ASSESSING OCEAN SALINITY AS NATURE’S RAIN GAUGE

Abstract

Changes in the hydrological cycle can have profound impacts on society, from more frequent and severe droughts to extreme flooding. However, monitoring changes in the hydrological cycle is difficult from current observing platforms (namely satellites), and even more difficult to identify secular changes in the often-noisy data. Near-surface ocean salinity patterns mirror the distribution of evaporation and precipitation over the ocean: regions dominated by evaporation are saltier, while regions dominated by precipitation are fresher. Recently, ocean salinity has gained attention as a proxy for tracking global hydrological changes. Altering salinity in the ocean through changes in the hydrological cycle can impact global ocean circulation, which could have major downstream impacts on global climate. Thus, this research focuses on the relationship between changing ocean salinity and the global hydrological cycle. The first chapter provides details on what ocean salinity is, why it is important, how it is measured, and what research gaps are addressed in this dissertation. The second chapter focuses on validating satellite-based surface salinity measurements with in situ observations to ensure that the global signals identified by satellites are reliable. The third chapter examines how surface salinity relates to evaporation and precipitation in the dynamic and climatically important North Atlantic. The fourth chapter leverages a vigorous methodology designed to minimize sampling biases and properly preserve and propagate uncertainties to estimate robust salinity pattern amplifications (salty gets saltier, fresh gets fresher) over short (< 20 years) and long (> 60 years) time periods. We find salinity patterns have amplified at a rate of 4.89% per 50 years over the 1957/61 – 2019/23 pentadal record. Furthermore, we identify a 30–40-year period for when secular changes are identifiable (e.g., salty areas become saltier and fresh areas become fresher), and we detect and quantify an acceleration in the salinity pattern amplifications which may be indicative of an acceleration in the amplification of the hydrological cycle. Finally, the fifth chapter addresses the future work we believe is critical to further our understanding of Earth’s climate through the lens of ocean salinity. One of the most important questions that has evolved from this dissertation is how will salinity impact the Atlantic Meridional Overturning Circulation (AMOC) in a warming climate. We identified changes in observed salinity that may help maintain/enhance the AMOC; however, it is unclear if increased meltwater and hydrologic amplifications will counter those changes. Thus, to address this question we must leverage both numerical simulations and observations. Since these changes are happening on multi-decadal time scales, it is critical that we continue to collect near-global ocean salinity measurements for the foreseeable future.