Critical Role of Continental Slopes in Halocline and Eddy Dynamics of the Ekman-Driven Beaufort Gyre

The Beaufort Gyre (BG) is a large‐scale bathymetrically constrained circulation driven by a surface Ekman convergence that creates a bowl‐shaped halocline and stores a significant portion of the Arctic Ocean's freshwater. Theoretical studies suggest that in the gyre interior, the halocline is equilibrated by a balance between Ekman pumping and counteracting mesoscale eddy transport energized by baroclinic instability. However, the strongest anticyclonic flows occur over steep continental slopes, and, despite bathymetric slopes being known to influence baroclinic instability, their large‐scale impacts on BG halocline remain unexplored. Here we use an idealized eddy‐resolving BG model to demonstrate that the existence of continental slopes dramatically affects key gyre characteristics leading to deeper halocline, stronger anticyclonic circulation, and prolonged equilibration. Over continental slopes, the magnitude of the Eulerian mean circulation is dramatically reduced due to the Ekman overturning being compensated by the eddy momentum‐driven overturning. The eddy thickness flux overturning associated with lateral salt transport is also weakened over the slopes, indicating a reduction of eddy thickness diffusivity despite the isopycnal slopes being largest there. Using a theoretical halocline model, we demonstrate that it is the localized reduction in eddy diffusivity over continental slopes that is critical in explaining the halocline deepening and prolonged equilibration time. Our results emphasize the need for observational studies of eddy overturning dynamics over continental slopes and the development of slope‐aware mesoscale eddy parameterizations for low‐resolution climate models

Critical Role of Continental Slopes in Halocline and Eddy Dynamics of the Ekman-Driven Beaufort Gyre

The Beaufort Gyre (BG) is a large‐scale bathymetrically constrained circulation driven by a surface Ekman convergence that creates a bowl‐shaped halocline and stores a significant portion of the Arctic Ocean's freshwater. Theoretical studies suggest that in the gyre interior, the halocline is equilibrated by a balance between Ekman pumping and counteracting mesoscale eddy transport energized by baroclinic instability. However, the strongest anticyclonic flows occur over steep continental slopes, and, despite bathymetric slopes being known to influence baroclinic instability, their large‐scale impacts on BG halocline remain unexplored. Here we use an idealized eddy‐resolving BG model to demonstrate that the existence of continental slopes dramatically affects key gyre characteristics leading to deeper halocline, stronger anticyclonic circulation, and prolonged equilibration. Over continental slopes, the magnitude of the Eulerian mean circulation is dramatically reduced due to the Ekman overturning being compensated by the eddy momentum‐driven overturning. The eddy thickness flux overturning associated with lateral salt transport is also weakened over the slopes, indicating a reduction of eddy thickness diffusivity despite the isopycnal slopes being largest there. Using a theoretical halocline model, we demonstrate that it is the localized reduction in eddy diffusivity over continental slopes that is critical in explaining the halocline deepening and prolonged equilibration time. Our results emphasize the need for observational studies of eddy overturning dynamics over continental slopes and the development of slope‐aware mesoscale eddy parameterizations for low‐resolution climate models

Rossby wave instability and apparent phase speeds in large ocean basins

Author Posting. © American Meteorological Society, 2007. 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 37 (2007): 1177-1191, doi:10.1175/jpo3054.1.The stability of baroclinic Rossby waves in large ocean basins is examined, and the quasigeostrophic (QG) results of LaCasce and Pedlosky are generalized. First, stability equations are derived for perturbations on large-scale waves, using the two-layer shallow-water system. These equations resemble the QG stability equations, except that they retain the variation of the internal deformation radius with latitude. The equations are solved numerically for different initial conditions through eigenmode calculations and time stepping. The fastest-growing eigenmodes are intensified at high latitudes, and the slower-growing modes are intensified at lower latitudes. All of the modes have meridional scales and growth times that are comparable to the deformation radius in the latitude range where the eigenmode is intensified. This is what one would expect if one had applied QG theory in latitude bands. The evolution of large-scale waves was then simulated using the Regional Ocean Modeling System primitive equation model. The results are consistent with the theoretical predictions, with deformation-scale perturbations growing at rates inversely proportional to the local deformation radius. The waves succumb to the perturbations at the mid- to high latitudes, but are able to cross the basin at low latitudes before doing so. Also, the barotropic waves produced by the instability propagate faster than the baroclinic long-wave speed, which may explain the discrepancy in speeds noted by Chelton and Schlax.PEI was supported by a postdoctoral grant from the Norwegian Research Council, JHL was supported under the Norwegian NOCLIM II program, and JP was partly supported by NSF OCE 0451086

Observed and modeled surface eddy heat fluxes in the eastern Nordic Seas

Large-scale budget calculations and numerical model process studies suggest that lateral eddy heat fluxes have an important cooling effect on the Norwegian Atlantic Current (NwAC) as it flows through the Nordic Seas. But observational estimates of such fluxes have been lacking. Here, wintertime surface eddy heat fluxes in the eastern Nordic Seas are estimated from surface drifter data, satellite data and an eddy-permitting numerical model. Maps of the eddy heat flux divergence suggest advective cooling along the path of the NwAC. Integrating the flux divergence over temperature classes yields consistent estimates for the three data sets; the waters warmer than about 6°C are cooled while the cooler waters are warmed. Similar integrations over bottom depth classes show that regions shallower than about 2000 m are cooled while deeper regions are warmed. Finally, integrating the flux divergence along the core of the NwAC suggests that the highest eddy-induced heat loss at the surface is along the steepest part of the continental slope, east of the Lofoten Basin. The model fields indicate that cooling of the current by lateral eddy fluxes is comparable to or larger than the local heat loss to the atmosphere

Ar-Ar Analysis of Chelyabinsk: Evidence for a Recent Impact

The Chelyabinsk meteorite is an LL5 ordinary chondrite that fell as a spectacular fireball on February 15th, 2013, over the Ural region in Russia. The meteoroid exploded at an altitude of 25-30 km, producing shockwaves that broke windowpanes in Chelyabinsk and surrounding areas, injuring some 1500 people. Analyses of the samples show that the meteorite underwent moderate shock metamorphism (stage S4; 25-35 GPa) [1]. Most of the samples have a fusion crust ranging from ~0.1-1mm thick, and roughly a third of the samples were composed of a dark fine-grained impact melt with chondrule fragments which were targeted for chronometry. A Pb-Pb age obtained by [2] of a shock-darkened and potentially melted sample of Chelyabinsk is reported as 4538.3 +/- 2.1 Ma, while a U-Pb study [3] gave an upper concordia intercept of 4454 +/- 67 Ma and a lower intercept of 585 +/- 390. Galimov et al. 2013 [1] suggest the Sm-Nd system records a recent impact event [~290 Ma] that may represent separation from the parent body, while the Rb-Sr isotopic system is disturbed and does not give any definitive isochron. In order to better understand its history, we have performed 40Ar-39Ar analysis on multiple splits of two Chelyabinsk samples; clast- rich MB020f,2 and melt-rich MB020f,5. The term "clast-rich" lithology is meant to indicate a mechanical mixture of highly shock-darkened and less shocked components, both with some shock melt veining

Tilførselsprogrammet 2012. Overvåking av tilførsler og miljøtilstand i Barentshavet og Lofotenområdet.

Denne rapporten beskriver beregninger av tilførsel av olje, miljøgifter og radioaktive stoffer fra åtte kilder til fire regioner i Lofotenområdet og den norske delen av Barentshavet. Dataene fra de ulike kildene brukes til å beregne konsentrasjoner av miljøgifter i vannsøylen i hele det definerte havområdet (3 dimensjoner) og beregne transport av Hg, PCB-153 og BaP inn og ut av hver av de fire regionene. Denne transportfluksen er stor sammenlignet med tilførslene og vil skiftevis være en netto kilde eller netto sluk til hver region. Utenom netto eksport/import fra tilstøtende havområder er hovedtrekkene at tilførslene domineres av relativt stort nedfall fra atmosfæren som modifiseres av sjøis og balanseres av nedbrytning og sedimentering. Transport fra sjø til atmosfære er ikke inkludert i nåværende utgave av modellen. Det er relativt liten tilførsel av miljøfarlige stoffer til disse fire regionene. Med få unntak gir tilførsel fra luft det største bidrag av kvikksølv, bly, kadmium, krom, arsen, PCB (PCB-153) og PAH (benzo[a]pyren) til alle regionene. Unntak gjelder bidrag fra land for krom i midt-Norge og bidrag fra havbunn for bly og krom fra mer sentrale deler av Norskehavet og for krom og arsen fra mer sentrale deler av Nordsjøen. Skipstrafikk dominerer mht. tilførsel av olje. Tilførsel fra luft gir det største bidraget av radionuklidene plutonium-239+240 i alle regionene og av strontium-90 i Barentshavet og den østlige delen av Norskehavet. Det er fortsatt store kunnskapsmangler og usikkerheter både i datagrunnlag og i estimatene av tilførsler. Det er spesielt viktig å få forbedrede tall for tilførsler til havområdene via luft og forbedring av de marine transport- og spredningsmodellene

Calibrating rates of early Cambrian evolution

An explosive episode of biological diversification occurred near the beginning of the Cambrian period. Evolutionary rates in the Cambrian have been difficult to quantify accurately because of a lack of high-precision ages. Currently, uranium-lead zircon geochronology is the most powerful method for dating rocks of Cambrian age. Uranium-lead zircon data from lower Cambrian rocks located in northeast Siberia indicate that the Cambrian period began at ~544 million years ago and that its oldest (Manykaian) stage lasted no less than 10 million years. Other data indicate that the Tommotian and Atdabanian stages together lasted only 5 to 10 million years. The resulting compression of Early Cambrian time accentuates the rapidity of both the faunal diversification and subsequent Cambrian turnover