The disappearing Antilles Current dominates the weakening meridional heat transport in the North Atlantic Ocean under global warming

The Antilles Current (AC) off the Bahamas Islands is an important component for both wind-driven and thermohaline circulation system in the North Atlantic. The evolution of AC intensity could exert substantial impacts on mid-latitude climate and surrounding environment. For instance, an anomalous weaker AC is found to decelerate the nutrient transport in the shelf regions, risking the deep-water corals. In addition, a weaker AC could reduce the poleward heat transport of the Gulf Stream and the North Atlantic Drift and further influence the climate in Western Europe. Based on nine high-resolution coupled climate models, we find a 3.8 Sv weakening of the AC, which is equivalent to 63% of its climatology transport during 1950–2050. The deceleration of AC introduces a −0.17 PW of heat transport decrement, dominating the total heat transport change across 26.5° N. Further analysis reveals that change of AC is mainly attributed to the evolution of thermohaline circulation in a changing climate and is partly influenced by wind stress curl in the North Atlantic. Our finding highlights the needs to establish a long-term monitoring network for the AC and a comprehensive understanding of associated impacts.


Introduction
The Antilles Current (AC) is a prominent component of the circulation system in the North Atlantic Ocean, serving as an efficient conveyor of mass and energy between the tropical and mid-latitude regions (Maloney 1967, Costin 1968, Tomczak and Godfrey 2003).Originating from the North Equatorial Current, it flows northward along the eastern coast of the Lesser Antilles and Bahamas Islands.Compared to the surface-intensified Loop Current and Florida Current (FC), AC is characterized by a subsurface core with velocity of 30-40 cm s −1 at 300 ∼ 400 m (Lee et al 1996, Johns et al 2008, Meinen et al 2019, Caínzos et al 2023).Its mean transport is estimated to be 4.7 Sv (1 Sv = 10 6 m 3 s −1 ) and temporal standard deviation is 7.5 Sv at 26.5 • N (Meinen et al 2019).To the north of Little Bahama Bank, it merges with the FC and forms the Gulf Stream (Tomczak and Godfrey 2003).
The anomalous changes in the intensity of AC exert substantial influence in the mid-latitude regions.Based on previous observation, the meridional heat transport (MHT) induced by AC is 0.37 PW (Fillenbaum et al 1997, McCarthy et al 2020) and accounts for nearly 20% of the total MHT by western boundary current (Shoosmith et al 2005, Johns et al 2011, 2023).Acceleration of the AC may lead to anomalous warming in the Northwest Atlantic Ocean (Wu et al 2012, Saba et al 2016) that leads to reduction in Atlantic cod recruitment (Pershing et al 2015).By contrast, deceleration of AC could reduce the poleward heat transport of the Gulf Stream and the North Atlantic Drift, which is important in maintaining the mid-latitude region (e.g. the Western Europe) warm and moist (Kwon et al 2010, Palter 2015).In addition to affecting the downstream regions, variability of AC can also exert direct and substantial impacts on surrounding environment.AC carries a large number of sediments and sufficient nutrients northward (Pelegri andCsanady 1991, Pelegrí et al 1996) that are important in shaping the sediment distribution and in feeding the deep-water corals (Correa et al 2012, Tournadour et al 2015).Its deacceleration would induce sediment redistribution and increase a risk for marine ecosystem.
In response to a warming climate, the western boundary current system in the North Atlantic has been found to undergo a poleward shift (Zhao 2017, Yang et al 2020), featuring an enhanced warming in its extension regions (Wu et al 2012, Cheng et al 2022).However, owing to the limited ocean current observations and coarse resolution of numerical models, the issue of how the intensity of AC and its MHT might change under global warming, which is crucial for comprehending the changes in mid-latitude region and coastal areas, remains unknown (Jackson et al 2016, 2020, Beadling et al 2018, Caesar et al 2018, Lobelle et al 2020, Tsubouchi et al 2021).Here, based on nine high resolution climate models, we show a rapid weakening of AC under global warming, which dominates the weakening oceanic MHT in the North Atlantic.

Model description and validation
To explore the structure and mass/heat transport of the AC, eight high-resolution climate models configured with an ocean grid equal to or higher than 0.25  Good et al 2013), and are spun up of 30-50 years under constant 1950s' forcing.After spin-up, the models are sequentially composed under time-varying historical forcing (historical run; 1950-2014), and the shared socioeconomic pathway 585 forcing (SSP585 run;2015-2050).Besides the two warming scenarios, a control-1950 run is designed in which each model is configured under constant 1950s' forcing during 1950-2050(1950-1990 for FGOALS-f3-H) for FGOALS-f3-H).In this study, monthly outputs (horizontal velocity, potential temperature, salinity and wind stress) of control-1950 run, historical run and SSP585 run during 1950-2050 from these models are used, among which two models with multiple members (HadGEM3-GC31-HM; HadGEM3-GC31-MM) are averaged before using.
Besides the HighResMIP products, monthly model outputs with horizontal resolution of 0. Above all, nine eddy-permitting climate models are involved in the model ensemble and used for explore the structure and transport of the AC.It should be noted that the HighresMIP models depict a relatively short spin-up period for meeting the computing time requirement, especially for the deep ocean (Roberts et al 2019).To eliminate the potential effect of model drift in trend estimation, all trends during 1950-2050 below are subtracted by the trend based on coinstantaneous control-1950 run (PI-CTRL for CESM1.3-iHESP).
Before exploring the change of MHT induced by AC, it is necessary to quantify whether the climate models can accurately simulate its structure and associated dynamical processes.Here, MHT is defined as vertically and zonally integrated temperature flux induced by mean flow as denoted in equation ( 1) where ρ is seawater density, c p is the seawater specific heat, v and θ represent meridional velocity and potential temperature, respectively.H 1 /H 2 and x e /x w denote the limits of integration in vertical and zonal directions, respectively.In observation, FC carries 2.6 PW heat poleward, while the interior ocean return flow depicts a 1.8 PW southward heat transport, consistent with model simulations.Based on the moorings on the Bahamian continental margin, total MHT induced by AC and AMOC bottom branch is estimated to be 0.12 PW (text S3 in supporting information), resembles the model-simulated value (0.13 PW) as well.It is noted that the standard deviation in figure 2(a) is much larger than that in figures 2(b) and (c), which is mainly caused by the highly unstable characteristic of AC.Based on observations, westward propagation of Rossby wave with a speed of 9 cm s −1 is responsible for its large variability (Lee et al 1996, DiNezio et al 2009, Meinen et al 2019).Overall, the high-resolution models reasonably capture the characteristics of AC and the related current systems.In the following sections, we will explore their changes under global warming.

Rapid weakening of AC dominates the MHT evolution
Figure 2(a) shows the trend of meridional velocity along 26.5 • N, in which the FC depict a negative velocity during 1950-2050 with maximum weakening rate of −0.12 m/s/century at sea surface.The weakening of the FC is also robust in observation, with a 1.2 Sv reduction in the past 40 years (Dong et al 2019, Piecuch 2020, Piecuch and Beal 2023), which is similar with the model simulated value in the same period (1.5 Sv).The AC also depicts a decelerating trend from surface to the depth of 1200 m, with a maximum weakening rate of −0.05 m/s/century.In addition, the FC reduces by 13% with a 4.2 Sv reduction (figure 2(b)).Despite the mean volume transport of the AC is one order of magnitude smaller than the FC, it experiences a 3.8 Sv decrement during one century (equivalent to 65% of its climatology mean) and contributes almost half of total upper western boundary volume transport change (figure 2(b)).By contrast, both the bottom branch of the AMOC and the return flow of the inner basin depict positive trends, compensating the reduced poleward volume transport induced by upper layer western boundary transport.In addition, the weakening of AMOC and the rapid decreasing AC can also be inferred from AMOC index and reanalysis data (text S4, figures S3 and S4).

Associated changes in wind forcing and overturning circulation
The above discussion highlights the dominant role of AC in MHT decrease across the North Atlantic under global warming.Two potential processes that could contribute to evolution of the AC are the wind field (Stommel 1948, Sen Gupta et al 2021) and thermohaline circulation intensity (Thomas et al 2012, Meinen et al 2019, Asbjørnsen andÅrthun 2023).The intensity of wind stress curl (WSC) significantly impacts the barotropic stream function of general calculation across the North Atlantic (Townsend et al 2000).The weakening thermohaline circulation under global warming results in the rapid reduction of both the surface Gulf Stream and the thermohaline circulation bottom branch (Chen et al 2019, Bryden et al 2023).As a separated branch of Gulf stream system, the AC is also supposed to weaken (Thomas et al 2012, Asbjørnsen and Årthun 2023).To explore the role of wind and thermohaline circulation intensity in governing the strength and heat transport of the AC, we proceed to evaluate their changes during 1950-2050.
According to the wind-driven circulation theory and previous studies, the total transport of the western boundary current is primarily determined by the negative WSC over the subtropical North Atlantic (figure 3 Regarding thermohaline circulation intensity, the zonal mean stream function across the Atlantic Ocean reveals a latitude-dependent pattern, with a maximum value over 25 Sv located at 30 • N, 1000 m (figure 3(c)).During 1950-2050, a 10%-20% decline of thermohaline circulation intensity is detected throughout the basin (figure 3(d)), consistent with previous study based on high-resolution models (Roberts et al 2020).Overall, a weakening wind stress curl and thermohaline circulation are detected in the North Atlantic during 1950-2050 based on multimodel ensemble.To compare the impacts of wind forcing and thermohaline circulation on the change of the AC, we perform a bilinear regression of AC transport on both thermohaline circulation stream function at 1000 m depth and the Sverdrup transport across 26.5 • N, which is expressed by the following equation: (2) Here, V Antilles , V thermocline and V Sverdrup indicate the anomaly of volume transport of AC, thermohaline circulation stream function and Sverdrup transport inferred from the wind field, while C A , C S represent the regression coefficient on thermohaline circulation and Sverdrup transport, respectively.
Figure 4 illustrates the results of bilinear regression of AC transport.It is found that the thermohaline circulation depicts a significant linear relation with AC (figure 4(a)), while the scatter plot of Sverdrup transport is rather chaotic.The regression coefficient of AC transport on thermohaline circulation is 0.78 ± 0.16 (figure 4

Summary and implications
Based on nine high-resolution models, this study highlights the role of AC in volume transport and MHT changes in the North Atlantic Ocean under global warming.In spite of its smaller climatology volume transport compared to that of FC, the weakening of AC (3.5 Sv) during 1950-2050 contributes 46% of western boundary volume transport change, equivalent to the evolution of FC.Besides, the AC also contributes over 65% (−0.17 PW) of the total MHT weakening in the North Atlantic along 26.5 • N, which is twice of that from FC.Further analysis reveals that the weakening of AC is strongly linked to the evolution of thermohaline circulation whereas the wind field plays a secondary role.
Our discovery in this study suggests profound implications in surrounding environment.A reduced nutrient supply east of the Bahama could induce anomalously low-nutrient conditions, as seen when an anomalous reduction in AC could severely affect deep-water corals in that region (Pelegri and Csanady 1991, Correa et al 2012, Tournadour et al 2015).In addition to local effect, weakened MHT of the Gulf Stream and the North Atlantic Drift is likely to occur and further limits the latent heat release in the midlatitude region (Minobe et al 2008, Kwon et al 2010, Palter 2015), deteriorating the climate in Western Europe.Therefore, our findings highlight the need for a comprehensive assessment of the impacts associated with the evolution of the AC off the Bahamas Islands.
Figures 1(a) and (b) present the ensemble-mean background northward velocity and volume transport across 26.5 • N based on control-1950 run.The AC velocity core locates at 400 m depth with northward velocity of 0.3 m s −1 , while the largest velocity is found at the Florida Channel with a value larger than 1 m s −1 , in consistence with previous observations (figure 1(a); Lee et al 1996, Johns et al 2008, Meinen et al 2019, Caínzos et al 2023).The corresponding volume transport of FC and AC are

Figure 1 .
Figure 1.(a) Time-mean meridional velocity (m s −1 ) and (b) volume transport (Sv) for the period of 1950-2050 based on high-resolution model ensemble.(c) Time-mean meridional temperature flux (vθ; • C m s −1 ) and (d) MHT (PW) during 1950-2050.The four black boxes in (a) and (c) mark the FC, AC, AMOC bottom branch and RF.The error bars in (b) and (d) denote the inter-model standard deviation.The velocity and temperature flux in RF region in (a) and (c) are artificially multiplied by 10 to show the value.

Figure 2 .
Figure 2. (a) Linear trend of meridional velocity (m/s/century) and (b) volume transport (Sv/century) for the period of 1950-2050 based on high-resolution model ensemble.(c) Linear trend of meridional temperature flux ( • C m/s/century) and (d) integrated heat transport (PW/century) during 1950-2050.The black dots in (a) and (c) denote that the trend is statistically significant above the 95% confidence level estimated by Student's T-test.The four black boxes in (a) and (c) mark the FC, AC, AMOC bottom branch and RF.The error bars in (b) and (d) denote the 95% confidence interval estimated by Student's T-test.The velocity and temperature flux trend in RF region in (a) and (c) are artificially multiplied by 10 to show the value.
Accompanied by the velocity, MHT of the currents also experiences significant changes, especially within the upper-layer western boundary (figure2(c)).During 1950-2050, MHT induced by FC reduces by 0.09 PW (figure 2(d)).Surprisingly, despite of its small background transport and MHT, deacceleration of AC lead to a 0.17 PW MHT reduction, which dominate the value of total MHT change across 26.5 • N and is 1.5 times the value of FC.This phenomenon may be attributed to the faster warming near the surface ocean than the ocean below (Durack et al 2014, Desbruyères et al 2017, Li et al 2020, Cheng et al 2022, Todd and Ren 2023).With large amount of heat injection into the current, MHT reduction associated with the decreased volume transport of FC is largely balanced by an increased potential temperature (1.89 • C/century; 0-100 m; equation (1)).In comparison, the warming rate in the vicinity of subsurface AC (300-400 m) is much smaller (0.65 • C/century), leading to a much weaker compensation effect of temperature and more significant MHT weakening rate for the AC.Similar to the FC, although the RF weakens dramatically, the change of MHT in the mid-ocean remains insignificant due to the compensation effect of enhanced surface heat injection (Wang et al 2016).As for the AMOC bottom branch, its MHT change is also negligible due to the low potential temperature at depth.
(a); Stommel 1948, Pedlosky 1987).Its spatial mean value across 26.5 • N is −7.1 × 10 −8 N m −3 , equivalent to a 17 Sv Sverdrup transport (integral between 75 • W and 22.5 • W based on control-1950 run; extreme values induced by boundaries and islands are omitted) which corresponds well with the total western boundary transport (18 Sv).During 1950-2050, the WSC depicts a positive trend and induces a weaker vorticity input (figure 3(b)), indicating a slow-down of subtropical gyre-circulation.The century-long trend of WSC between 1950 and 2050 depicts a value of 4.4 ± 2.2 × 10 −9 N m −3 /century (4.4 represent the trend value and 2.2 represents the 95% confidence interval estimated by student's T-test), consistent with the observed and simulated weakening of westerlies over the subtropical North Atlantic Ocean (Wang et al 2016, Chen et al 2019).
(a); 0.78 represent the trend value and 0.16 represents the 95% confidence interval estimated by student's T-test).Based on the regression analysis, a 4.2 Sv reduction of thermohaline circulation (indicated by the transport change of thermohaline circulation bottom branch shown in figure 2(b)) may lead the AC transport to diminish by 3.2 Sv, accounting for 80% of the total AC decrement.The mechanism for the thermohaline circulation to regulate the AC could be addressed to thermocline adjustment through boundary Kelvin wave.Previous studies have pointed out that the anthropogenic freshening signal will continue in the 2020s (Buckley et al 2023, Haine et al 2023) then enhance the stratification in the subduction zone.This phenomenon will further block the deep ventilation and the formation of North Atlantic Deep Water (Srokosz et al 2023), resulting in a deepening of the interface on the western boundary relative to the eastern boundary in the North Atlantic.The interface deepening signal propagates southward along the western boundary of the North Atlantic via coastal Kelvin waves and finally reaches 26.5 • N within 3 months (Sun et al 2020; figure 1 in Sun and Thompson 2020).To confirm whether the southward coastal Kelvin waves could influence the AC, we estimate the correlation between thermocline depth in the vicinity of AC and volume transport of thermohaline circulation.Here, depth of thermocline is defined by a representative potential temperature isotherm of 15 • C (Moum and Osborn 1986, Fiedler Paul 2010, Wang et al 2015).As is shown in figure S5, thermocline depth in AC is negatively related to the transport of thermohaline circulation, indicating the interface deepening signal induced by reduction in deep water formation can propagate equatorward along

Figure 4 .
Figure 4. (a) Scatter plot of the AC transport and thermohaline circulation transport, where colors in dots represents the chronological order from 1950 to 2050.Thick black line denotes the 2-dimensional projection of the linear regression relationship described in equation (2).(b) Is similar with (a) but for the relationship between AC transport and Sverdrup transport.The dashed line indicates the linear regression does not pass 95% student's T-test.