The downward spiralling nature of the North Atlantic Subtropical Gyre

The Atlantic Meridional Overturning Circulation (AMOC) plays a vital role in global heat distribution and climate regulation. Evidence suggests a slowdown in the AMOC during the 20th century, with projections indicating further deceleration in the coming decades. This slowdown has significant implications for both regional and global climate patterns, making it a subject of intense research. The Gulf Stream, a key component of the northward AMOC flow, is also integral to the North Atlantic Subtropical Gyre (hereafter 'Gyre'). Previous research has suggested a deep connection between the Subtropical and Subpolar Gyres, possibly involving recirculation within the Subtropical Gyre. The formation of North Atlantic Subtropical Mode Water (NASTMW) in the Gulf Stream's warm flank further complicates the understanding of this interconnected system. While the Gyre's importance to the AMOC is acknowledged, its precise role and extent remain unclear. The northward heat transport driven by the AMOC is responsible for the relatively mild climate in Northern Europe. This transport is significantly influenced by wind-driven ocean circulation, and future climate change is expected to impact both wind patterns and water mass properties (temperature and salinity) within the Gyre, potentially altering the AMOC and its heat transport capacity. This study uses Lagrangian trajectories to investigate the relationship between the Gyre's circulation and the AMOC, aiming to quantify the Gyre's influence and its implications for future climate projections.

Numerous studies have documented the slowing of the AMOC during the 20th century and projected its further decline. These studies highlight the AMOC's importance in climate regulation and the urgency of understanding the underlying mechanisms of its variability. Research on the Gulf Stream and its interaction with the North Atlantic Subtropical Gyre has shown the complex interplay of water masses and heat transport. The transition between the Subtropical and Subpolar Gyres, often assumed to occur at the surface, has been shown to be a deeper process potentially linked to recirculation within the Subtropical Gyre. The formation of North Atlantic Subtropical Mode Water (NASTMW) and its role in connecting the two gyres at depth has also been investigated. However, the extent to which the Gyre influences the AMOC remains poorly understood. Existing studies emphasize the need for a better understanding of the AMOC's mechanisms and the factors driving its variability.

This study employs Lagrangian trajectories simulated using data from the Earth System Model EC-Earth-Veg version 3.3.1. Two model resolutions were used: 1° (LR-case) and 1/4° (HR-case). The EC-Earth-Veg model incorporates the atmospheric-land surface module IFS, the dynamical vegetation model LPJ-GUESS, the ocean model NEMO (including OPA and LIM3 sea-ice modules), and has 75 depth levels. The Lagrangian trajectory model TRACMASS was used, leveraging mass flux, temperature, and salinity fields from EC-Earth-Veg. Trajectories were initiated at 17°S, where the flow was northward, and terminated either upon returning to 17°S or reaching 58°N. The analysis focused on trajectories reaching 58°N, representing water flowing northward as part of the AMOC and the Subtropical Gyre. Lagrangian stream functions were computed, as TRACMASS is mass-conserving. A 'color-clock' method was developed to compute the mean trajectory of the spiraling water within the Gyre, dividing the Gyre into 12 segments. The Lagrangian divergence of heat, salt, and density was calculated to quantify local changes experienced by water parcels. The instantaneous mixed layer depth was used to differentiate between air-sea interaction and interior mixing processes driving these changes. To assess robustness, the same analysis was repeated with the higher-resolution EC-Earth3P-HR model.

The study's key findings reveal a previously unrecognized spiraling pattern of water within the North Atlantic Subtropical Gyre. Approximately 70% of the water flowing northward as part of the AMOC first completes one or more circuits within the Gyre before continuing its northward journey. The number of circuits ranges from 1 to 14, with a mean of about 3-4 loops before the water becomes sufficiently dense to escape the Gyre and join the northern upper branch of the AMOC. Each circuit results in a net increase in water density and depth. The initial circuit shows strong cooling and densification due to air-sea interaction, primarily in the Gulf Stream and northern flank of the Gyre where NASTMW forms. Subsequent circuits are increasingly influenced by subsurface mixing. Analysis of Lagrangian divergence of heat, salt, and density highlights three key regions below the mixed layer: (1) the western Gulf Stream/northern Gyre, where strong cooling and freshening occur due to mixing with denser waters; (2) the middle and eastern Gyre, characterized by heat and salt gain, possibly due to mixing with Mediterranean waters; and (3) the southern Gyre, where cooling and freshening are density-compensated, suggesting isopycnal mixing. The total heat and salt changes are accumulated over multiple circuits. The analysis using a higher-resolution model (1/4°) confirms these findings, although the circuit times are shorter due to a narrower, faster Gulf Stream and eddy effects. The mean trajectory and the changes in heat, salt, density and depth in the high resolution run are very similar to the low resolution run. However, the higher resolution showed that the time it takes for the water to circulate is about half as long as the lower resolution run.

These findings demonstrate the critical role of the North Atlantic Subtropical Gyre in regulating the AMOC. The Gyre's recirculation not only increases water density, making northward transport possible, but also acts as a conduit to return water to the Gulf Stream for further densification. The processes driving density changes differ between the first and subsequent circuits, with air-sea interaction dominating the initial loop and subsurface mixing playing a more prominent role in later circuits. Future work should investigate the mixing processes within the Gyre in greater detail to quantify the contributions of vertical and lateral diffusion to heat and salt changes. The high-resolution model results reinforce the study's key findings, highlighting the robustness of the low-resolution model's ability to capture large-scale, time-mean ocean dynamics, even in the absence of resolved eddies. Similar spiraling patterns might exist in other ocean gyres, underscoring the need for further comparative research.

This study reveals a crucial, previously unknown role of the North Atlantic Subtropical Gyre in the AMOC. The spiraling nature of water parcels within the Gyre, resulting in density increase and subsequent northward flow, significantly impacts the AMOC's strength and heat transport. The findings underscore the need to consider Gyre dynamics when modeling and predicting future changes to the AMOC and its climatic influence. Future research could focus on more detailed investigations of the mixing processes in the Gyre and further analysis of this phenomenon in other ocean gyres.

The study relies on model simulations, which may not perfectly capture the complexities of real-world ocean processes. Future work should include further validation with observational data, particularly regarding the mixing processes below the mixed layer. The study focuses solely on the Subtropical Gyre, and expanding to other Gyres would enrich our understanding of global ocean circulation patterns.