| 24 Sep 2026
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.
The study aims to produce the first observational closure of the Arabian Gulf’s sea‑level budget by separating total sea‑level change into its manometric and steric components at both seasonal and multi‑year timescales. Using DUACS satellite altimetry together with the CSR and JPL GRACE/GRACE‑FO mascon solutions (2002 – 2025), the author quantifies the seasonal cycle, validates the steric estimate with independent ERA5‑SST and GLORYS12 reanalyses, and evaluates the long‑term trend while explicitly accounting for groundwater‑depletion leakage and the limited spatial resolution of regional mascons. The central conclusion is that the modest observed seasonal sea‑level amplitude (≈ 4.3 cm) is the residual of two larger, nearly antiphased signals—a ~3.9 cm mass cycle and a ~7 cm steric cycle, and that roughly half of the observed sea‑level rise cannot be explained by the summed mass and steric trends, highlighting the need for further investigation of dynamic or methodological sources of the residual. Although I agree on the fact that further investigation is needed, I have very strong doubts on the validity of the data considered. First, there is no discussion about the validity of DUACS product in a region where every most of the surface less than 50 km from the coast (Cazenave et al, 2026).
Second, The Unsuitability of JPL CRI and CSR mascons for this region. JPL mascon are built on a native spatial resolution of 3° even though they are delivered on a 0.5° grid. The chosen region is Even the author claims that both trends "are unphysical" (line 302). Yet the authour propose a leakage correction based on forward modelling without any further detail that "its trend is -2.2 +-0.3 mm/yr and a seasonal amplitude of 1.3cm" (line 305). Consequently, the regional gravimetric trend derived from these mascons is unreliable, and the residual should be interpreted as arising from processes that cannot be captured at the mascons’ native resolution rather than from a fundamental flaw in the mascon products themselves.
Moreover, the claim that the seasonal sea‑level signal is a clean residual of an anti‑phased 3.9 cm mass cycle and a 7 cm steric cycle is not robust. The antiphase relationship rests on a steric amplitude that is derived entirely from indirect estimates (ERA5 SST‑based column expansion and the GLORYS12 reanalysis) and is not backed by any in‑situ hydrographic observations; the Gulf lacks an Argo network and has only sparse temperature profiles, so the steric term remains highly uncertain (Sec. 4.2, 4.4).
On top of these points, the paper needs to be restructured, there are several future‑looking statements (“will be addressed in Sect. X”) interleaved with the material they refer to, and large blocks of text contain multiple unrelated topics without clear subsections, making the narrative difficult to follow (see the section 2.2 with links to secs 3.3 and 4.4) or the two subsections with the same title: 3.6 Trend budget and 4.5 The trend budget.
And yet, the author claims the principle to be simple (line 63) but it is not clear to me whether the authour fully understands the complexity of the sea-level budget. I strongly recommend to follow the community standard set out by Gregory et al. (2019), and avoid sentences like line 142 ( A2 comparable to A1) or the wrong assesment in figure 1 ( OBP cannot be defined in emerged land, figure includes GRACE and GRACEFO, and the colorscole is so badly chosen that the region of interest remains between the three colors around 0.