Onset Of El Research Articles

On the role of anomalous ocean surface temperatures for promoting the record Madden‐Julian Oscillation in March 2015

AbstractA Madden‐Julian Oscillation (MJO) event dramatically amplified at the beginning of March 2015 as the convective phase traversed an unusually warm central Pacific Ocean. This record amplification also resulted in record amplitude of the MJO based on index measurements since 1974. We explore the possible role of the anomalously high ocean surface temperatures in the equatorial central Pacific for promoting the extraordinary amplification of this MJO event. Forecast sensitivity experiments with the Predictive Ocean Atmosphere Model for Australia show that the enhanced growth of the MJO resulted from amplification of the convective anomaly as it encountered the unusually warm central Pacific. Our results indicate that anomalous sea surface temperature (SST) at the onset of El Niño 2015 promoted the intensification of the MJO. We suggest a two‐way interaction whereby initial SST anomalies promoted enhanced MJO activity which then possibly led to enhanced El Niño development.

Re-evaluating the resource potential of lomas fog oasis environments for Preceramic hunter–gatherers under past ENSO modes on the south coast of Peru

Lomas – ephemeral seasonal oases sustained by ocean fogs – were critical to ancient human ecology on the desert Pacific coast of Peru: one of humanity's few independent hearths of agriculture and “pristine” civilisation. The role of climate change since the Late Pleistocene in determining productivity and extent of past lomas ecosystems has been much debated.Here we reassess the resource potential of the poorly studied lomas of the south coast of Peru during the long Middle Pre-ceramic period (c. 8000–4500 BP): a period critical in the transition to agriculture, the onset of modern El Niño Southern Oscillation (‘ENSO’) conditions, and eustatic sea-level rise and stabilisation and beach progradation.Our method combines vegetation survey and herbarium collection with archaeological survey and excavation to make inferences about both Preceramic hunter–gatherer ecology and the changed palaeoenvironments in which it took place. Our analysis of newly discovered archaeological sites – and their resource context – show how lomas formations defined human ecology until the end of the Middle Preceramic Period, thereby corroborating recent reconstructions of ENSO history based on other data.Together, these suggest that a five millennia period of significantly colder seas on the south coast induced conditions of abundance and seasonal predictability in lomas and maritime ecosystems, that enabled Middle Preceramic hunter–gatherers to reduce mobility by settling in strategic locations at the confluence of multiple eco-zones at the river estuaries. Here the foundations of agriculture lay in a Broad Spectrum Revolution that unfolded, not through population pressure in deteriorating environments, but rather as an outcome of resource abundance.

Holocene Demographic Changes and the Emergence of Complex Societies in Prehistoric Australia.

A continental-scale model of Holocene Australian hunter-gatherer demography and mobility is generated using radiocarbon data and geospatial techniques. Results show a delayed expansion and settlement of much of Australia following the termination of the late Pleistocene until after 9,000 years ago (or 9ka). The onset of the Holocene climatic optimum (9-6ka) coincides with rapid expansion, growth and establishment of regional populations across ~75% of Australia, including much of the arid zone. This diffusion from isolated Pleistocene refugia provides a mechanism for the synchronous spread of pan-continental archaeological and linguistic attributes at this time (e.g. Pama-Nyungan language, Panaramitee art style, backed artefacts). We argue longer patch residence times were possible at the end of the optimum, resulting in a shift to more sedentary lifestyles and establishment of low-level food production in some parts of the continent. The onset of El Niño - Southern Oscillation (ENSO; 4.5-2ka) restricted low-level food production, and resulted in population fragmentation, abandonment of marginal areas, and reduction in ranging territory of ~26%. Importantly, climate amelioration brought about by more pervasive La Niña conditions (post-2ka), resulted in an intensification of the mobility strategies and technological innovations that were developed in the early- to mid-Holocene. These changes resulted in population expansion and utilization of the entire continent. We propose that it was under these demographically packed conditions that the complex social and religious societies observed at colonial contact were formed.

Role of subsurface entrainment temperature (Te) in the onset of El Niño events, as represented in an intermediate coupled model

An improved intermediate coupled model (ICM) is described for use in ENSO-related modeling in the tropical Pacific, with ten baroclinic modes included in the vertical and horizonatally varying stratification taken into account. One crucial component of the model is the way in which the subsurface entrainment temperature in the surface mixed layer (Te) is explicitly used to determine the sea surface temperature (SST) variability. An optimized procedure is developed to depict Te using inverse modeling from an SST anomaly equation and its empirical relationship with the sea surface pressure variability. The coupled system realistically produces interannual variability associated with ENSO cycles, with a dominant 4-year oscillation. The onset and development of El Nino events from this ICM are examined in view of the well-known delayed oscillator paradigm; an example is given for the evolution of La Nina conditions in model year 2 to El Nino conditions in year 4. Right after a La Nina event (e.g., in year 2), there is a clear signature of reflections at the western boundary in early year 2, with related equatorial signals propagating eastward along the equator into the eastern basin in middle year 2. However, these reflected signals on the equator do not directly lead to an onset of an El Nino event at that time. Instead, approximately 1-year delay, a major El Nino event is seen to develop in the following year (late year 3), at a time when there is no reflected signal explicitly from the western boundary, indicating that the origin of the El Nino event cannot be directly ascribed to the reflection processes. Instead, Kelvin waves in the ocean that actually triggers the El Nino event in early year 3 are generated by interior wind anomalies near the date line that are associated with the first appearance of warm SST anomalies off the equator. Persisted Te anomalies off the equator in the western tropical Pacific initiate the warm SST anomaly near the date line along the North Equatorial Countercurrent region, which induces wind anomalies and an ocean–atmosphere coupling, leading to the El Nino event in year 4. The relevance of these ICM-based results to other onset mechanisms of El Nino and observations is also discussed.

On the dynamical mechanisms explaining the western Pacific subsurface temperature buildup leading to ENSO events

AbstractDespite steady progress in the understanding of El Niño–Southern Oscillation (ENSO) in the past decades, questions remain on the exact mechanisms explaining the heat buildup leading to the onset of El Niño (EN) events. Here we use an ensemble of ocean and atmosphere assimilation products to identify mechanisms that are consistently identified by all the data sets and that contribute to the heat buildup in the western Pacific 18 to 24 months before the onset of EN events. Meridional and eastward heat advection due to equatorward subsurface mass convergence and transport along the equatorial undercurrent are found to contribute to the subsurface warming at 170°E–150°W. In the warm pool, instead, surface horizontal convergence and downwelling motion have a leading role in subsurface warming. The picture emerging from our results highlights a sharp dynamical transition at 170°E near the level of the thermocline.

Equatorial Pacific Easterly Wind Surges and the Onset of La Niña Events*

Abstract The processes responsible for the onset of La Niña events have not received the same attention as those responsible for the onset of El Niño events, for which westerly wind events (WWEs) in the tropical Pacific have been identified as important contributors. Results here show that synoptic-scale surface easterly wind surges (EWSs) play an important role in the onset of La Niña events, akin to the role of WWEs in the onset of El Niño events. It is found that EWSs are a substantial component of zonal wind stress variance along the equatorial Pacific. Using reanalysis wind stress fields, validated against buoy measurements, 340 EWS events are identified between 1986 and 2012. Their distributions in space, time, and El Niño–Southern Oscillation (ENSO) state are described. About 150 EWSs occur during ENSO-neutral conditions, during the months associated with La Niña initiation and growth (April–December). Composites of changes in sea surface temperature anomaly (SSTA) following these ~150 events show statistically significant cooling (0.1°–0.4°C) along the oceanic waveguide that persists for 2–3 months following the EWSs. Experiments with EWS forcing of an ocean general circulation model show SSTA patterns like those in the observations. It is suggested that EWSs play an important role in the onset of La Niña waveguide surface cooling and deserve additional study.

Climatic Cycles and Gradients of the El Niño Core Region in North Peru

Climatic processes in northern Peru are evaluated on surface observation independent of modelling studies. The region is characterized by regular oscillations, but episodic El Niño-events introduce strong disturbances. Conceptual models based on observations, remote sensing data, and output of regional climate models are compared with data from a new station network. The results show regular oscillations of all climate variables on the annual and daily time scale. The daily cycle is probably associated with thermotidal forcings, causing gravity waves to emanate from the Andes Cordillera. Main factors are the interaction of large scale pressure systems like the Southeast Pacific High and the intertropical convergence zone (ITCZ). Also, there are regional factors: an extended sea-breeze system, the barrier-effect of the Andes, additional energy input by elevated radiation absorption at the mountain slopes, local wind systems, and the variations of the sea surface temperature. At the coast, a low-level jet works as a thermodynamic energy sink, suppressing deep convection and supporting the aridity. Those patterns are found in most of the station data and the processes of this climate can generally be confirmed. The overturning of this stable system with the onset of El Niño-conditions is possibly caused by disruptions of the regional circulation.

El Niño adversely affected childhood stature and lean mass in northern Peru

El Niño is responsible for natural disasters and infectious disease outbreaks worldwide. During the 1997–1998 El Niño, northern Peru endured extreme rainfall and flooding. Since short stature may occur as a result of undernutrition or repeated infections during childhood, both of which are highly prevalent during natural disasters, we sought to determine if the 1997–1998 El Niño had an adverse effect on stature and body composition a decade later. In 2008–2009, we measured height, weight, and bioimpedance in a random sample of 2,095 children born between 1991 and 2001 in Tumbes, Peru. Height-for-age increased by 0.09 SD/year of birth between 1991 and 1997 (P < 0.001), indicating overall improvements in health over time in the study area; however, this rate fell to 0.04 SD/year of birth during and shortly after El Niño, less than half the rate prior to El Niño (P = 0.046). Height shortfalls were even greater in children residing in households most likely to be flooded after El Niño. Any improvement over time was completely blunted and became negative in children living in households with flood likelihoods of ≥7% (P = 0.001). In the subset of 912 children with bioimpedance measurements, those born after the onset of El Niño had less lean mass (P < 0.001), whereas fat mass was unaffected (P = 0.48). Children born during and after 1997–1998 El Niño were on average shorter and had less lean mass for their age and sex than expected had El Niño not occurred. The effects of El Niño on health are long lasting and, given its cyclical nature, may continue to negatively impact future generations.

Subseasonal Atmospheric Variability and El Niño Waveguide Warming: Observed Effects of the Madden–Julian Oscillation and Westerly Wind Events*

AbstractWesterly wind events (WWEs) have previously been shown to initiate equatorial Pacific waveguide warming. The relationship between WWEs and Madden–Julian oscillation (MJO) activity, as well as the role of MJO events in initiating waveguide warming, is reconsidered here over the 1986–2010 period. WWEs are identified in observations of near-surface zonal winds using an objective scheme. MJO events are defined using a widely used index, and 64 are identified that occur when the El Niño–Southern Oscillation (ENSO) is in its neutral state. Of these MJO events, 43 have one or more embedded WWEs and 21 do not. The evolution of sea surface temperature anomaly over the equatorial Pacific waveguide following the westerly surface wind phase of the MJO over the western equatorial Pacific is examined. Waveguide warming is found for the MJO with WWE events in similar magnitudes as following the WWEs not embedded in an MJO. There is very little statistically significant waveguide warming following MJO events that do not contain an embedded WWE. The observed SST anomaly changes are well reproduced in an ocean general circulation model forced with the respective composite wind stress anomalies. Further, it is found that the occurrence of an MJO event does not significantly affect the likelihood that a WWE will occur. These results extend and confirm the earlier results of Vecchi with a near doubling of the period of study. It is suggested that understanding the sources and predictability of tropical Pacific westerly wind events remains essential to improving predictions of the onset of El Niño events.

Impact of East Asian Winter and Australian Summer Monsoons on the Enhanced Surface Westerlies over the Western Tropical Pacific Ocean Preceding the El Niño Onset

Abstract Composite analysis from NCEP–NCAR reanalysis datasets over the period 1948–2007 indicates that stronger East Asian winter monsoons (EAWM) and stronger Australian summer monsoons (ASM) generally coexist in boreal winters preceding the onset of El Niño, although the EAWM tend to be weak after 1990, probably because of the decadal shift of EAWM and the change in El Niño events from cold-tongue type to warm-pool type. The anomalous EAWM and ASM enhance surface westerlies over the western tropical Pacific Ocean (WTP). It is proposed that the enhanced surface westerlies over the WTP prior to El Niño onset are generally associated with the concurrent anomalous EAWM and ASM. A simple analytical atmospheric model is constructed to test the hypothesis that the emergence of enhanced surface westerlies over the WTP can be linked to concurrent EAWM and ASM anomalies. Model results indicate that, when anomalous northerlies from the EAWM converge with anomalous southerlies from the ASM, westerly anomalies over the WTP are enhanced. This result provides a possible explanation of the co-impact of the EAWM and the ASM on the onset of El Niño through enhancing the surface westerly over the WTP.

Summer monsoon circulation and precipitation over the tropical Indian Ocean during ENSO in the NCEP climate forecast system

This study investigates the El Nino Southern Oscillation (ENSO) teleconnections to tropical Indian Ocean (TIO) and their relationship with the Indian summer monsoon in the coupled general circulation model climate forecast system (CFS). The model shows good skill in simulating the impact of El Nino over the Indian Oceanic rim during its decay phase (the summer following peak phase of El Nino). Summer surface circulation patterns during the developing phase of El Nino are more influenced by local Sea Surface Temperature (SST) anomalies in the model unlike in observations. Eastern TIO cooling similar to that of Indian Ocean Dipole (IOD) is a dominant model feature in summer. This anomalous SST pattern therefore is attributed to the tendency of the model to simulate more frequent IOD events. On the other hand, in the model baroclinic response to the diabatic heating anomalies induced by the El Nino related warm SSTs is weak, resulting in reduced zonal extension of the Rossby wave response. This is mostly due to weak eastern Pacific summer time SST anomalies in the model during the developing phase of El Nino as compared to observations. Both eastern TIO cooling and weak SST warming in El Nino region combined together undermine the ENSO teleconnections to the TIO and south Asia regions. The model is able to capture the spatial patterns of SST, circulation and precipitation well during the decay phase of El Nino over the Indo-western Pacific including the typical spring asymmetric mode and summer basin-wide warming in TIO. The model simulated El Nino decay one or two seasons later, resulting long persistent warm SST and circulation anomalies mainly over the southwest TIO. In response to the late decay of El Nino, Ekman pumping shows two maxima over the southern TIO. In conjunction with this unrealistic Ekman pumping, westward propagating Rossby waves display two peaks, which play key role in the long-persistence of the TIO warming in the model (for more than a season after summer). This study strongly supports the need of simulating the correct onset and decay phases of El Nino/La Nina for capturing the realistic ENSO teleconnections. These results have strong implications for the forecasting of Indian summer monsoon as this model is currently being adopted as an operational model in India.

Triggering of El Niño onset through trade wind–induced charging of the equatorial Pacific

AbstractSea surface temperature variations over the equatorial Pacific associated with the El Niño/Southern Oscillation (ENSO) produce changes in climates across the globe. Here we report evidence from observationally constrained ocean data for the initiation of warm ENSO events (El Niños) resulting from subsurface equatorial Pacific heat content increases related to tropical/extratropical sea level pressure (SLP) changes over the North Pacific. We hypothesize that the increase in heat content is a response to SLP‐generated variations in the North Pacific trade winds, which we term trade wind charging of the equatorial Pacific. Experiments using a high‐resolution numerical ocean model verify that the charging of subsurface heat content along the equatorial Pacific can be induced by SLP‐generated trade wind variations. Furthermore, analysis of the numerical model results and historical observations indicates that the trade wind–induced ocean heat content increases are sufficiently large to initiate the onset of El Niño events, which mature approximately 12 months after the trade wind forcing itself.

Are Tulas and Enso Linked In Australia?

The distinctive tool called ‘tula’ is an endemic adaptation, which was adopted by Aboriginal people across central and western Australia, encompassing some two-thirds of the continent. The tula is a hafted tool used for working hardwoods as well as other tasks including butchery and plant-processing. The geographic spread of tulas appears to have been rapid and no antecedent form has been identified. The sudden appearance of tulas was coincident with the onset of El Niño Southern Oscillation (ENSO) conditions. While we do not yet have the data to establish an unequivocal causal link, in this paper we hypothesise that the appearance of this new and specialised tool at c.3700 BP was very likely a human response to the intensification of ENSO. This intensification resulted in increased aridity and climatic variability lasting almost 2000 years. We posit that this technological adaptation, an element of a risk minimisation toolkit, was part of a wider economic and social strategy adopted by Aboriginal people to cope with increasing climatic uncertainty. This possibility has implications for the diversity of innovation processes operating in Australia during the Holocene, which is further explored in this paper with concluding suggestions for future research. We offer this discussion as a platform for these future, and what we believe, are very necessary critical studies.

A New Extratropical Tracer Describing the Role of the Western Pacific in the Onset of El Niño: Implications for ENSO Understanding and Forecasting

Abstract A complex empirical orthogonal function analysis was applied to sea surface temperature data in the southern high-latitude Pacific to identify and isolate primary processes related to the onset of El Niño (EN) events. Results were compared to those of a lead–lag composite analysis of a new tracer of EN events in the southern high-latitude Pacific, the Ross–Bellingshausen (RB) dipole. Both techniques successfully isolate the main low-frequency features in the interaction among the tropical and southern extratropical Pacific during the onset of recent eastward-propagating EN events. Particularly, positive RB peaks were followed by EN events around 9 months later, on average. In turn, RB maxima were anticipated by local warm anomalies in the western tropical Pacific a year in advance, which enhance local convection and upper-troposphere divergence and generate an anomalous wave train extending eastward and poleward in the southern extratropics. In addition, circulation changes lead to a warm SST region in the central tropical Pacific, which is then strengthened by suppressed equatorial easterlies. Convection thus starts to move to the central Pacific and so the Walker circulation weakens, activating the positive Bjerknes feedback that ultimately leads to the development of an EN event. These results highlight the enormous potential of the interaction between the tropics and this high-latitude region in the Southern Hemisphere to increase El Niño–Southern Oscillation understanding and to improve the long-lead prediction skill of EN phenomenon.

A Bayesian Regression Approach to Seasonal Prediction of Tropical Cyclones Affecting the Fiji Region

Abstract This study presents seasonal prediction schemes for tropical cyclones (TCs) affecting the Fiji, Samoa, and Tonga (FST) region. Two separate Bayesian regression models are developed: (i) for cyclones forming within the FST region (FORM) and (ii) for cyclones entering the FST region (ENT). Predictors examined include various El Niño–Southern Oscillation (ENSO) indices and large-scale environmental parameters. Only those predictors that showed significant correlations with FORM and ENT are retained. Significant preseason correlations are found as early as May–July (approximately three months in advance). Therefore, May–July predictors are used to make initial predictions, and updated predictions are issued later using October–December early-cyclone-season predictors. A number of predictor combinations are evaluated through a cross-validation technique. Results suggest that a model based on relative vorticity and the Niño-4 index is optimal to predict the annual number of TCs associated with FORM, as it has the smallest RMSE associated with its hindcasts (RMSE = 1.63). Similarly, the all-parameter-combined model, which includes the Niño-4 index and some large-scale environmental fields over the East China Sea, appears appropriate to predict the annual number of TCs associated with ENT (RMSE = 0.98). While the all-parameter-combined ENT model appears to have good skill over all years, the May–July prediction of the annual number of TCs associated with FORM has two limitations. First, it underestimates (overestimates) the formation for years where the onset of El Niño (La Niña) events is after the May–July preseason or where a previous La Niña (El Niño) event continued through May–July during its decay phase. Second, its performance in neutral conditions is quite variable. Overall, no significant skill can be achieved for neutral conditions even after an October–December update. This is contrary to the performance during El Niño or La Niña events, where model performance is improved substantially after an October–December early-cyclone-season update.

Precursors of the El Niño/La Niña onset and their interrelationship

In this study, the equatorial sea level, western Pacific zonal wind, and high‐frequency (2∼90 day) wind variability are investigated as precursors of the onset of El Niño/La Niña. To a large extent, it is shown that each variable is a good indicator of El Niño–Southern Oscillation (ENSO) onset by exhibiting significant correlation to the ENSO index with 9 month lag. However, it is demonstrated here that the three precursors are remarkably correlated with each other. Based on the statistical analysis, we suggest here that the three precursors are governed by strong coupling processes: (1) scale interaction within the atmosphere between the low‐frequency (LF, i.e., interannual) and high‐frequency (HF, i.e., intraseasonal) winds and (2) positive air‐sea coupled feedback among the western Pacific westerly, SST, and zonal mean thermocline. Our observational analysis and statistical prediction experiments suggest that the three precursors are not independent and they reflect the different aspects of the same coupling process during the ENSO onset phase.

Understanding ENSO dynamics through the exploration of past climates

The palaeoclimate record shows that significant changes in ENSO characteristics took place during the Holocene. Exploring these changes, using both data and models, provides a means of understanding ENSO dynamics. Previous modelling studies have suggested a mechanism whereby changes in the Earth's orbital geometry explain the strengthening of ENSO over the Holocene. Decreasing summer insolation over the Asian landmass resulted in a weakening of the Asian monsoon system. This led to a weakening of the easterly trade winds in the western Pacific, creating conditions more favourable for El Niño development. To explore this hypothesised forcing mechanism, we use a climate system model to conduct a suite of simulations of the climate of the past 8,000 years. In the early Holocene, we find that the Asian summer monsoon system is intensified, resulting in an amplification of the easterly trade winds in the western Pacific. The stronger trade winds represent a barrier to the eastward propagation of westerly wind bursts, therefore inhibiting the onset of El Niño events. The fundamental behaviour of ENSO remains unchanged, with the major change over the Holocene being the influence of the background state of the Pacific on the susceptibility of the ocean to the initiation of El Niño events.

A Coupled GCM Analysis of MJO Activity at the Onset of El Niño

Abstract The ocean dynamics of the Madden–Julian oscillation (MJO) and its interaction with El Niño–Southern Oscillation (ENSO) are assessed using a flux-corrected coupled model experiment from the Australian Bureau of Meteorology. The model demonstrates the correct oceanic Kelvin wave response to the MJO-related westerly winds in the western Pacific. Although there may be a role for the MJO in influencing the strength of El Niño, its impact is difficult to separate from that of strong heat content preconditioning of ENSO. Hence, the MJO–ENSO relationship is assessed starting from a background state of low heat content anomalies in the western Pacific that are also characteristic of recent observed El Niño events. The model shows a strong relationship between ENSO and the MJO near the peak of El Niño. At this time, the sea surface temperature (SST) anomaly is largest in the central Pacific, and it is difficult to separate cause and effect. Near the onset of El Niño, however, when Pacific Ocean SST anomalies are near zero, an increase in MJO activity is associated with Kelvin wave activity and stronger subsequent ENSO warming. A significant increase in the number of MJO events, rather than the strength of individual MJO events, leads to stronger eastern Pacific warming; the MJO appears not to be responsible for the occurrence of El Niño itself, but, rather, is important for influencing its development thus. This research supports a role for downwelling oceanic Kelvin waves and subsequent deepening of the thermocline in contributing to eastern Pacific warming during the onset of El Niño.

Validation and Variation of Upper Layer Thickness in South China Sea from Satellite Altimeter Data.

Satellite altimeter data from 1993 to 2005 has been used to analyze the seasonal variation and the interannual variability of upper layer thickness (ULT) in the South China Sea (SCS). Base on in-situ measurements, the ULT is defined as the thickness from the sea surface to the depth of 16°C isotherm which is used to validate the result derived from satellite altimeter data. In comparison with altimeter and in-situ derived ULTs yields a correlation coefficient of 0.92 with a slope of 0.95 and an intercept of 6 m. The basin averaged ULT derived from altimeter is 160 m in winter and 171 m in summer which is similar to the in-situ measurements of 159 m in winter and 175 m in summer. Both results also show similar spatial patterns. It suggests that the sea surface height data derived from satellite sensors are usable for study the variation of ULT in the semi-closed SCS. Furthermore, we also use satellite derived ULT to detect the development of eddy. Interannual variability of two meso-scale cyclonic eddies and one anticyclonic eddy are strongly influenced by El Niño events. In most cases, there are highly positive correlations between ULT and sea surface temperature except the periods of El Niño. During the onset of El Niño event, ULT is deeper when sea surface temperature is lower.

Implications of a 14 200 year contiguous fire record for understanding human—climate relationships at Goochs Swamp, New South Wales, Australia

This paper interprets macroscopic charcoal (&gt;250 μm), humification and loss-on-ignition over the last ~14200 cal. BP from Goochs Swamp, located to the west of Sydney in the Greater Blue Mountains World Heritage Area. This study aimed to investigate relationships between humans, climate and fire through time, primarily by comparison of these palaeoenvironmental indices with archaeological evidence from the region. Climatic forcing can explain all periods of change in the history of fire at Goochs Swamp: fire activity was variable during the Lateglacial—Holocene transition, low during the relatively stable climate of the early Holocene, and high but variable after the onset of modern El Niño from the mid Holocene. Although the dominant control on fire in this environment during the Holocene appears to be climate, fluctuations in the late Holocene may reflect anthropogenic fire or human responses to climate change. The archaeological record of the Blue Mountains and other parts of the Sydney Basin illustrates that Aboriginal people altered subsistence, resource and land-use patterns in the late Holocene. We propose that these cultural measures were adopted to overcome new risks as the frequency of ENSO events increased, and the natural fire regime and resource reliability changed. These strategies perhaps included a more systematic use of fire. The most parsimonious interpretation of the evidence for changes in fire activity at Goochs Swamp in the light of nearby archaeological evidence is that Aboriginal people used fire within a changing climatic framework.