Wind-driven flow over topography

The space and time scales over which wind forcing can directly drive flows over regions of closed topographic contours are explored using an idealized numerical model and theory. It is shown that stratification limits the vertical scale of the mean flow but also results in an enhanced recirculation strength in shallow water by distorting the isopycnals in the bottom boundary layer. Time-dependent forcing can drive flows that extend deeper than the mean flow because the initial response is primarily barotropic. This response is limited at low frequencies by baroclinic Rossby wave propagation. It is expected that these wind-driven flows might be important in the vicinity of islands and over large-scale topographic features

Topographic influences on the wind-driven exchange between marginal seas and the open ocean

Author Posting. © American Meteorological Society, 2021. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 51(12),(2021): 3663–3678, https://doi.org/10.1175/JPO-D-21-0058.1.The wind-driven exchange through complex ridges and islands between marginal seas and the open ocean is studied using both numerical and analytical models. The models are forced by a steady, spatially uniform northward wind stress intended to represent the large-scale, low-frequency wind patterns typical of the seasonal monsoons in the western Pacific Ocean. There is an eastward surface Ekman transport out of the marginal sea and westward geostrophic inflows into the marginal sea. The interaction between the Ekman transport and an island chain produces strong baroclinic flows along the island boundaries with a vertical depth that scales with the ratio of the inertial boundary layer thickness to the baroclinic deformation radius. The throughflows in the gaps are characterized by maximum transport in the center gap and decreasing transports toward the southern and northern tips of the island chain. An extended island rule theory demonstrates that throughflows are determined by the collective balance between viscosity on the meridional boundaries and the eastern side boundary of the islands. The outflowing transport is balanced primarily by a shallow current that enters the marginal sea along its equatorward boundary. The islands can block some direct exchange and result in a wind-driven overturning cell within the marginal sea, but this is compensated for by eastward zonal jets around the southern and northern tips of the island chain. Topography in the form of a deep slope, a ridge, or shallow shelves around the islands alters the current pathways but ultimately is unable to limit the total wind-driven exchange between the marginal sea and the open ocean.This research is supported in part by the China Scholarship Council (201906330102). H. G. is financially supported by the China Scholarship Council to study at WHOI for 2 years as a guest student. M. A. S. is supported by the National Science Foundation Grant OCE-1922538

The Simulated Biological Response to Southern Ocean Eddies via Biological Rate Modification and Physical Transport

We examine the structure and drivers of anomalous phytoplankton biomass in Southern Ocean eddies tracked in a global, multiyear, eddy-resolving, 3-D ocean simulation of the Community Earth System Model.We examine how simulated anticyclones and cyclones differentially modify phytoplankton biomass concentrations, growth rates, and physical transport. On average, cyclones induce negative division rate anomalies that drive negative net population growth rate anomalies, reduce dilution across shallower mixed layers, and advect biomass anomalously downward via eddy-induced Ekman pumping. The opposite is true in anticyclones. Lateral transport is dominated by eddy stirring rather than eddy trapping. The net effect on anomalous biomass can exceed 10–20% of background levels at the regional scale, consistent with observations. Moreover, we find a strong seasonality in the sign and magnitude of regional anomalies and the processes that drive them. The most dramatic seasonal cycle is found in the South Pacific Antarctic Circumpolar Current, where physical and biological processes dominate at different times, modifying biomass in different directions throughout the year. Here, in cyclones, during winter, anomalously shallow mixed layer depths first drive positive surface biomass anomalies via reduced dilution, and later drive positive depth-integrated biomass anomalies via reduced light limitation. During spring, reduced iron availability and elevated grazing rates suppress net population growth rates and drive the largest annual negative surface and depth-integrated biomass anomalies. During summer and fall, lateral stirring and eddy-induced Ekman pumping create small negative surface anomalies but positive depth-integrated anomalies. The same mechanisms drive biomass anomalies in the opposite direction in anticyclones

Recent trends in the wind-driven California current upwelling system

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this recordLong-term changes in the marine ecosystems of the Eastern Boundary Upwelling Systems (EBUS) are predicted due to anthropogenic climate change. In particular, global ocean acidification is having a profound effect on the coastal waters of the EBUS, affecting the entire trophic chain, net primary production (NPP) and related economic activities such as fisheries. Another predicted change related to human activity is that of upwelling dynamics with expected long-term changes in upwelling winds as proposed by Bakun (1990), Bakun et al. (2015) and Rykaczewski et al. (2015). Although these predicted long-term changes may emerge only later in the 21st century, this has fueled many studies using historical data. Long-term increase in upwelling winds has thus been a much debated topic, showing that there is considerable uncertainty depending on the EBUS considered, the effect of natural climate fluctuations, the choice of wind dataset, the time period considered, and the methodologies and significance tests applied. Therefore, there is an immediate interest in being able to monitor upwelling using verified and self-consistent wind data sets. This work focused on a sensitivity study of the estimated trends in upwelling winds in the California Current Upwelling System (CCUS), for the most recent period 1996–2018, using the two state-of-the-art satellite wind analyses and two atmospheric model re-analyses. Embedded into the strong modulation by natural climate fluctuations on interannual and decadal time scales, we do see an increase in upwelling-favorable winds in the core of the CCUS, with a local increase of more than 25% in seasonal upwelling transport for the period considered. In this central upwelling zone, a good agreement on stronger equatorward winds for the winter and spring seasons is found between the different datasets, although with different significance levels. Conversely, conflicting results are found in the southernmost part of the CCUS between the satellite analyses and the model reanalyses. Systematic, time-dependent differences are found between the wind products, highlighting the need to further investigate the poorly documented temporal stability of these widely used wind long-term climatology products. The observed spatial structuring of the estimated wind trends is consistent with the trend analysis of water chlorophyll-a, partial pressure of CO2, and basity (pH) analysis products. This result is consistent with changes being important for modulating the carbonate system within the CCUS.European Space Agenc

Drivers and impact of the seasonal variability of the organic carbon offshore transport in the Canary upwelling system

The Canary upwelling system (CanUS) is a productive coastal region characterized by strong seasonality and an intense offshore transport of organic carbon (Corg) to the adjacent oligotrophic offshore waters. There, the respiration of this Corg substantially modifies net community production (NCP). While this transport and the resulting coupling of the biogeochemistry between the coastal and open ocean has been well studied in the annual mean, the temporal variability, and especially its seasonality, has not yet been investigated. Here, we determine the seasonal variability of the offshore transport of Corg, its mesoscale component, latitudinal differences, and the underlying physical and biological drivers. To this end, we employ the Regional Ocean Modeling System (ROMS) coupled to a nutrient–phytoplankton–zooplankton–detritus (NPZD) ecosystem model. Our results reveal the importance of the mesoscale fluxes and of the upwelling processes (coastal upwelling and Ekman pumping) in modulating the seasonal variation of the offshore Corg transport. We find that the region surrounding Cape Blanc (21∘ N) hosts the most intense Corg offshore flux in every season, linked to the persistent, and far reaching Cape Blanc filament and its interaction with the Cape Verde Front. Coastal upwelling filaments dominate the seasonality of the total offshore flux up to 100 km from the coast, contributing in every season at least 80 % to the total flux. The seasonality of the upwelling modulates the offshore Corg seasonality hundreds of kilometers from the CanUS coast via lateral redistribution of nearshore production. North of 24.5∘ N, the sharp summer–fall peak of coastal upwelling results in an export of more than 30 % of the coastal Corg at 100 km offshore due to a combination of intensified nearshore production and offshore fluxes. To the south, the less pronounced upwelling seasonality regulates an overall larger but farther-reaching and less seasonally varying lateral flux, which exports between 60 % and 90 % of the coastal production more than 100 km offshore. Overall, we show that the temporal variability of nearshore processes modulates the variability of Corg and NCP hundreds of kilometers offshore from the CanUS coast via the offshore transport of the nearshore production

Course Manual Winter School on Structure and Functions of Marine Ecosystem: Fisheries

Marine ecosystems comprises of diverse organisms and their ambient abiotic components in varied relationships leading to an ecosystem functioning. These relationships provides the services that are essential for marine organisms to sustain in the nature. The studies examining the structure and functioning of these relationships remains unclear and hence understanding and modelling of the ecological functioning is imperative in the context of the threats different ecosystem components are facing. The relationship between marine population and their environment is complex and is subjected to fluctuations which affects the bottom level of an ecosystem pyramid to higher trophic levels. Understanding the energy flow within the marine ecosystems with the help of primary to secondary producers and secondary consumers are potentially important when assessing such states and changes in these environments. Many of the physiological changes are known to affect the key functional group, ie. the species or group of organisms, which play an important role in the health of the ecosystem. In marine environment, phytoplankton are the main functional forms which serves as the base of marine food web. Any change in the phytoplankton community structure may lead to alteration in the composition, size and structure of the entire ecosystem. Hence, it is critical to understand how these effects may scale up to population, communities, and entire marine ecosystem. Such changes are difficult to predict, particularly when more than one trophic level is affected. The identification and quantification of indicators of changes in ecosystem functioning and the knowledge base generated will provide a suitable way of bridging issues related to a specific ecosystem. New and meaningful indicators, derived from our current understanding of marine ecosystem functioning, can be used for assessing the impact of these changes and can be used as an aid in promoting responsible fisheries in marine ecosystems. Phytoplantkon is an indicator determining the colour of open Ocean. In recent years, new technologies have emerged which involves multidisciplinary activities including biogeochemistry and its dynamics affecting higher trophic levels including fishery. The winter school proposed will provide the insights into background required for such an approach involving teaching the theory, practical, analysis and interpretation techniques in understanding the structure and functioning of marine ecosystems from ground truth measurements as well as from satellite remote sensing data. This is organized with the full funding support from Indian council of Agricultural Research (ICAR) New Delhi and the 25 participants who are attending this programme has been selected after scrutiny of their applications based on their bio-data. The participants are from different States across Indian subcontinent covering north, east, west and south. They are serving as academicians such as Professors/ scientists and in similar posts. The training will be a feather in their career and will enable them to do their academic programmes in a better manner. Selected participants will be scrutinized initially to understand their knowledge level and classes will be oriented based on this. In addition, all of them will be provided with an e-manual based on the classes. All selected participants are provided with their travel and accommodation grants. The faculty include the scientists who developed this technology, those who are practicing it and few user groups who do their research in related areas. The programme is coordinated by the Fishery Resources Assessment Division of CMFRI. This programme will generate a team of elite academicians who can contribute to sustainable management of marine ecosystem and they will further contribute to capacity building in the sector by training many more interested researchers in the years to come

Computational analysis of the biophysical controls on Southern Ocean phytoplankton ecosystem dynamics

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Applied Ocean Science & Engineering at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2019.Southern Ocean net community productivity plays an out sized role in regulating global biogeochemical cycling and climate dynamics. The structure of spatial-temporal variability in phytoplankton ecosystem dynamics is largely governed by physical processes but a variety of competing pathways complicate our understanding of how exactly they drive net population growth. Here, I leverage two coupled, 3-dimensional, global, numerical simulations in conjunction with remote sensing data and past observations, to improve our mechanistic understanding of how physical processes drive biology in the Southern Ocean. In Chapter 2, I show how different mechanistic pathways can control population dynamics from the bottom-up (via light, nutrients), as well as the top-down (via grazing pressure). In Chapters 3 and 4, I employ a higher resolution, eddy resolving, integration to explicitly track and examine closed eddy structures and address how they modify biomass at the mesoscale. Chapter 3 considers how simulated eddies drive bottom-up controls on phytoplankton growth and finds that division rates are, on average, amplified in anticyclones and suppressed in cyclones. Anomalous division rates are predominately fueled by an anomalous vertical iron flux driven by eddy-induced Ekman Pumping. Chapter 4 goes on to describe how anomalous division rates combine with anomalous loss rates to drive anomalous net population growth. Biological rate-based mechanisms are then compared to the potential for anomalies to evolve strictly via physical transport (i.e. dilution, stirring, advection). All together, I identify and describe dramatic regional and seasonal variability in when, where, and how different mechanisms drive phytoplankton growth throughout the Southern Ocean. Better understanding this variability has broad implications to our understanding of how oceanic biogeochemisty will respond to, and likely feedback into, a changing climate. Specifically, the uncertainty associated with this variability should temper recent proposals to artificially stimulate net primary production and the biological pump via iron fertilization. In Chapter 5 I argue that Southern Ocean Iron Fertilization fails to meet the basic tenets required for adoption into any regulatory market based framework.The CESM project is supported by the National Science Foundation and the Office of Science (BER) of the U.S. Department of Energy. Computing resources were provided by the Climate Simulation Laboratory at NCAR’s Computational and Information Systems Laboratory (CISL), sponsored by the National Science Foundation and other agencies. This research was enabled by CISL compute and storage resources. TR was supported by an NDSEG graduate fellowship. TR and SCD acknowledge support from the National Aeronautics and Space Administration Ocean Biology and Biogeochemistry Program (NNX14AL86G). TR, SCD and MTK acknowledge support from the National Science Foundation Polar Programs award 1440435 (Antarctic Integrated System Science) to the Palmer LTER program. Please contact [email protected] for further questions or to access to data

The Origin and Fate of Subantarctic Mode Water and Antarctic Intermediate Water in the Southern Ocean

The global ocean plays a major role in moderating atmospheric temperature rise, thereby buffering climate change. Amongst the various oceanic regions undergoing warming, the Southern Ocean is a primary heat sink in the climate system. Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) are the dominant water masses in the upper Southern Ocean, and play a fundamental role in ocean ventilation and the uptake of heat and carbon into the ocean interior. This thesis focuses on understanding the geographic and seasonal variability in the formation of SAMW and AAIW, as well as the role of SAMW, AAIW, and other mode and intermediate waters in recent global ocean warming, using observationally based hydrography and estimates of mixing strength. Firstly, the mechanisms controlling the volumetric change of SAMW within the mixed layer and in the ocean interior are investigated separately. We find that the seasonal variability of SAMW volume in the mixed layer is governed by formation due to air-sea buoyancy fluxes (45%, lasting from July to August) and entrainment (35%), while the interior SAMW formation is controlled by subduction during August-October. The annual mean subduction estimate shows strong regional variability with hotspots of large SAMW subduction, consistent with the distribution and export pathways of SAMW over the central and eastern parts of the south Indian and Pacific Oceans. Secondly, a volume budget analysis is performed to identify the mechanisms governing the spatial and seasonal variability of AAIW. Firstly, Ekman pumping upwells the dense variety of AAIW into the mixed layer south of the Polar Front, which can be advected northward by Ekman transport into the subduction regions of lighter variety AAIW and SAMW. The subduction of light AAIW occurs mainly by lateral advection in the southeast Pacific and Drake Passage as well as eddy-induced flow between the Subantarctic and Polar Fronts. Secondly, the diapycnal transport from subducted SAMW into the AAIW layer is predominantly by mesoscale mixing near the Subantarctic Front and vertical mixing in the South Pacific, while AAIW is further replenished by transformation from Upper Circumpolar Deep Water by vertical mixing. Lastly, part of AAIW is exported out of the Southern Ocean. Our results suggest that the distribution of AAIW is set by its formation due to subduction and mixing, and its circulation eastward along the Antarctic Circumpolar Current (ACC) and northward into the subtropical gyres. Finally, the ocean absorbs >90% of anthropogenic heat in the Earth system. However, it remains unclear how this heat uptake is distributed across water masses. Here we show that ocean heat accumulation during 2010–2020 has more than doubled relative to 1990–2000. Of the total ocean heat uptake, 94% is found in global mode and intermediate water layers that have subsequently warmed and increased in volume. After factoring out volumetric changes, warming of mode and intermediate waters explains ~40% of net global ocean warming, despite occupying just ~16% of the total ocean volume. These water masses in the subtropical Pacific and Atlantic Oceans, as well as in the Southern Ocean, are responsible for a large fraction of total heat uptake, with important implications for ongoing ocean warming, sea-level rise, and climate impacts

Exploring the role of the “Ice-Ocean governor” and mesoscale eddies in the equilibration of the Beaufort Gyre: lessons from observations

Observations of Ekman pumping, sea surface height anomaly, and isohaline depth anomaly over the Beaufort Gyre are used to explore the relative importance and role of (i) feedbacks between ice and ocean currents, dubbed the “Ice-Ocean governor” and (ii) mesoscale eddy processes in the equilibration of the Beaufort Gyre. A two-layer model of the gyre is fit to observations and used to explore the mechanisms governing the gyre evolution from the monthly to the decennial time scale. The Ice-Ocean governor dominates the response on inter-annual timescales, with eddy processes becoming evident only on the longest, decadal timescales. Keywords: Ocean; Arctic; Eddies; Ekman pumping/transport; Atmosphere-ocean interactionNational Science Foundation (U.S.). Polar Program

Observations of Seasonal Upwelling and Downwelling in the Beaufort Sea Mediated by Sea Ice

We present observational estimates of Ekman pumping in the Beaufort Gyre region. Averaged over the Canada Basin, the results show a 2003-14 average of 2.3myr⁻¹ downward with strong seasonal and interannual variability superimposed: monthly and yearly means range from 30myrsu downward to 10myr⁻¹ upward. A clear, seasonal cycle is evident with intense downwelling in autumn and upwelling during the winter months, despite the wind forcing being downwelling favorable year-round. Wintertime upwelling is associated with friction between the large-scale Beaufort Gyre ocean circulation and the surface ice pack and contrasts with previous estimates of yearlong downwelling; as a consequence, the yearly cumulative Ekman pumping over the gyre is significantly reduced. The spatial distribution of Ekman pumping is also modified, with the Beaufort Gyre region showing alternating, moderate upwelling and downwelling, while a more intense, yearlong downwelling averaging 18myr⁻¹ is identified in the northern Chukchi Sea region. Implications of the results for understanding Arctic Ocean dynamics and change are discussed. Keywords: Ocean; Arctic; Arctic Oscillation; Atmosphere-ocean interaction; Ekman pumping/transport; Upwelling/downwellin