Showing papers in "Atmosphere-ocean in 2009"

Internal Wave Observations in the South China Sea: The Role of Rotation and Non-Linearity

Abstract: Observations of internal waves travelling across the deep basin of the South China Sea provide an opportunity for exploring the effects of rotation and non‐linearity on their evolution. Time series measurements using inverted echo‐sounders at three locations illustrate the progressive steepening of the internal tide generated in Luzon Strait and the subsequent development of short non‐linear internal wave trains. Potential mechanisms for internal tide generation are discussed in terms of tidal beam interaction with near‐surface stratification and mode 1 response to flow over a ridge. For transformation of an internal tide under the influence of non‐linearity and rotation, we apply Boyd's (2005) criterion for wave stability in a rotating flow to separate waves dominated by non‐linearity, which can be expected to steepen and break, from waves that are inhibited from breaking due to rotational dispersion of energy into internal inertial gravity waves. Wave breaking in this context refers to the poin...

On tidal resonance in the global ocean and the back‐effect of coastal tides upon open‐ocean tides

Abstract: The resonance of semi‐diurnal tidal elevations is investigated with a forward numerical forced damped global tide model and an analytical model of forced‐damped tides in a deep ocean basin coupled to a shelf The analytical model contains the classical half‐wavelength and quarter‐wavelength resonances in the deep ocean and shelf, respectively, as well as a forcing‐scale dependence which depends on the ratio of the phase speed of open‐ocean gravity waves to that of the astronomical forcing In the analytical model, when the deep ocean and shelf resonate separately at the same frequency, the resonance in the coupled system shifts to frequencies slightly higher and lower than the original frequency, such that a ‘double bump’ is seen in plots of elevation amplitude versus frequency The addition of a shelf to a resonant open ocean tends to reduce open‐ocean tides, especially when the shelf is also near resonance The magnitude of this ‘back‐effect’ is controlled by shelf friction A weakly damped res

Sensitivity of the Statistical DownScaling Model (SDSM) to reanalysis products.

Abstract: Numerous general circulation models (GCMs) have been designed by climate centres to predict future climate. An outstanding issue with the use of GCM output for local applications is the coarse spatial resolution. To produce accurate daily predictions of future climate variables at the regional scale, the Statistical DownScaling Model (SDSM) is a commonly used downscaling technique. The SDSM statistically identifies relationships between large‐scale predictors (i.e., GCM) and local‐scale predictands, using a multiple linear regression model. Reanalyses, such as those produced by the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) and the European Centre for Medium‐range Weather Forecasts (ECMWF), are important components for the structuring of the SDSM as they supply the predictor values for the calibration and validation of the model. It is well known that the reanalysis products contain biases which may subsequently affect the development of dow...

Skill Assessment of Seasonal Hindcasts from the Canadian Historical Forecast Project

Abstract: The performance of seasonal hindcasts produced with four global atmospheric models in the second phase of the Canadian Historical Forecasting Project is evaluated. Deterministic and probabilistic forecast skill assessments are carried out using common verification measures. Several methods of combining multi‐model output to produce deterministic and probabilistic forecasts of near‐surface air temperature, 500 hPa geopotential height, and 700 hPa temperature for zero‐month and one‐month leads are considered. A variance‐based weighting modestly improves the skill of deterministic and probabilistic hindcasts in some cases. A parametric Gaussian probability estimator is superior to a non‐parametric count‐method estimator for producing multi‐model probability forecasts. Statistical adjustment is beneficial for deterministic and probabilistic hindcasts of near‐surface temperature over the ocean but not always over land. Skill improves with the number of different models used for a given total ensemble ...

Extreme Wind Regime Responses to Climate Variability and Change in the Inner South Coast of British Columbia, Canada

Abstract: This study shows how information about climate variability can be valuable to the understanding of wind regime responses and improvement of wind forecasting skill. To this end we demonstrate the use and value of climate information in accurately determining extreme wind recurrences at three locations on the inner south coast of British Columbia (48°‐49°N, 123°W). The methodology is primarily based on approximating a Generalized Pareto Distribution (GPD) to extreme winds in the presence of climate variability covariates. The long‐term hourly wind speed data maintained by the Meteorological Service of Canada are used to evaluate the possible influence of climate variability on extreme wind response. Preliminary results suggest that there are significantly different extreme wind responses to warm and cold El Nino Southern Oscillation (ENSO) modes, with a tendency for high extreme winds to occur during the negative (i.e., cold) ENSO phase.

The Oceanic Variability Spectrum and Transport Trends

Abstract: Oceanic meridional transports evaluated over the width of the Pacific Ocean from altimetric observations become incoherent surprisingly rapidly with meridional separation. Even with 15 years of data, surface slopes show no significant coherence beyond 5° of latitude separation at any frequency. An analysis of the frequency/zonal‐wavenumber spectral density shows a broad continuum of motions at all time and space scales, with significant excess energy along a ‘non‐dispersive’ line extending from the barotropic to the first baroclinic mode Rossby waves. It is speculated that much of that excess energy lies with coupled barotropic and first mode Rossby waves. The statistical significance of apparent oceanic transport trends depends upon the existence of a reliable frequency/wavenumber spectrum for which only a few observational elements currently exist.

Tracing freshwater anomalies through the air‐land‐ocean system: A case study from the Mackenzie river basin and the Beaufort Gyre

Abstract: Mackenzie River discharge was at a record low in water year (WY) 1995 (October 1994 to September 1995), was near average in WY 1996, and was at a record high in WY 1997. The record high discharge in WY 1997, with above average flow each month, was followed by a record high flow in May 1998, then a sharp decline. Through diagnosing these changing flows and their expression in the Beaufort Sea via synthesis of observations and model output, this study provides insight into the nature of the Arctic's freshwater system. The low discharge in WY 1995 manifests negative anomalies in P‐E and precipitation, recycled summer precipitation, and dry surface conditions immediately prior to the water year. The complex hydrograph for WY 1996 reflects a combination of spring soil moisture recharge, buffering by rising lake levels, positive P‐E anomalies in summer, and a massive release of water held in storage by Bennett Dam. The record high discharge in WY 1997 manifests the dual effects of reduced buffering by ...

Climate Trends in a Seasonal Forecasting System

Abstract: Deterministic forecasts produced with the second Historical Forecasting Project (HFP2) multi‐model two‐tier seasonal forecasting system are analyzed in order to assess the presence and importance of long‐term trends on seasonal forecast skill. The results show that trends in land surface temperature, 850 hPa temperature and 500 hPa geopotential height in the National Centers for Environmental Prediction (NCEP) reanalysis data, with which the forecasts are initialized and verified, are much stronger than those in the forecasts. Since biases are removed from the forecast variables, this lack of trend is unlikely to be a consequence of model climate drift. Although the atmospheric initial conditions and oceanic boundary conditions for the forecasts contain greenhouse gas and aerosol forcing information, these forcing mechanisms are not explicitly represented in the atmospheric general circulation models with which the forecasts are made. It is possible, therefore, that long‐term externally forced tr...

Comparing Two Climatologies of the Labrador Sea : Geopotential and Isopycnal

Abstract: Two climatologies, one using an isopycnic approach and the other employing a more classical geopotential approach, are produced for the Labrador Sea region. These differ from existing climatologies through the use of smaller search radii, more data and a carefully chosen depth dependent correction scheme. This results in the preservation of the strong fronts that exist between cold, fresh boundary currents and warmer, more saline interior waters and, in general, less smoothing of features. The waters of the West Greenland Current, the Labrador Current and the interior are well represented, especially Labrador Sea Water and the Deep Western Boundary Current. We consider that our ‘best’ results are produced by the isopycnal climatology. Isopycnal averaging gives more realistic results by reducing artificial mixing of water properties and preserving the baroclinicity of the flow. We estimate the total transport, using the results from the isopycnal climatology in a diagnostic model driven by climato...

An examination of mixed layer sensitivity in the northeast Pacific Ocean from July 2001–July 2005 using the general ocean turbulence model and Argo data

Abstract: The mixed layer depth (MLD) at Ocean Station Papa (OSP) in the northeast Pacific Ocean has been shoaling for the past 50 years, with the shallowest maximum MLDs ever recorded occurring in the winters of 2002–03 and 2003–04. We use the General Ocean Turbulence Model (GOTM) to estimate the MLD and to determine how various atmospheric forcings from 2001–05 at four stations in the northeast Pacific affect the MLD. Three of the stations are in the Alaskan Gyre: OSP (50°N, 145°W) in the south; S16 (49°17'N, 134°40'W) at the southeastern edge; CAG (55°N, 145°W) in the centre; and one, NSG (40°N, 145°W), is south of the Alaskan Gyre. Interpolated Argo temperature and salinity, for both initial conditions and restoring, and atmospheric inputs (heat fluxes and wind stress) from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis were used. Experiments showed that in the winter, the MLD was most sensitive to increased winds at all stations and al...

Mesoscale Wind Climate Modelling in Steep Mountains

Abstract: Although the Mesoscale Community Compressible (MC2) model successfully reproduces the wind climate (for wind energy development purposes) of the Gaspe region, equivalent simulations for the steep moun- tainous southern Yukon have been unsatisfactory. An important part of the problem lies in the provision of suit- able boundary conditions in the lower troposphere. This paper will describe an alternative provision of boundary conditions to the MC2 model based partly on standard National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis statistics, however, with modified lower tropos- pheric conditions based on local radiosonde measurements. The MC2 model is part of the AnemoScope wind energy simulation toolkit which applies statistical-dynamical downscaling of basic large-scale weather situations (i.e., the NCEP/NCAR Reanalysis) to simulate the steady- state wind climate of a complex region. A case study summarized here imposes a typical mean winter tempera- ture inversion on the boundary conditions to reduce downward momentum transfer in the MC2 model over the Whitehorse region. In conjunction with this step, the geostrophic wind at the boundaries is held constant (with height) in speed and direction, based on the (observed) dominant southwesterly winds above the mountaintops. The resulting simulation produces wind directions within the modelled domain that are in much better agreement with the available measurements. However, despite the imposed atmospheric stability, downward transfer of hor- izontal momentum from aloft still appears to exceed that occurring in nature. It is recommended that (in future studies of this type regarding mountain wind climate) the input statistics processed from the NCEP/NCAR Reanalysis be modified by referencing the geostrophic winds to a level above the mountaintops. It is also suggested that converting to a height (z) coordinate system may reduce the erroneous downward momentum transfer found in the present terrain-following grid. RESUME (Traduit par la redaction) Meme si le modele de mesoechelle compressible communautaire (MC2) reproduit adequatement la climatologie du vent (pour les besoins du developpement eolien) dans la region de Gaspe, il n'y est pas parvenu de facon satisfaisante lors de simulations equivalentes dans les regions de montagnes escarpees du sud du Yukon. Une partie importante du probleme reside dans la fourniture de conditions aux limites appropriees dans la basse troposphere. Cet article decrira une autre facon de fournir des conditions aux limites du modele MC2 fondee en partie sur les statistiques normalisees des reanalyses des NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) avec, cependant, des conditions modifiees dans la basse troposphere en fonction des mesures locales obtenues par radiosondes. Le modele MC2 fait partie de la trousse de simulation de l'energie eolienne AnemoScope qui applique une reduction d'echelle statistique-dynamique a des situations meteorologiques a grande echelle (c.-a-d. les reanalyses des NCEP/NCAR) pour simuler la climatologie du vent en regime permanent dans une region au relief accidente. Une etude de cas que l'on resume ici impose une inversion thermique moyenne caracteristique de l'hiver comme conditions aux limites pour reduire le transfert de quantite de mouvement vers le bas dans le modele MC2 au-dessus de la region de Whitehorse. En meme temps, le vent geostrophique aux limites est maintenu constant (avec la hauteur) en vitesse et en direction et base sur les vents dominants du sud-ouest (observes) au-dessus du sommet des montagnes. La simulation resultante produit des directions du vent dans le domaine modelise qui s'accordent beaucoup mieux avec les mesures que nous avons. Cependant, malgre la stabilite atmospherique imposee, le transfert de quantite de mouvement vers le bas a partir des couches en altitude semble encore plus important que dans la realite. Nous recommandons que (dans les futures etudes de ce genre sur la climatologie du vent en regions montagneuses) les statistiques derivees des reanalyses des NCEP/NCAR fournies en entree soient modifiees en definissant les vents geostrophiques par rapport a un niveau situe au-dessus du sommet des montagnes. Nous pensons aussi que l'emploi d'un systeme de coordonnees de hauteur (z) pourrait reduire le transfert errone de quantite de mouvement vers le bas observe avec la grille actuelle, qui epouse le relief.

Chlorophyll Patches Observed during Summer in the Main Stream of the Kuroshio

Abstract: Two transects through the Oyashio‐Kuroshio frontal region were made off the coast of Japan using a towed CTD‐fluorometer (Aquashuttle). These show numerous patches of elevated chlorophyll concentration adjacent to the Kuroshio front. In particular, a few patches are found in the main stream of the Kuroshio. Making use of the Temperature and Salinity (T‐S) properties of the identified patches, the origin of the patches is inferred. It is confirmed that the patches appearing in the Kuroshio are not from the adjacent coastal waters. Their T‐S properties are similar to those in the upstream portion of the Kuroshio. These facts suggest that the elevated level of chlorophyll in the patches arise because of mixing that occurs while the Kuroshio passes over the Izu Ridge. The patches found in the main stream of the Kuroshio are transported to the Kuroshio extension, which is the nursery ground for many pelagic larval fish. These patches, if they appear during the spawning season, may provide food for the...

Gas ventilation of the Saguenay fjord by an energetic tidal front

Abstract: Dissolved noble gas samples were taken during a pilot study in the Saguenay Fjord, Quebec, Canada, in order to determine the contribution of different air‐sea gas exchange mechanisms in an estuary and to assess the contribution of tidal fronts to the aeration of subsurface waters. The noble gases He, Ne, Ar, Kr, and Xe span a large range of molecular diffusivities and solubilities and hence constitute a useful probe of various gas exchange and bubble injection processes. Samples were taken at flood tide upstream and downstream of an energetic tidal front that is generated by a hydraulically controlled flow over a shallow sill at the entrance to the Fjord. The results are interpreted with the help of hydrographic measurements of density and currents along cross‐sill transects describing the physical forcing at the sill. High gas saturations downstream of the sill indicate the aeration of water within the frontal region. An inverse model is used to compare the contribution of bubble injection in th...

Sampling Errors in Estimation of the Small Scales of Monthly Mean Climate

Abstract: Nous examinons les erreurs d'echantillonnage dans l'estimation aux petites echelles des moyennes mensuelles du climat atmospherique moyen, telles que percues dans l'energie cinetique moyenne. Nous analysons les relations existant entre l'energie cinetique moyenne et transitoire a petite echelle dans l'atmosphere et dans les simulations du flux atmospherique. Nous expliquons comment l'estimation de la moyenne depend du nombre de realisations ou de la duree de la periode des donnees. Nous effectuons nos analyses en nous basant a la fois sur un modele barotrope et le modele de circulation generale (MCG) Mark 3 du Commonwealth Scientific and Industrial Research Organisation (CSIRO), en nous concentrant sur les spectres a 500 hPa et moyennes verticalement. Nous presentons les resultats de simulations en janvier perpetuel pour des ensembles de 32, 62 et 1500 membres dans le modele barotrope et pour des integrations de 1, 10 et 60 mois avec le MCG. Nous trouvons qu'avec trop peu de realisations dans l'ensemble ou en etablissant la moyenne pour un seul mois, l'energie cinetique moyenne affiche un spectre faux, avec une loi de puissance similaire a celle de l'energie cinetique transitoire mais avec des valeurs plus faibles par environ deux ordres de grandeur. Pour les plus grands ensembles ou les plus longues periodes de calcul de moyenne, l'energie cinetique moyenne diminue plus rapidement que l'energie cinetique transitoire. Nos resultats menent a la conclusion que les spectres d'energie cinetique moyenne bases sur les donnees d'un seul mois, comme ceux presentes dans les etudes specialisees, et tout recemment par Boer (2003), sont domines par des erreurs d'echantillonnage aux petites echelles.

Ice‐band characteristics of the Antarctic seasonal ice zone observed using MOS MESSR images

Abstract: Ice‐band characteristics for the region off East Queen Maud Land in Antarctica were examined and their relationship with the wind conditions was assessed using a large number of Marine Observation Satellite (MOS) Multispectral Electronic Self Scanning Radiometer (MESSR) images received at Syowa Station during the period 1989–93. Analyses from 43 examples of bands captured from August to December suggest that ice‐band formation and band scale are affected by both wind speed and direction over approximately the preceding four days (defined as the effective wind). Ice‐band width and spacing are significantly correlated with the effective wind speed and the maximum wind speed during that period. The long axis of ice bands tends to be oriented at 70°‐90° (mean of 75°) to the right of the effective wind direction. The band scales decrease from winter (August) to summer (December) with typical band spacing of 4–6 km in winter and 1–2 km in summer. This seems to be primarily due to a decrease in ice floe...

Snow cover validation and sensitivity to CO2 in the UVic ESCM

Abstract: The University of Victoria's (UVic) Earth System Climate model is used to conduct equilibrium atmospheric CO2 sensitivity experiments over the range 200–1600 ppm in order to explore changes in northern hemisphere snow cover and feedbacks on terrestrial surface air temperature (SAT). Simulations of warmer climates predict a retreat of snow cover over northern continents, in a northeasterly direction. The decline in northern hemisphere global snow mass is estimated to reach 33% at 600 ppm and 54% at 1200 ppm. In the most northerly regions, annual mean snow depth increases for simulations with CO2 levels higher than present day. The shift in the latitude of maximum snowfall is estimated to be inversely proportional to the CO2 concentration. The northern hemisphere net shortwave radiation changes are found to be greater over land than over the ocean, suggesting a stronger albedo feedback from changes in terrestrial snow cover than from changes in sea ice. Results also reveal high sensitivity of the s...

Matching of coastal and open ocean wave models in a Mesoscale application over Lake Erie

Abstract: Three widely used wave models, namely, the open ocean wave model (Cycle‐4.5, hereinafter referred to as WAM4.5) and the coastal models, Simulation of WAves Nearshore (Cycle III version 40.31, hereinafter referred to as SWAN) and the K‐model, are applied to Lake Erie to simulate waves at a spatial resolution of about 4 km. The results of a three‐week hindcast study are compared with buoy observations in terms of integrated parameters, one‐dimensional (1‐D) and two‐dimensional (2‐D) energy spectra, scatter plots and statistical analyses of the wave fields. The time development of the 1‐D spectra by the models matches the buoy measurements well. All the wave models tend to overpredict the wave heights and underpredict (particularly the K‐model) the peak period. SWAN performs best for the wave heights and WAM4.5 for the peak periods and is computationally less demanding, whereas the spatial resolution applied to Lake Erie seems to be too coarse for an adequate use of the K‐model. In general, WAM4.5 h...

Numerical simulation of extreme waves during the storm of 20–22 January 2000 using winds generated by the CMC weather prediction model

Abstract: The storm of 20–22 January 2000 over Canada's Atlantic Provinces was an exceptional storm for several reasons, these include extremely high coastal ocean waves, widespread coastal damage due to the storm surge, very strong winds over a large area, an extremely fast deepening rate, and a very low central pressure. It produced unusually large waves which caused significant damage in communities along the south coast of Newfoundland and the eastern shores of Nova Scotia. Bottom scouring was observed around the feet of three mobile offshore oil and gas drilling platforms operating near Sable Island. Using buoy data enhanced with a detailed data set from one of the platforms, this study examines the growth of destructive waves and the performance of two state‐of‐the‐art third generation ocean wave models running in shallow water mode. The wave models perform well in numerically simulating the extreme waves associated with this storm. They correctly predict the growth of wind waves and handle the arriv...

Estimate of the steric contribution to global sea level rise from a comparison of the WOCE one‐time survey with 2006–2008 Argo observations

Abstract: It is well known from observations by altimetric satellites (predominantly TOPEX/Poseidon and Jason‐1) that global sea level is rising. What is less well known is exactly how the observed sea level rise is partitioned between a steric contribution (sea level rising because of changes in ambient temperature and salinity) and a contribution arising from the addition of new water mass to the oceans. Strictly speaking, such a separation is not possible because of the non‐linearity in the equation of state for sea water, but in practice the non‐linearities are sufficiently small to allow this separation as a very good first approximation. A careful comparison of the World Ocean Circulation Experiment (WOCE) one‐time survey with recent observations by the Argo array indicate a steric component to sea level rise of 2.2 mm y–1 between the early 1990s and 2006 to 2008. This is a significantly larger rise rate than previously estimated and, along with recent estimates of melt rate from ice sheets, is in mu...

Application of Canadian weather radar data to plant disease management schemes in southern Ontario.

Abstract: Management schemes based on the mean temperature and duration of wetness periods (dew or rain) have been developed to schedule fungicide applications rationally in a number of high‐value crops. Due to the spatial variability of convective rain events during the growing season in southern Ontario, data from an existing rain‐gauge network is inadequate for delivering advice to the farming community. Therefore, the use of both conventional and Doppler weather radar data to supplement rainfall data was investigated. Hourly measurements of leaf wetness duration and tipping bucket rainfall data were compared to data from the King City and Exeter radars in two disease warning schemes. When used in conjunction with the Penman‐Monteith model to estimate disease indices, the King City radar data were in better agreement with the leaf wetness sensor than the Exeter radar. There was no marked difference between conventional and Doppler data. When both conventional and Doppler radar data were used in combinat...