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Guomin Wang
and
Harry H. Hendon

Abstract

Australia typically experiences drought during El Niño, especially across the eastern two-thirds of the continent during austral spring (September–November). There have, however, been some interesting departures from this paradigm. For instance, the near-record-strength El Niño of 1997 was associated with near-normal rainfall. In contrast, eastern Australia experienced near-record drought during the modest El Niño of 2002. This stark contrast raises the issue of how the magnitude of the drought is related to the character and magnitude of El Niño, for instance as measured by the broadscale sea surface temperature (SST) anomaly in the equatorial eastern Pacific. Internal (unpredictable) atmospheric noise is one plausible explanation for this contrasting behavior during these El Niño events. Here, the authors suggest that Australian rainfall is sensitive to the zonal distribution of SST anomalies during El Niño and, in particular, the greatest sensitivity is to the SST variations on the eastern edge of the Pacific warm pool rather than in the eastern Pacific where El Niño variations are typically largest. Positive SST anomalies maximized near the date line in 2002, but in 1997 maximum anomalies were shifted well into the eastern Pacific, where their influence on Australian rainfall appears to be less. These findings provide a plausible physical basis for the view that forecasting the strength of El Niño is not sufficient to accurately predict rainfall variations across Australia during El Niño.

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Fiona Lo
and
Harry H. Hendon

Abstract

An empirical model that predicts the evolution of the Madden–Julian oscillation (MJO) in outgoing longwave radiation (OLR) and 200-mb streamfunction is developed. The model is based on the assumption that the MJO can be well represented by a pair of empirical orthogonal functions (EOFs) of OLR and three EOFs of streamfunction. With an eye toward using this model in real time, these EOFs are determined with data only subjected to filtering that can be applied in near–real time. Stepwise lag regression is used to develop the model on 11 winters of dependent data. The predictands are the leading two principal components (PCs) of OLR and the leading three PCs of streamfunction. The model is validated with five winters of independent data and is also compared to dynamic extended range forecasts (DERFs) made with the National Centers for Environmental Prediction’s Medium Range Forecast (MRF) model.

Skillful forecasts of the MJO in OLR and streamfunction with the empirical model are achieved out to about 15 days. Initial skill arises from autocorrelation of the PCs. Subsequent skill beyond about 1 week arises primarily from the cross correlation with the other PCs that define the MJO. Inclusion of PCs not associated with the MJO as predictors appears not to reliably improve skill. Skill is found to be substantially better when the MJO is active at the initial condition than when it is inactive. The empirical forecasts are also found to be more skillful than DERF from the MRF for lead times longer than about 1 week. Furthermore, skill of DERF from the MRF is found to be better when the MJO is quiescent than when it is active at the initial condition. It is suggested that significant improvement of tropical DERF could be achieved by improvement of the representation of the MJO in the dynamic forecast model.

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Sally Langford
and
Harry H. Hendon

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.

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Toshiaki Shinoda
,
Harry H. Hendon
, and
John Glick

Abstract

Reliability of the surface fluxes from National Centers for Environmental Prediction (NCEP) reanalyses is assessed across the warm pool of the western Pacific and Indian Oceans. Emphasis is given to the spatial distribution and coherence of the fluxes on intraseasonal (25–100 day) periods, as intraseasonal variability predominates the subseasonal variability across the warm pool. Comparison is made with surface fluxes estimated from data collected at a mooring during the Coupled Ocean–Atmosphere Response Experiment and with independent gridded estimates based on operational wind and surface pressure analyses and satellite observations of rainfall, shortwave radiation, and outgoing longwave radiation. In general, fluxes that depend primarily on surface wind variations (e.g., stress and latent heat flux) agree more favorably than fluxes that are largely dependent on fluctuations of convection (e.g., surface shortwave radiation and freshwater or precipitation). In particular, the intraseasonal variance of shortwave radiation and precipitation in the NCEP reanalyses is about half of that estimated from in situ observations and from satellite observations. Composite surface flux variations for the Madden–Julian oscillation, which is the dominant mode of intraseasonal variability in the warm pool, are also constructed. Again, the composite variations of wind stress and latent heat flux from the NCEP reanalyses agree reasonably well, both in magnitude and phasing, with the composite fluxes from the independent gridded data. However, the composite intraseasonal shortwave radiation and precipitation from the NCEP reanalyses, while agreeing in phase, exhibit less than half the amplitude of the satellite-based estimates.

The impact of the underestimation of these surface flux variations in the NCEP reanalyses on the intraseasonal evolution of sea surface temperature (SST) in the warm pool is investigated in the context of a one-dimensional mixed layer model. When forced with the intraseasonal surface fluxes from the NCEP reanalyses, the amplitude of the intraseasonal SST variation is some 30%–40% smaller than observed or than that from forcing with the independent gridded fluxes. This reduced amplitude is primarily caused by the underestimation of the intraseasonal shortwave radiation variations in the NCEP reanalyses.

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Wenju Cai
,
Harry H. Hendon
, and
Gary Meyers

Abstract

Coupled ocean–atmosphere variability in the tropical Indian Ocean is explored with a multicentury integration of the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Mark 3 climate model, which runs without flux adjustment. Despite the presence of some common deficiencies in this type of coupled model, zonal dipolelike variability is produced. During July through November, the dominant mode of variability of sea surface temperature resembles the observed zonal dipole and has out-of-phase rainfall variations across the Indian Ocean basin, which are as large as those associated with the model El Niño–Southern Oscillation (ENSO). In the positive dipole phase, cold SST anomaly and suppressed rainfall south of the equator on the Sumatra–Java coast drives an anticyclonic circulation anomaly that is consistent with the steady response (Gill model) to a heat sink displaced south of the equator. The northwest–southeast tilting Sumatra–Java coast results in cold sea surface temperature (SST) centered south of the equator, which forces anticylonic winds that are southeasterly along the coast, which thus produces local upwelling, cool SSTs, and promotes more anticylonic winds; on the equator, the easterlies raise the thermocline to the east via upwelling Kelvin waves and deepen the off-equatorial thermocline to the west via off-equatorial downwelling Rossby waves. The model dipole mode exhibits little contemporaneous relationship with the model ENSO; however, this does not imply that it is independent of ENSO. The model dipole often (but not always) develops in the year following El Niño. It is triggered by an unrealistic transmission of the model’s ENSO discharge phase through the Indonesian passages. In the model, the ENSO discharge Rossby waves arrive at the Sumatra–Java coast some 6 to 9 months after an El Niño peaks, causing the majority of model dipole events to peak in the year after an ENSO warm event. In the observed ENSO discharge, Rossby waves arrive at the Australian northwest coast. Thus the model Indian Ocean dipolelike variability is triggered by an unrealistic mechanism. The result highlights the importance of properly representing the transmission of Pacific Rossby waves and Indonesian throughflow in the complex topography of the Indonesian region in coupled climate models.

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Aihong Zhong
,
Harry H. Hendon
, and
Oscar Alves

Abstract

The evolution of the Indian Ocean during El Niño–Southern Oscillation is investigated in a 100-yr integration of an Australian Bureau of Meteorology coupled seasonal forecast model. During El Niño, easterly anomalies are induced across the eastern equatorial Indian Ocean. These act to suppress the equatorial thermocline to the west and elevate it to the east and initially cool (warm) the sea surface temperature (SST) in the east (west). Subsequently, the entire Indian Ocean basin warms, mainly in response to the reduced latent heat flux and enhanced shortwave radiation that is associated with suppressed rainfall. This evolution can be partially explained by the excitation of an intrinsic coupled mode that involves a feedback between anomalous equatorial easterlies and zonal gradients in SST and rainfall. This positive feedback develops in the boreal summer and autumn seasons when the mean thermocline is shallow in the eastern equatorial Indian Ocean in response to trade southeasterlies. This positive feedback diminishes once the climatological surface winds become westerly at the onset of the Australian summer monsoon.

ENSO is the leading mechanism that excites this coupled mode, but not all ENSO events are efficient at exciting it. During the typical El Niño (La Niña) event, easterly (westerly) anomalies are not induced until after boreal autumn, which is too late in the annual cycle to instigate strong dynamical coupling. Only those ENSO events that develop early (i.e., before boreal summer) instigate a strong coupled response in the Indian Ocean. The coupled mode can also be initiated in early boreal summer by an equatorward shift of the subtropical ridge in the southern Indian Ocean, which stems from uncoupled extratropical variability.

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S. Abhik
,
Harry H. Hendon
, and
Chidong Zhang

Abstract

The Madden–Julian oscillation (MJO) is often observed to weaken or sometimes completely decay as its convective anomaly moves from the Indian Ocean over to the Maritime Continent (MC), which is known as the MC barrier effect on the MJO. This barrier effect is often exaggerated in numerical models. Using 23 years of the retrospective intraseasonal forecast from two coupled model systems with useful MJO prediction skills, we show that the predictive skill of the real-time multivariate MJO (RMM) index for the continuously propagating MJO events across the MC region is higher than for the blocked MJO events. The greater prediction skill is not related to the higher initial RMM amplitude for the continuous MJO events. Rather the higher skill arises from the more persistent behavior of the propagating MJO events as the convective anomaly moves through the MC region into the western Pacific. The potential predictability is similar for both types of MJO events, suggesting the forecast models hardly differentiate the two types of MJO events in prediction; they only maintain higher RMM magnitudes of the continuously propagating events. The global reanalysis dataset indicates that the blocked events are often associated with persistent higher surface pressures over colder sea surface temperatures in the central Pacific, suggesting the large-scale environment plays a role in promoting or inhibiting the MJO propagation across the MC region. Caveats in the models to reproduce the observed MJO events are also discussed.

Open access
Toshiaki Shinoda
,
Harry H. Hendon
, and
John Glick

Abstract

Composites of sea surface temperature (SST), surface heat, momentum, and freshwater flux anomalies associated with intraseasonal oscillations of convection are developed for the warm pool of the western Pacific and Indian Oceans during 1986–93. The composites are based on empirical orthogonal function analysis of intraseasonally filtered outgoing longwave radiation (OLR), which efficiently extracts the Madden–Julian oscillation (MJO) in convection. Surface fluxes are estimated using gridded analyses from the European Centre for Medium-Range Weather Forecasts, weekly SST, OLR, microwave sounding unit precipitation, and the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) bulk flux algorithm. At intraseasonal timescales, these surface flux estimates agree reasonably well with estimates based on mooring observations collected during TOGA COARE.

The amplitude of the composite SST variation produced by the MJO is about 0.25°C in the western Pacific, 0.35°C in the Indonesian region, and 0.15°C in the Indian Ocean. The intraseasonal anomalies of SST and net surface heat flux propagate eastward at about 4 m s−1 along with the convective anomaly. The amplitude of the net surface heat flux variation is 50–70 W m−2 in the western Pacific, with anomalous insolation and latent heat flux making similar contributions. Across the Indian Ocean, the net surface heat flux anomaly is weaker (30–40 W m−2), and anomalous insolation appears to make a greater contribution than anomalous latent heat flux. Across the entire warm pool, the net surface heat flux leads the SST variation by about one-quarter cycle, which is consistent with the notion that surface heat flux variations are driving the SST variations at these intraseasonal timescales. The intraseasonal SST variation, however, is estimated to significantly reduce the amplitude of the latent and sensible heat fluxes produced by the MJO.

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Harry H. Hendon
,
Noel E. Davidson
, and
Bruce Gunn

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.

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Dennis L. Hartmann
,
Harry H. Hendon
, and
Robert A. Houze Jr.

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.

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