Abstract

The previously reported spectral peak near 50 days in time series of length of day (LOD) is shown to occur in conjunction with episodes of tropical convective activity associated with the Madden-Julian oscillation (MJO). When the convective signal of the MJO is absent, LOD exhibits a red spectrum at intraseasonal time-scales. LOD is shown to be in phase with the convective anomaly due to the MJO over the date line and out of phase with the convective anomaly over the Indian Ocean. A composite angular momentum budget, made relative to the convective signal of the MJO, reveals that the zonal surface stress only partially accounts for the observed tendency of LOD. Not only is the amplitude some 50% too weak, the phase is shifted ahead of the LOD tendency by about 1/8 cycle. Hence, in order to balance the angular momentum budget, an additional mountain torque is postulated to occur. This additional torque is required to lag the frictional torque by about 1/4 of a cycle, but be of similar amplitude.

The composite surface stress anomalies appear to result predominantly from zonal mean zonal wind anomalies. An important role for the zonally symmetric convective anomaly due to the MJO is suggested. The surface zonal wind anomalies at low latitudes, which exhibit a high degree of equatorial symmetry with zero amplitude on the equator, appear to be accounted for as the linear response to zonal mean convective heating in the presence of strong dissipation. The upper-tropospheric zonal wind anomalies, which mimic the angular momentum anomalies, are not accounted for by simple linear momentum balance. In particular, maximum zonal wind anomaly occurs on the equator, which suggests an important role for eddy fluxes of momentum during the life cycle of the MJO.

Abstract

The effects of a positive-only cumulus heating parameterization on equatorially trapped waves are investigated using both nonlinear and linear single vertical mode models. In the linear model the cumulus heating is shown to slow down the equatorial waves. In particular the eastward moving Kelvin wave is readily slowed from the free phase speed (45 m s⁻¹) to the intrinsic speed associated with the observed 40–50 day oscillation (<10 m s⁻¹). Slow eastward propagating Kelvin waves only occur for stable cumulus heating (stable meaning that adiabatic cooling is able to compensate for the cumulus heating). However these linear moist waves decay rapidly and are thus an unlikely explanation of the observed oscillation. The observed meridional wind perturbation is also unexplained by this mechanism. For unstable cumulus heating, growing linear CISK waves develop but remain stationary. Thus no satisfactory explanation of the 40–50 day oscillation is possible with this single vertical mode linear model.

For unstable cumulus heating in the nonlinear model, the growing CISK modes rapidly stabilize the atmosphere. The stability increases greatest to the west of the CISK heating which thus leads to eastward propagation. Upon equilibration, the CISK mode propagates eastward ≲10 m s⁻¹ for a wide range of parameters. The mode has a substantial meridional velocity perturbation and appears to be a horizontally coupled Rossby-Kelvin wave. These propagating modes are quite similar to the observed 40–50 day oscillation. Various experiments are conducted to elucidate the mode of propagation. Experiments relevant to the observed atmosphere (i.e., moisture parameters that are a function of space and time) are also discussed.

Abstract

No abstract available.

Abstract

The variability and time-mean response to orographic forcing are examined in a nonlinear atmospheric model. Distinct signatures from both high-frequency (synoptic-scale) and low-frequency (periods greater than 10 days) transients are seen in the temporal variance and eddy fluxes. Downstream of the orography, in the region of the time-mean jet stream, high-frequency transients are organized into a storm track and exhibit baroclinic energy conversions. The low-frequency transients, while producing, greater variability in the same region as the storm track, exhibit significantly less baroclinic energy generation. The structure of the low-frequency transients downstream of the orography is similar to the observed PNA pattern. The time scale of the eddies in this region appears to be longer than typical time scales associated with stationary Rossby wave dispersion. These eddies exhibit large local barotropic conversion of mean kinetic energy due to the large zonal gradient of the mean zonal wind. These barotropic processes downstream of the mountain give the appearance of low-frequency waves propagating out of the tropics even though there is no low latitude forcing in this model.

Midlatitude orography is shown to influence the tropical time-mean circulation; a weak easterly jet along the equator develops due south of the orography. The influence on the tropical variability is restricted to increased high-frequency variance with limited effects on the low-frequency transients.

Abstract

The response to steady tropical diabatic forcing in a nonlinear model of the atmosphere is analyzed. For sufficiently small diabatic heating, upper tropospheric anticyclones develop along the equator to the west of the heating, as predicted by linear theory. For sufficiently large diabatic heating, the anticyclones shift eastward to the same longitude as the heating (nonlinear response). The structure of the nonlinear response agrees more favorably with the observations than does the linear response. Inclusion of strong tropospheric dissipation causes the weaker diabatic forcing to produce the structural characteristics of the nonlinear response. Analysis of the time-mean vorticity budget reveals that the relative magnitude of the steady nonlinear flux divergence is substantially larger for the strong forcing as compared to the weak forcing. This appears to be the mechanism responsible for the marked difference between the two responses. Significant differences in the extratropical response exist for the two cases.

The tropical variability in the presence of the time-mean asymmetries in the basic state is examined. Strong maxima and minima in the transient kinetic energy are observed in the regions of equatorial westerlies and easterlies, respectively. The penetration into the regions of reduced easterlies by equatorward propagating extratropical waves is shown to be the major cause of the asymmetry in the variance. Most incident midlatitude waves are seen to be absorbed at their low-latitude critical line. Because the spectrum of atmospheric waves produced in midlatitudes includes some westward moving waves, the existence of equatorial westerlies is not required for the asymmetry to occur.

Abstract

The impact of air–sea coupling on the dynamics of the tropical Madden–Julian oscillation (MJO) is investigated with an atmospheric general circulation model (GCM) coupled to an ocean mixed layer model. In the uncoupled GCM, where climatological sea surface temperature (SST) is specified, realistic space–time spectra of near-equatorial zonal wind and precipitation are produced, with power concentrated at eastward wavenumbers 1–3 with periods of 35–90 days. However, the simulated MJO is roughly 50% stronger than observed, largely resulting from enormous activity during northern summer. Furthermore, during southern summer, when the observed MJO is most dominant across the Indian and western Pacific Oceans, intraseasonal variance in the uncoupled model is overly concentrated to the north and east of Australia with little activity extending into the equatorial Indian Ocean. Contrary to other recent modeling studies, coupling did not alleviate either of these problems nor did it have any other appreciable impact on the model’s MJO.

Feedback of the SST anomalies onto the MJO, both observed and diagnosed in other coupled models, appears to result from correlation of positive equatorial SST anomalies across the warm pool with surface low pressure to the east of the convective anomaly. This feedback is insignificant in the present coupled model because the SST anomalies, besides being too weak and not spatially coherent, do not systematically exhibit the requisite phasing with the surface pressure. The observed SST anomalies result from a combination of shortwave radiation and latent heat flux, whereby reduced shortwave radiation associated with enhanced convection slightly leads enhanced latent heat flux associated with increased surface westerlies. The model does produce realistic shortwave radiation anomalies, but its latent heat flux anomalies are too weak and do not constructively add with the shortwave radiation anomalies. It is concluded that coupling is not a panacea for problems of simulating the MJO in uncoupled GCMs and that coupling, if it is important, depends critically on the structure of the surface fluxes produced by the MJO.

Abstract

Relationships between Indonesian rainfall and Indo-Pacific sea surface temperatures (SSTs) and circulation anomalies are investigated using observations for 1951–97. Indonesia receives significant rainfall year-round but experiences a wet season that peaks in January and a dry season that peaks in August. Dry season rainfall anomalies are spatially coherent, strongly correlated with SST, and tightly coupled to El Niño–Southern Oscillation (ENSO) variations in the Pacific basin. Drought conditions typically occur during El Niño, when SSTs surrounding Indonesia are cool and the Walker circulation is weakened, resulting in anomalous surface easterlies across Indonesia. The opposite tends to occur during La Niña. Broadscale Indonesian rainfall and SST anomalies tend to not persist from the dry season into the wet season. Rainfall in the heart of the wet season tends to be uncorrelated with SST and spatially incoherent.

Seasonally varying feedback between Indonesian SST, winds, and rainfall explains the growth, persistence, and coherence of the local anomalies during the dry season and their decay or change in sign once the wet season commences. During the dry season anomalous surface easterlies, remotely driven by warm SSTs in the central Pacific during El Niño, act to increase local wind speed, cooling the ocean surrounding and to the east of Indonesia and thereby increasing the anomalous SST gradient across the Pacific. Hence, local rainfall and the Walker circulation are further reduced. Once the wet season commences and the climatological surface winds across Indonesia shift from southeasterly to northwesterly, the anomalous surface easterlies now act to reduce the wind speed. The initial cold SST anomaly is damped, reducing the negative rainfall anomalies and surface easterlies. The opposite scenario occurs during La Niña.

Indonesian rainfall variations during the dry season are also coupled to the development of an anomalous zonal SST gradient in the equatorial Indian Ocean. This anomalous gradient is strongly related to ENSO and is most prominent during the dry season. Once the wet season commences, the entire Indian Ocean tends to have the same-signed SST anomaly (positive during El Niño and negative during La Niña). Development and decay of this anomalous zonal SST gradient in the Indian Ocean is promoted by seasonally varying air–sea interaction in the eastern Indian Ocean in response to ENSO conditions in the Pacific. The eastern Indian Ocean SST changes are driven largely by induced surface heat flux variations (primarily changes in latent heat flux and net shortwave radiation). Biennial variations in the Indonesian region may also be induced by this seasonally varying air–sea interaction associated with ENSO.

Abstract

The Bureau of Meteorology, Australia, routinely analyzes the tropospheric winds over the Australian Tropical Region (40°S–40°N, 70°–180°E). These wind data are assimilated without the use of a forecast model. While being free of any model bias, the optimum interpolation scheme imposes no dynamical constraints on the winds. To assess the realism of the Australian Tropical Region analyses, a qualitative comparison with gridded ECMWF wind data and OLR (as a proxy for tropical convection) is conducted.

In general, the depiction of the large-scale tropical circulation of the Australian Tropical Region analyses is quite reasonable. The gross features of the Australian and Asian Monsoons seem equally captured by both the ECMWF and Australian analyses. The seasonal development of the two monsoons and the relationship between the vertical structure of the divergence and zonal wind depicted in the Australian analyses agree well with previous theoretical and observational studies. Subtle differences (such as with the phase of the upper level anticyclones relative to the divergence) between the theory and the dynamics inferred from the Australian analyses are highlighted. However, we conclude that these objectively analyzed tropospheric winds are a valuable data resource for both the comparison with forecast model assimilated data and for deduction of physical processes.

Abstract

Seasonal rainfall predictions for Australia from the Predictive Ocean Atmosphere Model for Australia (POAMA), version P15b, coupled model seasonal forecast system, which has been run operationally at the Australian Bureau of Meteorology since 2002, are overconfident (too low spread) and only moderately reliable even when forecast accuracy is highest in the austral spring season. The lack of reliability is a major impediment to operational uptake of the coupled model forecasts. Considerable progress has been made to reduce reliability errors with the new version of POAMA2, which makes use of a larger ensemble from three different versions of the model. Although POAMA2 can be considered to be multimodel, its individual models and forecasts are similar as a result of using the same perturbed initial conditions and the same model lineage. Reliability of the POAMA2 forecasts, although improved, remains relatively low. Hence, the authors explore the additional benefit that can be attained using more independent models available in the European Union Ensemble-Based Predictions of Climate Changes and their Impacts (ENSEMBLES) project.

Although forecast skill and reliability of seasonal predictions of Australian rainfall are similar for POAMA2 and the ENSEMBLES models, forming a multimodel ensemble using POAMA2 and the ENSEMBLES models is shown to markedly improve reliability of Australian seasonal rainfall forecasts. The benefit of including POAMA2 into this multimodel ensemble is due to the additional information and skill of the independent model, and not just due to an increase in the number of ensemble members. The increased reliability, as well as improved accuracy, of regional rainfall forecasts from this multimodel ensemble system suggests it could be a useful operational prediction system.

Abstract

A seasonally independent index for monitoring the Madden–Julian oscillation (MJO) is described. It is based on a pair of empirical orthogonal functions (EOFs) of the combined fields of near-equatorially averaged 850-hPa zonal wind, 200-hPa zonal wind, and satellite-observed outgoing longwave radiation (OLR) data. Projection of the daily observed data onto the multiple-variable EOFs, with the annual cycle and components of interannual variability removed, yields principal component (PC) time series that vary mostly on the intraseasonal time scale of the MJO only. This projection thus serves as an effective filter for the MJO without the need for conventional time filtering, making the PC time series an effective index for real-time use.

The pair of PC time series that form the index are called the Real-time Multivariate MJO series 1 (RMM1) and 2 (RMM2). The properties of the RMM series and the spatial patterns of atmospheric variability they capture are explored. Despite the fact that RMM1 and RMM2 describe evolution of the MJO along the equator that is independent of season, the coherent off-equatorial behavior exhibits strong seasonality. In particular, the northward, propagating behavior in the Indian monsoon and the southward extreme of convection into the Australian monsoon are captured by monitoring the seasonally independent eastward propagation in the equatorial belt. The previously described interannual modulation of the global variance of the MJO is also well captured.

Applications of the RMM series are investigated. One application is through their relationship with the onset dates of the monsoons in Australia and India; while the onsets can occur at any time during the convectively enhanced half of the MJO cycle, they rarely occur during the suppressed half. Another application is the modulation of the probability of extreme weekly rainfall; in the “Top End” region around Darwin, Australia, the swings in probability represent more than a tripling in the likelihood of an upper-quintile weekly rainfall event from the dry to wet MJO phase.