The Subpolar North Atlantic (SPNA) plays a crucial role in the global climate system. Its upper layer temperature responds to both short-term atmospheric events and decadal-scale changes in ocean circulation and heat transport. This region is vital for the Meridional Overturning Circulation (MOC), transporting heat and carbon globally. It's the primary gateway for warm Atlantic waters to the Arctic, influencing its heat and mass budgets. The SPNA also influences atmospheric regimes and continental weather patterns, potentially triggering long-term climate variability. Understanding the SPNA's temperature variability is therefore critical for predicting future climate changes. Previous research documented a warming-to-cooling reversal in the mid-2000s, attributed to changes in ocean circulation. This study focuses on the most recent reversal, a shift from cooling to warming that began sharply in 2016, to determine its underlying mechanisms and potential future implications.

Prior research has highlighted the SPNA's susceptibility to decadal-scale temperature fluctuations linked to changes in ocean circulation and heat transport. Studies have shown the impact of these changes on sea level, Arctic heat budgets, and even continental weather patterns. The mid-2000s saw a documented warming-to-cooling shift, with consequences extending to the East Atlantic. Theoretical work predicted a subsequent advection-driven warming reversal starting in 2016. This study builds upon these previous findings by using advanced observational and statistical techniques to provide a more comprehensive analysis of the 2016 shift and its causal mechanisms.

This study employed a multi-faceted approach combining various datasets and analytical techniques. First, it utilized an ocean analysis product (primarily using Argo-derived hydrographic profiles) to track temperature anomalies in the eastern SPNA from 2002 to 2019. Satellite altimetry data provided information on sea surface height (SSH) and geostrophic velocities to calculate mean kinetic energy (MKE). Atmospheric fluxes from NCEP/NCAR reanalysis were incorporated to assess the role of air-sea heat exchange. To determine the contribution of advection, a simple advection-diffusion model was applied to a passive tracer, simulating the movement of warm subtropical and cold subpolar waters into the SPNA using altimetry-derived surface geostrophic currents and observation-based mesoscale eddy diffusivities. The model output allowed for quantifying the proportion of subtropical-origin water (PSTG) in the SPNA over time. This was complemented by a similar analysis using the three-dimensional velocity field from the ECCOv4r4 ocean state estimate to confirm the surface-focused advective results were representative of changes in the interior ocean circulation. Finally, a machine learning technique—a profile classification model (PCM)—was used to statistically cluster ocean vertical profiles into subpolar and subtropical water masses, allowing an independent assessment of the changes in water mass proportions and their contribution to the observed temperature variations. The PCM utilized temperature and salinity profiles from the ISAS-15 dataset. The proportion of subtropical and subpolar water masses was calculated, linking those changes to observed changes in Ocean Heat Content (OHC).

The analysis revealed a significant cooling-to-warming transition in the SPNA starting in 2016. This warming was surface-intensified but also apparent in the deeper water column (down to 2000 m). The observed increase in ocean heat content (OHC) from 2016 to 2019 was largely attributed to enhanced ocean heat transport convergence. Spatial patterns showed large areas of positive temperature and SSH anomalies in the Iceland Basin and surrounding areas. A striking intensification of the subtropical portion of the North Atlantic Current (NAC) was observed, along with an increase in the northeastward transport of warm subtropical waters. The advection-diffusion model showed a strong correlation between the proportion of subtropical-origin water (PSTG) and the upper layer temperature anomalies, indicating a causal relationship. This result was corroborated by a similar analysis using the ECCOv4r4 data. The PCM analysis independently confirmed the increase in subtropical water mass proportion, as well as the importance of the changing relative proportion of subtropical and subpolar waters in driving the observed temperature changes. The dominant contribution to advection-driven temperature anomalies stemmed from shifts in the relative proportion of subtropical and subpolar waters, highlighting the key role of circulation shifts. The warming is expected to spread westward within the SPNA and to greater depths, potentially influencing water mass transformation and the Atlantic MOC.

The findings directly address the research question by demonstrating the causal link between a shift in ocean circulation and the recent warming trend in the SPNA. The strong correlation between PSTG and temperature anomalies, confirmed by two independent methodologies (advection-diffusion modeling and PCM analysis), strongly supports the advection-driven warming mechanism. This mechanism was identified as the primary driver of both the recent warming and previous cooling-to-warming transitions in the region. The results align with previous model-based analyses and highlight the importance of considering both the relative proportions and temperatures of the source waters when predicting SPNA temperature changes. The study contributes significantly to our understanding of decadal-scale climate variability in the North Atlantic, particularly regarding the inter-linkages between circulation shifts, heat transport, and temperature changes. The ongoing warming may alter the intensity of the MOC, affecting heat transport and long-term climate patterns.

This study provides robust evidence for a large-scale warming trend in the SPNA since 2016, primarily driven by enhanced northward transport of warm subtropical waters. Two independent analyses, a passive tracer model and a profile classification model, strongly support this conclusion. The advective origin of this warming and the SPNA's long ocean memory suggest its persistence in the coming years. Further research should focus on disentangling the atmospheric and oceanic drivers of the observed changes and exploring the potential impacts on the Atlantic Multidecadal Variability (AMV) and global climate.

While the study utilizes comprehensive datasets and sophisticated analytical techniques, some limitations exist. The advection-diffusion model simplifies complex ocean processes, neglecting certain factors that may influence water mass transport. The resolution of some datasets may limit the accuracy of regional-scale analysis. Additionally, the study focuses primarily on the eastern SPNA, and further research is needed to fully understand the broader impacts of the warming trend across the entire subpolar region.