the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Deoxygenation of the Gulf of Mexico thermocline linked to a decrease in the detachment frequency of Loop Current Eddies
José Gerardo Quintanilla
Juan Carlos Herguera
Julio Sheinbaum
Abstract. This study presents an oxygen time series of the Gulf Mexico deep water region spanning from 2010 to 2019 using data from 6 oceanographic cruises and one ARGO buoy. The data suggest a deoxygenation trend in the thermocline of the Gulf of Mexico. This deoxygenation trend seems to be connected to a reduction in the number of eddies that detached from the Loop Current from 2010 to 2019 observed with altimetry data from the last two decades, and although the average size of the mesoscale structures shows a slight increase, the average detached area per year almost halved from the 2000–2010 decade to the 2010–2020 decade. Using the oxygen measurements and the altimetry data, a simple box model was formulated to reproduce the measured oxygen temporal variability in the GoM main thermocline from 2000 to 2020. The results from the box model suggest that an average detached Loop Current Eddy area of about 90 000 km2 per year is needed in order to maintain constant oxygen levels in the main thermocline waters. This threshold wasn’t reached during the 2010 to 2020 decade and if the LCE detachment area per year continues to decrease in the future, oxygen concentrations in the Gulf of Mexico thermocline might continue to fall with still unknown effects in the ecological web structure at these depths.
José Gerardo Quintanilla et al.
Status: open (until 08 Jun 2023)
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RC1: 'Comment on egusphere-2023-751', Anonymous Referee #1, 14 May 2023
reply
Review comments on manuscript egusphere-2023-751, titled “Deoxygenation of the Gulf of Mexico thermocline linked to a decrease in the detachment frequency of Loop Current Eddies” submitted to EGUsphere by Quintanilla et al.
In this manuscript, the authors studied the deoxygenation in the thermocline of the Gulf of Mexico using data from research cruises and one Argo buoy. Using a simple box model, they came to the conclusion that deoxygenation between 2010 and 2020 is caused by the reduced Loop Current Eddy area based on altimetry data. There are several fundamental issues in this study, which must be addressed before the publication of the paper.
- Oxygen is not a conservative variable in the ocean. Oxygen concentration or oxygen content is strongly influenced by biological processes (production and consumption) and air-sea oxygen flux near the sea surface. Although these two processes mainly influence the oxygen concentration in the upper ocean, which causes strong seasonal variations in the surface mixed layer, they can also influence the oxygen concentration in the thermocline via vertical mixing/diffusion, entrainment, downwelling etc. Horizontal advection, horizontal diffusion and mixing can also change the oxygen concentration. These were all ignored in the simple box model. To use the simple box model, the authors need to prove Loop Current Eddies is the dominant process to supply oxygen in the thermocline and all other processes can be ignored. Otherwise, the proposed box model cannot be used.
- Even though the box model can be used, it takes time for the oxygen in the Loop Current eddies to be mixed into the common water. That time scale needs to be estimated, which should be a function of horizontal diffusion. If the time scale is longer than several years, the model cannot be used directly. A time lag needs to be considered. The deoxygenation may not be observed immediately after the reduction of Loop current Eddy annual area if there is a lag. This is also critical to determine whether the box model is reasonable or not.
- The Argo O2 profiles used in this study were not adjusted/QCed. Based on the Argo QC manual (https://archimer.ifremer.fr/doc/00354/46542/), it may be necessary to correct the oxygen profiles from Argo, at least some quality check/quality assurance should be conducted before those profiles can be used. Data quality assurance is critical to the detection of long-term trends. Non-QC’d data should not be used in the trend study because any shift/bias may lead to inaccurate conclusions.
- Stratification and warming influence on deoxygenation in the thermocline should be considered. There are several recent publications showed recent warming in the GOM (e.g. https://journals.ametsoc.org/view/journals/phoc/51/4/JPO-D-19-0295.1.xml; https://journals.ametsoc.org/view/journals/clim/36/8/JCLI-D-22-0409.1.xml). Warming could cause oxygen solubility changes and stratification changes, which may contribute to the deoxygenation in the Gulf of Mexico. To what extent the deoxygenation in the study region is controlled by warming should be investigated and discussed.
- Seasonal variations of dissoved oxygen in the thermocline needs to be considered. The thermocline can still be influenced by seasonal changes. To determine the long-term trend, the seasonal cycle typically should be removed first from the signal because seasonal variations can be orders of magnitude larger than the long-term trend.
- It seems like in the box model, oxygen in the thermocline is only supplied by the Loop Current Eddies. Is this a reasonable assumption? Let’s assume an extreme case that no Loop Current Eddies are generated in the Gulf of Mexico. Based on the box model, the oxygen concentration in the thermocline will decrease to zero due to oxygen consumption. That’s not true. Because oxygen was measured in other oceans without Loop Current eddies. Surface mixed layer is a major source for oxygen in the thermocline via ventilation, entrainment due to mixed layer seasonal cycle and mixing/diffusion etc. The authors have already assumed deoxygenation in the thermocline by ignoring oxygen exchange between the surface mixed layer and thermocline, which is invalid. At least a two layer model is needed to consider the oxygen exchange between the upper ocean and the thermocline. Maybe the oxygen exchange between the upper and lower thermoclines should be considered as well.
- The possible reasons for the reduction of LCE area after 2010 needs to be further discussed and investigated.
- The data shown in the paper demonstrated strong horizontal difference in dissloved oxygen concentration in the thermocline. Because the cruise data were not always measured in the same locations. Therefore, horizontal difference might cause year to year difference in the average oxygen values if the data are not collected at same stations. The influence of sampling locations should be considered in this study.
Citation: https://doi.org/10.5194/egusphere-2023-751-RC1 -
RC2: 'Comment on egusphere-2023-751', Anonymous Referee #2, 29 May 2023
reply
The authors demonstrated that the Gulf of Mexico thermocline is encountering a deoxygenation trend during the past decade (2010-2019) which is attributed to Loop Current Eddies (LCEs) activities. The findings are derived from observations from 6 oceanographic cruises and one BioARGO float and from a simple box model. Although the studied topic is interesting, I don’t think the manuscript is ready for publication.
(1) The thermocline shall not be demonstrated by potential density anomaly which is used for pycnoclines. If the “thermocline” is considered a studied object, potential temperature anomaly shall be shown and discussed. Otherwise, the authors would like to use “pycnocline” throughout the manuscript.
(2) The findings lack statistical support. The authors separated the manuscript into two main sections. In the first part, they provided evidence of deoxygenation of the gulf waters and changes in LC or LCE activity, while a box model is developed for further mechanistic studies in the second part. However, statistical tools like simple linear regression are lacking. For example, the trend of deoxygenation shall be detected by linear regression (with p values) not by the naked eye as the shown temporal differences in Table 1 are minor than the based values and are closed for different layers and geographical locations. So, the tune “seem to have” (e.g., Lines 212, 217, …) is weak and not convincing at all. Further, statistical evidence or mechanistic analysis is also needed to investigate the relationship between LCE and deoxygenated waters. The authors need to dig deeper when “building” such a linkage.
(3) More data is needed. The authors are studying mesoscale phenomena over a vast ocean basin from 2000 to 2020. So, I don’t think measurements from 6 cruises and 1 ARGO float are enough. I am not sure if there are any other cruise measurements over the south of 25°N, but, at least, there are many BioARGO profiles around the path of LC provided by other archive systems like the world ocean database. The author should try to explore more data.
(4) The temporal scale of interest may need to be reconsidered and clarified. The lifespan of LCE is typically within a year, however, the authors focus more on the decadal changes of LCE and its relationship to the decadal changes in dissolved oxygen concentration in the gulf. I think the relationship between LCE and gulf water deoxygenation should be stronger on an interannual scale. In addition, 20-yr data is not enough to study a decadal phenomenon but is more suitable for interannual or even intra-annual scales.
(5) Even though the authors can statistically “prove” that changes in the gulf DO concentration are related to changes in LCE activity, the study is carried out under a major consumption that other factors that contribute to DO changes like air-sea oxygen exchanges, biological processes in the upper ocean, and DO transports due to advection and diffusion are minor or negligible. To my understanding, it is usually not the case. Contributions by all these factors should be quantified and discussed before the authors can “prove” that LCE activity outcompetes others.
(6) Again, as factors like air-sea oxygen exchanges are ignored in this study, changes in oxygen solubility due to temperature changes are ignored. Many studies indicate that the Gulf of Mexico is warming in recent decades. Decadal changes in water temperature are apparent which will lead to a decrease of oxygen solubility and further water deoxygenation. Thus, the effect of warming water should be discussed.
(7) A simple box model is not enough to reveal the mechanism of how LCE activity affects the water deoxygenation in the main thermoclines as the model neglects or simplifies too much critical processes like air-sea oxygen exchanges, advection, diffusion, and biogeochemical processes. The authors may need to seek another numerical tool like a coupled 3D model to reproduce the oxygen dynamics and LCE activity.
Citation: https://doi.org/10.5194/egusphere-2023-751-RC2
José Gerardo Quintanilla et al.
Data sets
Deoxygenation of the Gulf of Mexico thermocline linked to a decrease in the detachment frequency of Loop Current Eddies José Quintanilla, Juan Carlos Herguera, and Julio Sheinbaum https://doi.org/10.5281/zenodo.7830465
José Gerardo Quintanilla et al.
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