Anti-phased steric and mass cycles and an unresolved dynamic trend in Arabian Gulf sea level

Abstract. The Arabian Gulf is a shallow, evaporative marginal sea whose sea level has been characterised almost entirely from tide gauges and satellite altimetry; its steric and ocean-mass components have never been separated observationally, because the basin has no continuous hydrographic record. We combine DUACS satellite altimetry with CSR and JPL GRACE/GRACE-FO mascon solutions over April 2002 to December 2025 to close the Gulf sea-level budget at seasonal and multi-year timescales. The seasonal cycle of total sea level (annual amplitude 4.3 ± 0.3 cm, maximum in November) is shown to be the residual of two larger, nearly anti-phased components: an ocean-mass cycle of 3.9 ± 0.4 cm with a climatological maximum in January and a steric cycle of 7.0 ± 0.3 cm with a climatological maximum in October. The inferred steric cycle is confirmed by two further estimates: a fully independent thermal-expansion calculation from ERA5 sea-surface temperature (7.2 cm, August) and the full-column steric height from the GLORYS12 reanalysis (5.3 cm, August; halosteric contribution 0.6 cm). GLORYS12 assimilates altimetry, so it is independent for the seasonal cycle but not for the trend. These bracket the budget value and bound its systematic uncertainty at about 1.5 cm. For the long-term trend we show that regional gravimetry cannot be used directly: leakage from groundwater depletion in the adjacent Arabian Peninsula and Iran imprints −2.2 ± 0.3 mm yr⁻¹ on Gulf mascon cells, and the two mascon products diverge by 10–15 cm after 2021. Using instead the global barystatic rate (2.1 mm yr⁻¹) and a steric trend of 0.3–0.9 mm yr⁻¹, 2.1–3.2 mm yr⁻¹ of the observed 5.2–5.6 mm yr⁻¹ rise — roughly two-fifths to just over half, depending on the end date (2002–2021 or 2002–2025) and the steric route — is accounted for by neither ocean-mass nor steric change. This implies a Gulf-mean manometric trend of order 4–5 mm yr⁻¹ that regional GRACE cannot independently confirm or refute; we weigh a dynamic, remotely forced origin against two alternatives in the Discussion. Gulf sea level rose in the year following each of the four strong positive Indian Ocean Dipole events of the record, consistent with remote forcing through the Strait of Hormuz.