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https://doi.org/10.5194/egusphere-2024-2848
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the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/egusphere-2024-2848
© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
24 Sep 2024
| 24 Sep 2024
Status: this preprint is open for discussion.
An evaluation of the Arabian Sea Mini Warm Pool's advancement during its mature phase using a coupled atmosphere-ocean numerical model
Abstract. A coupled atmosphere-ocean numerical model has been used to examine the relative contributions of atmospheric and oceanic processes in developing the Arabian Sea Mini Warm Pool (MWP). The model simulations were performed for three independent years, 2013, 2016, and 2018, through April–June, and the results were compared against observations. The model simulated sea surface temperature (SST) and salinity bias were less than 1.75 oC and 1 psu, respectively; this bias was minimal in the MWP region. Moreover, the model simulated results effectively represented the presence of the MWP across the three years. The mixed layer heat budget analysis indicates that the net heat flux raised the mixed layer temperature tendency of the MWP by a maximum of 0.1 °C/day during its development phase. The vertical processes, thereafter, exerted a cooling impact of −0.08 oC/day in the temperature tendency, causing it to dissipate. Further, four sensitivity numerical experiments were performed to investigate the comparative consequences of the ocean and atmosphere on the advancement of the MWP. The sensitivity experiments indicated that pre-April ocean conditions in years with a strong MWP result in a 136 % increase in MWP intensity in years when MWP SST was close to climatology, which shows the primary role of oceanic preconditioning in determining MWP strength during strong MWP years rather than the atmospheric forcing. However, once the oceanic preconditions are met, the atmospheric conditions of weak MWP years lead to an 82 % reduction in MWP intensity relative to normal years, highlighting the detrimental impact of atmospheric forcing under such circumstances. Additionally, atmospheric conditions, particularly wind, are critical in influencing the spatial evolution and dissipation of MWP in the SEAS by modulating vertical processes. A wind shadow zone, characterized by less turbulent kinetic energy that does not exist during weak MWP years, facilitates the spatial expansion of MWP in SEAS during moderate to strong MWP years.
How to cite. Lahiri, S. P., Prakash, K. R., and Pant, V.: An evaluation of the Arabian Sea Mini Warm Pool's advancement during its mature phase using a coupled atmosphere-ocean numerical model, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2024-2848, 2024.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.
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Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor
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RC1: 'Comment on egusphere-2024-2848', Anonymous Referee #1, 27 Nov 2024
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Prasad Lahiri et al. employs the use of a coupled ocean-atmosphere regional model (ROMS + WRF) to examine processes contributing to the mini warm pool in the Arabian Sea. They examine three events of the warm pool in their model and attempt to determine the relative contributions between ocean and atmospheric processes driving the strength and dissipation of these events.
While I find the concept of the study particularly interesting and potentially a good contribution to literature in the future. Nevertheless, significant revisions must be made before this manuscript can be reconsidered for publication.
As detailed throughout the comments below, there is a need to restructure the manuscript text in general to make it clear and concise. There is an extreme overuse of conjunctive adverbs such as “moreover”, “however”, “nonetheless” and many others that is in several cases is erroneous. There are missing references in several parts. The model description and validation are lacking. There are also several statements throughout the results section that do not appear to be supported by the figures. For example, the provided plots qualitatively comparing with a buoy do not support statements that the model compares well with the buoy data. Second, the statement on line 296: Figure 8f shows that the net surface heat flux is the dominant term driving the heat tendency with contributions from vertical processes remaining largely the same throughout the time series. It is my impression that several of the comments below can be addressed with a thorough revision of the text.
- Section 2.2: I find the description of the model components lacking and insufficient. For example: no information is given on the time-step of each component, no information is provided on how the open boundary conditions are specified except for the datasets, the horizontal advection scheme used in ROMS, and other important configuration details. Furthermore, the authors jump back and forth describing the model components which is quite confusing. I’m also curious about the selection of nested domains that are not fully overlapping each other between the components (WRF domain 2 does not fully overlap with the ROMS domain). See the references below for examples on complete model descriptions:
- Olabarrieta, M., Warner, J. C., Armstrong, B., Zambon, J. B., & He, R. (2012). Ocean–atmosphere dynamics during Hurricane Ida and Nor’Ida: An application of the coupled ocean–atmosphere–wave–sediment transport (COAWST) modeling system. Ocean Modelling, 43, 112-137.
- Castruccio, F. S., Curchitser, E. N., & Kleypas, J. A. (2013). A model for quantifying oceanic transport and mesoscale variability in the Coral Triangle of the Indonesian/Philippines Archipelago. Journal of Geophysical Research: Oceans, 118(11), 6123-6144.
- Ross, A. C., Stock, C. A., Adcroft, A., Curchitser, E., Hallberg, R., Harrison, M. J., ... & Simkins, J. (2023). A high-resolution physical-biogeochemical model for marine resource applications in the Northwest Atlantic (MOM6-COBALT-NWA12 v1. 0). Geoscientific Model Development Discussions, 2023, 1-65.
- Seijo-Ellis, G. G., Giglio, D., Marques, G. M., & Bryan, F. O. (2024). CARIB12: A Regional Community Earth System Model/Modular Ocean Model 6 Configuration of the Caribbean Sea. EGUsphere, 2024, 1-48.
- Table 1 and sensitivity experiements. Are the open boundary conditions for the first time step also replaced along with the initial conditions? If not, wouldn’t this generate noticeable discrepancies and noise near the boundaries?
- The authors omit important citations in several places, for example no reference is given to the ERA5 and SODA reanalysis datasets or the ROMS and WRF models. These should be cited in the text, not just the Data availability statement.
- ERA5: Hersbach, H., and Coauthors, 2020: The ERA5 global reanalysis. Q.J.R. Meteorol. Soc., 146, 1999–2049, https://doi.org/10.1002/qj.3803.
- SODA: Carton, J.A., G.A. Chepurin, and L. Chen (2018), SODA3: a new ocean climate reanalysis, J. Climate, 31, 6967-6983, https://doi.org/10.1175/JCLI-D-18-0149.1
- 136-137: It would be much clearer and more concise to say: "The first month of each simulation is used for spin-up and not included in the analysis.” On that note, is a 1-month spin-up realistically enough for this case? Are the ocean boundaries nudged or sponge layers used to help with initial noise at the ocean boundaries?
- Line 139: “We named this set of runs the control experiment (CNTRL).” What set of runs? The ones in Prakash & Pant (2017)? If so, the simulations must be described to some extent here, I have no idea what they did there.
- All datasets used should be described under Section 2. Data and Methodology not in the Results section. That includes the validation datasets and description of any processing done to them for the purposes of the validation comparison.
- Lines 190-202: Figure 4 results are described before Figure 2 and 3.
- Figure 4 should have a differences panel like Figures 2 and 3. This is important because there seems to be biases in the magnitude and spatial extent of some features. Current vectors could also be included in the difference panels similar to Figure 4 in Liu et al. (2015).
- Liu, Y., Lee, S. K., Enfield, D. B., Muhling, B. A., Lamkin, J. T., Muller-Karger, F. E., & Roffer, M. A. (2015). Potential impact of climate change on the Intra-Americas Sea: Part-1. A dynamic downscaling of the CMIP5 model projections. Journal of Marine Systems, 148, 56-69.
- Lines 94-: “The simulated SST effectively captured the cold SST along the Somalia coast across all the examined years, firmly aligning with AVHRR SST data (Fig 2). The SST bias remained within 1oC in all three experiments except in the northern Arabian Sea, where a cold bias patch appeared in the model simulated SST.” While the cold-tongue is present in the model, the statement as it is currently written is not supported by Figure 2 which shows some of the largest SST biases occur along the Somalian coast (particularly in 2016). Similarly, there are biases above 1degC (positive and negative) in other parts of the domain, not just in the Arabian Sea. While these biases are likely acceptable (the authors must convince the reader they are), it is still important to recognize them properly.
- Lines 196-198: “This cold bias is attributed to the dry anomalous wind originating from the northwestern region of the South Asian landmass, a pattern also detected in the CMIP models (S. Sandeep & Ajayamohan, 2014).” This doesn’t make sense to me. Have you tested this is true in your model? CMIP models are free-running global climate models while you are running a reanalysis forced model. Unless you tested this is true, there is no reason at all to believe the same bias would exist in your forced simulation. Sandeep and Ajayamohan (2014) show that this SST bias results from biases in the representation of large-scale circulation on CMIP models. Those biases would likely not be present in your model and very constrained in the reanalysis forcing.
- Lines 203-206: The figures in the supplemental materials must have a panel with the differences. The current qualitative comparison does not support the statement that the model results aligned well with the Buoy data. This part of the validation is important as the model must represent the temporal and vertical evolution of properties which are important to the processes the authors aim to understand better.
- Figure S3 SST: shows noticeable difference in the time extent and magnitude of the warm temperatures and the shallowing of colder waters towards the end of the simulation.
- Figure S3 SSS: shows even more noticeable discrepancies between the model and buoy data. The buoy misses a notable high salinity water mass around June 2018. The near-surface distribution of salinity is also quite different.
- Similar comments generally apply to Figures S4 and S5.
- The authors do not provide a reference to the buoy data.
- Lines 206-208: Some additional information would be useful. Is this taking from a single grid point in the model closest to the buoy location? Or is this a horizontal average? If it is a horizontal average, over what region?
- Line 138: Be consistent with the terminology, is it dissipation day or phase. One implies several days the other one a specific day. These terms seem to be used interchangeably throughout the manuscript which is confusing.
- The definition for the mature phase in Lines137-138 say “..is characterized by the day when the sea surface temperature (SST) within the MWP core (shown by the white box in Fig. 1) reaches its highest magnitude in May.” Why only in May? What if it reaches the highest SST within the MWP on June?
- Line 241: the use of “Furthermore” is not correct here.
- Line 242: “…the wind stress over the SEAS remained less.” Less than what? Sentence is incomplete.
- A table detailing the mature day and dissipation day for each event would help the reader. The table can include the threshold used to define each.
- The authors often omits the year when writing dates which makes the reading more confusing that it already is with the back and forth between the different years.
- The domain of interest should be included in ALL figures and panels showing maps like shown in panels c, f and g of Figure 2.
- Figure 6. Since the description of this figure in the text largely relies on the difference in the conditions between the mature and dissipation phases the authors could consider showing a similar figure with the difference between the time periods (i.e. dissipation – mature). This would be much more informative and relatable to the text in most of Section 3.2.
- Line 255-257: “The signal of the southeastward propagation…” Without the surface currents I can’t tell the low salinity signal is propagating southwestward, could as well be that the source of that low salinity is weaker and thus its extent is less.
- Line 271: “However, these components of the net heat flux could not justify the progress of the MWP in the SEAS.” How do the authors reach this conclusion? it seems obvious to me that the latent heat flux is an important contributor to the progression of the MWP: the pattern of the net surface heat flux during the dissipation phase is very similar to the combined patterns of latent and surface heat fluxes. This is not surprising, as wind stress increases it drives evaporation at the surface which acts to cool the ocean surface via release of latent heat. In fact, Figure 8f shows that the heat tendency is largely explained by the net surface heat fluxes.
- Line 292-294: “ In 2018, the net heat flux supplied…” This needs re-writing as it implies this is true for the full time series in Figure 8d which is not.
- Lines 296-300: While vertical mixing has a negative contribution to the heat tendency (i.e. cooling effect) that is true for the full time series. The vertical mixing curve remains largely flat with no noticeable trend driving the large variations shown for the heat tendency. The statement the authors make is only somewhat true for 2018. In 2013 its clear that contributions from horizontal advection and net surface fluxes drive most of the heat tendency during the dissipation phase. In 2016, the net surface heat fluxes also drive the heat tendency during the dissipation phase with the contribution from vertical mixing largely uniform throughout the time series.
- The panels in Figures 9 and 10 need numbering.
- Line 307: “…the intial pre-April ocean temperature…” Is the author referring to the initial condition mean SST? Please clarify this in the text.
- Lines 304-315: The description along these lines is extremely confusing and difficult to follow within the context of what the authors aim to describe. I would encourage rewriting. Start by reminding the reader what the first sensitivity experiment was, describe the results of the experiment, then connect with the results of the control simulation and what the results from the sensitivity experiment mean. Then do the same for the second sensitivity experiment.
- Line 314-315 seem to contradict the statements made before about net heat fluxes and vertical mixing in the control experiment, which I already commented about in #24 above.
- Figure 11: each panel should identify to which experiment is corresponds to. I shouldn’t need to read the caption to find this information.
- It would be more useful and informative if Figure 11 and 8 were combined.
- Line 333-334: “Moreover, the atmosphere was the primary driver of the vertical processes within the mixed layer that lead to the dissipation of the MWP.” I’m not sure what the author means by this. How is the atmosphere the primary driver of vertical mixing processes in the ocean? Furthermore, the net surface heat fluxes are the main contributor to the tendency 11d, with combined contributions from all terms in Figure 11b. Furthermore, as detailed before, the contribution by vertical processes remains mostly the same throughout the time series, so its hard to understand in Fig.11 b and d how vertical processes are the drive the dissipation phase.
- Lines 336-338 seem to me repetitions of Lines 330=331?
- Figure 12-15. Experiment must be identified in the figure in addition to the caption.
- Lines 373-374: shadow seem like an odd way to describe an area of high Ptke. Keep it simple and call it what it is.
- Figures 13-15 are all discussed in a single paragraph. I would recommend combining (by eliminating unnecessary panels). The authors do not describe and/or comment on the panels associated with the thermal buoyancy flux or haline buyoyancy flux driven turbulent energy.
- Line 409: “…revealed that net heat flux is the primary driver of the MWP development..” this statement (which I agree with) is not consistent with statements made in the results section (see comment 24 and others). Specifically line 271.
- Lines 411-413: “However, net heat flux alone did not fully account…” You mixed layer budget (Figure 8) does not support the statement that after the mature phase, vertical mixing in the ocean leads to a rapid dissipation of the MWP, except for 2018 to some extent.
- “Net heat flux” should always be “net surface heat flux”.
- Line 414: “…significantly impacted…” what defines a significant impact in this case? There is no statistical analysis done to determine significance. Furthermore, one resulted in an increase and the other in a decrease. This is rather inconclusive.
- Lines 423-424: “This contradicts previous studies, such as Kurian and Vinayachandran (2007), which suggested that MWP development in May was independent of the pre-April ocean conditions.” This needs further elaboration and comments by the authors. What are the differences between this study and the Kurian 2007 study that may lead to these different results?
- Figure 16 is potentially a nice way to wrap-up the manuscript. But the author make no effort to summarize and describe the formation and dissipation mechanisms of the MWP as shown in the figure. Describe in 1-2 simple sentences the processes as shown in Figure 16.
Citation: https://doi.org/10.5194/egusphere-2024-2848-RC1 - Section 2.2: I find the description of the model components lacking and insufficient. For example: no information is given on the time-step of each component, no information is provided on how the open boundary conditions are specified except for the datasets, the horizontal advection scheme used in ROMS, and other important configuration details. Furthermore, the authors jump back and forth describing the model components which is quite confusing. I’m also curious about the selection of nested domains that are not fully overlapping each other between the components (WRF domain 2 does not fully overlap with the ROMS domain). See the references below for examples on complete model descriptions:
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Latest update: 13 Dec 2024
Sankar Prasad Lahiri
CORRESPONDING AUTHOR
Centre for Atmospheric Sciences, Indian Institute of Technology Delhi, India
Kumar Ravi Prakash
Centre for Atmospheric Sciences, Indian Institute of Technology Delhi, India
Applied Physics Laboratory, University of Washington Seattle, WA, USA
Vimlesh Pant
Centre for Atmospheric Sciences, Indian Institute of Technology Delhi, India
Short summary
The Arabian Sea mini warm pool matures in the southeastern Arabian Sea in May and the influence of the ocean and atmosphere on its formation is debated for the past two decades. Using a coupled numerical model, our study concludes that both the oceanic and atmospheric conditions are necessary for its genesis. For instance, in a robust Mini Warm Pool year, the pre-April ocean condition primarily influences its formation, which is further favored by the presence of a 'wind shadow zone.'
The Arabian Sea mini warm pool matures in the southeastern Arabian Sea in May and the influence...