Tidally induced residual flows around an island due to both frictional and rotational effects
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A hydrodynamic numerical model developed in polar coordinates derives the tidal characteristics around an island. Due to the frictional stress over the sloping topography four residual eddies are generated. The effect of the Earth's rotation deflects the tidal flow in the shallow waters near the island and marginally adjusts the residual flows. Relationships for the M2 tidal vorticity are derived for these effects which can be used to estimate the rectified vorticity distribution associated with tidal flow around islands. The corresponding bottom stress distributions, which are important in sediment transport studies, are briefly discussed.Keywords:
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Considering our atmosphere as a steady viscous gaseous envelope that co-rotates with the Earth, we obtain a solution for the form in which this induced rotational effect decreases as a function of the distances to the centre of the Earth and to the rotation axis.
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Early Earth
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Submesoscale eddies form an important component of the circulation of the Southern California Bight (SCB), greatly impacting ecological processes. Despite their acknowledged significance in influencing ocean physics and biology, submesoscale eddies have been exceptionally hard to observe because of the technical challenges posed by both field and remote platforms. Here using a decade of high-frequency radar (HFR) surface current data we address this challenge for the SCB. Over the ten years of data, our research has mapped out the spatial distribution of submesoscale eddies and provided their seasonal and inter-annual variations. Between 2012 and 2021, a total of 235229 eddies were detected, averaging 452 eddies per week. Of these, 56% were cyclonic and 44% were anticyclonic. The contribution is roughly equal if eddies through their life spans are counted as one occurrence. This is because cyclonic eddies lived longer. The spatial distribution of eddies exhibited strong topographically related heterogeneity. Spatially coherent eddies, which reoccurred in certain locations over time, formed hotspots of eddy activity, largely in association with headlands. However, there were hotspots that did not seem to be associated with any typographic feature. Eddy temporal variations were examined at seasonal and interannual scales. On seasonal scales, eddies were found to be more numerous in the summer and early fall than in the spring. In August, the number of eddies was the highest, with 55% more observed eddies than in April, the least active month. The strong equatorward flow in the springtime seems to be linked with the reduced eddy activity at this time, likely due to the flow's suppressing effect on vortices and instabilities. At interannual scales, the eddy activity substantially increased in association with the 2014-2015 Blob event and the 2015-2016 El Niño. Observed eddies rose by 38% in 2014 compared to 2013 and remained high in 2015 and 2016. The results of this study are useful for the validation of numerical modeling studies in the SCB and could be of interest to the biological community to evaluate links between ecosystems and submesoscale activity along the highly productive coasts of the SCB.
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A number of authors have, in the past, been of the opinion that dust devil direction of rotation is controlled by the earth's rotation. While this contention can be easily attacked through theoretical arguments, actual observations become the deciding factor. The observations presented, believed to be the largest collection on record, show quite conclusively that dust devils in general have no preferred direction of rotation.
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Using satellite altimetry data, we have observed a series of anticyclonic eddies as they form at the Big Island of Hawaii and have tracked them as they move away from the island. While similar eddies have been observed near the Hawaiian Islands in previous studies, the fate of the anticyclonic eddies has previously been unclear. The eddies that we observed initially propagated to the southwest but consistently changed propagation direction to the northwest later in their lifetimes. This was intriguing to us, as theoretically, the decay of isolated anticyclonic eddies on a β plane should cause them to continually move toward the southwest. Such isolated eddy dynamics are unable to account for the observed change to northwestward eddy propagation, and the presence of the westward flowing North Equatorial Current turns out to be important to the Big Island eddy dynamics. The eddies are not passively advected by the North Equatorial Current; rather, the mean flow changes the propagation characteristics of the eddies. An existing theory that includes meridionally varying, purely zonal mean flow is shown to account for the observed propagation of the Big Island eddies if the zonal variation of the mean flow is considered.
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Warm-core eddies off the eastern coast of Australia are characterized by their isothermal core temperatures. For coastal eddies, core temperatures correlate with the latitude of the eddy at the end of winter. The isothermal temperature is used to identify and track eddies. Eddy positions from 1976 to 1981 have been charted to show patterns in their formation, drift and interactions with other eddies and the East Australian Current. To date, eddies have been named alphabetically in an ad hoc way that has caused confusion because of the unexpected behaviour of some eddies. To overcome this, a systematic way of naming eddies is suggested, which takes into account the eddy's history.
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<p>In ocean research, mesoscale eddies typically are detected through surface signatures based on satellite data. The assumption is that most eddies are surface intensified and have a vertical structure consistent with a surface intensified mode. However, in-situ eddy observations, especially in the tropical oceans, showed that the vertical eddy structure is often more complex than previously assumed (higher baroclinic modes), and a diverse subsurface eddy field is present, which does not show any surface signatures at all. Our objective here is a first step towards a quantification of the occurrence of subsurface relative to surface eddies.&#160;To do this, we use an actively eddying model to compare the subsurface eddy field to its surface signatures in order to be able to estimate which vertical eddy structures prevail and how much of the eddy field is hidden in the subsurface. In addition, the model results are compared against an unprecedented assemblage of observations of subsurface eddies in the tropical oceans.&#160;In a first step we focus on eddies in the model that are detectable at the surface for more than 120 days. We found that around 60 % of the detected eddies have a vertical structure associated with a surface intensified mode as previously assumed which are characterized by a strong surface signature. Around 40 % of the eddy field have a vertical structure associated to a higher baroclinic mode. They are often called &#8220;intrathermocline&#8221; eddies and are characterized by a rather weak surface signature. In a second step we track subsurface eddies (lifetime > 120 days) in the model by identifying density layer thickness anomalies and connect them with possible surface signatures. Around 30 % of the total eddy field of the model, are hidden in the subsurface with no detectable surface signature. In conclusion, our results show that subsurface eddies form a substantial contribution to the total eddy field. Consequently it is difficult to estimate the impact of the eddy field on the ocean when only working with surface based satellite data.</p>
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