Collapse of the Atlantic Meridional Overturning Circulation (AMOC) is often invoked as an explanation of major past climate changes and as a major risk for future climate. Many of these arguments ...appear, from an observer’s point of view, as far-more definitive than is warranted. In the hypothetical event of a future collapse, the implications may be much less severe than those from many other elements of global change already underway. The Gulf Stream system, and its required return flow of mass, implies that changed circulations will nonetheless continue to carry significant amounts of heat, carbon etc., poleward even without any AMOC.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
While the Atlantic Ocean is ventilated by high-latitude deep water formation and exhibits a pole-to-pole overturning circulation, the Pacific Ocean does not. This asymmetric global overturning ...pattern has persisted for the past 2-3 million years, with evidence for different ventilation modes in the deeper past. In the current climate, the Atlantic-Pacific asymmetry occurs because the Atlantic is more saline, enabling deep convection. To what extent the salinity contrast between the two basins is dominated by atmospheric processes (larger net evaporation over the Atlantic) or oceanic processes (salinity transport into the Atlantic) remains an outstanding question. Numerical simulations have provided support for both mechanisms; observations of the present climate support a strong role for atmospheric processes as well as some modulation by oceanic processes. A major avenue for future work is the quantification of the various processes at play to identify which mechanisms are primary in different climate states.
In contrast with the atmosphere, which is heated from below by solar radiation, the ocean is both heated and cooled from above. To drive a deep-reaching overturning circulation in this context, it is ...generally assumed that either intense interior mixing by winds and internal tides, or wind-driven upwelling is required; in their absence, the circulation is thought to collapse to a shallow surface cell. We demonstrate, using a primitive equation model with an idealized domain and no wind forcing, that the surface temperature forcing can in fact drive an interhemispheric overturning provided that there is an open channel unblocked in the zonal direction, such as in the Southern Ocean. With this geometry, rotating horizontal convection, in combination with asymmetric surface cooling between the north and south, drives a deep-reaching two-cell overturning circulation. The resulting vertical mid-depth stratification closely resembles that of the real ocean, suggesting that wind-driven pumping is not necessary to produce a deep-reaching overturning circulation, and that buoyancy forcing plays a more important role than is usually assumed.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Understanding the physical mechanisms behind the transport and accumulation of floating objects in the ocean is crucial to efficiently tackle the issue of marine pollution. The main sinks of marine ...plastic are the coast and the bottom sediment. This study focuses on the former, investigating the timescales of dispersal from the ocean surface and onto coastal accumulation areas through a process called “beaching.” Previous studies found that the Stokes drift can reach the same magnitude as the Eulerian current speed and that it has a long-term effect on the trajectories of floating objects. Two particle tracking models (PTMs) are carried out and then compared, one with and one without Stokes drift, named
PTM-SD
and
PTM-REF
, respectively. Eulerian velocity and Stokes drift data from global reanalysis datasets are used for particle advection. Particles in the
PTM-SD
model are found to beach at a yearly rate that is double the rate observed in
PTM-REF
. The main coastal attractors are consistent with the direction of large-scale atmospheric circulation (Westerlies and Trade Winds). After 12 years (at the end of the run), the amount of beached particles is 20% larger in
PTM-SD
than in
PTM-REF
. Long-term predictions carried out with the aid of adjacency matrices found that after 100 years all particles have beached in
PTM-SD
, while 8% of the all seeded particles are still floating in
PTM-REF
. The results confirm the need to accurately represent the Stokes drift in particle models attempting to predict the behaviour of marine debris, in order to avoid overestimation of its residence time in the ocean and effectively guide policies toward prevention and removal.
West Antarctic ice shelves have thinned dramatically over recent decades. Oceanographic measurements that explore connections between offshore warming and transport across a continental shelf with ...variable bathymetry toward ice shelves are needed to constrain future changes in melt rates. Six years of seal‐acquired observations provide extensive hydrographic coverage in the Bellingshausen Sea, where ship‐based measurements are scarce. Warm but modified Circumpolar Deep Water floods the shelf and establishes a cyclonic circulation within the Belgica Trough with flow extending toward the coast along the eastern boundaries and returning to the shelf break along western boundaries. These boundary currents are the primary water mass pathways that carry heat toward the coast and advect ice shelf meltwater offshore. The modified Circumpolar Deep Water and meltwater mixtures shoal and thin as they approach the continental slope before flowing westward at the shelf break, suggesting the presence of the Antarctic Slope Current. Constraining meltwater pathways is a key step in monitoring the stability of the West Antarctic Ice Sheet.
Key Points
Warm but modified CDW floods the continental shelf of the Bellingshausen Sea
A cyclonic trough circulation that carries heat toward the coast is detected
The mCDW flows westward at the shelf break, suggesting the Antarctic Slope Current
In the Amundsen Sea, warm saline Circumpolar Deep Water (CDW) crosses the continental shelf toward the vulnerable West Antarctic ice shelves, contributing to their basal melting. Due to lack of ...observations, little is known about the spatial and temporal variability of CDW, particularly seasonally. A new data set of 6,704 seal tag temperature and salinity profiles in the easternmost trough between February and December 2014 reveals a CDW layer on average 49 dbar thicker in late winter (August to October) than in late summer (February to April), the reverse seasonality of that seen at moorings in the western trough. This layer contains more heat in winter, but on the 27.76 kg/m3 density surface CDW is 0.32°C warmer in summer than in winter, across the northeastern Amundsen Sea, which may indicate that wintertime shoaling offshelf changes CDW properties onshelf. In Pine Island Bay these seasonal changes on density surfaces are reduced, likely by gyre circulation.
Plain Language Summary
In the Amundsen Sea, Antarctica, warm salty water crosses the continental shelf from the deep open ocean, toward the vulnerable West Antarctic ice shelves, bringing heat to help melt them from underneath. Due to lack of observations, little is known about how this flow of warm water varies in space and time, particularly seasonally. Between February and December 2014, in a trough in the eastern Amundsen Sea, 6,704 profiles were collected by sensors attached to seals, measuring temperature and salinity as the seals return from dives up to 1,200 m deep. These data showed that this warm (∼1°C) deep layer is on average ∼50 m thicker in late winter (August to October) than in late summer (February to April), the reverse seasonality of that seen within a trough in the western Amundsen Sea. This warm layer contains more heat in winter but on a surface of constant density is 0.32°C warmer in summer than in winter, across the northeastern Amundsen Sea. This may indicate that in winter the deep waters offshelf rise, allowing different water onto the continental shelf. In Pine Island Bay these seasonal changes on density surfaces are reduced, probably because here the water circulates and mixes.
Key Points
In the eastern Amundsen Sea, the CDW layer is thicker in winter than in summer and contains more heat and salt
On isopycnals, CDW is cooler and fresher in winter, likely the result of a rising core of offshelf CDW changing CDW properties onshelf
Farther south, in PIB, this seasonality is less pronounced, likely the result of mixing as the PIB gyre recirculates CDW of different ages
Large-scale breeding failures, such as offspring die-offs, can disproportionately impact wildlife populations that are characterized by a few large colonies. However, breeding monitoring-and thus ...investigations of such die-offs-is especially challenging in species with long reproductive cycles. We investigate two unresolved dramatic breeding failures that occurred in consecutive years (2009 and 2010) in a large king penguin Aptenodytes patagonicus colony, a long-lived species with a breeding cycle lasting over a year. Here we found that a single period, winter 2009, was likely responsible for the occurrence of breeding anomalies during both breeding seasons, suggesting that adults experienced poor foraging conditions at sea at that time. Following that unfavorable winter, the 2009 breeding cohort-who were entering the late stage of chick-rearing-immediately experienced high chick mortality. Meanwhile, the 2010 breeding cohort greatly delayed their arrival and egg laying, which would have otherwise started not long after the winter. The 2010 breeding season continued to display anomalies during the incubation and chick-rearing period, such as high abandonment rate, long foraging trips and eventually the death of all chicks in winter 2010. These anomalies could have resulted from either a domino-effect caused by the delayed laying, the continuation of poor foraging conditions, or both. This study provides an example of a large-scale catastrophic breeding failure and highlights the importance of the winter period on phenology and reproduction success for wildlife that breed in few large colonies.
Antarctic coastal polynyas are regions of persistent open water and are thought to be key bio-physical features within the sea-ice zone. However, their use by the upper trophic levels of ecosystems ...remains unclear. A unique bio-physical dataset recorded by southern elephant seals reveals that East Antarctic polynyas are a key winter foraging habitat for male seals. During their post-moult trips from Isles Kerguelen to the Antarctic continental shelf, a total of 18 out of 23 seals visited 9 different polynyas, spending on average 25 ± 20% (up to 75%) of their total trip time inside polynyas. Changes in seal foraging and diving behaviours are observed inside polynyas as compared to outside polynyas. Two polynya usages by seals are observed for the inactive and active polynya phases, pointing to different seasonal peaks in prey abundance. During the active polynya phase, we link seal foraging behaviour to changes in the physical stability of the water-column, which likely impact the seasonal biological dynamics within polynyas.
The oceanic mixed layer is the gateway for the exchanges between the atmosphere and the ocean; in this layer, all hydrographic ocean properties are set for months to millennia. A vast area of the ...Southern Ocean is seasonally capped by sea‐ice, which alters the characteristics of the ocean mixed layer. The interaction between the ocean mixed layer and sea‐ice plays a key role for water mass transformation, the carbon cycle, sea‐ice dynamics, and ultimately for the climate as a whole. However, the structure and characteristics of the under‐ice mixed layer are poorly understood due to the sparseness of in situ observations and measurements. In this study, we combine distinct sources of observations to overcome this lack in our understanding of the polar regions. Working with elephant seal‐derived, ship‐based, and Argo float observations, we describe the seasonal cycle of the ocean mixed‐layer characteristics and stability of the ocean mixed layer over the Southern Ocean and specifically under sea‐ice. Mixed‐layer heat and freshwater budgets are used to investigate the main forcing mechanisms of the mixed‐layer seasonal cycle. The seasonal variability of sea surface salinity and temperature are primarily driven by surface processes, dominated by sea‐ice freshwater flux for the salt budget and by air‐sea flux for the heat budget. Ekman advection, vertical diffusivity, and vertical entrainment play only secondary roles. Our results suggest that changes in regional sea‐ice distribution and annual duration, as currently observed, widely affect the buoyancy budget of the underlying mixed layer, and impact large‐scale water mass formation and transformation with far reaching consequences for ocean ventilation.
Key Points
Climatological seasonal cycle of under‐ice mixed layer in the Southern Ocean is produced with unprecented number of observations
Under‐ice seasonal cycle of mixed‐layer buoyancy content and stability is primarily driven by their haline contributions
Buoyancy content of under‐ice mixed layer is predominantly explained by ice‐ocean and vertical entrainment fluxes
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