Warming and salinity changes of the upper ocean Caribbean through-flow since 1960

Gradone, Joseph C.; Miles, T. N.; Palter, J. B.; Glenn, S. M.; Wilson, W. D.

doi:10.1038/s41598-025-05494-z

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Published: 02 July 2025

Warming and salinity changes of the upper ocean Caribbean through-flow since 1960

Joseph C. Gradone¹,
T. N. Miles¹,
J. B. Palter²,
S. M. Glenn¹ &
…
W. D. Wilson^3,4

Scientific Reports volume 15, Article number: 23157 (2025) Cite this article

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Physical oceanography

Abstract

The Caribbean through-flow (CTF) is a vital component of Earth’s climate system, facilitating and impacted by heat and salt fluxes from major circulation systems like the North Atlantic Subtropical Gyre (NASTG) and Atlantic Meridional Overturning Circulation (AMOC). Here, we show significant changes have occurred in upper ocean water mass properties of the CTF since 1960, including subsurface warming of ~ 0.2 °C decade⁻¹, surface freshening of ~ 0.13 g kg⁻¹ decade⁻¹, and subsurface salinification of ~ 0.05 g kg⁻¹ decade⁻¹. In the upper 0–200 m, temperature and stability increases are nearly 3 and 20 times larger than globally averaged trends, respectively, with implications for tropical cyclones, sea level rise, and marine ecosystems. We show these upper ocean changes are likely impacting water mass formation in the NASTG, thereby indirectly influencing the AMOC. These findings highlight the CTF as a bottleneck for climatically important water masses and emphasize the need for sustained subsurface observations here.

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Introduction

The oceanic conditions of the Caribbean Sea have profound impacts on the broader Intra-Americas Sea region, driving tropical cyclone generation and rapid intensification as well as sea level rise, both which are increasingly devastating coastal communities¹. The main circulation in the Intra-Americas Sea region, the Caribbean through-flow (CTF) (Fig. 1), is an important component of Earth’s climate system as it organizes and transports significant heat, salt, and freshwater fluxes from two major circulation systems, the North Atlantic Subtropical Gyre (NASTG) and the Atlantic Meridional Overturning Circulation (AMOC). As these flows combine, the resulting CTF and its properties are exported through the Yucatán Straits to feed the Florida Current and eventually forms an important component of the Gulf Stream. Though historical literature highlighting the importance of the Florida Current in carrying NASTG and AMOC transport northward over the past several decades has progressed our understanding of ocean circulation^2,3,4,5,6, this is largely due to the ease of measuring the Florida Current concentrated in the Florida Straits between the Florida and the Bahamas. However, the structure and magnitude of the Florida Current water mass transport are determined by processes that mix and modify water masses from the North and South Atlantic in the Caribbean Sea. Moreover, mass conservation requires that the sum of the CTF inflow pathways from the Atlantic through the complex bathymetry of the Antilles Island arc is equal to the transport of the Florida Current. In this sense, it is the CTF that represents a chokepoint for climatically important water masses.

Significant paleoceanographic evidence exists supporting this linkage between upper ocean water mass conditions in the CTF and both NASTG and AMOC strength^{7,8,9,10,11,12}. This connection is largely due to the impact of water mass origin, as North Atlantic Water, representing NASTG recirculation, and South Atlantic Water, representing northward AMOC return flow, have distinctly different temperature and salinity properties and communicate signals from both circulation systems. Water mass analyses have found the majority of the South Atlantic inflow is concentrated in the Caribbean’s southeastern Windward Island Passages^2,13 while the northeastern passages exhibit stronger inflow from North Atlantic Water^13,14. The generally accepted mean of ~ 17 Sv of South Atlantic Water needed to balance the AMOC deep southward interior flow^15,16 are believed to enter the Caribbean through the island passage of the Antilles Arc^2,3,13,14,17 and then contribute to the ~ 30 Sv transport through the Yucatán Straits¹⁸, representing the termination of the CTF. From this perspective, the CTF lies at a significant nexus point for heat, salt, and freshwater fluxes in the upper limb of the AMOC and NASTG¹⁹.

Recent evidence indicates the NASTG is changing^20,21,22,23 and the AMOC may be slowing^24,25,26,27. As the AMOC transports ~ 25% of the northern hemisphere’s northward atmospheric-ocean heat transport at 26.5° N²⁸, changes in this inter-hemispheric conveyor belt of ocean heat would lead to a profound reorganization of the modern climate system including impacts to both ecosystems and human health, livelihoods, and food and water security globally^29,30. Coastal communities, including those throughout the Caribbean, could experience accelerated regional sea-level rise, marine heatwaves, and extreme storms³¹, making them particularly susceptible to changes driven by these large-scale circulations. One of the most concerning teleconnections between oceanic conditions and climate processes is the coherent multi-decadal coupling between AMOC strength and Atlantic tropical cyclone activity³². While scientific understanding of the CTF was advanced substantially by ship-based campaigns in the 1990’s^2,4,5,6,13, the lack of a comprehensive, focused investigation into the CTF in the 2 decades since, especially considering the changes in Earth’s climate in the twenty-first century, represents a limitation in our ability to understand and decipher climatic variability in this important region and its larger scale impacts.

Here, we reveal rapid, multi-decadal temperature, salinity, and stratification trends in the CTF by combining all available temperature and salinity profiles collected in the Caribbean since 1960 from the EN4³³ database augmented with missing historical ship-based campaigns and autonomous underwater glider deployments, resulting in over 28,000 profiles (see “Methods” section). We also utilize subsurface velocity fields from the Copernicus Marine Environment Monitoring Service’s 1/12° Global Ocean Physics Reanalysis (GLORYS) to translate these trends in water mass properties to heat, salt, and freshwater transports, which allows for a more direct analysis of the role of these changes on the circulation of the greater Atlantic (see “Methods” section). This analysis reveals significant changes occurring in the water mass properties of the CTF. We provide evidence for changes in the CTF component of the heat, salt and freshwater transport at this latitude with magnitudes large enough to likely impact water mass formation in the NASTG, thereby indirectly influencing the AMOC. We discuss additional implications for these changes including local and regional sea-level rise, increasing oceanic conditions promoting tropical cyclone activity, and impacts to biological processes and productivity.

Results

Observed changes in water mass properties

For each profile in our combined dataset, we calculate anomalies by computing and subtracting maps of all-time mean temperature, salinity, and density profiles on depth levels (see “Methods” section). The resulting time series shows a statistically significant warming trend from the surface to 150 m with a mean trend of 0.19 ± 0.05 °C decade⁻¹ (Fig. 2). Warming peaks in the upper 5 m with a trend of 0.23 ± 0.1 °C decade⁻¹. Cooling does not occur at any depth. The mean upper 150 m warming trend is 1.3 ± 0.1 °C since 1960. The vertical structure of the decadal trends in salinity shows more nuance. The upper 5 m show a statistically significant freshening trend of − 0.13 ± 0.05 g kg⁻¹ decade⁻¹ whereas a statistically significant salinification trend occurs from 70 to 225 m with a mean trend of 0.035 ± 0.009 g kg⁻¹ decade⁻¹. The salinification peak at 125 m of 0.05 ± 0.009 g kg⁻¹ decade⁻¹ coincides with the depth of the salinity maximum waters. Mean salinity has increased by 0.2 g kg⁻¹ in the 70–225 m layer since 1960. In combination, these temperature and salinity trends result in a statistically significant mean density reduction of − 0.08 kg m⁻³ decade⁻¹ in the 0–60 m layer (Fig. 2). Below 60 m, temperature and salinity changes are density compensated. The density reduction peaks in the upper 5 m with a trend of − 0.17 ± 0.06 kg m⁻³ decade⁻¹. Mean density in the upper 5 m has decreased by 1.1 kg m⁻³ since 1970.

Our finding of a warming trend of 0.15 ± 0.06 °C decade⁻¹ in the upper 500 m is ~ 3 times greater than the globally averaged trend from 2006 to 2013 in the upper 500 m from Argo profiles³⁴. However, our finding of a warming trend of 0.18 ± 0.04 °C decade⁻¹ in the upper 250 m is approximately 1/3 of that observed from 2001 to 2023 in the Gulf Stream at the same depth³⁵. As the CTF constitutes the source waters for the Gulf Stream, the trends reported here both confirm prior evidence of enhanced warming over subtropical northwest Atlantic^35,36 and highlight potential processes occurring between the CTF and the Gulf Stream as important for further accelerating upper ocean warming (e.g. the Loop Current, Gulf of Mexico, etc.).

The statistically significant salinification trend in the 70–225 m range reported here is consistent in both sign and magnitude relative to observed salinity changes in the Florida Current³⁷ between 1982–1987 and 2001–2015 and Gulf Stream³⁵ from 2006 to 2013. For instance, the 0.16 ± 0.02 g kg⁻¹ salinification observed in the Florida Current at the depth of the salinity maximum (~ 125–150 m) agrees well with the 0.12 ± 0.02 g kg⁻¹ reported here over a comparable time and depth range. Due to the stark trend at these depths and the CTF’s strong connection to the subtropics where these subsurface salinity maximum waters are formed, we investigate this salinification further over yearly and decadal timescales (see “Methods” section). This focused examination of the salinity maximum waters confirmed a significant decadal salinification trend on the order of 0.05 ± 0.006 g kg⁻¹ decade⁻¹ (Fig. 3). This salinification trend is similar to that observed towards the formation region of these salinity maximum waters in the NASTG³⁸.

Increasing upper ocean stability and tropical cyclone heat potential

The observed changes in the water mass properties of the CTF reported here have resulted in changes to water column vertical structure and stability that have both local and global implications. Figure 2 also shows the vertical structure of the decadal mean and trend in buoyancy frequency. There is an increase in upper ocean stratification in the upper ~ 200 m as evident by the buoyancy frequency increase with a mean trend of 1.2 ± 0.93 × 10⁻⁵ s⁻² decade⁻¹ here. This trend is nearly a factor of 20 larger than the 1960–2018 global trend of 6.9 ± 4.4 × 10⁻⁷ s⁻² decade⁻¹ estimated from EN4 data over the same depth range³⁹. This dramatic increase in upper-ocean stability is likely due to the surface layer warming and freshening in concert with the subsurface salinity maximum waters salinification that increases the vertical density gradient of the upper ocean.

Potential energy anomaly (PEA) is an alternate measure of stratification and can be viewed as a measure of the energy required to fully mix a water column with a given vertical density gradient (see “Methods” section). Figure 3 shows a statistically significant increase in PEA in the 0–200 m layer with a mean trend of 27.13 ± 8.5 J m⁻³ decade⁻¹, which equates to an increase of approximately 123 J m⁻³ since 1960. We calculate PEA over the upper 200 m as this layer captures the net effect of warming and freshening the surface layer as well as the salinification of the subsurface salinity maximum on the water column stability.

The CTF is known to establish oceanographic conditions that promote tropical cyclone rapid intensification, such as warm, freshwater barrier layers that suppress upper ocean mixing^40,41. Therefore, the increasing the stability of the upper ocean in the CTF is particularly concerning considering the comparable rise in Tropical Cyclone Heat Potential (TCHP)⁴². The upper ocean warming has driven a decadal increase in TCHP with a mean trend of 4.7 ± 1.4 kJ cm⁻³ deacade⁻¹, which equates to a mean increase of approximately 31 kJ cm⁻³ since 1960. Further, upper ocean Turner angle⁴³ values in the upper 50 m have shifted from positive (temperature dominant contribution to stratification) to negative (salinity dominant contribution to stratification) since 1960. This change in the driver of stratification implies the CTF has shifted to a regime more dominated by surface freshwater barrier layers⁴⁴.

Changes in heat, salt, and freshwater transport

Because the Yucatan Straits (Fig. 1) act as a bottleneck for water masses in the Caribbean, exporting all ~ 30 Sv of the CTF northward^18,45, we are able to place the observed changes in temperature and salinity in the context of the circulation of the greater Atlantic by deriving trends in heat, salt, and freshwater transports relative to the net transport (see “Methods” section). GLORYS velocity fields are used to balance the mass flux of the Yucatan Straits with the mass flux along 21.5° N across the Atlantic interior (Supplementary Fig. 4). We apply the same methodology applied to the EN4 profiles across the CTF to all available EN4 profiles across 21.5° N to calculate corresponding decadal mean temperature and salinity profiles for the Atlantic interior. Then, with mass flux balanced, these decadal mean temperature and salinity profiles can be used to derive net meridional heat, salt, and freshwater transports.

There is significant uncertainty involved with the calculation of Atlantic interior decadal mean temperature and salinity profiles, largely due to poor data coverage in earlier decades in the open Atlantic. Further, the GLORYS reanalysis data used to compute volume transport only spans the period of 1993–2023 which adds an additional layer of uncertainty in using this dataset for transport calculations decades prior to this period. To overcome these challenges, we employ a Monte-Carlo simulation to propagate the uncertainty in these derived quantities to obtain a more robust estimation of heat, salt, and freshwater transports (see “Methods” section).

The Monte-Carlo simulations revealed a likely mean net heat, salt, and freshwater transport for the Yucatan Straits/21.5° N of 1.27 ± 0.11 PW, 52.5 ± 17.0 Sv, and − 0.009 ± 0.013 Sv, respectively (Supplementary Table 1). The net heat transport estimate here agrees well with the 1.2 ± 0.12 PW estimate at the 26.5° N RAPID array²⁸. Further, ocean reanalyses have suggested the peak net northward ocean heat transport occurs around 15° N with values exceeding 1.3 PW frequently⁴⁶. While we refrain from discussing any trends in the net transports revealed by the Monte-Carlo simulations, we assert the Yucatan Straits component of the net transports provide a valuable comparison point that allows for placing the trends in CTF water mass properties discussed here in the context of the circulation of the greater Atlantic.

As the mass transport of the Yucatan Straits contributes 88–100% of the source waters for the Florida Current transport⁴⁵, the Yucatan Straits component of the net heat, salt, and freshwater transport can be viewed as analogous to the Florida Current component of the greater RAPID array. Results from the Monte-Carlo simulations revealed the heat, salt, and freshwater transport of the Yucatan Straits is likely 84% ± 14%, 85% ± 15%, and 67% ± 11% of the Florida Current, respectively. The Monte-Carlo simulations also revealed the decadal trends in CTF water mass properties are driving decadal trends in the heat, salt, and freshwater transport of the Yucatan Straits that are statistically different from zero, implying a likely increase in these metrics. In light of our inability to conclusively determine trends in the net transports, we assert the likely increase in Yucatan Straits transport over the 1960–2023 time period relative to the net transport is a meaningful metric. In the following section, we show the likely 5%, 3%, and 2% increases in heat, salt, and freshwater transport of the Yucatan Straits relative to the respective net transports (Supplementary Table 1) are quite large and likely impacting the circulation of the greater Atlantic.

Discussion

Global processes driving recent rapid regional climate change are becoming well documented in many oceanographic regions^47,48,49. The CTF water mass property changes documented here expand the growing evidence connecting accelerated regional climate change with large-scale processes. We have divided the discussion below into two sections where we (1) highlight the implications of these changes on the Caribbean Sea locally and then (2) discuss the likely drivers of these water mass property changes and their relation to and impact on the circulation of the greater Atlantic.

Caribbean sea-level is the most directly impacted metric by the water mass property changes detailed here. Long-term sea-level rise trends and projections in the Caribbean Sea have been largely attributed to upper ocean steric variability^50,51 and rapid rates of sea-level rise in the Caribbean Sea from 2010 to 2016 have been attributed to an increase in upper ocean temperature and salinity⁵². While the vertical structure of these previously documented water mass changes agrees with the trends observed here, the contribution from surface freshwater loss differs from the decadal trend. Interestingly, however, our analysis shows a brief increase in surface salinity during the 2010’s decade, despite the overall freshening. Subsurface warming and redistribution of ocean mass onto the continental shelf was recently revealed as the dominant mechanism behind an acceleration in sea-level rise trends in the Gulf of Mexico⁵³. In addition, accelerated sea-level rise trends along the U.S. east coast south of Cape Hatteras was shown to be caused by a 0.2 °C year⁻¹ warming of the Florida Current⁵⁴. As the CTF forms the source waters for the inflow to the Gulf of Mexico and the Gulf Stream, the sea-level rise impacts from these water mass trends could also be expected downstream.

The CTF warming and resulting increase in TCHP is particularly concerning considering the accompanying increase in the stability of the upper ocean. Mean CTF TCHP has increased to well over the 60 kJ cm⁻² threshold suggested for hurricane intensification^55,56,57. However, this metric fails to capture the additional impact increased stratification can have on determining the oceanic conditions to promote tropical cyclone activity. Indeed, rapid intensification has recently been observed over low OHC waters when highly stratified⁵⁸. The increase in upper ocean stability and TCHP, in addition to the potential shift in the dominant driver of stratification observed here, has likely played a role in the observed increases in tropical cyclone peak intensification rates in the central Caribbean in the twenty-first century⁵⁹. This connection between a warmer, fresher, and more stable upper ocean and tropical cyclone intensification has been suggested for the Pacific⁶⁰. The most concerning aspect of these trends is the potential for exacerbated exposure of already highly vulnerable communities. During the twenty-first century, exposed demographics to tropical cyclones have been found to be older and more disadvantaged globally⁶¹. Of the 177 countries and regions affected by tropical cyclones between 2002 and 2019, 8 of the top 14 most vulnerable countries are Caribbean and Central American countries⁶¹.

The upper ocean warming, subsurface salinification, and increase in stratification detailed here may also significantly impact marine productivity and biodiversity locally. Upper ocean warming has been shown to be a major stressor on Caribbean coral reef ecosystems over the past century⁶². Increased stratification has been associated with global declines in nutrient delivery⁶³ and phytoplankton productivity⁶⁴, and thus, might also be a concern for marine ecosystems in the Caribbean. Further, the “salty get saltier” pattern observed in mode waters throughout the Atlantic has recently been shown to coincide with an increase in subtropical ventilation that opposes warming-driven loss of oxygen⁶⁵. As the CTF transports water mass anomalies formed throughout the greater Atlantic, whether the Caribbean is experiencing a deoxygenation as other tropical regions are experiencing remains an open question⁶⁵.

In the second part of this discussion, we highlight relevant trends observed elsewhere and discuss possibility of their impact on the CTF considering the circulation dynamics of this region. We focus the discussion this way as the observations utilized here do not allow us to directly disentangle the origin or mechanisms behind the changes in CTF upper ocean water mass properties. We use Fig. 4 as a guide to discuss the potential origins of the changes in the CTF based on the water mass class they occurred in. Further, Fig. 4 and Supplemental Fig. 6 highlight how these changes are occurring on isopycnals, rather than as a result of the vertical movement of isopycnals, and thus reflect changes in water mass properties themselves.

The magnitude of the Surface Water (SW in Fig. 4) warming agrees well with the sea surface temperature trends observed within the Caribbean Sea⁶⁶ and throughout the entire Atlantic⁶⁷. Associating the decreasing sea surface salinity trend reported here with relevant trends observed elsewhere in the Atlantic is not as straightforward as the trends in temperature. The advection of low salinity waters from the Amazon-Orinoco River systems has a first-order control on the surface salinity cycle of the western tropical Atlantic⁶⁸. Therefore, an increase in the strength of the North Brazil Current (NBC) transport, possibly through enhanced NBC ring activity, would theoretically be needed to drive a reduction in the sea surface salinity of the CTF. As no significant trends have been observed in the NBC over most of the time period analyzed here⁶⁹, an increase in local precipitation could provide an alternate explanation for the sea surface salinity decrease. Observations show a weak increase in precipitation over the Caribbean from 1986 to 2010, during which the Atlantic Multidecadal Oscillation was, and continues to be, in a positive phase preconditioning the Caribbean for wetter conditions⁷⁰. Whether this decrease in sea surface salinity of the CTF can be attributed to the weak increase in precipitation over the Caribbean is uncertain.

Below the surface layer, the potential drivers of the subsurface salinification observed here are more straightforward. In Fig. 4, Subtropical Under Water (STUW) and Subtropical Mode Water (STMW) are formed via surface fluxes in both hemispheres. The evaporation–precipitation imbalances these regions experience has increased with the intensification of the global hydrologic cycle (i.e. wet get wetter, dry get drier), an expected result of global warming^71,72,73. Therefore, the salinification reported for these water masses here is consistent with the “fresh gets fresher, salty gets saltier” pattern observed due to global warming^{38,65,74,75,76}. This increase in sea surface salinity has led to a well-documented changes in the subduction rate of both North Atlantic STUW and STMW over the past several decades^{22,38,77,78,79}. The corresponding increase in salinity of the subducted high salinity core of North Atlantic STUW and STMW has been reported at a similar, if not slightly lower, trend observed here. While trends in the South Atlantic STUW and STMW remain inconclusive primarily due to limited spatiotemporal resolution of salinity profiles⁸⁰, a general salinification from 0 to 700 m of approximately 0.025–0.05 g kg⁻¹ decade⁻¹ throughout the South Atlantic⁸¹ implies similar trends are occurring here as well. Further, the magnitude and structure of the subsurface warming reported here agree well with trends observed in both North and South Atlantic Subtropical Mode Waters^23,81,82.

Focusing on the impact of these changes on the circulation of the greater Atlantic, we now highlight how the magnitude of the observed CTF warming trend is consistent with recent observations of NASTG warming, leading to a surface density reduction and decline in subtropical mode water formation²³. The decline in subtropical mode water formation has been coupled with a decline in the strength of the AMOC²³. This coupling is mechanistically analogous to the downward cascade of subtropical mode waters in which cooling through each spiral of the NASTG increases the density of water parcels^83,84,85. Lagrangian tracer analysis of model data has suggested ~ 70% of the waters in the northward flowing component of the AMOC must recirculate in the NASTG at least once before continuing northward, intimately connecting the NASTG and the AMOC⁸⁵. This concept of a subsurface, rather than surface, transition from the subtropical to subpolar gyre has been suggested by others^{86,87,88,89,90,91} and supports a decadal timescale for transmission of water mass anomalies. Berglund et al.’s⁸⁵ Lagrangian tracer analysis of model data provides a useful comparison point as they suggests the first two spirals through the NASTG take approximately 60 years and result in a heat loss of 0.175 PW and a salt loss of 0.24 × 10⁶ kg s⁻¹. Over the 63-year time-period considered in this analysis, the Yucatan Straits heat and salt transport has increased by approximately 0.07 ± 0.05 PW and 1.46 ± 2.60 Sv (equivalent to 1.46 + 2.6 × 10⁶ kg s⁻¹), respectively. The increase in heat transport represents 40% of the heat lost and the increase in salt transport is ~ 6 times the salt lost in the first two loops through the NASTG. Interestingly, the trends reported here for the Yucatan Straits are ~ 10% of the Florida Current increase in heat transport¹⁵ since 2010 but commensurate in terms of the increase in salt transport³⁷ over the period from 1982–1987 to 2001–2015. These differences raise interesting questions about the processes occurring between these two regions yet support what appears to be more of an influence from warming, rather than salinification, on downstream water mass formation. Nonetheless, with this comparison we show the trends in Yucatan Straits heat and salt transport are quantitatively large enough to directly impact water mass formation in the NASTG.

Lastly, we highlight the trend in Yucatan Straits freshwater transport (− 0.004 Sv decade⁻¹) as commensurate with the freshwater forcing used in many freshwater hosing experiments to understand AMOC stability^92,93. More recent freshwater hosing experiments have shown a slowdown in AMOC strength of 0.5 Sv for every 0.01 Sv of freshwater added at a rate of 0.001 Sv year⁻¹, eventually resulting in an complete AMOC collapse⁹⁴. The degree to which tropical and subtropical salinification is relevant as a stabilizing feedback on the freshening trends projected to impact AMOC strength warrants further investigations. Again, we emphasize the uncertainty in the net transport estimates derived here and acknowledge these trends may be compensated by the southward Atlantic interior flow but use the above examples to highlight the potential impacts. Based on the results presented and the documented changes occurring elsewhere in the Atlantic, we conclude that, despite the mixing of interhemispheric water masses with distinctly different temperature and salinity properties, changes in water mass properties of the CTF are likely being communicated throughout the greater Atlantic and are influencing water mass formation in both the NASTG and the AMOC.

Using over 28,000 temperature and salinity profiles we document significant changes in the upper ocean water mass properties of the CTF since 1960. The implications of these changes highlight how the circulation dynamics of the CTF enhance this region’s already high climate sensitivity⁹⁵. We find evidence for changes in the CTF component of the heat, salt, and freshwater transport at this latitude with magnitudes large enough to impact water mass formation in the NASTG, thereby indirectly influencing the AMOC. We find several likely routes to where these changes are transmitted throughout the greater Atlantic, leading to many presently observable changes that are consistent in magnitude with the trends reported here. These changes are also likely contributing to local sea-level rise, increasing oceanic conditions promoting tropical cyclone activity, and impacting biological processes and productivity. These findings emphasize the importance of the CTF as an important component of Earth’s climate system and reinforce the need for dedicated, sustained, subsurface water mass property observations needed for improved understanding of our climate.

Methods

EN4 profiles

We utilize profiles from the UK-Hadley Centre’s EN4 subsurface ocean temperature and salinity observational data product³³ with bias corrections applied⁹⁶ to analyze decadal trends in water mass properties in the Caribbean Sea. To ensure continuous, ample data coverage, only profiles from 1960 to 2023 are used. The native EN4 quality control flags (flag = 4) are applied to only accept profiles where both temperature and salinity are not rejected. The remaining data is then geographically subset to a shapefile that represents to extents of the Caribbean Sea: bounded to the east by the Lesser Antilles from the U.S. Virgin Islands south to the northeastern coast of Venezuela, to the north by the Greater Antilles from Puerto Rico to Cuba, and to the west and south by Central America and the northern coast of South America (Fig. 1).

After visual inspection it was clear two important Caribbean datasets were missing from the EN4 profiles. The first dataset is from the National Atmospheric and Oceanic Administration Climate and Global Change Atlantic Climate Change Program sponsored Windward Island Passages monitoring Program (WIPP), which collected a comprehensive dataset of lowered Conductivity-Temperature-Depth (CTD) measurements in every major Caribbean island passage from 1991 to 2001. The second dataset is from autonomous underwater gliders deployed by Rutgers University and the University of the Virgin Islands in several eastern Caribbean passages from 2019 to 2023. These glider deployments have sampled over 5600 km over 270 days in the Anegada, Dominica, St. Lucia, and St. Vincent Passages. These datasets were combined with the EN4 profiles and the following processing steps were applied to all data. Temperature and salinity data are converted to potential temperature and practical salinity using the Internation Equator of State of Seawater 1980 (EOS-80)⁹⁷ and conservative temperature and absolute salinity using the International Thermodynamic Equation of Seawater 2010 (TEOS-10)⁹⁸. Daily mean profiles are calculated for these additional datasets to comply with the temporal resolution of the EN4 profile data. Each profile was linearly interpolated onto a common vertical grid, with 5-m resolution in the upper 100 m, 25-m resolution above 250 m and 50-m resolution above 1000 m. We limit the maximum depth of the mean profiles to 1000 m as the mean sill depth of the Caribbean island passages are 800 m and the maximum sampling depth of the gliders is 1000 m. Limiting the maximum depth to 1000 m also ensures for a more robust and complete dataset of Caribbean profiles as significantly smaller number of profiles extend below this depth.

As the EN4 profiles are separated by month in individual years, we combine the glider and WIPP profiles with the corresponding EN4 month and year before applying the subsequent steps which largely follow Gonçalves Neto et al.⁹⁹. To avoid giving disproportionate weight to profiles collected close together in space and time, means of profiles in the same month in a given year in a 0.5° latitude × 0.5° longitude spatial grid were calculated. Though this issue is not apparent in the EN4 profiles, this approach does not account for a sampling bias across the Caribbean basin, where there is also variability in the vertical structure of water mass properties. All-time mean temperature and salinity profiles were then calculated for each spatial grid cell. Anomaly profiles were then calculated by subtracting the all-time mean profiles from each individual monthly mean profile at the same spatial grid cell. Yearly mean and decadal mean anomaly profiles are calculated by averaging the individual anomaly profiles accordingly. Next, the all-time mean profiles are added back to the yearly mean and decadal mean anomalies to calculate yearly mean and decadal mean profiles. Finally, buoyancy frequency is calculated using the Nsquared function from the Python implementation of the Gibbs SeaWater (GSW) Oceanographic Toolbox of TEOS-10⁹⁸. Decadal trends and standard errors of the trend are then determined by fitting linear regressions to the respective decadal mean property profiles at every depth level over the 1960–2023 time-period used in this analysis. The trends are not weighted by the number of observations at a given depth.

The magnitude of the subsurface salinity maximum is calculated by sub-setting profiles to between 100 and 300 m and calculating yearly and decadal means and standard deviations. Tropical Cyclone Heat Potential (TCHP) is most relevant in during Hurricane season therefore only profiles from June to October are used for this calculation (Supplementary Fig. 1). TCHP is calculated following⁴²:

$$Q = \rho C_{p} \Delta T\Delta z$$

where Q represents TCHP in kJ cm², ρ represents density, $C_{p}$ represents the heat capacity of seawater taken to be 3985 J kg⁻¹ K⁻¹, $\Delta T$ represents the mean temperature different above 26 °C for a given depth increment $\Delta z$. We also calculate potential energy anomaly (PEA) only during hurricane season to ensure it is meaningful with the related TCHP calculation described above. PEA is calculated following¹⁰⁰:

$$\varphi = \frac{1}{H}\mathop \int \limits_{ - H}^{0} \left( {\overline{\rho } - \rho } \right)gz dz$$

where φ represent PEA in J m⁻³, ρ is density over a vertical profile with depth H, $\overline{\rho }$ is the depth averaged density, z is depth, and g is gravitational acceleration.

To ensure there is minimal potential spatial biases, averaging data in decadal bins allows for sufficient spatial coverage in the Caribbean Sea (Supplementary Fig. 2). To investigate potential seasonal sampling biases, salinity maximum values are computed, and means are calculated for each month in each decade (Supplementary Fig. 3). The general increase in salinity maximum across all months further supports the salinification trend shown here despite the dramatic increase in number of observations in the summer months after 2010.

Heat, salt, and freshwater transport

To calculate potential heat, salt, and freshwater transports of the Caribbean Through-Flow water mass trends documented in this analysis, we use a combination of the EN4 profiles detailed above and Copernicus Marine Environment Monitoring Service’s (CMEMS) 1/12° Global Ocean Physics Reanalysis (GLORYS). The GLORYS ocean model is based on the NEMO platform with 50 vertical levels with a resolution ranging from 1 to 450-m at 5000 m depth¹⁰¹. The model is forced at the surface by a 3-hourly European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim atmospheric reanalysis to reproduce diurnal cycling¹⁰¹. Large and Yeager bulk formulae¹⁰² are used to parameterized turbulent surface fluxes of momentum and heat. Precipitation and radiative flux biases are corrected by a satellite-based, large-scale adjustment¹⁰¹.

GLORYS has been shown to reproduce variability as well as vertical structure of temperature and salinity globally when compared to observation-based climatology¹⁰¹. Importantly, GLORYS was also shown to be in good agreement with meridional volume, heat, salt, and freshwater transports, particularly in the Atlantic basin^101,103.

Because the Yucatan Straits act as a bottleneck for water masses in the Caribbean, exporting all ~ 30 Sv of the CTF northward^18,45, we are able to place the observed changes in temperature and salinity in the context of the circulation of the greater Atlantic by deriving trends in heat, salt, and freshwater transports relative to the net transport. However, to calculate meaningful heat, salt, and freshwater transport the Yucatan Straits ~ 30 Sv net mass flux out of the Caribbean Sea^18,45 needs to be balanced.

We initially find an all-time mean mass flux between the Yucatan Straits (27.9 ± 4.47 Sv) in good agreement with prior estimates^18,45 and nearly balanced by the Atlantic interior section along 21.5° N (29.2 ± 3.50 Sv), a difference of just 1.3 Sv. The mean transport depth profile across the Yucatan Straits also agrees well with prior estimates^18,45. However, we limit the Yucatan Straits transport to 760 m as the sill depth of the Florida Straits¹⁰⁴ ultimately limit the depth to which waters can be transported northward. This reduces the all-time mean Yucatan Straits transport to 26.67 ± 2.47 Sv. We derive all-time mean velocity profiles across the Yucatan Straits and the Atlantic interior section along 21.5° N, interpolating them onto the same depth grid as the EN4 hydrographic data. To ensure mass balance between these two sections, we redistribute 0.0045 cm s⁻¹ to every grid cell throughout the Atlantic interior section. A correction of a similar magnitude is historically performed on the RAPID section as well¹⁰⁵. All-time mean velocity standard deviation at each depth level is also derived for use in the Monte-Carlo simulation detailed in the next section.

In a similar manner to the methodology applied to determine heat, salt, and freshwater transports at the RAPID-MOCHA array along 26.5° N^28,106, we instead use the interior transport across 21.5° N (Supplementary Fig. 4) to derive a basin-wide mass balance with the Yucatan Straits here. The same methodology applied on the hydrographic data in the Caribbean detailed above is applied to EN4 profile data from the Atlantic interior along 21.5° N, where a longitudinal grid spacing of 2° and latitudinal grid of 1.5° are used. Temperature and salinity observations in the Atlantic interior along 21.5° N are extremely sparse, particularly in the earlier decades of our analysis time-period. To overcome this issue, we calculate an all-time climatological mean temperature and salinity field along 21.5° N and fill gaps in the individual decadal means with this estimate. Implications of this gap-filling procedure are discussed in the following section.

Net heat transport ($Q_{NET}$) is calculated following²⁸:

$$Q_{NET} = {\iint }\rho c_{p} v\theta dx dz$$

where $\rho$ is seawater density, $c_{p}$ is the specific heat of seawater, $v$ is meridional velocity, and $\theta$ is potential temperature. The double integral is computed over the full area of the Yucatan Straits and Atlantic interior section along 21.5° N. We follow Johns et al.²⁸ by using a constant value of ρc_p = 4.1 × 10⁶ J kg⁻¹ °C⁻¹ in the heat transport calculation.

Net salt transport ($F_{S}$) is calculated following¹⁰⁶:

$$F_{S} = {\iint }vS dx dz$$

where $v$ is meridional velocity and $S$ is salinity and again the double integral is computed over the full area of the Yucatan Straits and Atlantic interior section along 21.5° N. We calculate salt transport on both the practical salinity scale (PSS-78) using units of practical salinity unit (PSU) as well as on the absolute salinity scale (TEOS-10) using units of g kg⁻¹. On the practical salinity scale, salinity is unitless and therefore salt transport has units of Sverdrups, whereas on the absolute salinity scale it has units of Sverdrups g kg⁻¹, which can effectively be converted to a mass flux of salt (g s⁻¹) if multiplied by ρ.

Net equivalent freshwater transport ($F_{e}$) is calculated following McDonagh et al.¹⁰⁶:

$$F_{e} = - \frac{1}{{\overline{s}}}\left( {F_{S} - \overline{s} T} \right)$$

where $\overline{s}$ is the section-averaged salinity, $F_{S}$ is salinity flux from above, and T is transport. Along 21.5° N, $\overline{s}$ is determined to be 35.2 which is in good agreement with the 35.17 determined at RAPID¹⁰⁶. As salinity is always positive, a negative freshwater flux indicates removal of freshwater or increasing salinity.

Monte-Carlo simulation

There are two main factors that introduce significant uncertainty into this calculation of heat, salt, and freshwater transport:

1.
The 1993–2023 GLORYS reanalysis record does not overlap with the entire 1960–2023 time-period used in this analysis.
2.
The gap filling procedure for the sparse temperature and salinity observations in the greater Atlantic along 21.5° N in the earlier decades of our analysis time-period acts to dampen variability and therefore any potential trends in properties considerably.

To overcome these challenges, we employ a Monte-Carlo simulation to propagate the uncertainty in the individual components of in the transport calculations to obtain the most robust estimates possible. We ran 10,000 simulations when calculating both the all-time mean and decadal trends in heat, salt, and freshwater transport where we varied each term in the respective equations randomly within their respective 95% confidence intervals. For terms like temperature or the trend in temperature, this procedure simply evaluates the impact of picking different values within the 95% confidence interval of the mean temperature or trend in temperature at each depth level. This procedure attempts to account for both the significant uncertainty in the hydrographic data from the greater Atlantic along 21.5° N and the range of trends at each depth level in Fig. 2 to the best of our ability.

For terms involving velocity, we randomly vary the mean transport within the all-time standard deviation (± 4.26 Sv) for each simulation. Mass balance is ensured for each simulation by redistributing this transport adjustment evenly over the respective areas of the Yucatan Straits and Atlantic interior sections. This procedure attempts to account for the limited overlap in the time-periods between the GLORYS reanalysis and EN4 hydrographic data to the best of our ability. Potential decadal trends in mass transport are beyond the scope of this analysis.

The 10,000 Monte-Carlo simulations result in a distribution, or probability, of the likely heat, salt, and freshwater transports and decadal trends in these transports given the uncertainty calculated for each input term (Supplementary Fig. 5). We evaluate these distributions by computing means and standard deviations. Further, the likelihood of a meaningful trend is determined by calculating a one-sided t-test between a given trend’s distribution and zero. A significant result implies the trend is statistically different from zero. Finally, we determine the decadal increase in the Yucatan Straits transport relative to the net transport (last column Supplementary Table 1) by taking the decadal trends from the Monte-Carlo simulations, multiplying them by the number of decades in our 1960–2023 analysis time-period, and dividing them by the respective net transports.

Data availability

All code and links to access data relevant to this manuscript can be found within the following GitHub repository: https://github.com/JGradone/CTF-WaterMass-Analysis.

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Acknowledgements

J.C.G discloses support for the research of this work from the Department of Defense National Defense Science and Engineering Graduate Fellowship. J.C.G., T.N.M., S.M.G., and W.D.W. disclose support for the research of this work from the G. Unger Vetlesen Foundation. T.N.M. and S.M.G. disclose support for the research of this work from Rutgers University. W.D.W. discloses support for the research of this work from VI-EPSCoR NSF award OIA# 1946412. J.B.P. discloses support for the research of this work from the National Oceanic and Atmospheric Administration Climate Variability Program Project #0008287. We thank Rebecca Jackson, the editors, and three anonymous reviewers for their feedback regarding this manuscript.

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Department of Marine and Coastal Sciences, Center for Ocean Observing Leadership, School of Environmental and Biological Sciences, Rutgers University, New Brunswick, NJ, USA
Joseph C. Gradone, T. N. Miles & S. M. Glenn
Graduate School of Oceanography, University of Rhode Island, Narragansett, RI, USA
J. B. Palter
Center for Marine and Environmental Studies, University of the Virgin Islands, Charlotte Amalie West, St. Thomas, VI, USA
W. D. Wilson
Ocean and Coastal Observing – Virgin Islands, Inc., St. Thomas, VI, USA
W. D. Wilson

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Joseph C. Gradone
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T. N. Miles
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J. B. Palter
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S. M. Glenn
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W. D. Wilson
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Contributions

J.G. performed the data analysis and visualization and wrote the main manuscript text. J.G., T.M. and J.P. contributed to the development of the methodology. All authors contributed to the interpretation of results, critical review, and approval of the final manuscript.

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Correspondence to Joseph C. Gradone.

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Gradone, J.C., Miles, T.N., Palter, J.B. et al. Warming and salinity changes of the upper ocean Caribbean through-flow since 1960. Sci Rep 15, 23157 (2025). https://doi.org/10.1038/s41598-025-05494-z

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Received: 16 January 2025
Accepted: 03 June 2025
Published: 02 July 2025
DOI: https://doi.org/10.1038/s41598-025-05494-z