Bottom Water Formation Research Articles

Warming of deep and abyssal water masses along the Greenwich meridian on decadal time scales: The Weddell gyre as a heat buffer

The Southern Ocean renders a significant contribution to the global overturning system through the formation of deep and bottom waters. The Weddell Sea is one of the most prominent regions in this respect. Data obtained between 1984 and 2008 from eight repeat hydrographic sections, moored instruments and profiling floats in the Weddell gyre on the Greenwich meridian – almost all of them collected with RV Polarstern – were used to identify variations in the Weddell system. Fluctuations in the water mass properties were detected in the Warm Deep Water, with a temperature maximum in the 1990s and a minimum in 2005, but also significant variations occurred in the Weddell Sea Deep and Bottom Waters, whereas the Warm Deep Water is dominated by decadal variations; the average temperature and salinity of the whole water column is subject to a positive trend over 24 years. The variations of the water mass properties are induced by variations of the inflow of Circumpolar Deep Water at the boundary. Due to asymmetric wind forcing at the northern and the southern limb of the gyre, variable in- and outflows occur at the open boundaries. Internal processes redistribute heat and salt in the gyre resulting in a long-term increase of the temperature and salinity in the whole water column. The transfer of heat to deeper layers assigns to the Weddell gyre the role of a buffer, with potential impact on the global climate.

Downflow of the Antarctic shelf water at the shelf and continental slope of the Commonwealth Sea in the summer season and its effect on the bottom water formation in the Southern Ocean

In the summer seasons of 2004–2007, the intensive runoff (cascading) of the Antarctic shelf water (ASW) down the shelf and continental slope was revealed thanks to the recording of numerous thermohaline profiles across the shelf and continental slope of the Commonwealth Sea and Prydz Bay. The quickly executed profiles (4–10 h) with submesoscale resolution (near the shelf’s edge, the scale was even eddy-determinative, i.e., within 1.9–5.6 km), in combination with the fine-structure sounding and fine vertical resolution of the near-bottom boundary layer, provided a qualitatively new level of understanding the natural data. The detailed analysis of the temperature, salinity, and density patterns revealed the regularities and peculiarities of the ASW shelf and slope cascading. The intensive ASW cascading near the shelf break and lower part of the slope can be forced (appearing as discrete frontal meanders) or free (appearing as discrete plumes) and often has a wave-eddy character. The field observational data confirmed the obtained representative estimates of the elements of the ASW slope cascading. The basic area of the ASW formation is near the Amery Shelf Ice, from where the ASW spreads to the northwest, goes around the Fram Bank, and flows down the continental slope. The evaluative contribution of the ASW slope cascading to the ventilation of the deep and slope water of the Southern Ocean (near the shelf break 70 km long where the ASW cascading was observed) is QK = 0.04–0.24 Sv, which agrees well with the analogous estimates obtained in other regions of the Antarctic.

Research Spotlight: Climate response to melting of the West Antarctic Ice Sheet

The West Antarctic Ice Sheet (WAIS) could collapse in the future as the rising sea level and warming climate destabilize the sheet. Previous studies have shown that melting of WAIS could contribute to several meters of global sea level rise over the next few centuries. Some studies have pointed to evidence that WAIS has collapsed in the past during previous interglacial periods, which could give clues to what might happen in the future. To gain additional insight into how the Earth's climate would respond to a collapse of WAIS, Menviel et al. simulated the effects on global climate and the carbon cycle of a massive meltwater discharge from the collapse of the ice sheet. They found that a large meltwater discharge into the Southern Ocean would lead to a substantial cooling of the Southern Ocean, causing a northward expansion of the sea ice margin. Southern Hemisphere westerly winds would intensify. Furthermore, the formation of cold Antarctic Bottom Water would be suppressed, and subsurface warming would take place in areas where Antarctic Bottom Water is formed under present‐day conditions. This subsurface warming would lead to positive feedback that would accelerate the melting of the ice sheet. (Paleoceanography, doi:10.1029/2009PA001892, 2010)

A multi-variable box model approach to the soft tissue carbon pump

Abstract. The canonical question of which physical, chemical or biological mechanisms were responsible for oceanic uptake of atmospheric CO2 during the last glacial is yet unanswered. Insight from paleo-proxies has led to a multitude of hypotheses but none so far have been convincingly supported in three dimensional numerical modelling experiments. The processes that influence the CO2 uptake and export production are inter-related and too complex to solve conceptually while complex numerical models are time consuming and expensive to run which severely limits the combinations of mechanisms that can be explored. Instead, an intermediate inverse box model approach of the soft tissue pump is used here in which the whole parameter space is explored. The glacial circulation and biological production states are derived from these using proxies of glacial export production and the need to draw down CO2 into the ocean. We find that circulation patterns which explain glacial observations include reduced Antarctic Bottom Water formation and high latitude upwelling and mixing of deep water and to a lesser extent reduced equatorial upwelling. The proposed mechanism of CO2 uptake by an increase of eddies in the Southern Ocean, leading to a reduced residual circulation, is not supported. Regarding biological mechanisms, an increase in the nutrient utilization in either the equatorial regions or the northern polar latitudes can reduce atmospheric CO2 and satisfy proxies of glacial export production. Consistent with previous studies, CO2 is drawn down more easily through increased productivity in the Antarctic region than the sub-Antarctic, but that violates observations of lower export production there. The glacial states are more sensitive to changes in the circulation and less sensitive to changes in nutrient utilization rates than the interglacial states.

Climate and biogeochemical response to a rapid melting of the West Antarctic Ice Sheet during interglacials and implications for future climate

interglacials onto the global climate‐carbon cycle system using the Earth system model of intermediate complexity LOVECLIM. Prescribing a meltwater pulse in the Southern Ocean that mimics a rapid disintegration of the WAIS, a substantial cooling of the Southern Ocean is simulated that is accompanied by an equatorward expansion of the sea ice margin and an intensification of the Southern Hemispheric Westerlies. The strong halocline around Antarctica leads to suppression of Antarctic Bottom Water (AABW) formation and to subsurface warming in areas where under present‐day conditions AABW is formed. This subsurface warming at depths between 500 and 1500 m leads to a thermal weathering of the WAIS grounding line and provides a positive feedback that accelerates the meltdown of the WAIS. Our model results further demonstrate that in response to the massive expansion of sea ice, marine productivity in the Southern Ocean reduces significantly. A retreat of the WAIS, however, does not lead to any significant changes in atmospheric CO2. The climate signature of a WAIS collapse is structurally consistent with available paleoproxy signals of the last interglacial MIS5e. Citation: Menviel, L., A. Timmermann, O. E. Timm, and A. Mouchet (2010), Climate and biogeochemical response to a rapid melting of the West Antarctic Ice Sheet during interglacials and implications for future climate, Paleoceanography, 25, PA4231, doi:10.1029/2009PA001892.

Overflow dynamics and bottom water formation in the western Ross Sea: Influence of tides

The tidal impact on overflow dynamics and bottom water production in the western Ross Sea is studied with the Finite Element Ocean Model, which allows embedding a mesh with 0.5 km resolution in a coarse resolution (30 km) setup without nesting. The simulated overflow properties inside and downstream of the western Ross Sea are described. The overflow exhibits pronounced variability at both daily and spring‐neap tidal time scales in the western Ross Sea. Tides increase mixing over both the outer shelf and upper slope there. Plume jets are shaped by tidal currents at a bathymetric bend west of the Drygalski Trough mouth, descending rapidly and supplying the bottom water. A fraction of shelf water remains over the shelf and propagates westward from the Ross Sea, but it does not contribute significantly to bottom water formation because of energetic mixing over the upper slope. Compared to a simulation without tidal forcing, tides (with the major K1 and O1 constituents) increase the outflow rate over the continental slope off Cape Adare by about 70%. A set of sensitivity experiments show that the rate of bottom water production is not a monotonic function of the tidal currents amplitude. Tidal forcing with intermediate strength leads to the most efficient bottom water formation.

Modeling ice shelf water overflow and bottom water formation in the southern Weddell Sea

Descent of Ice Shelf Water (ISW) overflowed from the Filchner Depression in the southern Weddell Sea and consequent Weddell Sea Bottom Water (WSBW) production is investigated through high resolution numerical experiments. The model reproduces the part of ISW which interacts with the submarine ridge near 36°W. The baroclinic instability wave developed near the exit of the Filchner Depression leads to intermittent outflow of ISW, and the intermittent passage of ISW‐origin dense water induces 6‐day period oscillation of velocity at the east side of the ridge which coincides well with mooring data. The existence of the ridge induces two different mechanisms for cross‐isobath transport of the ISW‐origin dense water: a strong downslope current at the east side of the ridge and offshore transport by eddies, which are generated when a gravity current makes a detour around the tip of the ridge, over a wide region to the west of the ridge. In a sensitivity experiment where thermobaricity is removed from the equation of state, there is almost no transport of the ISW‐origin water below 3000 m depth. WSBW production rate as a result of descent of the ISW‐origin dense water in the model domain is about ∼0.15 Sv.

Antarctic Bottom Water from the Adélie and George V Land coast, East Antarctica (140–149°E)

We report on observations of dense shelf water overflows and Antarctic Bottom Water (AABW) formation along the continental margin of the Adélie and George V Land coast between 140°E and 149°E. Vertical sections and bottom layer water mass properties sampled during two RVIB Nathaniel B Palmer hydrographic surveys (NBP00–08, December 2000/January 2001 and NBP04–08, October 2004) describe the spreading of cold, dense shelf water on the continental slope and rise from two independent source regions. The primary source region is the Adélie Depression, exporting high‐salinity dense shelf water through the Adélie Sill at 143°E. An additional eastern source region of lower‐salinity dense shelf water from the Mertz Depression is identified for the first time from bottom layer properties northwest of the Mertz Sill and Mertz Bank (146°E–148°E) that extend as far as the Buffon Channel (144.75°E) in summer. Regional analysis of satellite‐derived ice production estimates over the entire region from 1992 to 2005 suggests that up to 40% of the total ice production for the region occurs over the Mertz Depression and therefore this area is likely to make a significant contribution to the total dense shelf water export. Concurrent time series from bottom‐mounted Microcats and ADCP instruments from the Mertz Polynya Experiment (April 1998 to May 1999) near the Adélie Sill and on the upper continental slope (1150 m) and lower continental rise (3250 m) to the north describe the seasonal variability in downslope events and their interaction with the ambient water masses. The critical density for shelf water to produce AABW is examined and found to be 27.85 kg m−3 from the Adélie Depression and as low as 27.80 kg m−3 from the Mertz Depression. This study suggests previous dense shelf water export estimates based on the flow through the Adélie Sill alone are conservative and that other regions around East Antarctica with similar ice production to the Mertz Depression could be contributing to the total AABW in the Australian‐Antarctic Basin.

Modelling the impact of ocean warming on melting and water masses of ice shelves in the Eastern Weddell Sea

The Eastern Weddell Ice Shelves (EWIS) are believed to modify the water masses of the coastal current and thus preconditions the water mass formation in the southern and western Weddell Sea. We apply various ocean warming scenarios to investigate the impact on the temperature–salinity distribution and the sub-ice shelf melting in the Eastern Weddell Sea. In our numerical experiments, the warming is imposed homogeneously along the open inflow boundaries of the model domain, leading to a warming of the warm deep water (WDW) further downstream. Our modelling results indicate a weak quadratic dependence of the melt rate at the ice shelf base on the imposed amount of warming, which is consistent with earlier studies. The total melt rate has a strong dependence on the applied ocean warming depth. If the warming is restricted to the upper ocean (above 1,000 m), the water column (aside from the mixed surface layer) in the vicinity of the ice shelves stabilises. Hence, reduced vertical mixing will reduce the potential of Antarctic Bottom Water formation further downstream with consequences on the global thermohaline circulation. If the warming extends to the abyss, the WDW core moves significantly closer to the continental shelf break. This sharpens the Antarctic Slope Front and leads to a reduced density stratification. In contrast to the narrow shelf bathymetry in the EWIS region, a wider continental shelf (like in the southern Weddell Sea) partly protects ice shelves from remote ocean warming. Hence, the freshwater production rate of, e.g., the Filchner–Ronne Ice Shelf increases much less compared with the EWIS for identical warming scenarios. Our study therefore indicates that the ice-ocean interaction has a significant impact on the temperature-salinity distribution and the water column stability in the vicinity of ice shelves located along a narrow continental shelf. The effects of ocean warming and the impact of increased freshwater fluxes on the circulation are of the same order of magnitude and superimposed. Therefore, a consideration of this interaction in large-scale climate studies is essential.

Meridional Density Gradients Do Not Control the Atlantic Overturning Circulation

Abstract A wide body of modeling and theoretical scaling studies support the concept that changes to the Atlantic meridional overturning circulation (AMOC), whether forced by winds or buoyancy fluxes, can be understood in terms of a simple causative relation between the AMOC and an appropriately defined meridional density gradient (MDG). The MDG is supposed to translate directly into a meridional pressure gradient. Here two sets of experiments are performed using a modular ocean model coupled to an energy–moisture balance model in which the positive AMOC–MDG relation breaks down. In the first suite of seven model integrations it is found that increasing winds in the Southern Ocean cause an increase in overturning while the surface density difference between the equator and North Atlantic drops. In the second suite of eight model integrations the equation of state is manipulated so that the density is calculated at the model temperature plus an artificial increment ΔT that ranges from −3° to 9°C. (An increase in ΔT results in increased sensitivity of density to temperature gradients.) The AMOC in these model integrations drops as the MDG increases regardless of whether the density difference is computed at the surface or averaged over the upper ocean. Traditional scaling analysis can only produce this weaker AMOC if the scale depth decreases enough to compensate for the stronger MDG. Five estimates of the depth scale are evaluated and it is found that the changes in the AMOC can be derived from scaling analysis when using the depth of the maximum overturning circulation or estimates thereof but not from the pycnocline depth. These two depth scales are commonly assumed to be the same in theoretical models of the AMOC. It is suggested that the correlation between the MDG and AMOC breaks down in these model integrations because the depth and strength of the AMOC is influenced strongly by remote forcing such as Southern Ocean winds and Antarctic Bottom Water formation.

Numerical simulation of the Filchner overflow

The plume of Ice Shelf Water (ISW) flowing into the Weddell Sea over the Filchner sill contributes to the formation of Antarctic Bottom Water. The Filchner overflow is simulated using a hydrostatic, primitive equation three‐dimensional ocean model with a 0.5–2 Sv ISW influx above the Filchner sill. The best fit to mooring temperature observations is found with influxes of 0.5 and 1 Sv, below a previous estimate of 1.6 ± 0.5 Sv based on sparse mooring velocities. The plume first moves north over the continental shelf, and then turns west, along slope of the continental shelf break where it breaks up into subplumes and domes, some of which then move downslope. Other subplumes run into the eastern submarine ridge and propagate along the ridge downslope in a chaotic manner. The next, western ridge is crossed by the plume through several paths. Despite a number of discrepancies with observational data, the model reproduces many attributes of the flow. In particular, we argue that the temporal variability shown by the observations can largely be attributed to the unstable structure of the flow, where the temperature fluctuations are determined by the motion of the domes past the moorings. Our sensitivity studies show that while thermobaricity plays a role, its effect is small for the flows considered. Smoothing the ridges out demonstrate that their presence strongly affects the plume shape around the ridges. An increase in the bottom drag or viscosity leads to slowing down, and hence thickening and widening of the plume.

The circulation and water masses of the Antarctic shelf and continental slope between 30 and

Abstract The circulation and water masses from the Antarctic continental shelf to 62 ∘ S between 30 and 80 ∘ E are described using hydrographic data collected on seven hydrographic sections during the Baseline Research on Oceanography, Krill and the Environment-West (BROKE-West) experiment. The eastern limb of the Weddell Gyre dominates circulation between 30 and 40 ∘ E , and is significantly cooler and fresher than the region to the east. The Antarctic Circumpolar Current (ACC) extends from the north into the survey region east of 40 ∘ E , reaching as far south as 65 . 5 ∘ S at 60 ∘ E . This results in increasing observed maximum temperature and salinities progressively towards the east, peaking at 80 ∘ E due to the intrusion of the southern ACC Front (sACCF) to 63 ∘ S . This southward extension is steered by the southern end of Kerguelen Plateau, causing a horizontal shear of over 0.15 m s - 1 between the eastward ACC and westward-flowing Antarctic Slope Current (ASC). The ASC is observed at all six meridional sections immediately north of the shelf break. It is strongly barotropic and transports a total of 15.8 ± 7.4 Sv westwards, while the bottom referenced baroclinic component only contributes 1.3 ± 0.3 Sv . At each section this current intensifies to a narrow westward ‘jet’ with absolute velocities up to 0.3 m s - 1 over the steepest shelf slope gradients. At 70 ∘ E a ‘V’ shape is observed in the ASF. This, and the nearby presence of denser shelf water and ice-shelf water, is characteristic of Antarctic Bottom Water (AABW) formation, but no new AABW is found on this section. Instead, significantly warmer, saltier and less oxygenated AABW to the east and newly formed AABW high on the continental slope immediately to the west suggest a formation region just west of 70 ∘ E . This newly formed AABW progressively becomes warmer and saltier west of 60 ∘ E and is observed extending offshore and moving westward below eastward flowing water masses. ACC frontal positions are found to be 1–2° farther north in the survey region than suggested by historical climatology.

Temporal variations and trends of CFC11 and CFC12 surface-water saturations in Antarctic marginal seas: Results of a regional ocean circulation model

The knowledge of chlorofluorocarbon (CFC11, CFC12) concentrations in ocean surface waters is a prerequisite for deriving formation rates of, and water mass ages in, deep and bottom waters on the basis of CFC data. In the Antarctic coastal region, surface-layer data are sparse in time and space, primarily due to the limited accessibility of the region. To help filling this gap, we carried out CFC simulations using a regional ocean general circulation model (OGCM) for the Southern Ocean, which includes the ocean–ice shelf interaction. The simulated surface layer saturations, i.e. the actual surface concentrations relative to solubility-equilibrium values, are verified against available observations. The CFC surface saturations driven by concentration gradients between atmosphere and ocean are controlled mainly by the sea ice cover, sea surface temperature, and salinity. However, no uniform explanation exists for the controlling mechanisms. Here, we present simulated long-term trends and seasonal variations of surface-layer saturation at Southern Ocean deep and bottom water formation sites and other key regions, and we discuss differences between these regions. The amplitudes of the seasonal saturation cycle vary from 22% to 66% and their long-term trends range from 0.1%/year to 0.9%/year. The seasonal surface saturation maximum lags the ice cover minimum by two months. By utilizing observed bottle data the full seasonal CFC saturation cycle can be determined offering the possibility to predict long-term trends in the future. We show that ignoring the trends and using instead the saturations actually observed can lead to systematic errors in deduced inventory-based formation rates by up to 10% and suggest an erroneous decline with time.

Hemispheric sea ice extent dynamics as observed from MSMR

The ice covered regions of the polar seas influence the global climate in several ways. Any perturbation in the polar oceanic cryosphere affects the local weather and the global climate through modulation of the radiative forcing, the bottom water formation and the mass & the momentum transfer between Atmosphere-Cryosphere-Ocean System. The cold, harsh and inhospitable conditions in the polar regions prohibit the collection of extensive in situ data with sufficient spatial and temporal variation. However, satellite remote sensing is an ideal technique for studying the areas like the polar regions with synoptic and repetitive coverage. This paper discusses the analysis of the data obtained over the polar oceanic regions during the period June 1999 – September 2001 through the use of Multi-channel Scanning Microwave Radiometer (MSMR), onboard India’s first oceanographic satellite Oceansat-1. The MSMR observation shows that all the sectors in the Antarctic behave differently to the melting and formation of the sea ice. Certain peculiar features like the increase in sea ice extent during the melt season of 1999 – 2000 in the Indian Ocean sector, 15 – 20% decrease in the sea ice extent in the western Pacific sector during the ice formation period for the year 2000, melting spell within the formation phase of sea ice in B & A sector in the year 2000 were observed. On the other hand the northern polar sea ice extent is seen to be more dominated by the land characteristics. The ice formation in Kara and the Barent Sea sector is dominated by the ocean currents, where as the ice covered in the Japan and the Okhotsk Sea is dominated by the land processes. The sea ice extent in the Arctic Ocean show fluctuations from July to October and remain almost steady over other months. The global sea ice cover shows a formation phase from March to June and melting phase from November to February. In other months, i.e., from July – October the global sea ice cover is dominated by the hemispheric asymmetry of the ice growth and retreat.

Projected Changes to the Southern Hemisphere Ocean and Sea Ice in the IPCC AR4 Climate Models

Abstract Fidelity and projected changes in the climate models, used for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4), are assessed with regard to the Southern Hemisphere extratropical ocean and sea ice systems. While individual models span different physical parameterizations and resolutions, a major component of intermodel variability results from surface wind differences. Projected changes to the surface wind field are also central in modifying future extratropical circulation and internal properties. A robust southward shift of the circumpolar current and subtropical gyres is projected, with a strong spinup of the Atlantic gyre. An associated increase in the core strength of the circumpolar circulation is evident; however, this does not translate into robust increases in Drake Passage transport. While an overarching oceanic warming is projected, the circulation-driven poleward shift of the temperature field explains much of the midlatitude warming pattern. The effect of this shift is less clear for salinity, where, instead, surface freshwater forcing dominates. Surface warming and high-latitude freshwater increases drive intensified stratification, and a shoaling and southward shift of the deep mixed layers. Despite large intermodel differences, there is also a robust weakening in bottom water formation and its northward outflow. At the same time the wind intensification invigorates the upwelling of deep water, transporting warm, salty water southward and upward, with major implications for sequestration and outgassing of CO2. A robust decrease is projected for both the sea ice concentration and the seasonal cycling of ice volume, potentially altering the salt and heat budget at high latitudes.

Annual cycle of circulations in Lake Biwa, part 2: mechanisms

We investigate the mechanisms of circulations in Lake Biwa using model results that are driven by the momentum and heat fluxes evaluated from meteorological parameters observed around and on the lake. The kinetic energy budget shows that the wind stress over the lake, which has positive (negative) curl over the northern (southern) region of the lake throughout the year, generates the current system of multiple gyres in the stratified season (May–January), while the thermal forcing makes a minor contribution. This mechanism is quite different from the previous picture, in which the thermal forcing was dominant in forming the gyre system during the heating season. In the cooling season, the overturning of the lake occurs not in a simple one-dimensional (vertical) process, but in a time-dependent three-dimensional one. Cold water that is formed in the shallow eastern region descends the bottom slope to the deep lake in December to March while such a descent hardly occurs in the western region. A large shallow area (small heat capacity) as well as intense cooling in the eastern region effectively produces cold water. Wind again plays an important role by intensifying the cooling through its higher speed and by driving slope current. Further, the fluctuating wind direction causes significantly time-dependent behaviors of the slope current or bottom water formation.

Atmospheric Moisture Transport Moderates Climatic Response to the Opening of Drake Passage

Abstract The absence of the Drake Passage (DP) gateway in coupled models generally leads to vigorous Antarctic bottom water (AABW) formation, Antarctic warming, and the absence of North Atlantic deep-water (NADW) formation. Here the authors show that this result depends critically on atmospheric moisture transport by midlatitude storms. The authors use coupled model simulations employing geometries different only at the location of DP to show that oceanic circulation similar to that of the present day is possible when DP is closed and atmospheric moisture transport values enhanced by Southern Ocean storm activity are used. In this case, no Antarctic warming occurs in conjunction with DP closure. The authors also find that the changes in poleward heat transport in response to the establishment of the Antarctic Circumpolar Current (ACC) are small. This result arises from enhanced atmospheric moisture transport at the midlatitudes of the Southern Hemisphere (SH), although the values used remain within a range appropriate to the present day. In contrast, homogeneous or (near) symmetric moisture diffusivity leads to strong SH sinking and the absence of a stable Northern Hemisphere (NH) overturning state, a feature familiar from previous studies. The authors’ results show that the formation of NADW, or its precursor, may have been possible before the opening of the DP at the Eocene/Oligocene boundary, and that its presence depends on an interplay between the existence of the DP gap and the hydrological cycle across the midlatitude storm tracks.

Sea ice variability and primary productivity in the Ross Sea, Antarctica, from methylsulphonate snow record

The Ross Sea contains the most biologically productive continental shelf in Antarctica and is a region where the annual formation of sea ice drives substantial amounts of bottom water formation. We present snow pit chemistry data from Mt Erebus Saddle that provide a quantitative proxy to reconstruct summer sea ice conditions and rates of marine primary production. The methylsulphonate (MS) record is strongly correlated with changes in the area of open water (R2 = 0.903, p < 0.05) caused by differences in atmospheric circulation and the sea‐ice‐damming effect of large icebergs, B‐15 and C‐19, which calved from the Ross Ice Shelf in 2000 and 2002 respectively. Furthermore, MS and phytoplankton net primary production correlate significantly (R2 = 0.927, p < 0.01). Our results demonstrate the potential of the Mt Erebus Saddle ice core to reconstruct sea ice and primary productivity variability in the Ross Sea beyond the observational record.

Validation of three global ocean models in the Weddell Sea

We present a validation of three global z-level eddy-permitting/resolving ocean general circulation models against hydrographic observations and remote sensing sea ice data from the Weddell Sea. The Weddell Sea is a region in which complex processes such as water mass formation, sea ice formation and melt, and circulation under ice shelves take place. The representation of these processes is challenging even for the current generation of eddy-permitting ocean models. Simulating the hydrographic structure of this basin is a stringent test for models, notably so when considering the global influence of the regional processes. The performance of OCCAM (at two resolutions), ORCA025 and TPAC are tested regarding water mass properties, sea ice seasonality (OCCAM and ORCA025 only) and volume transport. OCCAM simulates the deep water masses reasonably well in both resolutions. The eddy-resolving run is not significantly better than the eddy-permitting simulation. ORCA025 and TPAC represent the surface layers and the Weddell Gyre circulation better but are generally too warm throughout the water column. All models underestimate Weddell Sea Bottom Water formation. Both OCCAM and ORCA025 struggle to correctly model the sea ice cover: OCCAM overestimates the summer ice extent while little multi-year sea ice remains in ORCA025. TPAC exhibits considerable drift in potential temperature during the model run. The choice of a model for a study has to be made carefully taking into account the model’s performance in the specific area, application and variable of interest. We identify several starting points for improving the models, namely model numerics and parameterisations of subgridscale processes, ensuring more accurate forcing datasets, correct initialisation, and ice–ocean interactions, all of which are likely to have larger benefits than simply increasing the horizontal resolution beyond eddy permitting.

The carbon cycle during the Mid Pleistocene Transition: the Southern Ocean Decoupling Hypothesis

Abstract. Various hypotheses were proposed within recent years for the interpretation of the Mid Pleistocene Transition (MPT), which occurred during past 2 000 000 years (2 Myr). We here add to already existing theories on the MPT some data and model-based aspects focusing on the dynamics of the carbon cycle. We find that the average glacial/interglacial (G/IG) amplitudes in benthic δ13C derived from sediment cores in the deep Pacific ocean increased across the MPT by ~40%, while similar amplitudes in the global benthic δ18C stack LR04 increased by a factor of two over the same time interval. The global carbon cycle box model BICYCLE is used for the interpretation of these observed changes in the carbon cycle. Our simulation approach is based on regression analyses of various paleo-climatic proxies with the LR04 benthic δ18C stack over the last 740 kyr, which are then used to extrapolate changing climatic boundary conditions over the whole 2 Myr time window. The observed dynamics in benthic δ13C cannot be explained if similar relations between LR04 and the individual climate variables are assumed prior and after the MPT. According to our analysis a model-based reconstruction of G/IG amplitudes in deep Pacific δ13C before the MPT is possible if we assume a different response to the applied forcings in the Southern Ocean prior and after the MPT. This behaviour is what we call the "Southern Ocean Decoupling Hypothesis". This decoupling might potentially be caused by a different cryosphere/ocean interaction and thus changes in the deep and bottom water formation rates in the Southern Ocean before the MPT, however an understanding from first principles remains elusive. Our hypothesis is also proposing dynamics in atmospheric pCO2 over the past 2 Myr. Simulated pCO2 is varying between 180 and 260 μatm before the MPT. The consequence of our Southern Ocean Decoupling Hypothesis is that the slope in the relationship between Southern Ocean SST and atmospheric pCO2 is different before and after the MPT, something for which first indications already exist in the 800 kyr CO2 record from the EPICA Dome C ice core. We finally discuss how our findings are related to other hypotheses on the MPT.