Harry H. Hendon

An All-Season Real-Time Multivariate MJO Index: Development of an Index for Monitoring and Prediction

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 RMM...

The life cycle of the Madden-Julian oscillation

Abstract A composite life cycle of the Madden–Julian oscillation (MJO) is constructed from the cross covariance between outgoing longwave radiation (OLR), wind, and temperature. To focus on the role of convection, the composite is based on episodes when a discrete signal in OLR is present. The composite convective anomaly possesses a predominantly zonal wavenumber 2 structure that is confined to the eastern hemisphere. There, it propagates eastward at about 5 m s−1 and evolves through a systematic cycle of amplification and decay. Unlike the convective anomaly, the circulation anomaly is not confined to the eastern hemisphere. The circulation anomaly displays characteristics of both a forced response, coupled to the convective anomaly as it propagates across the eastern hemisphere, and a radiating response, which propagates away from the convective anomaly into the western hemisphere at about 10 m s−1. The forced response appears as a coupled Rossby–Kelvin wave while the radiating response displays predom...

On the Remote Drivers of Rainfall Variability in Australia

Abstract This work identifies and documents a suite of large-scale drivers of rainfall variability in the Australian region. The key driver in terms of broad influence and impact on rainfall is the El Nino–Southern Oscillation (ENSO). ENSO is related to rainfall over much of the continent at different times, particularly in the north and east, with the regions of influence shifting with the seasons. The Indian Ocean dipole (IOD) is particularly important in the June–October period, which spans much of the wet season in the southwest and southeast where IOD has an influence. ENSO interacts with the IOD in this period such that their separate regions of influence cover the entire continent. Atmospheric blocking also becomes most important during this period and has an influence on rainfall across the southern half of the continent. The Madden–Julian oscillation can influence rainfall in different parts of the continent in different seasons, but its impact is strongest on the monsoonal rains in the north. Th...

Australian Rainfall and Surface Temperature Variations Associated with the Southern Hemisphere Annular Mode

Abstract Daily variations in Australian rainfall and surface temperature associated with the Southern Hemisphere annular mode (SAM) are documented using observations for the period 1979–2005. The high index polarity of the SAM is characterized by a poleward contraction of the midlatitude westerlies. During winter, the high index polarity of the SAM is associated with decreased daily rainfall over southeast and southwest Australia, but during summer it is associated with increased daily rainfall on the southern east coast of Australia and decreased rainfall in western Tasmania. Variations in the SAM explain up to ∼15% of the weekly rainfall variance in these regions, which is comparable to the variance accounted for by the El Nino–Southern Oscillation, especially during winter. The most widespread temperature anomalies associated with the SAM occur during the spring and summer seasons, when the high index polarity of the SAM is associated with anomalously low maximum temperature over most of central/easter...

Intraseasonal Behavior of Clouds, Temperature, and Motion in the Tropics

Abstract The spectral character of tropical convection is investigated in an 11-yr record of outgoing longwave radiation from the Advanced Very High Resolution Radiometer to identify interaction with the tropical circulation. Along the equator in the eastern hemisphere, the space–time spectrum of convection possesses a broad peak at wave-numbers 1–3 and eastward periods of 35–95 days. Significantly broader than the dynamical signal of the Madden–Julian oscillation (MJO), this quasi-discrete convective signal is associated with a large-scale anomaly that propagates across and modulates time mean or “climatological convection” over the equatorial Indian Ocean and western Pacific. Outside that region the convective signal is small, even though, under amplified conditions, coherence can be found east of the date line and in the subtropics. Having a zonal scale of approximately wavenumber 2, anomalous convection propagates eastward at some 5 m s−1 and suppresses as well as reinforces climatological convection ...

Intraseasonal Variability of Surface Fluxes and Sea Surface Temperature in the Tropical Western Pacific and Indian Oceans

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 ...

Some Implications of the Mesoscale Circulations in Tropical Cloud Clusters for Large-Scale Dynamics and Climate

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 c...

MJO Simulation Diagnostics

The Madden–Julian oscillation (MJO) interacts with and influences a wide range of weather and climate phenomena (e.g., monsoons, ENSO, tropical storms, midlatitude weather), and represents an important, and as yet unexploited, source of predictability at the subseasonal time scale. Despite the important role of the MJO in climate and weather systems, current global circulation models (GCMs) exhibit considerable shortcomings in representing this phenomenon. These shortcomings have been documented in a number of multimodel comparison studies over the last decade. However, diagnosis of model performance has been challenging, and model progress has been difficult to track, because of the lack of a coherent and standardized set of MJO diagnostics. One of the chief objectives of the U.S. Climate Variability and Predictability (CLIVAR) MJO Working Group is the development of observation-based diagnostics for objectively evaluating global model simulations of the MJO in a consistent framework. Motivation for this activity is reviewed, and the intent and justification for a set of diagnostics is provided, along with specification for their calculation, and illustrations of their application. The diagnostics range from relatively simple analyses of variance and correlation to more sophisticated space–time spectral and empirical orthogonal function analyses. These diagnostic techniques are used to detect MJO signals, to construct composite life cycles, to identify associations of MJO activity with the mean state, and to describe interannual variability of the MJO.

Seasonal Dependence of the MJO-ENSO Relationship

Abstract Observations of the development of recent El Nino events suggest a pivotal role for the Madden–Julian oscillation (MJO). Previous attempts to uncover a systematic relationship between MJO activity and the El Nino–Southern Oscillation (ENSO), however, have yielded conflicting results. In this study the MJO–ENSO relationship is stratified by season, and the focus is on MJO activity in the equatorial western Pacific. The results demonstrate that MJO activity in late boreal spring leads El Nino in the subsequent autumn–winter for the period 1979–2005. Spring is the season when MJO activity is least asymmetric with respect to the equator and displays the most sensitivity to SST variations at the eastern edge of the warm pool. Enhanced MJO activity in the western Pacific in spring is associated with an eastward-expanded warm pool and low-frequency westerly surface zonal wind anomalies. These sustained westerly anomalies in the western Pacific are hypothesized to project favorably onto a developing El N...

Sensitivity of Australian Rainfall to Inter-El Niño Variations

Australia typically experiences drought during El Nino, 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 Nino of 1997 was associated with near-normal rainfall. In contrast, eastern Australia experienced near-record drought during the modest El Nino of 2002. This stark contrast raises the issue of how the magnitude of the drought is related to the character and magnitude of El Nino, 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 Nino events. Here, the authors suggest that Australian rainfall is sensitive to the zonal distribution of SST anomalies during El Nino 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 Nino 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 Nino is not sufficient to accurately predict rainfall variations across Australia during El Nino.