Patterns & mechanisms of ocean salinity changes and their connection to the climate system
Date
2023
Authors
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Publisher
University of Delaware
Abstract
Salinity is a crucial variable in understanding ocean and climate systems. This dissertation focuses on investigating the complex mechanisms governing the ocean's salinity variability and its implications for the water cycle and global/regional climate change, primarily utilizing the ECCO estimate. ☐ The first topic I explored is the mixed layer salinity (MLS) budget. A comprehensive analysis of the controlling factors of MLS was conducted, employing unsupervised k-means clustering to identify distinct MLS budget regimes based on surface forcing, advection, and diffusion terms. The clustering approach effectively summarizes commonalities across different regions of the global ocean, offering insights into long-term and short-term variability. The advection process emerges as the primary driver of temporal MLS variability globally, while surface forcing plays a significant role in Western Boundary Current (WBC) regions. ☐ The utilization of k-means clustering enables the targeting of specific time windows and the generation of detailed spatial maps, elucidating regions influenced by freshwater flux or oceanic processes. These maps provide insights into the confidence level of reconstructing freshwater flux from salinity variations, facilitating the refinement of local Evaporation-Precipitation (E-P) estimates. By incorporating these spatial maps, the estimation of E-P can be enhanced, leading to a better understanding of water cycle dynamics in specific regions. ☐ With deepened understanding on salinity in the surface and upper layer, the second topic of this dissertation focused on the connection between the upper ocean and intermediate ocean layer. The dissertation investigated salinity changes and variations in the Mediterranean Sea and the Mediterranean Outflow Water (MOW). The salinity budget in the Mediterranean Sea is influenced by surface forcing, advection, and mixing, with surface freshwater flux playing a dominant role in salinity tendency variability at different scales. Interannual connectivity between the Mediterranean Sea and MOW is observed, with both water bodies exhibiting similar rates of salinification. ☐ Multiple climate indices, including the El Niño-Southern Oscillation (ENSO), are found to be present in both the Mediterranean Sea and MOW. The ENSO-related signal in the Mediterranean Sea explains a portion of the variance, with a similar correlation observed in MOW, suggesting the propagation of non-stationary ENSO signals into the deeper ocean through density-driven flows. The North Atlantic Oscillation (NAO) also influences the salinity budget in the MOW region, particularly during winter, with a moderate correlation observed in the advection process. ☐ Additionally, the study reveals discrepancies in ocean salinity variability and a significant increase in salinification in the upper ocean since 2015. These spurious signals are prominent in the Tropics and the Southern Hemisphere below 300 meters, extending as deep as 2000 meters. By comparing data-constrained and process-based ECCO estimates, the study further explores the dynamical basis for the substantial increase in salinity after 2015. The findings highlight the limitations of Argo-based data products in explaining these signals and the need for further investigations to improve our understanding of ocean dynamics and enhance the interpretability of observations. ☐ Overall, this dissertation sheds light on the intricate mechanisms governing the ocean's salinity budget, highlights the role of freshwater flux and oceanic processes, and explores the connectivity between different oceanic regions and climate indices. The insights gained from this research contribute to our understanding of the water cycle and its implications for climate variability.
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Keywords
Air-sea interaction, Mediterranean Sea, Ocean dynamics, Ocean mixed layer, Ocean salinity