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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 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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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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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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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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Ka-Ming Lau
and
Paul H. Chan

Abstract

Satellite-inferred short-term climate variability and atmospheric teleconnections are studied using seven years (1974–81) of Outgoing Longwave Radiation (OLR) data from NOAA polar orbiters. This study utilizes composite, partition-of-variance and multiple correlation techniques to investigate the simultaneous relationship in OLR fluctuation between remotely separated regions of the globe.

Results show that two dominant modes of variation are present in the monthly anomalous OLR time series fluctuation in the tropics. They are 1) variations of 2–3 month time scale associated with quasi-stationary fluctuation in the Walker Circulation and 2) an abrupt shift in the mean level of fluctuation related to the Southern Oscillation. The distribution of diabatic heat sources and sinks appears to exhibit a dipole-like oscillation that alternates between dry and wet periods over the equatorial central Pacific region and the maritime continent of Indonesia. The wet period is characterized by strong convection over the equatorial central Pacific, an eastward migration of the South Pacific convergence zone, and an equatorial migration of the ITCZ over the central and eastern Pacific, resulting in considerable shrinkage of the eastern Pacific dry zone. The dry period corresponds to intense convection over the maritime continent and an extensive eastern Pacific dry zone. While one major transition from dry to wet conditions is believed to occur from March 1976 to December 1977 associated with the 1975–76 El Niño, minor transitions between the two periods also occur frequently in the time scale of 2–3 months.

The major significant teleconnections using the equatorial central Pacific OLR fluctuation as reference are (identified by key geographical areas, with the sense of the correlation denoted by the signs in parentheses):
i1520-0469-40-12-2735-e1

It is suggested that the dry period over the equatorial central Pacific corresponds dynamically to stronger zonal teleconnections by equatorially trapped Kelvin and low-order Rossby-type response, and the wet periods, to stronger downstream meridional teleconnections characteristic of external higher-order Rossby-wave response to tropical diabatic forcing.

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Ka-Ming Lau
and
Paul H. Chan

Abstract

As a sequel to Part I of this study, lagged relationships in atmospheric teleconnections associated with outgoing longwave radiation (OLR) are investigated using Lagged Cross Correlations (LCC). The feasibility of extratropical seasonal-to-interannual predictions using satellite-derived observation is also quantitatively assessed. It is found that the global influence of teleconnectivity of the atmosphere is strongest for diabatic forcing located near the equatorial central Pacific, but much reduced for forcings over the maritime continent and to the east of the dateline. The LCC patterns show that at zero-lag, the OLR fluctuation over the equatorial central Pacific is associated with simultaneous excitation of quasi-stationary waves in the tropics. These tropics–tropics teleconnections eventually (in about 5 months) transform into tropics–midlatitude and midlatitude–midlatitude teleconnections associated with possible excitation of extratropical quasi-stationary waves in both hemispheres.

Analysis of the LCC pattern with the Southern Oscillation (SO) signal removed shows that during 1974–81, both the SO signal and the variability in the 2–3 month time scale contribute substantially to the observed LCC patterns. The presence of a convective heat source in the equatorial central Pacific is found to be important in forcing the tropics–midlatitude and the midlatitude–midlatitude teleconnections, which appear also to be phase-locked with the normal seasonal cycle. A mechanism is proposed to explain the observed lagged relationships. This mechanism is consistent with both internal atmospheric dynamics related to the seasonal cycle and with external influences such as sea surface temperature anomalies associated with the El Niño/Southern Oscillation.

Initial assessment of the predictability of regional climate using satellite-derived atmospheric teleconnection shows that about 30–40% of the wintertime OLR variance over the southeastern United States is accounted for by a 5-month antecedent OLR variation over the equatorial central Pacific. Because of the close relation between OLR variation and synoptic disturbances, the satellite-derived teleconnections described in Part I and Part II of this study can be used to identified regions with potentially higher seasonal-to-interannual predictability.

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Ka-Ming Lau
and
Paul H. Chan

Abstract

Intraseasonal and interannual variations of tropical convection are studied using 12 years of NOAA outgoing longwave radiation (OLR) data. The spatial patterns of OLR variance in the 1–5 day, 20–70 day and longer-than-180 day bands am examined separately for the northern summer (May–October) and winter (November&ndash April). In the 1–5 day band, the most prominent features are the Pacific and Atlantic ITCZs during the northern summer and the large variability over the western Pacific and the tropical continents during the northern winter. For the time scales beyond 10 days, it was found that the maximum variance shifts north and south of the equator from summer to winter but is generally located between 20°S and 20°N in the longitudinal sector between the Indian and Pacific oceans. Over the Indian Ocean and the western Pacific, variations with 30–60 day period are most pronounced. In the central and eastern Pacific variations with periods over one year are more prevalent.

The natural variability of tropical convection is estimated. Results show that the climate signal (interannual variability) in convection is 1argest and most easily detectable over the equatorial central Pacific. Results of empirical orthogonal function analyses show that the most dominant mode of variation in tropical convection exhibits multiple time scales that can be identified respectively with the 40–50 day oscillation, the annual cycle and an aperiodic variation with several year' time scale. These variations are associated with fluctuations of the equatorial Walker circulation. It is suggested that an interaction of these time scales may be instrumental in leading to the onset of ENSO. The possibility of the 40–50 day oscillation as a trigger to ENSO is further supported by the seemingly systematic frequency and amplitude modulation of these oscillations before and after the 1982–83 ENSO. Discussion of the results in light of recent development in the dynamics of the coupled ocean-atmosphere system is also presented.

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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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