the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 May 2024
Upper ocean changes with hurricane-strength wind events: a study using Argo profiles and an ocean reanalysis
Abstract. As the Earth’s climate is warming, the intensity and rain rate of tropical cyclones (TCs) is projected to increase. TCs intensify by extracting heat energy from the ocean, hence a better understanding of upper ocean changes with the TC passage is helpful to improve our understanding of air-sea interactions during and after the event. This work uses Argo float observations and the HYCOM ocean reanalysis to describe characteristics of upper ocean changes with hurricane-strength wind events. We study the association of upper ocean changes with the vertical structure of the salinity profile before the event (increasing versus decreasing), as well as the contribution of changes in salinity to upper ocean density changes in each case. Results show that in regions where pre-event salinity increases (decreases) with depth, there is a corresponding statistically significant increase (decrease) in upper ocean salinity. Consistent with previous studies, temperature decreases in both regions. As temperature decreases, upper ocean density increases and the increase is larger where pre-event salinity increases with depth. Changes in upper ocean properties (from Argo and HYCOM) are overall consistent with wind-driven vertical mixing of near-surface waters with colder and higher (or lower) salinity waters below. Resulting changes in ocean stratification have implications for air-sea interactions during and after the event, with potential impacts on weather events that follow.
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RC1: 'Comment on egusphere-2024-1202', Anonymous Referee #1, 22 May 2024
The authors examine how pre-existing salinity stratification impacts the vertical redistribution of salinity by TCs and weather events with TC-strength winds. A strength of the paper is the use of both HYCOM ocean reanalysis fields as well as in situ measurements by Argo profiling floats. The authors find that pre-event salinity profiles that increase with depth yield greater near-surface salinity increases and near-surface stratification decreases compared to pre-event salinity profiles that decrease with depth. The analysis and results are potentially interesting and informative to the TC and air-sea interaction communities, but several improvements to the writing and presentation are needed to make the paper suitable for publication.
First, the writing is rather negatively affected by the overuse of parenthetical statements, making it difficult to follow the logic and thought processes of the authors. Examples just within the Introduction can be found on lines 38, 46, 56, 57, 63, and 72, but this practice is pervasive throughout the entire manuscript. In general, text within parentheses should either be omitted, as it is not essential to convey the concept or finding, or should be fully incorporated into the sentence; the latter may require sentence restructuring. There are also several instances of run-on sentences (i.e., lines 60-63) that require attention. Additionally, the use of “condition (counter-condition) to describe state (counter-state)” is discouraged in scientific writing. See the 2010 Eos article by Alan Robock “Parentheses Are (Are Not) for References and Clarification (Saving Space)” for a discussion on this point.
Other points to address before publication relate to the motivation and framing of the paper, analysis methods, the use of supplemental figures to discuss temperature changes, and figure improvements.
Motivation/framing: I could not understand the rationale to analyze TC effects for increasing and decreasing salinity profiles instead of barrier layer presence, and how this classification might be similar to or different than previous work that has focused on barrier layers. To make a case for their approach, the authors should 1) show mean S or S’ profiles for each state and 2) include an analysis of barrier layer presence, and possibly thickness and strength (i.e., stability) as a function of salinity profile. A second point of confusion is related to the inclusion of non-TC weather events with TC wind speeds. These events are included in the analysis, but there is no discussion of their characteristics, prevalence, or distribution relative to TCs. A brief overview of non-TC high wind events should be given in the Introduction, and their distribution should be indicated somehow in Fig 1, or perhaps in a table that lists percentages of TC and non-TC wind events for each region shown in Fig. 1. Finally, their effects on ocean T, S, and potential temperature should be compared and contrasted to TCs, as they aren’t discussed in the latter parts of the paper.
Analysis methods: according to Section 3.2.4 (lines 175-183), T and S anomalies with respect to TCs are computed by subtracting the mean annual cycle of non-TC-classified profiles from TC profiles. First of all, it is not clear how the mean seasonal cycle is computed: is it computed for each 1x1 grid box or averaged over the boxes shown in Figure 1? Second, by subtracting the non-TC mean profile, what’s left is a profile that is potentially unique to the TC environment. Third, I wonder if it would be more informative to subtract the T or S profile averaged from days -2 to 0 from all days plotted in Figure 2 and similar figures. The reason being that these figures show T and S anomalies whose vertical distribution is nearly constant with time, which might simply reflect the classification scheme and not wind-generated vertical mixing effects. Subtracting the pre-event averaged anomaly might show changes in the DEPTH of the anomalies with TC passage, which is the expected signal.
Use of supplemental figures to discuss temperature changes: The authors dedicate a fair amount of text to discussing temperature changes for increasing and decreasing salinity profiles (i.e., lines 195-221, but the discussion refers to two multipanel plots of supplemental figures. I can understand the desire to limit the number of figures, but assessing changes to ocean stratification requires consideration of both salinity and temperature, so it might be better to combine these changes into one plot by adding temperature differences as contours to Figures 2 and 4.
Figure improvements: most figures could be improved through more informative panel labels. For instance, Figure 1 panel labels should include “HYCOM” in panels a and b, and “dS/dz >0” and “dS/dz<0” to help readers understand what they’re looking at (“d” should be partial derivative symbol). Figures 2-5 should include panel labels “dS/dz>0”, “dS/dz<0”, and “difference (a-b).”
This work has the potential to be quite informative to the TC and air-sea interaction communities and I hope that the authors can incorporate some of these suggestions to produce a paper that should be widely read among these communities.
Citation: https://doi.org/10.5194/egusphere-2024-1202-RC1 -
RC2: 'Comment on egusphere-2024-1202', Anonymous Referee #2, 04 Jun 2024
In this paper the authors discussed the trend of ocean salinity and density profiles before and after tropical cyclones using Argo float observations and HYCOM ocean reanalysis. The upper ocean changes and the vertical structures are presented, but all figures are discussing the pre-event salinity increases and decreases with depth without much background information. There is even no example showing how much the pre-event salinity changes. That being said, the results are very well presented and the manuscript is very well written. In my opinion, the paper can be published after addressing a few minor issues.
Here is the list of my concerns:
Line 65. Why do the authors look at the vertical structures of the ocean under tropical cyclone conditions under pre-event salinity changes? The authors should mention and discuss this more explicitly in the introduction. Do you want to investigate the response of the ocean under different initial conditions?
Line 126. Can the authors show an example of increasing and decreasing cases? Why use 50 meters below the mixing layer? How is the mixing layer determined in this work? What will happen if the differences for salinity or density are very small?
Line 145. I think this coordinate system is only used for Fig. 6 and the definition of cross-track angle is confusing. From Hu et al. 2024, isn’t it the shortest distance between Argo float and cyclone track? The word “angle” in Fig. 6 is also confusing because it may refer to some angles between -180/pi to 180/pi. Is there any reason for not using the distance in km or meters?
Line 150. How the Argo data and HYCOM data being used is still confusing to me. It seems that most figures are plotted directly using HYCOM data, but the pre-event conditions are determined based on the Argo floats?
Line 190. More technical details are needed for the statistical analysis. Are pre-TC and post-TC profiles having the same standard deviations?
Figure 2a. Any interpretation of the alternative blue red blue colors between 50 to 200 m from day -1 to day 2? How about the decrease of the density at day 0 in Figure 3a? If Figure 4 and Figure 2 are presenting the same result, why are the Argo data having much stronger increase at day 0?
Citation: https://doi.org/10.5194/egusphere-2024-1202-RC2
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