The Somali Current system in the western Arabian Sea reverses seasonally with the South Asian Monsoon and is associated with localized upwelling cells or cold wedges during the summer southwest ...monsoon. Drifter trajectories in boreal summer and fall 2014 provide rare observational evidence that the northward Somali Current and associated cold wedges can persist into the boreal fall intermonsoon period. The near‐surface circulation and sea surface temperatures further suggest that the wedge‐like surface signatures may intermittently be capped and then reappear at a later time. Our observations show that the northward Somali Current system rapidly decayed within 1 week after the onset of the winter northeast monsoon and the arrival of a cyclonic eddy at the coast in early November 2014. This eddy may not only have affected regional ocean‐atmosphere interactions but also biogeochemical processes and the marine ecosystem through the transport of water properties and locally induced upwelling.
Plain Language Summary
The Indian Ocean rim countries are home to about one third of the global population and depend on rain‐fed agriculture as well as fisheries supported by coastal upwelling systems, which are linked to monsoon variability. A better understanding of regional air‐sea phenomena is therefore needed, and we report here on recent drifter observations in the western Arabian Sea that provide new insights into the persistence of upwelling cells or cold wedges and associated circulation patterns in the southern part of the Somali Current system, which reverses seasonally with the monsoon. Our observations during the 2014 drought year reveal that the northward Somali Current and associated cold wedges can persist into the intermonsoon period following the summer southwest monsoon. The wedge‐like surface signatures in the surface circulation and sea surface temperature may intermittently be capped and then reappear at a later time. During our observational period, the northward Somali Current system of the summer southwest monsoon rapidly decayed within 1 week after the onset of the winter northeast monsoon and the arrival of a mesoscale feature at the coast in early November. The latter may not only have affected regional ocean‐atmosphere interactions but also biogeochemical processes and the marine ecosystem.
Key Points
First observational evidence that the northward Somali Current and associated cold wedges may persist into the boreal fall intermonsoon
Rapid breakdown of the Somali Current system after the onset of the winter northeast monsoon and the arrival of a cyclonic eddy
Persistence of cold wedges and associated circulations may affect air‐sea interactions and biogeochemical processes within the ecosystem
An array of surface drifters deployed ahead of Hurricane Michael measured the surface temperature, pressure, directional wind and wave spectra, and surface currents one day before it made landfall as ...a Category 5 Hurricane. The drifters, 25–50 km apart, spanned two counter‐rotating ocean eddies as Hurricane Michael rapidly intensified. The drifters measured the shift of wave energy between frequency bands in each quadrant of the storm, the response of upper ocean currents, and the resulting cold wake following Michael's passage. Wave energy was greatest in the front quadrants and rapidly decreased in the left‐rear quadrant, where wind and wave energy were misaligned, and components of the wave field were aligned with currents. Hurricane Michael's wave field agreed with previous studies of nondirectional wave spectra across multiple tropical cyclones but had some unique characteristics. The analysis demonstrates how co‐located surface wind and wave observations can complement existing airborne and satellite observations.
Plain Language Summary
Lagrangian drifters were air‐deployed ahead of Hurricane Michael and measured the direction and strength of waves and surface wind, sea surface temperature, and sea‐level pressure as the storm transited through the central Gulf of Mexico. As Hurricane Michael passed over the drifters, the drifters observed the cyclonic structure of the wind, the shift of wave energy from swell to wind‐sea, and the relative mismatch in direction of wind, waves, and ocean currents. In the rear quadrants of the storm, low‐frequency waves opposed the wind direction. The drifters, caught in counter‐rotating eddies, were ultimately entrained in different sides of the storm. The observations illustrate the importance of a suite of in situ surface observations to complement airborne observing strategies of tropical cyclones.
Key Points
Ten surface drifters measured temperature, pressure, currents, directional wave spectra, and wind under Hurricane Michael
Wave energy was greatest in the front quadrants with misalignment of wave energy by frequency band in rear‐quadrants of the storm
Waves agreed with fetch‐limited wave growth and there were significant differences in wind‐wave‐current alignment in each quadrant
Internal gravity waves, the subsurface analogue of the familiar surface gravity waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and ...the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis, sediment and pollutant transport and acoustic transmission; they also pose hazards for man-made structures in the ocean. Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking, making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects. For over a decade, studies have targeted the South China Sea, where the oceans' most powerful known internal waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variability and energy budget, however, owing to the lack of in situ data from the Luzon Strait, where extreme flow conditions make measurements difficult. Here we use new observations and numerical models to (1) show that the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena, (2) reveal the existence of >200-metre-high breaking internal waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean, (3) determine that the Kuroshio western boundary current noticeably refracts the internal wave field emanating from the Luzon Strait, and (4) demonstrate a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region. Together, these findings give a cradle-to-grave picture of internal waves on a basin scale, which will support further improvements of their representation in numerical climate predictions.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SBMB, SIK, UILJ, UKNU, UL, UM, UPUK
Oceanography relies heavily on observations to fuel new ideas and drive advances, creating a strong coupling between the science and the technological developments that enable new measurements. Novel ...observations, such as those that resolve new properties or scales, often lead to advances in understanding. Physical, biological, and chemical processes unfold over a broad range of scales—seconds to decades and millimeters to ocean basins—with critical interactions between scales. Observational studies work within a tradespace that balances spatial and temporal resolution, scope, and resource constraints. New platforms and sensors, along with the novel observational approaches they enable, address this challenge by providing access to an expanding range of temporal and spatial scales.
FROM THE GUEST EDITORS Lee, Craig M.; Paluszkiewicz, Theresa; Rudnick, Daniel L. ...
Oceanography (Washington, D.C.),
06/2017, Letnik:
30, Številka:
2
Journal Article
The air-sea interface is a key gateway in the Earth system. It is where the atmosphere sets the ocean in motion, climate/weather relevant air-sea processes occur and pollutants (i.e., plastic, ...anthropogenic carbon dioxide, radioactive/chemical waste) enter the sea. Hence, accurate estimates and forecasts of physical and biogeochemical processes at this interface are critical for sustainable blue economy planning, growth and disaster mitigation. Such estimates and forecasts rely on accurate and integrated in-situ and satellite surface observations. High-impact uses of ocean-surface observations of Essential Ocean/Climate Variables (EOVs /ECVs) include 1) assimilation into/validation of weather, ocean and climate forecast models to improve their skill, impact and value; 2) ocean physics studies (i.e., heat, momentum, freshwater and biogeochemical air-sea fluxes) to further our understanding and parameterization of air-sea processes; and 3) calibration and validation of satellite ocean products (i.e., currents, temperature, salinity, sea level, ocean color, wind, waves). We review strengths and limitations, impacts, and sustainability of in-situ ocean surface observations of several ECVs and EOVs. We draw a 10-year vision of the global ocean-surface observing network for improved synergy and integration with other observing systems (e.g., satellites), modeling/forecast efforts and for a better ocean observing governance. The context is both the applications listed above and the guidelines of frameworks such as IOC-WMO-UNEP-ICSU GOOS, GCOS, and WIGOS. Networks of multiparametric platforms, such as the global drifter array, offer opportunities for new and improved in-situ observations. Advances in sensor technology (e.g., low-cost wave sensors), high throughput communications, evolving cyberinfrastructures and data information systems with potential to improve the scope, efficiency, integration and sustainability of the ocean surface observing system are explored.
Understanding and sustainably managing complex environments such as marine ecosystems benefits from an integrated approach to ensure that information about all relevant components and their ...interactions at multiple and nested spatiotemporal scales are considered. This information is based on a wide range of ocean observations using different systems and approaches. An integrated approach thus requires effective collaboration between areas of expertise in order to improve coordination at each step of the ocean observing value chain, from the design and deployment of multi-platform observations to their analysis and the delivery of products, sometimes through data assimilation in numerical models. Despite significant advances over the last two decades in more cooperation across the ocean observing activities, this integrated approach has not yet been fully realized. The ocean observing system still suffers from organizational silos due to independent and often disconnected initiatives, the strong and sometimes destructive competition across disciplines and among scientists, and the absence of a well-established overall governance framework. Here, we address the need for enhanced organizational integration among all the actors of ocean observing, focusing on the occidental systems. We advocate for a major evolution in the way we collaborate, calling for transformative scientific, cultural, behavioral, and management changes. This is timely because we now have the scientific and technical capabilities as well as urgent societal and political drivers. The ambition of the United Nations Decade of Ocean Science for Sustainable Development (2021–2030) and the various efforts to grow a sustainable ocean economy and effective ocean protection efforts all require a more integrated approach to ocean observing. After analyzing the barriers that currently prevent this full integration within the occidental systems, we suggest nine approaches for breaking down the silos and promoting better coordination and sharing. These recommendations are related to the organizational framework, the ocean science culture, the system of recognition and rewards, the data management system, the ocean governance structure, and the ocean observing drivers and funding. These reflections are intended to provide food for thought for further dialogue between all parties involved and trigger concrete actions to foster a real transformational change in ocean observing.
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