A likely important feature of the poorly understood aerosol‐cloud interactions over the Southern Ocean (SO) is the dominant role of sea spray aerosol, versus terrestrial aerosol. Ice nucleating ...particles (INPs), or particles required for heterogeneous ice nucleation, present over the SO have not been studied in several decades. In this study, boundary layer aerosol properties and immersion freezing INP number concentrations (nINPs) were measured during a ship campaign that occurred south of Australia (down to 53°S) in March–April 2016. Ocean surface chlorophyll a concentrations ranged from 0.11 to 1.77 mg/m3, and nINPs were a factor of 100 lower than historical surveys, ranging from 0.38 to 4.6 m−3 at −20 °C. The INP population included organic heat‐stable material, with contributions from heat‐labile material. Lower INP source potentials of SO seawater samples compared to Arctic seawater were consistent with lower ice nucleating site densities in this study compared to north Atlantic air masses.
Plain Language Summary
The Southern Ocean is known for a prevalence of clouds that contain both liquid and ice, which are one of the most poorly understood cloud regimes in the climate system. A large gap in understanding important processes in these clouds is a lack of knowledge regarding particles (e.g., sea spray) required for forming ice crystals, termed ice nucleating particles. In a ship‐based monthlong field study, several instruments were deployed in efforts to characterize the ice nucleating particles present over the Southern Ocean for the first time in over four decades. Abundances of ice nucleating particles throughout the voyage were extremely low compared to other ocean regions, and concentrations were 2 orders of magnitude lower than the most recent survey conducted in the 1970s. We report that the ocean‐derived ice nucleating particles observed in this study were organic in nature, supporting a hypothesized link between ice nucleating particles and organic particles associated with phytoplankton blooms. The data from this study provide a desperately needed benchmark for constraining the number of ice crystals that may form in the remote and poorly understood clouds occurring over the Southern Ocean.
Key Points
Number concentrations of ice nucleating particles over the Southern Ocean in March 2016 were a factor of 100 lower than historical surveys
The ice nucleating particle source strength of Southern Ocean seawater was lower than previous measurements in northern hemisphere seawater
Ice nucleation site densities were lower over the Southern Ocean compared to measurements of pristine air masses from other ocean basins
Nonlinear mesoscale eddies can influence biogeochemical cycles in the upper ocean through vertical and horizontal advection of nutrients and marine organisms. The relative importance of these two ...processes depends on the polarity of an eddy (cyclones versus anticyclones) and the initial biological conditions of the fluid trapped in the core of the eddy at the time of formation. Eddies originating in the eastern South Indian Ocean are unique in that anticyclones, typically associated with downwelling, contain elevated levels of chlorophyll‐a, enhanced primary production and phytoplankton communities generally associated with nutrient‐replete environments. From analysis of 9 years of concurrent satellite measurements of sea surface height, chlorophyll, phytoplankton carbon, and surface stress, we present observations that suggest eddy‐induced Ekman upwelling as a mechanism that is at least partly responsible for sustaining positive phytoplankton anomalies in anticyclones of the South Indian Ocean. The biological response to this eddy‐induced Ekman upwelling is evident only during the Austral winter. During the Austral summer, the biological response to eddy‐induced Ekman pumping occurs deep in the euphotic zone, beyond the reach of satellite observations of ocean color.
Key Points
Elevated CHL is observed in anticyclones of the South Indian Ocean during winter
Eddy‐induced Ekman pumping can sustain elevated CHL in anticyclones
CHL anomalies in eddy are likely too deep to be observed by satellites in summer
A Road Map to IndOOS-2 Beal, L. M.; Vialard, J.; Roxy, M. K. ...
Bulletin of the American Meteorological Society,
11/2020, Letnik:
101, Številka:
11
Journal Article
Recenzirano
Odprti dostop
The Indian Ocean Observing System (IndOOS), established in 2006, is a multinational network of sustained oceanic measurements that underpin understanding and forecasting of weather and climate for ...the Indian Ocean region and beyond. Almost one-third of humanity lives around the Indian Ocean, many in countries dependent on fisheries and rain-fed agriculture that are vulnerable to climate variability and extremes. The Indian Ocean alone has absorbed a quarter of the global oceanic heat uptake over the last two decades and the fate of this heat and its impact on future change is unknown. Climate models project accelerating sea level rise, more frequent extremes in monsoon rainfall, and decreasing oceanic productivity. In view of these new scientific challenges, a 3-yr international review of the IndOOS by more than 60 scientific experts now highlights the need for an enhanced observing network that can better meet societal challenges, and provide more reliable forecasts. Here we present core findings from this review, including the need for 1) chemical, biological, and ecosystem measurements alongside physical parameters; 2) expansion into the western tropics to improve understanding of the monsoon circulation; 3) better-resolved upper ocean processes to improve understanding of air–sea coupling and yield better subseasonal to seasonal predictions; and 4) expansion into key coastal regions and the deep ocean to better constrain the basinwide energy budget. These goals will require new agreements and partnerships with and among Indian Ocean rim countries, creating opportunities for them to enhance their monitoring and forecasting capacity as part of IndOOS-2.
During the 1997-98 El Niño, the equatorial Pacific Ocean retained 0.7 × 10$^{15}$ grams of carbon that normally would have been lost to the atmosphere as carbon dioxide. The surface ocean became ...impoverished in plant nutrients, and chlorophyll concentrations were the lowest on record. A dramatic recovery occurred in mid-1998, the system became highly productive, analogous to coastal environments, and carbon dioxide flux out of the ocean was again high. The spatial extent of the phytoplankton bloom that followed recovery from El Niño was the largest ever observed for the equatorial Pacific. These chemical and ecological perturbations were linked to changes in the upwelling of nutrient-enriched waters. The description and explanation of these dynamic changes would not have been possible without an observing system that combines biological, chemical, and physical sensors on moorings with remote sensing of chlorophyll.
Sea ice and oceanic boundaries have a dominant effect in structuring Antarctic
marine ecosystems. Satellite imagery and historical data have identified the
southern boundary of the Antarctic ...Circumpolar Current as a
site of enhanced biological productivity. Meso-scale surveys
off the Antarctic peninsula have related the abundances of Antarctic krill
(Euphausia superba) and salps (Salpa thompsoni) to inter-annual
variations in sea-ice extent. Here we have examined the ecosystem
structure and oceanography spanning 3,500 km of the east Antarctic
coastline, linking the scales of local surveys and global observations. Between
80° and 150° E there is a threefold variation in the extent
of annual sea-ice cover, enabling us to examine the regional effects of sea
ice and ocean circulation on biological productivity. Phytoplankton, primary
productivity, Antarctic krill, whales and seabirds were concentrated where
winter sea-ice extent is maximal, whereas salps were located where the sea-ice
extent is minimal. We found enhanced biological activity south of the southern
boundary of the Antarctic Circumpolar Current rather than in association with
it. We propose that along this coastline ocean circulation
determines both the sea-ice conditions and the level of biological productivity
at all trophic levels.
Southern Ocean mesoscale eddies play an important role in ocean circulation and biogeochemical cycling, but their biological characteristics have not been well quantified at the basin scale. To ...address this, we combined a 15‐year tracked eddy data set with satellite observations of ocean color, sea surface temperature, and autonomous profiling floats to quantify the surface and subsurface properties of eddies. Anomalies of surface temperature and chlorophyll were examined in eddy‐centric composite averages constructed from thousands of eddies. Normalized surface chlorophyll anomalies (chlnorm) vary seasonally and geographically. Cyclones typically show positive chlnorm, while anticyclones have negative chlnorm. The sign of chlnorm reverses during late summer and autumn for eddies between the Subtropical and Polar Fronts. The reversal is most obvious in the Indian sector, and we attribute this to a combination of eddy stirring (deformation of surface gradients by the rotational velocity of an eddy) and deeper winter mixing in anticyclones. Both chlnorm and sea surface temperature anomalies transition from dipole structures north of the Subtropical Front to monopole structures south of the Subantarctic Front. Sea surface temperature and chlnorm composites provide evidence for eddy trapping (transporting of anomalies) and eddy stirring. This research provides a basin‐scale study of surface chlorophyll in Southern Ocean eddies and reveals counterintuitive biogeochemical signals.
Plain Language Summary
Ocean eddies are spinning parcels of water about 100 km across and 1,500‐m deep. They occur everywhere in the ocean. In the Southern Hemisphere, eddies that spin clockwise are cooler than the surrounding ocean because their rotation causes cold, deep water to move upward. This upwelling brings nutrients essential for photosynthesis to the surface and makes clockwise‐rotating eddies more productive. Satellites can measure this productivity by sensing differences in ocean color, which result from the increased plankton. By analyzing thousands of Southern Ocean eddies, we found that in summer and autumn, eddies behave opposite to our expectations. That is, clockwise rotating eddies have lower plankton concentrations compared to neighboring waters and counterclockwise rotating eddies have higher concentrations. To explain this, we examined how deep these eddies mix the ocean in the preceding months. We found that counterclockwise rotating eddies mix the ocean deeper in winter, allowing more nutrients to enter their interiors, leading to higher productivity. This work is important because eddy productivity plays a significant role in the exchange of carbon between the ocean and the atmosphere. Carbon exchange in the Southern Ocean is thought to be changing, and this work helps explain an important piece of that process.
Key Points
We quantify the seasonal and geographical variabilities in the physical and biological characteristics of Southern Ocean mesoscale eddies
Atypical chlorophyll anomalies are observed in eddies between the Subtropical Front and the Polar Front during summer and autumn
Eddy stirring, eddy trapping, and eddy pumping all contribute to physical and biological anomalies, depending on the region and season
In this paper we examine time-series measurements of near-surface chlorophyll concentration from a mooring that was deployed at 80.5°E on the equator in the Indian Ocean in 2010. These data reveal at ...least six striking spikes in chlorophyll from October through December, at approximately 2-week intervals, that coincide with the development of the fall Wyrtki jets during the transition between the summer and winter monsoons. Concurrent meteorological and in situ physical measurements from the mooring reveal that the chlorophyll pulses are associated with the intensification of eastward winds at the surface and eastward currents in the mixed layer. These observations are inconsistent with upwelling dynamics as they occur in the Atlantic and Pacific oceans, since eastward winds that force Wyrtki jet intensification should drive downwelling. The chlorophyll spikes could be explained by two alternative mechanisms: (1) turbulent entrainment of nutrients and/or chlorophyll from across the base of the mixed layer by wind stirring or Wyrtki jet-induced shear instability or (2) enhanced southward advection of high chlorophyll concentrations into the equatorial zone. The first mechanism is supported by the phasing and amplitude of the relationship between wind stress and chlorophyll, which suggests that the chlorophyll spikes are the result of turbulent entrainment driven by synoptic zonal wind events. The second mechanism is supported by the observation of eastward flows over the Chagos–Laccadive Ridge, generating high chlorophyll to the north of the equator. Occasional southward advection can then produce the chlorophyll spikes that are observed in the mooring record. Wind-forced biweekly mixed Rossby gravity waves are a ubiquitous feature of the ocean circulation in this region, and we examine the possibility that they may play a role in chlorophyll variability. Statistical analyses and results from the OFAM3 (Ocean Forecasting Australia Model, version 3) eddy-resolving model provide support for both mechanisms. However, the model does not reproduce the observed spikes in chlorophyll. Climatological satellite chlorophyll data show that the elevated chlorophyll concentrations in this region are consistently observed year after year and so are reflective of recurring large-scale wind- and circulation-induced productivity enhancement in the central equatorial Indian Ocean.
In the Southern Ocean, polynyas exhibit enhanced rates of primary productivity and represent large seasonal sinks for atmospheric CO2. Three contrasting east Antarctic polynyas were visited in late ...December to early January 2017: the Dalton, Mertz, and Ninnis polynyas. In the Mertz and Ninnis polynyas, phytoplankton biomass (average of 322 and 354 mg chlorophyll a (Chl a)/m2, respectively) and net community production (5.3 and 4.6 mol C/m2, respectively) were approximately 3 times those measured in the Dalton polynya (average of 122 mg Chl a/m2 and 1.8 mol C/m2). Phytoplankton communities also differed between the polynyas. Diatoms were thriving in the Mertz and Ninnis polynyas but not in the Dalton polynya, where Phaeocystis antarctica dominated. These strong regional differences were explored using physiological, biological, and physical parameters. The most likely drivers of the observed higher productivity in the Mertz and Ninnis were the relatively shallow inflow of iron‐rich modified Circumpolar Deep Water onto the shelf as well as a very large sea ice meltwater contribution. The productivity contrast between the three polynyas could not be explained by (1) the input of glacial meltwater, (2) the presence of Ice Shelf Water, or (3) stratification of the mixed layer. Our results show that physical drivers regulate the productivity of polynyas, suggesting that the response of biological productivity and carbon export to future change will vary among polynyas.
Key Points
In summer 2016–2017, phytoplankton biomass and NCP in the Mertz and Ninnis polynyas were 3 times those measured in the Dalton polynya
Iron‐rich Circumpolar Deep Water and sea ice meltwater best explained this productivity contrast.
The productivity contrast between the three polynyas could not be explained by the meteoric water, the Ice Shelf Water, or stratification
The physical factors that have been reported to affect primary and secondary production in the Southern Ocean are examined and critically reviewed. Long time series of physical measurements from the ...Southern Ocean are available and there is a theoretical base from which models can be constructed. In contrast, there are few large‐scale measurements of biological parameters and a paucity of long‐term biological data sets for the Antarctic region. The absence of predictive models for the biological systems of the region is underpinned by the absence of theoretical understanding of the variations in the physical environment and their effects on primary, secondary, or tertiary production. To further this understanding, we have examined some of the major seasonal and interannual physical data available for the region (sea ice extent and retreat rate, wind stress, and surface ocean circulation patterns) and have examined their relationship to spatial and temporal variation in satellite‐derived proxies of primary productivity (Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) ocean color data). The results indicate that there are regional differences in the dominant physical forcings and that simple models will fail to replicate the observed patterns of primary production. We have also used the dynamics of Antarctic krill in the South Atlantic as an example to develop a model and explore the various hypotheses that have been put forward to explain interannual variability in this region. Results from this model indicate that the physical system may change in ways that cause periodic shifts in the relative importance of the factors that affect secondary production. The implications for the design of future research programs are explored.
Profiling floats with optical sensors can provide important complementary data to satellite ocean color determinations by providing information about the vertical structure of ocean waters, as well ...as surface waters obscured by clouds. Here we demonstrate this ability by pairing satellite ocean color data with records from a profiling float that obtained continuous, high-quality optical data for 3 yr in the North Atlantic Ocean. Good agreement was found between satellite and float data, and the relationship between satellite chlorophyll and float-derived particulate backscattering was consistent with previously published data. Upper ocean biogeochemical dynamics were evidenced in float measurements, which displayed strong seasonal patterns associated with phytoplankton blooms, and depth and seasonal patterns associated with an increase in pigmentation per particle at low light. Surface optical variables had shorter decorrelation timescales than did physical variables (unlike at low latitudes), suggesting that biogeochemical rather than physical processes controlled much of the observed variability. After 2.25 yr in the subpolar North Atlantic between Newfoundland and Greenland, the float crossed the North Atlantic Current to warmer waters, where it sampled an unusual eddy for 3 months. This anticyclonic feature contained elevated particulate material from surface to 1000-m depth and was the only such event in the float's record. This eddy was associated with weakly elevated surface pigment and backscattering, but depth-integrated backscattering was similar to that previously observed during spring blooms. Such seldom-observed eddies, if frequent, are likely to make an important contribution to the delivery of particles to depth.
PDF
