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Frontal Zone (frontal + zone)

Distribution by Scientific Domains

Earth and Environmental Science	91%

Selected Abstracts

Differing body size between the autumn and the winter,spring cohorts of neon flying squid (Ommastrephes bartramii) related to the oceanographic regime in the North Pacific: a hypothesis

FISHERIES OCEANOGRAPHY, Issue 5 2004
Taro Ichii
Abstract The neon flying squid (Ommastrephes bartramii), which is the target of an important North Pacific fishery, is comprised of an autumn and winter,spring cohort. During summer, there is a clear separation of mantle length (ML) between the autumn (ML range: 38,46 cm) and the winter,spring cohorts (ML range: 16,28 cm) despite their apparently contiguous hatching periods. We examined oceanic conditions associated with spawning/nursery and northward migration habitats of the two different-sized cohorts. The seasonal meridional movement of the sea surface temperature (SST) range at which spawning is thought to occur (21,25°C) indicates that the spawning ground occurs farther north during autumn (28,34°N) than winter,spring (20,28°N). The autumn spawning ground coincides with the Subtropical Frontal Zone (STFZ), characterized by enhanced productivity in winter because of its close proximity to the Transition Zone Chlorophyll Front (TZCF), which move south to the STFZ from the Subarctic Boundary. Hence this area is thought to become a food-rich nursery ground in winter. The winter,spring spawning ground, on the other hand, coincides with the Subtropical Domain, which is less productive throughout the year. Furthermore, as the TZCF and SST front migrate northward in spring and summer, the autumn cohort has the advantage of being in the SST front and productive area north of the chlorophyll front, whereas the winter,spring cohort remains to the south in a less productive area. Thus, the autumn cohort can utilize a food-rich habitat from winter through summer, which, we hypothesize, causes its members to grow larger than those in the winter,spring cohort in summer. [source]

Incursion and excursion of Antarctic biota: past, present and future

GLOBAL ECOLOGY, Issue 2 2006
D. K. A. Barnes
ABSTRACT Aim, To investigate the major paradigms of intense isolation and little anthropogenic influence around Antarctica and to examine the timings and scales of the modification of the southern polar biota. Location, Antarctica and surrounding regions. Methods, First, mechanisms of and evidence for long-term isolation are reviewed. These include continental drift, the development of a surrounding deep-water channel and the Antarctic Circumpolar Current (ACC). They also include levels of endemism, richness and distinctiveness of assemblages. Secondly, evidence for past and modern opportunities for species transport are investigated. Comparative levels of alien establishments are also examined around the Southern Ocean. Discussion, On a Cenozoic time-scale, it is clear that Gondwana's fragmentation led to increasing geographical isolation of Antarctica and the initiation of the ACC, which restricted biota exchange to low levels while still permitting some movement of biota. On a shorter Quaternary time-scale, the continental ice-sheet, influenced by solar (Milankovitch) cycles, has expanded and contracted periodically, covering and exposing terrestrial and continental shelf habitats. There were probably refugia for organisms during each glacial maxima. It is also likely that new taxa were introduced into Antarctica during cycles of ice sheet and oceanic front movement. The current situation (a glacial minimum) is not ,normal'; full interglacials represent only 10% of the last 430 ka. On short (ecological) time-scales, many natural dispersal processes (airborne, oceanic eddy, rafting and hitch-hiking on migrants) enable the passage of biota to and from Antarctica. In recent years, humans have become influential both directly by transporting organisms and indirectly by increasing survival and establishment prospects via climate change. Main conclusions, Patterns of endemism and alien establishment are very different across taxa, land and sea, and north vs. south of the Polar Frontal Zone. Establishment conditions, as much as transport, are important in limiting alien establishment. Three time-scales emerge as important in the modification of Antarctica's biota. The natural ,interglacial' process of reinvasion of Antarctica is being influenced strongly by humans. [source]

Larval lobster (Homarus americanus) distribution and drift in the vicinity of the Gulf of Maine offshore banks and their probable origins

FISHERIES OCEANOGRAPHY, Issue 2 2005
G. C. HARDING
Abstract Surveys for lobster larvae in offshore waters of the north-eastern Gulf of Maine in 1983, 1987 and 1989 confirm that local hatching occurs mainly at depths <100 m over the banks, including Georges and Browns Banks. Detailed studies in the vicinity of Georges Bank in late July of both 1987 and 1989 indicate that the first and second moult stages were located primarily over the bank whereas stages III and IV lobster were collected both over and off the bank. At times stage IV lobster were more abundant off the bank than over it. The condition of stage III and IV lobster, as measured by a lipid index, was better off than over Georges Bank in 1988 and 1989 indicating a possible physiological advantage to being off the bank. In addition, the higher surface temperatures off Georges Bank would shorten larval development time to settlement. To determine the probable hatch sites of stage IV lobster collected off of Browns Bank in 1983 and off of Georges in 1987 and 1989, a 3-D circulation model of the Gulf of Maine was used to simulate larval lobster drift backwards in time. In all cases, areas off Cape Cod, MA, and off Penobscot Bay, ME were suggested as the source of the larvae, although most of the larval trajectories never reached these near-shore waters that are well-known, larval hatching areas. The model-projected larval release times match most closely the observed inshore hatch off Massachusetts but model uncertainties mean that coastal Maine cannot be ruled out as a source. Georges Bank is also a potential source because the present model does not take into account short-term wind events, off-bank eddy transport or the possibility of directed off-bank larval swimming. Examination of weather records prior to and during our 1988 and 1989 sampling periods indicates that winds were not of sufficient intensity and duration to induce larval transport off Georges Bank. The shedding of eddies from the northern flank of Georges Bank into the Gulf of Maine are a relatively common phenomenon during summer but not enough is known about them to evaluate their contribution to possible cross-bank transport of lobster larvae. Directed larval swimming is another possible source for the stage IV lobster found near Georges Bank. Plankton distributions across the northern frontal zone of Georges Bank in 1988 were used as proxies for the scarce larval lobsters. The more surface distribution of the microplankton, in particular, supports the possibility that wind and eddy events may be important in the transport of stage III and IV lobsters off of Georges Bank. Further studies are needed to evaluate these possible additional sources of advanced stage lobster larvae found off of the offshore banks. [source]

Variability in sea-surface temperature and winds in the tropical south-east Atlantic Ocean and regional rainfall relationships

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 1 2009
J. C. Hermes
Abstract Variability in sea-surface temperature (SST) and winds in the Angola Benguela frontal zone (ABFZ) in the tropical south-east Atlantic Ocean has previously been shown to be important for regional fisheries and for seasonal rainfall anomalies over Angola/Namibia in austral summer and coastal West Africa in boreal summer. This study investigates intraseasonal variability in winds and SST over this region using QuikSCAT and tropical rainfall measuring mission (TRMM) satellite data for 1999,2004. Wavelet analyses reveal periods of relatively strong power in the 20,30 or 30,64 day frequency bands throughout the record but that there is substantial interannual variability in the occurrence of these intraseasonal oscillations. The implications of this variability for seasonal rainfall anomalies during the main rainy seasons in southern Africa (austral summer) and coastal West Africa (boreal summer) are discussed. Copyright © 2008 Royal Meteorological Society [source]

The influence of an upper-level frontal zone on the Mack Lake Wildfire environment

METEOROLOGICAL APPLICATIONS, Issue 2 2007
Tarisa Zimet
Abstract Meteorological assessment of wildland fire danger has traditionally involved the identification of several synoptic weather types empirically determined to influence wildfire spread. Specifically, in the Great Lakes Region, high wildfire danger is often witnessed in association with northwesterly synoptic-scale flow aloft. Such synoptic-scale flow is regularly associated with the development of upper-level frontal zones also known as upper-level jet/front systems, which are often characterised by intrusions of stratospheric air into the troposphere. The notion that upper-frontal development can play an important role in promoting wildfire spread is advanced through interrogation of the output from a fine-scale numerical simulation of a documented explosive wildfire case; the Mack Lake Fire of May 1980. The Mack Lake case was characterised by a developing upper-level front embedded within a shortwave trough in the vicinity of the fire location. The upper-level front originated in northwesterly flow in central Canada as an upper-tropospheric ridge amplified over western North America. A thermally indirect circulation at the jet exit region both contributed to the intensification of the front and was associated with a maximum in quasi-geostrophic descent at mid-levels upstream of the fire region. The subsidence ushered dry air from the middle and upper-troposphere downward along sloping isentropes adiabatically warming and drying it along the way. A well-developed dry air intrusion associated with the operation of these processes extended to nearly the 750 hPa level far downstream from the actual upper-frontal zone supplying the fire environment with dry air that originated in the upper-troposphere/lower stratosphere. The organised subsidence was also responsible for downward advection of high momentum air from within the frontal zone into the fire environment, further influencing the wildfire spread. We conclude that upper-frontal processes, characteristic of northwesterly synoptic-scale flow, are likely a contributing factor to the prevalence of wildfire spread under such synoptic-scale conditions. Copyright © 2007 Royal Meteorological Society [source]

Cloud-resolving model simulations of multiply-banded frontal clouds

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 611 2005
M. Pizzamei
Abstract An idealized two-dimensional cloud-resolving model is used to investigate the formation and temporal evolution of multiply-banded clouds in frontal zones. Radar observations often show both upright and slantwise convection in the circulations associated with such bands. The aim is to examine the interaction between upright and slantwise convection and to determine the mechanisms leading to multiple banding. A warm bubble is used to initiate convection in the frontal zone, which has an initial thermodynamic profile based on observations. Further triggering occurs and banded clouds evolve. The initially upright plumes become tilted due to the so-called ,M adjustment process (upscale development). Observed multiple bands in frontal zones are frequently attributed to the release of conditional symmetric instability (CSI). However, in these simulations, there is no evidence of the release of CSI despite the fragmentation of slantwise bands into multiple layers in the mid-troposphere. Successive triggering of upright convection is instead associated with a spreading cold pool driven by evaporative cooling in the slanted downdraughts. Triggering can occur on both the warm- and cold-air sides of the frontal zone, and is sensitive to the microphysical parametrization used. Copyright © 2005 Royal Meteorological Society. [source]

Convective mixing in a tropopause fold

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 599 2004
H. J. Reid
Abstract We present a case study of the passage of a tropopause fold over the UK behind a cold front, with emphasis on the mixing caused by convection extending into the fold. The event took place on 15,16 January 1999, and was the subject of intensive observations using the Met Office C130 aircraft and the mesosphere,stratosphere,troposphere (MST) radar at Aberystwyth. Here we concentrate on radar and satellite observations during the afternoon of 16 January, when the surface cold front had passed over the UK. A tongue of moist air moved north-eastwards over Wales at 700 hPa at this time, which, because of the very dry air in the fold above, resulted in potential instability. The resulting convection was clearly observed in NOAA satellite images. The MST radar depicted the passage of the cold front and tropopause fold as a layer of high-echo power and vertical wind shear ascending with time. Spectral widths showed the fold to be free of turbulence until 1200 UTC on 16 January, when convection was observed reaching into the frontal zone and generating turbulence. Eddy dissipation and diffusivity rates of 8.6 mW kg,1 and 8.5 m2s,1, respectively, were derived for this event. To place these figures in context, they are compared with corresponding rates derived for sixteen other passages of tropopause folds over the radar, each resulting from shear rather than convective instability. The convective event is found to be comparable to the strongest shear events, and to correspond to moderate turbulence as experienced by an aircraft. This process is of potential importance for atmospheric chemistry because it mixes boundary layer air directly with stratospheric air over a timescale of 1,2 hours. Copyright © 2004 Royal Meteorological Society [source]

A cloud-resolving regional simulation of tropical cyclone formation

ATMOSPHERIC SCIENCE LETTERS, Issue 1 2002
Jordan G. Powers
Abstract The development of Tropical Cyclone Diana (1984) is simulated with a mesoscale model using 1.2 km grid spacing over a regional-scale (>1000 km) domain in the first known experiment of this kind. With only a synoptic-scale disturbance in the initial conditions, the model first develops a mesoscale convective system along a remnant frontal zone, which yields a mesoscale vortex. After a period of quiescence, banded convection organizes about the vortex from isolated, grid-resolved cells, with the system becoming warm-core and intensifying into Tropical Storm Diana. Copyright © 2002 Royal Meteorological Society. [source]

Hydrographic features and seabird foraging in Aleutian Passes

FISHERIES OCEANOGRAPHY, Issue 2005
CAROL LADD
Abstract Strong tidal currents crossing over the abrupt topography of the Aleutian Passes result in regions with high horizontal property gradients. These frontal regions vary with the tidal cycle and form the boundary between vertically mixed and stratified regions. Concentrations of seabirds were associated with convergence zones in the mixed water (MW) and with the front between North Pacific (NP) water and MW. Species that were foraging by picking at prey from the surface were associated with surface convergences that appeared to be associated with Langmuir circulation cells or tidal features (all fulmar aggregations) in the central passes (Samalga, Seguam). In contrast, subsurface foraging puffins and small alcids were mostly observed in areas of turbulent, well-mixed water near the shallow regions of the passes. Short-tailed shearwater flocks that were plunge-diving for prey were associated with the front between the NP water and MW in the passes. On our transects, we observed no significant aggregations of seabirds associated with Bering Sea water or NP water away from the frontal zones. The interaction of strong currents with bathymetric features results in zones of vertical advection, mixing, and surface convergences that make island passes attractive foraging regions for seabirds. Deep passes lacking these features, such as many of the passes in the western Aleutian Archipelago, are not as likely to facilitate trophic transfer to top predators as shallow passes, such as those found in the eastern Aleutian Islands. [source]