Date of Award

2016

Document Type

Open Access Thesis

Department

Marine Science

Sub-Department

College of Arts and Sciences

First Advisor

Subrahmanyam Bulusu

Abstract

There is evidence that the global water cycle has been undergoing an intensification over several decades as a response to increasing atmospheric temperatures, particularly in regions with skewed evaporation – precipitation (E-P) patterns such as the oceanic subtropical gyres. However, observational data (rain gauges, etc.) can be quite sparse over such areas due to the inaccessibility of open ocean regions. This study utilizes in situ data, reanalysis, and model outputs to infer interannual to decadal scale trends in surface freshwater forcing within remote, evaporation-dominated subtropical regions of the ocean as they pertain to the past and present state of the global water cycle. Emphasized in this study is the importance of utilizing a wide range of ocean parameters to strengthen and validate the inferences made from any one proxy of a given parameter.

A positive trend in sea surface salinity in the subtropical gyres revealed evidence for decadal intensification in the surface forcing of these regions. Zonal drift in the location of the salinity maximum of the south Pacific, north Atlantic, and south Indian regions implies a change in the mean near-surface currents responsible for advecting high salinity waters into the region. Additionally, a comparison of satellite, in situ, and model salinity datasets was conducted to highlight the potential applications of Aquarius and SMOS satellite-derived salinity products over oceanic regions of low observational density.

Spatial and temporal salinity trends in the five subtropical gyre regions were also analyzed over the past six decades, with a focus on the subsurface salinity of the upper 1000 m of the ocean. Our results indicate an overall salinity increase within the mixed layer, and a salinity decrease at depths greater than 200m in the global subtropical gyres over 61 years. Our analysis of decadal variability of depth-integrated mixed layer fluxes into and out of the gyres reveals little change in the strength of the mean currents through this region despite an increase in both the annual import and export of salt in the southern hemisphere gyres. This suggests that the salt content of E-P maximum waters advected into the subtropical gyres is increasing over time.

Another method of monitoring the marine branch of the global water cycle is through measurement of time variable mass over the Earth. We analyzed interannual freshwater fluxes inferred from the GRACE satellite mission and obtained striking agreement with trends observed through salinity and altimetry. The strengths of the mass concentration (mascon) processing technique relative to spherical harmonics is demonstrated. We suggest that discrepancies between sea level based and gravity based mass flux estimates are due primarily to an undersampling of the subsurface ocean, and not attributed to errors in GRACE measurement or retrieval.

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