Search

In this paper, we report on the frequency of false onsets of wet season rainfall in the case of the Northern Australian wet season and investigate the role of large‐scale tropical climate processes such as the El Niño–Southern Oscillation, Indian Ocean Dipole (IOD) and Madden–Julian Oscillation. A false onset occurs when a wet season rainfall onset criterion is met, but follow‐up rainfall is not received for weeks or months later. Our analysis of wet season rainfall data from 1950 through 2020 shows a false onset occurs, on average, between 20 and 30% of wet seasons across all of northern Australia. This increases at a regional and local level such as at Darwin, the Northern Territory (NT), and parts of Queensland's north coast to over 50%. Seasonal climate influences, such as a La Niña pattern and a negative IOD that typically expedite the wet season rainfall onset, also increase the likelihood of a false onset over northern Australia. Our analysis also finds that periods of false onsets can sometimes, but not always, coincide with periods of rapid soil moisture depletion. The false rainfall onsets that develop into flash drought can be potentially disruptive and costly and are of potential significance for agriculture and fire management in northern Australia, and in other monsoonal climates that also typically experience a slow build‐up to the seasonal monsoon. In conclusion, effective rainfall indicates that many seasons experience 'false onsets' with dry conditions after early rainfall. We propose that false onsets are a physical characteristic of the climate of northern Australia which occurs with relatively high frequency. In addition, these false onsets may sometimes co‐occur with a flash drought. [ABSTRACT FROM AUTHOR]

The Sea Slug Census program in Australia engages with citizen scientists to record the diversity and distribution of sea slugs across multiple locations. The program has consistently recorded shifts in distribution patterns but a recent, nine-day census in subtropical eastern Australia recorded unprecedented range extensions of tropical species. Seven species (six chromodorids and one polycerid) were found further south of their previously known distribution with Hypselodoris bertschi being recorded for the first time in Australia. These observations suggested the recent transport of larvae via the East Australian Current with recruitment to coastal sites possibly promoted by a protracted period of strong onshore winds associated with the 2021/22 La Niña in the western Pacific. With the increasing frequency of poleward range extensions of marine taxa, citizen science programs such as the Sea Slug Census provide the opportunity to substantially increase monitoring efforts. Linking with iNaturalist strengthens the value of the observations through online peer review to confirm species identities as well as the incorporation of substantiated (Research Grade) records into international biodiversity databases such as GBIF. [ABSTRACT FROM AUTHOR]

The El Niño‐Southern Oscillation (ENSO) is considered an important driver of rainfall variability in Australia, amongst many other global locations. Despite knowledge of the expected modulation of seasonal rainfall by ENSO, there is no consistently used method to quantify the role that specific ENSO events play in driving the observed anomalous rainfall. In this manuscript we adapt the Fraction of Attributable Risk (FAR) method, commonly used to identify the anthropogenic impact on a particular event, to quantify the impact of ENSO on the occurrence of monthly rainfall anomalies. We also explicitly calculate the ENSO induced change in risk and the FAR for all observed spring rainfall rates for our eastern Australian regions. A prominent role for ENSO in driving the large spring 2022 rainfall anomalies is identified. Though we choose to focus on ENSOs impact on rainfall in various Eastern Australian regions, the results are applicable to other climate modes, regions and climatic variables. Plain Language Summary: The El Niño‐Southern Oscillation (ENSO) is considered an important driver of rainfall variability in Australia, amongst many other global locations. Despite understanding how ENSO is expected to alter rainfall, we do not currently quantify the role ENSO played in driving a observed rainfall anomaly in any given season. In this manuscript we adapt a method that is commonly used to identify the anthropogenic impact on a particular event; and instead, we quantify the impact of ENSO on the occurrence of monthly rainfall anomalies. We then calculate the ENSO‐induced change in risk for all observed spring rainfall rates for our selected eastern Australian regions. A prominent role for ENSO in driving large rainfall anomalies of spring 2022 is also identified. Though we choose to focus on ENSOs impact on rainfall in various eastern Australian regions in this study, the results are applicable to other climate modes, regions and climatic variables. Key Points: We adapt the commonly used Fraction of Attributable Risk method to attribute rainfall variability to the El Niño‐Southern OscillationWe present the ENSO induced change in risk and the FAR for all observed spring rainfall rates for three eastern Australian regionsThe increased spring 2022 East Australian rainfall was >5 times more likely, and largely attributed to the La Niña conditions present [ABSTRACT FROM AUTHOR]

El Niño–Southern Oscillation (ENSO) is often characterized through the use of sea surface temperature (SST) departures from their climatological values, as in the Niño-3.4 index. However, this approach is problematic in a changing climate when the climatology itself is varying. To address this issue, van Oldenborgh et al. proposed a relative Niño-3.4 SST index, which subtracts the tropical mean SST anomaly from the Niño-3.4 index and multiplies by a scaling factor. We extend their work by providing a simplified calculation procedure for the scaling factor, and confirm that the relative index demonstrates reduced sensitivity to climate change and multidecadal variability. In particular, we show in three observational SST datasets that the relative index provides a more consistent classification of historical El Niño and La Niña oceanic conditions that is more robust across climatological periods compared to the nonrelative index. Forecast skill of the relative Niño-3.4 index in the North American Multimodel Ensemble (NMME) and ACCESS-S2 is slightly reduced for targets during the first half of the year because subtracting the tropical mean removes a source of additional skill. For targets in the second half of the year, the relative and nonrelative indices are equally skillful. Observed ENSO teleconnections in 200-hPa geopotential height and precipitation during key seasons are sharper and explain more variability over Australia and the contiguous United States when computed with the relative index. Overall, the relative Niño-3.4 index provides a more robust option for real-time monitoring and forecasting ENSO in a changing climate. Significance Statement: The goal of this study is to further explore a relative sea surface temperature index for monitoring and prediction of El Niño–Southern Oscillation. Sea surface temperature indices are typically computed as a difference from a 30-yr climatological average, and El Niño and La Niña events occur when values exceed a certain threshold. This method is suitable when the climate is stationary. However, because of climate change and other lower-frequency variations, historical El Niño and La Niña events are reclassified depending on which climatological period is selected. A relative index is investigated to ameliorate this problem. [ABSTRACT FROM AUTHOR]

Australia has long been observed to experience the ENSO (El Niño Southern Oscillation) of periodic alternation of flooding and drought. Australia has experienced consecutive years of the La Niña cycle of heavy rain and flooding. At some stage, this will revert to one or more years of drought, often severe. In previous years of drought, consideration was given to alternative water supply, including recycled wastewater. This paper reflects on the case of the previous period of El Niño drought in South East Queensland (SEQ), aiming to contribute to drought mitigation planning through reviving consideration of recycled water. • In depth case study on Queensland response to drought conditions. • Includes analysis of attempts to introduce other water sources, particularly recycled water for potable and non potable uses. • Examines reasons for failure and non-acceptance and uses comparisons with successful schemes to suggest policy changes for a way forward. [ABSTRACT FROM AUTHOR]

In this paper, the gravity recovery and climate experiment (GRACE), altimetry, ARGO, TRMM (tropical rainfall measurement mission) data and multivariate ENSO index (MEI) from January 2005 to December 2016 are adopted to analyze the effects of ENSO on the Western Pacific and its coastal areas, from the aspects of sea level, oceanic mass, steric and the terrestrial water storage change. During the GRACE data processing, we remove the effect of tectonic signals, and use an iterative forward model- ing method to recover time-varying signal. The result shows, the changes in the Western Pacific mean sea level (WPMSL) in response to ENSO are as follows: The sea level is lower during El Niño and higher dur- ing La Niña; in particular, during 2014—2016 El Nino, the WPMSL dropped by nearly 26.2 mm. The ste- ric change is the dominant factor affecting the nonseasonal anomalies of the WPMSL. The nonseasonal anomalies of water storage in Australia and Indochina Peninsula also respond well to the ENSO, and the correlation coefficients with the MEI are －0.61 (lag three months) and －0.65 (lag eight months). The re- sponse of the Yangtze River basin to ENSO is mainly in the middle and lower basin, 2014—2016 El Niño caused the water storage in the Yangtze River basin to reach the maximum value in recent years. [ABSTRACT FROM AUTHOR]

This study examines the multiscale modulation of mean and extreme rainfall in Northeast (NE) Australia under different background modes of variability, which is a new aspect given the high-resolution and long-term observational datasets. Daily rainfall probability is significantly modified by the Madden–Julian oscillation (MJO), and its influence varies with the seasons and is associated with atmospheric circulation anomalies. Rainfall generally decreases during El Niño and increases during La Niña years; however, there is a notable spatial nuance to El Niño–Southern Oscillation (ENSO)-associated extreme rainfall, with some locations showing the opposite precipitation response to the typical ENSO–rainfall relationship. Despite more precipitation overall in La Niña years, the mean and extreme precipitation responses to the MJO appear to be stronger and more often statistically significant during El Niño compared to La Niña periods. The impact of ENSO on the MJO–rainfall relationship is stronger than the variation of the MJO itself with ENSO, and likely reflects a change in the MJO modulation of rain-bearing atmospheric processes. During El Niño periods, diurnal rainfall amplitude is generally stronger in the central and southern subtropical parts of the study area than during La Niña periods, while the opposite tendency occurs in the northern tropical part. The diurnal cycle of both mean and extreme precipitation is amplified during suppressed convection phases compared to enhanced convection phases of the MJO. In general, the peak time of diurnal cycle does not change with MJO regimes, but there are some notable differences in rainfall propagation between enhanced and suppressed MJO phases. Significance Statement: This study presents a new perspective on the relationship between precipitation in Northeast (NE) Australia and two important climate modes, the Madden–Julian oscillation (MJO) and El Niño–Southern Oscillation (ENSO). Rainfall in NE Australia is strongly influenced by the MJO, ENSO, and their interaction, suggesting that climate models need to capture both individual climate modes and their interactions for reliable rainfall projections. These findings have societal climate risk implications, as NE Australia is an area prone to catastrophic flooding due to extreme rainfall. [ABSTRACT FROM AUTHOR]

A long-term archive of cloud properties (cloud top pressure, CTP; and cloud effective emissivity, ε) determined from High-resolution Infrared Radiation Sounder (HIRS) data is investigated for evidence of regional cloud cover change. In the 17 years between 1985 and 2001, different cloud types are analysed over the Australian region (10° S–45° S, 105° E–160° E) and areas of change in total cloud frequency examined. Total cloud frequency change over the Australian region between two adjacent eight-year time periods (1994 to 2001 minus 1985 to 1992) shows the largest increases (ranges between 6% and 12%) of average HIRS total cloud cover occurring over the offshore regions to the northwest and northeast of the continent. Over land, the largest reduction of average HIRS total cloud frequency is in the southwestern region of Australia (between 2% and 8%). Through central Australia, there is a 2% to 7% increase in average HIRS total cloud frequency when comparing these eight-year periods. This paper examines the regional cloud changes in 17 years over Australia that are embedded in global cloud statistics. Examining total HIRS cloud cover frequency over Australia and comparing two different eight-year time periods, has highlighted notable areas of average change. Preliminary reporting of satellite-derived HIRS cloud products and Global Precipitation Climatology Project (GPCP) rainfall products during La Niña seasons between 1985 and 2001 has also been undertaken. [ABSTRACT FROM AUTHOR]

We assess seasonally varying impacts of the Madden–Julian oscillation (MJO) on Australian maximum and minimum temperature anomalies and extremes, and their modulation by El Niño–Southern Oscillation (ENSO), for the period June 1974–May 2022. Our composite-based approach uses observed temperatures from the Australian Gridded Climate Data, and 850-hPa wind data from the NCEP–NCAR reanalysis, to show how relationships to temperature and circulation evolve over the eight-phase life cycle of the MJO, which we derive from the real-time multivariate MJO index. The MJO has significant impacts on Australian temperatures and winds in all parts of the country at various times throughout the year, and to varying degrees. Two of the most pronounced impacts are 1) daytime warming across southeastern Australia in MJO phase 3 during spring associated with a strong anomalous anticyclone and 2) nighttime cooling over Queensland in MJO phase 7 during winter associated with anomalous advection of cool dry continental air. La Niña acts to significantly lessen both of these impacts, while El Niño enhances both the phase 3 warming over southern Australia in spring and the phase 7 overnight cooling over southern Queensland in winter. We show how the MJO can combine with El Niño and La Niña to have strong compounding influences, thus highlighting the importance of understanding interactions between multiple modes of climate variability and how they relate to Australian temperatures and extremes. [ABSTRACT FROM AUTHOR]

Global Positioning System (GPS) deformation measurements were combined with groundwater level data to examine the spatiotemporal variability of groundwater storage in the Lachlan catchment located in central New South Wales (Australia). After correcting for effects of glacial isostatic adjustment, non‐tidal oceanic and atmospheric loading as well as hydrologic loading using existing models, we show that the seasonal and interannual variability of ground deformation and hydraulic head level data, extracted using wavelet time‐frequency analysis, exhibits an in‐phase behavior, indicating that the observed surface deformation is the poroelastic response to groundwater pressure change in aquifer system. Combination of GPS displacement and groundwater level change enables the estimation of elastic skeletal specific storage coefficients, which were then used for estimating groundwater storage changes. The estimated groundwater storage changes clearly reflect the four climate events of the Lachlan catchment since 1996: (a) the Millennium drought over 1996–2009, (b) the 2011–2012 La Nina and two significant floods in 2012 and 2016, (c) the drought conditions from mid‐2017 to late‐2019, and (d) the return of La Nina conditions since early 2020. We also found annual and long‐term groundwater storage variations of respectively ∼25±2.7GL $\sim 25\,\pm \,2.7\,\mathrm{G}\mathrm{L}$ and ∼−5±0.57GL/yr $\sim -5\,\pm \,0.57\,\mathrm{G}\mathrm{L}/\mathrm{y}\mathrm{r}$ over the period 2012–2021. Moreover, we show that groundwater level fluctuations can be predicted from GPS displacement measurements and storage coefficients with sufficient accuracy (80% correlation and 70% RMS reduction when compared in terms of seasonal cycle). This study provides essential information that can contribute to future groundwater planning, management, and control over the Australian continent. Plain Language Summary: Groundwater extraction or recharge changes the height of the Earth's surface, which can be obtained from displacement measurements derived from Global Positioning System (GPS). We combined the GPS displacement measurements over the Lachlan aquifer, located in central New South Wales (Australia), with the groundwater level change to study groundwater storage variation. The estimated groundwater storage changes reflect the major climate conditions over the area since 1996 such as the Millennium drought and the significant La Nina events in 2011 and 2020. We further analyze the aquifer response to the rainfall and found a delay ranging from 3 weeks to 2 months between the rainfall and groundwater wells levels, depending on the depth of the aquifer. Key Points: We estimate groundwater storage variation by combining Global Positioning System (GPS) vertical displacement and groundwater level dataInterannual variability measured by GPS vertical deformation is mainly driven by poroelasticity effect due to groundwater storage changesThe method is able to model seasonal groundwater level fluctuation using GPS displacement measurement and storage coefficient of the aquifer [ABSTRACT FROM AUTHOR]

Terrestrial ecosystems are fully coupled with the climate. The planet has been greening owing to the increased vegetation growth in response to the changing atmosphere, which in turn has feedback on the climate. Greening has slowed down the rise in global land-surface air temperature mainly through a coincident increase of evapotranspiration and precipitation in wet regions. In dry regions, greening intensifies the decrease in soil moisture induced by greening-enhanced transpiration. Uncertain, however, is how the climate effects of greening in semi-arid lands might differ for variable wet and dry conditions. Here, we focus on the biosphere–atmosphere interactions in Australia by modeling the perturbation of vegetation changes under various states of sea surface temperature (SST), including the climatology mean, El Niño, and La Niña conditions. For the dry conditions of El Niño, greening exacerbates water stress and largely depletes the soil moisture, while for the wet conditions of La Niña, greening-enhanced evapotranspiration and precipitation resupply the soil moisture. For the normal conditions using the climatology mean SST, a small decrease in soil moisture occurs but with large spatial contrast because of heterogeneous changes of evapotranspiration and precipitation induced by greening. We emphasize that the alternating dry and wet conditions modulated by the large-scale climate variability are vital to understanding the response of climate to greening. Furthermore, vegetation-based warming mitigation policies need to be cautious when inferring distinct climate effects associated with greening. [ABSTRACT FROM AUTHOR]

Streamflow in Australia's northern rivers has been steadily increasing since the 1970s, most likely due to increased intensity in the Indo‐Australian monsoon. However, because of limited data availability, it is hard to assess this recent trend and therefore contextualize potential future climatic changes. In this study, we used a network of 63 precipitation‐sensitive tree‐ring chronologies from the Indo‐Australian and Asian monsoon regions to reconstruct streamflow in the Daly catchment in the Northern Territory of Australia from 1413 to 2005 CE. We used a novel wavelet‐based method to transform the variance structure of the tree‐ring chronologies to better match the hydroclimate prior to reconstruction with a hierarchical Bayesian regression model. Our streamflow reconstruction accounts for 72%–78% of the variance in the instrumental period and closely matches both historical flood events and independent proxy records, increasing confidence in its validity. We find that while streamflow has been increasing since the 1800s, the most recent 40‐year period is unprecedented in the last ∼600 years. Comparison to an independent coral‐based streamflow record shows regional coherency in this trend. Extreme high flows were found to be linked to La Niña events, but we found no significant relationship between streamflow and El Niño events, or streamflow and other regional climatic drivers. More work is therefore needed to understand the drivers of the recent streamflow increase, but, regardless of the cause, water managers should be aware of the paleoclimatic context before making decisions on water allocations. Plain Language Summary: Large‐scale agricultural development has been proposed for the Daly catchment in the Northern Territory of Australia. Since the start of record keeping in the Daly catchment in the 1970s, streamflow has been steadily increasing, most likely due to increases in Australian monsoon rainfall. However, because of the limited amount of data, it is hard to assess whether this recent increase in streamflow is unusual, part of a longer trend, or a natural cycle in monsoon rainfall. In this study, we used rainfall‐sensitive tree growth and a statistical model to reconstruct Daly River streamflow over the past 592 years. Our streamflow reconstruction closely matches known past flood events, increasing confidence in its validity. We find that streamflow has increased since the 1800s but that the most recent 40‐year period is unprecedented in the last 600 years. More work is needed to understand the drivers of this increased streamflow but regardless of the cause, water managers should be aware of the Daly River's history before making decisions on allocating water to agriculture or other large users. Key Points: 592‐year annual streamflow reconstruction of the Daly River Catchment, Australia, derived using a tree‐ring networkWavelet‐based techniques to transform tree ring spectral characteristics improve the reconstruction abilityHigh streamflow in the twentieth Century is regionally coherent and unprecedented in the preceding five centuries [ABSTRACT FROM AUTHOR]

Using observation/reanalysis data, influences of the Interdecadal Pacific Oscillation (IPO) on the frequency of the locally amplified Ningaloo Niño (i.e., events with local air‐sea interactions playing an important role) and their mechanisms are investigated in this study. It is found that the ratio of the locally amplified Ningaloo Niño increases during the negative phase of the IPO. This is due to positive interdecadal sea surface temperature anomalies to the northwest of Australia that promote deep atmospheric convection and anomalous cyclone off the west coast of Australia through a Matsuno‐Gill response. Based on a mixed layer heat budget analysis using outputs from a regional ocean model, it is shown that an enhanced warming of anomalously thin mixed layer by the shortwave radiation contributes dominantly to the interdecadal sea surface temperature anomalies. The shallow mixed layer depth is suggested to be generated by anomalously weak winds associated with the negative IPO. Plain Language Summary: Ningaloo Niño is an anomalously warm ocean condition along the west coast of Australia and causes heat stress to marine species (e.g., coral bleaching). There are two mechanisms that give rise to the Ningaloo Niño. One is atmospheric and oceanic remote forcing, which is associated with La Niña, and the other is associated with local air‐sea interactions. A Ningaloo Niño event in which the local air‐sea interactions play an important role is called "locally amplified Ningaloo Niño." In this study, it is confirmed that the Ningaloo Niño occurs more frequently during the negative Interdecadal Pacific Oscillation (IPO) than the positive IPO. Moreover, it is revealed for the first time that the ratio of the locally amplified events increases during the negative IPO due to intensified air‐sea interaction associated with warm ocean condition to the northwest of Australia. These results and mechanisms may contribute to improve decadal prediction of the Ningaloo Niño. Key Points: Ningaloo Niño occurs more frequently during the negative phase of the Interdecadal Pacific Oscillation (IPO)The ratio of the locally amplified Ningaloo Niño increases during the negative IPOPositive interdecadal SST anomalies to the northwest of Australia during the negative IPO enhance the local air‐sea interaction [ABSTRACT FROM AUTHOR]

Climate strongly influences agricultural profitability. Climate risks on agriculture can be managed by moving production to other areas, but this may be costly and reduce profitability if climatic conditions are dynamic. Here, we test whether integrating spatial diversification and climate information could minimize climate risk, while not sacrificing profitability. We use 27 years of farm business profit and climate data (1991–2018) from four of Australia's most climatically diverse regions where grazing underpins socioeconomic activity. We show that spatial diversification coupled with seasonal climate information from El Niño-Southern Oscillation (ENSO) provides better estimates of optimized risk-profit tradeoffs in different ENSO years compared to estimates without climate information. Conditional Value-at-Risk (CVaR) (a measure of financial risk) is lower when climate information is used in El Niño (1.23 $/ha), La Niña (1.19 $/ha), and Neutral years (1.22 $/ha), compared to when no climate information (1.57 $/ha) is used. When targeting high profits, CVaR is reduced by 15, 86, and 22% in El Niño, La Niña, and Neutral years, respectively, compared to a 5% reduction when no climate information is used. When aiming to minimize CVaR in drought (El Niño), profit is higher using climate-informed spatial diversification (expected profits of 2.71 $/ha), relative to when it is not (expected profits of 2.30 $/ha). Climate-informed spatial diversification also provides options to graziers to achieve much higher gains (expected profits of up to ~ 4.13 $/ha) under La Niña versus 2.62 $/ha when no climate information is used. Here, we show for the first time that seasonal climate information coupled with spatial diversification can provide strategies to help graziers reduce risk and increase profitability. Our approach is applicable to other parts of the world and could be used to decrease climate risk and increase profitability for other agricultural sectors exposed to variable climatic conditions. [ABSTRACT FROM AUTHOR]

This study assesses whether, the extent to which and why the Pacific Meridional Mode (PMM) modulates rainfall in Australia and the Maritime Continent. We find a statistically significant time-lagged association between March-to-May (MAM) PMM and September-to-November (SON) rainfall in the Maritime Continent and Australia. The association is largely caused by the contribution of PMM to the El Niño Southern Oscillation (ENSO). Positive (negative) MAM PMM is generally followed by El Niño (La Niña) events in the following SON and December-to-February (DJF), which then suppresses (enhances) rainfall in the Maritime Continent and Australia. The suppression (enhancement) of rainfall is closely tied to the dynamical changes of moisture flux using moisture flux potential and its divergent components. Following the positive (negative) PMM phases, there is a strong moisture flux potential sink (source) in SON over the Maritime Continent and Australia, which act to suppress (enhance) rainfall there. Using MAM PMM as a predictor for SON rainfall in the Maritime Continent and Australia, the prediction skill is comparable to the North American Multimodel Ensemble project (NMME) forecasts initialized in June over the period 1981–2014. This suggests that MAM PMM may be used as a predictor for SON rainfall in the Maritime Continent and Australia. [ABSTRACT FROM AUTHOR]

A multi-year ocean glider dataset, obtained along a representative cross-shelf transect along the Rottnest continental shelf, south-west Australia, was used to characterise the seasonal and inter-annual variability of water column properties (temperature, salinity, and chlorophyll fluorescence distribution). All three variables showed distinct seasonal and inter-annual variations that were related to local and basin-scale ocean atmosphere processes. Controlling influences for the variability were attributed to forcing from two spatial scales: (1) the local scale (due to Leeuwin Current and dense shelf water cascades, DSWC) and (2) the basin scale (El Niño–Southern Oscillation, ENSO, events). In spring and summer, inner-shelf waters were well mixed due to strong wind mixing, and deeper waters (>50 m) were vertically stratified in temperature that contributed to the presence of a subsurface chlorophyll maximum (SCM). On the inner shelf, chlorophyll fluorescence concentrations were highest in autumn and winter. DSWCs were also the main physical feature during autumn and winter. Chlorophyll fluorescence concentration was higher closer to the seabed than at the surface in spring, summer, and autumn. The seasonal patterns coincided with changes in the wind field (weaker winds in autumn) and air–sea fluxes (winter cooling and summer evaporation). Inter-annual variation was associated with ENSO events. Lower temperatures, higher salinity, and higher chlorophyll fluorescence (>1 mg m -3) were associated with the El Niño event in 2010. During the strong La Niña event in 2011, temperatures increased and salinity and chlorophyll fluorescence decreased (<1 mg m -3). It is concluded that the observed seasonal and inter-annual variabilities in chlorophyll fluorescence concentrations were related to the changes in physical forcing (wind forcing, Leeuwin Current, and air–sea heat and moisture fluxes). [ABSTRACT FROM AUTHOR]

An ensemble of general circulation model simulations and dynamical mode analyses of important weather systems contributing to Australia's severe rainfall and flooding events, in January 1974 and January 2011, are described. Dispersion relation techniques have also been used to extract the leading weather systems in observations and general circulation model (GCM) simulations, including Kelvin waves and intraseasonal oscillations. We find that the severe rainfall and flooding events over northern Australia in January 1974 and January 2011 coincided with significant intraseasonal oscillation and Kelvin wave activity that constructively interfered on the critical days of very high rainfall. The CSIRO Mk3L spectral GCM has been used to simulate both events. Particular simulations from 1870 to 2011, forced by observed SSTs and increasing CO, agree well with observations in both the timing and patterns of these disturbances. The growth and structures of the leading dynamical normal modes have also been determined within a two-level primitive equation instability model. Our results show explosively growing intraseasonal oscillations and atmospheric Kelvin waves convectively coupled in the Australian monsoonal region. Monsoon disturbances, associated blocking over the Tasman Sea, and tropical-extratropical interactions also significantly contributed to the heavy rainfall. Model simulations and analyses of the dynamical modes are consistent with the synoptic situation seen in observational data for both severe precipitation events, and provide a more complete description of the reasons for the extreme impact of both events. [ABSTRACT FROM AUTHOR]

Australia experienced record high rainfall in austral spring 2010, which has previously been attributed to the concurrence of a strong La Niña event and a strong positive excursion of the Southern Annular Mode (SAM). In this study, we examine the role of the sea surface temperature (SST) trend over the recent 50 years, which has large warming over the tropical Indian, western Pacific and North Atlantic Oceans, in driving the extraordinary climate conditions of spring 2010, using the Australian Bureau of Meteorology coupled model seasonal forecast system. Four forecast sensitivity experiments were designed by using randomly chosen atmospheric initial conditions but with: (1) observed ocean initial conditions for 1 September 2010; (2) the same ocean initial conditions except the linear temperature trend over the period 1960-2010 was removed; (3) ocean initial conditions in which the trend was added to the climatological ocean state for 1 September; and (4) climatological ocean conditions only. A synergistic response to the La Niña SST anomalies and the SST trend was detected: the tropical rainfall anomalies were amplified over the western side of the Indo-Pacific warm-pool, which led to a significant increase of tropical upper tropospheric warming and a resultant increase of meridional temperature gradient in the Southern Hemisphere (SH) extratropics. Consequently, the SH eddy-driven jet was shifted poleward (i.e. positive phase of the SAM), which induced rainfall over subtropical Australia. Our findings highlight that the interaction of interannual anomalies and the trend may play an important role in the amplification of extreme events. [ABSTRACT FROM AUTHOR]

The variability in the number of severe floods that occurred in coastal catchments in southeastern Australia since the mid-nineteenth century, along with the variability in both the frequency of the weather types that triggered the floods and the associated death tolls, is analyzed. Previous research has shown that all of the severe floods identified were associated with one of two major weather types: east coast lows (ECLs) and tropical interactions (TIs). El Niño-Southern Oscillation (ENSO) is shown to strongly modulate the frequency of severe coastal flooding, weather types, and the number of associated deaths. The analysis presented herein, which examines links over more than a century, provides one of very few known statistically significant links between ENSO and death tolls anywhere in the world. Over the period 1876/77-2013/14 the average numbers of coastal floods, ECLs, TIs, and deaths associated with freshwater drowning in La Niña years are 92%, 55%, 150%, and 220% higher than the corresponding averages in El Niño years. The average number of deaths per flood in La Niña years is 3.2, which is 66% higher than the average in El Niño years. Death tolls of 10 or more occurred in only 5% of El Niño years, but in 27% of La Niña years. The interdecadal Pacific oscillation also modulates the frequency of severe floods, weather types, and death tolls. The results of this study are consistent with earlier research over shorter periods and broader regions, using less-complete datasets. [ABSTRACT FROM AUTHOR]

The association between Southern Hemisphere cyclones and anticyclones and the El Niño–Southern Oscillation (ENSO), southern annular mode (SAM), Antarctic sea ice extent (SIE), and rainfall in Perth and Melbourne is explored. Those cities are, respectively, located in the southwestern and southeastern corners of Australia, where substantial decreasing rainfall trends have been observed over the last decades. The need for a more unified understanding of large-scale anomalies in storm indicators associated with the climate features itemized above has motivated this study. The main aim is to identify cyclone-anomalous areas that are potentially important in characterizing continental rainfall anomalies from a hemispheric perspective, focusing on midlatitude Australia. The study covers the “satellite era” from 1979 to 2003 and was conducted for the southern winter when midlatitude rainfall is predominantly baroclinic. The results indicate a well-organized hemispheric cyclone pattern associated with ENSO, SAM, SIE, and rainfall anomalies. There is a moderate large-scale, high-latitude resemblance between La Niña, negative SAM, and reduced SIE in some sectors. In particular, there is a suggestion that SIE anomalies over the Indian Ocean and Western Australia sectors are associated with a large-scale pattern of cyclone/anticyclone anomalies that is more pronounced over the longitudes of Australia and New Zealand. Spatial correlation analysis suggests a robust link between cyclone density over the sectors mentioned above and rainfall in Perth and Melbourne. Statistical analyses of rainfall and SIE show modest correlations for Perth and weak correlations for Melbourne, generally corroborating the above. It is proposed that SAM and SIE are part of a complex physical system that is best understood as a coupled mechanism, and that their impacts on the circulation can be seen as partially independent of ENSO. While SAM and SIE have greater influence on the circulation affecting rainfall in the western side of Australia, ENSO is the dominant influence on the eastern half of the country. A contraction of the sea ice seems to be accompanied by a southward shift of high-latitude cyclones, which is also hypothesized to increase downstream cyclone density at midlatitudes via conservation of mass, similarly to what is observed during the extreme positive phase of the SAM. These associations build on previous developments in the literature. They bring a more unified view on high-latitude climate features, and may also help to explain the declining trends in Australian rainfall. [ABSTRACT FROM AUTHOR]

The influence of the El Niño-Southern Oscillation (ENSO) on NSW coastal storm severity was investigated by comparing the yearly mean Southern Oscillation Index, SOI, with the yearly averaged storm severity Ω measured at the NSW coast. It was found that Ω correlates well with SOI, i.e., Ω generally increases with SOI. The probability distributions of storm peak wave height and yearly mean SOI were also studied and were found to follow a Gumbel distribution. A simple linear relationship between extreme wave height and yearly mean SOI was then derived. At the NSW coast or at other coasts with similar wave climates, coastal recession and increased coastal damage may be expected in La Niña years, while beach recovery and reduced coastal threat may be expected in El Niño years. [ABSTRACT FROM AUTHOR]

Quasi-synoptic surveys along the east coast of Australia between 28 and 34°S show that the heterotrophic dinoflagellate, Noctiluca scintillans, occurs along this entire stretch of the coast. Areas of relatively high abundance of Noctiluca were observed downstream of regions predisposed to current-induced upwellings as a consequence of alongshore topographic variations. High-resolution temporal and spatial sampling of upwelling events showed that Noctiluca was abundant (up to 28 cells l−1) within mature upwelled waters. A high proportion (>80%) of fed Noctiluca cells (cells with prey in their vacuoles) was observed in the mature upwelled waters indicating that the observed increase in abundance of Noctiluca was associated with increased feeding activity. The absolute abundance of Noctiluca in upwelled waters was, however, found to vary from one upwelling location to another and between seasons. In particular, highest abundances of Noctiluca were recorded south of 31.5°S, where the East Australian Current (EAC) characteristically separates from the coast. The high abundances partly arise from southward advection and retention of the Noctiluca cells, and partly from upwelling inshore of the separated EAC driven by cross-shelf boundary layer fluxes. The temperature of the EAC was also found to influence absolute abundances. Surface water temperatures during our summer cruise were anomalously high due to a strong La Niña phase, and up to 4°C warmer than during our spring cruise. We found that the warmer surface water temperatures were associated with relatively lower average abundances of Noctiluca in the near shore zone. [ABSTRACT FROM AUTHOR]