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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

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

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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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

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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David M. Lawrence
and
Peter J. Webster

Abstract

The summertime intraseasonal oscillation (ISO) is an important component of the south Asian monsoon. Lagged regressions of intraseasonally filtered (25–80 days) outgoing longwave radiation (OLR) reveal that centers of convection move both northward and eastward from the central equatorial Indian Ocean subsequent to the initiation of an ISO. Eastward movement of convection is also seen at Indian subcontinent latitudes (10°–20°N). Based on the regression results, the summertime ISO convection signal appears as a band tilting northwestward with latitude and stretching from the equator to about 20°N. Viewed along any meridian, convection appears to propagate northward while equatorial convection propagates to the east. To examine the robustness of the connection between eastward and northward movement, individual ISOs are categorized and composited relative to the strength of the large-scale eastward component of convection in the central equatorial Indian Ocean. It is found that the majority of ISOs that exhibit northward movement onto the Indian subcontinent (42 out of 54 ISOs, or 78%) also exhibit eastward movement into the western Pacific Ocean. It is also found that when convection in the central Indian Ocean is not followed within 10–20 days by convection in the western Pacific Ocean (12 out of 54 ISOs, or 22%), the independent northward movement of convection in the Indian Ocean region is somewhat stunted.

The link between the eastward and northward movement of convection is consistent with an interpretation of the summertime ISO in terms of propagating equatorial modes. The northward moving portion of convection is forced by surface frictional convergence into the low pressure center of the Rossby cell that is excited by equatorial ISO convection. A similar convergence pattern is seen for the northern winter ISO, but it does not generate poleward movement due to relatively cool SSTs underlying the surface convergence.

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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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Peter J. Webster
and
Song Yang

Abstract

We seek relationships between the perturbation kinetic energy (PKE), the zonal wind, and outgoing longwave radiation. It is found that the mean low-level PKE extratropical maxima are located at the “atmospheric centers of action” (e.g., the Aleutian and Iceland lows), which lie beneath the high-level PKE-maxima downstream of the westerly jet streams. As expected, this means that the PKE maxima in the middle latitudes are closely related to the propagating disturbances. In the tropics, however, the high-level PKE maxima and minima are located in the strongest upper tropospheric westerlies and easterlies, respectively, while the low-level PKE maxima and minima are located in the monsoon systems and the easterly trade winds, respectively. It is also found that the maxima and minima of the PKE in the middle latitudes, at least in the Northern Hemisphere, are in phase in the vertical but completely out of phase in the tropics. These features are closely linked to the structure of the zonal wind component.

The hypothesis is posed that in the tropics, the PKE maxima in the upper troposphere are related to the tropical-extratropical atmospheric interaction, to the equatorially trapped transient modes which are presumably created in the convective regions, or to a combination of both processes.

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John Fasullo
and
Peter J. Webster

Abstract

The warm tropical oceans underlie the most convective regions on earth and are a critical component of the earth’s climate, yet there are differing opinions on the processes that control warm pool SST. The Indo–Pacific warm pool is characterized by large-scale variations in SST approaching 30°C on intraseasonal timescales. In this study, surface heat flux anomalies associated with composite warm episodes over three spatial scales in both the Pacific and Indian Ocean basins are examined.

The current study benefits from the recently available National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis dataset that enables the examination of variability in surface evaporation with moderate confidence. Solar flux estimates from the reanalysis are somewhat less reliable than evaporation estimates, however, and two techniques that infer surface shortwave radiation from satellite retrievals of cloud properties are considered. Error in all measurements is quantified.

Both shortwave and evaporative flux variability play significant roles in modifying the temperature of the warm pool, though the relative importance of individual flux anomalies depends on SST tendency and geographical location. There also exist differences in the relative heating roles of the flux anomalies among episodes within a fixed location, though in instances the resolved differences are less than likely flux estimation error. Differences also exist between the ocean basins. A more pronounced annual cycle exists in the eastern Indian Ocean, and SST there is less sensitive to surface thermal forcing. Finally, the analysis offers evidence that SST is not regulated by a simple atmospheric thermodynamic response to the surface. Instead, the relationship between warm pool variability and large-scale dynamical features of the Tropics (e.g., intraseasonal oscillation and the seasonal monsoon) is demonstrated. The conclusions are shown to be robust to spatial scale and are consistent with a recent analysis of Tropical Oceans and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment observations.

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