Abstract

Recent calculations suggest that mature cloud clusters produce a vertical distribution of diabatic beating with a sharp maximum near 400 mb and very weak heating below 600 mb. It is demonstrated that when such a vertical distribution of heating is applied to a linear steady-state model it results in simulated tropical east-west circulations which are in much better agreement with observations than those which are produced with a more conventional heating profile. In particular, the mature cloud cluster heating profile produces a Walker Circulation with its centers at the observed altitudes and with the observed westward tilt with height. It is suggested that the mature cloud cluster beating profile may provide the appropriate vertical distribution of tropical diabatic heating in many cases. since the presence or absence of mature cloud clusters is often the proximate cause of heating variations in the tropical atmosphere. Some further implications of this suggestion for large-scale dynamics and climate are discussed.

Abstract

The relationship between the Madden–Julian oscillation (MJO), the dominant mode of intraseasonal variability in the tropical troposphere, and the Kelvin waves that dominate the variability of the equatorial thermocline in the central and eastern Pacific Oceans is explored. The Kelvin waves have period near 70 days, which is distinctly longer than the dominant period of the MJO (40–50 days). Their zonal wavelength is roughly the width of the Pacific basin, which is about twice the zonal scale of the zonal stress anomalies produced by the MJO across the western Pacific. Their eastward phase speed is about 2.3 m s⁻¹, which is indistinguishable from the gravest baroclinic mode using the observed stratification in the Pacific.

The stress anomalies that force the Kelvin waves are shown to be associated with the lower-frequency components of the MJO (i.e., periods greater than about 60 days). These stress anomalies move eastward at less than 5 m s⁻¹ from the Indian Ocean to the date line, where their local wavelength is about 15000 km. East of the date line, where the convective component of the MJO weakens, the phase speed of the stress anomalies increases to greater than 10 m s⁻¹. The similarity of the phase speeds of the MJO west of the date line and of the gravest baroclinic Kelvin wave is shown to result in near-resonant forcing by the relatively weak, but zonally broad, stress anomalies induced by the MJO. Despite the large increase in phase speed east of the date line, the MJO-induced stress anomalies are shown to continue to positively project onto the Kelvin waves to about 130°W, which is where the observed thermocline perturbations are the largest. East of this longitude, the MJO-induced stress anomalies detract from the amplitude of the Kelvin waves. The large spatial scale of the zonal stress anomalies produced by the MJO and the near-resonant forcing west of the date line helps explain the observed spectral peak near 70 days for the Kelvin waves despite the higher central frequency of the MJO.

Abstract

Interaction between the large-scale circulation and the convective pattern is investigated in a coupled system governed by the linearized primitive equations. Convection is represented in terms of two components of heating:A “climatological component” is prescribed stochastically to represent convection that is maintained by fixed distributions of land and sea and SST. An “induced comonent” is defined in terms of the column-integrated moisture flux convergence to represent convection that is produced through feedback with the circulation. Each component describes the envelope organizing mesoscale convective activity.

As SST on the equator is increased, induced heating amplifies in the gravest zonal wavenumbers at eastward frequencies, where positive feedback offsets dissipation. Under barotropic stratification, a critical SST of 29.5°C results in positive feedback exactly cancelling dissipation in wavenumber 1 for an eastward phase speed of 6 m s⁻¹. The neutral disturbance is dominated by Kelvin structure along the equator and Rossby gyres in the subtropics of each hemisphere. Heating induced by the neutral disturbance is magnified in a neighborhood of the equator, where nearly geostrophic balance in the boundary layer gives way to frictional balance. Moisture convergence induced by the Kelvin and Rossby structures fuels heating that is positively correlated with the temperature anomaly. Induced heating then generates eddy available potential energy, which offsets dissipation in the neutral disturbance. This sympathetic interaction between the circulation and the induced heating is the basis for “frictional waveCISK,” which is distinguished from classical wave-CISK by rendering the gravest zonal dimensions most unstable. Under baroclinic stratification, the coupled system exhibits similar behavior. The critical SST is only 26.5°C for conditions representative of equinox, but in excess of 30°C for conditions representative of solstice. However, the neutral disturbance is then no longer confined to the tropical troposphere. Forced by the induced heating, wave activity radiates poleward into extratropical westerlies and vertically into the stratosphere.

Having the form of an unsteady Walker circulation, the disturbance produced by frictional wave-CISK compares favorably with the observed life cycle of the Madden–Julian oscillation (MJO). SST above the critical value produces an amplifying disturbance in which enhanced convection coincides with upper-tropospheric westerlies and is positively correlated with temperature and surface convergence. Conversely, SST below the critical value produces a decaying disturbance in which enhanced convection coincides with upper-tropospheric easterlies and is nearly in quadrature with temperature and surface convergence. The observed convective anomaly, which propagates across the Eastern Hemisphere at some 5 m s⁻¹, undergoes a similar shift between amplifying and decaying stages of the MJO. During the same transition, enhanced convection remains phase-locked to inviscid convergence above the boundary layer, as does induced heating in the calculations. Frictional wave-CISK also predicts seasonality in accord with that observed. The coupled system is most unstable under equinoctial conditions, for which climatological convection and maximum SST neighbor the equator. While sharing essential features with the MJO in the Eastern Hemisphere, frictional wave-CISK does not explain observed behavior in the Western Hemisphere, where the convective signal is largely absent. Comprised of Kelvin structure with the same frequency, observed behavior in the Western Hemisphere can be understood as a propagating response that is excited in and radiates away from the fluctuation of convection in the Eastern Hemisphere.

Synoptic images of the global cloud field have been created from infrared measurements taken aboard four geostationary and two polar-orbiting platforms simultaneously observing the earth. A series of spatial and temporal interpolations together with data reliability criteria are used to composite data from the individual satellites into synoptic images of the global cloud pattern. The composite Global Cloud Imagery (GCI) have a horizontal resolution of about half a degree and a temporal resolution of 3 h, providing an unprecedented view of the earth's cloud field. Each composite image represents a nearly instantaneous snapshot of the global cloud pattern. Collectively, the composite imagery resolve, on a global basis, most of the variability associated with organized convection, including several harmonics of the diurnal cycle.

The dense and 3-dimensional nature of the GCI make them a formidable volume of information to treat in a practical and efficient manner. To facilitate analysis of global cloud behavior, the GCI has been constructed with certain homogeneous properties. In addition to synoptic coverage of the globe, data are spaced uniformly in longitude, latitude, and time, and contain no data voids. An interactive Image Analysis System (IAS) has been developed to investigate the space-time behavior of global cloud activity. In the IAS, data, hardware, and software are integrated into a single system capable of providing a variety of space-time covariance analyses. Because of its customized architecture and the homogeneous properties of the GCI, the IAS can perform such analyses on the 3-dimensional data with interactive speed. Statistical properties of cloud variability are presented along with other preliminary results derived from the GCI.

Abstract

The seasonal-mean variance of the Madden–Julian oscillation (MJO) in austral summer has recently been shown to be significantly (p < 5%) enhanced during easterly phases of the quasi-biennial oscillation (QBO). The impact is large, with the mean MJO variance increasing by ~50% compared to the QBO westerly phase. In contrast, we show using observed outgoing longwave radiation that seasonal variations for convectively coupled equatorial Kelvin, Rossby, and mixed Rossby–gravity waves are insensitive to the QBO. This insensitivity extends to all high-frequency (2–30-day period) and the non-MJO component of the intraseasonal (30–120-day period) convective variance. However, convectively coupled Kelvin wave variability shows a modest increase (~13%) that is marginally significant (p = 10%) during easterly phases of the QBO in austral autumn, when Kelvin wave activity is seasonally strongest along the equator. The mechanism of impact on the Kelvin wave appears to be similar to what has previously been argued for the MJO during austral summer. However, the more tilted and shallower vertical structure of the Kelvin waves suggests that they cannot tap into the extra destabilization at the tropopause provided by the easterly phase of the QBO as effectively as the MJO. Lack of impact on the convectively coupled Rossby and mixed Rossby–gravity waves is argued to stem from their horizontal structure that results in weaker divergent anomalies along the equator, where the QBO impact is greatest. Our results further emphasize that the MJO in austral summer is uniquely affected by the QBO.

Abstract

Predictive skill for El Niño in the equatorial eastern Pacific across a range of forecast models declined sharply in the early twenty-first century relative to what was achieved in the late twentieth century despite ongoing improvements of forecast systems. This decline coincided with a shift in Pacific climate to an enhanced east–west surface temperature gradient across the Pacific and a stronger Walker circulation at the end of the twentieth century. Using seasonal forecast sensitivity experiments with the Australian Bureau of Meteorology coupled model POAMA2.4, the authors show that this shift in background climate acted to weaken key ocean–atmosphere feedbacks that amplify eastern Pacific El Niño, thus resulting in weaker variability that is less predictable. These results indicate that extreme El Niños, such as those that occurred in 1982/83 and 1997/98, were conditioned by the background climate and so were favored to occur in the late twentieth century. However, anticipating future changes in El Niño variability and predictability is an outstanding challenge because causes and prediction of low-frequency variations of Pacific climate have not yet been demonstrated.

Abstract

The date of Australian summer monsoon onset (ASMO) is found to be well correlated with the monsoon rainfall of India during the preceding June to September. Years of below (above) normal Indian summer monsoon rainfall (ISMR) are followed by delayed (early) ASMO. Sea surface temperature (SST) anomalies during the September to November season over the tropical Indian Ocean, the equatorial eastern Pacific Ocean, and the ocean north of Australia also correlate significantly with the date of the following ASMO. Delays in ASMO are associated with cold SST north of Australia and warm SST in the tropical Indian and equatorial cast Pacific oceans.

Previous studies have shown that a warm SST is created over the tropical Indian Ocean in years of poor ISMR. We hypothesize that a warm SST anomaly over the Indian Ocean delays the seasonal southward and eastward migration of the cloudiness maximum. A delay in the southeastward movement of cloudiness results in a delayed ASMO. A similar hypothesis already has been suggested to explain the variability of the date of monsoon onset over India.

Weak ISMR often is associated with the contemporaneous presence of El Niño, although many weak monsoons occur without El Niño. Thus warm SSTs in the eastern equatorial Pacific are related to a delayed ASMO through the Indian monsoon. Another signature of El Niño is the presence of negative SST anomalies north of Australia, adding to the delay in ASMO. Warm SSTs in the central and eastern Pacific may also act directly to delay ASMO by causing convection near and east of the date line and subsidence near Australia.

Abstract

Interannual variations of upper-ocean salinity in the tropical Pacific and relationships with ENSO are investigated using the Bureau of Meteorology (Australia) POAMA Ensemble Ocean Data Assimilation System (PEODAS) reanalyses. Empirical orthogonal function (EOF) analysis reveals the systematic evolution of salinity and temperature during ENSO. EOF1 and EOF2 of both temperature and salinity capture the mature phase of El Niño and the discharge and recharge phase, respectively. Typical El Niño and La Niña evolution captured by the leading pair of EOFs depicts eastward or westward migration of the eastern edge of the warm/fresh pool in the western Pacific. Increased or decreased freshness in the western Pacific mixed layer occurs in the recharge/discharge phase. EOF3 captures extreme El Niño, when the strong positive temperature anomaly extends to the South American coast and the fresh pool detaches from the western Pacific and shifts into the central Pacific. Large loadings on EOF3 occurred only during 1982/83 and 1997/98, which suggests that eastern Pacific El Niño is actually the exception, whereas moderate central Pacific El Niño and La Niña are more typical. The eastward expansion of the warm/fresh pool during El Niño is also associated with a continuous eastward displacement of the barrier layer, indicating an active role of the barrier layer not just at the onset of an event. The barrier layer and fresh pool shift much farther eastward during strong El Niño, which could contribute to the eastward shift of strong events. The prior enhancement of the barrier layer in the western Pacific is also more concentrated and stronger, which might portend development of extreme El Niño.

Abstract

Interannual variability of the Madden–Julian oscillation (MJO), the dominant mode of intraseasonal variability in the Tropics, is investigated during the extended austral summer season November–March, which is when the MJO is most prominent. Indexes of the level of MJO activity are developed using outgoing longwave radiation and zonal wind analyses at 850 mb for 1974–98. Based on these indexes, interannual variations in the level of MJO activity are found to be primarily associated with changes in the number of discrete MJO events each year and with changes in the intensity of intraseasonal convection across the Indian and western Pacific Oceans, where the MJO is normally prominent. An eastward shift of MJO activity east of the date line does occur during El Niño events. However, the overall level of MJO activity is found to be uncorrelated with El Niño, except during exceptional warm events when MJO activity is diminished. The level of MJO activity is shown to be weakly related to sea surface temperature anomalies in the equatorial Indian and western Pacific Oceans, but the weak correlations imply that much of the year-to-year variability of the MJO is internally generated, independent of any slowly varying boundary forcing. Such year-to-year variations of the intensity of the MJO are, however, associated with changes in the distribution of seasonal mean convection across the tropical Indian and western Pacific Oceans. This interannual variation of convection unrelated to SST variability may thus act as a limit to seasonal predictions that rely heavily on equatorial Pacific SST anomalies.

Abstract

Onset of the Australian summer monsoon during 1986/87 is examined using the gridded tropospheric winds and surface pressure data from the Australian Bureau of Meteorology Tropical Region Analyses. Dramatic and sudden circulation changes on the synoptic and larger scale are known to occur at onset of the ,monsoon. The enhanced observing network during AMEX 1987 afforded an excellent opportunity to study these changes as the onset occurred during the experiment.

During November 1986 through early January 1987 anomalous convection and low-level westerly winds persisted in the central equatorial Pacific associated with an El Niño/Southern Oscillation event. This anomalous convection forced a large-scale sinking, dry southeasterly flow over the Australian tropics, inhibiting the onset of the monsoon. In early January the anomalous ENSO convection in the central Pacific diminished rapidly, apparently due to the passage of the downward branch or a 40–50 day oscillation event. Thus the attendant dry sinking motion over the Australian tropics diminished, and monsoon onset subsequently occurred in the second week of January.

Onset in the Australian region appears to have been triggered by the arrival of a westward moving equatorial wave disturbance. This synoptic scale disturbance appears to have originated in the ENSO Westerlies in the central Pacific just before their temporary demise in early January. It propagated at least 5000 km across the Pacific into the Australian region. Synoptic-scale low-level moist northerlies due to the wave drastically increased the low level humidity while the wave's convergence appears to have initiated organized convection over the Australian region. Subsequently, in a matter of one to two days, monsoon convection and low-level westerlies developed over a longitudinal span of ∼40°. The synoptic-scale wave disturbances also appear to be associated with formation of tropical cyclones in the region soon after onset. The subsequent expansion of the monsoon westerlies after onset results from a combination of an amplification of the synoptic-scale wave disturbance in the Doppler-shifted group velocity direction (eastward) and by the passage of a 40-50 day oscillation event

From a thermodynamic viewpoint, substantial but unrealizable conditional instability existed over the Australian tropics prior to the monsoon onset. Because of the lack of low-level saturation (due to the dry sinking circulation produced by the ENSO anomaly) substantial lifting was required to achieve buoyancy prior to onset. The onset process appears to be one where initial development of convection induced by synoptic convergence and moisture advection cools and further moistens the lower troposphere and thereby reduces the degree of lifting required to achieve buoyancy. Onset occurs as a dramatic blowup of convection over a large longitudinal span (∼40°) when the lifting required for buoyancy achieves a minimum but prior to the loss of deep conditional instability due to the vertical transport of heat and moisture by the monsoon convection.