Marine heatwaves and cold spells are extreme surface temperature events that have been associated with adverse societal and ecosystem impacts in several regions around the globe. Predicting these events presents a challenge because of their generally short-lived nature and dependence on air-sea interactions, both locally and remotely. Here we analyze oceanic propagating features that promote the occurrence of marine heatwaves and cold spells in the western subtropical South Atlantic. The main interannual feature detected from satellite sea level data since 1993 shows a westward propagating zonal pattern with a periodicity of 3–5 years. The pattern has a significant in-phase correlation with sea surface temperature (SST) anomalies in the western South Atlantic, explaining 82% of the daily extreme warm (90th percentile) and cold (10th percentile) SST anomalies and consequently modulating interannual variations in the intensity and duration of marine heatwave and cold spell events. It is found that meridional oceanic advection plays an important role in the regional heat budget associated with the westward-propagating mode, modulating the meridional exchange of tropical (warm) and extratropical (cold) waters in the western subtropical South Atlantic region and thereby setting a baseline for temperature extremes on interannual timescales. This propagating mode is well correlated (r > 0.6) with the strength of the meridional overturning circulation at 25°S and 30°S with a lag of approximately 3–9 months. The lagged response provides a potential source of predictability of extreme events in the western South Atlantic.

Changes in the Atlantic Meridional Overturning Circulation (AMOC) and associated Meridional Heat Transport (MHT) can affect climate and weather patterns, regional sea levels, and ecosystems. However, despite its importance, direct observations of the AMOC are still limited spatially and temporally, particularly in the South Atlantic. The main goal of this study is to implement a cost-effective trans-basin section to estimate for the first time the AMOC at 22.5°S, using only sustained ocean observations. For this, an optimal mapping method that minimizes the difference between surface in-situ dynamic height and satellite altimetry was developed to retrieve monthly temperature and salinity profiles from Argo and XBT data along the 22.5°S section. The mean AMOC and MHT for 22.5°S were estimated as 15.55±2.81 Sv and 0.68±0.18 PW, respectively, and are stronger during austral fall/winter and weaker in spring. The high-resolution XBT data available at the western boundary are vital for capturing the highly variable Brazil Current, and our section shows a significant improvement when compared to Argo database. The mean values, interannual and seasonal time series of AMOC and MHT were compared with other products. At 22.5S the North Atlantic Deep Water is divided into two cores that flow along both western and eastern boundaries near 2500 m depth. Our results suggest a greater influence of western boundary system on the AMOC variability at 22.5°S; highlight the importance of high resolution in situ data for AMOC estimations; and contribute for a better understanding of AMOC and MHT variability in the South Atlantic.

The Subpolar North Atlantic plays a critical role in the formation of the deep water masses which drive Atlantic Meridional Overturning Circulation (AMOC). Labrador Sea Water (LSW) is formed in the Labrador Sea and exported predominantly via the Deep Western Boundary Current (DWBC). The DWBC is an essential component of the AMOC advecting deep waters southward, flowing at depth along the continental slope of the western Atlantic. By combining sustained hydrographic observations from the Labrador Sea, Line W, Bermuda basin, and offshore of Abaco Island along 26.5°N, we investigate the signal propagation and advective timescales of LSW via the DWBC from its source region to the Tropical Atlantic through various approaches using robust neutral density classifications. Two individually-defined LSW classes are observed to advect on timescales that support a new plausible hydrographically-observed advective pathway. We find each LSW class to advect on independent timescales, and validate a hypothesized alternative-interior advection pathway branching from the DWBC by observing LSW outside of the DWBC in the Bermuda basin just prior to or on the same timescale as at 26.5°N- 10-15 years after leaving the source region. Advective timescales estimated herein indicate that this interior pathway is likely the main advective pathway; it remains uncertain whether a direct pathway plays a significant advective role. Using LSW convective signals as advective tracers along the DWBC permits the estimation of advective timescales from the subpolar to tropical latitudes, illuminating deep water advection pathways across the North Atlantic and the lower-limb of AMOC as a whole.

The nearly four-decades-long quasi-continuous daily measurements of the Florida Current (FC) volume transport at 27ºN represents the longest climate record of a boundary current in existence. Given the extremely high utility of this submarine cable-collected time series for monitoring the Atlantic meridional overturning circulation, as well as for improving understanding and prediction of the regional weather, climate phenomena, coastal sea-level, and ecosystem dynamics, efforts are underway to establish a suitable backup observing system in case the cable becomes inoperable in the future. This study explores the utility of along-track satellite altimetry measurements since 1993 as a potential cable backup by establishing the relationship between the cross-stream sea surface height gradients and the FC volume transport derived from cable measurements and ship sections. We find that despite the lower temporal resolution, satellite altimetry can indeed serve as a decent but limited backup observing system. The FC transport inferred from satellite altimetry captures about 60% of the variability observed in the concurrent cable estimates, and the estimated error bars for the altimetry-derived transport are larger than those of the cable transport (2.1 Sv versus 1.5 Sv). We nevertheless demonstrate that satellite altimetry reproduces the seasonal, intra-seasonal, and inter-annual variability of the FC transport fairly well, as well as large transport anomalies during extreme weather events, such as tropical storms and hurricanes. The altimetry-derived transport can be provided in near-real time and serve the need to fill in data gaps in the cable record and assess its quality over time.

The properties and generation mechanisms of the Florida Current subseasonal variability (20 – 100 days) are evaluated from in-situ and satellite observations. The Florida Current volume transport estimates from submarine cable measurements reveal that subseasonal variability accounts for 37% of the total transport variance. It is most active through September - November and marked by quasi-monthly variation. Here we show that coastal-trapped waves generated by alongshore winds off the southern Mid-Atlantic Bight coast are the primary driver of the Florida Current transport subseasonal variability. In contrast, the role of local winds is insignificant. The subseasonal coastal-trapped waves cover a waveguide from Cape May to Port Isabel within 15 days with an average phase speed of 2.5 ± 0.4 m s-1. While transiting in the Florida Straits, the subseasonal waves modulate the Florida Current transport by up to 2.6 Sv, on average, close to the standard deviation of the total transport variability of 3.4 Sv. Under strong stratification in the Florida Straits, manifested in the Rossby deformation radius exceeding the cross-shelf length scale by a factor of 5, the waves exhibit Kelvin wave properties expected from theory. As the waves propagate into the Gulf of Mexico, their energy substantially dissipates. The wave amplitude at Cape May of up to 15 cm is more than five times higher than at Port Isabel.