Great Salinity Anomaly

The Great Salinity Anomalies of the 1970s and 1980s were well-documented decadal-scale events, where minima in salinity (and temperature) were observed successively in different basins around the northern North Atlantic Ocean.^[4] The fact that the anomaly was observed in different basins after each other indicates that this was an advective event, accounted for by the movement of a fresh (and cold) anomaly along main ocean currents. For the 1970s GSA, the propagation was traceable around the Atlantic sub-polar gyre from its origins in the northeast of Iceland in the mid- to late- 1960s until its return to the Greenland Sea in 1981–82.^[5] The 1980s GSA began with the anomaly being advected by the West Greenland Current In 1982 and ending up back in north Icelandic waters in 1989–90.

Salinity is a measure of how 'salty' water is, or the amount of dissolved matter within seawater. This is measured by passing seawater through a very fine filter to remove particulate matter. Historically, this was measured using a glass fibre filter with a nominal pore size of 0.45 ${\displaystyle \mu m}$. More recently, though, smaller and smaller pores have been used.

Salinity is difficult to measure directly as dissolved matter in seawater is a complicated mixture of virtually every known element and it is impossible to measure to complete composition of every water sample. Originally, chlorinity (the measure of the chloride content, by mass, of seawater^[6]) was measured and converted to salinity using a simple linear function. Since the early 1980s, the value calculated as salinity is actually the Practical Salinity, which is a proxy for the true salinity. The new seawater standard TEOS-10 defined a better measure of salinity used since 2010, named Absolute Salinity. This is measured by first measuring the electrical conductivity, temperature, and pressure of a water parcel.^[7] The electrical conductivity of a water sample is influenced by the concentration and composition of dissolved salts, as salts increase the ability of a solution to conduct an electrical current.^[8]

For the GSA's, the difference in salinity compared to a reference salinity is used in order to identify the anomaly, and salinity is measured using the practical salinity values, which are unitless.

In the North Atlantic Ocean, the high salinity of northward-flowing upper waters leads to the formation of deep, cold, dense waters at the high latitudes. This is a vital driver of the meridional overturning circulation (MOC). Increasing the influx of fresh water (which is less dense than saltier water) lowers the salinity of the upper layers, leading to a cold, fresh, light upper layer once cooled by the atmosphere. In turn, this deep water driver of the MOC is weakened, in turn weakening the MOC.^[9]

The GSA's observed could have different driving causes. For the anomaly in the late 1960s and early 1970s, the main cause of the anomaly was by a freshwater and sea ice pulse which came from the Arctic Ocean via the Fram Strait. Studies show an indirect cause of this pulse to be abnormally strong northern winds over the Greenland Sea, which brought more cold and fresh polar water to Iceland, which was in turn caused by a high pressure anomaly cell over Greenland in the 1960s.^[10] This is known as a remote cause of GSA's. However, local conditions such as cold weather are also important for the preservation of a GSA, in order to stop the anomaly being mixed out and allowing it to propagate as the GSA of the 1970s did.

As for the anomaly of the 1980s, the cause is likely to be more local. This GSA was likely caused by the extremely severe winters of the early 1980s in the Labrador Sea and the Baffin Sea. However, as with the earlier GSA, there is also the remote aspect - the GSA was likely supplemented by Arctic freshwater outflow.^[11]