CMIP5 models. To understand how differences in ocean circulation may affect this response, we use two techniques to diagnose the role of salt transports in the present-day climate. The first is a salt budget within the surface mixed layer, which identifies major transport processes. The second is a Lagrangian particle tracking tool, used to understand the regional connectivity of water masses. From this analysis, we find that anomalous freshwater signals become well mixed within the ocean gyres, but can be isolated on larger scales. The subpolar Atlantic salinity response generally shows freshening at the surface, but is sensitive to the transport of anomalously salty water from the subtropics, a largely eddy-driven process. As CMIP5 models use a range of eddy parameterizations, this is likely a source of uncertainty in the salinity response. Finally, we investigate the effect of salinity changes on the deep overturning cells and other circulations, and find a complex influence that also depends on the details of advective pathways. In a warming scenario, water cycle amplification actually works to strengthen the Atlantic meridional overturning circulation due to the influence of enhanced subtropical evaporation.