the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Dec 2022
Observed multi-decadal trends in subsurface temperature adjacent to the East Australian Current
Abstract. Sea surface temperature observations have shown that western boundary currents, such as the East Australian Current (EAC), are warming faster than the global average. However, we know little about coastal temperature trends inshore of these rapidly warming regions, particularly below the surface. In addition to this, warming rates are typically estimated linearly, making it difficult to know how these rates have changed over time. Here we use long-term in situ temperature observations through the water column at five coastal sites between approximately 27.3–42.6° S to estimate warming trends between the ocean surface and the bottom. Using an advanced trend detection method, we find accelerating warming trends at multiple depths in the EAC extension region at 34.1 and 42.6° S. We see accelerating trends at the surface and bottom at 34.1° S, but similar trends at 3 depths in the top 50 m at 42.6° S. We compare several methods, estimate uncertainty, and place our results in the context of previously reported trends, highlighting that magnitudes are depth-dependent, vary across latitude, and are sensitive to the data time period chosen. The spatial and temporal variability in the long-term temperature trends highlight the important role of regional dynamics against a background of broad-scale ocean warming. Moreover, considering that recent studies of ocean warming typically focus on surface data only, our results show the necessity of subsurface data for the improved understanding of regional climate change impacts.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
(10957 KB)
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2022-1336', Anonymous Referee #1, 30 Dec 2022
Observed multi-decadal trends in subsurface temperature adjacent to the East Australian Current
This paper utilizes temperature measurements near the surface of ocean waters to determine whether a trend exists. The East Australian Current is on the western side of the South Pacific subtropical gyre. There have been hints that this current may be strengthening over time. These changes along with the impact of these waters both regionally and for the globe makes this subject important for the scientific community.
Can the authors explain why the two particular sites are chosen? Are they selected because of their measurement quality? Because they are the only sites available? Because they are at particularly important locations? What is the reason?
I see a discussion about the accuracy of sensors that are on each of the observing platforms. But what about systematic biases?
I don’t see a reference or a discussion of figure 3 prior to the appearance of the figure. Perhaps I missed it?
In the second paragraph in Section 3.1, what I think are surprising results are discussed. That the warming rates at deeper layers exceed those at shallower layers. This behavior is discussed in depth in Section 4.1 with plausible physical mechanisms.
Did you mention any QC that is used on these measurements? If that was done, I missed it.
I also wonder how valuable it is to compare against quite old studies (Bindoff 1997 for example). I would think that limiting to studies done in the past decade or so would be wise. Perhaps they are not available?
Citation: https://doi.org/10.5194/egusphere-2022-1336-RC1 - AC1: 'Reply on RC1', Michael Hemming, 17 Feb 2023
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RC2: 'Comment on egusphere-2022-1336', Anonymous Referee #2, 25 Mar 2023
Review of Observed multi-decadal trends in subsurface temperature adjacent to the East Australian Current by Hamming et al.
The present work investigates the depth-structure of multi-decadal ocean warming along the southeastern Australian shelf and assesses how these rates have changed over time. The authors employ several trend detection methods to two long-term in-situ temperature (T) time series, and three shorter-term T time series, that sit inshore of a global “hotspot” for long-term ocean warming along the East Australian Current (EAC).
Whilst previous work has investigated surface T trends inshore of the EAC, this new work investigates the subsurface T trends over the inner- to mid-continental shelf, and here it makes a nice contributrion. I was not previously familiar with the EMMD method, and the authors also provide a great recipe for others to employ the technique, with a particularly thorough methodology section. In these respects, the work is likely to be of broad interest to the oceanographic community.
Overall, the manuscript is of high quality, is well-written and does not have any structural problems. An aspect that I struggle with, however, is the acceleration of warming that the authors refer to throughout. In particular, I question whether there is sufficient statistical support for an acceleration of warming trends, given what seem to be quite large uncertainties in these trends. Revision of the manuscript, focusing on the acceleration aspect(s), is therefore recommended before the work is considered for publication. The issues that need to be clarified by the authors are given below.
Major
1. Section 3.2 presents decadal temperature trends computed from the in-situ T timeseries. The methods used to assess the uncertainties and statistical significance of the trends, calculated from EEMD (L380-402) and TSSE (L122-129), seem appropriate. Multiple reference is also made in this section to finding accelerating warming trends using EMMD (e.g. L156-157, L159, L160-161), and again later in Section 4 (e.g. L215, 243).
The inference that warming is accelerating seems to be (?) based on the magnitude of decadal trends, given in Figure 4, getting larger when computed for more recent decades. However, I struggle to see what statistical evidence there is for acceleration, since you’ve not explicitly tested for a change in the rate of warming.
At the very least, uncertainties are needed on the EEMD decadal trends shown in Figure 4, and some evaluation of these is needed so that the reader can understand whether the trend magnitude during one period (e.g. 2010s) is statistically distinguishable from that during another (e.g. 1950s). After all, you go on to state that there is “high uncertainty” that the warming rate at 2 m at Port Hacking and Maria Island has accelerated over time (L215), but it’s not particularly clear to the reader how large these uncertainties are, or how you’ve reached that conclusion. I note that an uncertainty as large as 0.5 °C decade-1 is given later (L263) for the EMMD trends. Can acceleration of warming between decades actually be distinguished from these in-situ T timeseries, given such large uncertainties in the warming trends themselves?
Minor
2. L145-147. I think (?) you mean that the decadal trend is estimated by taking “…the mean of the first order temporal monthly derivative of the EMMD R(t) for each decade…”, rather than “…the mean of the first order temporal monthly derivative of the EMMD temperature trend for each decade…”.
3. L152. It’s really hard to see the white dashed lines in Figure 3.
4. L160-161. Can you please clarify how you are “detecting” acceleration here? Presumably based on the trend magnitude increasing between decades? This sentence also implies that the acceleration is not statistically significant until the 1990s, but it’s unclear in the manuscript what method you are using to assess this?
5. L247. I’m not sure that Shears and Bowen (2017) “emphasize acceleration”. The decadal trends at Maria Island they present during 1982-2016 are larger than between 1946-2016, but the uncertainties on these trends are large and they overlap with one another. Therefore, it's questionable whether any acceleration is present and/or statistically supported.
6. L355-364. Based on the text in Appendix B, I was confused as to how you extract a final value (e.g. °C decade-1) for the “monotonic trend” of x(t) using EMMD. I think this information is given in the main text on L145-147: it is the mean of the first order temporal monthly derivative of R(t). Please clarify how you end up with trends in °C decade-1 in Appendix B.
Citation: https://doi.org/10.5194/egusphere-2022-1336-RC2 - AC2: 'Reply on RC2', Michael Hemming, 29 Apr 2023
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RC3: 'Comment on egusphere-2022-1336', Anonymous Referee #3, 28 Mar 2023
The paper entitled, "Observed multi-decadal trends in subsurface temperature adjacent to the East Australian Current" uses subsurface temperatures to show warming trends in five locations along the east Australian coastline. The use of methods other than linear trends to look at the datasets allows for a more robust assessment of warming trends in the East Australian Current. Overall, the paper is well presented and comments are given to help avoid confusion for the reader.
General comments:
Section 2.1 - the paper makes use of a number of different instruments and datasets with varying instrument accuracies. However it is not clear how these different instrument accuracies are accounted for and whether these may or may not impact the overall trend analysis.
The acronyms are unnecessarily complicated. The author also uses the acronym and the full name for the sample site interchangeably within the text.
Line 129 - avoid confusion by renaming the combined trend method after one of the trends used.
Line 149 - "expect trends to be sensitive to the time period choice" - do you show this anywhere in the paper by way of explanation?
The Discussion would be easier to understand if you follow the same format as your Results section. It reads a little disjointedly. Section 4.2 would probably fit better within the Methods section. There is also a repetition of information through the Discussion section (e.g. line 274-279).
Line 300 - You refer here to "Sydney" which is not noted on your Figure 1, nor previously in text. Please amend as not all your readers will be local.
Specific comments -- in general the paper is well written and so there are not many issues grammatically.
Line 8 - "at in the top 20 m" ?
Line 21 - comma after "(...2019)"
Line 73 - "...starting in the 1940/50s to present"
Line 126 - Finish sentence after (Hamed and Rao, 1998), Start new sentence with "As..."
Citation: https://doi.org/10.5194/egusphere-2022-1336-RC3 - AC3: 'Reply on RC3', Michael Hemming, 29 Apr 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2022-1336', Anonymous Referee #1, 30 Dec 2022
Observed multi-decadal trends in subsurface temperature adjacent to the East Australian Current
This paper utilizes temperature measurements near the surface of ocean waters to determine whether a trend exists. The East Australian Current is on the western side of the South Pacific subtropical gyre. There have been hints that this current may be strengthening over time. These changes along with the impact of these waters both regionally and for the globe makes this subject important for the scientific community.
Can the authors explain why the two particular sites are chosen? Are they selected because of their measurement quality? Because they are the only sites available? Because they are at particularly important locations? What is the reason?
I see a discussion about the accuracy of sensors that are on each of the observing platforms. But what about systematic biases?
I don’t see a reference or a discussion of figure 3 prior to the appearance of the figure. Perhaps I missed it?
In the second paragraph in Section 3.1, what I think are surprising results are discussed. That the warming rates at deeper layers exceed those at shallower layers. This behavior is discussed in depth in Section 4.1 with plausible physical mechanisms.
Did you mention any QC that is used on these measurements? If that was done, I missed it.
I also wonder how valuable it is to compare against quite old studies (Bindoff 1997 for example). I would think that limiting to studies done in the past decade or so would be wise. Perhaps they are not available?
Citation: https://doi.org/10.5194/egusphere-2022-1336-RC1 - AC1: 'Reply on RC1', Michael Hemming, 17 Feb 2023
-
RC2: 'Comment on egusphere-2022-1336', Anonymous Referee #2, 25 Mar 2023
Review of Observed multi-decadal trends in subsurface temperature adjacent to the East Australian Current by Hamming et al.
The present work investigates the depth-structure of multi-decadal ocean warming along the southeastern Australian shelf and assesses how these rates have changed over time. The authors employ several trend detection methods to two long-term in-situ temperature (T) time series, and three shorter-term T time series, that sit inshore of a global “hotspot” for long-term ocean warming along the East Australian Current (EAC).
Whilst previous work has investigated surface T trends inshore of the EAC, this new work investigates the subsurface T trends over the inner- to mid-continental shelf, and here it makes a nice contributrion. I was not previously familiar with the EMMD method, and the authors also provide a great recipe for others to employ the technique, with a particularly thorough methodology section. In these respects, the work is likely to be of broad interest to the oceanographic community.
Overall, the manuscript is of high quality, is well-written and does not have any structural problems. An aspect that I struggle with, however, is the acceleration of warming that the authors refer to throughout. In particular, I question whether there is sufficient statistical support for an acceleration of warming trends, given what seem to be quite large uncertainties in these trends. Revision of the manuscript, focusing on the acceleration aspect(s), is therefore recommended before the work is considered for publication. The issues that need to be clarified by the authors are given below.
Major
1. Section 3.2 presents decadal temperature trends computed from the in-situ T timeseries. The methods used to assess the uncertainties and statistical significance of the trends, calculated from EEMD (L380-402) and TSSE (L122-129), seem appropriate. Multiple reference is also made in this section to finding accelerating warming trends using EMMD (e.g. L156-157, L159, L160-161), and again later in Section 4 (e.g. L215, 243).
The inference that warming is accelerating seems to be (?) based on the magnitude of decadal trends, given in Figure 4, getting larger when computed for more recent decades. However, I struggle to see what statistical evidence there is for acceleration, since you’ve not explicitly tested for a change in the rate of warming.
At the very least, uncertainties are needed on the EEMD decadal trends shown in Figure 4, and some evaluation of these is needed so that the reader can understand whether the trend magnitude during one period (e.g. 2010s) is statistically distinguishable from that during another (e.g. 1950s). After all, you go on to state that there is “high uncertainty” that the warming rate at 2 m at Port Hacking and Maria Island has accelerated over time (L215), but it’s not particularly clear to the reader how large these uncertainties are, or how you’ve reached that conclusion. I note that an uncertainty as large as 0.5 °C decade-1 is given later (L263) for the EMMD trends. Can acceleration of warming between decades actually be distinguished from these in-situ T timeseries, given such large uncertainties in the warming trends themselves?
Minor
2. L145-147. I think (?) you mean that the decadal trend is estimated by taking “…the mean of the first order temporal monthly derivative of the EMMD R(t) for each decade…”, rather than “…the mean of the first order temporal monthly derivative of the EMMD temperature trend for each decade…”.
3. L152. It’s really hard to see the white dashed lines in Figure 3.
4. L160-161. Can you please clarify how you are “detecting” acceleration here? Presumably based on the trend magnitude increasing between decades? This sentence also implies that the acceleration is not statistically significant until the 1990s, but it’s unclear in the manuscript what method you are using to assess this?
5. L247. I’m not sure that Shears and Bowen (2017) “emphasize acceleration”. The decadal trends at Maria Island they present during 1982-2016 are larger than between 1946-2016, but the uncertainties on these trends are large and they overlap with one another. Therefore, it's questionable whether any acceleration is present and/or statistically supported.
6. L355-364. Based on the text in Appendix B, I was confused as to how you extract a final value (e.g. °C decade-1) for the “monotonic trend” of x(t) using EMMD. I think this information is given in the main text on L145-147: it is the mean of the first order temporal monthly derivative of R(t). Please clarify how you end up with trends in °C decade-1 in Appendix B.
Citation: https://doi.org/10.5194/egusphere-2022-1336-RC2 - AC2: 'Reply on RC2', Michael Hemming, 29 Apr 2023
-
RC3: 'Comment on egusphere-2022-1336', Anonymous Referee #3, 28 Mar 2023
The paper entitled, "Observed multi-decadal trends in subsurface temperature adjacent to the East Australian Current" uses subsurface temperatures to show warming trends in five locations along the east Australian coastline. The use of methods other than linear trends to look at the datasets allows for a more robust assessment of warming trends in the East Australian Current. Overall, the paper is well presented and comments are given to help avoid confusion for the reader.
General comments:
Section 2.1 - the paper makes use of a number of different instruments and datasets with varying instrument accuracies. However it is not clear how these different instrument accuracies are accounted for and whether these may or may not impact the overall trend analysis.
The acronyms are unnecessarily complicated. The author also uses the acronym and the full name for the sample site interchangeably within the text.
Line 129 - avoid confusion by renaming the combined trend method after one of the trends used.
Line 149 - "expect trends to be sensitive to the time period choice" - do you show this anywhere in the paper by way of explanation?
The Discussion would be easier to understand if you follow the same format as your Results section. It reads a little disjointedly. Section 4.2 would probably fit better within the Methods section. There is also a repetition of information through the Discussion section (e.g. line 274-279).
Line 300 - You refer here to "Sydney" which is not noted on your Figure 1, nor previously in text. Please amend as not all your readers will be local.
Specific comments -- in general the paper is well written and so there are not many issues grammatically.
Line 8 - "at in the top 20 m" ?
Line 21 - comma after "(...2019)"
Line 73 - "...starting in the 1940/50s to present"
Line 126 - Finish sentence after (Hamed and Rao, 1998), Start new sentence with "As..."
Citation: https://doi.org/10.5194/egusphere-2022-1336-RC3 - AC3: 'Reply on RC3', Michael Hemming, 29 Apr 2023
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Michael P. Hemming
Neil Malan
Amandine Schaeffer
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(10957 KB) - Metadata XML