This project will extend the now 52 year long series of the ocean hydrographic station near Bermuda, known as Station S or the Panulirus Station for another 5 years. These time-series data have already been extensively used by investigators in many disciplines to examine fluctuations at periods from weeks to years in the western Atlantic Ocean and producing over 100 publications over the past 10 years. Hydrostation S allows a bi-weekly context for the biogeochemical measurements that are part of the more complex multi-disciplinary Bermude AtlanticTime-series Station program and also helps provide a better understanding of the short term variability associated with mesoscale eddy dynamics in this region. Patterns of long term change and decadal variability have emerged from these data. In particular, long term warming of the deeper ocean (especially at the depth of mode water 200-400m) has been documented. However, a similar long term increase has not occurred in the surface temperature record over the whole 52 year period although there have been significant trends in both temperature and salinity over decadal time scales. Thus these data show the importance of sampling the full ocean depth over very long time periods rather than just using sea surface temperature for proxies of climate change. These data have proven to be an important link in global ocean monitoring and have allowed scientists to establish direct linkages with climate change. The extension of this time series will further deduce the significance of these climate related signals and to further understand processes effecting ocean variability. The bi-weekly sampling periodicity is essential to resolve various oceanographic processes and the unique position of this site off Bermuda means that each full ocean depth station can be accomplished with just one ship day. In addition to the core measurements, ancillary users have enhanced the measurements at this site to provide a rich array of data available to the ocean sciences community. The investigators propose to actively continue this collaboration. The only change to this time-series project is to sample the full ocean depth (~3300m) rather than the current bottom depth of 2600m. This change will require an additional CTD cast and will generate another six discrete depths for water samples. The availability of the new BBSR vessel R/V Atlantic Explorer will allow us to effectively implement this change without any impact on cruise logistics or funding amounts.
Broader Impacts During the past 5 years, numerous student programs have used the Hydrostation S project as resource for training and conducting student fieldwork. This interest continues to grow such that there are now have several Universities seeking to make use of the BBSR vessel R/V Atlantic Explorer and the Hydrostation S program as explicit part of their course work. This project will continue and extend this facilitation and further, a graduate student component will be added to this program. Hydrostation S cruises have also proved to be an invaluable resource for training oceanography technicians form BBSR (over 15 in past 5 years) and other institutions and this activity will also continue. Another advantage of the single day cruises is that it lends itself for the inclusion of journalists and media personnel from both Bermuda and international locations.
Hydrostation ‘S’ (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 ‘S’ 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 ‘S’ 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 ‘S’ 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 ‘S’ 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 ‘S’ 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 ‘S’ for years 1955-2011 although clear differences do exist. It appears that the increase in salinity at Hydrostation ‘S’ 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 ‘S’ value. For the lower mode water and central waters, the increase in salinity at Hydrostation ‘S’ 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 difference in mixed layer temperatures was also very pronounced where for years 2007 to 2009 temperatures remained above 19°C while in early 2010 and 2011 mixed layer temperatures were below 18.5 °C. Further, the integrated heat content for the upper 400m for February through April in 2010 was the lowest for the full 58-year record. Finally, a recent review of time-series programs emphasizes the importance of these sustained time-series observations in helping build a framework that lends itself to better understanding of ocean variability for assessing climate processes. Now approaching its seventh decade of observations, the Hydrostation ‘S program provides an important multi-decadal synthesis context for many oceanography programs in the North Atlantic while helping educate the next generation of oceanographers. Data from this program are considered as a service to the community as a whole, being openly distributed. All data for this past award have been processed and data through December 2011 are available for access to both the scientific community and general public.
