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Submarine melting is an important contributor to the mass balance of tidewater glaciers in Greenland, and has been suggested as a trigger for their widespread acceleration. Our understanding of this process is limited, however. It generally relies on the simplified model of subglacial discharge in a homogeneous ocean, where the melting circulation consists of an entraining, buoyant plume at the ice edge, inflow of ocean water at depth, and outflow of a mixture of glacial meltwater and ocean water at the surface. Here, we use oceanographic data collected in August 2009 and March 2010 at the margins of Helheim Glacier, Greenland to show that the melting circulation is affected by seasonal runoff from the glacier and by the fjord’s externally forced currents and stratification. The presence of light Arctic and dense Atlantic waters in the fjord, in particular, causes meltwater to be exported at depth, and influences the vertical distribution of heat along the ice margin. Our results indicate that the melting circulation is more complex than hypothesized and influenced by multiple external parameters. We conclude that the shape and stability of Greenland’s glaciers may be strongly influenced by the layering of the Arctic and Atlantic waters in the fjord, as well as their variability. Submarine melting has been suggested as a trigger for the widespread acceleration of tidewater glaciers in Greenland. An analysis of oceanographic data from the fjord off Helheim Glacier, Greenland, suggests the presence of light Arctic and dense Atlantic waters in the fjord and that the melting circulation is more complex than thought.

Declining salinities signify that large amounts of fresh water have been added to the northern North Atlantic Ocean since the mid-1960s. We estimate that the Nordic Seas and Subpolar Basins were diluted by an extra 19,000 ± 5000 cubic kilometers of freshwater input between 1965 and 1995. Fully half of that additional fresh water--about 10,000 cubic kilometers--infiltrated the system in the late 1960s at an approximate rate of 2000 cubic kilometers per year. Patterns of freshwater accumulation observed in the Nordic Seas suggest a century time scale to reach freshening thresholds critical to that portion of the Atlantic meridional overturning circulation.

The oceans are a global reservoir and redistribution agent for several important constituents of the Earth's climate system, among them heat, fresh water and carbon dioxide. Whereas these constituents are actively exchanged with the atmosphere, salt is a component that is approximately conserved in the ocean. The distribution of salinity in the ocean is widely measured, and can therefore be used to diagnose rates of surface freshwater fluxes 1 , freshwater transport 2 and local ocean mixing 3 —important components of climate dynamics. Here we present a comparison of salinities on a long transect (50° S to 60° N) through the western basins of the Atlantic Ocean between the 1950s and the 1990s. We find systematic freshening at both poleward ends contrasted with large increases of salinity pervading the upper water column at low latitudes. Our results extend a growing body of evidence indicating that shifts in the oceanic distribution of fresh and saline waters are occurring worldwide in ways that suggest links to global warming and possible changes in the hydrologic cycle of the Earth.

The subpolar North Atlantic is a center of variability of ocean properties, wind stress curl, and air–sea exchanges. Observations and hindcast simulations suggest that from the early 1970s to the mid-1990s the subpolar gyre became fresher while the gyre and meridional circulations intensified. This is opposite to the relationship of freshening causing a weakened circulation, most often reproduced by climate models. The authors hypothesize that both these configurations exist but dominate on different time scales: a fresher subpolar gyre when the circulation is more intense, at interannual frequencies (configuration A), and a saltier subpolar gyre when the circulation is more intense, at longer periods (configuration B). Rather than going into the detail of the mechanisms sustaining each configuration, the authors’ objective is to identify which configuration dominates and to test whether this depends on frequency, in preindustrial control runs of five climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). To this end, the authors have developed a novel intercomparison method that enables analysis of freshwater budget and circulation changes in a physical perspective that overcomes model specificities. Lag correlations and a cross-spectral analysis between freshwater content changes and circulation indices validate the authors’ hypothesis, as configuration A is only visible at interannual frequencies while configuration B is mostly visible at decadal and longer periods, suggesting that the driving role of salinity on the circulation depends on frequency. Overall, this analysis underscores the large differences among state-of-the-art climate models in their representations of the North Atlantic freshwater budget.

Manifold changes in the freshwater cycle of high-latitude lands and oceans have been reported in the past few years. A synthesis of these changes in freshwater sources and in ocean freshwater storage illustrates the complementary and synoptic temporal pattern and magnitude of these changes over the past 50 years. Increasing river discharge anomalies and excess net precipitation on the ocean contributed ~20,000 cubic kilometers of fresh water to the Arctic and high-latitude North Atlantic oceans from lows in the 1960s to highs in the 1990s. Sea ice attrition provided another ~ 15,000 cubic kilometers, and glacial melt added ~2000 cubic kilometers. The sum of anomalous inputs from these freshwater sources matched the amount and rate at which fresh water accumulated in the North Atlantic during much of the period from 1965 through 1995. The changes in freshwater inputs and ocean storage occurred in conjunction with the amplifying North Atlantic Oscillation and rising air temperatures. Fresh water may now be accumulating in the Arctic Ocean and will likely be exported southward if and when the North Atlantic Oscillation enters into a new high phase.

Protists represent the majority of the eukaryotic diversity in the oceans. They have different functions in the marine food web, playing essential roles in the biogeochemical cycles. While the available data is rich in horizontal and temporal coverage, little is known on their vertical structuring, particularly below the photic zone. The present study applies V4 18S rDNA metabarcoding to samples collected over three years in conjunction with the BATS time-series to assess marine protist communities in the epipelagic and mesopelagic zones (0-1000 m). The protist community showed a dynamic seasonality in the epipelagic, responding to hydrographic yearly cycles. Mixotrophic lineages dominated throughout the year. However, autotrophs bloomed during the rapid transition between the winter mixing and the stratified summer, and heterotrophs had their peak at the end of summer, when the base of the thermocline reaches its deepest depth. Below the photic zone, the community, dominated by Rhizaria, is depth-stratified and relatively constant throughout the year, although they followed local hydrographic and biological features such as the oxygen minimum zone. The results suggest a dynamic partitioning of the water column, where the niche vertical position for each community changes throughout the year in the epipelagic, likely depending on nutrient availability, the mixed layer depth, and other hydrographic features. At depth, the protist community closely tracked mesoscale events (eddies), where the communities followed the hydrographic uplift, raising the deeper communities for hundreds of meters, and compressing the communities above.

Observational evidence is presented for interannual to interdecadal variability in the intensity of the North Atlantic gyre circulation related to the atmospheric North Atlantic Oscillation (NAO) patterns. A two-point baroclinic pressure difference between the subtropical and subpolar gyre centers--an oceanic analogue to the much-uses sea level pressure (SLP)-based atmospheric NAO indices--is constructed from time series of potential energy anomaly (PEA) derived from hydrographic measurements in the Labrador Basin and at Station S near Bermuda.

The linkages between dissolved organic matter (DOM) dynamics and bacterial activity (BAct) are key to regulating carbon fluxes in marine food webs. While transcriptional activities have shown diel patterns, the connections between BAct and DOM on the diel timescale remain unclear. This study explored how bacterioplankton transform DOM over diel cycles in the euphotic and upper twilight zones (to 300 m) of the North Atlantic subtropical gyre. BAct peaked at night in the euphotic zone, following daytime maxima in chlorophyll fluorescence, suggesting a delayed bacterial response to photosynthetic activity. Our data show nighttime BAct exceeded daytime rates 34-47% under dark incubations. Total dissolved amino acids (TDAA) concentrations were lowest at night in the upper twilight waters, indicating nighttime DOM consumption by bacterioplankton. Additionally, we observed that diel vertical migrators could contribute to the oscillation of labile DOM, such as TDAA, although their irregularity requires more detailed studies. BAct shifted DOM composition from fresher to more degraded forms, likely driven by bacterioplankton lineages such as SAR11, Rhodospirillaceae, and SAR202. Our findings show that daytime photosynthesis drives DOM production, while enhanced nighttime heterotrophic BAct facilitates its consumption and transformation, highlighting notable fluctuations in microbial processes and carbon cycling on a diel scale. Diel study revealed fine scale coupling between microbial activity and DOM biogeochemistry in the oligotrophic ocean.

The tropical Atlantic basin is one of seven global regions where tropical cyclones (TCs) commonly originate, intensify, and affect highly populated coastal areas. Under appropriate atmospheric conditions, TC intensification can be linked to upper-ocean properties. Errors in Atlantic TC intensification forecasts have not been significantly reduced during the last 25 years. The combined use of in situ and satellite observations, particularly of temperature and salinity ahead of TCs, has the potential to improve the representation of the ocean, more accurately initialize hurricane intensity forecast models, and identify areas where TCs may intensify. However, a sustained in situ ocean observing system in the tropical North Atlantic Ocean and Caribbean Sea dedicated to measuring subsurface temperature, salinity, and density fields in support of TC intensity studies and forecasts has yet to be designed and implemented. Autonomous and Lagrangian platforms and sensors offer cost-effective opportunities to accomplish this objective. Here, we highlight recent efforts to use autonomous platforms and sensors, including surface drifters, profiling floats, underwater gliders, and dropsondes, to better understand air-sea processes during high-wind events, particularly those geared toward improving hurricane intensity forecasts. Real-time data availability is key for assimilation into numerical weather forecast models.

The spatial distributions of certain sea-surface properties, such as temperature, fluctuate on timescales from months to decades and in synchrony with the main regional atmospheric patterns comprising the global climate system 1 . Although it has long been assumed that the ocean is submissive to the dictates of the atmosphere, recent studies raise the possibility of an assertive, not merely passive, oceanic role in which water-mass circulation controls the timescales of climate fluctuations 2 , 3 , 4 , 5 , 6 . Previously held notions of the immutability of the physical and chemical characteristics of deep water masses are changing as longer time series of ocean measurements indicate that the signatures of varying sea-surface conditions are translated to deep waters 4 , 7 . Here we use such time-series measurements to track signals ‘imprinted’ at the sea surface in the North Atlantic Ocean's subpolar Labrador Basin into the deep water of the subtropical basins near Bermuda, and infer an approximately 6-year transit time. We establish a geographic and temporal context for a portion of the long-term warming trend reported for mid-depth subtropical waters over the past 40 or so years 8 , 9 , and we predict that waters at these depths will continue to cool well into the next decade.