Intraseasonal Variability in the Persian Gulf Revealed by GRACE and Altimetry

Abstract:

The Persian Gulf is a semi-enclosed marginal sea of the Indian Ocean. It connects to the Arabian Sea through the Gulf of Oman and the Strait of Hormuz. The Persian Gulf has a large coastal population, and is relevant economically and geopolitically, and so it is important to understand sea-level changes in the region. We use satellite data to study intraseasonal sea-level variation over the Persian Gulf during 2002-2015. We interrogate the spatial scales and forcing functions of the variation and its relation to large-scale circulation and climate over the Indian Ocean. Empirical orthogonal function analysis applied to sea-level data from satellite altimetry reveals that the intraseasonal sea-level variation in the Persian Gulf is dominated by a basin-wide, single-signed mode of fluctuation. Maximum covariance analysis applied to altimetry and Gravity Recovery and Climate Experiment (GRACE) satellite retrievals shows that these basin-wide intraseasonal sea-level fluctuations are largely barotropic in nature and coupled to variations in bottom pressure. To interpret the results, we develop a simple linear barotropic theory based on volume and momentum conservation. The theory describes Persian Gulf sea level and bottom pressure in terms of freshwater flux over the region, wind stress along the Strait of Hormuz, and sea level in the Gulf of Oman. To test this theory, we perform a complex multiple linear regression using these regional freshwater-flux, wind-stress, and sea-level quantities as inputs, and Persian Gulf sea level or bottom pressure as outputs. The regression model explains ~70% of the intraseasonal Persian Gulf sea-level variance and ~60% of the bottom-pressure variance. The magnitudes and phases of the coefficients determined from the regression model are consistent with expectations from the simple theory. The Gulf of Oman sea level boundary condition shows lagged correlation with intraseasonal sea level upstream along the Indian Subcontinent, Maritime Continent, and equatorial Indian Ocean. This hints at a large-scale circulation and climate influence on intraseasonal variation of the Persian Gulf mediated by waves propagating along equatorial and coastal waveguides. This study highlights the value of GRACE satellite retrievals for understanding sea level in an understudied marginal sea.