No abstract

The NASA announcement of record surface melting of the Greenland ice sheet in July 2012 led us to examine the atmospheric and oceanic climatic anomalies that are likely to have contributed to these exceptional conditions and also to ask the question of how unusual these anomalies were compared to available records. Our analysis allows us to assess the relative contributions of these two key influences to both the extreme melt event and ongoing climate change. In 2012, as in recent warm summers since 2007, a blocking high pressure feature, associated with negative NAO conditions, was present in the mid-troposphere over Greenland for much of the summer. This circulation pattern advected relatively warm southerly winds over the western flank of the ice sheet, forming a 'heat dome' over Greenland that led to the widespread surface melting. Both sea-surface temperature and sea-ice cover anomalies seem to have played a minimal role in this record melt, relative to atmospheric circulation. Two representative coastal climatological station averages and several individual stations in south, west and north-west Greenland set new surface air temperature records for May, June, July and the whole (JJA) summer. The unusually warm summer 2012 conditions extended to the top of the ice sheet at Summit, where our reanalysed (1994-2012) DMI Summit weather station summer (JJA) temperature series set new record high mean and extreme temperatures in 2012; 3-hourly instantaneous 2-m temperatures reached an exceptional value of 2.2°C at Summit on 11 July 2012. These conditions translated into the record observed ice-sheet wide melt during summer 2012. However, 2012 seems not to be climatically representative of future 'average' summers projected this century. © 2013 Royal Meteorological Society

Open-ocean deep convection, one of the processes by which deep waters of the world's oceans are formed, is restricted to a small number of locations (for example, the Mediterranean and Labrador seas). Recently, the southwest Irminger Sea has been suggested as an additional location for open-ocean deep convection. The deep water formed in the Irminger Sea has the characteristic temperature and salinity of the water mass that fills the mid-depth North Atlantic Ocean, which had been believed to be formed entirely in the Labrador basin. Here we show that the most likely cause of the convection in the Irminger Sea is a low-level atmospheric jet known as the Greenland tip jet, which forms periodically in the lee of Cape Farewell, Greenland, and is associated with elevated heat flux and strong wind stress curl. Using a history of tip-jet events derived from meteorological land station data and a regional oceanic numerical model, we demonstrate that deep convection can occur in this region when the North Atlantic Oscillation Index is high, which is consistent with observations. This mechanism of convection in the Irminger Sea differs significantly from those known to operate in the Labrador and Mediterranean seas.

No abstract

No abstract

[1] The near-surface circulation of the Nordic Seas is basically cyclonic and consists of jets and recirculation cells, which are tightly linked to the bottom topography. Variable forcing by the large-scale rotation of the wind leads to a modulation in the strength of the gyres and their interconnecting jets. This is seen in drifter and altimeter data. Currents are stronger during winter and during phases of high North Atlantic Oscillation Index. The exchanges between the North Atlantic and the Nordic Seas do not seem to be directly affected by this variable forcing. The narrow boundary currents and the intergyre jets are subject to instability, causing mesoscale current fluctuations, which contribute to the stirring and mixing of Polar and Atlantic water masses.

The Atlantic Meridional Overturning Circulation (AMOC) is an important component of ocean thermohaline circulation. Melting of Greenland's ice sheet is freshening the North Atlantic; however, whether the augmented freshwater flux is disrupting the AMOC is unclear. Dense Labrador Sea Water (LSW), formed by winter cooling of saline North Atlantic water and subsequent convection, is a key component of the deep southward return flow of the AMOC. Although LSW formation recently decreased, it also reached historically high values in the mid-1990s, making the connection to the freshwater flux unclear. Here we derive a new estimate of the recent freshwater flux from Greenland using updated GRACE satellite data, present new flux estimates for heat and salt from the North Atlantic into the Labrador Sea and explain recent variations in LSW formation. We suggest that changes in LSW can be directly linked to recent freshening, and suggest a possible link to AMOC weakening.

The forcings behind the rapid increase in mass loss from the Greenland Ice Sheet in the early 2000s (ref. 1) are still debated. It is unclear whether the mass loss will continue in the near future and, if so, at what rate. These uncertainties are a consequence of our limited understanding of mechanisms regulating ice-sheet variability and the response of fast-flowing outlet glaciers to climate variability. In southeast Greenland, Helheim Glacier, one of the regions largest glaciers, thinned, accelerated and retreated during the period 2003-2005 (ref. 4) and although it has since slowed down and readvanced 9 , it has still not returned to its pre-acceleration flow rates. It has been suggested that warming 8,10 and/or inflow variability 11,12 of the nearby subsurface ocean currents triggered the acceleration, but to establish a causal relationship between glacier and climate variability, long-term records are needed. Here we present three high-resolution (1-3 years per sample) sedimentary records from Sermilik Fjord ( Fig. 1 and Supplementary Information) that capture the 2001-2005 episode of mass loss, and use them to reconstruct the calving variability of Helheim Glacier over the past 120 years. Next, this record is compared with records of climate indices.