A review of studies tackling problems on marine stratiform clouds is presented. The strong effect of marine stratocumulus clouds on the top-of-the-atmosphere radiation budget is demonstrated using 5 years of Earth Radiation Budget Experiment (ERBE) data. A relationship between albedo and sea surface temperature (SST) is shown to exist in two areas of the globe that tend to be covered by marine stratocumulus. Albedo increases when SST decreases and vice-versa in both regions when examined on an annual, interannual or spatial basis. The magnitude of the albedo response to a given SST change (1) varies within the regions; (2) differs between the two regions; (3) depends on the type of variability examined; (4) depends on the SST. No useful relations were found between albedo and other meteorological variables. Climatic implications on a global scale arising from the albedo-SST anticorrelation are also discussed.

The Canadian Regional Climate Model (CRCM) has been used to investigate the effects of Gulf of Mexico sea surface temperature (SST) anomalies on the regional climate. Three sets of experiments have been performed, each consisting of a control run and with two different imposed Gulf SST anomalies. The first is a uniform increase or decrease of the SST by 5K over the entire Gulf. The second and third experiments use 5K SST anomalies of smaller extent, characteristic of warm core rings shed off the Loop Current in the Gulf. The experiments are carried out for either 15 or 30 days, and statistics are computed after discarding the first 5 days of the integration. / The response of the SST anomalies are qualitatively similar in the three cases, except the response to the anomaly over the entire Gulf is stronger due to the much larger extent of the anomaly. For a positive SST anomaly, precipitation and moisture over the Gulf and southeastern U.S. both increase. The 1000mb temperature field shows a clear warming over the Gulf and adjacent areas, delineating the imposed SST anomaly. A low level cyclonic circulation forms over the Gulf and southeastern U.S. region, while an anticyclonic circulation develops at the upper levels. The negative SST anomaly experiments show a qualitatively similar response, except it is of opposite sign with a smaller magnitude.

The relationship between Arctic sea ice concentration anomalies, particularly those associated with the "Great Salinity Anomaly" of 1968-1982, and atmospheric circulation anomalies is investigated. Empirical orthogonal function (EOF) analyses are performed on winter and summer sea ice concentrations, sea-level pressure, 500 hPa heights and 850 hPa temperatures: these data cover the Northern Hemisphere north of 45$ sp circ$N during the post-World War II era. Spatial maps of temporal correlation coefficients between EOF 1 of winter sea ice concentration and the atmospheric anomaly fields are calculated. Significant correlations (at 95 and 99% levels) were found to exist between EOF 1 of winter sea ice and the atmospheric anomaly fields at zero lag, and with ice leading by one and one-and-a-half years, and ice lagging by one year. The main emphasis of the thesis is to identify connections between Arctic sea ice and atmospheric circulation anomalies at interannual timescales.

The various modes of atmospheric mass redistribution characterize the principal variations of the general circulation of the atmosphere. Interhemispheric exchanges of atmospheric mass occur with considerable regularity on intraseasonal time-scales. Observational evidence from previous studies indicates that anomalous and persistent regional atmospheric mass distributions (e.g., atmospheric blocking) may often be related to interhemispheric atmospheric mass exchange. / Using the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis, we identify significant events when the northern hemisphere (NH) loses dry atmospheric mass on subseasonal time-scales during the boreal winter from 1968 to 1997. A total of 25 events is found, with a preferred time-scale of 9 days. The linear correlation coefficient between the dry atmospheric mass anomalies for the NH and southern hemisphere (SH) is -0.91 for the 25 significant events, indicating very strong interhemispheric compensation and increasing our confidence in the suitability of this dataset for the study of interhemispheric dry atmospheric mass exchange. / Positive sea-level pressure anomalies are found over northern Eurasia, the North Pacific and the North Atlantic prior to the onset of the composite NH dry atmospheric mass collapse event. Over northern Eurasia the positive atmospheric mass anomaly associated with the building of the Siberian high is found to be a statistically significant precursor to the events. The breakdown of NH dry atmospheric mass occurs in association with the decay of the positive atmospheric mass anomaly in the North Pacific as a cyclone deepens explosively in the Gulf of Alaska. Pressure surges over Southeast Asia and North America, associated with statistically significant positive atmospheric mass anomalies, are mechanisms that act to channel the atmospheric mass equatorward on a rapid time-scale (~4 days). The dry atmospheric mass increase in the SH is manifested as enhanced surface ridging over the South Pacific and South Indian Oceans. / Preferential interhemispheric interaction is found in the region between 100°E and 130°E, and over the central Pacific in the vicinity of the dateline. A prominent channel of southeastward dry atmospheric mass flux, emanating from the Australian continent, combined with a southward channel from the equatorial central Pacific, contribute to the atmospheric mass buildup over the South Pacific. / The role of a Southeast Asian pressure surge was examined for a representative event. A large evacuation of atmospheric mass from northern Eurasia occurs as the atmospheric mass surges equatorward and into the SH. Along the west coast of Australia, a southerly pressure surge extends equatorward and converges with the northerly surge to create a pronounced near equatorial zonal pressure gradient. A low-level westerly wind burst develops in response to this enhanced zonal pressure gradient as part of the onset of an active phase of the Australian summer monsoon. We show that three prominent anticyclonic circulations intensify in the southern hemisphere extratropics, stretching from the South Indian Ocean to the South Pacific, beneath regions of upper tropospheric dry atmospheric mass convergence, originating from the monsoon convection outflow. These anticyclonic circulations are largely responsible for the dry atmospheric mass increase in the SH.

Side-scan sonar imagery, 40 in$ sp3$ sleeve gun and 3.5 kHz high-resolution profiles reveal a dichotomy of the upper Labrador Slope into (i) a low-relief sector off Hudson Strait that is dominated by mass-transport deposits resulting from the direct input of subglacially derived debris, and (ii) a high-relief sector to the south resulting mainly from deposition out of turbid surface plumes, and headward canyon erosion. This dichotomy continues in the Labrador Basin, where the leveed Northwest Atlantic Mid-Ocean Channel (NAMOC) is flanked by a sandy submarine braidplain in the east, which is the basinward extension of the differences on the upper slope. The floodplain west of NAMOC contains extensive debris-flow deposits generated on the Labrador Slope. / The meandering low-sinuosity, low-gradient NAMOC contains three segments: In the upper equilibrium channel, channel morphology is in equilibrium with spill-over from low-velocity, low-density upper portions of turbidity currents flowing in the channel; in the middle modified equilibrium channel, equilibrium morphology is altered by the confluence with tributary channels, and in the lower segment, channel morphology and position is controlled by basement topography (oceanic fracture zones and seamounts). The NAMOC levees interfinger with the braidplain deposits to the east and prograde southeastward in 7-8 packages. Most packages show a strong cross-channel decrease in thickness, and a strong asymmetry between the left and the right levee, as a Coriolis effect. The braidplain east of the NAMOC, which developed simultaneously with and prior to the NAMOC, shows a north to south gradient from laterally very extensive to highly channelized deposits. In the channelized part, channel positions change rapidly and deposits vary from sandy to gravelly. Extensive sheet-like turbidity currents on the braidplain may have been caused by catastrophic outbursts of subglacial lakes during times of ice-stream surges. / Quantitative morphological analysis shows that the NAMOC is characterized by particularly low sinuosities due to very low channel-gradients compared to other submarine channels. The channel displays several features not recognized before in the deep-sea, or not in the same detail; including submarine hanging valleys and chute pools, submarine point-bars within the channel, coarse-grained wash-over fans on the lee-side of the levees, and terraces within the channel. The talweg is variable in depth and meanders within the channel. An up to 80 m deep talweg is present in the distal NAMOC resulting from flow restrictions due to seamounts. Channel morphology as well as grain size of spill-over deposits suggest that the flow tops of turbidity currents in the NAMOC are slow ($<$0.7 m/s) and dilute ($<$12 kg/m$ sp3$). Gravel deposits in the channel require velocities of 6.5-8 m/s for suspension transport, suggesting a strong vertical velocity and density gradient.

We have developed a C-grid primitive equation ocean general circulation model with Cartesian and $ beta$-plane geometry. Temperature is the only state variable. The C-grid gives better results than the B-grid in reproducing the growth rates of linear unstable modes of the Raileigh-Benard equations. A semi-implicit scheme is used to treat the Coriolis term, which is important for efficient integration in coarse resolution large scale modelling studies. A new viscosity term which has a damping effect only on the divergence field associated with gravity waves is also introduced. The model can reproduce sucessfully most of the coarse resolution model results of other studies. The biharmonic and Smagorinsky frictional parameterizations are not as efficient as our scheme in eliminating noise in the vertical velocity field. / This model is used to study the effects of no slip or free slip boundary conditions on the energetics and northward heat transport in the eddy resolving regime. The divergence dissipation term is used only in the subpolar gyre region, where the Rossby radius of deformation is not well resolved. This term has little effect elsewhere in the model domain. The eddy energetics is sensitive to the lateral boundary conditions used. Increasing vertical resolution can increase the basin average and midlatitude free jet energetics, but its effect is much less than that due to different lateral boundary conditions. The northward heat transports by eddies and mean flow are also examined. / The effect of a restoring condition is compared to a zero heat capacity atmospheric model as a surface boundary condition for the eddy resolving model. Two significant differences are found with the use of the zero heat capacity atmospheric model. First, both eddy and mean kinetic energy near the midlatitude free jet are increased. Second, the vertical profiles of standard temperature deviation (eddy available potential energy) become more realistic. / An analysis of the mean advection and eddy convergence terms in the mean momentum equations shows that both enhanced horizontal resolution and the zero heat capacity atmospheric model can increase the midlatitude jets in the surface and deep layers. The eddy momentum convergence in midlatitudes is the dominant ageostrophic contribution to both the mean zonal flow and its variation. The mean advection is consistently less important. The effects of eddies have been further investigated by using the mean vorticity equation. The results again show that the eddy convergence term is the most important ageostrophic term, and can be as important as the geostrophic effect. The mean vorticity equation budget shows a similar sensitivity to the horizontal resolution and zero heat capacity atmospheric model as for the momentum equations.

A zonally averaged coupled atmosphere-ocean model for climate studies is developed. The ocean component is the Wright and Stocker two-dimensional thermohaline circulation (THC) model, and the atmospheric component is a zonally averaged energy-moisture balance model for the atmosphere. Both single- and multi-basin configurations of the model are considered. / The results obtained with this coupled model are compared with those from an ocean-only model that employs mixed boundary conditions. The differences in the steady states of the two models and their linear stability are examined over a wide range of parameters, for both one- and two-basin ocean models. The presence of additional feedbacks between the ocean circulation and the atmosphere and hydrological cycle in the coupled model produces significant differences between the latter and the ocean-only model. The two models generally have different (though similar) equilibria, but, more importantly for the issue of climate change, the variability in the models near similar steady states is quite different. These differences indicate that to perform relevant investigations of long-term climatic variability, a coupled model is necessary. / Next the coupled model with three-ocean basins is applied to last glacial maximum (LGM) conditions. It is found that to achieve realistic results, it is necessary to add a thermodynamic sea ice model into the coupled atmosphere-ocean model. The variability of the LGM conveyor circulation in the coupled ocean-sea ice-atmosphere model is then examined, and the model is subjected to a range of freshwater perturbation experiments. The conveyor state circulation is quite sensitive to the interbasin atmospheric transport of water vapour from the Atlantic to the Pacific. In particular, increasing this transport makes the conveyor state more robust. The LGM model circulation does not exhibit internal century-to-millennial scale variability, nor can the latter be excited by steady freshwater forcing. However, rapid climatic change on a timescale of decades can be generated through transient freshwater forcing of the northern North Atlantic. Perturbations in the ocean circulation are also found to propagate from the Atlantic Ocean to the Pacific Ocean in a few decades. Stochastic, white noise forcing of the model results in a mainly red noise response but also excites a natural mode of THC variability with a timescale of about 150 years.

The interannual variability of the sea surface temperature (SST) in the western South Atlantic Ocean is explored by applying the composite analysis method to a 40-year period of COADS data (1953-1992). Interactions between the ocean and the overlying atmosphere, as well as oceanic processes, are investigated in order to explain the variability in the SST associated with the two converging western boundary currents in this region: the Brazil and the Malvinas Currents. / The pattern of interannual variability of SST displays a dipole-like structure in the vicinity of the confluence of the two currents. That is, the SST fluctuations in both currents are roughly out-of-phase: warm episodes in one current accompany cold episodes in the other. It is suggested that the variability in the oceanic advection of the currents accounts for part of the observed variability in SST in the confluence region, the dipole pattern being associated with the strength of the temperature gradient in the confluence. / The pattern of variability of the air-sea heat exchange associated with the observed SST fluctuations displays a distribution of ocean-to-atmosphere flux anomalies roughly out-of-phase with those of SST. It is concluded that the SST fluctuations associated with the Brazil and Malvinas Currents are related to the variability in both the air-sea heat exchange and the oceanic advection of the currents. / The warm and cold events observed in the currents are associated with distinct atmospheric circulation patterns. Thus, it is suggested that changes in the atmospheric circulation on interannual timescales induce anomalies in the air-sea heat exchange, which have been found to be partly responsible for the variability in SST of the Brazil and Malvinas Currents.

Several numerical experiments are carried out using the Bryan-Cox Ocean General Circulation Model to investigate the variability of the North Atlantic thermohaline circulation under steady, non-zonal, surface forcing and realistic geometry. To this end the annual mean surface forcing fields were derived from the climatological data sets of Levitus (1982), Hellerman and Rosenstein (1983) and, Schmitt et al. (1989). Further, Arctic freshwater flux, an important part of the hydrological cycle within the North Atlantic Deep Water formation region, is taken into account. / It is found that under present-day climatological surface forcing the system may oscillate at interdecadal period. The mechanism driving the oscillations is linked to changes in both the horizontal and vertical extent of convection in the northern "Labrador Sea". The structure of the surface freshwater flux forcing plays a major role in both the initiation and sustenance of the interdecadal oscillations. Allowing for a freshwater flux into the northern region of the "Labrador Sea" inhibits the interdecadal variability. The oscillations, however, appear, relatively insensitive to Arctic fresh water transport into the "Greenland Sea". / A detailed three-dimensional discussion of the physics behind the interdecadal oscillations is presented.

Using a dataset of weekly ice cover in the Gulf of St. Lawrence, the intraseasonal and interannual variabilities of the mean ice cover fraction were investigated over the Gulf and six of its subregions for the 1963-90 period. Climatological seasonal cycles were calculated by averaging the weekly ice cover fraction means over the sampling period. The interannual variability was investigated using seasonal and monthly means. The major findings are the positive trend of the seasonal means and the prominence of variability at the interdecadal scale (12-15 yr). / Relationships with runoff, monthly surface air temperature, monthly air circulation, autumnal water salinity and temperature profiles, and particular climatic events are investigated to determine the major causes of the variability. / From the original 83-cell grid data, the monthly-averaged icefields of the Gulf were calculated and their maps generated for the months of January, February, March, April and May for every sample year.