Scott B. Power

The Predictability of Interdecadal Changes in ENSO Activity and ENSO Teleconnections

Abstract El Nino–Southern Oscillation (ENSO) in a century-long integration of a Bureau of Meteorology Research Centre (BMRC) coupled general circulation model (CGCM) drives rainfall and temperature changes over Australia that are generally consistent with documented observational changes: dry/hot conditions occur more frequently during El Nino years and wet/mild conditions occur more frequently during La Nina years. The relationship between ENSO [as measured by Nino-4 or the Southern Oscillation index (SOI), say] and all-Australia rainfall and temperature is found to be nonlinear in the observations and in the CGCM during June–December: a large La Nina sea surface temperature (SST) anomaly is closely linked to a large Australian response (i.e., Australia usually becomes much wetter), whereas the magnitude of an El Nino SST anomaly is a poorer guide to how dry Australia will actually become. Australia tends to dry out during El Nino events, but the degree of drying is not as tightly linked to the magnitude...

Robust twenty-first-century projections of El[thinsp]Nino and related precipitation variability

The El Niño–Southern Oscillation (ENSO) drives substantial variability in rainfall, severe weather, agricultural production, ecosystems and disease in many parts of the world. Given that further human-forced changes in the Earth’s climate system seem inevitable, the possibility exists that the character of ENSO and its impacts might change over the coming century. Although this issue has been investigated many times during the past 20 years, there is very little consensus on future changes in ENSO, apart from an expectation that ENSO will continue to be a dominant source of year-to-year variability. Here we show that there are in fact robust projected changes in the spatial patterns of year-to-year ENSO-driven variability in both surface temperature and precipitation. These changes are evident in the two most recent generations of climate models, using four different scenarios for CO2 and other radiatively active gases. By the mid- to late twenty-first century, the projections include an intensification of both El-Niño-driven drying in the western Pacific Ocean and rainfall increases in the central and eastern equatorial Pacific. Experiments with an Atmospheric General Circulation Model reveal that robust projected changes in precipitation anomalies during El Niño years are primarily determined by a nonlinear response to surface global warming. Uncertain projected changes in the amplitude of ENSO-driven surface temperature variability have only a secondary role. Projected changes in key characteristics of ENSO are consequently much clearer than previously realized.

A Tripole Index for the Interdecadal Pacific Oscillation

Abstract A new index is developed for the Interdecadal Pacific Oscillation, termed the IPO Tripole Index (TPI). The IPO is associated with a distinct ‘tripole’ pattern of sea surface temperature anomalies (SSTA), with three large centres of action and variations on decadal timescales, evident in the second principal component (PC) of low-pass filtered global SST. The new index is based on the difference between the SSTA averaged over the central equatorial Pacific and the average of the SSTA in the Northwest and Southwest Pacific. The TPI is an easily calculated, non-PC-based index for tracking decadal SST variability associated with the IPO. The TPI time series bears a close resemblance to previously published PC-based indices and has the advantages of being simpler to compute and more consistent with indices used to track the El Niño–Southern Oscillation (ENSO), such as Niño 3.4. The TPI also provides a simple metric in physical units of °C for evaluating decadal and interdecadal variability of SST fields in a straightforward manner, and can be used to evaluate the skill of dynamical decadal prediction systems. Composites of SST and mean sea level pressure anomalies reveal that the IPO has maintained a broadly stable structure across the seven most recent positive and negative epochs that occurred during 1870–2013. The TPI is shown to be a robust and stable representation of the IPO phenomenon in instrumental records, with relatively more variance in decadal than shorter timescales compared to Niño 3.4, due to the explicit inclusion of off-equatorial SST variability associated with the IPO.

More extreme swings of the South Pacific convergence zone due to greenhouse warming

The South Pacific convergence zone (SPCZ) is the Southern Hemisphere’s most expansive and persistent rain band, extending from the equatorial western Pacific Ocean southeastward towards French Polynesia. Owing to its strong rainfall gradient, a small displacement in the position of the SPCZ causes drastic changes to hydroclimatic conditions and the frequency of extreme weather events—such as droughts, floods and tropical cyclones—experienced by vulnerable island countries in the region. The SPCZ position varies from its climatological mean location with the El Niño/Southern Oscillation (ENSO), moving a few degrees northward during moderate El Niño events and southward during La Niña events. During strong El Niño events, however, the SPCZ undergoes an extreme swing—by up to ten degrees of latitude toward the Equator—and collapses to a more zonally oriented structure with commensurately severe weather impacts. Understanding changes in the characteristics of the SPCZ in a changing climate is therefore of broad scientific and socioeconomic interest. Here we present climate modelling evidence for a near doubling in the occurrences of zonal SPCZ events between the periods 1891–1990 and 1991–2090 in response to greenhouse warming, even in the absence of a consensus on how ENSO will change. We estimate the increase in zonal SPCZ events from an aggregation of the climate models in the Coupled Model Intercomparison Project phases 3 and 5 (CMIP3 and CMIP5) multi-model database that are able to simulate such events. The change is caused by a projected enhanced equatorial warming in the Pacific and may lead to more frequent occurrences of extreme events across the Pacific island nations most affected by zonal SPCZ events.

Decadal climate variability in Australia during the twentieth century

High quality rainfall and surface temperature records for Australia during the period 1910–1993 are examined to quantify the relative importance of decadal variability and to ascertain if there is any relationship with sea-surface temperature (SST) variability over adjacent oceans on the same time scale. The decadal signal was estimated by low-pass filtering detrended annual averages of gridded data covering the entire continent to eliminate all spectral contributions with periods less than or equal to 8 years. Such variability typically accounts for 10–60% of the total variance. Low-pass filtering does not appear to affect the structure of the leading empirical orthogonal functions (EOFs) of rainfall and temperature, and the variability of all-Australia averages of these quantities are reasonably well-modeled as red noise, for which there is no preference for decadal time scales. Decadal variability in Indian Ocean SST south of 40°S is associated with rainfall variability over eastern Australia. A tendency for increased Tasman Sea SST (south of 15°S) to coincide with the above normal central and eastern Australian rainfall on both interannual and decadal time scales is also evident. The first EOF of interannual Pacific SST is associated with rainfall variability over Australia. This is not surprising as the EOF has an amplitude that tends to be out of phase with the Southern Oscillation Index (SOI). A similar relationship exists between the EOF of decadal SST variability and decadal fluctuations in both rainfall and the SOI. The first EOF of decadal Pacific SST has a broad spatial structure extending into the North and South Pacific. It has a time coefficient that is well-represented by the decadal component of the (northern) winter-time SST EOF produced in a separate study, which is available back to the turn of the century. When the decadal SST EOF warms the central Pacific, Australian rainfall is reduced, and both the daily maximum temperature and the diurnal temperature range over Australia are increased. These changes are consistent with radiative and evaporative changes associated with fluctuations in rainfall, cloud cover and soil moisture.

Attribution of the late-twentieth-century rainfall decline in southwest Australia

Abstract There was a dramatic decrease in rainfall in the southwest of Australia (SWA) in the mid-1960s. A statistical method, based on the idea of analogous synoptic situations, is used to help clarify the cause of the drying. The method is designed to circumvent error in the rainfall simulated directly by a climate model, and to exploit the ability of the model to simulate large-scale fields reasonably well. The method uses relationships between patterns of various atmospheric fields with station records of rainfall to improve the simulation of the local rainfall spatial variability. The original technique was developed in a previous study. It is modified here for application to two four-member ensembles of simulations of the climate from 1870 to 1999 performed with the Parallel Climate Model (PCM). The first ensemble, called “natural,” is forced with natural variations in both volcanic activity and solar forcing. The second ensemble, called “full forcing,” also includes three types of human-induced for...

Evaluation of the South Pacific Convergence Zone in IPCC AR4 Climate Model Simulations of the Twentieth Century

Abstract Understanding how the South Pacific convergence zone (SPCZ) may change in the future requires the use of global coupled atmosphere–ocean models. It is therefore important to evaluate the ability of such models to realistically simulate the SPCZ. The simulation of the SPCZ in 24 coupled model simulations of the twentieth century is examined. The models and simulations are those used for the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC). The seasonal climatology and interannual variability of the SPCZ is evaluated using observed and model precipitation. Twenty models simulate a distinct SPCZ, while four models merge intertropical convergence zone and SPCZ precipitation. The majority of models simulate an SPCZ with an overly zonal orientation, rather than extending in a diagonal band into the southeast Pacific as observed. Two-thirds of models capture the observed meridional displacement of the SPCZ during El Nino and La Nina events. The four models that use ...

Consensus on Twenty-First-Century Rainfall Projections in Climate Models More Widespread than Previously Thought

AbstractUnder global warming, increases in precipitation are expected at high latitudes and near major tropical convergence zones in some seasons, while decreases are expected in many subtropical and midlatitude areas in between. In many other areas there is no consensus among models on the sign of the projected change. This is often assumed to indicate that precipitation projections in these regions are highly uncertain.Here, twenty-first century precipitation projections under the Special Report on Emissions Scenarios (SRES) A1B scenario using 24 World Climate Research Programme (WCRP)/Coupled Model Intercomparison Project phase 3 (CMIP3) climate models are examined. In areas with no consensus on the sign of projected change there are extensive subregions where the projected change is “very likely” (i.e., probability > 0.90) to be small (relative to, e.g., the size of interannual variability during the late twentieth century) or zero. The statistical significance of and interrelationships between method...

Rainfall Variability at Decadal and Longer Time Scales: Signal or Noise?

Rainfall variability occurs over a wide range of temporal scales. Knowledge and understanding of such variability can lead to improved risk management practices in agricultural and other industries. Analyses of temporal patterns in 100 yr of observed monthly global sea surface temperature and sea level pressure data show that the single most important cause of explainable, terrestrial rainfall variability resides within the El Nino-Southern Oscillation (ENSO) frequency domain (2.5-8.0 yr), followed by a slightly weaker but highly significant decadal signal (9-13 yr), with some evidence of lesser but significant rainfall variability at interclecadal time scales (15-18 yr). Most of the rainfall variability significantly linked to frequencies tower than ENSO occurs in the Australasian region, with smaller effects in North and South America, central and southern Africa, and western Europe. While low-frequency (LF) signals at a decadal frequency are dominant, the variability evident was ENSO-like in all the frequency domains considered. The extent to which such LF variability is (i) predictable and (ii) either part of the overall ENSO variability or caused by independent processes remains an as yet unanswered question. Further progress can only be made through mechanistic studies using a variety of models.

The Influence of Climate Science on Water Management in Western Australia: Lessons for Climate Scientists

Abstract Water flow into dams that supply Perth in Western Australia (WA) has fallen by 50% since the mid-1970s, and this has severely tested water managers. Climate change scenarios available since the 1980s have suggested that global warming will reduce rainfall over southern Australia, including Perth. Water managers recognize the uncertainties associated with the projections, including the significant differences that exist between the timing and magnitude of the observed changes and modeled projections. The information has, nevertheless, influenced their decision making. To understand why, we need to consider the broader environment in which the water managers operate. One key factor is that the imposition of severe water restrictions can lead to significant economic loss and increased unemployment. Prolonged restrictions can therefore create strong debate in the wider community. In recognition of this, state government policy requires that water managers ensure that the chance of having severe restr...