Estimates of basal melt from Greenland: a driver for sea level changes

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Mass loss from the Greenland Ice Sheet (GrIS) increased significantly over the past two decades. Besides the growing influence on sea level rise, the pathways and the amount of GrIS melt water imported into the interior of the ocean could have crucial consequences for the ventilation and formation of deep water and hence the strength of the climate-relevant Atlantic meridional overturning circulation (AMOC). In most climate and coupled ocean-sea ice models weak convection leads to a weaker AMOC and a regionally changing sea level. It is thought that the arrival of additional melt water in the deep-water formation region could strengthen the density stratification and thus weaken or even suppress deep convection. Some publications concluded that the GrIS melt has already slowed down the AMOC in the twentieth century, while others found that the additional freshwater from GrIS has not exerted any influence on deep water formation yet. The main result of the first phase is, that for the first time, submarine melt water (SMW) from GrIS was unambiguously identified outside of fjords. SMW fractions are highest in the upper 200 – 300m below the mixed layer and mostly confined at the Greenland and Canadian slope and shelf. The strong dilution of SMW in the upper ocean and the relatively small SMW percentages compared to the abundance of fresh Polar Water indicate that SMW is not a main player for LSW formation yet. The mechanism leading to the episodic occurrence of He excess anomalies in the deep water masses is uncertain. Based on the salinity – SMW relation, it could herald episodic East Greenland Current spill jets events that contains SMW bearing water with densities larger than 27.5. However, these results are based on a small noble gas data set.The objectives for the continuation phase are: (1) Quantify SMW fractions from GrIS in the Greenland boundary current, the Labrador and the Irminger Sea using much more detailed data sets (He, Ne, CFCs, SF6) than was available until now and by applying the noble gas excess method and an OMP (2) Assess the SMW signature in the area of origin of the EGC spill jet, evaluate which density classes are affected and whether more events with SMW in spill jets are found in 2018 and 2019 compared to 1994 and 2015(3) Calculate SMW fluxes using velocity measurements and the ventilation time inferred from transient tracers as CFCs and SF6(4) Assess how much the formation of LSW and the ventilation of other relevant water masses in the Labrador and Irminger Sea changed. Analyze the processes responsible and estimate how much SMW needs to be present on which key locations to significantly influence formation and ventilation of the relevant water masses. With this information estimate the time period until the SMW signal is sufficiently strong to fulfil this criterion.