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Ngar-Cheung Lau

Abstract

The principal modes of month-to-month variability of the wintertime storm tracks over the North Pacific and North Atlantic are identified by empirical orthogonal function analysis of the root-mean-square statistics of bandpass (2.5–6 day) filtered geopotential height data for 19 yr. One of the two leading modes depicts fluctuations in the level of synoptic-scale activity without any noticeable spatial displacement of the storm track axes, whereas the other mode is associated with meridional shifts of the storm tracks from their time-averaged positions. Higher order modes are indicative of diversion or truncation of cyclone tracks in particular geographical regions.

It is demonstrated that the leading storm track modes are linked to some of the best-known monthly averaged teleconnection patterns. The dipolar western Pacific and western Atlantic patterns for the monthly mean flow are seen to be accompanied by marked changes in the intensity of the storm tracks over the western oceans, whereas the more wave-like Pacific/North American and eastern Atlantic teleconnection patterns are coincident with north–south displacements of the storm track axes over the eastern oceans. The representative synoptic scenarios for various storm track modes are portrayed using composite charts. These patterns illustrate the strong modulation of the trajectory of weather systems by the intensity and steering action of the monthly averaged flow field, so that the storm tracks are preferentially located at and slightly downstream of the quasi-stationary troughs.

The shape and propagation of the synoptic scale eddies along the changing storm tracks, as well as the barotropic interactions between these disturbances and the monthly mew flow, are diagnosed using composite patterns of extended Eliassen-Palm vectors and eddy-induced geopotential tendencies at 300 mb. It is seen that the synoptic-scale fluctuations are typically crescent-shaped, and sometimes undergo noticeable deformation when they encounter quasi-stationary ridges. In the upper troposphere, enhanced eddy activity is accompanied locally by eastward acceleration, as well as by positive geopotential tendency immediately to the south, and negative geopotential tendency to the north, and vice versa. The distributions of eddy-induced geopotential tendency for individual storm track modes indicate a near inphase relationship between the synoptic scale barotropic forcing and the quasi-stationary flow pattern at 300 mb. The characteristic time scale for this forcing is approximately 7–10 days.

The characteristic circulations at sea level associated with various storm track modes are examined using composite charts of the sea level pressure field. Some of these composites resemble the patterns associated the North Pacific and North Atlantic Oscillations.

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Ngar-Cheung Lau

A review is given of the processes contributing to variability of the atmosphere-ocean system on interannual timescales. Particular emphasis is placed on the relationships between midlatitude atmospheric fluctuations and sea surface temperature (SST) anomalies in various geographical sites. Various hypotheses are tested using output from a coordinated set of general circulation model experiments, which are subjected to time-varying SST forcing observed during 1946-88 in different parts of the world's oceans. It is demonstrated that tropical Pacific SST fluctuations associated with El Nino-Southern Oscillation (ENSO) episodes produce a strong extratropical response in the model atmosphere, whereas the atmospheric signal associated with midlatitude SST anomalies is less robust. Analysis of a 100-yr control experiment, which is conducted in the absence of any interannual SST forcing, indicates that a substantial fraction of the simulated atmospheric variability may be attributed to internal dynamical processes alone.

The observed coexistence of tropical ENSO events with SST anomalies in the extratropical North Pacific is successfully reproduced by forcing the model atmosphere with tropical Pacific SST variations and allowing the atmospheric perturbations thus generated to drive a simple ocean mixed layer model inserted at ocean grid points outside the tropical Pacific. This simulation affirms the role of the atmospheric circulation as a "bridge" linking SST changes in different parts of the world's oceans. The midlatitude model responses in the presence of local air-sea interactions are noticeably stronger than the corresponding responses without such interactions. This finding is indicative of the positive feedback processes inherent in extratropical air-sea coupling.

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Ngar-Cheung Lau

Abstract

The three-dimensional structure of temporal mean, variance and covariance fields of the Northern Hemisphere wintertime circulation is documented by making use of twice-daily hemispheric analyses at 10 pressure levels (1000–100 mb) for 11 winters (1965–76). The Northern Hemisphere is partitioned into seven longitudinal sectors which correspond, respectively, to 1) the entrance regions of the two principal jet streams; 2) the two major oceanic storm tracks (areas with pronounced geopotential height variability in the 2.5–6 day period range); 3) the western portions of the North American and Eurasian land masses; and 4) the central part of the Asian continent. Meridional cross sections of various seasonal mean statistics averaged over meridians comprising these individual sectors are displayed. The fields presented in this manner include 1) time-averaged zonal wind and temperature; 2) temporal variance statistics of zonal and meridional wind components, temperature, and geopotential height; 3) meridional fluxes of heat, geopotential energy, westerly momentum and potential vorticity by transient eddies; and 4) poleward component of the time averaged ageostrophic flow. The hemispheric distributions of temperature statistics, meridional heat flux and mean potential vorticity in the vicinity of the tropopause (200 and 300 mb) are also shown.

The structure of eddy statistics and the implied energetics in the oceanic storm tracks are in good agreement with the characteristics of developing baroclinic waves. The strongest variability in geopotential height and wind occurs at 300 mb. The geopotential height fluctuations in the 2.5–6 day period range exhibit a secondary maximum at the ground level, so that the 850 mb level corresponds to a relative minimum. Thermal variability is strongest in the middle troposphere and just above the tropopause. The primary maximum for the heat flux statistics is located at 850 mb, with a weaker secondary maximum at 200 mb. The distribution of eddy momentum flux is characterized by strong convergence of westerly momentum into the storm track axis.

The western parts of the two major land masses exhibit quite distinct dynamical features in middle latitudes. The upper troposphere is characterized by equatorward heat fluxes, whereas the direction of heat transport in the lower stratosphere is poleward. The momentum fluxes in these regions are strong and poleward at the jet stream level. This configuration of heat and momentum fluxes is consistent with observed strong poleward fluxes of potential vorticity in the vicinity of the tropopause. Strong supergeostrophic winds in the eddies over the western continents are shown to induce conversion from eddy kinetic energy to eddy available potential energy in these regions.

The relative importance of the mean flow forcing by the transient eddies is discussed. It is seen that the jet streams in the entrance regions are maintained by strong poleward ageostrophic flows at the tropopause level, while strong equatorward ageostrophic flows over the oceanic storm tracks act to decelerate the jets at these longitudes. The intensity of these local meridional circulations is about an order of magnitude stronger than that of the zonally averaged Ferrel cell in midlatitudes. The local forcing of the mean zonal flow by the Coriolis acceleration accompanying these time-mean ageostrophic flows is shown to be much larger than the forcing due to eddy momentum flux convergence.

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Ngar-Cheung Lau

Abstract

The hemispheric distributions of wintertime circulation statistics derived from the forecast fields of vertical motion are presented. The dominant features in the pattern for time-mean vertical velocity are consistent with the existence of thermally direct meridional circulations over the entrance regions of the principal jet streams, and thermally indirect circulations over the jet exit regions. Rising motions in the transient disturbances are seen to display positive temporal correlations with temperature and geopotential height over the major oceanic storm track regions. On the other hand, the western portion of the continents and the adjacent oceanic areas are characterized by downward eddy transports of geopotential energy at 850 and 500 mb, as well as much reduced temporal correlations between the vertical motion and temperature fields.

The vertical phase structure of the transient disturbances at various geographical locations is studied by performing a cross-spectral analysis of the time series of geopotential height fields at 850, 500 and 250 mb. The local geopotential height fluctuations at different pressure levels are strongly coherent. Over the sites characterized by enhanced development of transient waves, geopotential height perturbations of synoptic temporal scales are seen to lag by about 60° (1/6 cycle) between the tropopause and 850 mb levels. The corresponding phase lag is reduced to about 25° over the western portion of the continents, and the disturbances acquire a barotropic character at these longitudes.

The results of a detailed diagnosis of the local, time-averaged budgets of time-mean and transient eddy kinetic energy at 300 mb are discussed. The kinetic energy of the intensified time-mean flow at the jet stream cores is primarily maintained by the local, time-averaged ageostrophic circulations, which dominate over the effects due to eddy-mean flow interactions. The energy generated in these source regions is transported by the time-mean flow to the jet exit regions, where the thermally indirect circulations function as local sinks of mean kinetic energy. Analogously, eddy kinetic energy at the jet stream level is generated by ageostrophic motions in the transient disturbances over the western oceans, it is then advected to the western portion of the continents by the time-averaged flow, and is eventually dissipated by the super-geostrophic flow in the eddies at those longitudes.

The regional character of the transient eddy statistics presented in this and earlier papers is interpreted in the light of the results from a recent modeling study by Simmons and Hoskins (1978) on the life cycle of nonlinear baroclinic waves.

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Ngar-Cheung Lau

Abstract

The spatial structure and temporal characteristics of prominent anomalies occurring in a 15-year simulation with a GFDL spectral general circulation model are examined using empirical orthogonal functions, teleconnection patterns, composite charts, lagged correlation functions and frequency spectra.

Despite the absence of any nonseasonal perturbation in the prescribed forcing such as sea surface temperature, insolation and cloud cover, the simulated circulation exhibits an appreciable degree of temporal variability on monthly time scales. The standing oscillation in the Northern Hemisphere winter which accounts for the largest fraction of this variance has a coherent three-dimensional structure. In the middle and upper troposphere, this preferred mode of oscillation is characterized by a wavelike pattern with multiple centers of action. The corresponding anomaly pattern at the sea level is dominated by north–south pressure seesaws over the North Atlantic and North Pacific. The flow patterns associated with these pressure anomalies are consistent with the principal temperature anomaly pattern in the lower troposphere. The large-scale features of the above anomaly patterns are similar to those associated with the most prevalent standing oscillation observed in the atmosphere. The synoptic behavior and hydrological processes in the model atmosphere during the outstanding anomalous episodes are internally consistent.

The spatial structure of the principal mode in the simulation is rather insensitive to the averaging period of the model data. The autocorrelation function and frequency spectrum of the first principal component, as determined from daily data, are characteristic of persistent phenomena with no preferred periodicity. The autocorrelation time scale associated with this anomaly pattern is estimated to be ∼ 15 days.

The principal anomaly pattern in the Northern Hemisphere summer is relatively less organized, while those for the Southern Hemisphere and the tropics are noted for their zonal symmetry. The east–west sea level pressure seesaw associated with the observed Southern Oscillation over the Pacific is not simulated by the model, thus suggesting the potential role of nonseasonal forcing mechanisms (such as sea surface temperature anomalies) in that phenomenon.

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Ngar-Cheung Lau

Abstract

The El Niño–Southern Oscillation (ENSO) phenomenon is one of the most prominent modes of atmospheric and oceanic variability on interannual and interdecadal time scales. The essential ENSO signals originate from the tropical Pacific (TP). However, the impacts of ENSO are felt in many other parts of the world. Major ENSO events are accompanied by notable changes in the extratropical atmospheric circulation in both hemispheres, various monsoon systems located beyond the TP, global air temperature and precipitation patterns, and the sea surface temperature (SST) distribution throughout the World Ocean. During the past several decades, this author has participated in a sustained effort to study the processes contributing to these “teleconnections” between ENSO forcing in the TP and variations in the atmosphere–ocean system elsewhere. These investigations are based on a large suite of experiments with several generations of general circulation models (GCMs) at the Geophysical Fluid Dynamics Laboratory (GFDL). These experiments are specifically designed to reveal various facets of the teleconnections with ENSO. In this lecture, a summary is given of the principal findings of this series of model studies. An account is given of the train of thought underpinning the sequence of experiments described herein, so as to illustrate how certain experimental setups have been motivated by the problem or hypothesis at hand.

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Ngar-Cheung Lau

Abstract

Two 15-year atmospheric GCM integrations are conducted with the lower boundary over the tropical Pacific being forced by observed month-to-month sea surface temperature (SST) changes during the period 1962–76. A descriptive account is given on selected aspects of the 30-year model climatology, as well as the anomalous model behavior through the life cycles of El Niño–Southern Osicillation (ENSO) episodes centered in the years 1965, 1969 and 1972. These model results are compared with available observations reported in the published literature. Particular attention is devoted to the timing of various simulated meteorological phenomena with respect to the spatially and temporally evolving SST forcing, and to the climatological seasonal cycle.

An assessment is made of the capability of the model to simulate the seasonal dependence of various climatological features relevant to ENSO. The phenomena examined include the flow field and rainfall in different monsoon regions, the planetary scale waves in the extratropics, and the low-level convergence zones in the tropical Pacific Basin.

The evolutionary response of the model atmosphere in a typical ENSO event is examined using time series of selected circulation indices, composite charts and Hoymöller diagrams. As the warm SST anomaly appears in the eastern equatorial Pacific during the boreal spring and subsequently spreads across the ocean basin, a well-defined sequence of meteorological events is evident in the model atmosphere. The most notable atmospheric response over the tropical Pacific Basin includes weakening of the east-west surface pressure gradient and easterly trades eastward displacement of the South Pacific Convergence Zone, southward displacement of the Intertropical Convergence Zone, above normal precipitation at and east of the date line, and below normal precipitation over the Indonesian Archipelago. The strongest anomalies am simulated in the northern winter following a warming off the Peruvian coast. The model response in this mature stage is characterized by tropospheric warming throughout the entire tropical zone, and by the appearance in the tropical upper troposphere of a pair of Pacific anticyclones straddling the equator. These anticyclonic centers appear as the starting points of well-organized wave trains spanning the midlatitude zones of both hemispheres. The Northern Hemisphere wave pattern in the Pacific-North American sector bears a strong resemblance to that reported in recent observational studies.

The warm Pacific SST anomaly tends to be replaced a year later by a cold anomaly. The polarities of meteorological anomalies simulated during the cold phase of the ENSO cycle are mostly opposite to those occurring during the warm phase.

Time series analysis of different circulation indices. as well as comparison between simulated amplitudes of atmospheric variability in this experiment and in a “control” experiment without any prescription of interannual SST variations, indicate that the impact of equatorial Pacific SST anomalies on the tropical circulation is much greater than that on the flow patterns in middle latitudes. In particular, the temporal variance of 200 mb height in this perturbed SST experiment is larger than the corresponding quantity in the control experiment by a factor of 2–6 over the tropics; whereas the same SST fluctuations are much less effective in enhancing the variability in middle and higher latitudes. Moreover, perturbations in the equatorial Pacific SST are more strongly correlated with circulation changes in the tropical atmosphere than with changes in the extratropics.

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Ngar-Cheung Lau

Abstract

The physical structure and the associated transport properties of stationary waves in the troposphere are described using circulation statistics compiled from twice-daily hemispheric analyses covering 11 winters. The distributions of standing eddy meridional transports in middle latitudes are characterized by momentum flux convergence and equatorward geopotential energy transports in the upper troposphere, and by poleward heat fluxes at the lower levels.

The contributions of steady and transient motions to the local, time-averaged budget of vorticity at 300 mb are evaluated. The dominant terms in the time-averaged vorticity equation are the local advection of relative vorticity by the stationary flow and the divergence term. The advection of planetary vorticity by the mean flow (the β-effect) and the convergence of vorticity fluxes by transient eddies appear to be of secondary importance. The hemispheric distributions of the principal terms in the vorticity balance are closely related to topographical features at the lower boundary.

The hemispheric field of stationary flow divergence at various levels is determined as a residual in the time-averaged vorticity balance. This diagnosed divergence field is used to 1) demonstrate the feasibility of retrieving essential stationary flow features in the upper troposphere through solution of the linearized vorticity equation with a prescribed divergence forcing; 2) deduce the velocity potential field; and 3) compute the mean vertical motion field through vertical integration of the continuity equation.

Standing eddy statistics involving vertical motions are described. Mean vertical motions in middle latitudes are found to be positively correlated with mean meridional motions and with mean temperature. The distributions of meridional and vertical transports of geopotential energy and westerly momentum in the meridional plane are presented in a vectorial format. The pattern depicting geopotential energy fluxes suggests that the enhanced standing wave kinetic energy over the subtropics is maintained by geopotential energy transports which originate from higher latitudes.

A diagnosis of the local, time-averaged balance of heat at 1000 and 700 mb is performed. The heat transports by the transient eddies in the lower troposphere exhibit a strong tendency to destroy the zonally asymmetric component of the stationary temperature field. This dissipative mechanism acts on a time scale of several days. The hemispheric distributions of the diabatic heating deduced from the heat budget are indicative of the central role of geographically fixed influences such as ocean currents and sea-land contrast.

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Guoxiong Wu and Ngar-Cheung Lau

Abstract

No abstract available.

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Yochanan Kushnir and Ngar-Cheung Lau

Abstract

A general circulation model was integrated with perpetual January conditions and prescribed sea surface temperature (SST) anomalies in the North Pacific. A characteristic pattern with a warm region centered northeast of Hawaii and a cold region along the western seaboard of North America was alternately added to and subtracted from the climatological SST field. Long 1350-day runs, as well as short 180-day runs, each starting from different initial conditions, were performed. The results were compared to a control integration with climatological SSTs.

The model's quasi-stationary response does not exhibit a simple linear relationship with the polarity of the prescribed SST anomaly. In the short runs with a negative SST anomaly over the central ocean, a large negative height anomaly, with an equivalent barotropic vertical structure, occurs over the Gulf of Alaska. For the same SST forcing, the long run yields a different response pattern in which an anomalous high prevails over northern Canada and the Alaskan Peninsula. A significant reduction in the northward heat flux associated with baroclinic eddies and a concomitant reduction in convective heating occur along the model's Pacific storm track. In the runs with a positive SST anomaly over the central ocean, the average height response during the first 90-day period of the short runs is too weak to be significant. In the subsequent 90-day period and in the long run an equivalent barotropic low occurs downstream from the warm SST anomaly. All positive anomaly runs exhibit little change in baroclinic eddy activity or in the patterns of latent heat release. Horizontal momentum transports by baroclinic eddies appear to help sustain the quasi-stationary response in the height field regardless of the polarity of the SST anomaly. These results emphasize the important role played by baroclinic eddies in determining the quasi-stationary response to midlatitude SST anomalies. Differences between the response patterns of the short and long integrations may be relevant to future experimental design for studying air-sea interactions in the extratropies.

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