The Impact of Climate Variability on Internal Tides: Mechanisms and Applications

Author:

Philip Thompson (University of Hawaii, Honolulu)

Co-Investigator(s):

Adam Devlin (University of Hawaii, Honolulu)

Project Summary:

Understanding the connections between climate variability and the internal tide field of the ocean is critical for advancing our knowledge of ocean dynamics with important implications for climate-driven changes in coastal and island regions. This project seeks to uncover these interactions on a global scale by investigating how internal tides are influenced by transient variations in ocean stratification associated with internal (or "natural") climate variability. This is important, because variations in internal tides can cause modulations of coastal surface tides and because internal climate variability also affects mean sea level. Thus, transient climate-driven variations in ocean stratification can lead to simultaneously higher sea levels and greater tidal amplitudes, which produces a compounding effect on the potential for coastal flooding. By combining satellite altimetry, long-term tide gauge records, high-resolution ocean reanalysis data from GLORYS12 and ECCOv4r4, as well as other supporting datasets, we aim to identify the physical mechanisms that connect modes of climate variability to changes in ocean stratification and hence non-stationarity in the internal tide field. Our goal is to connect these processes to major climate modes, such as the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO), providing new insights into how climate variability influences tidal dynamics across the global ocean.

Our approach involves detailed harmonic analyses of major tidal constituents (M2, S2, N2, O1, K1) using tide gauges and satellite altimetry data over the altimetry era (1993-present). Ocean density profiles will be calculated from reanalysis data to diagnose stratification and determine pycnocline depth variability that can modulate generation of internal tides. Empirical orthogonal function (EOF) methods will be applied to identify dominant modes of tidal and stratification co-variability, using regionally adaptive analyses to capture localized effects. We will investigate how climate indices modulate these variations through key physical drivers, such as wind-induced mixing and upper-ocean warming. Finally, we will provide two proof-of-concept applications of this work. The first is to use IPCC climate-model ensembles to understand how projected changes in ocean stratification will affect internal tides with implications for coastal surface tides that connect to flooding risks. The second is to use near-real-time altimetry and the identified modes of co-variability between stratification, MSL, and tides to dynamically adjust traditional harmonic-based tide predictions in the near term. Thus, this research can form the basis for applications that provide actionable insights into future tidal evolution and contribute to coastal risk management, both now and in the future.

This project will advance scientific understanding of the connections between internal tides, ocean density, and transient climate variability. It will inform climate science and risk management and by demonstrating the impacts of an under-appreciated aspect of climate variability on coastal and island regions. It will also improve operational tide predictions by integrating real-time stratification effects into tidal predictions to yield more accurate forecasts of critical water levels. These outcomes align with NASA's Ocean Surface Topography Science Team (OSTST) objectives to enhance understanding of large-scale ocean dynamics and interannual-to-decadal variability, to improve understanding of internal tide dynamics, and to contribute to the development of applications that benefit society.