Award: OCE-0648016

Hydrostation æSÆ (also known as the Panularis site) located some 25km south east of Bermuda in the Sargasso Sea represents the site of the longest maintained oceanographic hydrography timeseries. Initiated by Dr. Henry Stommel (WHOI) and co-workers in 1954, this invaluable oceanographic biweekly timeseries extends to nearly 58 years (total of 1197 hydrographic stations) on completion of this past award in March 2012. Historically, the Hydrostation æSÆ program through its core hydrography measurements and other ancillary work has facilitated a diversified list of research topics. The dominant theme of these studies has been long-term change relating to climate issues using the unprecedented near six decades of physical hydrography and 29-year record of upper ocean CO2 measurements. In addition to local convective mixing it also appears that the very cold mixed layers in 2010 and 2011 may have been enhanced on interaction with cold cyclonic mesoscale eddies that propagated through the local area at this time. Analysis of these deep cold mixed layers of 2010 and 2011 is currently on going in particular with reference to the Bermuda Atlantic Time-series Study Site (BATS, located 80 km south east of Bermuda) site in order to help better quantify the nutrient flux during this time period. The implied new production for 2010 and 2011 is significantly higher (order of magnitude) than previous years and likely commensurate with regions well to the north of Hydrostation æSÆ in the subtropical mode water (STMW) formation region of the Sargasso Sea. Observations of the integrated oxygen budget in the upper ocean for these years (2010 and 2011), clearly suggests enhanced productivity. Variability in the vertical mixing as observed in these past five years at Hydrostation æSÆ has profound implications for understanding the transfer of heat, fresh water and carbon (through both physical and biological processes) in the upper ocean and clearly highlights the importance of these time-series observations. Although substantial inter-annual variability is observed in the upper ocean at this time-series location, significant long-term trends in the temperature and salinity of the upper ocean at this site are clearly evident. For the upper 400 m, temperature is increasing by approximately 0.01 °C yr-1 while salinity is rising at approximately 0.002 yr-1. The level of significance for both temperature and salinity increases with depth reaching maximal values coincident with the average depth of STMW (core at ~ 300m). Interestingly, if these trend analyses are restricted to the time span of the BATS time-series (1988 to 2012) then different scenarios emerge for the upper ocean and surprisingly, BATS and Hydrostation æSÆ exhibit differing trends at fixed depths. For the upper 300 m neither location reveal long term signals in temperature although for deeper depths (400 to 600m) extending from STMW to central waters Hydrostation æSÆ data show a significant warming of ~ 0.01 to 0.02 °C yr-1. For salinity, the more recent trends (1989-2012) at the two sites are similar to the trends documented at Hydrostation æSÆ for years 1955-2011 although clear differences do exist. It appears that the increase in salinity at Hydrostation æSÆ for the upper 600 m has accelerated in the past two decades while at BATS an increase is only evident between 200 and 500m at a rate similar to the previously documented long term Hydrostation æSÆ value. For the lower mode water and central waters, the increase in salinity at Hydrostation æSÆ is almost double that observed at BATS. For this past pentad, substantial variability in the upper ocean has been observed which can be broadly characterized by relatively shallow winter mixing in years 2007, 2008 and 2009 followed by intense mixing in 2010 and 2011. The contrast between the winters of 2008/2009 and 2009/2010 was striking where maximal mixed later depths were 210 m and 450 m, respectively. Additionally, the diff...