Heat And Freshwater Controlling Processes On The Northern Gulf Of Alaska Shelf

Thesis (Ph.D.) University of Alaska Fairbanks, 2009We examined conditions and processes that control the distribution of heat and freshwater on the northern Gulf of Alaska (GOA) shelf. Cross-shelf heat gradients are weak throughout the year, while salinity gradients are substantial due to the impact of coastal freshwater runoff. Outer shelf water properties are influenced by large anticyclonic eddies, while the inner and middle shelves may be regulated by wind and freshwater runoff dynamics around the Alaska Coastal Current (ACC). On the outer shelf, anticyclonic eddies propagate from the eastern GOA southwestward along the continental slope, where they favor on-shelf (off-shelf) transport of saline and nutrient-rich (fresh and iron-rich) waters Certain along-shelf locations are identified where low-salinity coastal waters are found near the shelfbreak within reach of eddies and may be regions of enhanced cross-shelf freshwater transport. The eddies have lifetimes of ~5 years and increase in size and sea level anomaly west of the Seward Line, which implies more vigorous eddy cross-shelf exchange in the northwestern GOA. By comparison, on the inner shelf the heat and freshwater distribution is dominated by large coastal river runoff, which forces the ACC and controls the vertical distribution of temperatures through stratification. In May 2007, the coastal GOA revealed some of the lowest ocean temperatures since the early 1970s, initiated by strong atmospheric cooling and reduced coastal runoff in November 2006. Stepwise regression shows that 81% of the variability of deep temperatures is explained by salinity stratification and air-sea heat fluxes. Weak baroclinic flow in May 2007 likely aided the cooling through reduced along-shore heat transport. A more detailed examination of heat transport indicated that along-shore heat flux convergence in the ACC may re-supply 10-35% of the heat removed by air-sea fluxes throughout the coastal GOA cooling season, while the annual mean cross-shore heat flux convergence is insignificant. Spatial gradients show increasing heat fluxes from off- to on-shore and from east to west. The cross-shore gradients result from wind speed gradients due to ageostrophic near-shore wind jets near coastal mountains, while the along-shore gradients result from larger-scale pressure systems. While the ACC advects coastal freshwater around the GOA shelf its waters are subjected to disproportional heat loss west of the Seward Line

The impact of tides on mixing and freshwater export in the Laptev Sea

The vast and shallow Laptev Sea shelf is seasonally ice covered and receives large amounts of freshwater runoff from the Lena River. This shelf is an important export region for sea ice and freshwater to the Arctic basin, and features strong vertical and horizontal gradients which separate the saline basin waters from the fresh coastal waters. Processes promoting shear instabilities and diapycnal mixing are therefore of interest for physical and biogeochemical properties. The Laptev Sea shelf features considerable shear in under-ice currents largely dominated by the baroclinicity in semidiurnal tides. We present an investigation into semidiurnal tides based on year-round oceanographic moorings from different locations across the Laptev Sea shelf. Harmonic analysis of ADCP records shows a strong depth-dependence in the clockwise tidal currents that can be linked to stratification and further shows large spatial and seasonal variability of tides. Total current magnitudes are stronger on the outer than on the inner shelf, and tides overall explain >80% of the current’s variance throughout the year. On the inner shelf, tides play a comparatively greater role under sea ice (40-70%) than during open water periods (20-50%) when wind-induced inertial motions dominate. The ADCP records are further complemented by two cross-shelf microstructure transects which show episodes of intense turbulent kinetic energy dissipation in the pycnocline following the alignment of the semidiurnally rotating shear-vector and the surface forcing, hence underlining the potential influence of tides on diapycnal mixing. Our results highlight the potential of tides to vertically transport freshwater, heat and nutrients, and provide some first order insights into how the physical environment of this shelf may change with changing sea ice conditions

Baroclinic tides and their possible impact on bottom boundary layer evolution and vertical mixing in the Laptev Sea

3 consecutive years of moored ADCP and bottom temperature and salinity records at a ∼40 m deep location on the Laptev Sea shelf show strongly amplified internal tides with a period of ∼14 days during two highly stratified winters of 2009 and 2010, while no internal tides were identified during winter of 2008 when conditions were barotropic. The observations likely result from the combined effect of stratification induced by the Lena river freshwater plume (2009) or near-bottom inflow of denser waters (2010) with the proximity of the critical latitude of the M2 tide. The high velocity core found 10-15 m above the bottom during spring tide cycles appears to migrate upward in the water column, which suggests that the bottom boundary layer thickness increases due to shear instability beneath the pycnocline. This potentially has important consequences on the vertical distribution of heat and freshwater in the water column. In addition, measurements show that nutrients are available in near-bottom waters while depleted near the surface, hence upward mixing of nutrients by baroclinic tide-induced turbulence in winter may be a key mechanism for the success of the spring bloom. Currently, one-dimensional numerical experiments are performed to verify the suggested mechanisms and to further investigate the impact of baroclinic tides on bottom boundary layer evolution and water column stability in the Laptev Sea

Historical variability of the density stratification in the Laptev Sea

The Laptev Sea is a key region for sea ice formation and export to the inner Arctic. During winter, wind-ice-dynamics repeatedly produce open water areas (polynyas) with extensive heat fluxes, sea ice formation and water mass modification. In summer, the oceanic processes are strongly influenced by the enormous freshwater discharge of Siberian rivers. All this influences the density distribution. We therefore present a comprehensive analysis of the historical temperature and salinity data which is episodically available since around 1910. The variability of the density stratification is important for mixing across the water column

Amplified Arctic Surface Warming and Sea Ice Loss Due to Phytoplankton and Colored Dissolved Material

Optically active water constituents attenuate solar radiation and hence affect the vertical distribution of energy in the upper ocean. To understand their implications, we operate an ocean biogeochemical model coupled to a general circulation model with sea ice. Incorporating the effect of phytoplankton and colored dissolved organic matter (CDOM) on light attenuation in the model increases the sea surface temperature in summer and decreases sea ice concentration in the Arctic Ocean. Locally, the sea ice season is reduced by up to one month. CDOM drives a significant part of these changes, suggesting that an increase of this material will amplify the observed Arctic surface warming through its direct thermal effect. Indirectly, changing advective processes in the Nordic Seas may further intensify this effect. Our results emphasize the phytoplankton and CDOM feedbacks on the Arctic ocean and sea ice system and underline the need to consider these effects in future modeling studies to enhance their plausibility

Sudden, local temperature increase above the continental slope in the southern Weddell Sea, Antarctica

Around most of Antarctica, the Circumpolar Deep Water (CDW) shows a warming trend. At the same time, the thermocline is shoaling, thereby increasing the potential for CDW to enter the shallow continental shelves and ultimately increase basal melt in the ice shelf cavities that line the coast. Similar trends, on the order of 0.05 ∘C and 3 m per decade, have been observed in the Warm Deep Water (WDW), the slightly cooled CDW derivative found at depth in the Weddell Sea. Here, we report on a sudden, local increase in the temperature maximum of the WDW above the continental slope north of the Filchner Trough (74∘ S, 25–40∘ W), a region identified as a hotspot for both Antarctic Bottom Water formation (AABW) and potential changes in the flow of WDW towards the large Filchner–Ronne Ice Shelf. New conductivity–temperature–depth profiles, obtained in summer 2021, and recent (2017–2021) mooring records show that the temperature of the warm-water core increased by about 0.1 ∘C over the upper part of the slope (700–2750 m depth) compared with historical (1973–2018) measurements. The temperature increase occurred relatively suddenly in late 2019 and was accompanied by an unprecedented (in observations) freshening of the overlying winter water. The AABW descending down the continental slope from Filchner Trough is sourced by dense ice shelf water and consists to a large degree (60 %) of entrained WDW. The observed temperature increase can hence be expected to imprint directly on deep-water properties, increasing the temperature of newly produced bottom water (by up to 0.06 ∘C) and reducing its density.publishedVersio

Editorial: Towards a Unifying Pan-Arctic Perspective of the Contemporary and Future Arctic Ocean

An international symposium addressing pan-Arctic perspectives of the marine ecosystems of the Arctic Ocean took place in October 2017 and this editorial introduces the publications that derived from the conference. The symposium focused in particular upon physical forcing and biogeochemical cycling in surface waters of the Arctic Ocean, connectivity between surface and deep waters in the central basins and adjacent slopes and the ecology of the lesser-known shelf ecosystems. The symposium was the fourth in a sequence that has pan-Arctic integrations of Arctic Ocean ecosystems at its core. The series started in 2002 and its first volume was published under the title Structure and function of contemporary food webs on Arctic shelves (Wassmann, 2006). At the 2002-meeting, a suite of marine Arctic researchers from the main nations that work in the Arctic Ocean started applying the now-ubiquitous term pan-Arctic. The term underlined that the applied research goals and directions were more than a circumarctic perspective, but distinctly considered the entire expanse of the Arctic Ocean. Based upon this exercise, increased interest in the Arctic and some of the scientific endeavors of the 4th International Polar Year central projects and key oceanographers operating in the pan-Arctic region convened at the 2nd pan-Arctic integration symposium, entitled Arctic Marine Ecosystems in an Era of Rapid Climate Change in 2009 (Wassmann, 2011). After a decade of pan-Arctic research and building upon the foundation presented in Wassmann (2006, 2011) a 3rd conference was initiated in 2012, entitled Overarching perspectives of contemporary and future ecosystems in the Arctic Ocean (Wassmann, 2015). This Research Topic brings together 13 publications from the 4th pan-arctic integration symposium held in 2017, entitled Toward a Unifying Pan-Arctic Perspective of the Contemporary and Future Arctic Ocean. We, the editors of the Research Topic, are delighted with the breadth, quality and diversity of the papers. We introduce the essence of the publications under three, summarizing headlines • Physical connectivity, yet regionality • What shapes pan-Arctic primary production • The fate of production. Toward the end we incorporate the knowledge presented in this volume into the overall progress. and status of pan-Arctic marine ecosystem integration that has been achieved, so far, through the four pan-Arctic integration symposia

On the Nature of Winter Cooling and the Recent Temperature Shift on the Northern Gulf of Alaska Shelf

[1] In spring 2006 and 2007, northern Gulf of Alaska (GOA) shelf waters were ∼1.5°C below average throughout the similar to ∼250 m deep shelf and the salinity-dependent winter stratification was anomalously weak due to above (below) average surface (bottom) salinities. Spring 2007 and 2008 temperatures were also similar to ∼-1.5°C below average, but the anomalies were confined to the upper 100 m due to moderate salt stratification. Shelf temperatures in these 2 years were among the lowest observed since the early 1970s, thus interrupting an approximately 30-year warming trend. We examined winter cooling processes using historical conductivity-temperature-depth (CTD) profiles and mooring data from hydrographic station GAK1. The 2006 and 2007 cooling was associated with anomalously strong atmospheric heat loss in November 2006 and March 2007 and below-average fall runoff, which weakened winter stratification and allowed the late cooling to penetrate throughout the water column. In 2007 and 2008, early winter cooling was weak, fall runoff large, and stratification moderate at 100 m so that spring temperature anomalies were trapped to the upper 100 m. Analysis of the 40 year GAK1 CTD record indicates that winter averaged air-sea heat flux and salinity stratification anomalies explain 81% of the variation in deep (100-250 m) GOA temperatures. Although the timing and magnitude of winter runoff influences the shelf temperature distribution, temperature anomalies are a consequence of three-dimensional circulation and mixing processes. These involve the complex, but poorly understood, interplay among the air-sea heat flux; the ocean heat flux convergences; the stabilizing influence of runoff; and the destabilizing effects of cooling, vertical mixing, and the wind-driven cross-shelf buoyancy flux