| 24 Jun 2026
Fine-scale Thermohaline Stratification in the Near-Surface Layer Under Weak Wind Conditions with Indications of Salt Fingering
Abstract. The near-surface ocean regulates air–sea exchange of heat, momentum, and gases, while its fine-scale thermohaline structure remains poorly characterized, particularly under weak wind conditions in tidally influenced shelf seas. High-resolution Lagrangian observations of temperature and salinity in the upper two meters of the German Bight (North Sea) are presented, acquired during a period of weak winds and strong solar radiation. Two minimally invasive Lagrangian surface drifters equipped with a vertical sensor chain enabled continuous measurements within the same water mass, avoiding ship-induced disturbances and resolving the temporal evolution of near-surface stratification in a tidally energetic environment.
During the calm period, a pronounced diurnal warm layer developed, with temperature differences of up to 2.5 °C over less than two meters. Concurrently, a distinct salinity anomaly emerged, characterized by higher salinity at 0.55 m compared to 1.75 m depth. Despite these pronounced thermohaline gradients, the water column remained statically stable throughout the observation period, as indicated by the density structure and consistently positive buoyancy frequencies.
Temperature and salinity exhibited variability on timescales of seconds to minutes, indicating the presence of fine-scale processes such as shear-induced interleaving and intermittent vertical motions operating within an otherwise stable near-surface layer. Diagnostics based on the Turner angle and density ratio further suggest conditions favorable for salt-finger-type double-diffusive processes during the calm phase. A comparison with a one-dimensional water column turbulence model shows that while the model reproduces the bulk evolution of the diurnal warm layer, it does not capture the observed fine-scale thermohaline variability.
These observations demonstrate that the near-surface layer in tidally influenced shelf seas can exhibit complex and rapidly evolving thermohaline structures under weak wind conditions and high solar radiation. The results underscore the importance of high-resolution Lagrangian measurements for characterizing near-surface processes and for improving the representation of air–sea exchange with fine-scale processes in coastal and shelf-sea environments.
The paper deals with observations by two drifters instrumented in the top 150 cm in a very low wind environment during a few hours with heat flux estimates from a nearby ships. Comparisons are made with a one-D numerical model simulations.
Major comments
1: The drifters are equipped with sensors first along the hull of the drifters (near 20 cm and 40 cm), thus influences by possible mixing/circulation along the hull, and then with sensors along a thin line. The T, C sensors are measured by RBR Brevio at 0.55 and 1.75 m. These sensors are designed to be used as a CTD during CTD casts (w close to 1 m/s) and not necessarily as is here. At 55 cm, the sensors are at a level of rather high vertical T gradient during the low wind morning period. I wonder what kind of noise, biases (due to differences between the measured T, compared to the ‘effective’ T in the volume of the inductive cell in which C is measured) this could induce when estimating S at such high frequency and strong vertical gradients (‘average’ gradients are up to ~0.5°C in the 15 cm between 40 and 55 cm, at time according to figure 4; thus a mismatch of 0.05°C is probably in the range of what is likely, and that could induce the kind of noise/variability in the 55 cm salinity, as well). Also, the high frequencies of S can certainly not be taken at face-value as the T measurements is not made at the same location (horizontally) as the C one, and one needs to know what frequency range can be considered as accurate in S.
I strongly believe that to be more convincing on the double diffusive regime, there needs to be tests on what the RBRbrevio (used in the same vertical configuration as shown here for the 55 cm sensor) measures in such an environment (against maybe salinity samples from a pumping line at the same depth). This is typically the kind of environment where micro T-C cells (such as on some turbulence sensors) are used in profiling mode -not such a macro-instrument !This is not to say that there cannot be higher near surface salinity due to differential advection, as suggested here, but I don’t find the evidence supported enough, unfortunately.
I think that the statement as is reported here is too strongly worded, based on what remaining uncertainties there are on the fluxes and the model formulation which could induce deviations from the observed profile… (even if it is just sheared horizontal advection, as is argued for the source of S variability, but could also influence T). In particular I am not fully convinced by the fit of the observed T profile to the shortwave penetration parameterization on line 185, which I find a rather rough estimate, as we don’t know what the real absorption is (pigments, etc…) and, as this is just based on fitting the temperature profile (which could be affected by the mixing processes, including double diffusion, or horizontal advection).
Also, notice the uncertainty of not having the T-profile in top 20 cm, and thus difficulties of having the proper surface boundary conditions, and the whole profile. I was actually wondering whether the top drifter structure in the presence of horizontal current shear would not induce a little bit more of mixing. I am wondering about that to some extent because of figure 4, and the rather homogeneous signals at 20 and 40 cm depth until 12:30 ‘and the strong gradient with the next level at 55 cm(and with S) which is below the surface buoy (same for Figure S3). Also, this make me wonder what are the parameters that really enter in the COARE algorithm, which is not that well-tuned for strong near surface T-gradients with very weak winds (I am thinking to the warm layer, more than the skin effect, albeit if there are slicks, this could also be non-obvious…), but that’s probably a minor effect, compared to the huge incoming short-waves during the early low wind period.
Do you have some idea of what is the resulting uncertainty associated with it. I am aware that this is certainly not easy to assess, but maybe some assumptions can be made to give a range of its effects.
3: l. 213: mention is done on ‘pure surface current’ pathway. I find that ambiguous, as we are in certainly stratified near-surface. Thus, there is certainly a near-surface vertical shear, even with this low wind (diurnal jets, and wind is not that low, for it not to partially set, at least based on the S1 photographs and the wind values reported). Do you think that the buoy drift might be representative of a drift at a level close to 20-cm depth in this low wind environment, or something slightly deeper? That shear might be larger than the direct wind slip. (afterward, I have read the 4.1 discussion, but this does not fully answer this important question)
4: l. 390-400: I am surprised that salt fingers extend over such a vertical scale exceeding 1-m in this kind of environment (what is mentioned is that this is supported by the temperature of Fig. 4, but it might seem less clear from figure 6e, 6f?). There is a large temperature diffusion that would need to occur, due to the high thermal stratification. I wonder what kind of horizontal scale would be involved (and how compatible it is with possible vertical shear in the surface warm layer (NSL)? (to state it shortly, I doubt that this is possible over such a thick stratified and possibly sheared layer)
Minor comments:
Interestingly figure S2 shows quite a lot of jellyfish. Do you think that these organisms (their density seems non-negligible) could induce vertical mixing (or heat transport) through their horizontal/vertical motion, or are they mostly passive at this time of day. I am aware that the photograph might not be the ‘average’ situation observed, but an extreme (is that the case?)
Is time in Figure 3, local sun time, or some other time (GMT?)
l. 238. I would fully expect a delay in the response of enhanced vertical mixing to the surface wind increase: time to set the short surface waves, and their breaking or interaction with near surface currents. (I would roughly expect 15 minutes for that, and there are some papers on this non-stationary situation and time to set the wind sea that you could refer to). Notice that as far as I know, this is also something not included in GOTM.
l. 353: I don’t believe that the heat is vertically distributed by turbulent mixing across the entire thickness of the DWL in all other studies with instances of low wind. See for example, the study by W. Asher, A.T. Jessup and D. Clark, 2014, JGR: Oceans, DOI: 10.1002/2014JC009808
Allthough heat and temperature budget are not explicitely discussed in this paper, which was dedicated to salinity surface increase in a low wind high radiative heating situation, you can see the high T-stratification right to the surface (for example diff of C between 10 and 20 cm; and the fact that S daily increase is found at 10 cm, but very minimal at 50 cm)
l. 385: is there some evidence from the gradient between the two drifters of horizontal T-gradients indeed at short spatial scale. timing of peaks/intermittency is different on the two drifters (at the beginning near 11:15, the data (if one trusts them) would suggest a lower S(buoy 2) at 55 cm than at buoy 1 (but also a higher T at buoy 2; by the way, this is among the features which made me a little worried about the potential T mismatches with the ones in the conductivity cell). Wind at this initial time seems from the south or south-east. Would it bring saltier or fresher surface water? (any idea on that?)
l. 272: ‘… suggests that…’
l. 332: ‘in terms of double-diffusive parameters’.
l. 434: I am not sure that the low modeled SST (that T at 20or 40 cm on the buoys) is necessarily the result of the surface slicks for the reasons outlined on line 431-432 (we are not talking of the same levels, I fear…). I would assume that turbulence would be low in this low-wind morning situation even without slicks, and that if it happened, it would not reach below 20 or 40 cm (the surface heat loss is not that large in all cases)
l. 543: in the reference ‘and R. Carpenter, J.’ includes an extra initial?
l. 555: the reference is incomplete.