The Life Cycle and Variability of Antarctic Weak Polar Vortex Events

Motivated by the strong Antarctic sudden stratospheric warming (SSW) in 2019, a survey on the similar Antarctic weak polar events (WPV) is presented, including their life cycle, dynamics, seasonality, and climatic impacts. The Antarctic WPVs have a frequency of about four events per decade, with the 2002 event being the only major SSW. They show a similar life cycle to the SSWs in the Northern Hemisphere but have a longer duration. They are primarily driven by enhanced upward-propagating wavenumber 1 in the presence of a preconditioned polar stratosphere, i.e., a weaker and more contracted Antarctic stratospheric polar vortex. Antarctic WPVs occur mainly in the austral spring. Their early occurrence is preceded by an easterly anomaly in the middle and upper equatorial stratosphere besides the preconditioned polar stratosphere. The Antarctic WPVs increase the ozone concentration in the polar region and are associated with an advanced seasonal transition of the stratospheric polar vortex by about one week. Their frequency doubles after 2000 and is closely related to the advanced Antarctic stratospheric final warming in recent decades. The WPV-resultant negative phase of the southern annular mode descends to the troposphere and persists for about three months, leading to persistent hemispheric scale temperature and precipitation anomalies.

On the Southern Hemisphere stratospheric response to ENSO and its impacts on tropospheric circulation

As the leading mode of Pacific variability, the El Niño-Southern Oscillation (ENSO) causes vast and wide-spread climatic impacts, including in the stratosphere. Following discovery of a stratospheric pathway of ENSO to the Northern Hemisphere surface, here we aim to investigate if there is a substantial Southern Hemisphere (SH) stratospheric pathway in relation to austral winter ENSO events. Large stratospheric anomalies connected to ENSO occur on average at high SH latitudes as early as August, peaking at around 10 hPa. An overall colder austral spring Antarctic stratosphere is generally associated with the warm phase of the ENSO cycle, and vice versa. This behavior is robust among reanalysis and six separate model ensembles encompassing two different model frameworks. A stratospheric pathway is identified by separating ENSO events that exhibit a stratospheric anomaly from those that don’t and comparing to stratospheric extremes that occur during neutral-ENSO years. The tropospheric eddy-driven jet response to the stratospheric ENSO pathway is the most robust in the spring following a La Niña, but extends into summer, and is more zonally-symmetric compared to the tropospheric ENSO teleconnection. The magnitude of the stratospheric pathway is weaker compared to the tropospheric pathway and therefore when it is present, has a secondary role. For context, the magnitude is approximately half that of the eddy-driven jet modulation due to austral spring ozone depletion in the model simulations. This work establishes that the stratospheric circulation acts as an intermediary in coupling ENSO variability to variations in the austral spring and summer tropospheric circulation.

Antarctic ozone depletion measured by balloonsondes at Maitri - 1992

ABSTRACT. Profiles from a series of balloon borne ozonesonde ascents are used to chart the development of the Antarctic depletion over Maitri in the austral spring of 1992. The vertical structure of the ozone layer is discussed, including the presence of stratification, which occurs at all stages of development. The main feature of 1992 ozonesonde flights is depletion of 97% in the months of September and October between 15-23 km, which is unique.

Recruitment of Ostrea chilensis (Philippi, 1844) in Foveaux Strait, Southern New Zealand

<p>This research evaluates fundamental ecological processes to facilitate an understanding of recruitment in Ostrea chilensis from Foveaux Strait, southern New Zealand. Foveaux Strait represents an extreme habitat for oysters that differs to the sheltered nearshore, muddy habitats of most other oyster populations. O. chilensis exhibits the extreme end of brooding strategies in Ostreinae, does not form extensive reefs, and comprises putative self-recruiting populations. The Foveaux Strait oyster fishery is nationally important. Recurrent disease mortality in these populations has put greater onus on understanding recruitment. To evaluate the strength of a recruit-per-spawner relationship in oysters, seasonality in the settlement of larvae was determined. Most recruitment in any given year, over a 6-year period, occurred in the austral spring and summer (November to February). Fishery-wide, recruitment varied significantly between years, with most variation (50.8%) explained by a year effect that represents the combined influences of climatic and biological conditions. Spawner densities and fishery areas explain further variation (13.8% and 11.6%, respectively), with further 2-way interactions between these factors. Recruits-per-spawner declined serially over time, despite similar or increasing densities of spawning-sized oysters. Average recruitment was lowest when spawner densities were highest; this suggests a more complex relationship between recruitment and density that has implications for management of this oyster fishery. Recruitment to the O. chilensis fishery declined abruptly to low levels in 2010 and remained low until 2017. Relatively high spawning-stock sizes over this period had previously supported high recruitment. Density and oyster mortality from Bonamia exitiosa (a proxy for one or more infections) and their two-way interaction were the main determinants of recruitment. The highest recruitment occurred at times of low mortality and low density, suggesting reduced effects of disease on gametogenesis and reduced disease transmission. The contributions of climate factors were minor; however, a 3-way interaction between oyster density, mortality, and climate is likely to drive variation in recruitment. Pathobiomes (multiple infections in populations) may be important determinants of shellfish recruitment and population dynamics. This research evaluates the hypothesis of self-recruitment from distributions of recruit densities around an isolated natal population, and from the relationship between recruitment and brooding-sized oyster densities. Distance from the natal population, direction along or across the tidal current, or brooders did not predict recruit densities. Recruit distributions imply greater dispersal and larval mixing than previously reported. The swift tidal currents and possibility of more variable pelagic larval durations may enhance mixing and connectivity between populations in Foveaux Strait. Post-settlement mortality is the primary determinant of spatial structure in Foveaux Strait oysters. Productive fishery areas comprise mostly stable substrates of shells, sand, and gravel, with no or little other epifauna. Most (66.8%) post-settlement survivors were on the heavy shells of both live and dead O. chilensis, which suggests an unusual recruit-adult relationship based on survival rather than settlement. Recruits and 1+ year spat grew larger and had lower mortality at eastern sites with the lowest exposure to oceanic swells and putative lowest sediment movement. Moreover, recruits on spat collectors also grew larger and had lower mortality at heights ≥ 12 cm than those 2 cm off the seabed. This research suggests the effects of disease on brooding percentages and thereby larval supply may be the main determinant of the variation in recruitment in O. chilenesis, and the spatial structure of oyster populations in Foveaux Strait shaped by abiotic as well as biotic post-settlement mortality.</p>

Recruitment of Ostrea chilensis (Philippi, 1844) in Foveaux Strait, Southern New Zealand

<p>This research evaluates fundamental ecological processes to facilitate an understanding of recruitment in Ostrea chilensis from Foveaux Strait, southern New Zealand. Foveaux Strait represents an extreme habitat for oysters that differs to the sheltered nearshore, muddy habitats of most other oyster populations. O. chilensis exhibits the extreme end of brooding strategies in Ostreinae, does not form extensive reefs, and comprises putative self-recruiting populations. The Foveaux Strait oyster fishery is nationally important. Recurrent disease mortality in these populations has put greater onus on understanding recruitment. To evaluate the strength of a recruit-per-spawner relationship in oysters, seasonality in the settlement of larvae was determined. Most recruitment in any given year, over a 6-year period, occurred in the austral spring and summer (November to February). Fishery-wide, recruitment varied significantly between years, with most variation (50.8%) explained by a year effect that represents the combined influences of climatic and biological conditions. Spawner densities and fishery areas explain further variation (13.8% and 11.6%, respectively), with further 2-way interactions between these factors. Recruits-per-spawner declined serially over time, despite similar or increasing densities of spawning-sized oysters. Average recruitment was lowest when spawner densities were highest; this suggests a more complex relationship between recruitment and density that has implications for management of this oyster fishery. Recruitment to the O. chilensis fishery declined abruptly to low levels in 2010 and remained low until 2017. Relatively high spawning-stock sizes over this period had previously supported high recruitment. Density and oyster mortality from Bonamia exitiosa (a proxy for one or more infections) and their two-way interaction were the main determinants of recruitment. The highest recruitment occurred at times of low mortality and low density, suggesting reduced effects of disease on gametogenesis and reduced disease transmission. The contributions of climate factors were minor; however, a 3-way interaction between oyster density, mortality, and climate is likely to drive variation in recruitment. Pathobiomes (multiple infections in populations) may be important determinants of shellfish recruitment and population dynamics. This research evaluates the hypothesis of self-recruitment from distributions of recruit densities around an isolated natal population, and from the relationship between recruitment and brooding-sized oyster densities. Distance from the natal population, direction along or across the tidal current, or brooders did not predict recruit densities. Recruit distributions imply greater dispersal and larval mixing than previously reported. The swift tidal currents and possibility of more variable pelagic larval durations may enhance mixing and connectivity between populations in Foveaux Strait. Post-settlement mortality is the primary determinant of spatial structure in Foveaux Strait oysters. Productive fishery areas comprise mostly stable substrates of shells, sand, and gravel, with no or little other epifauna. Most (66.8%) post-settlement survivors were on the heavy shells of both live and dead O. chilensis, which suggests an unusual recruit-adult relationship based on survival rather than settlement. Recruits and 1+ year spat grew larger and had lower mortality at eastern sites with the lowest exposure to oceanic swells and putative lowest sediment movement. Moreover, recruits on spat collectors also grew larger and had lower mortality at heights ≥ 12 cm than those 2 cm off the seabed. This research suggests the effects of disease on brooding percentages and thereby larval supply may be the main determinant of the variation in recruitment in O. chilenesis, and the spatial structure of oyster populations in Foveaux Strait shaped by abiotic as well as biotic post-settlement mortality.</p>

El Niño-Southern Oscillation and Indian Ocean Dipole Modes: Their Effects on South American Rainfall during Austral Spring

This paper examines the effects of the tropical Pacific Ocean (TPO) and Indian Ocean Dipole (IOD) modes in the interannual variations of austral spring rainfall over South America (SA). The TPO mode refers to the El Niño-Southern Oscillation (ENSO). The isolated effects between IOD and TPO were estimated, events were chosen from the residual TPO (R-TPO) or residual IOD (R-IOD), and the IOD (TPO) effects for the R-TPO (R-IOD) composites were removed from the variables. One relevant result was the nonlinear precipitation response to R-TPO and R-IOD. This feature was accentuated for the R-IOD composites. The positive R-IOD composite showed significant negative precipitation anomalies along equatorial SA east of 55° W and in subtropical western SA, and showed positive anomalies in northwestern SA and central Brazil. The negative R-IOD composite indicated significant positive precipitation anomalies in northwestern Amazon, central–eastern Brazil north of 20° S, and western subtropical SA, and negative anomalies were found in western SA south of 30° S. This nonlinearity was likely due to the distinct atmospheric circulation responses to the anomalous heating sources located in longitudinally distinct regions: the western tropical Indian Ocean and areas neighboring Indonesia. The results obtained in this study might be relevant for climate monitoring and modeling studies.

Oceanographic anomalies coinciding with humpback whale super-group occurrences in the Southern Benguela

Seasonal feeding behaviour of humpback whales (Megaptera novaeangliae) has been observed in the coastal waters of the Southern Benguela where the species has been observed forming super-groups during the austral spring in recent years since 2011. Super-groups are unprecedented densely-packed aggregations of between 20 and 200 individuals in low-latitude waters and their occurrences indicate possible changes in feeding behaviour of the species. We accessed published data on super-groups occurrence in the study area in 2011, 2014 and 2015, and investigated oceanographic drivers that support prey availability in this region. We found that enhanced primary production is a necessary but not sufficient condition for super-groups to occur. Positive chlorophyll anomalies occurring one month prior to the super-group occurrences were identified, but only a concurrent significantly reduced water volume export from the region throughout October were conducive to the aggregations in the specific years. Hydrodynamic model results attributed the anomalous decreased volume export to the strength and orientation of the Goodhope Jet and associated eddy activity. The combination of random enhanced primary production typical of the region and emerging anomalous conditions of reduced water export in October since 2011 resulted in favourable food availability leading to the unique humpback whale aggregations. The novelty of this grouping behaviour is indicative of the lack of such oceanographic conditions in the past. Given the recency of the events, it is difficult to attribute this reduction in ocean transport to climatic regime shifts, and the origin should be likely investigated in the distant water mass interaction with the greater Agulhas system rather than in local intensifications of the upwelling conditions. A positive trend in the humpback whale population abundance points to the need to monitor the exposure of the species to the changing climate conditions.

Oxygen and Nutrient Trapping in the Southern Benguela Upwelling System

The Benguela Upwelling System in the southeast Atlantic Ocean is of crucial socio-economic importance due to its high productivity. However, predicting its response to global change and understanding past changes are still great challenges. Here, we compile data obtained from a research cruise and an oceanographic mooring to demonstrate that a topographically steered nutrient trapping zone develops in a narrow belt along the coast during the main upwelling season in austral spring and summer in the southern Benguela Upwelling System. High nutrient concentrations within this zone increase the impact of upwelling on the productivity of the southern Benguela Upwelling System, but the efficient nutrient trapping operates at the expense of decreasing oxygen concentrations. This enhances the probability of anoxic events emerging toward the end of the upwelling season. However, at the end of the upwelling season, the front that separates the coastally trapped waters from open shelf waters weakens or even collapses due to upwelling cessation and the reversing current regime. This, in addition to a stronger vertical mixing caused by winter cooling, fosters the ventilation of the nutrient trapping zone, which reestablishes during the following upwelling season. The postulated intensification of upwelling and changes in the ecosystem structure in response to global warming seem to reduce the nutrient trapping efficiency by increasing offshore advection of surface waters and plankton blooms. The intensified upwelling and resulting lower biological oxygen consumption appears to mask the expected impacts of global warming on the oxygen minimum zone (OMZ) in the southern Benguela Upwelling System. In contrast to other OMZs, including those in northern Benguela Upwelling Systems, the OMZ in the southern Benguela Upwelling System reveals so far no detectable long-term decrease in oxygen. Thus, the nutrient trapping efficiency seems to be a critical feature mitigating global change impacts on the southern Benguela Upwelling System. Since it is topographically steered, regional impacts on the nutrient trapping efficiency appear also to explain varying responses of upwelling systems to global change as the comparison between southern and northern Benguela Upwelling System shows. This emphasizes the need for further and more comparable studies in order to better understand the response of Eastern Boundary Upwelling Systems and their ecosystem services to global change.

Why Australia was not wet during spring 2020 despite La Niña

The austral spring climate of 2020 was characterised by the occurrence of La Niña, which is the most predictable climate driver of Australian springtime rainfall. Consistent with this La Niña, the Bureau of Meteorology’s dynamical sub-seasonal to seasonal forecast system, ACCESS-S1, made highly confident predictions of wetter-than-normal conditions over central and eastern Australia for spring when initialised in July 2020 and thereafter. However, many areas of Australia received near average to severely below average rainfall, particularly during November. Possible causes of the deviation of rainfall from its historical response to La Niña and causes of the forecast error are explored with observational and reanalysis data for the period 1979–2020 and real-time forecasts of ACCESS-S1 initialised in July to November 2020. Several compounding factors were identified as key contributors to the drier-than-anticipated spring conditions. Although the ocean surface to the north of Australia was warmer than normal, which would have acted to promote rainfall over northern Australia, it was not as warm as expected from its historical relationship with La Niña and its long-term warming trend. Moreover, a negative phase of the Indian Ocean Dipole mode, which typically acts to increase spring rainfall in southern Australia, decayed earlier than normal in October. Finally, the Madden–Julian Oscillation activity over the equatorial Indian Ocean acted to suppress rainfall across northern and eastern Australia during November. While ACCESS-S1 accurately predicted the strength of La Niña over the Niño3.4 region, it over-predicted the ocean warming to the north of Australia and under-predicted the strength of the November MJO event, leading to an over-prediction of the Australian spring rainfall and especially the November-mean rainfall.

Impacts of CP- and EP-El Niño events on the Antarctic sea ice in austral spring

Based on observational data analyses and idealized modeling experiments, we investigated the distinctive impacts of central Pacific (CP-) El Niño and eastern Pacific (EP-) El Niño on the Antarctic sea ice concentration (SIC) in austral spring (September to November). The tropical heat sources associated with EP-El Niño and the co-occurred positive phase of Indian Ocean Dipole (IOD) excite two branches of Rossby wave trains that propagate southeastward, causing an anomalous anticyclone over the eastern Ross-Amundsen-Bellingshausen Seas. Anomalous northerly (southerly) wind west (east) of the anomalous anticyclone favor poleward (offshore) movements of sea ice, resulting in a sea ice loss (growth) in the eastern Ross-Amundsen Seas (the Bellingshausen-Weddell Seas). Meanwhile, the anomalous northerly (southerly) wind also advected warmer and wetter (colder and drier) air into the eastern Ross-Amundsen Seas (the Bellingshausen-Weddell Seas), causing surface warming (cooling) through the enhanced (reduced) surface heat fluxes and thus contributing to the sea ice melting (growth). CP-El Niño, however, forces a Rossby wave train that generates an anomalous anticyclone in the eastern Ross-Amundsen Seas, 20° west of that caused by EP-El Niño. Consequently, a positive SIC anomaly occurs in the Bellingshausen Sea. A dry version of the Princeton atmospheric general circulation model was applied to verify the roles of anomalous heating in the tropics. The result showed that EP-El Niño can remotely induce an anomalous anticyclone and associated dipole temperature pattern in the Antarctic region, whereas CP-El Niño generates a similar anticyclone pattern with its location shift westward by 20° in longitudes.