scholarly journals Sea-ice Induced Southern Ocean Subsurface Warming and Surface Cooling in a Warming Climate

2020 ◽  
Author(s):  
F. Alexander Haumann ◽  
Nicolas Gruber ◽  
Matthias Münnich
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Heat Content ◽  
Ocean Surface ◽  
Open Ocean ◽  
Global Ocean ◽  
Content Increase ◽  
Surface Cooling ◽  
Cold Surface

<p>Much of the Southern Ocean surface south of 55° S cooled and freshened between at least the early 1980s and the early 2010s. Many processes have been proposed to explain the unexpected cooling, including increased winds or increased surface freshwater fluxes from either the atmosphere or glacial meltwater. However, these mechanisms so far failed to fully explain the surface trends and the concurrently observed warming of the subsurface (100 to 500 m). Here, we argue that these trends are predominantly caused by an increased wind-driven northward transport of sea ice, enhancing the extraction of freshwater near Antarctica and releasing it in the open ocean. This conclusion is based on factorial experiments with a regional ocean model. In all experiments with an enhanced northward transport of sea ice, the open-ocean surface between the Subantarctic Front and the sea-ice edge is cooled by strengthening the salinity dominated oceanic stratification. The strengthened stratification reduces the downward mixing of cold surface water and the upward heat loss of the warmer waters below, thus warming the subsurface. This sea-ice induced subsurface warming mostly occurs around West Antarctica, where it likely enhances ice-shelf melting. Moreover, it could account for about 8±2% of the global ocean heat content increase between 1982 and 2011. Antarctic sea-ice changes thereby may have contributed to the slowdown of global surface warming over this period. The important role of sea-ice in driving changes in the high-latitude Southern Ocean are robust across all considered sensitivity cases, although their magnitude is sensitive to the forcing and the role of salinity in controlling the vertical stratification in the mean state. It remains yet unclear whether these sea-ice induced changes are associated with natural variability or a response to anthropogenic forcing.</p>

scholarly journals Southern Ocean heat storage, reemergence, and winter sea ice decline induced by summertime winds

2020 ◽  
pp. 1-47
Author(s):  
Edward W. Doddridge ◽  
John Marshall ◽  
Hajoon Song ◽  
Jean-Michel Campin ◽  
Maxwell Kelley
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Mixed Layer ◽  
Vertical Mixing ◽  
Heat Content ◽  
Ice Cover ◽  
Sea Surface ◽  
Surface Mixed Layer ◽  
Surface Cooling ◽  
Westerly Winds

AbstractThe observational record shows a substantial 40-year upward trend in summertime westerly winds over the Southern Ocean, as characterised by the Southern Annular Mode (SAM) index. Enhanced summertime westerly winds have been linked to cold summertime sea surface temperature (SST) anomalies. Previous studies have suggested that Ekman transport or upwelling is responsible for this seasonal cooling. Here, another process is presented in which enhanced vertical mixing, driven by summertime wind anomalies, moves heat downwards, cooling the sea surface and simultaneously warming the subsurface waters. The anomalously cold SSTs draw heat from the atmosphere into the ocean, leading to increased depth-integrated ocean heat content. The subsurface heat is returned to the surface mixed layer during the autumn and winter as the mixed layer deepens, leading to anomalously warm SSTs and potentially reducing sea ice cover. Observational analyses and numerical experiments support our proposed mechanism, showing that enhanced vertical mixing produces subsurface warming and cools the surface mixed layer. Nevertheless, the dominant driver of surface cooling remains uncertain; the relative importance of advective and mixing contributions to the surface cooling is model dependent. Modeling results suggest that sea ice volume is more sensitive to summertime winds than sea ice extent, implying that enhanced summertime westerly winds may lead to thinner sea ice in the following winter, if not lesser ice extent. Thus, strong summertime winds could precondition the sea ice cover for a rapid retreat in the following melt season.

Recent trajectory of ocean heat uptake estimated from novel 1972-2017 ocean sea-ice model hindcast simulations

2021 ◽  
Author(s):  
Maurice Huguenin ◽  
Ryan Holmes ◽  
Matthew England
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Heat Content ◽  
Anthropogenic Heat ◽  
Global Ocean ◽  
Ocean Heat Uptake ◽  
Heat Uptake ◽  
The Tropics ◽  
And Storage ◽  
Ice Model

<p>Uptake and storage of heat by the ocean plays a critical role in modulating the Earth's climate system. In the last 50 years, the ocean has absorbed over 90% of the additional energy accumulating in the Earth system due to radiative imbalance. However, our knowledge about ocean heat uptake (OHU), transport and storage is strongly constrained by the sparse observational record with large uncertainties. In this study, we conduct a suite of historical 1972–2017 hindcast simulations using a global ocean-sea ice model that are specifically designed to account for a cold start climate and model drift. The hindcast simulations are initialised from an equilibrated control simulation that uses repeat decade forcing over the period 1962-1971. This repeat decade forcing approach is a compromise between an early unobserved period (where our confidence in the forcing is low) and later periods (which would result in a shorter experiment period and a smaller fraction of the total OHU). The simulations are aimed at giving a good estimate of the trajectory of OHU in the tropics, the extratropics and individual ocean basins in recent decades. Many modelling studies that look at recent OHU rates so far use a simpler approach for the forcing. For example, they use repeating cycles of 1950-2010 Coordinated Ocean Reference Experiment (CORE) forcing that is consistent with the Ocean Model Intercomparison Project 2 (OMIP-2). However, this approach cannot account for model drift. The new simulations here highlight the dominant role of the extratropics, and in particular the Southern Ocean in OHU. In contrast, little heat is absorbed in the tropics and simulations forced with only tropical trends in atmospheric forcing show only weak global ocean heat content trends. Almost 50% of the heat taken up from the atmosphere in the Southern Ocean is transported into the Atlantic Ocean. Two-thirds of this Southern Ocean-sourced heat is then subsequently lost to the atmosphere in the North Atlantic but nevertheless this basin gains heat overall. Our results help to estimate the large-scale cycling of anthropogenic heat within the ocean today and have implications for heat content trends under a changing climate.</p>

Subpolar Southern Ocean response to changes in the surface momentum, heat and freshwater fluxes under 2xCO2

2021 ◽  
Author(s):  
Fabio Boeira Dias ◽  
Catia Domingues ◽  
Simon Marsland ◽  
Stephen Rintoul ◽  
Petteri Uotila ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Climate Models ◽  
Deep Convection ◽  
Open Ocean ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Surface Warming ◽  
Freshwater Fluxes ◽  
Poleward Shift

<ul><li>The subpolar Southern Ocean (sSO) around Antarctica has fundamental climate importance. The densest water mass in the global ocean, Antarctica Bottom Water (AABW), originates in the sSO and supplies the lower limb of the Meridional Overturning Circulation (MOC), occupying about 36% of the ocean’s volume. However, climate models struggle to represent the processes involved in formation of AABW on the continental shelf, resulting in large differences between models and observations and a wide spread in projections of sea level and other properties. We explore the source of these persistent model biases by examining the response of the sSO to perturbations in surface forcing. Using an ocean-sea ice model (ACCESS-OM2) that forms AABW both on the shelf and in open-ocean (similar to other coarse resolution models), we investigate the sSO response to individual and combined perturbations of surface heat, freshwater and momentum fluxes following the  FAFMIP-protocol. The wind perturbation (i.e. a poleward shift and intensification of the Southern Ocean Westerlies) has the dominant effect, enhancing AABW formation and accelerating the MOC. This occurs through upwelling of warm waters and inhibition of sea-ice growth during winter, which triggers large open-ocean polynya events with associated deep convection. These events occur in the Weddell and Ross Seas and their variability is associated with the heat available at mid-depth; open-ocean polynyas cease when the heat reservoir is depleted. The effects of surface warming and freshening only partially compensate the changes due to wind by increasing the ocean stratification and reducing AABW formation. These results are relevant for the interpretation of climate change projections, suggesting that other coarse  models might respond in similar way and present an opposite trend than those seen from observations.</li> </ul>

scholarly journals Sinking Diatom Assemblages as a Key Driver for Deep Carbon and Silicon Export in the Scotia Sea (Southern Ocean)

2021 ◽  
Vol 9 ◽  
Author(s):  
D. Zúñiga ◽  
A. Sanchez-Vidal ◽  
M. M. Flexas ◽  
D. Carroll ◽  
M. M. Rufino ◽  
...  
Keyword(s):  
Carbon Sequestration ◽  
Southern Ocean ◽  
Sea Ice ◽  
Environmental Conditions ◽  
Model Simulation ◽  
Deep Ocean ◽  
Global Ocean ◽  
Carbon Export ◽  
Scotia Sea

Physical and biogeochemical processes in the Southern Ocean are fundamental for modulating global climate. In this context, a process-based understanding of how Antarctic diatoms control primary production and carbon export, and hence global-ocean carbon sequestration, has been identified as a scientific priority. Here we use novel sediment trap observations in combination with a data-assimilative ocean biogeochemistry model (ECCO-Darwin) to understand how environmental conditions trigger diatom ecology in the iron-fertilized southern Scotia Sea. We unravel the role of diatoms assemblage in controlling the biogeochemistry of sinking material escaping from the euphotic zone, and discuss the link between changes in upper-ocean environmental conditions and the composition of settling material exported from the surface to 1,000 m depth from March 2012 to January 2013. The combined analysis of in situ observations and model simulation suggests that an anomalous sea-ice episode in early summer 2012–2013 favored (via restratification due to sea-ice melt) an early massive bloom of Corethron pennatum that rapidly sank to depth. This event drove high biogenic silicon to organic carbon export ratios, while modulating the carbon and nitrogen isotopic signals of sinking organic matter reaching the deep ocean. Our findings highlight the role of diatom ecology in modulating silicon vs. carbon sequestration efficiency, a critical factor for determining the stoichiometric relationship of limiting nutrients in the Southern Ocean.

scholarly journals The Effect of the Sea Ice Freshwater Flux on Southern Ocean Temperatures in CCSM3: Deep-Ocean Warming and Delayed Surface Warming

2011 ◽  
Vol 24 (9) ◽  
pp. 2224-2237 ◽  
Author(s):  
Clark H. Kirkman ◽  
Cecilia M. Bitz
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Heat Transport ◽  
Deep Ocean ◽  
First Century ◽  
Freshwater Flux ◽  
Freshwater Input ◽  
Surface Cooling ◽  
Surface Warming

Abstract This study explores the role of sea ice freshwater and salt fluxes in modulating twenty-first-century surface warming in the Southern Ocean via analysis of sensitivity experiments in the Community Climate System Model, version 3 (CCSM3). In particular, the role of a change in these fluxes in causing surface cooling, expanding sea ice, and increasing deep oceanic storage of heat in the Southern Ocean is investigated. The results indicate that in response to the doubling of CO2 concentrations in the atmosphere in CCSM3, net freshwater input from sea ice to the ocean increases south of 58°S (owing to less growth) and decreases from 48° to 58°S (owing to less melt). The freshwater source from changing precipitation in the model is considerably less than from sea ice south of 58°S, but it serves to compensate for the reduction in sea ice melt near the ice edge, leaving almost no net freshwater flux change between about 48° and 58°S. As a result, freshwater input principally from sea ice reduces ocean convection, which in turns reduces the entrainment of heat into the mixed layer and reduces the upward heat transport along isopycnals below about 1000 m. The reduced upward heat transport (from all sources) causes deep-ocean heating south of 60°S and below 500-m depth, with a corresponding surface cooling in large parts of the Southern Ocean in the model. These results indicate that changing sea ice freshwater and salt fluxes are a major component of the twenty-first-century delay in surface warming of the Southern Ocean and weak reduction in Antarctic sea ice in model projections.

On the impact of sea ice in a global ocean circulation model

1997 ◽  
Vol 25 ◽  
pp. 111-115 ◽  
Author(s):  
Achim Stössel
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Ocean Circulation ◽  
General Circulation ◽  
Thermohaline Circulation ◽  
Circulation Model ◽  
Deep Ocean ◽  
Global Ocean ◽  
Convective Activity ◽  
The Impact

This paper investigates the long-term impact of sea ice on global climate using a global sea-ice–ocean general circulation model (OGCM). The sea-ice component involves state-of-the-art dynamics; the ocean component consists of a 3.5° × 3.5° × 11 layer primitive-equation model. Depending on the physical description of sea ice, significant changes are detected in the convective activity, in the hydrographic properties and in the thermohaline circulation of the ocean model. Most of these changes originate in the Southern Ocean, emphasizing the crucial role of sea ice in this marginally stably stratified region of the world's oceans. Specifically, if the effect of brine release is neglected, the deep layers of the Southern Ocean warm up considerably; this is associated with a weakening of the Southern Hemisphere overturning cell. The removal of the commonly used “salinity enhancement” leads to a similar effect. The deep-ocean salinity is almost unaffected in both experiments. Introducing explicit new-ice thickness growth in partially ice-covered gridcells leads to a substantial increase in convective activity, especially in the Southern Ocean, with a concomitant significant cooling and salinification of the deep ocean. Possible mechanisms for the resulting interactions between sea-ice processes and deep-ocean characteristics are suggested.

scholarly journals Modeling the effect of Ross Ice Shelf melting on the Southern Ocean in quasi-equilibrium

2018 ◽  
Vol 12 (9) ◽  
pp. 3033-3044 ◽  
Author(s):  
Xiying Liu
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Ross Sea ◽  
Global Ocean ◽  
Freshwater Flux ◽  
Quasi Equilibrium ◽  
Ice Shelf ◽  
Sea Ice Concentration ◽  
Ross Ice Shelf ◽  
Ice Concentration

Abstract. To study the influence of basal melting of the Ross Ice Shelf (BMRIS) on the Southern Ocean (ocean southward of 35∘ S) in quasi-equilibrium, numerical experiments with and without the BMRIS effect were performed using a global ocean–sea ice–ice shelf coupled model. In both experiments, the model started from a state of quasi-equilibrium ocean and was integrated for 500 years forced by CORE (Coordinated Ocean-ice Reference Experiment) normal-year atmospheric fields. The simulation results of the last 100 years were analyzed. The melt rate averaged over the entire Ross Ice Shelf is 0.25 m a−1, which is associated with a freshwater flux of 3.15 mSv (1 mSv = 103 m3 s−1). The extra freshwater flux decreases the salinity in the region from 1500 m depth to the sea floor in the southern Pacific and Indian oceans, with a maximum difference of nearly 0.005 PSU in the Pacific Ocean. Conversely, the effect of concurrent heat flux is mainly confined to the middle depth layer (approximately 1500 to 3000 m). The decreased density due to the BMRIS effect, together with the influence of ocean topography, creates local differences in circulation in the Ross Sea and nearby waters. Through advection by the Antarctic Circumpolar Current, the flux difference from BMRIS gives rise to an increase of sea ice thickness and sea ice concentration in the Ross Sea adjacent to the coast and ocean water to the east. Warm advection and accumulation of warm water associated with differences in local circulation decrease sea ice concentration on the margins of sea ice cover adjacent to open water in the Ross Sea in September. The decreased water density weakens the subpolar cell as well as the lower cell in the global residual meridional overturning circulation (MOC). Moreover, we observe accompanying reduced southward meridional heat transport at most latitudes of the Southern Ocean.

The distribution and abundance of viruses in the Southern Ocean during spring

2000 ◽  
Vol 12 (4) ◽  
pp. 414-417 ◽  
Author(s):  
Harvey Marchant ◽  
Andrew Davidson ◽  
Simon Wright ◽  
John Glazebrook
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Surface Waters ◽  
Significant Relationship ◽  
Polar Front ◽  
Open Ocean ◽  
Bacterial Concentration ◽  
Viral Abundance ◽  
Chroococcoid Cyanobacteria ◽  
Bacterial Concentrations

The concentrations of viruses, bacteria, chroococcoid cyanobacteria and chlorophyll a were determined in surface waters of the Southern Ocean during spring. Viral concentrations declined southward from around 4 × 106 ml−1 near Tasmania to a minimum of around 1 × 106 ml−1 at the Polar Front. South of the Front, virus concentrations increased again, reaching around 4 × 106 ml−1 in the sea-ice zone south of 60°S. Bacterial concentration decreased southwards across the Southern Ocean from around 6.5 × 105 ml−1 near Tasmania to < 1.0 × 105 ml−1 in the sea-ice zone. Cyanobacteria accounted for < 8% of the prokaryotes. There was no significant relationship between viral abundance and eithercyanobacterial or chl a concentration. Viral and bacterial concentrations were not significantly correlated north (P {0.10 < r < 0.20}) or south (P {0.20 < r < 0.5}) of the Polar Front. The virus to bacteria ratio (VBR) was between 3 and 15 in the open ocean but varied between 15 and 40 in the sea-ice region. These virus concentrations and VBRs indicate that viruses are no less important in Southern Ocean ecosystems than elsewhere in the world's oceans.

scholarly journals Sustaining observations in the polar oceans

2014 ◽  
Vol 372 (2025) ◽  
pp. 20130337 ◽  
Author(s):  
E. P. Abrahamsen
Keyword(s):  
Sea Ice ◽  
Ocean Circulation ◽  
Ice Cover ◽  
Global Ocean ◽  
Shelf Seas ◽  
Polar Oceans ◽  
Global Ocean Circulation ◽  
Coordination And Collaboration ◽  
Temperature Depth

Polar oceans present a unique set of challenges to sustained observations. Sea ice cover restricts navigation for ships and autonomous measurement platforms alike, and icebergs present a hazard to instruments deployed in the upper ocean and in shelf seas. However, the important role of the poles in the global ocean circulation provides ample justification for sustained observations in these regions, both to monitor the rapid changes taking place, and to better understand climate processes in these traditionally poorly sampled areas. In the past, the vast majority of polar measurements took place in the summer. In recent years, novel techniques such as miniature CTD (conductivity–temperature–depth) tags carried by seals have provided an explosion in year-round measurements in areas largely inaccessible to ships, and, as ice avoidance is added to autonomous profiling floats and gliders, these promise to provide further enhancements to observing systems. In addition, remote sensing provides vital information about changes taking place in sea ice cover at both poles. To make these observations sustainable into the future, improved international coordination and collaboration is necessary to gain optimum utilization of observing networks.

scholarly journals Sea‐Ice Induced Southern Ocean Subsurface Warming and Surface Cooling in a Warming Climate

AGU Advances ◽  
2020 ◽  
Vol 1 (2) ◽  
Author(s):  
F. Alexander Haumann ◽  
Nicolas Gruber ◽  
Matthias Münnich
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Surface Cooling ◽  
Warming Climate
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