Temperature-based Diagnosis of the Gulf Stream Path Overestimates its Northward Shift in a Warming Ocean

Garcia-Suarez, Lina; Fennel, Katja; Mehendale, Neha; Kemena, Tronje Peer; Keller, David Peter

doi:https://doi.org/10.22541/essoar.173758192.24328151/v2

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https://doi.org/10.22541/essoar.173758192.24328151/v2

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15 Jul 2025

| 15 Jul 2025

Status: this preprint is open for discussion and under review for Ocean Science (OS).

Temperature-based Diagnosis of the Gulf Stream Path Overestimates its Northward Shift in a Warming Ocean

Lina Garcia-Suarez, Katja Fennel, Neha Mehendale, Tronje Peer Kemena, and David Peter Keller

Abstract. A northward shift in the Gulf Stream (GS) path is considered a fingerprint of a weakening Atlantic Meridional Overturning Circulation (AMOC) and has been linked to recent ecosystem alterations in the northwest Atlantic Ocean. Temperature-based criteria, widely used as proxies for GS location, suggest a northward shift. This study uses high-resolution climate models to show that these criteria, especially those based on the North Wall, overestimate the shift under high-emission scenarios by a factor of two to three. In contrast, a sea surface height (SSH)-based criterion remains more closely aligned with the true GS path, providing a more reliable estimate. The rising seawater temperature biases isotherm-based assessments, creating a misleading indication of a GS migration. These results call into question the notion that warming in the northwest North Atlantic is causally related to a northward migration of the GS and emphasize the need for more robust indicators of its position.

Received: 03 Jul 2025 – Discussion started: 15 Jul 2025

Lina Garcia-Suarez, Katja Fennel, Neha Mehendale, Tronje Peer Kemena, and David Peter Keller

Status: open (until 25 Oct 2025)

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RC1:
'Comment on egusphere-2025-3172', Anonymous Referee #1, 05 Sep 2025 reply
Summary
This manuscript discusses several indicators of the position of the Gulf Stream and finds that those based on following a specific isotherm at a specific depth (i.e., 15ºC at 200 m or 12ºC at 400) become biased northward relative to the maximum velocity core in models under warming scenarios. This result is perhaps a bit obvious, but it is worthwhile to make this point explicitly as well as quantify the size of the bias. The manuscript is clearly written and thoroughly documented. I am not convinced, however, that the location of the maximum velocity core is the best indicator for the “true” latitude of the Gulf Stream (see comment 1) and the statistical tests are not necessarily appropriate to the hypotheses being tested (see comment 2). The second comment should be straightforward to address and it is possible the first can be addressed by providing additional motivation in the introduction.
Specific Comments
The authors motivate interest in knowing the latitude of the Gulf Stream by invoking the Gulf Stream’s connection to the AMOC and the Gulf Stream’s impact on ecosystems. They then adopt the location of the maximum surface velocity (i.e., the SSH front at the surface) as the “true” location of the Gulf Stream, but it is not clear that the location of the SSH front is relevant to the AMOC or ecology. Biology, in particular, is much more sensitive to temperature than SSH or current speed, so the location of the Gulf Stream temperature front (at various depths) is likely more relevant to biology and regional climate than the location of the SSH front. Chi et al. (2019) showed that the two fronts have distinct spatiotemporal variability—especially downstream of the New England Seamounts—such that motion of one is not necessarily correlated with motion of the other. It is likely that the 12 & 15ºC isotherms also become biased north of the location of the temperature front under warming scenarios, but the size of the bias is likely to be different than the bias between these isotherms and the location of the SSH front. In light of these facts, I suggest that the authors provide stronger motivation for their interest in the location of the maximum velocity at the surface.

While both the changes and slopes of the various indicators are all different from each other, it is not clear that these differences are statistically significant. Tables S1 and S2 give p-values for the slopes, but these only indicate that the slopes are significantly different from zero. Since the trends are being compared to each other, rather than zero, it would be more appropriate to give confidence intervals for the slopes so that the reader can assess whether or not the confidence intervals overlap.

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Citation: https://doi.org/10.5194/egusphere-2025-3172-RC1

Lina Garcia-Suarez, Katja Fennel, Neha Mehendale, Tronje Peer Kemena, and David Peter Keller

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Short summary

This study shows that regional ocean warming can make the Gulf Stream appear to shift north, even when its path remains stable in a changing climate. Temperature-based proxies, like the Gulf Stream North Wall, overestimate changes in its position. Methods based on sea surface height provide a more accurate view. These results help improve how we track changes in ocean currents and avoid misinterpreting signs of climate-related shifts.


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