|Comments on “Role of wind, mesoscale dynamics and coastal circulation in the interannual variability of South Vietnam Upwelling, South China Sea. Answers from a high-resolution ocean model” by Tai To Duy et al. in Ocean Science|
This is the second round review of the manuscrdipt. The authors have tried to addressed my concerns in the last review. Some major comments are still needed to be addressed before being acceptable.
1. Section 2.3:
Regarding the definition of the upwelling index, I have three points of criticism.
a) In lines 229-232, “The threshold temperature below which upwelling happens is defined from the analysis of the occurrence of cold surface water: it is defined as the temperature that allows to cover the largest number of upwelling occurrences but avoids to include cold water advected between areas. We obtain To = 27.6°C”. The authors obtained T0 based on the two conditions: (1) “the largest number of upwelling occurrences” and (2) “excluding advected cold water from other boxes”; however, no further explanation about how they dealt with two two conditions was given. Did the authors conduct any sensitive tests for the value of T0? Da et. al (2019) set up the threshold of T0=27.5 by applying sensitive tests of the frequency of occurrence but did not mention advected water. Did they use the same method or have any improvements?
b) In the revised manuscript, the authors decided to use one Tref for the four boxes of BoxSC, BoxNC, BoxOF and BoxMK to help UIy change continuously in space. And, the authors stated that the difference between the upwelling indices using old and new Tref is small (2%) based on a simple calculation (Tref,new - Tref,old)/Tref,old = (29.2-29.7)/29.7 = -0.017 ~ -2%). I think this argument is not correct. In fact, from Table A of Response, I find that difference between new and old UImean in BoxNC is 19.6% ((0.163 – 0.195)/0.163) and the difference between the standard deviation of new and old UI in BoxNC is 18.6% ((0.118-0.14)/0.118). In sum, the difference between the upwelling indices using old and new Tref is around 20%, not as small as they thought (just 2%).
c) In Response and Section 5.1 of revised manuscript, the authors have addressed reasons why they chose the constant Tref but not the interannually varying Tref: (1) “Tref can be influenced by the advection water from the other box, not only the interannual variation of SST”, (2) “a varying Tref would increase the weak values and decrease the strong values, hence reducing the interannual variability”, and (3) “differences between UIs using constant Tref and interannually varying Tref are small”. I think the main reason of the authors should be the third one and is reasonable; however, I still have two comments on the the choice of Tref.
- First. Tref in SST-based upwelling index represents the reference SST that the upwelling effect on this value is small. That is, the authors should re-consider the location of the reference box if “Tref is still strongly influenced by the advected upwelling water”.
- Second. I don’t agree with the second reason proposed by the authors that “using the interannual Tref will reduce the interannual variability of the upwelling”. The interannual variability of the SCS SST is closely related to the ENSO events. In the cases of summers 2010 and 2018 that the authors had mentioned, summer 2010 was on the decay phase of the El Nino event 2009/2010, while summer 2018 followed the La Niña event 2017/2018 [Liu and Chu, 2019]. This fact explained why SST and Tref in 2018 were cooler than those in 2010, and ENSO is considered the main factor. If the same Tref is used for these years, upwelling is underestimated in 2010 but overestimated in 2018. Therefore, subtracting SST by time-varying Tref can help to eliminate climate variability coexisting in SST and Tref. In addition, due to the fact that the input SST for UI is daily data, using daily Tref consistently could make UI more accurate.
2. Section 3
a) The authors used several sources of data to prove the occurrence of upwelling in BoxMK, and they convinced me. However, there is an issue that the percentage of covering days of JAXA over BoxMK in summer 2018 is low (below 60%), and on some days such as July 16th, the number of valid pixels in the satellite image is inadequate for a significant statistics, making values of minimum SST meaningless in Figure 5b. For this reason, I suggest the authors to use a scatterplot for JAXA as they did in Response, and mark values derived from the data with a low percentage of coverage by another color.
b) Section 3.2: The length of this Section can be optimized by reducing the repetition of information that has been mentioned in the Introduction.
c) The authors presented TS characteristics simulated by SYMPHONIE in the whole Section 3.3 (Line 353-401), with the aim of highlighting the capability of SYMPHONIE in simulating water masses in the coastal areas and open-sea as well. The description did not make any further contribution to upwelling analysis afterwards, which turns this Section into a disconnection message from the general structure of the paper.
3. Section 4
a) The authors used the spatial integral of the positive relative vorticity over BoxOF in JJAS (ζ+) as an indicator of the intensity of the summer circulation (AC/C dipole + eastward jet) in the offshore region. I don’t think this is a good indicator. The negative and positive vortices of a dipole do not always develop in conjunction, so the intensity of a dipole cannot be evaluated just by one side. For example, in the years 2011, 2013, and 2015, values of ζ+ were similar but the dipole evolved in three different patterns. As a result, the direction and intensity of the jet in these years were various.
b) One of the important conclusions (or new findings) is that NCU is not primarily driven by the intensity of the summer wind over the SVU of BoxNC region, which is drawn due to the low correlation coefficient between UI of BoxNC and WS averaged over the same area (I “assume” the area is around [108.5-110E, 12-15N] because no detailed information was provided). Besides some issues in the definition of UI as mentioned above, I can try to figure out certain differences between their results and Ngo and Hsin (2021), which is the study they compared with.
Ngo and Hsin (2021) used the wind-based upwelling index to find that wind stress-induced upwelling-favorable condition changes correspondingly with the change of orientation of the coastline along the southern Vietnamese coast, and the roles of two components of wind stress (WSU and WSV) also change in driving Ekman transport along the coast. In the NCU, the favorable condition of wind stress for the upwelling is mainly contributed by the WSV because the WSV induces Ekman transport pushing water offshore. Whereas, the WSU plays a role in restricting the development of the coastal upwelling (i.e. pulling water onshore). Thus, it appears clearly that the role of wind stress on the coastal upwelling cannot be illuminated if the total wind stress is just considered.
In addition, Ngo and Hsin (2021) analyzed the spatial distributions of correlation coefficients between SSTUI in their NCU with both WSU and WSV over the whole SCS, instead of only presenting the averaged WS over a chosen region, whose size or location is sensitive to the result of correlation analysis. Their results showed that the the SSTUI in the NCU is positively correlated with the local WSV (R=0.4~0.67) and has a negative remote connection with WSV in the southern SCS (R=-0.4~-0.7).
Based on the above two points, I think that the role of wind on NCU has been investigated carefully by Ngo and Hsin (2021) and are more reliable than the the authors’ argument unless the detailed evidence can be provided. Besides, the dissimilarity in the number of upwelling events in NCU (in other upwelling regions as well) also contributes to the variance in results between the two studies.
c) Influences of the alongshore and offshore currents on the NCU have a better presence. The finding that the enhancements of SCU and NCU could relate to the appearances of the secondary dipole east of BoxNC and the associated jet in-between is novel and interesting. However, the condition for the appearance of the secondary dipole and the associated jet is still unclear and needs more quantitative analysis to get a solid conclusion. Besides, the authors used the current in the upper 1m depth for their analysis, which is primarily driven by wind forcing. Can the current at the sea surface demonstrate properly (sub)mesoscale circulation, OIV, or ocean dynamics in the SCS?
d) In the MKU, the authors showed a strong relationship between UI and summer wind and summer offshore circulation based on high correlation coefficients; however, physical mechanisms for this upwelling region is lack.
e) The authors revealed that “the daily to intra-seasonal chronology of wind stress contributes to the summer average of SCU and OFU intensity, and their interannual variability”. I totally agree that but this is not a new finding. The summer SCU or OFU is calculated by averaging the daily SCU or OFU which has a strong correlation with the daily wind stress. Thus, summer SCU and OFU obviously are influenced by daily or intra-seasonal wind stress.
f) By examing three summers of 2009, 2012, and 2018, the authors concluded that the summer OFU is stronger when wind peaks occur during the core of the summer season (July-August) than at the beginning (June) or end (September). My question is whether it true for every year when the summer monsoon matures in July-August?
4. Section 5.2
The authors performed a simulation using summer wind stress in 2018 to exam role of intraseasonal variability of atmospheric forcing on the SVU. I think this part should be presented in another paper with more details on input wind data and the modelling results.
Line 70: “Ngo et al. 2021” should be “Ngo and Hsin 2021”
Line 220-221: “both in observed and simulated …” should be “both in observed and in simulated …”
Line 255: “vs.” is not suitable here.
Line 281: “the SSH temporal average over 2009-2018” should be “the averaged SSH over 2009-2018”
Line 413-414: should be “ … horizontal surface current rotation”
Line 288: remove “very”
Line 291-299: “Under …2000” and “In winter … datasets”. These sentences prolong the paragraph unnecessarily.
Line 327-331: “As explained …Figure 2k-l”. I think this part should be shortened.
Line 424: “….and related to the summer monsoon wind”. This is an obvious argument.
Line 469: “all” should be replaced by “both”
Line 485: “double” should be removed.
Line 485: “classically”. This word is not appropriate when describing the dipole and the eastward jet.