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Abstract
Solutions of the Stommel equation are presented which take the form of free waves in the interior of the ocean basin, driven by convergences and divergences in coastal transports brought about by the variation of the longshore wind stress around the coast. These waves have been termed “coastal waves” and result from the beta-effect in the presence of a uniform frictional process, such as the loss of momentum by the ocean to the atmosphere. The coastal waves which typically transport 5 Sv extend significantly into the interior of the ocean from all boundaries except the western boundary, and also drive a westward nonlinear current, appear to be an important feature in the general circulation. A good example of a quasi-steady wavefield induced by intermediate-scale coastline geography occurs in the Flinders Current off the south coast of Australia.
The western boundary current, of course, compensates for imbalances in interior transport. Its structure results from forcing, both by this transport and the longshore wind stress on the western coast itself, which produces no net transport outside of the boundary layer.
Abstract
Solutions of the Stommel equation are presented which take the form of free waves in the interior of the ocean basin, driven by convergences and divergences in coastal transports brought about by the variation of the longshore wind stress around the coast. These waves have been termed “coastal waves” and result from the beta-effect in the presence of a uniform frictional process, such as the loss of momentum by the ocean to the atmosphere. The coastal waves which typically transport 5 Sv extend significantly into the interior of the ocean from all boundaries except the western boundary, and also drive a westward nonlinear current, appear to be an important feature in the general circulation. A good example of a quasi-steady wavefield induced by intermediate-scale coastline geography occurs in the Flinders Current off the south coast of Australia.
The western boundary current, of course, compensates for imbalances in interior transport. Its structure results from forcing, both by this transport and the longshore wind stress on the western coast itself, which produces no net transport outside of the boundary layer.
Abstract
A series of two-layer quasigeostrophic solutions for the ocean circulation driven by a steady wind in a channel with topography and in a flat bottom rectangular basin are presented in which the atmosphere and ocean are inertially coupled through the surface stress relation. The only other frictional processes are biharmonic lateral friction (under free-slip boundary conditions) and topographic form stress; there is no bottom friction involved. The results indicate that realistic momentum balances can be obtained on this physical basis. Two types of solutions are obtained, which are called (i) the I series in which the inertial coupling relation is applied directly in the earth reference frame with no current averaging and almost steady stream fields occur and (ii) the S series in which the inertial coupling relation is applied for long current averaging periods, of the order 100 days, rather than instantaneously. The solutions for the longer current averaging periods produce vigorous eddy fields, but their time-mean is very similar to the corresponding solution with no current averaging. Surface Stokes drift streamfields are also generated by the inertial coupling mechanism. Some implications of the results for general circulation modeling are discussed.
Abstract
A series of two-layer quasigeostrophic solutions for the ocean circulation driven by a steady wind in a channel with topography and in a flat bottom rectangular basin are presented in which the atmosphere and ocean are inertially coupled through the surface stress relation. The only other frictional processes are biharmonic lateral friction (under free-slip boundary conditions) and topographic form stress; there is no bottom friction involved. The results indicate that realistic momentum balances can be obtained on this physical basis. Two types of solutions are obtained, which are called (i) the I series in which the inertial coupling relation is applied directly in the earth reference frame with no current averaging and almost steady stream fields occur and (ii) the S series in which the inertial coupling relation is applied for long current averaging periods, of the order 100 days, rather than instantaneously. The solutions for the longer current averaging periods produce vigorous eddy fields, but their time-mean is very similar to the corresponding solution with no current averaging. Surface Stokes drift streamfields are also generated by the inertial coupling mechanism. Some implications of the results for general circulation modeling are discussed.
Abstract
In March 2001, a hybrid low pressure system, unofficially referred to as Donald (or the Duck), developed in the Tasman Sea under tropical–extratropical influence, making landfall on the southeastern Australian coast. Here, it is shown that atmospheric blocking in the Tasman Sea produced a split in the subtropical jet, allowing persistent weak vertical wind shear to manifest in the vicinity of the developing low. It is hypothesized that this occurred through sustained injections of potential vorticity originating from higher latitudes. Hours before landfall near Byron Bay, the system developed an eye with a short-lived warm core at 500 hPa. Cyclone tracking revealed an erratic track before the system decayed and produced heavy rains and flash flooding.
A three-dimensional air parcel backward-trajectory scheme showed that the air parcels arriving in the vicinity of the mature cyclone originated from tropical sources at lower levels and from the far extratropics at higher levels, confirming the hybrid characteristics of this cyclone. A high-resolution (0.15°) nested simulation showed that recent improvements in the assimilation scheme used by the Australian models allowed for accurately simulating the system’s trajectory and landfall, which was not possible at the time of the event. Compared to the first South Atlantic hurricane of March 2004, the large-scale precursors were similar; however, the Duck was exposed to injections of upper-level potential vorticity and favorable surface heat fluxes for a shorter period of time, resulting in it achieving partial tropical transition only hours prior to landfall.
Abstract
In March 2001, a hybrid low pressure system, unofficially referred to as Donald (or the Duck), developed in the Tasman Sea under tropical–extratropical influence, making landfall on the southeastern Australian coast. Here, it is shown that atmospheric blocking in the Tasman Sea produced a split in the subtropical jet, allowing persistent weak vertical wind shear to manifest in the vicinity of the developing low. It is hypothesized that this occurred through sustained injections of potential vorticity originating from higher latitudes. Hours before landfall near Byron Bay, the system developed an eye with a short-lived warm core at 500 hPa. Cyclone tracking revealed an erratic track before the system decayed and produced heavy rains and flash flooding.
A three-dimensional air parcel backward-trajectory scheme showed that the air parcels arriving in the vicinity of the mature cyclone originated from tropical sources at lower levels and from the far extratropics at higher levels, confirming the hybrid characteristics of this cyclone. A high-resolution (0.15°) nested simulation showed that recent improvements in the assimilation scheme used by the Australian models allowed for accurately simulating the system’s trajectory and landfall, which was not possible at the time of the event. Compared to the first South Atlantic hurricane of March 2004, the large-scale precursors were similar; however, the Duck was exposed to injections of upper-level potential vorticity and favorable surface heat fluxes for a shorter period of time, resulting in it achieving partial tropical transition only hours prior to landfall.
Abstract
Climate variability is often studied in terms of fluctuations with respect to the mean state, whereas the dependence between the mean and variability is rarely discussed. Here, a new climate metric is proposed to measure the relationship between means and standard deviations of annual surface temperature computed over nonoverlapping 100-yr segments. This metric is analyzed based on equilibrium simulations of the Max Planck Institute Earth System Model (MPI-ESM): the last-millennium climate (800–1799), the future climate projection following the A1B scenario (2100–99), and the 3100-yr unforced control simulation. A linear relationship is globally observed in the control simulation and is thus termed intrinsic climate variability, which is most pronounced in the tropical region with negative regression slopes over the Pacific warm pool and positive slopes in the eastern tropical Pacific. It relates to asymmetric changes in temperature extremes and associates fluctuating climate means with increase or decrease in intensity and occurrence of both El Niño and La Niña events. In the future scenario period, the linear regression slopes largely retain their spatial structure with appreciable changes in intensity and geographical locations. Since intrinsic climate variability describes the internal rhythm of the climate system, it may serve as guidance for interpreting climate variability and climate change signals in the past and the future.
Abstract
Climate variability is often studied in terms of fluctuations with respect to the mean state, whereas the dependence between the mean and variability is rarely discussed. Here, a new climate metric is proposed to measure the relationship between means and standard deviations of annual surface temperature computed over nonoverlapping 100-yr segments. This metric is analyzed based on equilibrium simulations of the Max Planck Institute Earth System Model (MPI-ESM): the last-millennium climate (800–1799), the future climate projection following the A1B scenario (2100–99), and the 3100-yr unforced control simulation. A linear relationship is globally observed in the control simulation and is thus termed intrinsic climate variability, which is most pronounced in the tropical region with negative regression slopes over the Pacific warm pool and positive slopes in the eastern tropical Pacific. It relates to asymmetric changes in temperature extremes and associates fluctuating climate means with increase or decrease in intensity and occurrence of both El Niño and La Niña events. In the future scenario period, the linear regression slopes largely retain their spatial structure with appreciable changes in intensity and geographical locations. Since intrinsic climate variability describes the internal rhythm of the climate system, it may serve as guidance for interpreting climate variability and climate change signals in the past and the future.
Abstract
Presented here is an objective approach to identify, characterize, and track Southern Hemisphere mobile fronts in hemispheric analyses of relatively modest resolution, such as reanalyses. Among the principles in its design were that it should be based on broadscale synoptic considerations and be as simple and easily understood as possible. The resulting Eulerian scheme has been applied to the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA)–Interim and a climatology of frontal characteristics, at both the 10-m and 850-hPa levels, derived for the period 1 January 1989–28 February 2009. The knowledge of the character of these features is central to understanding weather and climate over the hemisphere.
In both summer and winter the latitude belt 40°–60°S hosts the highest frequency of frontal points, but there are significant zonal asymmetries within this band. The climatology reveals that the longest fronts are in the Indian Ocean where mean lengths exceed 2000 km. The mean frontal intensity over the hemisphere tends to be greater at 850 hPa than at 10 m, and greater in winter than in summer. The frontal intensity also shows its maximum in the Indian Ocean. In the mean, the meridional tilt of these fronts is northwest–southeast over much of the midlatitudes and subtropics, and increases with latitude toward the equator. The tilts are of overwhelmingly opposite sign in the coastal Antarctic and subantarctic regions.
Broadly speaking, the number of fronts and their mean length and mean intensity exhibit maxima in winter in the midlatitudes (30°–50°S), but show a sizeable semiannual variation (maxima in fall and spring) during the year at higher latitudes.
Abstract
Presented here is an objective approach to identify, characterize, and track Southern Hemisphere mobile fronts in hemispheric analyses of relatively modest resolution, such as reanalyses. Among the principles in its design were that it should be based on broadscale synoptic considerations and be as simple and easily understood as possible. The resulting Eulerian scheme has been applied to the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA)–Interim and a climatology of frontal characteristics, at both the 10-m and 850-hPa levels, derived for the period 1 January 1989–28 February 2009. The knowledge of the character of these features is central to understanding weather and climate over the hemisphere.
In both summer and winter the latitude belt 40°–60°S hosts the highest frequency of frontal points, but there are significant zonal asymmetries within this band. The climatology reveals that the longest fronts are in the Indian Ocean where mean lengths exceed 2000 km. The mean frontal intensity over the hemisphere tends to be greater at 850 hPa than at 10 m, and greater in winter than in summer. The frontal intensity also shows its maximum in the Indian Ocean. In the mean, the meridional tilt of these fronts is northwest–southeast over much of the midlatitudes and subtropics, and increases with latitude toward the equator. The tilts are of overwhelmingly opposite sign in the coastal Antarctic and subantarctic regions.
Broadly speaking, the number of fronts and their mean length and mean intensity exhibit maxima in winter in the midlatitudes (30°–50°S), but show a sizeable semiannual variation (maxima in fall and spring) during the year at higher latitudes.
Abstract
The authors present an analytical climate model, which has the features that (i) the atmosphere is a simple oscillator for all periods ≤1 year, (ii) the ocean stores heat, (iii) the ocean exchanges momentum with the atmosphere, and (iv) random forcing exists due to atmospheric thermodynamics and oceanic dynamics. The piecewise analytical integration of coupled linear equations for sea temperature, air-sea temperature difference, and air-sea velocity difference generates experimental climates. The exchange parameters of the algorithm, except for the exchange coefficient for heat with the deep ocean, am calibrated to the observed climate using the annual cycle, and random forcing is applied over intervals of one year. The atmospheric random forcing leads to bounded random walks, the extent of which increases as the exchange coefficient with the deep ocean decreases, and the oceanic random forcing generates a stationary response. It is found that the observed statistics of the global temperature series can be reproduced by either a relatively large heat exchange coefficient with the deep ocean and little oceanic variability or a smaller exchange coefficient with a larger oceanic variability. Plausible exchange coefficient values imply random walk lengths of at least a century-long timescale.
Abstract
The authors present an analytical climate model, which has the features that (i) the atmosphere is a simple oscillator for all periods ≤1 year, (ii) the ocean stores heat, (iii) the ocean exchanges momentum with the atmosphere, and (iv) random forcing exists due to atmospheric thermodynamics and oceanic dynamics. The piecewise analytical integration of coupled linear equations for sea temperature, air-sea temperature difference, and air-sea velocity difference generates experimental climates. The exchange parameters of the algorithm, except for the exchange coefficient for heat with the deep ocean, am calibrated to the observed climate using the annual cycle, and random forcing is applied over intervals of one year. The atmospheric random forcing leads to bounded random walks, the extent of which increases as the exchange coefficient with the deep ocean decreases, and the oceanic random forcing generates a stationary response. It is found that the observed statistics of the global temperature series can be reproduced by either a relatively large heat exchange coefficient with the deep ocean and little oceanic variability or a smaller exchange coefficient with a larger oceanic variability. Plausible exchange coefficient values imply random walk lengths of at least a century-long timescale.
Abstract
The Australian marine research, industry, and stakeholder community has recently undertaken an extensive collaborative process to identify the highest national priorities for wind-waves research. This was undertaken under the auspices of the Forum for Operational Oceanography Surface Waves Working Group. The main steps in the process were first, soliciting possible research questions from the community via an online survey; second, reviewing the questions at a face-to-face workshop; and third, online ranking of the research questions by individuals. This process resulted in 15 identified priorities, covering research activities and the development of infrastructure. The top five priorities are 1) enhanced and updated nearshore and coastal bathymetry; 2) improved understanding of extreme sea states; 3) maintain and enhance the in situ buoy network; 4) improved data access and sharing; and 5) ensemble and probabilistic wave modeling and forecasting. In this paper, each of the 15 priorities is discussed in detail, providing insight into why each priority is important, and the current state of the art, both nationally and internationally, where relevant. While this process has been driven by Australian needs, it is likely that the results will be relevant to other marine-focused nations.
Abstract
The Australian marine research, industry, and stakeholder community has recently undertaken an extensive collaborative process to identify the highest national priorities for wind-waves research. This was undertaken under the auspices of the Forum for Operational Oceanography Surface Waves Working Group. The main steps in the process were first, soliciting possible research questions from the community via an online survey; second, reviewing the questions at a face-to-face workshop; and third, online ranking of the research questions by individuals. This process resulted in 15 identified priorities, covering research activities and the development of infrastructure. The top five priorities are 1) enhanced and updated nearshore and coastal bathymetry; 2) improved understanding of extreme sea states; 3) maintain and enhance the in situ buoy network; 4) improved data access and sharing; and 5) ensemble and probabilistic wave modeling and forecasting. In this paper, each of the 15 priorities is discussed in detail, providing insight into why each priority is important, and the current state of the art, both nationally and internationally, where relevant. While this process has been driven by Australian needs, it is likely that the results will be relevant to other marine-focused nations.
