The coastal shelf region of East Antarctica is hypothesized to be shielded from the offshore heat of Circumpolar Deep Water (CDW) due to the dynamic barrier of the Antarctic Slope Front. Yet modified ...CDW (mCDW) intrudes into the coastal environment in key locations, with impacts on dense shelf water formation and ocean/ice shelf interaction that remain largely unquantified. Using moored measurements and conductivity‐temperature‐depth‐instrumented seal hydrographic data collected in Prydz Bay, East Antarctica, we find buoyancy‐driven upwelling of mCDW into the subsurface (~50 m) layer of the southeastern embayment. Wintertime convection extends as deep as 300 m, entraining heat of the upwelled mCDW to the surface. Accumulated sensible heat supply to the surface through deep convection during June–July reduces the potential sea ice production by 45% in the Davis Polynya, demonstrating that stronger/warmer mCDW intrusions onto the shelf will likely reduce the shelf water density and threaten Antarctic Bottom Water formation.
Plain Language Summary
Sea ice formation in key coastal polynyas (areas of open water or newly formed thin ice in the middle of the extensive pack ice) around Antarctica is critical to Antarctic Bottom Water (AABW) production. The intrusion of warm, modified Circumpolar Deep Water (mCDW) onto the continental shelf in East Antarctica conveys heat toward the shelf region at intermediate depth, capable of impacting sea ice formation in coastal polynyas. Here we use moored measurements and conductivity‐temperature‐depth‐instrumented seal hydrographic data to shed new light on the interaction between sea ice formation and the heat flux from these mCDW intrusions. Due to the buoyancy contrast with the cold, dense shelf water, the warmer mCDW upwells to shallower depths in the coastal regions. When the winter freezing season begins, surface cooling and brine rejection due to sea ice formation drive convection, deepening the mixed layer and entraining heat of the upwelled mCDW to the surface, which results in a negative feedback that reduces sea ice production. The processes identified in this study have strong implications for AABW production, given the projected increase of mCDW intrusions in the future.
Key Points
Buoyancy forcing drives the upwelling of modified Circumpolar Deep Water (mCDW) in the southeastern shelf region
Deep convection entrains sensible heat from the warm mCDW at mid‐depths into the surface layer during the freezing season
Sensible heat from mCDW causes a 45% reduction in sea ice production in the Davis Polynya region
Hydrographic top-to-bottom CTDs and profiling float profiles were collected during 2003–2020 periods in the south-west Indian Ocean sector of the Antarctic margin. Those calibrated dissolved oxygen, ...temperature and salinity records were used to document the distribution, pathway and changes of the newly-formed Cape Darnley Bottom Water (CDBW). We found that the newly-formed CDBW, with high fractions (50–90%) in the bottom 300 m, primarily appeared on the continental slope (64.5°-70°E) and the abyssal ocean (57°-64.5°E) in the eastern Cooperation Sea. Those distributions of CDBW fractions suggested a pathway originating from the Cape Darnley Polynya (CDP) and descending down Wild and Daly Canyons, and finally accumulating in the northwest abyssal ocean. The newly-formed CDBW presented decadal changes in potential temperature, salinity, dissolved oxygen, and neutral density between 2003-2006 and 2013–2020. It showed significant cooling (∼0.098 °C/decade), freshening (∼0.013/decade), increasing dissolved oxygen (∼11.8 μmol/kg/decade) rates on the continental slope just off CDP, while warming (0.018–0.038 °C/decade), freshening (0.0044–0.0055/decade), decreasing density (0.009–0.014 kg/m3/decade) and increasing dissolved oxygen (4–12 μmol/kg/decade) in the Daly Canyon. The most likely explanation of those high change rates is the enhanced CDBW formation caused by increased cascading Dense Shelf Water (DSW) plumes in Daly Canyon, and subsequently enhanced entrainment of warmer mid-depth Modified Circumpolar Deep Water (MCDW). In the northwest abyssal plain, the potential temperature and dissolved oxygen show little change while the freshening and more buoyant signals remain considerable, which might be influenced by the changes of AABWs from different sources. Finally, we relate the recent changes of newly-formed CDBW to the Sea Ice Production (SIP) change in the CDP. The SIP in the CDP shows significant increasing trends (0.52 m/decade) during 2002–2020. It increased by 12.5% during this period, which indicates an increased formation of regional DSW driven by the sea-ice formation and associated brine rejection, thus increasing formation of CDBW. Those processes were likely responsible for the decadal changes of the recently-formed CDBW.
Dense shelf water (DSW) produced in the western Ross Sea (RS) is one of the major sources of Antarctic bottom water (AABW). Thus the understanding of long-term variability of DSW salinity and its ...controlling factors in the western RS is critical to assess the variability of globally distributed AABW. Here we analyze a long time record of hydrographic data (1984-2020) collected in the western RS, as well as sea ice drift vectors, surface wind speed, sea level pressure and Amundsen Sea low (ASL) indices. We confirm recent findings that there is a rapid increase of DSW salinity in the western RS after a minimum in 2013, although the DSW has experienced substantial freshening in the past few decades, indicating a significant multidecadal variability of DSW salinity in the western RS. Over the past four decades, multidecadal variability in the DSW salinity has been strongly coupled with westward zonal flow changes along the coastal current, and the post-2013 rapid enhancement of DSW salinity is accompanied by reduced freshwater input due to weakening of the westward zonal flow from the upstream Amundsen Sea (AS) into the RS. Large-scale circulation determining the strength of the zonal flow is closely linked to the ASL variability. The accelerated deepening of the ASL and the resulting southwestward extension of low pressure induce an eastward coastal current anomaly. This reduces the freshwater input from the AS to the RS and is responsible for the subsequent enhancement of DSW salinity in recent years in the western RS. These dynamical processes demonstrated here explain how the ASL changes modulate the DSW salinity in the western RS, and will help to understand the implication of climate changes in the Southern Ocean on AABW formation.
The Southeast Tropical Indian Ocean (SETIO), dominated by the Indian Ocean monsoon, is an important source region for strong mesoscale eddies. To date, the impacts of the Indian Ocean monsoon on ...mesoscale eddies have not been clarified. Here we report on the dipole response of mesoscale eddy formation to monsoon transition in the SETIO, using satellite and reanalysis data sets. During the summer monsoon season, anticyclonic eddies are mainly concentrated north of 12°S, while cyclonic eddies are south of 12°S. This situation reverses during the winter monsoon season. We attribute this dipole feature to the oceanic perturbations and current shear during the different monsoon periods. A geographical boundary along 12°S aligns with meridional changes in eddy potential energy, which delineates the generation and direction of the newly‐formed eddies. The hot spot region, rich in eddy energy properties, tends to promote eddy formation and endurance during the monsoon periods.
Plain Language Summary
The Southeast Tropical Indian Ocean (SETIO) is a typical region of strong mesoscale (∼10–100 km) eddy generation. Eddies are circular currents that are important in moving heat, nutrients, and marine life around the ocean. The SETIO is also dominated by the Indian Ocean monsoon, which is a seasonal weather pattern that typically occurs in two main phases: the southwest monsoon from June to September, and the northeast monsoon from December to March. To date, the impacts of the Indian Ocean monsoon on the mesoscale eddies remain unclear. Based on satellite and reanalysis data sets, we found that there is a natural latitudinal change in the direction of eddies (anticlockwise/clockwise) formed north/south of 12°S in the summer monsoon, and that this pattern switches in the winter monsoon. The monsoon transition and associated changes to the ocean and its currents drives the dual‐pattern. The geographical boundary along 12°S occurs because it aligns with latitudinal changes in the energy stored in the eddies, which delineates a change in the direction of the newly‐formed eddies. This hot spot region, rich in eddy energy properties, promotes eddies formation and endurance during the monsoon periods.
Key Points
Strong mesoscale eddies in the Southeast Tropical Indian Ocean are generated in a clear seasonal cycle
The eddies present a distinct dipole response to the monsoon transition in the region
Changes in oceanic perturbation and current shear modulated by monsoon transition is responsible for this dipole response of eddies
The offshore ocean heat supplied to the Antarctic continental shelves by warm eddies has the potential to greatly impact the melting rates of ice shelves and subsequent global sea level rise. While ...featured in modeling and some observational studies, the processes around how these warm eddies form and overcome the dynamic sub-surface barrier of the Antarctic Slope Front over the upper continental slope has not yet been clarified. Here we report on the detailed observations of persistent eddies carrying warm modified Circumpolar Deep Water (CDW) onto the continental shelf of Prydz Bay, East Antarctica, using subsurface mooring and hydrographic section data from 2013-2015. We show the warm-eddy transport is most active when the summer westerlies strengthen, which promotes the upwelling of CDW and initiates eddy formation and intrusions. Our study highlights the important role of warm eddies in the melting of Antarctica's ice shelves, both now and into the future.
While the effects of the Southern Annular Mode (SAM), a dominant climate variability mode in the Southern Ocean, on ocean acidification have been examined using models, no consensus has been reached. ...Using observational data from south of Tasmania, we show that during a period with positive SAM trends, surface water pH and aragonite saturation state at 60°-55° S (Antarctic Zone) decrease in austral summer at rates faster than those predicted from atmospheric CO
increase alone, whereas an opposite pattern is observed at 50°-45° S (Subantarctic Zone). Together with other processes, the enhanced acidification at 60°-55° S may be attributed to increased westerly winds that bring in more "acidified" waters from the higher latitudes via enhanced meridional Ekman transport and from the subsurface via increased vertical mixing. Our observations support climatic modulation of ocean acidification superimposed on the effect of increasing atmospheric CO
.
Mesoscale eddies are a prominent oceanic phenomenon that plays an important role in oceanic mass transport and energy conversion. Characterizing by rotational speed, the eddy intensity is one of the ...most fundamental properties of an eddy. However, the seasonal spatiotemporal variation in eddy intensity has not been examined from a global ocean perspective. In this study, we unveil the seasonal spatiotemporal characteristics of eddy intensity in the global ocean by using the latest satellite-altimetry-derived eddy trajectory data set. The results suggest that the eddy intensity has a distinct seasonal variation, reaching a peak in spring while attaining a minimum in autumn in the Northern Hemisphere and the opposite in the Southern Hemisphere. The seasonal variation of eddy intensity is more intense in the tropical-subtropical transition zones within latitudinal bands between 15° and 30° in the western Pacific Ocean, the northwestern Atlantic Ocean, and the eastern Indian Ocean because baroclinic instability in these areas changes sharply. Further analysis found that the seasonal variation of baroclinic instability precedes the eddy intensity by a phase of 2–3 months due to the initial perturbations needing time to grow into mesoscale eddies.
Intrusion of warm modified Circumpolar Deep Water (mCDW) is critical to heat transport onto the continental shelf around Antarctica, with substantial impacts on basal melting of ice shelves and sea ...ice production. We use hydrographic data collected in Prydz Bay, East Antarctica, and atmospheric and ocean reanalyses to investigate the seasonality of mCDW intrusions onto the East Antarctic continental shelf. Mooring measurements confirm that there is significant intrusion of mCDW over the inner continental shelf of Prydz Bay in March–July (austral autumn and early winter). The warming signal of intruded mCDW over the inner shelf is correlated with wind changes north of the shelf break, with a significant lag. This suggests that the autumn‐winter mCDW intrusions over the inner shelf are significantly affected by the wind regime north of the shelf break in January–May, when a southward displacement of the westerly winds occurs. The southward shift of westerly winds drives the shallowest depth of upwelled mCDW to move poleward and contributes more warm water to the shelf break. This results in the shoaling of mCDW near the shelf break in January–May and promotes mCDW intrusions onto the continental shelf. The dynamic barrier imposed on mCDW intrusions by the strong Antarctic Slope Front in January–May is offset by this shoaling of mCDW. This study indicates the strong sensitivity of continental shelves in East Antarctica to atmospheric forcing changes over the Southern Ocean.
Plain Language Summary
Warm modified Circumpolar Deep Water (mCDW) from the Southern Ocean flows onto the continental shelf in many regions around Antarctica. These intrusions transport great amounts of heat to the continental shelves, affecting basal melting of ice shelves and sea ice formation. Data collected over the continental shelf of Prydz Bay, East Antarctica show that these intrusions occur seasonally. When we combine these observations with results from atmospheric model, we find that the strongest intrusion of mCDW over the inner shelf in March–July is related to the variability of westerly winds north of the shelf break in January–May. This is the period when the westerly winds over the Southern Ocean move southward. This causes the mCDW to become shallower near the shelf break and allows more warm water to flow to the inner shelf. Although an intensified Antarctic Slope Front in January–May should block the intrusion of mCDW, the blocking effect is overwhelmed by the shallowing of mCDW. Our findings provide insights into how seasonal variability of atmospheric forcing in the open ocean will affect warm water inflow and heat transport onto continental shelves in East Antarctica, and indicate potential effects of future changes in winds around Antarctica.
Key Points
Warm mCDW intrusions in March–July in Prydz Bay are linked to the wind regime in January–May north of the shelf break
The southward shift of westerly winds in January–May brings about the shoaling of mCDW over the shelf break
The barrier effect of the strong Antarctic Slope Front on mCDW intrusions is offset by shoaling of mCDW in January–May
Using observational data collected south of Tasmania during 14 austral summer cruises during 1993–2011, we examined the response of sea surface fugacity of carbon dioxide (fCO2) to the Southern ...Annular Mode (SAM) shift, which occurred around 2000. In the southern part of the Southern Ocean (SO) or the Polar Zone (PZ) and the Polar Frontal Zone (PFZ), fCO2 increased faster at the sea surface than in the atmosphere before the SAM shift, but not after the shift. In the northern part of the SO or the Subantarctic Zone (SAZ), however, surface fCO2 increased faster than atmospheric fCO2 both before and after the shift. The SAM shift had an important influence on the surface fCO2 trend in the PZ and PFZ but not in the SAZ, which we attribute to differences in regional oceanographic processes (upwelling versus nonupwelling). The SAM shift may have reversed the negative trend of SO CO2 uptake.
Key Points
CO2 response to SAM was different between zones in the south and north
This difference was associated with regional oceanographic processes
SAM shift may reverse the negative trend of Southern Ocean CO2 uptake
The seasonality of mesoscale eddy intensity in the southeastern tropical Indian Ocean (SETIO) is investigated using the latest eddy dataset and marine hydrological reanalysis data. The results show ...that the eddy intensity in an area to the southwest coast of the Java Island has prominent seasonality—eddies in this area are relatively weak during the first half of the year but tend to enhance in August and peak in October. Further analysis reveals that the strong eddies in October are actually developed from the ones mainly formed in July to September, and the barotropic instability and baroclinic instability are the key dynamics for eddy development, but each plays a different role at different development stages. The barotropic instability resulting from the horizontal shear of surface current plays an important role in the early stage of eddy development. However, in the late development stage, the baroclinic instability induced by the sloping pycnocline becomes the major energy contributor of eddy development.
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