Evidence of a Weakening Gulf Stream from In-situ Expendable Bathythermograph Data, 1996-2013

Abstract:

A weakening of the Gulf Stream, the upper branch of the Atlantic Meridional Overturning Circulation system, has been hypothesized to accelerate sea level rise on the east coast of the United States, caused by changes in the Gulf Stream strength and, hence, sea level difference across the current. It still remains unclear if the Gulf Stream has in fact weakened or remains stable, along with the potential role of natural long-term variability. Tide gauges along the east coast show an accelerated sea level rise from Cape Hatteras to Cape Cod that is 3-4 times higher than global sea level rise. Satellite altimetry shows a weakening gradient in Gulf Stream sea surface height that is highly correlated (r=-0.85) with east coast sea level rise, however, direct velocity measurements showed no significant decrease in Gulf Stream strength over a similar time period. We introduce another in-situ dataset to examine the issues between these conflicting results. Expendable bathythermographs (XBTs) measure temperature at depth directly, and then depth and salinity can be inferred, along with geostrophic velocity and transport. XBT data has been used to measure transport in various current systems, however, the Gulf Stream transport has not been analyzed using the newest high-density XBT data made available since 1996. The trend in sea level difference is determined to be 3.3 +/- 3.2 mm/yr, resulting in an overall decrease of 5.2 cm in sea level from 1996-2013. This result agrees with satellite altimetry results that show a significant decrease in recent years. This data also shows a changing Gulf Stream core position, based on the 15°C isotherm at 200 m, of 0.03°N/yr that is negatively correlated with surface transport (r=-0.25). Issues remain in defining the core and width of the Gulf Stream and with eliminating the possibility of natural variability in the current system.