Six years of simultaneous moored observations near the western and eastern boundaries of the South Atlantic are combined with satellite winds to produce a daily time series of the basin‐wide meridional overturning circulation (MOC) volume transport at 34.5°S. The results demonstrate that barotropic and baroclinic signals at both boundaries cause significant transport variations, and as such must be concurrently observed. The data, spanning ~20 months during 2009–2010 and ~4 years during 2013–2017, reveal a highly energetic MOC record with a temporal standard deviation of 8.3 Sv, and strong variations at time scales ranging from a few days to years (peak‐to‐peak range = 54.6 Sv). Seasonal transport variations are found to have both semiannual (baroclinic) and annual (Ekman and barotropic) timescales. Interannual MOC variations result from both barotropic and baroclinic changes, with density profile changes at the eastern boundary having the largest impact on the year‐to‐year variations.

We present the first-ever daily time series of the full-depth volume transports by the overturning circulation cells at 34.5°S.

Hurricane Michael formed on October 7, 2018, in the Northwestern Caribbean Sea, and quickly traveled northward through the Gulf of Mexico, making landfall on the Florida panhandle as a devastating Category 5 hurricane only 3 days later. Before landfall, Michael underwent rapid intensification, despite unfavorable atmospheric conditions. Using observations, we characterized the key ocean features encountered by Michael along its track, which are known for favoring hurricane intensification: high sea surface temperatures, upper ocean heat content and low salinity barrier layer conditions. Ocean observations were consistent with suppressed hurricane‐induced upper ocean cooling, which could only be observed by underwater gliders, and showed that Hurricane Michael constantly experienced sea surface temperatures above 28°C. We carried out ocean Observing System Experiments, which demonstrate that the combined assimilation of in situ and satellite ocean observations into a numerical ocean model led to the most realistic representation of the ocean conditions. They also suggest that, when using the Cooper‐Haines (1996) method to assimilate altimetry observations, assimilating temperature observations is necessary to constrain the model upper ocean vertical structure. We also performed coupled hurricane‐ocean simulations to assess the impact of ocean initial conditions on forecasting Michael. These simulations demonstrate that the ocean conditions, in particular the high sea surface temperatures north of 24°N, played a crucial role in the intensification of Michael. Coupled simulations initialized with the most realistic ocean conditions, constrained by field and satellite observations, show a ∼56% error reduction in wind intensity prior to landfall compared to simulations initialized without data assimilation.