Environment Counts | Reconstruction of North Atlantic surface temperature and salinity from 818 to 1780 AD
Author: Geoff Zeiss – Published At: 2014-07-20 12:39 – (1208 Reads)
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The Atlantic Meridional Overturning Circulation (AMOC) is a global ocean current system which transports heat and nutrients throughout most of the Earth’s oceans and has a strong influence on the Earth’s climate. A key driver is the the North Atlantic Current (NAC) which carries salt to the northern ocean and is essential for maintaining the high salinity of surface waters in the Nordic and Labrador seas. The sinking of highly saline water in the North Atlantic is responsible for deep water formation which the AMOC carries to the Earth’s oceanic basins. In this article the authors report a reconstruction of temperature and salinity south of Iceland over about a thousand years from 818 to 1780 AD. The reconstruction found several abrupt shifts each occurring over several decades in the temperature of the NAC waters of ~3.5 ± 1.1°C between AD 818 to 1780. Some correlation between the timing of the temperature shifts in the sediment record and variations in solar irradiance was found at time scales of decades to centuries. Nature Geoscience 7, 275–278 (2014) doi:10.1038/ngeo2094
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The North Atlantic Current (NAC) carries salt to higher latitudes and is essential for maintaining the high salinity of surface waters in the Nordic and Labrador seas. High salinity in the North Atlantic is a critical part of the Atlantic meridional overturning circulation (AMOC). The sinking of saline waters at high latitudes is responsible for deep water formation. The deep dense water flows into the Earth’s ocean basins. Most of it upwells in the Southern Ocean, but some of it upwells in the North Pacific. The cycle time for some of these waters can reach on the order of a century. The water masses transport heat and matter around the globe which has a large impact on the climate of the Earth. For example, the heat released from the NAC, transported by westerly winds, contributes to warming the climate of Europe.
In this article the authors report a reconstruction of temperature and salinity in the Iceland Basin region south of Iceland over about a thousand years from AD 818 to 1780 from oxygen-18/oxygen-16 and Mg/Ca ratio measurements of single-celled marine protozoan shells from a marine sediment core sampled at 2,303 m water depth.
Figure Sea surface temperature map for January 2008 showing the surface circulation of the North Atlantic. Solid arrows indicate the warm salty current from the tropics, the North Atlantic Current (NAC) and its main branches. The dashed arrows indicate the cold south-flowing waters such as the East Greenland Current, West Greenland Current and Labrador Current. Locations of sediment cores RAPiD-17-5P (2,303 m water depth) and RAPiD-35-25B (3,486 m water depth) are marked with black circles.
Delta-oxygen-18, which is a measure of the ratio of stable isotopes oxygen-18 and oxygen-16, is commonly used as a proxy for temperature. The concentration of Mg in the calcite shells of single-cell protozoans is also an established proxy for temperature. When it is combined with the Delta-oxygen-18 ratio of the same calcite, it is possible to derive the temperature and salinity of the sea water in which the protozoans lived. The upper 600 m of the ocean at the core site RAPiD-17-5P is dominated by the northward-flowing NAC so the sediments collected there record the temperature and salinity of the NAC. The chronology for the core was obtained using radiocarbon dates with a resolution of ~6 years between AD 818 and 1780 years.
Figure Shaded areas highlight the well-known periods of solar radiation minima. a Solar irradiance (orange) and global volcanic stratospheric aerosols (grey) b and c Temperature and salinity derived from paired Mg/Ca and delta-oxygen-18 measurements of shells from RAPiD-17-5P core. SMOW, standard mean ocean water. d Comparison of temperature record from sea sediments (black) and total solar irradiance (orange)
The reconstruction of the temperature and salinity of surface waters (to 600 m) of the North Atlantic waters responsible for deep water formation is an important result. The reconstruction reveals substantial variability in temperature and salinity of the NAC waters during the past millennium. In particular, there are several abrupt shifts in temperature and salinity of ~3.5 ± 1.1°C and ~1.2 ± 0.8, respectively. Note that salinities here are measured using the Practical Salinity Scale 1978 (PSS-78) which does not have units.
Correlating temperature and salinity with solar irradiance
The authors attempt to relate temperature and salinity to variations in solar irradiance. Solar irradiance is a measure of electromagnetic radiation (sunlight) hitting any given point on the Earth’s surface, usually expressed as watts/square meter. Total solar irradiance (TSI), is a measure of the total amount of solar radiative energy incident on the entirety of the Earth’s upper atmosphere.
By comparing temperature and salinity with total solar irradiance (TSI) they find that the timing of the abrupt shifts in the sediment record exhibits some correlation with TSI variability, minima in solar irradiance generally correspond to cold and fresh conditions in the NAC and solar irradiance maxima correlate with warm and salty conditions in the NAC.
The “Pearson correlation coefficient” is a measure of the strength of the linear relationship between two variables. In this case a computed Pearson’s correlation coefficient of 0.51 (n = 77) indicates some correlation between the sediment temperature record and TSI, but also clearly shows that TSI does not explain all the variance in the sediment temperature record.
Wavelet transform analysis is a technique for analyzing a signal, in this case temperature and salinity, for repeating patterns corresponding to certain frequencies. Wavelet transform analysis of the temperature record reveals a roughly 200-year cycle which is especially strong between AD 1200 and 1650. Further analysis finds that that sediment temperature record and TSI are in synchrony above the 90% confidence level in the frequency range 177–227 years. This provides support for the correlation found between the sediment temperature record and TSI over the past millennium.
However, the argument in support of the correlation between the sediment temperature record and TSI is weakened by the similarity in timing of volcanic eruptions and solar irradiance minima during the past millennium. This makes the separation of their relative climatic influence difficult.
Relationship to atmospheric circulation
The authors argue that variation in temperature and salinity in the North Atlantic are likely to be linked to shifts in atmospheric circulation. There is evidence that small-scale atmospheric patterns in the Northeast Atlantic, such as atmospheric blocking events, may contribute to driving North Atlantic surface circulation. Atmospheric blocking events are mid-latitude weather systems where a high-pressure system located in the Northeast Atlantic modifies the flow of the westerly winds by blocking or diverting their pathway. The impacts of the frequency and magnitude of these small-scale atmospheric systems are not restricted to the ocean but also have important effects on European temperatures, as they block the warm maritime winds which are replaced by cold northeasterlies. For example, Atlantic blocking events are thought to have been responsible for several recent cold European winters (for example, 1963, 2009, 2010 and 2013).
