the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Dec 2023
Observed change and the extent of coherence in the Gulf Stream system
Abstract. By transporting warm and salty water poleward, the Gulf Stream system maintains a mild climate in northwestern Europe while also facilitating the dense water formation that feeds the deep ocean. The sensitivity of North Atlantic circulation to future greenhouse gas emissions seen in climate models has prompted an increasing effort to monitor the various ocean circulation components in recent decades. Here, we synthesise available ocean transport measurements from several observational programs in the North Atlantic and Nordic Seas, as well as an ocean state estimate (ECCOv4-r4), for an enhanced understanding of the Gulf Stream and its poleward extensions as an interconnected circulation system. We see limited coherence between the records on interannual time scales, highlighting the local oceanic response to atmospheric circulation patterns and variable recirculation time scales within the gyres. On decadal time scales, we find a weakening subtropical circulation between the mid-2000s and mid-2010s, while the inflow and circulation in the Nordic Seas remained stable. Differing decadal trends in the subtropics, subpolar North Atlantic, and Nordic Seas warrant caution in using observational records at a single latitude to infer large-scale circulation change.
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RC1: 'Comment on egusphere-2023-2963', Christopher Piecuch, 10 Jan 2024
Review of "Observed change and the extent of coherence in the Gulf Stream system" by Asbjørnsen et al. (egusphere-2023-2963)
The authors use available data and model products to quantify correlation between different components of the Gulf Stream system during the past couple decades. The main take-home message is that, during recent interannual and decadal periods, the system shows gyre-scale structure: subtropical circulation features—Florida Current, western boundary current, Oleander Gulf Stream—are correlated with one another to varying degrees, but uncorrelated with elements in the Nordic Seas—Barents Sea Opening, Svinøy, Greenland-Scotland Ridge inflow. The authors also identify potential forcing mechanisms related to large-scale modes of surface climate variation as well as whether these circulation features exhibit significant trends.
This is an excellent paper. The authors study an important question relevant to large-scale observing systems of the North Atlantic. By focusing on particular flow features within the Gulf Stream system, rather than overturning streamfunction, the authors provide a valuable new perspective on the meridional coherence of North Atlantic Ocean circulation—a topic of longstanding interest. The paper is well-written and clear, the reasoning is logical, and the conclusions follow naturally from the results. From what I can tell, the methodology and analysis are scientifically sound.
I found very little (if anything) to criticize here. My minor comments are given below. The paper should be suitable for publication after minor revisions.
Congratulations to the authors on a very nice study
Best,
Chris Piecuch, Woods Hole
* References to "the Bahamas" (lower-case "t") should be changed to "The Bahamas" (upper-case "T")
* I found this sentence near lines ~90-95 confusing: "along a transect from New Jersey to Bermuda (Rossby et al., 2005). The ADCP measurements reach 250-400m depth for the 1992-2004 period, and 500-600m from 2005 and onwards (Sanchez-Franks et al., 2014)." To me, it reads like the ADCP measurements are made *over the range* 250-400 m for 1992-2004 and *over the range* 500-600 m from 2005 onwards. Rather, I think what the authors mean is that the ADCP measures velocities *down to* 250-400 m and *down to* 500-600 m from 2005 onwards. I suggest to clarify.
* ~Line 134 "... ocean bottom pressure FROM GRACE AND GRACE-FO"
* I think the figure reference on line 142 should be to Figure 4?
* It'd be informative to explain how the authors compute "Florida Current Transport" in ECCOv4-r4 and how thy distinguish it from the total western boundary current transport. The resolution of ECCOv4-r4 is very coarse, and the model bathymetry in that region is heavily modified, such that the "Florida Straits" in the model are much broader than in reality, and the depiction of The Bahamas very unrealistic.
* Can the authors explain why they normalize the ECCOv4-r4 and observational time series for comparison? I think the result would be more powerful if the authors didn't divide by the standard deviation.
* Minor point on ~Line 205 and Table S1. I suggest to place the correlation values in the lower-left part of the matrix rather than the upper-right so that it's easier for the reader to go back and forth comparing to the corresponding values in Table 2.
* ~Line 235 the authors should also cite work by Lobelle et al. on this point (https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2020GL089974)
* Line 241 table -> tables
* Line 259ff and Figure 5. The authors' analysis here, being based on correlation coefficients, only considers in-phase or anti-phase relationships between transport and sea-level pressure. But earlier, the authors talked about the various mechanisms that mediate the ocean response, which include processes that impart lags and phase differences between forcing and response. Are the authors confident that figures like Figure 5 entirely capture relationships between transport and sea-level pressure? It may be informative also to consider the relationship between transport and the Hilbert transform of sea-level pressure.
* Line 259 and elsewhere (e.g., Table 2). The authors refer to their analysis as measuring "coherence" between different quantities. This isn't strictly true. Coherence is a measure in the frequency domain. I suggest the authors adjust the language to more clearly say theirs is a correlation analysis not a coherence analysis.
* Line 279. indecies -> indices
* Discussion section. The paper focuses on volume transports. But, with the exception of coastal sea level, what we really care about from a climate perspective is the transport of heat and other tracers like carbon. Based on their results, can the authors briefly speculate on the potential meridional coherence of heat transport or transport of other tracers?
Citation: https://doi.org/10.5194/egusphere-2023-2963-RC1 - AC1: 'Reply on RC1', Helene Asbjørnsen, 09 Apr 2024
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RC2: 'Comment on egusphere-2023-2963', Anonymous Referee #2, 12 Mar 2024
Comments on "Observed change and the extent of coherence in the Gulf Stream system" by Asbjørnsen et al.
In this manuscript, the authors present new evidence of a lack of coherence between the subtropical gyre, subpolar gyre and Nordic Seas in the North Atlantic, while also discussing possible trends in the strength of the circulation. They take advantage of the observational data available between 24N and 75N, also relying on models. Instead of focusing on AMOC transports, the authors have provided results using components of the Gulf Stream System along the three gyre structures. Moreover, they propose mechanisms of interannual to decadal variability linked to atmospheric forcing.
The authors have presented a well written paper with a clear methodology. Their new approach using certain components of the upper ocean circulation instead of the AMOC integrated view is of high interest for understanding the meridional coherence of the North Atlantic circulation.
As a result, I find this manuscript should be suitable for publication after addressing some minor comments given below.
- AC2: 'Reply on RC2', Helene Asbjørnsen, 09 Apr 2024
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