Characteristics and Variability of Antarctic Intermediate Water in the UKESM1-0-LL CMIP6 model

<p>Antarctic Intermediate Water (AAIW) is the dominant intermediate water mass in the Southern Hemisphere. AAIW plays a key role in the hydrological cycle and also contributes to the replenishment of nutrients at low latitudes. It is characterised by a mid-depth salinity minimum. Although its salinity minimum signature can be clearly identified, the formation mechanisms and how its properties evolve with climate change are unclear.</p><p>The aim of this study is to assess the ability of the UKESM1-0-LL CMIP6 model to represent the key characteristics and variability of AAIW and to evaluate its evolution under radiative forcing (with the SSP5-8.5 and SSP2-4.5 scenarios).</p><p>A diagnostic is developed to identify the core of AAIW in the different basins and scenarios. AAIW can be identified in the UKESM1-0-LL model but it is lighter than in observations. The Pacific, Atlantic and Indian type of AAIW have core density values of 26.5 kg/m<sup>3</sup>, 26.6 kg/m<sup>3</sup> and 26.9 kg/m<sup>3</sup> respectively. AAIW presents different properties across each basin with different depth, temperature and salinity properties. The Pacific type of AAIW is lighter and fresher than the Atlantic and Indian types of AAIW. Under radiative forcing, it is found that AAIW shoals and becomes warmer. The largest changes in temperature, salinity and density are found in the Pacific. The outcrop location of the salinity minimum remains constant in the different scenarios in spite of the surface conditions changing with climate change.</p><p>A change in depth could have major implications on the overturning circulation. Ongoing and future work focuses on identifying which mechanisms need to be improved in CMIP6 models to reduce the bias observed in AAIW.</p>

Deep convection in the Lofoten Basin: ARGO vs MITgcm

<p>The Lofoten basin (the LB) contains relatively warm and salty waters regarding border basins such as Greenland and Barents Seas. Variability of the processes inside occurring in the basin reflects on the climate as on the mesoscales as on the interannual scales. We use a term mixed layer depth (MLD) as a border of the pycnocline in the water column, this parameter lets us evaluate the intensity of the convection in the region. Several methods of MLD calculations are tested in the current study: Kara, Montegut, and Dukhovskoy. The convection in the basin destructs stratification and forms massive intermediate water mass. The MITgcm data for 1993-2012 and over 5000 in-situ Argo T, S profiles for 2001-2017 were used in the calculations of the MLD.</p><p>We consider the maximum MLD (mMLD) in the region and its spatial distribution. The mMLD is higher in the central part of the LB and corresponds to the location of the Lofoten basin eddy (the LBE). Here the mMLD reaches 1000 meters, the medium maximum is 400 meters based both on the in-situ and model data. The maximum mixed layer depth ​​varies in the range of 400-1000 meters according to both datasets were used. The MLD over 400 meters is observed from January to May every year.</p><p><strong>Acknowledgments: </strong>The authors acknowledge the support of the Russian Science Foundation (project No. 18-17-00027). The results of the MITgcm were provided by D.L. Volkov, Cooperative Institute for Marine and Atmospheric Studies, University of Miami, USA.</p>

Seasonal variability of intermediate water masses in the Gulf of Cádiz: implications of the Antarctic and subarctic seesaw

Global circulation of intermediate water masses has been extensively studied; however, its regional and local circulation along continental margins and variability and implications on sea floor morphologies are still not well known. In this study the intermediate water mass variability in the Gulf of Cádiz (GoC) and adjacent areas has been analysed and its implications discussed. Remarkable seasonal variations of the Antarctic Intermediate Water (AAIW) and the Subarctic Intermediate Water (SAIW) are determined. During autumn a greater presence of the AAIW seems to be related to a reduction in the presence of SAIW and Eastern North Atlantic Central Water (ENACW). This interaction also affects the Mediterranean Water (MW), which is pushed by the AAIW toward the upper continental slope. In the rest of the seasons, the SAIW is the predominant water mass reducing the presence of the AAIW. This seasonal variability for the predominance of these intermediate water masses is explained in terms of the concatenation of several wind-driven processes acting during the different seasons. Our finding is important for a better understanding of regional intermediate water mass variability with implications in the Atlantic Meridional Overturning Circulation (AMOC), but further research is needed in order to decode their changes during the geological past and their role, especially related to the AAIW, in controlling both the morphology and the sedimentation along the continental slopes.

Seasonal variability of intermediate water masses in the Gulf of Cadiz: implications of the Antarctic and Subarctic seesaw model

Global circulation of intermediate water masses has been extensively studied; however, its regional and local circulation along continental margins and variability and implications on sea floor morphologies are still not well known. In this study the intermediate water mass variability in the Gulf of Cádiz and adjacent areas has been analysed and its implications discussed. Remarkable seasonal variations of the Antarctic Intermediate Water (AAIW) and the Subarctic Intermediate Water (SAIW) are determined. During autumn a greater presence of the AAIW seems to be related to a reduction in the presence of SAIW and Eastern North Atlantic Central Water (ENACW). This interaction also affects the Mediterranean Outflow Water (MOW), which is pushed by the AAIW toward the upper continental slope. In the rest of the seasons, the SAIW is the predominant water mass reducing the presence of the AAIW. This seasonal variability for the predominance of these intermediate water masses is explained by a novel model based on the concatenation of several wind-driven processes acting during the different seasons. Our finding is important for a better understanding of regional intermediate water mass variability with implications in the Atlantic Meridional Overturning Circulation (AMOC) but further research is needed in order to decode their changes during the geological past and their role, especially related to the AAIW, in controlling both the morphology and the sedimentation along the continental slopes.

The influence of seawater pH on U / Ca ratios in the scleractinian cold-water coral <i>Lophelia pertusa</i>

The increasing pCO2 in seawater is a serious threat for marine calcifiers and alters the biogeochemistry of the ocean. Therefore, the reconstruction of past-seawater properties and their impact on marine ecosystems is an important way to investigate the underlying mechanisms and to better constrain the effects of possible changes in the future ocean. Cold-water coral (CWC) ecosystems are biodiversity hotspots. Living close to aragonite undersaturation, these corals serve as living laboratories as well as archives to reconstruct the boundary conditions of their calcification under the carbonate system of the ocean. We investigated the reef-building CWC Lophelia pertusa as a recorder of intermediate ocean seawater pH. This species-specific field calibration is based on a unique sample set of live in situ collected L. pertusa and corresponding seawater samples. These data demonstrate that uranium speciation and skeletal incorporation for azooxanthellate scleractinian CWCs is pH dependent and can be reconstructed with an uncertainty of ±0.15. Our Lophelia U / Ca–pH calibration appears to be controlled by the high pH values and thus highlighting the need for future coral and seawater sampling to refine this relationship. However, this study recommends L. pertusa as a new archive for the reconstruction of intermediate water mass pH and hence may help to constrain tipping points for ecosystem dynamics and evolutionary characteristics in a changing ocean.

The influence of seawater pH on U / Ca ratios in the scleractinian cold-water coral <i>Lophelia pertusa</i>

The increasing pCO2 in seawater is a serious threat for marine calcifiers and alters the biogeochemistry of the ocean. Therefore, the reconstruction of past-seawater properties and their impact on marine ecosystems is an important way to investigate the underlying mechanisms and to better constrain the effects of possible changes in the future ocean. Cold-water coral (CWC) ecosystems are biodiversity hotspots. Living close to aragonite-undersaturation, these corals serve as living laboratories as well as archives to reconstruct the boundary conditions of their calcification under the carbonate system of the ocean. We investigated the reef-building CWC Lophelia pertusa as a recorder of intermediate ocean seawater pH. This species-specific field calibration is based on a unique sample set of live in-situ collected L. pertusa and corresponding seawater samples. These data demonstrate that uranium speciation and skeletal incorporation for azooxanthellate scleractinian CWCs is pH dependent. However, this also indicates that internal pH up-regulation of the coral does not play a role in uranium incorporation into the majority of the skeleton of L. pertusa. This study suggests L. pertusa provides a new archive for the reconstruction of intermediate water mass pH and hence may help to constrain tipping points for ecosystem dynamics and evolutionary characteristics in a changing ocean.

Control of Mode and Intermediate Water Mass Properties in Drake Passage by the Amundsen Sea Low

The evolution of the physical properties of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the Drake Passage region is examined on time scales down to intraseasonal, within the 1969–2009 period. Both SAMW and AAIW experience substantial interannual to interdecadal variability, significantly linked to the action of the Amundsen Sea low (ASL) in their formation areas. Observations suggest that the interdecadal freshening tendency evident in SAMW over the past three decades has recently abated, while AAIW has warmed significantly since the early 2000s. The two water masses have also experienced a substantial lightening since the start of the record. Examination of the mechanisms underpinning water mass property variability shows that SAMW characteristics are controlled predominantly by a combination of air–sea turbulent heat fluxes, cross-frontal Ekman transport of Antarctic surface waters, and the evaporation–precipitation balance in the Subantarctic zone of the southeast Pacific and Drake Passage, while AAIW properties reflect air–sea turbulent heat fluxes and sea ice formation in the Bellingshausen Sea. The recent interdecadal evolution of the ASL is consistent with both the dominance of the processes described here and the response of SAMW and AAIW on that time scale.

Temperature and salinity variability in the Greek Seas based on POSEIDON stations time series: preliminary results

Temperature and salinity time series provided by three POSEIDON monitoring stations (buoys) are examined in order to study the seasonal and interannual variability of the water mass characteristics. The sites at Athos (North Aegean Sea), E1M3A (Central Cretan Sea) and Pylos (Eastern Ionian Sea) were chosen, as these buoys provide measurements at various depths, while they represent 3 major basins respectively. The study of the T and S characteristics reveals important seasonal changes and highlights the particular characteristics of each basin. Dense water production in the Northern Aegean is found to be hindered by the presence of the surface Black Sea Water (BSW) mass. On the other hand, the intermediate water mass in the Cretan Sea is shown to be ventilated during the winter season. A significant temperature and salinity increase has been monitored over both the Central Cretan and Eastern Ionian Seas starting from the middle of 2008 and 2009 respectively. This could possibly be attributed to changes in the intermediate water masses of the Eastern Mediterranean, without ruling out the possibility of water mass exchanges between the two basins.