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
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
Abstract
Horizontal kinematic properties, such as vorticity, divergence, and lateral strain rate, are estimated from drifter clusters using three approaches. At submesoscale horizontal length scales
..., kinematic properties become as large as planetary vorticity
f
, but challenging to observe because they evolve on short time scales
. By simulating surface drifters in a model flow field, we quantify the sources of uncertainty in the kinematic property calculations due to the deformation of cluster shape. Uncertainties arise primarily due to (i) violation of the linear estimation methods and (ii) aliasing of unresolved scales. Systematic uncertainties (iii) due to GPS errors, are secondary but can become as large as (i) and (ii) when aspect ratios are small. Ideal cluster parameters (number of drifters, length scale, and aspect ratio) are determined and error functions estimated empirically and theoretically. The most robust method—a two-dimensional, linear least squares fit—is applied to the first few days of a drifter dataset from the Bay of Bengal. Application of the length scale and aspect-ratio criteria minimizes errors (i) and (ii), and reduces the total number of clusters and so computational cost. The drifter-estimated kinematic properties map out a cyclonic mesoscale eddy with a surface, submesoscale fronts at its perimeter. Our analyses suggest methodological guidance for computing the two-dimensional kinematic properties in submesoscale flows, given the recently increasing quantity and quality of drifter observations, while also highlighting challenges and limitations.
Significance Statement
The purpose of this study is to provide insights and guidance for computing horizontal velocity gradients from clusters (i.e., three or more) of Lagrangian surface ocean drifters. The uncertainty in velocity gradient estimates depends strongly on the shape deformation of drifter clusters by the ocean currents. We propose criteria for drifter cluster length scales and aspect ratios to reduce uncertainties and develop ways of estimating the magnitude of the resulting errors. The findings are applied to a real ocean dataset from the Bay of Bengal.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
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
Abstract
A cluster of 45 drifters deployed in the Bay of Bengal is tracked for a period of four months. Pair dispersion statistics, from observed drifter trajectories and simulated trajectories based ...on surface geostrophic velocity, are analyzed as a function of drifter separation and time. Pair dispersion suggests nonlocal dynamics at submesoscales of 1–20 km, likely controlled by the energetic mesoscale eddies present during the observations. Second-order velocity structure functions and their Helmholtz decomposition, however, suggest local dispersion and divergent horizontal flow at scales below 20 km. This inconsistency cannot be explained by inertial oscillations alone, as has been reported in recent studies, and is likely related to other nondispersive processes that impact structure functions but do not enter pair dispersion statistics. At scales comparable to the deformation radius
L
D
, which is approximately 60 km, we find dynamics in agreement with Richardson’s law and observe local dispersion in both pair dispersion statistics and second-order velocity structure functions.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
In the subtropical western North Pacific Ocean, the Kuroshio delivers heat, salt, and momentum poleward, much like its North Atlantic analog, the Gulf Stream. Though the Kuroshio generally flows ...along the western boundary from Taiwan to southeastern Japan as an "attached" current, the Kuroshio's strength, vertical structure, and horizontal position undergo significant temporal and spatial variability along this entire route. Ubiquitous mesoscale eddies and complicated topography associated with a string of marginal seas combine to make the western North Pacific a region with complex circulation. Here, we synthesize results from the recent US Origins of the Kuroshio and Mindanao Currents and Taiwan Observations of Kuroshio Transport Variability observational programs with previous findings to build a comprehensive picture of the Kuroshio on its route from northeastern Taiwan to southeastern Japan, where the current finally transitions from a western boundary current into the Kuroshio Extension, a vigorously meandering free jet.
Salinity measurements from drifters constitute an important in situ dataset for the calibration and validation of the sea surface salinity satellite missions. A total of 114 satellite-tracked ...salinity drifters were deployed within the framework of the first Salinity Processes in the Upper Ocean Regional Study (SPURS) experiment in the subtropical North Atlantic focusing on the period August 2012-April 2014. In this study, a subset of 83 drifters, which provided useful salinity measurements in the central SPURS region from a few weeks to more than one year, is evaluated and an ad hoc quality-control procedure based on previously published work and the new observations is described. It was found that the sampling algorithm of the drifters introduces a predominantly fresh bias in the noise level of the salinity data, probably caused by the presence of air bubbles within the measuring cell. Since such noise is difficult to eliminate using statistical methods, extensive editing was done manually instead. Such quality-control procedures cannot be routinely applied to the real-time data stream from the drifters. Therefore, a revision of the sampling algorithm of the drifter's salinity sensor is needed. Comparisons of the drifter's salinity measurements with independent datasets further indicate that the sensor can provide reliable observations for up to one year. Finally, little evidence was found that the quality of the drifter's salinity measurements depends on the presence of the drogue.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Data from QuikSCAT, ERA5, WAVERYS, drifters, and HYCOM + NCODA Reanalysis in the northern Pacific are analyzed under tropical cyclones (TCs) and extratropical cyclones (ECs). The EC (TC) has the ...stable (unstable) wind directions and wider (narrower) fetch resulting in a larger (smaller) significant wave height (
H
s
). For modest and weak wind speeds (
W
≤ 26 m s
−1
), the current speed (
U
) is higher in low latitudes than in mid-latitudes since the oceanic mixed-layer thickness (ℎ) is not large enough. For strong wind speed (
W
> 26 m s
−1
), ℎ deepens more in low latitudes and causes lower
U
.
Measuring vertical motions represent a challenge as they are typically 3–4 orders of magnitude smaller than the horizontal velocities. Here, we show that surface vertical velocities are intensified ...at submesoscales and are dominated by high frequency variability. We use drifter observations to calculate divergence and vertical velocities in the upper 15 m of the water column at two different horizontal scales. The drifters, deployed at the edge of a mesoscale eddy in the Alboran Sea, show an area of strong convergence (O $\mathcal{O}$(f)) associated with vertical velocities of −100 m day−1. This study shows that a multilayered‐drifter array can be an effective tool for estimating vertical velocity near the ocean surface.
Plain Language Summary
The study of vertical motions in the ocean is a key challenge as they are difficult to measure and predict although their impact is crucial on the exchange of water properties in the water column. Here, we use high resolution drifter observations to calculate vertical velocities in the upper layers of a surface density front. This study shows high values of downward speed combined with a high temporal variability at the surface layer.
Key Points
Horizontal divergence and vertical vorticity computed from drifter observations
Maximum downward speeds of 100 m day−1 in the upper 15 m on a subducting region
The vertical velocity time series shows a high temporal variability, varying from 50 to −100 m day−1 over 4 hr
Plastics and other artificial materials pose new risks to health of the ocean. Anthropogenic debris travels across large distances and is ubiquitous in the water and on the shorelines, yet, ...observations of its sources, composition, pathways and distributions in the ocean are very sparse and inaccurate. Total amounts of plastics and other man-made debris in the ocean and on the shore, temporal trends in these amounts under exponentially increasing production, as well as degradation processes, vertical fluxes and time scales are largely unknown. Present ocean circulation models are not able to accurately simulate drift of debris because of its complex hydrodynamics. In this paper we discuss the structure of the future integrated marine debris observing system (IMDOS) that is required to provide long-term monitoring of the state of the anthropogenic pollution and support operational activities to mitigate impacts on the ecosystem and safety of maritime activity. The proposed observing system integrates remote sensing and in situ observations. Also, models are used to optimize the design of the system and, in turn, they will be gradually improved using the products of the system. Remote sensing technologies will provide spatially coherent coverage and consistent surveying time series at local to global scale. Optical sensors, including high-resolution imaging, multi- and hyperspectral, fluorescence, and Raman technologies, as well as SAR will be used to measure different types of debris. They will be implemented in a variety of platforms, from hand-held tools to ship-, buoy-, aircraft-, and satellite-based sensors. A network of in situ observations, including reports from volunteers, citizen scientists and ships of opportunity, will be developed to provide data for calibration/validation of remote sensors and to monitor the spread of plastic pollution and other marine debris. IMDOS will interact with other observing systems monitoring physical, chemical, and biological processes in the ocean and on shorelines as well as state of the ecosystem, maritime activities and safety, drift of sea ice, etc. The synthesized data will support innovative multi-disciplinary research and serve diverse community of users.
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