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PETER J. WEBSTER

Abstract

A theoretical analysis is made of the large-scale, stationary, zonally asymmetric motions that result from heating and the orographic effect in the tropical atmosphere. The release of latent heat dominates the sensible and radiational heating and the latter two effects are ignored. The first linear model is a continuous stratified atmosphere in solid westward rotation with no dissipation. Of all the modes, only the rotationally trapped Kelvin wave exhibits a significant response. Because the Kelvin wave response does not compare well with the observed flow, we concluded that the neighboring westerlies in the real atmosphere are important even if the forcing is in low latitudes.

The second linear model is a two-layer numerical model including parameterized dissipation and realistic basic currents. Realistic forcing is considered, following an analysis of the response to especially simple forms of heating and orographic forcing. Dissipative effects close to the Equator are very important in this model. The dominant forcing at very low latitudes is the latent heating; at higher latitudes, the advective terms and the effects of rotation become more important and the influences of orography and heating are more nearly equal. A study of the energetics shows that the response near the Equator is due to both local latent heating and the effect of steady, forced motions at subtropical latitudes.

Comparison of the response of the model with observed motion fields and with the results of other studies suggests that most of the time-independent circulation of low latitudes is forced by heating and orography within the Tropics and subtropics. In the subtropics, however, forcing from higher latitudes must be of importance.

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PETER J. WEBSTER

Abstract

An analysis is made of low-latitude, large-scale, zonally asymmetric motions that result from the influence of stationary extratropical disturbances. A linear, two-layer, primitive-equation model in spherical coordinates with parameterized dissipation and realistic basic flows is used. Midlatitude effects are included by applying conditions at the lateral boundaries of the model near 40°N and 40°S.

A series of hypothetical cases is considered in which the roles of dissipation and various basic fields are studied for their effect on the equatorward propagation of energy. The interaction of seasonal forcing functions and basic states in December, January, and February and in June, July, and August is studied. The response near the Equator is found to depend on both the basic state and the magnitude of the forcing, although generally the midlatitude effects dominate the subtropics, whereas local forcing is of greater importance in low latitudes.

A comparison of the computed composite state of the tropical atmosphere (due to both local and remote forcing) with observed fields and previous studies indicates a successful simulation of many features of the seasonal mean tropical atmosphere.

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Peter J. Webster

Abstract

Two apparently contradictory situations are provided by observations of the atmospheric response to sea surface temperature anomalies. These are: (i) The extratropical regions of the winter hemisphere appear to possess strong teleconnections with equatorial forcing but weak or non-existent connections with local (extratropical) heating anomalies. (ii) The extratropical regions of the, summer hemisphere are quite sensitive to local thermal forcing but apparently unaffected by the remote forcing from equatorial regions. An attempt is made to provide a consistent physical picture which simultaneously embraces these two situations.

With the aid of a simple linear model it is shown that the summer hemisphere is more sensitive to local forcing than the winter hemisphere because it is closer to the diabatic limit of Webster (1981), thus allowing an efficient energy generation. The winter hemisphere is much closer to the adjective limit. The sensitivity of the midlatitudes to remote (equatorial) forcing is shown to be a function of the relative location of the SSTA (sea surface temperature anomaly) to the zeros of the basic flow and the magnitude of the midlatitude westerlies. A hemisphere will become excited by remote forcing if at least part of the low-latitude sea surface temperature anomaly is located in the weak subtropical westerlies. Given that the latter criterion is met it is shown that the amplitude of the response and the latitude to which a particular mode is transmitted depends upon the distribution of westerly winds. The specific situation of El Niño sea surface temperature forcing is considered relative to realistic seasonal mean zonal wind fields. The model response is compared with the gross features of the observed anomalous atmosphere during El Niño years and a correspondence found.

Finally, it is argued that the explanation of seasonality in atmospheric response offered in this paper will allow seasonal climate forecasting to be approached with an a priori physical expectation.

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PETER J. WEBSTER

Abstract

The response of the tropical atmosphere to steady forcing for all four seasons is computed using a simple two-layer linear primitive-equation model allowing an arbitrary basic flow with two-dimensional shear. The character of each seasonal response is studied, and two distinct forms of behavior are found. Near the Equator, the forcing excites a slowly varying Kelvin wave of the time scale of months while, in midlatitudes, the atmosphere responds via the Rossby mode. Both regimes are separated by the critical latitudes existing on the poleward limits of the easterly channel of the basic flow. The long-wave scale response of the equatorial motions noted by Krishnamurti are shown to be the results of the natural filtering of the slowly varying Kelvin wave. The temporal variation of this zonal circulation is discussed.

The analysis is extended to infer the behavior and location of transient disturbances in the upper tropical troposphere for time scales much less than seasonal. Also shown is that the zonal mean flow plus the standing eddies are locally barotropically unstable, providing preferred geographical locations for the development and maintenance of transient disturbances. Such locations are shown to vary seasonally.

Variations of the tropical atmosphere of time scales much greater than seasonal also are investigated. It is shown that the correlations of Walker may be thought of as long-term variations in the seasonal standing eddies, which themselves provide a mode of communication throughout the tropical atmosphere. Also suggested is that the Walker circulation of the tropical Pacific Ocean may be thought of as the slowly varying filtered Kelvin wave response, within the easterly channel of the basic flow, weighted toward the long-wave heating distribution of low latitides.

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Peter J. Webster

Abstract

Observations indicate that monsoon systems are characterized by orderly large-scale and low-frequency variations. With a time scale of two weeks and sometimes longer, regions of ascending motion are observed to form to the north of the equator and propagate slowly northward across southeast Asia. The propagation appears to be associated with the “active-break sequence” of the summer monsoon which acts as a modulator on the activity of the synoptic-scale disturbances.

A zonally symmetric non-linear two-layer model containing an interactive ocean and a “continent” poleward of 18°N is used to investigate the mechanisms which produce the observed low-frequency variability. Only when a full hydrology cycle is considered does the model product variations which resemble the observed structures. Mechanisms are traced to include the interaction of the components of the total heating function. The sensible heat input in the boundary layer, although considerably smaller than the other heating components, destabilizes the atmosphere ahead of the ascending zone allowing the moist convective heating component to move northward slightly ahead of the band of precipitation. The poleward encroachment of these components of the heating forces the vertical velocity, which is proportional to the total heating, to move poleward also. The poleward movement is aided by the evaporative cooling of the precipitation moistened ground on the equatorial side of the rising motion which reduces the sensible heat input and effectively stabilizes the troposphere and thus reduces the convective heating in that sector while at the same time reducing the latent heat flux. The time scale of the event is determined by the rate of evaporative drying behind the ascent and the formation of a new zone of ascent in the vicinity of the coastal margin. A schematic representation of heating intercomponent interaction and dynamic feedback is given and the generality of the mechanism to other observed situations is considered. The hypotheses developed and tested in this study underline the importance of the role of ground hydrology related processes in large-scale atmospheric dynamics.

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Peter J. Webster

Abstract

A simple model is used to study the mechanisms which control the local and remote (teleconnection) response of the atmosphere to the thermal forcing resulting from sea surface temperature (SST) anomalies located at various latitudes. The model chosen is a linear baroclinic spherical primitive equation model containing a zonally symmetric basic state with horizontal and vertical shear. An iterative procedure is developed in which the total diabatic heating resulting from the initial heating by the SST anomaly is calculated via feedbacks between the heating and the dynamic response of the system.

Depending on the latitudinal location of the SST anomaly, two major limits of atmospheric response may be identified. The first, the “diabatic limit”, occurs with the SST anomaly embedded in weak low-latitude basic flow and results in a strong enhancement of the initial anomaly response through a vigorous positive dynamics-diabatic beating feedback. Strong teleconnections are evident between low and high latitudes. The second domain, the “advective limit”, occurs when the SST anomaly is placed at higher latitudes in the vicinity of the westerly maximum. The local response is extremely small due to the creation of an indirect zonal circulation in the vicinity of the anomaly which is related to the strength of the local basic flow and the latitude of the forcing due to rotational limitations on the relative scale of the vertical velocity. In contrast to the diabatic limit, the form of the principal forced mode appears unimportant in determining the final response. That is, the dynamics-diabatic heating feedback is weak and only marginally positive.

The form of the remote high-latitude response in all cases is scale selective and only the largest scale transmitted modes are excited. It is argued that these are closest to resonance in latitudes of strong basic zonal flow. The remote response shows a distinct structural difference on either side of the westerly maximum, being highly baroclinic on the equatorial side but barotropic on the polar side. Limited cross-equatorial propagation occurs due to the existence of critical latitudes on the equatorial side of the forcing.

The model results are used to interpret the experimental results obtained from general circulation models (GCM) and provide a rationale for the existence of teleconnections found between low and high latitudes when SST anomalies were imposed in the equatorial oceans. Furthermore, the results suggest why “super-anomalies” were required in midlatitudes in some GCM experiments in order to produce a response which was measurable above the noise level of the model.

It is shown that it is possible to resolve the apparent paradox between the minimal response of GCMs to the imposition of middle latitude SST anomalies and the observations of Namias (1976a) and Davis (1978) who related atmospheric anomalies at least relative to summer SST anomalies. It is argued that only at times of small basic flow (i.e., summer) will a significant response arise in midlatitudes. Finally, the relevancy of the model results to such features as the South Pacific cloud band and the Southern Oscillation is discussed.

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Galina Chirokova and Peter J. Webster

Abstract

The work in this paper builds upon the relatively well-studied seasonal cycle of the Indian Ocean heat transport by investigating its interannual variability over a 41-yr period (1958–98). An intermediate, two-and-a-half-layer thermodynamically active ocean model with mixed layer physics is used in the investigation. The results of the study reveal that the Indian Ocean heat transport possesses strong variability at all time scales from intraseasonal (10–90 days) to interannual (more than one year). The seasonal cycle dominates the variability at all latitudes, the amplitude of the intraseasonal variability is similar to the seasonal cycle, and the amplitude of the interannual variability is about one-tenth of the seasonal cycle. Spectral analysis shows that a significant broadband biennial component in the interannual variability exists with considerable coherence in sign across the equator. While the mean annual heat transport shows a strong maximum between 10° and 20°S, interannual variability is relatively uniform over a broad latitudinal domain between 15°N and 10°S. The heat transport variability at all time scales is well explained by the Ekman heat transport, with especially good correlations at the intraseasonal time scales. The addition of the Indonesian Throughflow does not significantly affect the heat transport variability in the northern part of the ocean.

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Christopher Torrence and Peter J. Webster

Abstract

The El Niño–Southern Oscillation (ENSO) and Indian monsoon are shown to have undergone significant interdecadal changes in variance and coherency over the last 125 years. Wavelet analysis is applied to indexes of equatorial Pacific sea surface temperature (Niño3 SST), the Southern Oscillation index, and all-India rainfall. Time series of 2–7-yr variance indicate intervals of high ENSO–monsoon variance (1875–1920 and 1960–90) and an interval of low variance (1920–60). The ENSO–monsoon variance also contains a modulation of ENSO–monsoon amplitudes on a 12–20-yr timescale.

The annual-cycle (1 yr) variance time series of Niño3 SST and Indian rainfall is negatively correlated with the interannual ENSO signal. The 1-yr variance is larger during 1935–60, suggesting a negative correlation between annual-cycle variance and ENSO variance on interdecadal timescales.

The method of wavelet coherency is applied to the ENSO and monsoon indexes. The Niño3 SST and Indian rainfall are found to be highly coherent, especially during intervals of high variance. The Niño3 SST and Indian rainfall are approximately 180° out of phase and show a gradual increase in phase difference versus Fourier period. All of the results are shown to be robust with respect to different datasets and analysis methods.

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Weiqing Han and Peter J. Webster

Abstract

A nonlinear, 4½-layer reduced-gravity ocean model with active thermodynamics and mixed layer physics is used to investigate the causes of sea level interannual variability in the Bay of Bengal, which may contribute to flooding and cholera outbreaks in Bangladesh. Forcing by NCEP–NCAR reanalysis fields from 1958 to 1998 yields realistic solutions in the Indian Ocean basin north of 29°S. Controlled experiments elucidate the roles of the following forcing mechanisms: interannual variability of the Bay of Bengal wind, equatorial wind, river discharges into the bay, and surface buoyancy flux including precipitation minus evaporation (heat fluxes + PE).

Sea level changes in the bay result largely from wind variability, with a typical amplitude of 10 cm and occasionally 10–25 cm at an interannual timescale. Near the eastern and northern boundaries, sea level anomalies (SLAs) are predominantly caused by equatorial wind variability, which generates coastal Kelvin waves that propagate into the bay along the eastern boundary. Near the western boundary the bay wind has a comparable influence as the equatorial wind, especially during the southwest monsoon season, owing to the counterclockwise propagation of coastal Kelvin waves forced by the large-scale alongshore wind stress in the bay. In the bay interior, SLAs are dominated by the equatorial wind forcing in the central bay, result almost equally from the equatorial and the bay wind in the southwestern bay, and are dominated by the bay wind forcing in the southwestern bay during the southwest monsoon. The westward intensification of the bay wind influence is associated with the westward propagation of Rossby waves forced by the large-scale wind curl in the interior bay. The effect of heat fluxes + PE is generally small. Influence of interannual variability of river discharges is negligible.

SLAs caused by the equatorial wind at the equator and that caused by the bay wind along the northern and western boundaries as well as in the southwestern bay are significantly correlated, reflecting the anomalous wind pattern associated with the dipole mode event in the tropical Indian Ocean. Given the dominance of equatorial wind forcing near the northern bay boundary, SLAs (or alternatively westerly wind anomalies) in the equatorial ocean may serve as a potential index for predicting Bangladesh flooding and cholera.

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Peter J. Webster and Roger Lukas

Despite significant progress in the Tropical Ocean–Global Atmosphere (TOGA) program, a number of major hurdles remain before the primary objective, prediction of the variability of the coupled ocean-atmosphere system on time scales of months to years, can be achieved. Foremost among these hurdles is understanding the physics that maintains and perturbs the western Pacific warm pool, the region of the warmest sea surface temperature in the open oceans, which coexists with the largest annual precipitation and latent heat release in the atmosphere. Even though it is believed that the warm pool is a “center of action” for the El Nino-Southern Oscillation (ENSO) phenomena in the ocean and the atmosphere, successful simulation of the warm pool has remained an elusive goal.

To gain a clear understanding of global climate change, the ENSO phenomenon, and the intraseasonal variability of the coupled atmosphere–ocean system, it is clear that a better specification of the coupling of the ocean and the atmosphere is required. An observational and modeling program, the TOGA Coupled Ocean–Atmosphere Response Experiment (TOGA COARE), has been designed to work toward this goal.

The scientific goals of COARE are to describe and understand:

  • 1) the principal processes responsible for the coupling of the ocean and the atmosphere in the western Pacific warm-pool system;

  • 2) the principal atmospheric processes that organize convection in the warm-pool region;

  • 3) the oceanic response to combined buoyancy and wind-stress forcing in the western Pacific warm-pool region; and

  • 4) the multiple-scale interactions that extend the oceanic and atmospheric influence of the western Pacific warm-pool system to other regions and vice versa.

To carry out the goals of TOGA COARE, three components of a major field experiment have been defined: interface, atmospheric, and oceanographic. An intensive observation period (IOP), embedded in a period of enhanced meteorological and oceanographic monitoring, will occur from November 1992 through February 1993 in the western Pacific region bordered by 10°N, 10°S, 140°E, and the date line. The experimental design calls for a complex set of oceanographic and meteorological observations from a variety of platforms that will carry out remote and in situ measurements. The focus of the observational effort will be in an intensive flux array (I FA) centered at 2°S and 156°E. The resulting high-quality dataset is required for the calculation of the interfacial fluxes of heat, momentum, and moisture, and to provide ground truth for a wide range of remotely sensed variables for the calibration of satellite-derived algorithms. The ultimate objective of the COARE dataset is to improve air–sea interaction and boundary-layer parameterizations in models of the ocean and the atmosphere, and to validate coupled models.

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