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Seasonal Hydrography of Ameralik: A Southwest Greenland Fjord Impacted by a Land‐Terminating Glacier - Stuart‐Lee - 2021 - Journal of Geophysical Research: Oceans - Wiley Online Library
Abstract . Greenland's coastal zone encompasses a large number of fjords, many of which are impacted by glacial meltwater runoff from land-terminating glaciers. This type of fjord has received limited research attention, yet may represent the future of other fjords currently impacted by marine-terminating glaciers that are retreating. In this study we describe the seasonal hydrography of Ameralik, a fjord on the southwest coast of Greenland impacted by a land-terminating glacier. To complement this analysis we compare our results with observations from the neighbouring Godth?bsfjord, which receives meltwater from both land- and marine-terminating glaciers. We find that the absence of subglacial discharge and glacial ice in Ameralik has a strong impact on the inner fjord density profiles and on circulation. The mean temperature of the upper 50?m layer was lower in Ameralik than Godth?bsfjord in May, but by September was 2°C higher in Ameralik. Dense coastal inflows occur in the late winter months in Ameralik, flushing the fjord and contributing to the return to a weakly stratified state. During the runoff period the surface waters are subject to estuarine circulation and wind forcing, while at intermediate depths a density gradient between the inner and outer fjord regions produces an intermediate baroclinic circulation, resulting in the exchange of water in this layer and the deepening of isopycnals. During summer a large fraction of the meltwater runoff is retained within the fjord rather than being exported. A substantial export of this summer accumulated freshwater occurs in connection with coastal inflows during winter. Plain Language Summary . Marine-terminating glaciers around Greenland are retreating, impacting the adjacent fjords with potential changes in the pathways taken by glacial meltwater. Few studies focus on fjords that receive meltwater only from land-terminating glaciers, although knowledge of these systems can help us better understand how water from melting glaciers eventually ends up in the ocean in a future warmer climate. Here we present a full year of temperature and salinity observations from one such fjord on the southwest coast of Greenland and use these measurements to describe seasonal changes in water properties and circulation patterns. We compare the results with observations from a neighbouring fjord which receives meltwater from both land- and marine-terminating glaciers. This comparison allows us to link differences observed between the fjords with the water movements associated with the type of glacier. While physical properties of the fjords, such as sill depth, play an important role in the circulation, the meltwater pathway also influences fjord hydrography, most prominently in the surface layers. This study will help us to understand the potential future state for many Greenland fjords with glaciers that are presently marine-terminating. 1 Introduction . As a consequence of rapid climate change in the Arctic region, the Greenland Ice Sheet (GrIS) has suffered substantial mass loss in recent decades (Shepherd et?al.,? 2020 ; Simonsen et?al.,? 2021 ; Smith et?al.,? 2020 ). For the surrounding fjord systems this translates to large scale glacial retreat with increased freshwater fluxes into fjords (e.g., King et?al.,? 2020 ; Rignot et?al.,? 2016 ). Changes in fjord hydrography subsequently affect the range of roles performed by these systems that act as transition zones between the GrIS and the open ocean. They support complex and productive ecosystems (Arendt et?al.,? 2016 ; Meire et?al.,? 2017 ), play a key role in regulating heat transport to glaciers, and transform and export freshwater from the GrIS to the continental shelf (Beaird et?al.,? 2018 ; Mortensen et?al.,? 2018 ; Straneo & Cenedese,? 2015 ). A major challenge in predicting the response of fjords to glacial retreat, as well as other consequences of climate change, lies in the inherent diversity of these systems. Hydrography in fjords is not easily generalized, owing in part to the contributions of regional differences (e.g., ocean water masses, local climate) and variation in physical features (e.g., fjord bathymetry, freshwater sources, tides). This drives the need for studies into a range of fjords across Greenland. Changes in marine-terminating glaciers are responsible for most of Greenland's contribution to sea level rise in recent decades (King et?al.,? 2020 ; Mouginot et?al.,? 2019 ). The fjords in which these glaciers terminate have received considerable research attention, with particular focus on subglacial discharge and its related upwelling mechanism which contributes to the unique hydrography of these fjords (e.g., Mankoff et?al.,? 2016 ; Mortensen et?al.,? 2020 ; Sciascia et?al.,? 2013 ; Straneo et?al.,? 2011 ). Although fjords impacted by land-terminating glaciers represent a substantial remaining proportion of Greenland's coast and key export routes for meltwater from the Greenland Ice Sheet to the ocean, few studies focus on this type of fjord (e.g., Dmitrenko et?al.,? 2015 ; Nielsen et?al.,? 2010 ). One such example is Kangerlussuaq, a fjord on the west coast (66.8°N, 51.5°W), which only receives glacial meltwater via river runoff. The inner part of Kangerlussuaq is segregated from the ocean by a 100?km long shallow outer section, almost decoupling the inner fjord from coastal dynamics. As such, fjord hydrography is strongly determined by meltwater runoff in summer, driving stratification and estuarine circulation, and by sea ice formation in winter, forming dense water through brine release (Lund-Hansen et?al.,? 2018 ; Monteban et?al.,? 2020 ; Nielsen et?al.,? 2010 ). In addition, research has been carried out in the Young Sound/Tyrolerfjord system on the northeast coast (74.4°N, 20.4°W) which is also impacted only by meltwater runoff via land. Estuarine circulation resulting from this runoff characterises this system in summer, while polynyas and tide-driven fjord-shelf exchange are drivers of a two-layer circulation in winter that extends up to ?150?m depth (Bendtsen et?al.,? 2014 ; Boone et?al.,? 2017 ; Dmitrenko et?al.,? 2015 ). Young Sound has a very shallow outer sill of ?45?m depth that is a topographic barrier to the ocean (Rysgaard et?al.,? 2003 ), and its location on the east coast means that it is impacted by different coastal water masses (associated with the East Greenland Current) than those found on the west coast (Rysgaard et?al.,? 2020 ). Variation observed in seasonal climate, coastal water masses and other physical differences such as bottom topography leads to a need for high resolution and seasonal studies of other fjords impacted by land-terminating glaciers. Here we describe for the first time the seasonal hydrography, inferred circulation patterns and meteorological conditions of Ameralik, a fjord impacted by a land-terminating glacier, which is located next to Godth?bsfjord on the southwest coast of Greenland (Figure? 1a ). Transects from March–December 2019 and observations from March 2020 provide a unique description of changing seasonal conditions in Ameralik. We compare our data to observations from Godth?bsfjord, which, in contrast to Ameralik, is impacted by three marine-terminating glaciers in addition to three land-terminating glaciers. The seasonal hydrography of Godth?bsfjord has been described extensively (Mortensen et?al.,? 2011 ,? 2013 ,? 2014 ,? 2018 ), providing a breadth of existing knowledge including a baseline study on the surrounding water masses to aid the analysis of Ameralik. Additionally, hydrographic surveys conducted in 2019 allow a closer comparison. We investigate the roles of land- and marine-terminating glaciers in order to highlight fundamental differences between the two systems and contribute toward our understanding of potential future fjord transitions under glacial retreat. Figure 1 Open in figure viewer PowerPoint (a) Map of Ameralik and Godth?bsfjord. Standard CTD station locations are indicated by solid black circles. Selected stations are labeled in black (prefixed with “GF” for Godth?bsfjord or “AM” for Ameralik). Other sites are labeled in blue: Nuuk, the Greenland ice sheet (“GrIS”), the weather station in Ameralik (“WS”), and the river Naajat Kuuat (“NK”). The study area is identified on the inset map of Greenland. (b) Bathymetry section of Ameralik with approximate station positions indicated. 2 Study Area and Methodology . 2.1 Regional Setting . Ameralik is located on the southwest coast of Greenland close to Nuuk (Figure? 1a ). The fjord is ?75?km long, 5–7?km wide and has an area of ?400?km 2 . A ?110?m deep sill is found at the entrance to the fjord and the central part is characterized by a sequence of deep basins reaching a maximum depth of ?700?m (Figure? 1b ). The tidal range for Nuuk is ?1–5?m (Richter et?al.,? 2011 ). The fjord remains largely ice-free throughout the year, with sea ice only found close to the river delta in the inner part of the fjord during winter. No glaciers terminate directly in the fjord, though glacial meltwater is delivered via runoff from rivers. Meltwater is primarily supplied by the glacial river Naajat Kuuat (Figure? 1a ), which drains a catchment area of ?356?km 2 of the Greenland Ice Sheet. For 2012, discharge from the river was estimated as 0.78?km 3 ?yr ?1 (Overeem et?al.,? 2015 ). Godth?bsfjord (Nuup Kangerlua) is the neighbour fjord to Ameralik on the northern side (Figure? 1a ). It has a length of ?190?km and a surface area of 2,013?km 2 extending over several fjord branches and containing multiple sills. The main sill is located at the entrance with a depth of ?200?m and the deepest point is 620?m (Mortensen et?al.,? 2011 ,? 2018 ). In Godth?bsfjord, meltwater and glacial ice are delivered by three marine-terminating glaciers and three land-terminating glaciers via rivers. Estimates from a regional climate model for freshwater input are 18.4?±?5.8?km 3 ?yr ?1 for the marine-terminating glaciers, excluding solid ice discharge, and 7.5?±?2.1?km 3 ?yr ?1 for the land-terminating glaciers (based on the period 2002 to 2012, Langen et?al.,? 2015 ). Solid ice discharge from the three marine-terminating glaciers is estimated at 7–10?km 3 w.e. yr ?1 (Van As et?al.,? 2014 ). Sea ice is present seasonally in the innermost part of Godth?bsfjord (from near station GF13 inwards, Figure? 1a ). The coastal waters outside of Ameralik and Godth?bsfjord are characterized by distinct water masses (winter mode waters: Rysgaard et?al.,? 2020 ). The upper layer is composed of relatively cool and fresh southwest Greenland coastal water ( θ-S properties ?0°C and 33) arriving from the south with an origin in the East Greenland Current and runoff from Greenland. In summer 2016 this coastal water was identified in the upper 200?m of the water column at 64°N, close to Ameralik and Godth?bsfjord (Rysgaard et?al.,? 2020 ). At greater depths, warmer and more saline subpolar mode water of Atlantic origin is present, which is subdivided into upper subpolar mode water ( θ-S properties ?6°C and 35) and deep subpolar mode water ( θ-S properties ?4°C and 34.7) (Lin et?al.,? 2018 ; Rysgaard et?al.,? 2020 ). The thickness of the combined subpolar mode water layer varied between 0 and 600?m along the southwest coast of Greenland in summer 2016 (Rysgaard et?al.,? 2020 ). The seasonal presence of cold and relatively saline Baffin Bay polar water ( θ-S properties ??1.8°C and 33.6) of northern origin can occasionally be observed in a diluted form outside the fjords (Rysgaard et?al.,? 2020 ). 2.2 Sampling . Air temperature, photosynthetically active radiation (PAR), wind speed and wind direction data were measured with 10?min resolution in 2019 for Ameralik using an Onset weather station located on land in the inner part of the fjord on the north side (64°13.42'N, 50°18.12'W, Figure? 1a ). In its vicinity a SBE microcat mooring (SBE 37-SMP, SeaBird) was fixed to the bedrock at a mean depth of ?6?m, just outside the intertidal zone, measuring the surface layer water temperature, salinity and pressure with 10?min resolution. Air temperature and wind speed were obtained from the meteorological station in Nuuk (Asiaq, Greenland Survey). Due to practical limitations it was not possible to assess potential rotational effects in Ameralik. However, earlier work based on cross-fjord sampling in Godth?bsfjord found limited cross-fjord variation with the dominant flow being in the along-fjord direction (Mortensen et?al.,? 2014 ). As Ameralik is narrower than Godth?bsfjord, cross-fjord variation is presumed to also be limited, resulting in the focus on the along-fjord gradient. Seasonal measurements were made at 12 standard stations in Ameralik spaced between 3 and 13?km from one another (AM1 to AM12 in Figure? 1a ) in May, July and September 2019. Longer term fjord sampling was also carried out at monthly intervals from March–December 2019 at selected stations (AM3, AM5, AM7, AM10, AM11 and AM12) and in March 2020 at station AM5. Additionally, sampling was conducted at 13 standard stations in Godth?bsfjord (GF1 to GF13 in Figure? 1a ) in May, July and September 2019. Favourable ice conditions in the inner fjord during the September campaign allowed the sampling of 4 further standard stations (GF14 to GF17). Longer term fjord sampling in Godth?bsfjord was also carried out at monthly intervals from February to December 2019 at selected stations from GF5 and inwards. At every station, depth profiles of conductivity, temperature and pressure were collected using a SeaBird SBE19plus CTD and averaged over 1?m vertical depth intervals. Sensors were calibrated annually by the manufacturer and salinity precision was typically within the range of 0.005–0.010. Sampling took place aboard the Greenland RV Sanna and Avataq, and the commercial Greenland vessels Polar Dive and Tulu. For comparison with previous field programs, potential temperature (ITS-90) and practical salinity are used throughout the text. Freshwater content (FWC) represents the portion of the water column composed of freshwater, expressed in meters. This was calculated for salinity profiles with 1?m resolution at depth ranges 0–50, 50–200 and 200–500?m at station AM5 according to the equation: where z is the depth, a and b are the top and bottom depths, S is the measured salinity, and S ref is the reference salinity, which was set at 33.3. This value is the maximum salinity measured in Ameralik across the sampled period (at station AM5 in March 2019). The stratification index ( φ , J m ?3 ) represents the energy required to completely mix the water column (Simpson,? 1981 ) and was calculated according to the equation of MacKenzie and Adamson?( 2004 ): integrating from a constant depth h to the sea surface and where ρ is the measured density at the vertical coordinate z , is the mean density from the sea surface to h , and g is acceleration due to gravity (9.81?m?s ?2 ). To compare differences in the surface layer, this calculation was based on density profiles from the upper 50?m for all stations except AM12, for which the upper 36?m was used (maximum depth of the station). All processing of data was done using the open-source programming language R (R Core Team,? 2013 ). 3 Results . 3.1 Weather Observations . Data from the weather station in the inner part of Ameralik (Figure? 1a ) show low incoming solar radiation in the winter of 2019, with a mean PAR of ?16?μmol?m ?2 ?s ?1 in January (Figure? 2a ). Air temperatures in winter reached a low of ?18°C (Figure? 2c ). The gradual increase in incoming radiation during spring resulted in positive monthly mean temperatures starting from April and peaking in July at 12°C. Compared to similar measurements collected in Nuuk in 2019, air temperatures in the inner part of Ameralik were colder in winter and warmer in summer (Figure? 2c ). The mean temperature in January was ?8.3°C at the Ameralik station, and ?6.4°C in Nuuk, and in June these were 10.3°C and 6.5°C respectively. This is in line with similar observations in the region showing a clear land-ocean gradient (Abermann et?al.,? 2019 ). Figure 2 Open in figure viewer PowerPoint Weather conditions for Ameralik and Nuuk in 2019. Weekly mean values (solid circles, thick lines), and daily means (thin lines) for (a) photosynthetically active radiation at Ameralik weather station, (b) sea surface temperature (black) and sea surface salinity (red) at Ameralik mooring, (c) air temperature, and (d) wind speed at Ameralik weather station (black) and Nuuk weather station (blue). Month labels on the x -axes of (a–d) appear in the middle of the month. (e) Quarterly windroses for Ameralik. Distinct seasonal changes were also found in the wind patterns in Ameralik, with the highest mean daily wind speeds occurring between November and April. Wind speeds further out of the fjord were generally higher, as can be seen from a comparison with Nuuk, close to the coast (Figure? 2d ). Throughout the year, wind was usually oriented along the fjord axis, as observed in other Arctic fjords (e.g., Svendsen et?al.,? 2002 ), with the steep mountainous terrain of Ameralik contributing with orographic effects (Figures? 1a and? 2e ). From January to March, and from October to December (roughly corresponding to winter months), winds blew predominantly out-fjord (easterly wind) throughout the day (Figure? 2e ). From April to September, as the land became warmer than the sea, the dominant wind direction was in-fjord (westerly wind) (Figure? 2e ). Transitions between these two modes took place during April and late August to September. The summer winds were less uni-directional and changes in wind direction occurred during the day due to differential heating between land and sea. Generally there was in-fjord wind during the day and weaker out-fjord wind at night. 3.2 Seasonal Hydrographic Observations in Ameralik . Owing to its mostly glacial source, freshwater runoff to the fjord is highly seasonal and increases approximately in line with air temperature once above 0°C. This corresponds to little to no runoff between October and April (Van As et?al.,? 2014 ). In March 2019, air temperature remained well below 0°C, corresponding to low freshwater runoff. Under these conditions, increased wind speeds, together with tides, promoted mixing of the water column. This is reflected in a relatively uniform cold (?0°C) and saline (?33.1) water column with weak stratification and little horizontal or vertical variation (Figures? 3a and? 3b ). Figure 3 Open in figure viewer PowerPoint Bimonthly potential temperature and practical salinity length sections of upper 500?m of Ameralik from March to November 2019, from outer fjord (left) to inner fjord (right). March and November data are from long term Ameralik monitoring; May to September data are from the seasonal campaigns with higher horizontal spatial resolution. As air temperature and insolation rose through spring and summer, the surface water warmed and produced an along-fjord surface gradient with temperatures increasing from outer to inner fjord (Figures? 3c and? 3e ). Mooring data demonstrate the time lag between sea surface layer temperature and insolation (Figures? 2a and? 2b ). Sea surface layer temperature at the mooring (?6?m depth, depending on the tide) increased quickly in May, with a rise of 7.5°C in the weekly mean. In May, a shallow thermocline started to develop in the upper 10?m, most prominently from the midfjord (AM5) to the innermost station (AM12). In this range the mean potential temperature of the upper 10?m layer was 1.9°C and the mean salinity was 32.8. At the mouth of the fjord (AM1 and AM2), vertical gradients near the surface were limited. In subsequent months, increased freshwater runoff further strengthened the surface stratification through the establishment of a thin layer of relatively fresh (mean salinity <31) water in the upper 10?m of the central and inner fjord stations (AM5 to AM12). Driven by the estuarine circulation, which results in a net out-fjord flow in the surface layer, the freshwater runoff originating in the inner fjord was transported out-fjord. By July, following high levels of freshwater runoff, this surface layer was well defined along Ameralik with salinity as low as 6 in the inner part of the fjord (station AM12) and a strong halocline in the upper 6?m of the central and inner fjord stations (AM5 to AM12, Figures? 3e and? 3f ). The upper 10?m layer of this range had a mean potential temperature of 8.0 ? C and mean salinity of 28.6. The intermediate depths (50–200?m) freshened and warmed during this period (Figures? 3c–3f ,? 4a and? 4b ), which we explain in Section? 4.1 . Figure 4 Open in figure viewer PowerPoint Time series of water properties at station AM5 in 2019: (a) potential temperature at 100, 200 and 400?m, (b) practical salinity at 100, 200 and 400?m, (c) potential density anomaly at 100 and 200?m, and (d) potential density anomaly at 400?m. Line types represent depths: dotted for 100?m, dashed for 200?m, and solid for 400?m. Note the difference scales used in (c) and (d) for potential density anomaly. As September arrived, the air temperatures dropped below those of the surface waters (Figures? 2b and? 2c ) and freshwater input will have declined strongly (Van As et?al.,? 2014 ), leading to higher salinity in the surface layer and a less pronounced halocline. Together with increased winter storms (evidenced by higher daily mean gust speeds in the winter months, Figure? S1 ), this resulted in a much weaker surface stratification throughout the fjord by the end of the year (Figures? 3i and? 3j ). The upper 10?m layer from station AM5 to AM12 had a mean potential temperature of 6.3°C and mean salinity of 30.2 by September. At intermediate depths (50–200?m) we observe a gradual deepening of isopycnals during autumn (September–November, Figures? 3g–3j ), determined primarily by the changes in salinity (dashed line representing the 200?m depth in Figures? 4b and? 4c ). 3.3 Retention of Summer Accumulated Freshwater in Ameralik . To assess changes in freshwater, freshwater content (FWC) was calculated for a station in the central fjord (AM5) using the maximum salinity in Ameralik in March 2020 as the reference value. A significant freshening took place at station AM5 between June and August, with an increase in FWC of the upper 500?m layer of 4.4?m (Figure? 5 ). This was followed by a decrease of 1.2?m between August and December. Considering individual layers during this period of decline in the second half of the year, the surface layer (0–50?m) FWC decreased by 2.1?m, while that of the intermediate layer (50–200?m) increased by 0.7?m, and that of the deep layer (200–500?m) increased by 0.3?m. This shows that a large fraction of the freshwater discharged during the summer months remained within the fjord rather than being directly exported. Figure 5 Open in figure viewer PowerPoint Monthly Freshwater Content (FWC, m) at AM5 in Ameralik from March 2019 to March 2020. Calculated with a practical salinity reference value of 33.3. 3.4 Winter Fjord Water Renewal in Ameralik . The deep water (below 400?m) is fairly isolated owing to the combination of a shallow entrance sill (?110?m) and deep basin (Figure? 1b ). As such, this water is decoupled from the seasonal changes observed in the upper water column and its properties remained stable for much of 2019 (Figure? 4 ). Due to diffusion processes the density of the basin water gradually lowers (Figure? 4d ), reducing the density difference between the coastal water and the fjord. In this way, the basin water becomes preconditioned for dense coastal inflows, whereby coastal water enters over the sill and replaces the resident fjord water. Mid-fjord measurements are not available for January and February 2019, but we identify dense coastal inflow in late winter/early spring 2019 from the increase in potential density anomaly of 0.03?kg m ?3 at 400?m between March and May (Figure? 4d ) and associated increase in salinity of 0.26 (Figure? 4b ). This increase cannot be attributed to the process of brine rejection due to the lack of sea ice and drifting sea ice in Ameralik during winter. Nor can wind forcing or convection be the primary cause as this would result in decreased salinity. Winter dense coastal inflow has also been observed in the neighbouring Godth?bsfjord (Mortensen et?al.,? 2018 ). Following this period of deep water renewal in Ameralik, the density of the basin water was at its highest, and further dense coastal inflow events are not observed during the summer months. There was a substantial decrease of 1.9?m in FWC between December 2019 and March 2020 (Figure? 5 ), which may be representative of an annual pattern, i.e., a large freshwater export occurring each winter. If this is the case, the period can be characterized by a flushing of the summer accumulated freshwater throughout the fjord. From the approximated surface area of ?400?km 2 and the difference of 1.9?m in FWC at AM5 between December 2019 and March 2020, we estimate an export of ?0.8?km 3 . This is very close to the estimated annual river discharge of 0.78?km 3 reported in Section? 2.1 . The estimated export is most pronounced in the intermediate layer (50–200?m), with a 1.5?m decrease between December 2019 and March 2020. We hypothesise that a fraction of the meltwater discharged to the surface layer is retained within the fjord system and only flushed out of the intermediate layer after the summer, indicating that the winter period plays an important role for the freshwater export. 3.5 Water Masses in Ameralik and Godth?bsfjord . Potential temperature and salinity ( θ-S ) curves from a selection of stations in Ameralik and Godth?bsfjord in July and December 2019 are shown in Figure? 6 . Changes in insolation and volumes of meltwater input result in a large difference in temperature and salinity ranges between summer and winter. In July, the θ-S curves show the relatively warm and fresh summer surface water from the surface layers of the inner fjords (Figure? 6a ). This summer surface water originated from runoff (i.e., freshwater runoff to the surface layer) and net precipitation, which was subsequently warmed by solar radiation. In Godth?bsfjord, a prominent dip in temperature and salinity identifies the body of cool and fresh subglacial water below the summer surface water in the inner part of the fjord (Figure? 6a ). This subglacial water, which is not found in Ameralik, has its origin in the subglacial discharge of freshwater discharged at the grounding line of marine-terminating glaciers. The discharge undergoes modification by entrainment as it ascends toward the surface, and then by cooling from the ice mélange as it is transported away from the glacier, and ends up below the summer surface layer (Bendtsen et?al.,? 2015 ; Mankoff et?al.,? 2016 ; Mortensen et?al.,? 2020 ). Figure 6 Open in figure viewer PowerPoint Potential temperature and practical salinity observations from selected stations in Ameralik (dashed lines) and Godth?bsfjord (solid lines) in (a) July, and (b) December 2019. Each line represents the depth profile from one station. The July plot is cropped to exclude observations of practical salinity below 27 and potential temperature above 9°C. Approximate positions of coastal (Rysgaard et?al.,? 2020 ) and fjord (Mortensen et?al.,? 2011 ) water masses are indicated with acronyms: BBPW, Baffin Bay polar water; CW, southwest Greenland coastal water; dSPMW, deep subpolar mode water; uSPMW, upper subpolar mode water; SgW, subglacial water; SrW, sill region water; sSW, summer surface water; wSW, winter surface water; BW, basin water. The gray basin water rectangles indicate water range in 2019 below 400?m. The gray dashed lines are isopycnals. The layer below the subglacial water contained sill region water, an intermediate water mass which originates in the outer sill region where tidal-induced diapycnal mixing takes place between a number of near surface water masses (i.e., summer surface water, subglacial water and coastal water) (Mortensen et?al.,? 2011 ). In Godth?bsfjord, sill region water can be identified by the temperature maximum relative to the cooler subglacial water layer found above (Mortensen et?al.,? 2011 ). Because Ameralik does not contain subglacial water, the profiles lack the corresponding subsurface temperature minimum from which to identify the sill region water. It can however be approximated from the overlap in properties with sill region water in Godth?bsfjord (Figure? 6a ). The similarity of these properties suggests that this is the same water mass, and that the same mechanism is therefore involved in its formation in Ameralik as in Godth?bsfjord, which is further discussed in Section? 4.1 . Below the layer containing sill region water was the relatively cool and saline basin water, identified in Figure? 6 with gray boxes. This occupied the water column down to the bottom, remaining almost stagnant for most of the year with intermittent periods of renewal during winter. In winter, cooling and reduced freshwater runoff was responsible for the formation of winter surface water and fjord properties were dominated by basin water, a fjord water mass with close connection to the coastal water masses (Figure? 6b ).Observations from sampled central fjord stations in March 2019 show that the mean properties of the upper 10?m layer were slightly cooler and more saline in Ameralik (?0.1°C, 33.1) than in Godth?bsfjord (0.3°C, 32.9). 3.6 Fjord Comparison of Upper Water Column Properties . The seasonal hydrography of Godth?bsfjord is well documented (Mortensen et?al.,? 2011 ,? 2013 ,? 2018 ,? 2020 ) and the focus here is on the comparison with Ameralik. Figure? 7 presents the mean potential temperature of the upper 50?m layer at each station through the studied months. Temperatures near the shared mouth region of the two fjords remained similar, but further in-fjord diverged considerably. In May, at the start of the melt season, the upper 50?m layer of Ameralik (mean of stations AM1 to AM12) was 0.6°C cooler than in Godth?bsfjord (mean of stations GF1 to GF13), which relates to the differences in entrance sill depth and associated basin water temperatures. As discussed below in Section? 4.2 , the basin water of Ameralik in 2019 was cooler than that of Godth?bsfjord. During winter flushing, a portion of the basin water was displaced upwards (e.g., Mortensen et?al.,? 2018 ), consequently affecting the temperature of the upper water layers. The lower temperatures in the basin water of Ameralik and inflow of cooler water are both likely to have contributed to the colder subsurface layer compared to Godth?bsfjord. During winter and spring, icebergs in Godth?bsfjord remained mostly trapped in the innermost fjord by sea ice (i.e., in the ice mélange), which may have limited their cooling influence at that time. However, large changes in surface layer properties between the studied months show the influence of the strong meltwater input in Godth?bsfjord (composed of runoff, subglacial freshwater discharge and net precipitation) as well as the presence of large quantities of icebergs further out-fjord (observed during fjord sampling). As a result, by July the upper 50?m layer of Ameralik was warmer than the upper 50?m of Godth?bsfjord, and by September the difference in mean temperature of the upper 50?m was 2°C (Figure? 7 ). Figure 7 Open in figure viewer PowerPoint Mean potential temperature in the upper 50?m depth layer of each station in (a) Ameralik, and (b) Godth?bsfjord in May, July and September 2019. The extra freshwater input in Godth?bsfjord does not lead to increased overall stratification: the stratification index is higher in Ameralik than in Godth?bsfjord in the mid- and outer fjord in July (Figure? 8 ). This can be explained by the presence of subglacial discharge in Godth?bsfjord, whereby the freshwater released at depth forms buoyant plumes that entrain surrounding ambient water, bringing deep fjord water toward the surface and mixing the water column (Bendtsen et?al.,? 2015 ). The subglacial water forms an out-fjord flow below the shallow surface layer that originates from runoff (?10–30?m depth range, Figures? 9c and? 9e ). In this way, subglacial discharge has an impact on the vertical structure of the water column throughout the fjord. Increases in runoff can have a similar effect by causing a thinning of the surface layer (Bendtsen et?al.,? 2014 ). In May, the surface layer in Ameralik was shallower than in Godth?bsfjord, but by July, as freshwater input increased, this layer was shallower in Godth?bsfjord than in Ameralik (Figures? 10b and? 10c ). Figure 8 Open in figure viewer PowerPoint Stratification index ( φ ) for upper 50?m water depth layer of Ameralik (stations AM3 to AM12) and Godth?bsfjord (stations GF3 to GF13). Figure 9 Open in figure viewer PowerPoint Potential temperature and practical salinity length sections of the upper 500?m of Godth?bsfjord in May, July and September 2019, from outer fjord (left) to inner fjord (right). Note that the length represented is greater in the September section, when favourable ice conditions allowed inner fjord stations GF14-17 to be sampled. Figure 10 Open in figure viewer PowerPoint Potential density anomaly in upper 50?m water depth layer of Ameralik and Godth?bsfjord in (a) March, (b) May, (c) July, and (d) September 2019. Note that different scales are used for the x -axes. 4 Discussion . 4.1 Fjord Circulation Comparison . By winter, seasonal change in Ameralik led to the disappearance of the summer stratification, whereas in Godth?bsfjord some stratification remained (Figure? 10a ). These seasonal changes are influenced by a combination of mixing mechanisms including tides and dense coastal inflows, which in Ameralik resulted in a flushing of the fjord, as has been inferred from the increasing basin water density discussed in Section? 3.4 . Godth?bsfjord also experiences dense coastal inflows, but in Godth?bsfjord the deep water does not completely renew (e.g., Mortensen et?al.,? 2018 ). The presence of sea ice also contributes to the perseverance of stratification in Godth?bsfjord by providing a barrier to wind mixing (Meire et?al.,? 2015 ). As observed in Ameralik, it may be that during the winter period (when dense coastal inflow is active) Godth?bsfjord also undergoes a large export of summer accumulated freshwater. The FWC calculations in 2008–2009 show that FWC at intermediate depths (30–277?m) dropped from 4.1?m in November to 1.5?m in May (based on a reference salinity of 33.56, Mortensen et?al.,? 2011 ). In the intermediate water layer (50–200?m) of Godth?bsfjord in July there was an inflow of a warm tongue extending from the fjord entrance (Figure? 9c ). From station GF5 inwards this can be observed beneath the cooled layer of subglacial water (?10–30?m) that is itself below the warmer surface layer (?0–10?m). This is referred to as intermediate baroclinic circulation by Mortensen et?al.?( 2011 ) and develops with a compensation out-flow below, lowering the isopycnals in the fjord. This circulation was later supported by direct current measurements (Mortensen et?al.,? 2014 ). It is present year-round in Godth?bsfjord (Figures? 11b and? 11d ), driven by density differences between the inner and outer fjord and the shelf/coast. It is the most dominant circulation in summer when the combination of tidal mixing in the outer sill region and warming and freshening of surface fjord water results in the outer fjord water becoming less dense than the inner fjord (Mortensen et?al.,? 2014 ). Figure 11 Open in figure viewer PowerPoint Conceptual diagrams of circulation modes and coastal water masses during summer and winter in Ameralik (a, c) and Godth?bsfjord (b, d). Thick black arrows represent freshwater input (surface runoff and subglacial meltwater discharge). Thin black arrows represent entrainment into the subglacial plume. Thin colored arrows represent net effects of circulation modes: yellow for estuarine circulation, red for intermediate baroclinic circulation, gray for subglacial circulation and blue for dense coastal inflows. We hypothesise that the intermediate baroclinic circulation is also present in Ameralik. Ameralik also displayed steady warming and freshening in its intermediate layer during summer (Figures? 3e and? 3f ) and associated deepening of the isopycnals. In addition, mid-fjord (station AM5) potential density anomaly measurements show that there was greater change between May and December in intermediate depths (100 and 200?m, Figure? 4c ) than deeper in the water column (400?m, Figure? 4d ). The change in density from May to December observed at 400?m at AM5 (?0.024?kg m ?3 ) is indicative of diffusion processes and is an order of magnitude smaller than at 200?m (?0.325?kg m ?3 ) and 100?m (?0.514?kg m ?3 ) depths, indicating the existence of another exchange mechanism at 100 and 200?m depths. This supports our hypothesis, which is further evidenced by the finding made in Section? 3.5 that sill region water was present in both outer and inner fjord stations of Ameralik and Godth?bsfjord. We hypothesise that, after forming in the outer sill region, this water mass was transported into the fjord by the intermediate baroclinic circulation, as has been verified in Godth?bsfjord through current measurements (Mortensen et?al.,? 2014 ). 4.2 Impact of Sill Depth on Bottom Water Properties . A comparison of deep water properties reveals another important difference between these two fjords. Between 50 and 400?m, Ameralik was fresher (by 0.2) and cooler (by between 0.7 and 1.1°C) than Godth?bsfjord for each of the studied months (May, July and September). This difference also applied to the water below 400?m, as indicated by the gray rectangles in Figure? 6 . The difference in deep water properties is related to the sill depth, which determines the inflow of coastal water and differs between the two fjords. The entrance sill of Ameralik is ?110?m deep, compared to the ?200?m deep entrance sill of Godth?bsfjord. As a result, Ameralik receives less of the deeper, warmer and more saline subpolar mode water from the coastal region than Godth?bsfjord does. As this water makes its way into the fjord and mixes with local waters it eventually becomes part of the basin water. As such, the reduced inflow of subpolar mode water in Ameralik compared to Godth?bsfjord can explain the cooler and fresher deep water observed in Ameralik. This has important implications for the ecology due to distinct environmental preferences of species, as well as variation in the species transported in these different water masses (e.g., Grainger,? 1963 ; Hirche,? 1991 ; Hirche & Mumm,? 1992 ). 4.3 Implication of Fjord Comparison . Comparing Ameralik with Godth?bsfjord demonstrates how the absence of marine-terminating glaciers, and thus of subglacial freshwater discharge, impacts the surface layer of the fjord (0–50?m), with Ameralik found to be more stratified on average than Godth?bsfjord in the summer (Figure? 8 ). Increased summer stratification may be reflected in the future of fjords undergoing a retreat of glaciers onto land. This is in line with modeling predictions (e.g., Torsvik et?al.,? 2019 ). The upwelling mechanism resulting from the subglacial discharge plumes has further implications for the nutrient distribution near the termini, which sustains primary production (Hopwood et?al.,? 2018 ; Kanna et?al.,? 2018 ; Meire et?al.,? 2017 ) and extends support to the wider food web, including marine mammals and seabirds (Lydersen et?al.,? 2014 ; Urbanski et?al.,? 2017 ). However, despite the higher summer stratification, the absence of a marine-terminating glacier does not necessarily mean a less dynamic fjord. We argue that there are three circulation modes in Ameralik, illustrated in Figures? 11a and? 11c . Dense coastal inflows across the sill during the winter of 2019–2020 had a major impact in the flushing of Ameralik through water renewal and induced internal upwelling. This prevented basin water from remaining stagnant through the whole year and contributed to the return of a weakly stratified state in winter. Meanwhile, the surface layer of Ameralik was modified through estuarine circulation (driven by freshwater from river runoff) and wind forcing. Tides are likely to have had an impact on mixing in Ameralik, with its location in the Davis Strait as one of the regions of Greenland's coast with the highest tidal ranges (Padman et?al.,? 2018 ) and its shared entrance with Godth?bsfjord, which is highly turbulent (Mortensen et?al.,? 2018 ). The tidal-induced mixing in the outer sill region most likely contributed to the setup of an intermediate baroclinic circulation, which resulted in the exchange of water at intermediate depths and consequently plays an important role in redistributing heat and freshwater in the fjord (Mortensen et?al.,? 2014 ). The circulation activity in fjords impacted only by land-terminating glaciers is set by boundary conditions including the sill depth, fjord geometry, coastal water masses, tidal currents, annual freshwater input (glacial water and net precipitation) and regional climatic conditions, all of which vary considerably across the fjords of Greenland. In contrast to the bottom water renewal observed in Ameralik, long term mooring data from the Young Sound/Tyrolerfjord system on the northeast coast of Greenland show that bottom water renewal did not occur in the period between 2004 and 2014 (Boone et?al.,? 2018 ). The relatively shallow (?45?m) outer sill strongly restricts deep water renewal. Ongoing freshening in the coastal water in East Greenland further reduces the chance that it will exceed the density of the fjord basin water, which is necessary for coastal water to be able to reach these depths (Boone et?al.,? 2018 ; Sejr et?al.,? 2017 ). 5 Summary and Conclusions . Ameralik, in southwest Greenland, represents the stage of a fjord after its glaciers have retreated from a marine-terminating to a land-terminating position. In this study we provide the first description of its hydrography, and compare our findings with the neighboring Godth?bsfjord, which is impacted by both land- and marine-terminating glaciers. The absence of a marine-terminating glacier, and therefore of glacial ice and subglacial discharge, has implications for the hydrography and circulation. In 2019, spring and summer months in Ameralik were characterized by the transition from a weakly stratified fjord to a fresher, highly stratified fjord with a strong halocline and an estuarine circulation resulting from freshwater runoff. We found that a large fraction of the freshwater input is retained inside the fjord during the summer and autumn months, which we propose occurred through intermediate baroclinic circulation. During this period the upper 50?m water layer was considerably warmer in Ameralik than in Godth?bsfjord. Autumn and winter represented a steady return toward the pre-spring conditions, as freshwater runoff was strongly reduced, and export of summer accumulated freshwater from the fjord occurred, which we propose was related to coastal inflows. This resulted in flushing of the fjord and restored the system to a weakly stratified state. The shallower main sill of Ameralik with respect to that of Godth?bsfjord resulted in the intrusion of less of the subpolar mode water that is warmer and more saline compared to the overlying southwest Greenland coastal water. This produced differences in the fjord water properties which may be important for fjord ecology. Tides are likely to play a major role in mixing in the fjord, with tidal-induced mixing contributing to the setup of an intermediate baroclinic circulation in the fjord, as identified from the downward temporal movement of isopycnals at mid-depths and the inferred in-fjord transportation of sill region water in Ameralik. This case study demonstrates the importance of physical features, such as bathymetry and glacier type, as well as coastal water masses for the hydrography of Greenland fjords. Acknowledgments . L. Meire was funded by research program VENI with project 016.Veni.192.150 from the Dutch Research Council (NWO). Meteorological data from Nuuk were provided by Asiaq Greenland Survey, Nuuk, Greenland. We would like to thank Flemming Heinrich and the crew of RV SANNA, Peter Rosvig Pedersen of the Polar Diver, Anders Pedersen and the crew of the Tulu for field assistance. Open Research . . Data Availability Statement . Processed CTD, mooring and weather data are available at the World Data Center PANGAEA (Stuart-Lee et al., 2021a , 2021b , 2021c ). .
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