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Ka-Ming Lau

Abstract

Coupling between large-scale atmospheric and oceanic equatorial Kelvin waves is shown to be relevant in the climatic time scale related to equatorial ocean/atmosphere processes. The present analyses show that the inclusion of air-sea coupling into the linearized shallow-water equations can result in two types of dispersion relations for the Kelvin waves. The first type (Mode 1) has fast phase speed and is mostly manifest in the atmospheric response. This mode is relatively unaffected by air-sea coupling. The second type (Mode II) has slow phase speed and is the predominant mode in the time-scale of the ocean. A resonant stationary wave is shown to exist as an intrinsic mode in the coupled system, the length scale of which is determined by the strength of the coupling and the magnitude of the damping. The positive feedback mechanism shown to exist between these coupled Kelvin waves in the Mode II regime is suggested to play an important role in relation to observed low-latitude teleconnections.

Results of the numerical experiments using the coupled model show that an El Niño-type oscillation can occur in a baroclinic ocean-atmosphere system as a result of a prolonged period of strengthening and subsequent weakening in the barotropic component of the wind. The weakening and the eastward shift of the rising branch of the Walker cell, identified as the atmospheric component of a coupled quasi-stationary Kelvin wave, during El Niño provide a positive feedback favoring warm water formation in the eastern Pacific and contribute to the large amplitude of the oscillation.

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Ka-Ming Lau

Abstract

Day-today convective activity in the equatorial regions (10°N–10°S and 90–180°E) during December 1974 and January 1975 was estimated from NOAA satellite visible imageries using simple visual techniques. The daily convective estimates were composited with respect to the phase of individual cold surge episodes occurring during the two-month period. By decomposing the data into symmetric and anti-symmetric components with respect to the equator, it was shown that eastward and westward propagating cloud clusters had phase speeds ∼10 m s−1 and ∼5 m s−1, respectively, and were generated near southwestern Borneo following a cold surge onset. The strong equatorially trapped character of the eastward moving components suggested the possibility of Kelvin wave responses. The eastward moving convective clusters interacted strongly with westward moving disturbances which may be identified with low-order symmetric Rossby waves originating from the western Pacific. The predominantly larger response amplitude of the symmetric compared with that of the antisymmetric component, especially near the equator, suggested that the most strongly trapped of these waves were likely generated from heat sources concentrated near the equator.

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Ka-Ming Lau

Abstract

In this paper, we present the elements of a theory of the long-term variability of the El Niño/Southern Oscillation (ENSO). Three basic processes, i.e., unstable air-sea interaction, the seasonal variation and stochasfic forcings from high-frequency (relative to ENSO) transients are identified to be the crucial factors leading to the long-term behavior of the tropical ocean-atmosphere system. We hypothesize that the occurrence of the ENSO is the result of an inherent instability in the tropical ocean-atmosphere triggered by stochastic forcings. Such a process is strongly modulated by the seasonal variation.

For interannual time scale of the ENSO, the tropical ocean-atmosphere system is formally described in terms of a stochasticaly forced climate system. In a simple prototype model, we demonstrate how the abovemendoned processes can interact to produce many of the salient features of the long-term variability of the Southern Oscillation, including frequency of occurrence of ENSO, autocorrelation, spectral characteristics nd phase-locking properties The proposed theory provides an interesting framework for further investigation into the long-term variability of the tropical ocean-atmosphere system, as well as more general coupled dynamical climate systems.

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Ka Ming W. Lau

Abstract

In a series of numerical experiments using a simplified, domain-averaged, coupled air-sea model, some aspects of the low-latitude large-scale interaction between the atmosphere and the ocean are investigated. Experiments are designed with a view toward elucidating basic mechanisms involved in the coupled processes. In this paper, we focus on the response of coupled system to 1) steady forcing, 2) seasonal variations and 3) large perturbations and continuous short-period random forcings.

Results indicated that the sea surface temperature (SST) distribution, which is strongly controlled by the oceanic upwelling, is the primary factor in determining the location and transition of the tropical rainbelt. The strongest convective activities in the ITCZ, however, depend mainly on the moisture supply from horizontal convergence and the static stability of lower atmosphere, and do not necessarily coincide with the occurrence of maximum SST. It is also demonstrated that the positive feedback processes between latent heat release in the ITCZ convection and the Hadley cell are opposed by oceanic upwelling and the concomitant cooling of the mixed layer through a stabilization of the lower atmosphere and a decrease of moisture supply from the tropical oceans. Further, results of the forced oscillation experiment suggest that the statistical effect of a large number of short-term atmospheric disturbances is capable of generating large-scale low-frequency variabilities in the coupled system. The separation in time scale is possibly the manifestation of a red-shifted spectral response in a multi-time-scale coupled climate system.

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Ka Ming W. Lau

Abstract

A study of the seasonal variation of the climatic states of the atmosphere and the ocean is made using a coupled atmosphere-ocean model. The “domain-averaged” nature of the model enables the inclusion of effects due to continent-ocean-ice distribution in a quasi-two-dimensional framework. While the atmosphere is described by simplified “domain-average” primitive equations, the ocean is represented as simple advective mixed layer. Large-scale circulation and upwelling in the ocean are modeled in terms of a wind-driven and a thermally driven component.

Integration is carried out for the coupled model until a repeatable annual cycle is observed in the mean climatic states. Results of the experiment show that large differences in the time scale and amplitude of the response exist between the land and ocean domains. Features of the mean atmospheric circulation such as the Hadley cell, the Ferrel cell, tropical easterly jet and monsoon transition are well simulated. In the model ocean the domain-averaged spatial and temporal response of sea surface temperature and mixed layer depth are simulated fairly realistically. The large-scale circulation in the ocean shows some interesting features such as the gyre circulation, the western boundary transports, and a decrease in sea surface temperature in eastern and equatorial oceans as a result of oceanic upwelling. In the overall heat budget of the combined system, the ocean is found to dominate in energy storage. Between 20 and 30°S, the maximum oceanic poleward heat transport is computed in February and March and is as large as or larger than the corresponding atmospheric heat transport at the same latitudes.

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Ka-Ming Lau and Hock Lim

Abstract

The linearized shallow-water equatorial β-plane equation was solved for a subset of approximate solutions applicable to thermally driven large-scale tropical circulation. In particular, the heat-induced monsoon circulations during Southeast Asian northeasterly cold surges are investigated. It was demonstrated that the response of the tropical atmosphere to a localized heat source consists of forced Rossby waves propagating westward and Kelvin waves eastward along the equator away from the region of forcing. In general, for any source/sink distribution, the heat-induced motion can have the characteristics of a Walker-type (ν = 0 at the equator) or a Hadley-type (u = 0 at the equator) response or a combination of both, depending on the latitudinal location of the forcing. Away from the equator, a forcing corresponding to the sudden imposition of mass at the lower layer, or equivalently in our model a rapid cooling of the lower troposphere, produces a sudden local surface pressure rise and strong anticyclonic flow to the west of the forcing. Strong NE-SW till in the axis of the anticyclone is observed and can be understood in terms of the dispersion of the various wave modes excited. The low-latitude response is, as expected, dominated by Kelvin and the gravest Rossby wave modes.

Coupled with an equatorial heat source, the sudden cooling of the lower troposphere over a localized area in the subtropics gives rise to a northeasterly wind surge and large-scale Walker and Hadley circulations reminiscent of periods of strong cold surges over East Asia. Finally, the effect of the presence of a mean wind is shown to modify the spatial extent of the equatorial circulation with a mean easterly wind favoring the formation of equatorially trapped Walker cells.

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Ka-Ming Lau and Hock Lim

Abstract

The dynamics of equatorially forced climate teleconnections on the sphere is studied using quasi-geostrophic wave theory and numerical models. Using the concept of a refractive index for meridional propagation of energy, it is demonstrated that for zonal mean flow with no horizontal shear, steady-state atmospheric teleconnections are composed of radiating Rossby modes which are forced in weak westerly zonal mean flow and evanescent modes in easterly zonal mean wind. For horizontally sheared zonal mean flow, westerly shear will lead to initial transient growth of wave packets with northwest–southeast tilt. These wave packets will move initially northward from the source region new the equator and subsequently become damped after turning southward at various critical latitudes. In contrast, easterly shear will always cause monotonic decay of all northbound wave packets from the tropics. The results imply that, in the case of a barotropically stable mean flow, kinematic shearing effect will focus or defocus wave energy from tropics to midlatitudes depending on whether the ambient horizontal shear is westerly or easterly. This mechanism also explains why tropical wave energy is naturally drawn toward the exit region of climatological winter jet streams.

Experiments with a nonlinear barotropic spectral model with equatorial forcing shows that wave energy can still propagate away from regions of initially weak tropical easterly mean flow by the shear-induced growth mechanism which modifies the zero-wind line downstream of the source. The interaction of the winter subtropical westerly jet and the wave disturbance generated by diabatic forcing source over the equator produces a quasi-stationary wave pattern reminiscent of the Pacific–North America pattern. The gross features of the tropics and extratropical steady-state response in the radiating mode are in qualitative agreement with that predicted from linear theory.

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Ngar-Cheung Lau and Ka-Ming Lau

Abstract

The onset dates for 11 individual cold-air outbreaks over the East Asian seaboard during the Winter Monsoon Experiment (1 December 1978–28 February 1979) are used for constructing composite synoptic charts. The three-dimensional structure and energetics of disturbances with time males shorter than ∼5 days are distinguished from the corresponding properties of more slowly varying fluctuations by using time-filtering techniques.

It is seen that the high-frequency disturbances accompanying the cold surges experience systematic structural changes as they migrate along a well-defined storm track from East Asia to the Gulf of Alaska. The typical life cycle of such extratropical storms is characterized by a barolinic growth phase coinciding with the polar outbreaks, and a decay phase in which barotropic processes play an active role. The propagation of low-frequency fluctuations is oriented toward lower latitudes, with new vorticity centers developing downstream and equatorward of the primary disturbances associated with the outbreaks. The shapes of the disturbances appearing in the composite charts indicate that a strong degree of anisotropy exists in both the high-frequency and low-frequency disturbances. The fluctuations with short time scales are elongated in the meridional direction, whereas those with long time scales are elongated in the zonal direction.

The findings of this composite study are seen to be consistent with circulation statistics derived from continuous climatological records. The behavior of the fluctuations with short and long time scales is also reminiscent of the characteristics of baroclinically unstable waves and Rossby-wave trains, respectively, appearing in model experiments.

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Ngar-Cheung Lau and Ka-Ming Lau

Abstract

The three-dimensional structure and temporal evolution of quasi-periodic, planetary-scale tropospheric oscillations simulated by a 15-wavenurnber GCM are investigated by applying cross-spectral, eigenvector, composite and temporal correlation techniques to 12 years of model output. Evident from this diagnostic study is the presence in the model tropics of well-defined eastward traveling features with spatial scales of zonal wavenumbers 1 and 2, and with temporal scales of 25–40 and 15–20 days, respectively. The flow pattern associated with the oscillations of both spatial scales is characterized by circulation cells oriented along the equatorial zonal plane, with the zonal wind and geopotential height fluctuations near the sea level being negatively correlated with the corresponding fluctuations in the upper troposphere. The movement of these zonal circulation cells along the equatorial bell is accompanied by systematic migration of the global-scale horizontal divergence field, and by dipole-like precipitation structures within the Indonesian/Pacific sector. The preferred sites for such oscillatory behavior exhibit a notable seasonal dependence, with the most active zonal circulation cells being located in the summer hemisphere.

During the northern summer, the 25–40 day oscillations coincide with the occurrence of northward moving, zonally elongated rainbands over the monsoon region of South Asia. During the northern winter, the 25–40 day phenomena in the tropics are linked to well-organized extratropical wave trains spanning the Eurasian and Pacific/North American sectors.

The principal characteristics of the model-generated phenomena analyzed in this study are compared with corresponding results reported in the observational literature. Although the period of the simulated wavenumber-1 phenomena is somewhat shorter than the corresponding observed values, it is demonstrated that the spatial structure, propagation characteristics and seasonal dependence of the model features are consistent with observations. The model findings are also interpreted in terms of current theoretical understanding of tropical and extratropical motions.

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Chung-Hsiung Sui and Ka-Ming Lau

Abstract

An improved treatment of diabatic heating due to moist convection is introduced into the dynamical model used in Part I of this paper to further investigate the origin of intraseasonal oscillations in the tropics. The convective heating in the model is parameterized by a simple one-dimensional cloud model which takes into account the available moisture supply in the lower troposphere and the mean thermodynamic states for the entire troposphere. Consequently, the spatial distribution of convective heating in the model can be determined internally as a function of the sea surface temperature consistent with observed convection-SST relationship in the tropics. The periods of low-frequency oscillations excited in the numerical simulations range from 20 to 50 days depending primarily on the vertical distribution of heating through condensation-moisture-convergence feedback or “mobile wave-CISK” The “fast” wave (period around 20 days) is excited by deep convection which has heating maximum at or above the 500 mb level. The “slow” wave (period near 50 days) is excited by heating maximized in the lower troposphere between 500 and 700 mb. A crude parameterization of lower boundary forcing due to heat flux from the ocean surface is incorporated in the model. The boundary forcing tends to further destabilize the mobile wave-CISK modes. It is also found that the boundary forcing plays an important role in sustaining the propagation of intraseasonal oscillations around the globe, especially over the eastern part of ocean where SST is cold and deep convection is strongly inhibited.

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