Capitalizing on Observations from the CMV Oleander and Explorer of the Seas to Examine Causes and Consequences of Long-Term Variability in the Northwestern North Atlantic

The Gulf Stream serves both as the return flow for the North Atlantic’s wind-driven subtropical gyre and carries the warm limb of the Atlantic Meridional Overturning Circulation poleward.  Here we consider the causes and consequences of low frequency variability in the western North Atlantic detected using a stream coordinates approach to identify the meandering Gulf Stream front which separates cold fresh waters and the shallow thermocline in the Slope Sea from the warm salty waters and the deep thermocline in the Sargasso Sea. This analysis capitalizes on observations collected from the CMV Oleander – a ship of opportunity that regularly crosses the Gulf Stream along a transect between New Jersey and Bermuda – with expendable bathythermographs (XBTs) deployed since the late 1970s and velocity profiles collected since the early 1990s using a hull-mounted acoustic Doppler current profiler (ADCP). Additional information from Argo floats, shipboard hydrographic casts and satellite altimetry provides spatial and temporal context for the Oleander measurements.  Observations show that the Slope Sea area is shrinking as the Gulf Stream axis (identified by the location of the velocity maximum at 55 m depth along the Oleander Line) shifts northward and that the ambient Slope Sea waters are experiencing surface-intensified warming that reaches to about 750 m depth.  This upper-ocean warming may reflect increased mixing of waters carried into the Slope Sea by warm core rings shed from the Gulf Stream and is consistent with a reported regime shift in 2000 when the average number of warm core rings shed annually from the Gulf Stream nearly doubled.  The shrinking area inferred from the Oleander Line velocity measurements is consistent with maps of the kinetic energy based on altimetry, which suggest that this region of northward shifted Gulf Stream stretches from Cape Hatteras (~75°W) to about 69°W. In the Sargasso Sea, warming is concentrated within the “Eighteen Degree Water” (the subtropical North Atlantic mode water, STMW) which has warmed to 19°C.  This warming of STWM is accompanied by a decrease in thickness of the STMW layer and deepening of the top of this layer.  Global mean sea level is reportedly increasing by about 2.9 mm/yr, however, the increase in sea surface height (SSH) in the North Atlantic is not uniform in the region spanning the Gulf Stream between 68° and 58° W. In the Slope Sea, SSH is increasing by 1.6 mm/year while in the Sargasso Sea SSH is increasing by 4.0 mm/yr.  The CMV Oleander observations, together with occasional velocity transects to 1000-m depth from an ADCP mounted on the Explorer of the Seas cruise ship, provide valuable benchmarks for comparison to ocean reanalysis products and state estimates.  CMV Oleander continues to make regular measurements with an expanded sensor suite that includes meteorological and biological sampling.