Long-term monitoring of the NW Atlantic through altimetry and an acoustic Doppler current profiler

Monitoring long-term variability in the vicinity of the Gulf Stream provides insight into the larger-scale gyre circulation in the Atlantic including northward heat and mass transport by the western boundary current. The containership M/V Oleander has an acoustic Doppler current profiler (ADCP) attached to its hull and has measured upper-ocean current during its weekly route between New Jersey and Bermuda since late 1992. The transit covers three distinct regions of the northwest Atlantic including the Slope Sea, Gulf Stream, and Sargasso Sea. The 16-year ADCP record provides an excellent data set to create a mean geostrophic sea surface height (SSH) field across this area for comparison with the mean SSH product used with the altimeter SSH anomalies. The mean fields show good agreement, but the derived SSH mean product shows a wider Gulf Stream with a 132 cm difference in SSH across a 320 km wide region, while the ADCP mean SSH field shows a 110 cm difference in sea surface height across a 260 km wide region. In addition, the Gulf Stream core in the derived product appears to be located approximately 30-50 km to the northwest. The ADCP mean field depicts a slower and broader return flow in the Sargasso Sea, and the slope is about half of that from the derived SSH mean product. Surface-layer fluxes can be calculated across the Oleander line with very high horizontal resolution. Altimeter SSH, due to its global coverage, potentially provides a regional context to interpret the Oleander fluxes. It is important to show that the two data sets estimating fluxes complement one another. This study utilizes a mapped altimeter SSH product. Through geostrophy, surface fluxes are calculated from SSH and compared to the in-situ measurements provided by M/V Oleander in the Slope Sea, Gulf Stream, and Sargasso Sea. Comparing annual fluxes within subregions defined by dynamic boundaries proved challenging due to rapid shifts in the narrow, intense Gulf Stream that were not resolved by the altimeter and the mean SSH product depicting a stronger and wider Gulf Stream than the ADCP mean SSH field. Defining fixed regions across the Oleander line whose endpoints were local minima in SSH variability led to surface-flux estimates with strikingly similar results and correlations of 0.94 and greater in the Slope and Sargasso Seas. Total fluxes across the breadth of the Oleander transit determined by the ADCP and altimeter, however, show differences that reduce the correlation to 0.58 and even exceed the standard error estimates in one year. These differences most likely result from ageostrophic contributions measured by the ADCP including Ekman and inertial fluxes. ADCP flux variance exceeds that of the altimeter and can be attributable to ageostrophic influences. Estimated inertial and Ekman fluxes produce variance estimates that account for 38–94% of the variance in the ADCP fluxes. Both measurement systems contribute to monitoring interannual and decadal variability of the region