Observing the cooling of the North Atlantic Ocean during the last decade, using weather satellites.
17 September 2021
06 September 2021
By Rob Roebeling, Viju John, José Prieto, Vesa Nietosvaara, Christine Traeger-Chatterjee, Hayley Evers-King and Ben Loveday
Since the beginning of last century, temperatures around the world have mostly increased (IPCC, 2013). However, there are still areas where temperatures are cooling, , for example large parts of the North Atlantic Ocean. North Atlantic temperatures still warmed from the 1980s until the mid-2000s. However, these temperatures reached their peak round 2004, after that year they started cooling and continued to do so until today (Ruiz-Barradas et al., 2018).
Cooling temperatures in a global warming climate may seem counterintuitive, but can be explained by increased melting of the Greenland ice sheet. The area of cooler and fresher waters is located southeast of Greenland (approximately 55°N, 35°W). Scientists associate this cooling with slowing down of the Atlantic Gulf Stream, or Atlantic Meridional Overturning Circulation (AMOC), which reduces the transport of heat from the tropics in the North Atlantic (Rahmstorf et al., 2015). This cooling and weakening of the Atlantic Gulf Stream may have significant future implications for climate in the North Atlantic region (Openheimer et al., 2021, Ruiz-Barradas et al., 2018).
The Atlantic Gulf Stream moves warm, salty water from the equator to the North Atlantic (Figure 1).
This warm and salty water flows over cooler water that is less salty but more dense. As the warm and salty water cools, it becomes more dense and heavier than the surrounding water, and starts sinking to great depths. This powerful deep convection takes place in a few locations in the North Atlantic — such as the Labrador Sea and the Greenland–Iceland–Norway (GIN) Sea — and drives the circulation of millions of cubic metres of water in the ocean (Figure 2).
The melting of Greenland ice sheet, pouring freshwater into the North Atlantic, drives the weakening of the Atlantic Gulf Stream through decreasing the salinity and density of ocean waters, and increasing the buoyancy of ocean waters in the region. This hinders new warm, salty water from the equator from streaming into the North Atlantic because the, now, more buoyant cooler waters cannot clear out of the way. The weakening of the Atlantic Gulf Stream impacts weather in northern Europe, as well as in the tropics, and can, when it persists, lead to a cooler climate in northern Europe and a warmer climate in other regions (Kelly and Dong, 2004).
Observing sea surface from space
With modern climate data records, it is possible to observe the past evolution and present state of water temperatures, water salinity, and masses of melt water from the Greenland ice sheet, three important indicators of the state of the Atlantic Gulf Stream.
Warming of ocean waters can be observed along the coast of Greenland, consistent with studies on the impacts of sea ice loss (Pedersen and Christensen, 2019), and glacier retreat (Wood et al., 2021)
A cooling of ocean waters, observed from satellite and weather forecasting model reanalysis sea surface temperatures (Figure 3), could hint at a reduced inflow of warm waters from the equator. A sweetening of ocean waters, as observed from ocean surface salinities, signals an increased inflow of freshwater into the North Atlantic (Figure 5).
Recent cooling of the North Atlantic Ocean
The Ocean and Sea Ice Satellite Application Facility (OSI SAF) provides data records of Sea Surface Temperatures (SST) derived from cloud cleared Advanced Very High Resolution Radiometer (AVHRR) observations from the Metop and NOAA satellites. Similarly, the ECMWF makes available a Sea Surface Temeprature climate data record through their fifth ECMWF reanalysis (ERA-5).
Although the large-scale patterns of sea surface temperature over the North Atlantic may seem unchanged during the period 2004–2020, a more detailed analysis shows that the surface of the North Atlantic has cooled in large parts by up to 3 Kelvin since 2004. This is clearly visible from the sea surface temperature differences images in Figure 3.
What is striking in these figures is the strong cooling that occurs over very large parts of the Atlantic Ocean. The largest area with cooling occurs southeast of Greenland (SE) (see area indicated with the circle in Figure 3). Since 2014, the seawater in this area has cooled rapidly, by about 3 Kelvin (see Figure 4, left panel). A smaller area with cooling occured northwest of Greenland in Labrador Sea (see area indicated with the arrow in Figure 3). As this area is closer to the Greenland coast, its cooling could mainly be attributed to strong outflows of fresh melt water. Because annual variation in outflows of freshwater drive changes in seawater temperatures in the Labrador Sea, the cooling patterns in this area are more 'bumpy' (see Figure 4, right panel). Remarkably, the only areas where cooling seems absent are the waters of the Greenland–Iceland–Norway Sea.
Note that the sea surface temperatures in the region are also linked with the North Atlantic Oscillation, (Wang et al., 2004). Interconnected ocean and atmosphere forcing can cause feedback on seasonal scales, and links have been shown between the NAO index, and preceding winter SSTs, particularly with regards to the Gulf Stream region.
Variations in salinity levels over the North Atlantic Ocean
As stated above, part of the observed cooling is associated with freshwater inflow from melting Greenland ice. Besides cooling the ocean, the ample inflow of fresh water also leads to decreased surface ocean salinities (Openheimer et al., 2021). Animations of North Atlantic Ocean surface salinities over the period 1993–2020 (see Figure 5), taken from the Copernicus Marine Environment Monitoring Service (CMEMS), confirm that the inflow of fresh melt water lowers these salinities close to the coast of Greenland. This is the case during the month with minimum ice melt (March) and with the month with maximum ice melt (September). However, the lowest ocean surface salinities occur in September along the coast of Greenland (see Figure 5, right panel).
Overall, no systematic increasing or decreasing trend in ocean surface salinities occurred during the period 1993–2020 (Figure 6).
What can be seen though is that year-to year variations in ocean surface salinities have increased dramatically since the year 2005, i.e., the year when the North Atlantic Sea Surface Temperatures peaked. Northwest of Greenland very low surface salinities occurred during September 2008 and 2017 (see Figure 6, left panel), while southeast of Greenland very low surface salinities occurred during September 2010 and 2015 (see Figure 6, right panel).
Greenland lost a considerable amount of ice over the period 2006-2020m with the largest losses occurring between 2006–2013 and 2016–2017 (Sasgen, I., et al., 2020). As stated, the outflows of melt water lead to lower salinities of ocean surface waters that, in turn, could trigger a slowdown, or even a shutdown, of the Atlantic Gulf Stream. Such types of shutdowns are not uncommon and, for example, already happened in the Labrador Sea in the 1970s (Rahmstorf et al., 2015).
The ocean salinity profiles of the last 10 years confirm the strong decrease in salinities that manifests from the ocean surface down to more than 2000m (see Figure 7).
Due to the lack of salty water at the ocean surface, further cooling prevents this water from becoming heavier than the layers of water underneath, stopping it from sinking to great depths and, in effect, slowdown, or even shutdown, deep convection.
Although major uncertainties remain about the evolution of the Atlantic Gulf Stream for lack of direct measurement, indirect evidence from multiple sources provides a consistent picture, linking together the time evolution of sea surface temperatures, ocean salinities and Greenland ice melt volumes. If the interconnections between these components continue, as we believe, the ongoing melting of the Greenland ice sheet may lead to further freshening of the North Atlantic in the decades to come. This might lead to further weakening of the Atlantic Gulf Stream, and, possibly, even more permanent shutdown of Labrador Sea convection, as has been predicted by some climate models (Rahmstorf et al., 2015).
The recent IPCC Annual Report 6 and the special report on the ocean and cryosphere, state that it is likely the AMOC will weaken over the 21st century, however, distinction between observed weakening, decadal variability, and long term trends in the AMOC is currently challenging due to lack of observations. The combined use of sea surface temperature and ocean surface salinity data records can help to increase our understanding of the underlying processes governing the past evolution and present state of the AMOC. The quality of the OSI SAF and ERA-5 data records of sea surface temperature is confirmed by their quantitative agreement as their consistency in spatial patterns.
- Satellite-based (AVHRR on Metop-A, -B, and NOAA-19) data record of Sea Surface Temperatures from 2006–2020, provided by the OSI SAF. DOI: http://doi.org/10.15770/EUM_SAF_OSI_0004
- A reanalysis-based climate data record of Skin Temperatures from 2006–2020, from ECMWFs fifth reanalysis (ERA-5). This reanalysis provides the most complete picture currently possible of past weather and climate. They are a blend of observations with past short-range weather forecasts rerun with modern weather forecasting models. They are globally complete and consistent in time and are sometimes referred to as ‘maps without gaps’. DOI: https://doi.org/10.24381/cds.e2161bac
- Satellite and in-situ measurements-based climate data record of monthly ocean surface salinities from 1993–2020, from the CMEMS (Guinehut et al., 2012, Mulet et al. 2012).
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