the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Mar 2025
Abrupt meteorological changes reverse thermohaline features in the skin layer
Abstract. This study provides unique data on temperature and salinity anomalies between the skin layer (upper first millimetre) and a depth of 100 cm during abrupt meteorological changes—that is, shifts in air temperature, wind speed, precipitation, and heat fluxes. We determined how these abrupt meteorological changes forced the anomalies and altered the conditions at the air–sea boundary layer during three events monitored by an autonomous surface vehicle. Two events were observed in the harbour of Bremerhaven and one event in the North Sea. The skin layer, which covers the upper millimetre of the sea surface, interacts with the atmosphere, including heat, gas, and freshwater fluxes. The characteristics of the skin layer regulate the exchange of heat and gases between the atmosphere and the ocean. Global climate change increases extreme weather events, highlighting the need for observations during abrupt shifts to better estimate heat flux changes. However, there is a lack of small-scale mechanistic understanding of these fluxes, especially under abrupt meteorological changes, due to observational challenges during stormy conditions in the open sea. Here, we show that the skin layer instantly reacts to abrupt meteorological changes. The average temperature change in the skin layer was almost 50 % higher than that at a depth of 100 cm. An abrupt change in meteorological conditions, shifting the net heat flux from positive to negative, can turn a warm skin layer into a cooler layer compared with the 100 cm depth. The effect of abrupt meteorological changes, including freshwater fluxes, on salinity anomalies was less pronounced in the harbour than in the North Sea event. The current velocities showed that changes in wind direction could alter the surface current direction, and that the backscatter signal consistently reflects wind-induced mixing, with higher backscatter observed during increased wind conditions. This study reveals the complex relationships between atmospheric conditions and oceanic responses and provides valuable information for understanding air–sea interactions and their implications for climate dynamics.
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Status: open (until 22 May 2025)
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RC1: 'Comment on egusphere-2025-1307', Mingxi Yang, 25 Apr 2025
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This paper describes observations of the very surface of the ocean (skin layer as well as the subskin water), especially how temperature, salinity, current velocities and backscattering respond to abrupt changes in meteorology. The measurements were made from a state of the art, autonomous platform HALOBATES, and the intricate details observed are impressive.
However, I feel that as it stands, the paper is rather descriptive, and wish more scientific insights can be teased out from this novel dataset. For example, how good is the state of the art model for capturing the cool skin effect and changes in near surface hydrodynamics? Quantitatively to what extent and how quickly does the near surface hydrodynamics respond to atmospheric forcing? Was the atmosphere also responding to changes in the surface ocean (as might be expected for a coupled atmosphere-ocean system)? Or were you only seeing the atmosphere driving oceanic changes?
Specific comments:
Line 32. Poor grammar: It’s the skin layer that is response, not the characteristics that are responsible.
Line 44. Whether precipitation enhances gas transfer velocity (via mixing induced by falling droplets) or reduces gas transfer velocity (by making the surface ocean more stratified) depends on varies, depending on environmental conditions, I think.
Line 57-68. At a glance, this paragraph says pretty much the same thing as the paragraph above
Line 90. Thickness of the skin layer sampled? Is this a true skin temperature measurement? How can you be sure that the temperature of the sampled ‘skin water’ doesn’t change during sampling (e.g. due to exposure to atmosphere during transfer)? Have you compared this temperature with typical measurements of skin temperature by IR methods?
Line 93. The order of the two numbers are reversed before respectively
Line 107. I don’t think Fairall et al 2003 is the correct reference for COARE3.6
To compute heat flux in the COARE model, there is an option to 1) use subskin temperature measurement and turn on the skin effect, or 2) use skin temperature measurement and turn off the skin effect. Which approach was used? Why not comparing the two approaches?
Line 107 how was shortwave measured?
Figure 1d. one useful output from the COARE model is the cool skin effect, dter (or temperature anomaly here). It would be insightful to compare the modelled cool skin effect and the observation here. I’m a bit surprised that you have a cool skin at all during the first period of this event, given that all the heat fluxes seem positive (ocean heating).
Figure 1f, which component of the longwave heat flux is shown here? I’m somewhat surprised by its small magnitude as well as sign
Here and for other examples, did you have any measurements of the atmospheric boundary layer structure?
Line 185. Do you have further evidence that the increased backscatter is due to deposited particles? I would’ve guessed that increased wind speed led to more wave breaking and production of bubbles near the surface. What does a plot of ‘surface’ backscattering vs. wind speed look like? Does the depth of backscattering-‘cline’ increase with wind speed? Were there measurements of whitecap fraction?
Figure 2. The near surface ADCP measurements are very interesting. Perhaps more can be done with the data. For example how does current velocity and direction change with wind speed and direction? Were there temperature or density measurements over the first 5 m that gives an indication for the degree of stratification?
Section 3.4 I don’t find this section very useful. The equations for estimating heat flux and ocean/atmosphere variables such as temperatures and winds are pretty well known. So of course there will be correlations.
With regard to how in water variables such as temperature anomaly, salinity, and backscatter respond to meteorological variables, one easy and potentially useful analysis may be a lag correlation analysis. Do in water variables respond immediately or is there a short time lag?
Line 358-360. This strikes me as unlikely, as the air temperature was > water temperature (suggesting warmer precipitation temperature), and also the skin salinity > subskin salinity
Section 4.1. This section is very descriptive still and doesn’t really read like a discussion, but more of an extension to ‘results.’ The COARE model is known to be decent at reproducing the cool skin effect. Why not comparing the model vs. observation here and highlight places where the model may be improved? I understanding that the cool skin is dynamic, but does the model can capture the dynamics already?
Line 465. There is an important distinction between the thermal skin (or boundary) layer and the mass skin layer. Because heat diffuses much faster than mass, the thermal skin layer is quite a bit thicker.
Citation: https://doi.org/10.5194/egusphere-2025-1307-RC1
Data sets
High-resolution measurements of essential climate variables in the North Sea from the autonomous surface vehicle HALOBATES during RV Heincke cruise HE609 L. Gassen et al. https://doi.pangaea.de/10.1594/PANGAEA.968800
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