Wind Reversal Research Articles

The Downward Influence of Stratospheric Sudden Warmings*

Abstract The coupling between the stratosphere and the troposphere following two major stratospheric sudden warmings is studied in the Canadian Middle Atmosphere Model using a nudging technique by which the zonal-mean evolution of the reference sudden warmings are artificially induced in an ~100-member ensemble spun off from a control simulation. Both reference warmings are taken from a freely running integration of the model. One event is a displacement, the other is a split, and both are followed by extended recoveries in the lower stratosphere. The methodology permits a statistically robust study of their influence on the troposphere below. The nudged ensembles exhibit a tropospheric annular mode response closely analogous to that seen in observations, confirming the downward influence of sudden warmings on the troposphere in a comprehensive model. This tropospheric response coincides more closely with the lower-stratospheric annular mode anomalies than with the midstratospheric wind reversal. In addition to the expected synoptic-scale eddy feedback, the planetary-scale eddies also reinforce the tropospheric wind changes, apparently responding directly to the stratospheric anomalies. Furthermore, despite the zonal symmetry of the stratospheric perturbation, a highly zonally asymmetric near-surface response is produced, corresponding to a strongly negative phase of the North Atlantic Oscillation with a much weaker response over the Pacific basin that matches composites of sudden warmings from the Interim ECMWF Re-Analysis (ERA-Interim). Phase 5 of the Coupled Model Intercomparison Project models exhibit a similar response, though in most models the response’s magnitude is underrepresented.

Observations of stratospheric sudden warmings in Earth rotation variations

AbstractStratospheric sudden warmings (SSWs) are extreme events in the polar stratosphere that are both caused by and have effects on the tropospheric flow. This means that SSWs are associated with changes in the angular momentum of the atmosphere, both before and after their onset. Because these angular momentum changes are transferred to the solid Earth, they can be observed in the rate of the Earth's rotation and the wobble of its rotational pole. By comparing observed Earth rotation variations to reanalysis data, we find that an anomaly in the orientation of the Earth's rotational pole, up to 4 times as large as the annual polar wobble, typically precedes SSWs by 20–40 days. The polar motion signal is due to pressure anomalies that are typically seen before SSW events and represents a new type of observable that may aid in the prediction of SSWs. A decline in the length of day is also seen, on average, near the time of the SSW wind reversal and is found to be due to anomalous easterly winds generated in the tropical troposphere around this time, though the structure and timing of this signal seems to vary widely from event to event.

Impact of atmospheric boundary layer depth variability and wind reversal on the diurnal variability of aerosol concentration at a valley site

The development of the atmospheric boundary layer (ABL) plays a key role in affecting the variability of atmospheric constituents such as aerosols, greenhouse gases, water vapor, and ozone. In general, the concentration of any tracers within the ABL varies due to the changes in the mixing volume (i.e. ABL depth). In this study, we investigate the impact on the near-surface aerosol concentration in a valley site of 1) the boundary layer dilution due to vertical mixing and 2) changes in the wind patterns. We use a data set obtained during a 10-day field campaign in which a number of remote sensing and in-situ instruments were deployed, including a ground-based aerosol lidar system for monitoring of the ABL top height (zi), a particle counter to determine the number concentration of aerosol particles at eight different size ranges, and tower-based standard meteorological instruments.Results show a clearly visible decreasing trend of the mean daytime zi from 2900m AGL (above ground level) to 2200m AGL during a three-day period which resulted in increased near-surface pollutant concentrations. An inverse relationship exists between the zi and the fine fraction (0.3–0.7μm) accumulation mode particles (AMP) on some days due to the dilution effect in a well-mixed ABL. These days are characterized by the absence of daytime upvalley winds and the presence of northwesterly synoptic-driven winds. In contrast, on the days with an onset of an upvalley wind circulation after the morning transition, the wind-driven local transport mechanism outweighs the ABL-dilution effect in determining the variability of AMP concentration. The interplay between the ABL depth evolution and the onset of the upvalley wind during the morning transition period significantly governs the air quality in a valley and could be an important component in the studies of mountain meteorology and air quality.

Tropical response to stratospheric sudden warmings and its modulation by the QBO

Major Stratospheric Sudden Warmings (SSWs) are characterized by a reversal of the zonal mean zonal wind and an anomalous warming in the polar stratosphere that proceeds downward to the lower stratosphere. In the tropical stratosphere, a downward propagating cooling is observed. However, the strong modulation of tropical winds and temperatures by the quasi-biennial oscillation (QBO) renders accurate characterization of the tropical response to SSWs challenging. A novel metric based on temperature variations relative to the central date of the SSW using ERA-Interim data is presented. It filters most of the temperature structure related to the phase of the QBO and provides proper characterization of the SSW cooling amplitude and downward propagation tropical signal. Using this new metric, a large SSW-related cooling is detected in the tropical upper stratosphere that occurs almost simultaneously with the polar cap warming. The tropical cooling weakens as it propagates downward, reaching the lower stratosphere in a few days. Substantial differences are found in the response to SSWs depending on the QBO phase. Similar to what is observed in the polar stratosphere, tropical SSW-associated temperatures persist longer during the west QBO phase at levels above about 40 hPa, suggesting that the signal is mainly controlled by changes in the residual mean meridional circulation associated with SSWs. Conversely, in the lower stratosphere, around 50–70 hPa, enhanced cooling occurs only during QBO east phase. This behavior seems to be driven by anomalous subtropical wave breaking related to changes in the zero-wind line position with the QBO phase.

Circulation changes in the stratosphere and mesosphere during the stratospheric sudden warming event in January 2009

In this study, general circulation changes are investigated during the stratospheric sudden warming event in January 2009 by the use of Aura Earth Observing System Microwave Limb Sounder observations to show dynamical coupling of stratosphere and mesosphere. It is revealed that the reversal from westerly winds to easterly winds occurred earlier in the mesosphere in comparison with the stratopause level and suggested that the earlier reversal in the mesosphere may be caused by large-scale waves formed in the lower mesosphere due to barotropic and/or baroclinic instability there. Such instability is considered to be brought about by the intensification of the westerly jet with double maxima in the polar night stratosphere and subtropical mesosphere before the warming occurrence. Furthermore, it is shown that an independent meridional circulation cell might be formed by the earlier reversal of zonal winds in the mesosphere.

Larval dispersal and population connectivity among a network of marine reserves

AbstractA major challenge in marine ecology is describing patterns of larval dispersal and population connectivity, as well as their underlying processes. We used a biophysical model to simulate dispersal of eastern oyster, Crassostrea virginica, larvae and connectivity among a network of 10 no‐take reserves in a shallow, wind‐driven estuary to assess the relative importance of spawning location, spawning date, larval behavior, larval mortality, and adult reproductive output to predicted dispersal and connectivity patterns. The location (i.e., natal reserve) and date of spawning relative to physical processes, particularly frequency of wind reversals, were the dominant drivers of dispersal and connectivity patterns. To a lesser extent, larval behavior (i.e., 3D vertical advection and ontogenetic depth regulation) and mortality modified dispersal and connectivity, whereas spatiotemporal variability in adult reproductive output was of minimal importance. Over a 21‐day larval duration, mean dispersal distance of passive surface particles ranged from 5 to 40 km. Reserves were too small (1 km2) relative to mean dispersal distances to promote extensive local retention (median 2%) and spaced too far apart (typically ~50 km) to promote extensive inter‐reserve connectivity (median 2%). Limited connectivity and local retention may preclude the network from being self‐sustainable, thereby limiting its long‐term conservation and management benefits. In reserve systems characterized by limited connectivity, management efforts should focus on increasing connectivity by increasing the number or size of reserves to realize the benefits of improved adult demographics within reserves.

On the Role of Planetary-Scale Waves in the Abrupt Seasonal Transition of the Northern Hemisphere General Circulation

Abstract The role of planetary-scale waves in the abrupt seasonal transition of the Northern Hemisphere (NH) general circulation is studied. In reanalysis data, the winter-to-summer transition involves the growth of planetary-scale wave latent heat and momentum transports in the region of monsoons and anticyclones that dominate over the zonal-mean transport beginning in midspring. The wave-dominated regime coincides with an abrupt northward expansion of the cross-equatorial circulation and reversal of the trade winds. In the upper troposphere, the transition coincides with the growth of cross-equatorial planetary-scale wave momentum transport and a poleward shift of subplanetary-scale wave transport and jet stream. The dynamics of the seasonal transition are captured by idealized aquaplanet model simulations with a prescribed subtropical planetary-scale wave sea surface temperature (SST) perturbation. The SST perturbation generates subtropical planetary-scale wave streamfunction variance and transport in the lower and upper troposphere consistent with quasigeostrophic theory. Beyond a threshold SST, a transition of the zonal-mean circulation occurs, which coincides with a localized reversal of absolute vorticity in the NH tropical upper troposphere. The transition is abrupt in the lower troposphere because of the quadratic dependence of the wave transport on the SST perturbation and involves seasonal-time-scale feedbacks between the wave and zonal-mean flow in the upper troposphere, including cross-equatorial wave propagation. The zonal-mean vertical and meridional flows associated with the circulation response are in balance with the planetary-scale wave momentum and latent heat meridional flux divergences. The results highlight the leading-order role of monsoon–anticyclone transport in the seasonal transition, including its impact on the meridional extent of the Hadley and Ferrel cells. They can also be used to explain why the transition is less abrupt in the Southern Hemisphere.

Exchanges of nutrients and chlorophyll a through two inlets of Ria Formosa, South of Portugal, during coastal upwelling events

The Ria Formosa is a shallow and multi-inlet coastal lagoon system in southern Portugal, with six permanent connections to the Atlantic Ocean. The western sector of Ria Formosa comprises three of the main inlets, Ancão, Faro-Olhão and Armona, which contribute ~90% of the total tidal prism of the lagoon. Of those three inlets, Ancão and Faro-Olhão supply the minimum and maximum contribution for these exchanges, respectively. Four field surveys were conducted at these two inlets during the upwelling season (from spring to autumn) to determine the influence of upwelling process upon the hydrographic characteristics in spring tidal conditions: i) spring 2009 (Ancão inlet), ii) autumn 2011 (both inlets) and iii) spring 2012 (Faro-Olhão inlet). Water samples were collected hourly (at three levels), along complete semi-diurnal tidal cycles, to determine chlorophyll a (chl a) and nutrient concentrations. At the same time, the water velocity was measured along the cross section to estimate the tidal prism, nutrients and chl a transports. Ancão inlet contributes a much lower volume to the total tidal prisms than Faro-Olhão inlet. The temporal data variability reveals that chl a and nutrient dynamics through these inlets depends not only on the tidal influence and characteristics of the boundary waters, but also on the intensity of the atmospheric and oceanic forcings acting on the adjacent coastal zone. During pulses of coastal upwelling events, more evident in spring season (mainly in April 2009 and May 2012), the two inlets imported chl a and phosphate. In contrast, in the absence of upwelling conditions, due to wind reversals and/or long periods of wind relaxation, both inlets exported nitrate and phosphate, fertilizing the coastal ocean. However, the Ria Formosa is a highly complex hydrodynamic system, and hydrographic temporal differences can be expected at different scales as a result of changes in: i) morphological configuration of the natural inlets, ii) interconnectivity of channels, and iii) the seasonal meteorological and oceanographic conditions.

Stratospheric Polar Vortex Splitting in December 2009

The 2009–10 Arctic stratospheric winter, in comparison with other recent winters, is mainly characterized by a major Sudden Stratospheric Warming (SSW) in late January associated with planetary wavenumber 1. This event led to a large increase in the temperature of the polar stratosphere and to the reversal of the zonal wind. Unlike other major SSW events in recent winters, after the major SSW in January 2010 the westerlies and polar vortex did not recover to their pre-SSW strength until the springtime transition. As a result, the depletion of the ozone layer inside the polar vortex over the entire winter was relatively small over the past 20 years. The other distinguishing feature of the 2010 winter was the splitting of the stratospheric polar vortex into two lobes in December. The vortex splitting was accompanied by an increase in the temperature of the polar stratosphere and a weakening of the westerlies but with no reversal. The splitting occurred when, in addition to the high-pressure system over northeastern Eurasia and the northern Pacific Ocean, the tropospheric anticyclone over Europe amplified and extended to the lower stratosphere. Analysis of wave activity in the extratropical troposphere revealed that two Rossby wave trains propagated eastward to the North Atlantic several days prior to the vortex splitting. The first wave train propagated from the subtropics and mid-latitudes of the eastern Pacific Ocean over North America and the second one propagated from the northern Pacific Ocean. These wave trains contributed to an intensification of the tropospheric anticyclone over Europe and to the splitting of the stratospheric polar vortex.

Effects of meteorological conditions on the concentration and dispersion of an accidental release of H2S in Canada

This study aims to determine the effects of the land's meteorological conditions on the dispersion of an accidental release of H2S using the CALPro software. The three Canadian cities or towns of Edmonton, Yarmouth and Whitehorse, which are all of different meteorological conditions, were chosen as the domains of study. Hourly geophysical, surface and upper air meteorological data were used with CALMET to model the wind field of the three domains for the modeling period of March 11, 2012 from 00h00 to 23h00 LST. Individual 5-h modeling periods where the wind field showed the most significant variations were chosen for each region of study. CALPUFF was used to model the dispersion effects of an accidental release of H2S from a single point source due to an accidental vessel puncture using time-varying emission data modified to suit each region's modeling period. Despite the wind reversal encountered in Edmonton, its relatively flat terrain allowed H2S to disperse outwards, causing concentrations to accumulate lower than the other two regions but still to sever levels and a much greater population. Differences between the effect of land and sea breeze on H2S dispersion in Yarmouth's coastal region caused concentrations to accumulate higher than the other two regions and to life threatening levels around the source. The mountainous terrain of Whitehorse shaped the plume trajectory, causing H2S concentrations to accumulate to levels that can cause irreversible health effects at various times and locations. Results show that each area's meteorological conditions will have different impacts on dispersion.

Hydrodynamic characterization of the bottom boundary layer in a coastal upwelling system (Ría de Vigo, NW Spain)

The hydrodynamics of the bottom boundary layer (BBL) in the Ría de Vigo (NW Iberian Peninsula) are studied for the first time, identifying their possible forcing factors and filling a previous lack of knowledge about the coastal upwelling system of NW Iberian Peninsula. For tackling this subject, high resolution time series of bottom currents by means of a downwards looking Acoustic Doppler Current Profiler (3m above the bottom) were recorded in the inner part of the Ría de Vigo covering the four seasons of the climate year 2004–2005.Our results point to a coupling among bottom residual currents, shelf winds and runoff supporting previous works not directly focused on BBL. Most of the time, 64.5%, 75.4% and 61.6% for residual, tidal and total currents respectively, the bottom currents respond to a logarithmic profile being possible to apply the law of the wall and estimate shear stress.Shear stress variability is controlled by the frequency of wind reversals and modulated by runoff and thermal stratification. Rapid shifts of wind direction favors the generation of shear stress. Intense summer stratification or strong winter mixing of the water column reduces shear stress levels. Runoff by increasing the variability of the velocity pulse direction can favor the development of shear stress. Bottom shear stress reaches levels that are strong enough (~5% of the time in autumn) to re-suspend fine cohesive sediments. This fact comes out to support previous biogeochemistry studies suggesting the occurrence of intense resuspension processes during the autumn season.

Mesoscale and Nutrient Conditions Associated with the Massive 2008 Cochlodinium polykrikoides Bloom in the Sea of Oman/Arabian Gulf

Cochlodinium polykrikoides formed large blooms in the coastal waters of Oman from October 2008 through mid-January 2009, and satellite images from Aqua-MODIS and region-wide reports suggest that this bloom was found throughout the Arabian Gulf and Sea of Oman for more than 10 months. The unusual occurrence of this species appears to have supplanted the more regularly occurring bloom species, Noctiluca scintillans, in 2008–2009. For the first 2 weeks of the coastal Omani bloom, C. polykrikoides abundance was near monospecific proportions, with cell densities ranging from 4.6 × 103 to 9 × 106 cells L−1 and very high levels of chlorophyll a (78.0 μg L−1) were also recorded. The regional progression of the bloom likely began with stronger than normal upwelling along the Iranian and northern Omani coasts during the southwest monsoon in late summer, followed by discharge of unusually warm coastal plume water along the coast of Oman with the reversal of monsoonal winds in late October. The occurrence and persistence of high densities of C. polykrikoides in Oman coastal water were also significantly influenced by an elevated nutrient load and warmer than normal temperatures. Concentrations of nutrients, especially NH4 +, urea, PO4 3−, and organic nitrogen and phosphorus, were manyfold higher than observed in the year prior or since. These findings suggest that mesoscale features were important in bloom dynamics more regionally, but locally the bloom was sustained by nutrient enrichment supplemented by its mixotrophic capabilities.

The impact of planetary waves on the latitudinal displacement of sudden stratospheric warmings

Abstract. The Northern Hemispheric winter is disturbed by large scale variability mainly caused by Planetary Waves (PWs), which interact with the mean flow and thus result in Sudden Stratospheric Warmings (SSWs). The effects of a SSW on the middle atmosphere are an increase of stratospheric and a simultaneous decrease of mesospheric temperature as well as a wind reversal to westward wind from the mesosphere to the stratosphere. In most cases these disturbances are strongest at polar latitudes, get weaker toward the south and vanish at mid-latitudes around 50° to 60° N as for example during the winter 2005/06. However, other events like in 2009, 2010 and 2012 show a similar or even stronger westward wind at mid- than at polar latitudes either in the mesosphere or in the stratosphere during the SSW. This study uses local meteor and MF-radar measurements, global satellite observations from the Microwave Limb Sounder (MLS) and assimilated model data from MERRA (Modern-ERA Retrospective analysis for research and Applications). We compare differences in the latitudinal structure of the zonal wind, temperature and PW activity between a "normal" event, where the event in 2006 was chosen representatively, and the latitudinal displaced events in 2009, 2010 and 2012. A continuous westward wind band between the pole and 20° N is observed during the displaced events. Furthermore, distinctive temperature differences at mid-latitudes occur before the displaced warmings compared to 2006 as well as a southward extended stratospheric warming afterwards. These differences between the normal SSW in 2006 and the displaced events in 2009, 2010 and 2012 are linked to an increased PW activity between 30° N and 50° N and the changed stationary wave flux in the stratosphere around the displaced events compared to 2006.

Is the trend in TEJ reversing over the Indian subcontinent?

AbstractIndian Summer Monsoon (ISM) is mainly characterized by seasonal wind reversal in low level jet stream and tropical easterly jet (TEJ) among several other elements of monsoon systems. TEJ is observed in general between 100 and 150 hPa during June–September over the Indian region and its strength is directly related to the monsoon rainfall. In the context of changing climate, large reduction in its extent and weakening of its strength were reported. Using high resolution measurements, we report here the observation of a sharp strengthening of the TEJ during the recent warmest decade (2001–2010), reaching its 1970s value. We also show that this change is reflected in the tropical cyclone systems and finally on the precipitation patterns over the Indian region as they are interlinked. We attribute this unusual change partly to the change in the circulation due to the tropospheric warming and lower stratospheric ozone recovery.

Waves in the Red Sea: Response to monsoonal and mountain gap winds

An unstructured grid, phase-averaged wave model forced with winds from a high resolution atmospheric model is used to evaluate wind wave conditions in the Red Sea over an approximately 2-year period. The Red Sea lies in a narrow rift valley, and the steep topography surrounding the basin steers the dominant wind patterns and consequently the wave climate. At large scales, the model results indicated that the primary seasonal variability in waves was due to the monsoonal wind reversal. During the winter, monsoon winds from the southeast generated waves with mean significant wave heights in excess of 2m and mean periods of 8s in the southern Red Sea, while in the northern part of the basin waves were smaller, shorter period, and from northwest. The zone of convergence of winds and waves typically occurred around 19–20°N, but the location varied between 15 and 21.5°N. During the summer, waves were generally smaller and from the northwest over most of the basin. While the seasonal winds oriented along the axis of the Red Sea drove much of the variability in the waves, the maximum wave heights in the simulations were not due to the monsoonal winds but instead were generated by localized mountain wind jets oriented across the basin (roughly east–west). During the summer, a mountain wind jet from the Tokar Gap enhanced the waves in the region of 18 and 20°N, with monthly mean wave heights exceeding 2m and maximum wave heights of 14m during a period when the rest of the Red Sea was relatively calm. Smaller mountain gap wind jets along the northeast coast created large waves during the fall and winter, with a series of jets providing a dominant source of wave energy during these periods. Evaluation of the wave model results against observations from a buoy and satellites found that the spatial resolution of the wind model significantly affected the quality of the wave model results. Wind forcing from a 10-km grid produced higher skills for waves than winds from a 30-km grid, largely due to under-prediction of the mean wind speed and wave height with the coarser grid. The 30-km grid did not resolve the mountain gap wind jets, and thus predicted lower wave heights in the central Red Sea during the summer and along the northeast coast in the winter.

The structure of the mesosphere during sudden stratospheric warmings in a global circulation model

Mesospheric coolings and wind reversals during sudden stratospheric warmings are studied in a 570 day permanent January simulation with the Kühlungsborn Mechanistic Circulation Model (KMCM). The model is run at a high resolution in combination with an advanced parameterization of turbulence, allowing for an explicit description of gravity‐wave effects. Self‐generated major sudden stratospheric warmings are quantified with benchmark tests of Charlton and Polvani (2007) and are found to be nearly realistic. Extension of the tests into the mesosphere gives rise to a benchmark diagram for the entire middle atmosphere, showing that the mesospheric cooling is robust while the mesospheric wind response is variable. The same features appear in a composite constructed from five stratospheric warming events. The zonal‐mean structures indicate an old stratopause descending with about 6 km per day during about 5 days before the central date. About 10 days after the central date, an elevated new stratopause develops in the upper mesosphere and subsequently progresses downward, which is driven by westward gravity waves. Temporal characteristics are deduced from 7 day smoothed daily time series from representative stratospheric and mesospheric zonal‐mean quantities. A correlation analysis shows that the temperature signals are anticorrelated, while the wind signals are uncorrelated. The begin, peak, and end times of the events are diagnosed with two objective methods. Both methods reveal nearly equal‐time appearance of stratospheric warming and wind deceleration (within 2 days), followed by mesospheric cooling with a delay of less than 5 days. This rules out mesospheric precursors of sudden stratospheric warmings in our simulation.

A climatology of frozen‐in anticyclones in the spring arctic stratosphere over the period 1960–2011

During springtime, following the stratospheric final warming, intrusions from low latitudes can become trapped at polar latitudes in long‐lived anticyclones. Such “frozen‐in” anticyclones (FrIACs) have occasionally been observed to persist as late as August, advected by summer easterlies.In this study, the high‐resolution advection contour model MIMOSA is used to advect a pseudo‐potential vorticity tracer. The model is driven by ERA‐40 and the ERA‐Interim reanalyses over the period 1960–2011. We first identify a remarkable FrIAC event in spring 2011. In addition, we developed a method to detect the characteristic size of low‐latitude intrusions into the polar region at the time of the spring transition, over the period 1960–2011. Years are classified as either Type‐A when the intrusions are small or as Type‐B when intrusions are large, potentially evolving into FrIACs. For a FrIAC to occur, we require an additional criterion based on the in‐phase character of the core of the intrusions and the anticyclone.During the 52 analyzed years, 9 events have been identified: 1 in the 1960s, 1 in the 1980s, 2 in the 1990s, and 5 from 2002. FrIAC are predominantly long‐lived intrusions, which occur in association with abrupt and early reversal to summer easterlies with a large heat flux pulse around the date of this wind reversal. Finally, the results are discussed in a climatological context.

A climatology of elevated stratopause events in the whole atmosphere community climate model

Elevated stratopause (ES) events occurring during Northern Hemisphere winter are identified in four climate simulations of the period 1953–2005 made with the Whole Atmosphere Community Climate Model (WACCM). We find 68 ES events in 212 winters. These events are found in winters when the middle atmosphere is disturbed and there are zonal wind reversals in the stratosphere at high latitudes. These disturbances can be associated with both major and minor stratospheric sudden warming events (SSWs). The ES events occur under conditions where the stratospheric jet, the gravity wave forcing, and the residual circulation remain reversed longer than in those winters where an SSW occurs without an ES. We compare ES events with the type of SSW (vortex splitting and vortex displacement) and find that 68% of ES events form after vortex splitting events. We also present a climatology of ES events based on NASA's Modern‐Era Retrospective Analysis for Research and Applications reanalysis data from 1979 to 2012 and compare it to the model results. WACCM composites of major SSW and ES also show enhanced Eliassen‐Palm flux divergences in the upper mesosphere after the stratospheric warming, immediately before the formation of an ES. However, the formation of an ES in WACCM is due primarily to adiabatic heating from gravity wave‐driven downwelling, which follows the reestablishment of the eastward jet in the upper stratosphere. We find nine winters where an ES forms in the absence of any significant planetary wave activity in the upper mesosphere and illustrate one such event.

The age and the flushing time of the Great Barrier Reef waters

A numerical model of the Great Barrier Reef (GBR) was verified using water current data from twenty sites, and applied to estimate the flushing time and age of waters. These timescales were calculated under different wind and oceanic inflow conditions. The age of oceanic waters intruding in the GBR was estimated to be between 3 and 5 months on leaving the GBR, depending on the location; the largest residence time prevailed in the southern GBR matrix where the age was the highest within the high density reef matrix, indicating veering of the mean currents around the GBR matrix. The flushing time depends on the size of the domain, and was estimated to be 67 days for the whole central GBR. For the flushing of coastal waters the wind had two effects. Firstly, it increased the flushing times by generating wind-driven currents, transporting some water back to the source; this process is comparable to that of an estuary, where water that leaves the estuary at ebb tide may return at flood tide, a process parameterized by the return coefficient. The return coefficient in the GBR due to wind reversals may be as large as 50% at the timescale of the wind. Secondly, in the southern and central regions of the GBR, the southeasterly tradewind deflected the southward flowing oceanic inflow seaward, away from the inner shelf and towards the outer shelf, making room for a wind-driven current of opposite direction on the inner shelf. Thus the intrusion of oceanic water in the GBR depends on the wind over the GBR. The veering of the mean currents around the GBR reef matrix and the wind over the shelf influencing the oceanic circulation demonstrate small scales (the GBR shelf) influencing the large scale oceanic circulation.

Fate of river Tiber discharge investigated through numerical simulation and satellite monitoring

Abstract. The aim of this study was to determine the dispersion of passive pollutants associated with the Tiber discharge into the Tyrrhenian Sea using numerical marine dispersion models and satellite data. Numerical results obtained in the simulation of realistic discharge episodes were compared with the corresponding evolution of the spatial distributions of MODIS diffuse light attenuation coefficient at 490 nm (K490), and the results were discussed with reference to the local climate and the seasonal sub-regional circulation regime. The numerical model used for the simulation of the sub-tidal circulation was a Mediterranean sub-regional scale implementation of the Princeton Ocean Model (POM), nested in the large-scale Mediterranean Forecasting System. The nesting method enabled the model to be applied to almost every area in the Mediterranean Sea and also to be used in seasons for which imposing climatological boundary conditions would have been questionable. Dynamical effects on coastal circulation and on water density due to the Tiber discharge were additionally accounted for in the oceanographic model by implementing the river estuary as a point source of a buoyant jet. A Lagrangian particle dispersion model fed with the POM current fields was then run in order to reproduce the effect of the turbulent transport of passive tracers mixed in the plume with the coastal flow. Two significant episodes of river discharge in both winter and summer conditions were discussed in this paper. It was found that the winter regime was characterized by the presence of a strong coastal jet flowing with the ambient current. In summer the prevailing wind regime induced coastal downwelling conditions, which tended to confine the riverine waters close to the shore. In such conditions sudden wind reversals due to local weather perturbations, causing moderate local upwelling, proved to be the only effective way to disperse the tracers offshore, moving the plume from the coast and detaching large pools of freshwater.