Two hydrographic open seas record 40 years of change in the subtropical North Atlantic Ocean.
New research published in Nature communication Soil and environment today (October 1
The study used data sets from the Hydrostation ‘S’ and Bermuda Atlantic Time-series Study (BATS) projects at the Bermuda Institute of Ocean Sciences (BIOS). Both are led by Professor Nicholas Bates, senior researcher in BIOS and project lead researcher (PI), and Rod Johnson, BIOS assistant scientist and project co-PI. Together, these time series represent the two longest continuous records of data from the global open sea.
“The four decades of data from BATS and Hydrostation ‘S’ show that the ocean does not change uniformly over time and that the carbon sink in the ocean has not been stable in recent times with variation from decade to decade,” Bates said.
Of the two places, Hydrostation ‘S’ is the oldest, located approx. 25 km southeast of Bermuda and consisting of repeated two weeks of hydrographic observations of temperature, salinity and dissolved oxygen conducted through the water column since 1954. The Bermuda Atlantic Time Series Survey Site (BATS) is located approx. 80 km southeast of Bermuda. It consists of monthly sampling of physics, chemistry and biology of the entire water column since 1988. The study data set represents more than 1381 cruises to Hydrostation ‘S’ from 1954 to 2020 and more than 450 cruises to BATS from 1988 to the end of 2019.
The results showed that the surface temperatures in the Sargasso Sea have increased by 0.85 +/- 0.12 ° C over the last 40 years, with summer surface temperatures rising higher than winter. In addition, the winter state (<22 ° C) sea level has become shorter by almost a month, while the summer season (with water warmer than 25 ° C) has become longer. During the same period, the surface salinity also increased by ~ 0.11 +/- 0.02. Importantly, these data show signs of decadal variation; over the last decade (2010-2019), however, rapid warming of 1.18 ° C and salinization at 0.14 place have been found.
The data also show a trend of decrease in dissolved oxygen (DO) in the Sargasso Sea since the 1980s, which represents a loss of ~ 2% per year. Decade. Given the observed ocean warming in the Sargasso Sea, the researchers estimate that the warming effect on DO solubility would probably have contributed to approx. 13% of the total decline in DO over the last almost 40 years. The remaining deoxygenation (~ 87%) must have been the result of the combined effect of changes in marine biology and physics.
BATS and Hydrostation ‘S’ time series data enable direct detection of the ocean acidification signal in the surface waters of the North Atlantic Ocean. The typical pH range of surface water in the 1980s ranged from winter highs of ~ 8.2 to summer temperatures of ~ 8.08-8.10, where the sea was currently slightly alkaline (~ 7.98-8.05). The pH change rate is ~ 0.0019 +/- 0.0001 year-1, which is a more negative rate than previously reported and represents a 20% increase in hydrogen ion concentration since 1983. These changes were accompanied by significant increases in dissolved inorganic carbon and CO2 and falls into both calcite and aragonite saturation states.
“For forty years, seawater has CO2“Carbonate chemical conditions are now changing in addition to the seasonal chemical changes observed in the 1980s,” Johnson said. “The change of seawater CO2 carbonate chemistry will continue with future man-made CO2 emissions. ”
Observations off Bermuda reveal the significant decadal variations and highlight the need for long-term data to determine trends in other physical and biogeochemical properties of the ocean, especially when linking local measurements with changes in the basin scale. Long-term data on marine chemistry and physical from time series sites such as Hydrostation ‘S’ and BATS provide critically needed and unique observations that, when combined with ocean atmosphere models, allow for a more complete understanding of drivers for the global carbon cycle.
Reference: “Acceleration of ocean warming, saltinification, deoxygenation and sourification in the subtropical North Atlantic Ocean surface” by Nicholas Robert Bates and Rodney J. Johnson, October 16, 2020, Communication Earth and environment.
DOI: 10.1038 / s43247-020-00030-5