The first 250 years of the Heinrich 11 iceberg discharge: Last Interglacial HadGEM3-GC3.1 simulations for CMIP6-PMIP4

The Heinrich 11 event is simulated using the HadGEM3 model during the Last Interglacial period. We apply 0.2 Sv of meltwater forcing across the North Atlantic during a 250 years long simulation. We find that the strength of the Atlantic Meridional Overturning Circulation is reduced by 60 % after 150 years of meltwater forcing, with an associated decrease of 0.2 to 0.4 PW in meridional ocean heat transport at all latitudes. The changes in ocean heat transport affect surface temperatures. The largest increase in the meridional surface temperature gradient occurs between 40–50 N. This increase is associated with a strengthening of 20 % in 850 hPa winds. The stream jet intensification in the Northern Hemisphere in return alters the temperature structure of the ocean heat through an increased gyre circulation, and associated heat transport (+0.1–0.2 PW), at the mid-latitudes, and a decreased gyre ocean heat transport (−0.2 PW) at high-latitudes. The changes in meridional temperature and pressure gradients cause the Intertropical Convergence Zone (ITCZ) to move southward, leading to stronger westerlies and a more positive Southern Annual Mode (SAM) in the Southern Hemisphere. The positive SAM influences sea ice formation leading to an increase in Antarctic sea ice. Our coupled-model simulation framework shows that the classical "thermal bipolar see-saw'' has previously undiscovered consequences in both Hemispheres: these include Northern Hemisphere gyre heat transport and wind changes; alongside an increase in Antarctic sea ice during the first 250 years of meltwater forcing.

Different mechanisms of Arctic and Antarctic sea ice response to ocean heat transport

Understanding drivers of Arctic and Antarctic sea ice on multidecadal timescales is key to reducing uncertainties in long-term climate projections. Here we investigate the impact of ocean heat transport (OHT) on sea ice, using pre-industrial control simulations of 20 models participating in the latest Coupled Model Intercomparison Project (CMIP6). In all models and in both hemispheres, sea ice extent is negatively correlated with poleward OHT. However, the similarity of the correlations in both hemispheres hides radically different underlying mechanisms. In the northern hemisphere, positive OHT anomalies primarily result in increased ocean heat convergence along the Atlantic sea ice edge, where most of the ice loss occurs. Such strong, localised heat fluxes ($$\sim {}100~\text {W}~\text {m}^{-2}$$ ∼ 100 W m - 2 ) also drive increased atmospheric moist-static energy convergence at higher latitudes, resulting in a pan-Arctic reduction in sea ice thickness. In the southern hemisphere, increased OHT is released relatively uniformly under the Antarctic ice pack, so that associated sea ice loss is driven by basal melt with no direct atmospheric role. These results are qualitatively robust across models and strengthen the case for a substantial contribution of ocean forcing to sea ice uncertainty, and biases relative to observations, in climate models.

Antarctic sea ice types from active and passive microwave remote sensing

Polar sea ice is one of the Earth’s climate components that has been significantly affected by the recent trend of global warming. While the sea ice area in the Arctic has been decreasing at a rate of about 4 % per decade, the multi-year ice (MYI), also called perennial ice, is decreasing at a faster rate of 10 %–15 % per decade. On the other hand, the sea ice area in the Antarctic region was slowly increasing at a rate of about 1.5 % per decade until 2014 and since then it has fluctuated without a clear trend. However, no data about ice type areas are available from that region, particularly of MYI. Due to differences in physical and crystalline structural properties of sea ice and snow between the two polar regions, it has become difficult to identify ice types in the Antarctic. Until recently, no method has existed to monitor the distribution and temporal development of Antarctic ice types, particularly MYI throughout the freezing season and on decadal time scales. In this study, we have adapted a method for retrieving Arctic sea ice types and partial concentrations using microwave satellite observations to fit the Antarctic sea ice conditions. The first circumpolar, long-term time series of Antarctic sea ice types; MYI, first-year ice and young ice is being established, so far covering years 2013–2019. Qualitative comparison with synthetic aperture radar data, with charts of the development stage of the sea ice, and with Antarctic polynya distribution data show that the retrieved ice types, in particular the MYI, are reasonable. Although there are still some shortcomings, the new retrieval for the first time allows insight into the evolution and dynamics of Antarctic sea ice types. The current time series can in principle be extended backwards to start in the year 2002 and can be continued with current and future sensors.

Inter-annual variations observed in spring and summer Antarctic sea ice extent in recent decade

The growth and decay of sea ice are complex processes and have important feedback onto the oceanic and atmospheric circulation. In the Antarctic, sea ice variability significantly affects the primary productivity in the Southern Ocean and thereby negatively influences the performance and survival of species in polar ecosystem. In present days, the awareness on the sea ice variability in the Antarctic is not as matured as it is for the Arctic region. The present paper focuses on the inter-annual trends (1999-2009) observed in the monthly fractional sea ice cover in the Antarctic at 1 × 1 degree level, for the November and February months, derived from QuikSCAT scatterometer data. OSCAT scatterometer data from India’s Oceansat-2 satellite were used to asses the sea ice extent (SIE) observed in the month of November 2009 and February 2010 and its deviation from climatic maximum (1979-2002) sea ice extent (CMSIE). Large differences were observed between SIE and CMSIE, however, trend results show that it is due to the high inter-annual variability in sea ice cover. Spatial distribution of trends show the existence of positive and negative trends in the parts of Western Pacific Ocean, Ross Sea, Amundsen and Bellingshausen Seas (ABS), Weddell Sea and Indian ocean sector of southern ocean. Sea ice trends are compared with long-term SST trends (1982-2009) observed in the austral summer month of February. Large-scale cooling trend observed around Ross Sea and warming trend in ABS sector are the distinct outcome of the study.

The Light Responses of Proteorhodopsin-bearing, Antarctic Sea-ice Bacteria

<p>Although homogenous in appearance, Antarctic sea ice forms a complex habitat that is characterised by steep vertical gradients of temperature, irradiance and salinity. Despite these harsh and variable environmental conditions, numerous microbial organisms prosper within Antarctic sea ice. In 2010, bacteria bearing the proteorhodopsin (PR) gene were found within Antarctic sea ice. PR is a photoactive membrane protein that functions as a light-driven proton pump. The hydrogen ion membrane gradient that PR establishes has the potential to drive ATP synthesis, thus allowing PR-bearing bacteria to obtain energy from solar radiation. Although this gene is present in up to 80% of marine bacteria, the active contribution of PR in vivo is debatable. Light induced growth or enhanced survival is generally observed only when PR-bearing bacteria are grown under sub-optimum conditions, such as limited nutrients or carbon, or variations in salinity. This has lead to the general hypothesis that PR has multiple functions, becoming most influential under conditions of stress. In this way, Antarctic sea-ice bacteria may utilise PR to promote survival and enhance energy inputs, when exposed to the harsh conditions of this environment. To explore this hypothesis, potential PR-bearing isolates were cultured from samples of Antarctic sea-ice bacteria. Using 16S rRNA gene sequencing as well as a comparison of phenotypic and environmental characteristics, the isolates were identified as; Psychrobacter nivimaris, Polaribacter dokdonensis, Paracoccus marcusii and Micrococcus sp. These species, along with Psychroflexus torquis (an Antarctic sea-ice bacterium known to possess PR) were examined for the presence of the PR gene. This gene was identified in P. torquis, Ps. nivimaris and Po. dokdonensis. To my knowledge, this is the first time PR has been found in Ps. nivimaris. To assess the influence of irradiance on these species, a series of culture based experiments were undertaken. In 2012, a preliminary field experiment was conducted in which a mixed culture of PR-bearing and non PR-bearing bacteria; Ps. nivimaris, Po. dokdonensis, Pa. marcusii and Micrococcus sp., was incubated in situ in the annual sea ice surrounding Ross Island, Antarctica. The method developed for these experiments is unique, in that cultures of sea-ice bacteria have not before been incubated within their natural environment. No major differences in growth patterns were observed when bacteria were incubated under different wavelengths and light intensities, however, valuable insight into methodological improvement was obtained. Using these refinements, a second in situ incubation experiment was conducted at the same field site, in 2013. Over this 2 week incubation, monocultures of P. torquis grown in full strength media grew most readily under 50%- and blue-light treatments, with red- and green-light yielding lower biomasses, and no growth occurring in the dark. Ambient sea-ice irradiance resulted in highly variable growth, attributed to high irradiance growth-inhibition. These results indicate that P. torquis utilises low levels of light in order to increase its growth in Antarctic sea ice. The influence of light on the growth of P. torquis, Ps. nivimaris and Po. dokdonensis was examined in a laboratory-based experiment, in which media strength and temperature were varied. When cultured at 12°C, Ps. nivimaris grown under constant irradiance reached a higher biomass than in darkness. This trend was most pronounced when this species was cultured in a 10% media concentration. A trend of decreased exponential-growth was observed in light-incubated cultures of Ps. nivimaris, grown at 4°C or -1°C. Elevated maximum growth of Po. dokdonensis was observed under irradiated conditions in the 10% media treatment. This species however, only grew at 12°C; an unexpected result for an Antarctic microbe. P. torquis was not affected by irradiance under any culture conditions and did not grow at -1°C. This last result contrasts the results of the in situ incubations and may have been affected by factors such as culture age. This research demonstrates multiple examples of light-enhanced growth occurring in PR-bearing Antarctic sea-ice bacteria, with the most prominent trends occurring in reduced concentration media. Therefore, this work agrees with the overarching hypothesis that PR is most influential under conditions of stress. The varying effect of temperature on the influence of PR suggests that some species are able to use this protein at low temperatures, whilst others cannot. Therefore, PR likely provides a selective advantage to some species, depending on a variety of physicochemical factors, including nutrient and carbon availability, salinity and temperature.</p>

The Light Responses of Proteorhodopsin-bearing, Antarctic Sea-ice Bacteria

<p>Although homogenous in appearance, Antarctic sea ice forms a complex habitat that is characterised by steep vertical gradients of temperature, irradiance and salinity. Despite these harsh and variable environmental conditions, numerous microbial organisms prosper within Antarctic sea ice. In 2010, bacteria bearing the proteorhodopsin (PR) gene were found within Antarctic sea ice. PR is a photoactive membrane protein that functions as a light-driven proton pump. The hydrogen ion membrane gradient that PR establishes has the potential to drive ATP synthesis, thus allowing PR-bearing bacteria to obtain energy from solar radiation. Although this gene is present in up to 80% of marine bacteria, the active contribution of PR in vivo is debatable. Light induced growth or enhanced survival is generally observed only when PR-bearing bacteria are grown under sub-optimum conditions, such as limited nutrients or carbon, or variations in salinity. This has lead to the general hypothesis that PR has multiple functions, becoming most influential under conditions of stress. In this way, Antarctic sea-ice bacteria may utilise PR to promote survival and enhance energy inputs, when exposed to the harsh conditions of this environment. To explore this hypothesis, potential PR-bearing isolates were cultured from samples of Antarctic sea-ice bacteria. Using 16S rRNA gene sequencing as well as a comparison of phenotypic and environmental characteristics, the isolates were identified as; Psychrobacter nivimaris, Polaribacter dokdonensis, Paracoccus marcusii and Micrococcus sp. These species, along with Psychroflexus torquis (an Antarctic sea-ice bacterium known to possess PR) were examined for the presence of the PR gene. This gene was identified in P. torquis, Ps. nivimaris and Po. dokdonensis. To my knowledge, this is the first time PR has been found in Ps. nivimaris. To assess the influence of irradiance on these species, a series of culture based experiments were undertaken. In 2012, a preliminary field experiment was conducted in which a mixed culture of PR-bearing and non PR-bearing bacteria; Ps. nivimaris, Po. dokdonensis, Pa. marcusii and Micrococcus sp., was incubated in situ in the annual sea ice surrounding Ross Island, Antarctica. The method developed for these experiments is unique, in that cultures of sea-ice bacteria have not before been incubated within their natural environment. No major differences in growth patterns were observed when bacteria were incubated under different wavelengths and light intensities, however, valuable insight into methodological improvement was obtained. Using these refinements, a second in situ incubation experiment was conducted at the same field site, in 2013. Over this 2 week incubation, monocultures of P. torquis grown in full strength media grew most readily under 50%- and blue-light treatments, with red- and green-light yielding lower biomasses, and no growth occurring in the dark. Ambient sea-ice irradiance resulted in highly variable growth, attributed to high irradiance growth-inhibition. These results indicate that P. torquis utilises low levels of light in order to increase its growth in Antarctic sea ice. The influence of light on the growth of P. torquis, Ps. nivimaris and Po. dokdonensis was examined in a laboratory-based experiment, in which media strength and temperature were varied. When cultured at 12°C, Ps. nivimaris grown under constant irradiance reached a higher biomass than in darkness. This trend was most pronounced when this species was cultured in a 10% media concentration. A trend of decreased exponential-growth was observed in light-incubated cultures of Ps. nivimaris, grown at 4°C or -1°C. Elevated maximum growth of Po. dokdonensis was observed under irradiated conditions in the 10% media treatment. This species however, only grew at 12°C; an unexpected result for an Antarctic microbe. P. torquis was not affected by irradiance under any culture conditions and did not grow at -1°C. This last result contrasts the results of the in situ incubations and may have been affected by factors such as culture age. This research demonstrates multiple examples of light-enhanced growth occurring in PR-bearing Antarctic sea-ice bacteria, with the most prominent trends occurring in reduced concentration media. Therefore, this work agrees with the overarching hypothesis that PR is most influential under conditions of stress. The varying effect of temperature on the influence of PR suggests that some species are able to use this protein at low temperatures, whilst others cannot. Therefore, PR likely provides a selective advantage to some species, depending on a variety of physicochemical factors, including nutrient and carbon availability, salinity and temperature.</p>

Bacterial Community Structure, Function and Diversity in Antarctic Sea Ice

<p>Antarctic sea ice is an important feature of the southern ocean where at its maximum it can cover 8 % of the Southern Hemisphere. It provides a stable environment for the colonisation of diverse and highly specialised microbes which play a central role in the assimilation and regulation of energy through the Antarctic food web. Polar environments are sensitive to changes in the environment. Small changes in temperature can have large effects on sea ice thickness and extent and Antarctic sea ice cover is expected to shrink by 25 % over the next century. It is unknown how the sea ice microbiota will respond. In order to understand the effects of climate change on the sea ice ecosystem it is necessary to obtain information about the community structure, function and diversity and their reactions with the environment. Studies have focused on algal diversity and physiology in Antarctic sea ice and in comparison studies on the prokaryotic community are few. Although prokaryotic diversity has been investigated using clone libraries and culture based methods, it is likely that certain species have still not been described. Almost nothing is known about the Antarctic sea ice bacterial community spatial and temporal dynamics under changing abiotic and biotic conditions or their role in biogeochemical cycles. This is the first study linking Antarctic bacterial communities to function by using statistics to investigate the relationships between environmental variables and community structure. Bacterial community structure was investigated by extracting both the DNA and RNA from the environment to understand both the metabolically active (RNA) and total (DNA) bacterial community. The thickness of the sea ice and nutrient concentrations were key factors regulating bacterial community composition in Antarctic sea ice. Sea ice thickness is likely to have an effect on the physiological responses of algae leading to changes in photosynthate concentrations and composition of dissolved organic matter (DOM). Further investigations into the relationships between enzymatic activity and community structure revealed that the composition of the DOM drove variation between bacterial communities. There was no relationship between bacterial abundance and chlorophyll-a (as a measure of algal biomass), suggesting a un-coupling of the microbial loop. However bacteria were actively involved in the hydrolysis of polymers throughout the sea ice core. Investigations using quantitative PCR (qPCR) found that the functional genes involved in denitrification and light energy utilisation were in low abundance therefore these processes are minor in Antarctic sea ice. These results confirm that sea ice bacteria are predominantly heterotrophs and have a major role in the cycling of carbon and nitrogen through the microbial loop ...</p>