Andrew G. Marshall

Decadal increase in Ningaloo Niño since the late 1990s

Ningaloo Nino refers to the episodic occurrence of anomalously warm ocean conditions along the subtropical coast of Western Australia (WA). Ningaloo Nino typically develops in austral spring, peaks in summer, and decays in autumn, and it often occurs in conjunction with La Nina conditions in the Pacific which promote poleward transport of warm tropical waters by the Leeuwin Current. Since the late 1990s, there has been a marked increase in the occurrence of Ningaloo Nino, which is likely related to the recent swing to the negative phase of the Interdecadal Pacific Oscillation (IPO) and enhanced El Nino–Southern Oscillation variance since 1970s. The swing to the negative IPO sustains positive heat content anomalies and initiates more frequent cyclonic wind anomalies off the WA coast so favoring enhanced poleward heat transport by the Leeuwin Current. The anthropogenically forced global warming has made it easier for natural variability to drive extreme ocean temperatures in the region.

Initiation and amplification of the Ningaloo Niño

AbstractnMarine heat waves along the Western Australian coast are potentially damaging to the marine environment especially coastal fisheries and the Ningaloo Reef. Initiation and amplification mechanisms for marine heat waves (referred to as ‘Ningaloo Niño’ events) are explored using ocean and atmosphere reanalyses for the period 1960–2011. We find that the onset stage from October to November is promoted by wind-evaporation-SST feedback that operates to the northwest of the coast on the north-eastern flank of the Mascarene subtropical high: cyclonic anomalies act to reduce the surface wind speed and warm the ocean surface, thereby driving increased rainfall and stronger cyclonic anomalies. The growth and southward expansion of positive SST anomalies along the Australian west coast is further supplemented by anomalous poleward advection of heat by the Leeuwin Current, which is coupled with the cyclonic anomalies off the coast. The strongest Ningaloo Niño events, such as the record strong 2011 event, occur in conjunction with La Niña conditions in the Pacific, which drives westerly wind anomalies to the northwest of Australia that can promote the WES feedback and accelerate the Leeuwin Current via transmission of thermocline anomalies from the western Pacific onto the west Australian coast. However, many Ningaloo Niño events occur independent of La Niña and some Ningaloo Niño events even occur during certain El Niños. We explain this general independence from ENSO because the triggering of Ningaloo Niño events from the Pacific is most sensitive to antecedent SST anomalies in the far western Pacific, rather than in the central Pacific where ENSO typically has greatest magnitude.

Impact of the quasi-biennial oscillation on predictability of the Madden–Julian oscillation

The Madden–Julian oscillation (MJO) during boreal winter is observed to be stronger during the easterly phase of the quasi-biennial oscillation (QBO) than during the westerly phase, with the QBO zonal wind at 50xa0hPa leading enhanced MJO activity by about 1xa0month. Using 30xa0years of retrospective forecasts from the POAMA coupled model forecast system, we show that this strengthened MJO activity during the easterly QBO phase translates to improved prediction of the MJO and its convective anomalies across the tropical Indo-Pacific region by about 8xa0days lead time relative to that during westerly QBO phases. These improvements in forecast skill result not just from the fact that forecasts initialized with stronger MJO events, such as occurs during QBO easterly phases, have greater skill, but also from the more persistent behaviour of the MJO for a similar initial amplitude during QBO easterly phases as compared to QBO westerly phases. The QBO is thus an untapped source of subseasonal predictability that can provide a window of opportunity for improved prediction of global climate.

Subseasonal Prediction of Australian Summer Monsoon Anomalies

Subseasonal prediction of Australian summer monsoon anomalies is assessed using 30u2009years of retrospective forecasts from version 2 of the Predictive Ocean Atmosphere Model for Australia. Active and break monsoon rainfall episodes are associated with large-scale cyclonic westerly and anticyclonic easterly winds, respectively, for which the Madden-Julian oscillation (MJO) makes a dominant contribution and thus is a source of predictability. Although the forecast model can predict the local large-scale zonal wind anomalies for lead times beyond 4u2009weeks, predictive skill of the monsoon rainfall anomalies is limited to about 2u2009weeks. We show that improving the prediction of the MJO and its local expression in the summer monsoon leads to improved monsoon rainfall predictions at multiweek timescales.

Visualizing and Verifying Probabilistic Forecasts of the Madden-Julian Oscillation

We describe a new approach for presenting probabilistic forecasts of the Madden-Julian Oscillation (MJO) based on the community standard Real-time Multivariate MJO (RMM) index, using forecasts from version 2 of the Predictive Ocean Atmosphere Model for Australia. This new display overcomes the difficulty of interpreting a dispersive ensemble plume and directly quantifies the probability for the MJO to occur in each of its eight RMM-defined phases as well as the weak phase. Beyond monitoring and interpreting predictions of the MJO, this new approach also provides a basis for forecast verification using probability-based skill scores. Here we present a clear and concise quantitative summary of this innovative method for accessing probability of the state of the MJO in an ensemble forecast. This new method compliments the traditional MJO ensemble forecast display and verification and will benefit global forecasting centers, international MJO working groups, and the World Meteorological Organization Subseasonal to Seasonal Project.

Multi-week prediction of the Madden–Julian oscillation with ACCESS-S1

We assess the ability of the Bureau of Meteorology’s new ACCESS-S1 dynamical forecast system to predict the MJO using retrospective forecasts for the period 1990–2012. Compared to the benchmark POAMA-2 system, ACCESS-S1 demonstrates improved skill in predicting the ensemble mean bivariate RMM index by about 4xa0days lead time in austral summer and 5xa0days in boreal summer. Probabilistic forecast scores further demonstrate improved skill in predicting MJO amplitude by at least 7xa0days, and MJO phase by about 9xa0days. However, the ensemble from ACCESS-S1 for the MJO is underdispersed, indicating further gains in forecast skill can still be achieved. Improvements in the regional depiction of MJO rainfall in ACCESS-S1 over POAMA-2 include a more realistic southward extension of austral summer rainfall over Northern Australia, and a better overall spatial distribution and eastward extension of boreal summer rainfall over the tropical Indo-Pacific region. Both models depict well the northward propagation of boreal summer rainfall over the Indian Ocean warm pool. Overall, ACCESS-S1 simulates the MJO signature in global rainfall at least as well as, if not better than, POAMA-2.