| 12 Jan 2026
Ocean salinity across space-time scales: From water cycle indicator to dynamical driver
Abstract. Ocean salinity plays a complementary role in the climate system: it integrates changes in the global water cycle while also helping drive ocean circulation through its control on seawater density. Salinity has long been viewed as the ocean’s “rain gauge,” a largely passive recorder of surface evaporation, precipitation, and runoff. Yet salinity also shapes the currents and mixing that redistribute heat and freshwater, raising a central question: when does salinity mainly record climate forcing, and when does it actively influence climate dynamics? This review synthesizes two decades of satellite and in situ observations within a regime-dependent framework in which salinity’s function is set by the competition among freshwater forcing, advection, and mixing timescales. At basin scales (>1000 km) over decades, salinity tracks water-cycle change through pattern amplification, with fresh regions freshening and salty regions becoming saltier. At regional to mesoscale (∼10–500 km) and seasonal-to-interannual timescales, salinity traces circulation pathways; subsurface anomalies often reflect subduction and ventilation histories from years earlier. At submesoscales (O(10 km)) and synoptic timescales (hours to days), salinity becomes dynamically active, sharpening density fronts, modulating stratification, and altering mixing in ways that feed back on its own transport and air–sea exchange. Understanding ocean climate response requires resolving regime boundaries where these balances shift. The critical observational gap is global sea-surface salinity at O(10 km), where salinity transitions from passive tracer to active driver yet current satellite products cannot resolve this scale. Observations at regime boundaries would show how water-cycle intensification and ocean circulation changes interact, improving projections of climate change, ocean heat storage and distribution, and ecosystem dynamics at regional and global scales.
This is an interesting and timely paper, summarizing both the past work of Lisan Yu at larger scales, and what happens at shorter scales (~10 km) which is a challenge for future satellite missions, as well as for the observations.
My comments are mostly minor:
Section 3: Interannual time scales. I suggest to start by indicating how interannual is defined here (detrended and deseasonalized is too vague here (it could be both subannual periods or longer periods; also for figure 3, only 10 years are considered: how stable are the statistics presented for Corr Interannual variability ‘p stat’ are probably not enough…)
In the river plume discussion (possibly 4.1.3, it could be good to also cite Olivier et al (2023)
Olivier, L., G. Reverdin, J. Boutin, R. Laxenaire, D. Iudicone, S. Pesant, Paulo Calil, J. Horstmann, D. Couet, J. M. Erta, A. Koch-Larrouy A. Bertrand, P. Rousselot, J.-L. Vergely, S. Speich, M. Araujo, 2024. Late summer northwestward Amazon plume pathway under the action of the North Brazil Current rings. Rem. Sens Res., 307, 114165, ISSN 0034-4257.
This fits well in the ’ tf/tadv ~1 regime (if here one adopts tf on order of month (or months)
This may hint to a sight difficulty of the reading of table 2. Forcing of a river outflow might be of time scales of days, but can also be on the scale of a few weeks, even months (for the Amazone, given as an example later, for example). May be that should be mentioned beforehand. Thus near field will vay in spatial scale depending on the time scale considered. In the table, maybe use for that tf, the symbol to (tau of outflow). This would make the comparison with the previous dscussion in the open ocean easier to follow. In table 4, for subsurface tvmix tadv, I consider that a particular type of subsurfce signal. In the whole subduction regime, this is not so different (well, depends where, I agree).
I suspect there is an ambiguity exemplified on line 421, with mid-field tf is larger. What is meant is that the response time to plume change is on these time scales, not really the earlier river discharge time scale. The ambiguity in the whole par is that tf defined as discharge, which should have same time scale wherever one is… Same issue on line 427… One needs to redefine tf…
4.2.1: not so clear that there is subsurface intensification on Fig. 6 (specially in Atlantic Ocean), whereas it is shifted spatially (OK; in particular in southern Pacific). Wondering whether isopycnal representation would help (instead of Z-representation)
L 568 is awkward: ‘Solar heating preferentially warms colder patches, because…’. The net feedback is not just solar heating (I guess there it was the longwave fluxes (a net cooling term; maybe use ‘radiative cooling’…) that were implied), but in the other terms (sensible and latent heat), and it is not the lower ocean heat content that is in itself the cause of differential heating, but the lower surface temperature… Maybe instead of ‘solar heating’ use ‘solar heat fluxes’ (thi is clear later on in the paper)
l.578-582: however, if one starts from a ‘T-dominated’ front, there is a limit to this ‘stronger frontogenesis’, as when one gets close to pi/4, as salinity gradients increase relative to temperature gradients of these initially ‘T-dominated’ fronts, the horizontal surface temperature gradients tend to 0 , thus weakening the initial frontogenesis. I guess after what you refer to and describe in this paragraph is the descent towards ‘salinity dominated’ fronts…
612 and 614. I understand the difference of what is meant here, but I believe that ‘because forcing persists longer than circulation can export them…’ and ‘lateral transport is faster than changes in forcing’ have many things in common, so one has to be a bit more careful in the separation of time-space to use the two (times and space) in a very clear fashion, which is somewhat ambiguous here.
By line 674, the spatial scale context is also recognized… (which is important)
L 694: ‘mix downward… at the 10 km scale’. Another issue is also the role of these processes in intensifying surface signals (due to its overall influence on stratification)