Southern Hemisphere Atmospheric Circulation Research Articles

Beyond the First Tipping Points of Southern Hemisphere Climate

Analysis of observations, reanalysis, and model simulations, including those using machine learning methods specifically designed for regime identification, has revealed changes in aspects of the Southern Hemisphere (SH) circulation and Australian climate and extremes over the last half-century that indicate transitions to new states. In particular, our analysis shows a dramatic shift in the metastability of the SH climate that occurred in the late 1970s, associated with a large-scale regime transition in the SH atmospheric circulation, with systematic changes in the subtropical jet, blocking, zonal winds, and storm tracks. Analysis via nonstationary clustering reveals a regime shift coincident with a sharp transition to warmer oceanic sea surface temperatures and increased baroclinicity in the large scales of the Antarctic Circumpolar Circulation (ACC), extending across the whole hemisphere. At the same time, the background state of the tropical Pacific thermocline shoaled, leading to an increased likelihood of El Niño events. The SH climate shift in the late 1970s is the first hemispheric regime shift that can be directly attributed to anthropogenic climate change. These changes in dynamics are associated with additional regional tipping points, including reductions in mean and extreme rainfall in south-west Western Australia (SWWA) and streamflow into Perth dams, and also with increases in mean and extreme rainfall over northern Australia since the late 1970s. The drying of south-eastern Australia (SEA) occurred against a background of accelerating increases in average and extreme temperatures across the whole continent since the 1990s, implying further inflection points may have occurred. Analysis of climate model simulations capturing the essence of these observed shifts indicates that these systematic changes will continue into the late 21st century under high greenhouse gas emission scenarios. Here, we review two decades of work, revealing for the first time that tipping points characteristic of regime transitions are inferred to have already occurred in the SH climate system.

Synoptic Moisture Intrusion Provided Heavy Isotope Precipitations in Inland Antarctica During the Last Glacial Maximum

AbstractStable water isotopes in inland Antarctic ice cores are powerful paleoclimate proxies; however, their relationship with dynamical atmospheric circulations remains controversial. Using a water isotope climate model (MIROC5‐iso), we assessed the influence of the Last Glacial Maximum (LGM; ∼21,000 years ago) sea surface temperatures (SST) and sea ice (SIC) on Antarctic precipitation isotopes (δ18Op) through atmospheric circulation. The results revealed that the synoptic circulation mostly maintained southward moisture transport, reaching inland Antarctica. The steepened meridional SST gradient in the mid‐latitudes increased δ18Op in inland Antarctica with the enhanced baroclinic instability and synoptic moisture transport. In contrast, expanded SIC distribution decreased δ18Op over Antarctica by enhanced preferential removal of heavy isotopes during vapor transport due to the increased transport distance and enhanced surface cooling. These findings propose to use Antarctic ice cores to describe the southern hemisphere atmospheric circulation, represented by the westerly jets, during the LGM and other past climates.

Seasonal climate summary for the southern hemisphere (summer 2019–20): a summer of extremes

This is a summary of the southern hemisphere atmospheric circulation patterns and meteorological indices for summer 2019–20; an account of seasonal rainfall and temperature for the Australian region is also provided. The antecedent climate conditions and climatic drivers for summer 2019–20 resulted in unprecedented extremes for Australia, with many heat and fire weather extremes. The austral summer of 2019–20 was staged to be hot and dry, with climate drivers supporting higher than average temperatures and lower than average rainfall. These conditions contributed to the highest recorded monthly accumulated national Forest Fire Danger Index. As the dominant climate influence for December receded during the season, dynamic (weather) processes dominated for changeable conditions – particularly in the mid-latitudes. Both January and February were among the 10 hottest on record, although several mid-latitude sites experienced unusually cool days. Across the rest of the hemisphere, conditions were also extreme, with notable drought conditions persisting from spring over large parts of South America. Temperature anomalies for land and ocean areas of the southern hemisphere were respectively the third and second highest on record.

The role of large-scale drivers in the Amundsen Sea Low variability and associated changes in water isotopes from the Roosevelt Island ice core, Antarctica

Here we examine the water stable-isotope data from the Roosevelt Island Climate Evolution (RICE) ice core. Roosevelt Island is an independent ice rise located at the northeastern margin of the Ross Ice Shelf. In this study, we use empirical orthogonal function (EOF) analysis to investigate the relationship between RICE ice-core oxygen-18 isotopes (δ18O) and Southern Hemisphere atmospheric circulation during the extended austral winter (April–November). The RICE δ18O record is correlated with Southern Annular Mode (SAM) and Pacific–South American pattern 1 (PSA1), which both project onto the Amundsen–Bellingshausen Sea (ABS) geopotential height field. Pacific sector Southern Ocean, eastern Ross Sea, and West Antarctic’s atmospheric circulation, sea ice, and surface air temperature (SAT) anomalies, as well as RICE δ18O, are strongest when El Niño–Southern Oscillation (ENSO) and SAM are “in-phase”. That is when the SAM − /PSA1 + (El Niño) and SAM + /PSA1 − (La Niña) phasing prevails. When in-phase, the δ18O correlation with the 500-hPa geopotential height (Z500) is strong in regions (e.g., the Amundsen Sea) where their anomalies associated with SAM and PSA1 show the same sign. SAM − /PSA1 + (El Niño) and SAM + /PSA1 − (La Niña) is associated with positive and negative δ18O anomalies, respectively. RICE δ18O can aid in establishing past natural variability of the strength of the SH high-latitude Pacific sector ENSO-SAM connection and associated atmospheric circulation, sea ice, and SAT extremes.

Combined Effects of Global Warming and Ozone Depletion/Recovery on Southern Hemisphere Atmospheric Circulation and Regional Precipitation

AbstractOzone depletion led to a positive trend in the summertime Southern Annular Mode (SAM) during the last decades of the 20th century. During the present century, global warming (GW) is expected to contribute to a positive SAM trend while ozone recovery is expected to act in the opposite direction. Here, Southern Hemisphere (SH) circulation and regional precipitation change are studied with a methodology that separates the effects from GW and ozone depletion/recovery. Our results show that a “tug‐of‐war” between ozone and GW occurs in the summertime stratosphere, propagating to the troposphere where it is manifest in the SAM. However, at the regional scale this “tug‐of‐war” is not as relevant as the combined effects of other remote drivers of circulation change, which force different kinds of precipitation changes in the SH. For regional precipitation changes, the uncertainty in future circulation change is as important as the uncertainty in the GW level.

Effects of prescribed CMIP6 ozone on simulating the Southern Hemisphere atmospheric circulation response to ozone depletion

Abstract. The Antarctic ozone hole has led to substantial changes in the Southern Hemisphere atmospheric circulation, such as the strengthening and poleward shift of the midlatitude westerly jet. Ozone recovery during the twenty-first century is expected to continue to affect the jet's strength and position, leading to changes in the opposite direction compared to the twentieth century and competing with the effect of increasing greenhouse gases. Simulations of the Earth's past and future climate, such as those performed for the Coupled Model Intercomparison Project Phase 6 (CMIP6), require an accurate representation of these ozone effects. Climate models that use prescribed ozone fields lack the important feedbacks between ozone chemistry, radiative heating, dynamics, and transport. In addition, when the prescribed ozone field was not generated by the same model to which it is prescribed, the imposed ozone hole is inconsistent with the simulated dynamics. These limitations ultimately affect the climate response to ozone depletion. This study investigates the impact of prescribing the ozone field recommended for CMIP6 on the simulated effects of ozone depletion in the Southern Hemisphere. We employ a new state-of-the-art coupled climate model, Flexible Ocean Climate Infrastructure (FOCI), to compare simulations in which the CMIP6 ozone is prescribed with simulations in which the ozone chemistry is calculated interactively. At the same time, we compare the roles played by ozone depletion and by increasing concentrations of greenhouse gases in driving changes in the Southern Hemisphere atmospheric circulation using a series of historical sensitivity simulations. FOCI captures the known effects of ozone depletion, simulating an austral spring and summer intensification of the midlatitude westerly winds and of the Brewer–Dobson circulation in the Southern Hemisphere. Ozone depletion is the primary driver of these historical circulation changes in FOCI. The austral spring cooling of the polar cap in the lower stratosphere in response to ozone depletion is weaker in the simulations that prescribe the CMIP6 ozone field. We attribute this weaker response to a prescribed ozone hole that is different to the model dynamics and is not collocated with the simulated polar vortex, altering the strength and position of the planetary wavenumber one. As a result, the dynamical contribution to the ozone-induced austral spring lower-stratospheric cooling is suppressed, leading to a weaker cooling trend. Consequently, the intensification of the polar night jet is also weaker in the simulations with prescribed CMIP6 ozone. In contrast, the differences in the tropospheric westerly jet response to ozone depletion fall within the internal variability present in the model. The persistence of the Southern Annular Mode is shorter in the prescribed ozone chemistry simulations. The results obtained with the FOCI model suggest that climate models that prescribe the CMIP6 ozone field still simulate a weaker Southern Hemisphere stratospheric response to ozone depletion compared to models that calculate the ozone chemistry interactively.

A historical climate dataset for southwestern Australia, 1830–1875

AbstractSouthwestern Australia is a high priority area for climate research as there has been a significant, anthropogenically influenced winter rainfall decline detected in the region since 1970. Here we present the oldest daily historical climate dataset for Perth, southwestern Australia, to provide an extended record for analysing pre‐industrial climate variability and extremes from the region. This newly digitized record contains sub‐daily observations of temperature, barometric pressure, wind direction and weather remarks, including rain days, from 1830 to 1875. Following quality control and homogeneity adjustments, we demonstrate that the historical dataset reliably resolves modern characteristics of Perth's weather and climate variability, including the seasonal cycle and the ability to capture daily extremes like storm and heatwave events. We compare covariations in temperature, pressure, wind and rain days to cross‐verify our historical dataset against modern observations for Perth. We also use the historical dataset, modern Australian Bureau of Meteorology observations, the Twentieth Century Reanalysis, and historical documents to further validate our results. The multivariate method introduced in this study demonstrates that using multiple variables and data sources is a useful approach to reconstruct past variability, extremes and their societal impacts from historical observations. This is the first long instrumental record to be developed for southwestern Australia; an important region for observing pre‐industrial Indian and Southern Ocean variability and extremes. The new record from Perth provides 19th century data from a data sparse region of the Southern Hemisphere that can now be compared with daily observations from South Africa, New Zealand and southeastern Australia. This comparison will improve our understanding of contemporary changes in Southern Hemisphere atmospheric circulation.

Seasonal climate summary for Australia and the southern hemisphere (summer 2018–19): extreme heat and flooding prominent

This is a summary of the southern hemisphere atmospheric circulation patterns and meteorological indices for summer 2018–19; an account of seasonal rainfall and temperature for the Australian region is also provided. January 2019 was Australia’s hottest month on record, nearly 1°C warmer than any previous month. Impacts of heavy rain and floods were reported in Australia, New Zealand and South American nations. Extreme terrestrial and maritime heatwaves occurred in and around Australia and New Zealand. Case studies of the Australian heatwave, Queensland floods in January and February, and a tide-driven coastal inundation event are considered.

Dynamics of Southern Hemisphere Atmospheric Circulation Response to Anthropogenic Aerosol Forcing

AbstractAnthropogenic aerosols, concentrated largely in the Northern Hemisphere, not only affect the local climate but also induce pronounced changes in atmospheric circulation that extend into the Southern Hemisphere (SH). In coupled historical single‐forcing simulations, aerosol forcing induces a deceleration of both the subpolar jet (SPJ) and the subtropical jet (STJ) in SH in austral winter. Atmospheric general circulation model experiments indicate that the STJ is weakened by an interhemisphere gradient in the zonal mean sea surface temperature (SST) and an anomalous cross‐equatorial Hadley circulation, while the SPJ response shares similar feedbacks with the greenhouse gas forcing. Specifically, atmospheric eddy adjustments are important for the SPJ change. The atmospheric response unique to anthropogenic aerosol forcing (e.g., cross‐equatorial Hadley cell and the weakened SH STJ) can be exploited for climate change attribution.

Seasonal climate summary for the southern hemisphere (spring 2018): positive Indian Ocean Dipole and Australia’s driest September on record

This is a summary of the southern hemisphere atmospheric circulation patterns and meteorological indices for spring 2018; an account of seasonal rainfall and temperature for the Australian region and broader southern hemisphere is also provided. A positive phase of the Indian Ocean Dipole developed during the season, and the central and eastern equatorial Pacific were warmer than average without reaching El Niño thresholds. It was Australia’s driest September on record, before rainfall of closer to average in October and November. It was warmer than average, especially in northern Australia, with coastal Queensland affected by extreme heat and wildfires in November. It was one of the three warmest springs on record for the southern hemisphere as a whole, and was notably dry in southern and eastern Africa.

Seasonal climate summary for the southern hemisphere (spring 2017): equal-fifth warmest spring on record, with rainfall mixed

This is a summary of the southern hemisphere atmospheric circulation patterns and meteorological indices for spring 2017; an account of seasonal rainfall and temperature for the Australian region and the broader southern hemisphere is also provided. The tropical Pacific Ocean cooled from mid-winter, with an unusually late La Niña becoming established by the end of spring. Spring was the equal-fifth warmest on record, in terms of area-averaged national mean temperature, and was characterised by exceptional September warmth over eastern Australia, while rainfall was mixed geographically and from month-to-month, but below average overall.

Seasonal climate summary for the southern hemisphere (winter 2018): fifteenth-warmest and fourteenth-driest

This is a summary of the southern hemisphere atmospheric circulation patterns and meteorological indices for winter 2018; an account of seasonal rainfall and temperature for the Australian region and the broader southern hemisphere is also provided. The climate influences from the El Niño–Southern Oscillation and the Indian Ocean Dipole were weak, with both demonstrating neutral conditions over the season. It was a dry and warm winter for Australia, being the fourteenth-driest and fifteenth-warmest (in terms of mean temperature) in a record of 119 and 109 years respectively. The warm and dry conditions were particularly pronounced over eastern Australia during July. Maximum temperatures were above average while minimum temperatures were below average.

Effect of prescribed sea surface conditions on the modern and future Antarctic surface climate simulated by the ARPEGE atmosphere general circulation model

Abstract. Owing to increase in snowfall, the Antarctic Ice Sheet surface mass balance is expected to increase by the end of the current century. Assuming no associated response of ice dynamics, this will be a negative contribution to sea-level rise. However, the assessment of these changes using dynamical downscaling of coupled climate model projections still bears considerable uncertainties due to poorly represented high-southern-latitude atmospheric circulation and sea surface conditions (SSCs), that is sea surface temperature and sea ice concentration. This study evaluates the Antarctic surface climate simulated using a global high-resolution atmospheric model and assesses the effects on the simulated Antarctic surface climate of two different SSC data sets obtained from two coupled climate model projections. The two coupled models from which SSCs are taken, MIROC-ESM and NorESM1-M, simulate future Antarctic sea ice trends at the opposite ends of the CMIP5 RCP8.5 projection range. The atmospheric model ARPEGE is used with a stretched grid configuration in order to achieve an average horizontal resolution of 35 km over Antarctica. Over the 1981–2010 period, ARPEGE is driven by the SSCs from MIROC-ESM, NorESM1-M and CMIP5 historical runs and by observed SSCs. These three simulations are evaluated against the ERA-Interim reanalyses for atmospheric general circulation as well as the MAR regional climate model and in situ observations for surface climate. For the late 21st century, SSCs from the same coupled climate models forced by the RCP8.5 emission scenario are used both directly and bias-corrected with an anomaly method which consists in adding the future climate anomaly from coupled model projections to the observed SSCs with taking into account the quantile distribution of these anomalies. We evaluate the effects of driving the atmospheric model by the bias-corrected instead of the original SSCs. For the simulation using SSCs from NorESM1-M, no significantly different climate change signals over Antarctica as a whole are found when bias-corrected SSCs are used. For the simulation driven by MIROC-ESM SSCs, a significant additional increase in precipitation and in winter temperatures for the Antarctic Ice Sheet is obtained when using bias-corrected SSCs. For the range of Antarctic warming found (+3 to +4 K), we confirm that snowfall increase will largely outweigh increases in melt and rainfall. Using the end members of sea ice trends from the CMIP5 RCP8.5 projections, the difference in warming obtained (∼ 1 K) is much smaller than the spread of the CMIP5 Antarctic warming projections. This confirms that the errors in representing the Southern Hemisphere atmospheric circulation in climate models are also determinant for the diversity of their projected late 21st century Antarctic climate change.

The effects of model climate bias on ENSO variability and ensemble prediction

New methods are presented for determining the role of coupled ocean-atmosphere model climate bias on the strength and variability of the El Nino-Southern Oscillation (ENSO) and on the seasonal ensemble prediction of El Nino and La Nina events. An intermediate complexity model with a global atmosphere coupled to a Pacific basin ocean is executed with parallelised algorithms to produce computationally efficient year-long forecasts of large ensembles of coupled flow fields, beginning every month between 1980 and 1999. Firstly, the model is provided with forcing functions that reproduce the average annual cycle of climatology of the atmosphere and ocean based on reanalysed observations. We also configure the model to generate realistic ENSO fluctuations. Next, an ensemble prediction scheme is employed which produces perturbations that amplify rapidly over a month. These perturbations are added to the analyses and give the initial conditions for the ensemble forecasts. The skill of the forecasts is presented and the dependency on the annual and ENSO cycles determined. Secondly, we replace the forcing functions in our model with functions that reproduce the averaged annual cycles of climatology of two state of the art, comprehensive Coupled General Circulation Models. The changes in skill of subsequent ensemble forecasts elucidate the roles of model bias in error growth and potential predictability.
 
 References C. S. Frederiksen, J. S. Frederiksen, and R. C. Balgovind. ENSO variability and prediction in a coupled ocean-atmosphere model. Aust. Met. Ocean. J., 59:35–52, 2010a. URL http://www.bom.gov.au/jshess/papers.php?year=2010. C. S. Frederiksen, J. S. Frederiksen, and R. C. Balgovind. Dynamic variability and seasonal predictability in an intermediate complexity coupled ocean-atmosphere model. In Proceedings of the 16th Biennial Computational Techniques and Applications Conference, CTAC-2012, volume 54 of ANZIAM J., pages C34–C55, 2013a. doi:10.21914/anziamj.v54i0.6296. C. S. Frederiksen, J. S. Frederiksen, J. M. Sisson, and S. L. Osbrough. Trends and projections of Southern Hemisphere baroclinicity: the role of external forcing and impact on Australian rainfall. Clim. Dyn., 48:3261–3282, 2017. doi:10.1007/s00382-016-3263-8. J. S. Frederiksen, C. S. Frederiksen, and S. L. Osbrough. Seasonal ensemble prediction with a coupled ocean-atmosphere model. Aust. Met. Ocean. J., 59:53–66, 2010b. URL http://www.bom.gov.au/jshess/papers.php?year=2010. J. S. Frederiksen, C. S. Frederiksen, and S. L. Osbrough. Methods of ensemble prediction for seasonal forecasts with a coupled ocean-atmosphere model. In Proceedings of the 16th Biennial Computational Techniques and Applications Conference, CTAC-2012, volume 54 of ANZIAM J., pages C361–C376, 2013b. doi:10.21914/anziamj.v54i0.6509. P. R. Gent, G. Danabasoglu, L. J. Donner, M. M. Holland, E. C. Hunke, S. R. Jayne, D. M. Lawrence, R. B. Neale, P. J. Rasch, M. Vertenstein, P. H. Worley, Z.-L. Yang, and M. Zhang. The community Climate System Model version 4. J. Clim., 24:4973–4991, 2011. doi:10.1175/2011JCLI4083.1. S. Grainger, C. S. Frederiksen, and X. Zheng. Assessment of modes of interannual variability of Southern Hemisphere atmospheric circulation in CMIP5 models. J. Clim., 27:8107–8125, 2014. doi:10.1175/JCLI-D-14-00251.1. E. Kalnay, M. Kanamitsu, R. Kistler, W. Collins, D. Deaven, L. Gandin, M. Iredell, S. Saha, G. White, J. Woollen, Y. Zhu, M. Chelliah, W. Ebisuzaki, W. Higgins, J. Janowiak, K. C. Mo, C. Ropelewski, J. Wang, A. Leetmaa, R. Reynolds, R. Jenne, and D. Joseph. The NCEP/NCAR 40-year reanalysis project. B. Am. Meteorol. Soc., 77:437–472, 1996. doi:10.1175/1520-0477(1996)077<0437:TNYRP>2.0.CO;2. H. A. Rashid, A. Sullivan, A. C. Hirst, D. Bi, X. Zhou, and S. J. Marsland. Evaluation of El Nino-Southern Oscillation in the ACCESS coupled model simulations for CMIP5. Aust. Met. Ocean. J., 63:161–180, 2013. doi:10.22499/2.6301.010. K. E. Taylor, R. J. Stouffer, and G. A. Meehl. An overview of CMIP5 and the experiment design. Bull. Am. Meteorol. Soc., 93:485–498, 2012. doi:10.1175/BAMS-D-11-00094.1.

Detecting Regime Transitions in Time Series Using Dynamic Mode Decomposition

We employ the framework of the Koopman operator and dynamic mode decomposition to devise a computationally cheap and easily implementable method to detect transient dynamics and regime changes in time series. We argue that typically transient dynamics experiences the full state space dimension with subsequent fast relaxation towards the attractor. In equilibrium, on the other hand, the dynamics evolves on a slower time scale on a lower dimensional attractor. The reconstruction error of a dynamic mode decomposition is used to monitor the inability of the time series to resolve the fast relaxation towards the attractor as well as the effective dimension of the dynamics. We illustrate our method by detecting transient dynamics in the Kuramoto-Sivashinsky equation. We further apply our method to atmospheric reanalysis data; our diagnostics detects the transition from a predominantly negative North Atlantic Oscillation (NAO) to a predominantly positive NAO around 1970, as well as the recently found regime change in the Southern Hemisphere atmospheric circulation around 1970.

Seasonal climate summary for the southern hemisphere (summer 2016–17): a season of extremes despite neutral ENSO, IOD

This is a summary of the southern hemisphere atmospheric circulation patterns and meteorological indices for summer 2016–17; an account of seasonal rainfall and temperature for the Australian region is also provided. Although indices for the El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) represented typical neutral condition for these drivers, evidence of other climate drivers can be found in the land, ocean and atmosphere data from this time. The Southern Annular Mode appeared to have had some effect on rainfall in the east of Australia, and the Madden–Julian Oscillation active periods produced heavy rain in the tropical north. Despite neutral ENSO and IOD, extreme temperatures, in some areas highest on record, occurred in northern NSW and southern Queensland. High sea-surface temperatures caused further severe bleaching on the Great Barrier Reef.

Seasonal climate summary for the southern hemisphere (winter 2017): exceptionally warm days for Australia

This is a summary of the southern hemisphere atmospheric circulation patterns and meteorological indices for austral winter 2017; an account of seasonal rainfall and temperature for the Australian region is also provided. The El Niño–Southern Oscillation was neutral during winter 2017, as was the Indian Ocean Dipole. A positive Southern Annular Mode influenced the climates of southern hemisphere countries at times during winter. Despite the lack of large-scale ocean influences, mean temperatures for the season were very much above average across large areas of Australia, New Zealand, southern Africa and South America. Precipitation during the season was below average across much of Australia, South Africa and western areas of Chile and Argentina, but above average in some southern and eastern areas of South America.

Seasonal climate summary for the southern hemisphere (summer 2017–18): an exceptionally warm season for Australia – a short-lived and weak La Niña

This is a summary of the southern hemisphere atmospheric circulation patterns and meteorological indices for summer 2017–18; an account of seasonal rainfall and temperature for the Australian region is also provided. A short-lived and weak La Niña developed but decayed by the end of February 2018. Sea-surface temperatures were exceptionally warm in the Tasman Sea from late 2017 to early 2018. It was an exceptionally warm summer for Australia, and the third-warmest mean temperature on record for the nation. Summer rainfall was close to the long-term average for Australia, with aboveaverage rainfall in west and below-average rainfall in the east.

Seasonal climate summary for the southern hemisphere (winter 2016): a strong negative Indian Ocean Dipole brings wet conditions to Australia

This is a summary of the southern hemisphere atmospheric circulation patterns and meteorological indices for winter 2016; an account of seasonal rainfall and temperature for the Australian region and the broader southern hemisphere is also provided. One of the strongest negative phases on record of the Indian Ocean Dipole (IOD) developed during the season, contributing to Australia's second wettest winter on record, with rainfall above average over the vast majority of the continent. Neutral conditions prevailed in the tropical Pacific following the end of a strong El Nino event in autumn 2016, but the continuing effect of the 2015-16 El Nino was still evident in southern hemisphere temperatures, which were at or near record high levels.

Seasonal climate summary for the southern hemisphere (autumn 2016): El Niño slips into neutral and a negative Indian Ocean Dipole develops

This is a summary of the southern hemisphere atmospheric circulation patterns and meteorological indices for autumn 2016; an account of seasonal rainfall and temperature for the Australian region is also provided. While autumn began with a weak El Nino signal in the Pacific, the decay of the El Nino was evident with subsurface temperatures in the central and eastern Pacific continuing to cool. Later in the season, the Indian Ocean Dipole (IOD) transitioned to a negative phase. The negative IOD combined with warm water to Australia’s north channeled warm, moisture-laden air over the continent; unseasonable rainfall ensued, over eastern and northern Australia and New Zealand’s western coastal areas during May. Temperatures averaged over the southern hemisphere were record warm for autumn, both for land and ocean areas; separately or combined. For Australia, autumn arrived during a significant and prolonged heatwave that contributed to the warmest autumn on record for Australia. The elevated sea surface temperatures (SSTs) recorded in the Australian region earlier in the year persisted, and were warmest on record for autumn. Warm SSTs led to a global coral bleaching event affecting reefs in tropical waters; while, in extra-tropical waters, diminished kelp forests were observed. In the Australian region, reefs off the northwestern coast and, in northern areas of the Great Barrier Reef, were bleached. The most severe marine heatwave since records began was recorded in the Great Barrier Reef lagoon.