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In recent decades, wave power (WP) energy from the ocean is one of the cleanest renewable energy sources associated with oceanic warming. In Indo-Pacific Ocean, the WP is significantly influenced by natural climate variabilities, such as El Niño Southern Oscillation (ENSO), Indian Ocean Dipole (IOD) and Pacific Decadal Oscillation (PDO). In this study, the impact of major climate variability modes on seasonal extreme WP is examined over the period 1979–2019 using ERA5 reanalysis data and the non-stationary generalized extreme value analysis is applied to estimate the climatic extremes. Independent ENSO influence after removing the IOD trends (ENSO|IOD) on WP are evident over the eastern and central Pacific during December–February (DJF) and March–May (MAM), respectively, which subsequently shifts towards the western Pacific in June–August (JJA) and September–November (SON). The ENSO|PDO impact on WP exhibits similar yet weaker intensity year round compared to ENSO. Extreme WP responses due to the IOD|ENSO include widespread decreases over the tropical and eastern Indian Ocean (IO), with localized increases only over the South China and Philippine (SCP) seas and Bay of Bengal (BOB) during JJA, and the Arabian Sea during SON. Lastly, for the PDO|ENSO, the significant increases in WP are mostly confined to the Pacific, and most prominent in the North Pacific. Composite analysis of different phase combinations of PDO (IOD) with El Niño (La Niña) reveals stronger (weaker) influences year-round. The response patterns in significant wave height (SWH), peak wave period (PWP), sea surface temperatures (SST), and sea level pressure (SLP) helps to explain the seasonal variations in WP.

Extreme ocean waves can have devastating impacts on many populous coastal regions or offshore islands. Yet, knowledge of how ocean waves are likely to respond to future climate change remains limited. To assess potential increases in risk associated with extreme ocean waves, future changes in seasonal mean and extreme significant wave height (SWH) are examined over the Indian Ocean (IO) using 18 Coupled Model Intercomparison Project Phase 5 (CMIP5) models forced with representative concentration pathway (RCP) 4.5 and 8.5 scenarios. The seasonal maxima are fit to the generalized extreme value (GEV) distribution and corresponding 10-year return values are estimated for the present-day (1981–2010) and future periods (2070–2099). Overall, projected changes in IO SWH exhibit noticeable seasonality. Under the high emissions RCP8.5 scenarios, mean and extreme SWH in the Arabian Sea (AS) and Bay of Bengal (BOB) are projected to increase during all seasons except December–February (DJF). In the western tropical IO (TIO), mean and extreme SWHs are projected to increase during June–August (JJA) and September–November (SON) in line with the projected circulation changes toward an Indian Ocean Dipole (IOD) positive phase-like mean state. Southern IO (SIO) SWHs exhibit a strong zonal shift, with large increases over high-latitudes and decreases over mid-latitudes, which is related to future changes in the Southern Annular Mode (SAM) toward its positive phase. Interestingly, some regions like the western TIO show significantly less increases in SWH under the lower emissions RCP4.5 scenarios, highlighting avoidable future risk through global warming mitigation efforts.

This study provides the first quantitative assessment of observed long-term changes in summer-season timing and length in the Southern Hemisphere (SH) and its subregions over the past 60 years, enabling a global completeness by complementing such characteristics previously reported for the Northern Hemisphere (NH). Using an objective algorithm that is based on temperature indices, relative measures of summer onset, withdrawal, and duration are determined at each land location over the period 1953–2012. Significant widespread summer-season lengthening, due to earlier onset and delayed withdrawal, has occurred across the SH, a longer period for extreme heat-wave events and wildfires to potentially occur. The asymmetric magnitude (onset vs withdrawal) in summer-season lengthening is slightly less over the SH than over the NH. Contributions of anthropogenic and natural factors to the observed trends in summer-season characteristics were investigated using phase 5 of the Coupled Model Intercomparison Project (CMIP5) multimodel simulations integrated with observed external forcings [anthropogenic plus natural (ALL)], greenhouse gas forcing only (GHG), and natural forcing only [solar and volcanic activities (NAT)]. Overall, consistent with the NH, increased greenhouse gases were the main cause of observed changes in the SH, with negligible contribution from other external forcings. ALL and GHG simulations also reproduced a slight tendency for earlier summer onset to contribute more to summer lengthening. Proportions of observed regional trends in summer-season indices attributable to trends in long-term internal variability in the SH, namely, the interdecadal Pacific oscillation (IPO) and southern annular mode (SAM), suggests such variability can only explain up to ~12%, supporting the dominant role of greenhouse gas forcing.

This study is motivated by the potential improvement in water supply reliability and better forecasts of extreme rainfall and floods linked to Atmospheric rivers (ARs) in New Zealand. Results indic...

Atmospheric Rivers (ARs) are filamentary channels of strong poleward water vapour transport in the midlatitudes. Recent studies have found that ARs are northwesterly and northeasterly orientated an...

Future precipitation changes include contributions from both thermodynamic and dynamic processes. Given that precipitation in the tropics is commonly associated with convergence lines, we construct a simple linear regression model relating the convergence line frequency and strength to precipitation at sub-daily time-scales, and use it to show that changes in the convergence lines are related to the dynamical change in the precipitation. Given GCM-predicted convergence line changes, we predict precipitation changes using the regression model. The so predicted precipitation change is equivalent to the dynamical component of the precipitation change identified in earlier studies that used very different methods. The difference between the precipitation change in GCMs and that predicted from changes in convergence lines accounts for thermodynamic and other potentially important dynamical contributions. More accurate predictions of future precipitation therefore require the accurate simulations of the relatively short-lived weather features responsible for convergence lines in the tropics in GCMs.

This study quantifies the impact of atmospheric rivers (ARs) on rainfall in New Zealand. Using an automated AR detection algorithm, daily rainfall records from 654 rain gauges, and various atmospheric reanalysis datasets, we investigate the climatology of ARs, the characteristics of landfalling ARs, the contribution of ARs to annual and seasonal rainfall totals, and extreme rainfall events between 1979 and 2018 across the country. Results indicate that these filamentary synoptic features play an essential role in regional water resources and are responsible for many extreme rainfall events on the western side of mountainous areas and northern New Zealand. In these regions, depending on the season, 40–86% of the rainfall totals and 50–98% of extreme rainfall events are shown to be associated with ARs, with the largest contributions predominantly occurring during the austral summer. Furthermore, the median daily rainfall associated with ARs is 2–3 times than that associated with other storms. The results of this study extend the knowledge on the critical roles of ARs on hydrology and highlight the need for further investigation on the landfalling AR physical processes in relation to global circulation features and AR sources, and hydrological hazards caused by ARs in New Zealand.

Precipitation is often organized along coherent lines of low-level convergence, which at longer time and space scales form well-known convergence zones over the world’s oceans. Here, an automated, objective method is used to identify instantaneous low-level convergence lines in reanalysis data and calculate their frequency for the period 1979–2013. Identified convergence lines are combined with precipitation observations to assess the extent to which precipitation around the globe is associated with convergence lines in the lower troposphere. It is shown that a large percentage of precipitation (between 65% and 90%) over the tropical oceans is associated with such convergence lines, with large regional variations of up to 30% throughout the year, especially in the eastern Pacific and Atlantic Oceans. Over land, the annual-mean proportion of precipitation associated with convergence lines ranges between 30% and 60%, and the lowest proportions (less than 15%) associated with convergence lines occur on the eastern flank of the subtropical highs. Overall, much greater precipitation is associated with long coherent lines (greater than 300 km in length) than with shorter fragmented lines (less than 300 km), and the majority of precipitation associated with shorter lines occurs over land. The proportion of precipitation not associated with any convergence line primarily occurs where both precipitation and frequency of convergence lines are low. The high temporal and spatial resolution of the climatology constructed also enables an examination of the diurnal cycle in the relationship between convergence lines and precipitation. Here an example is provided over the tropical Maritime Continent region.

Special issue The 2nd International Indian Ocean Expedition (IIOE-2): Motivating New Exploration in a Poorly Understood Ocean Basin (Volume 1).-- 15 pages, 7 figures, 5 tables, 2 appendices, In the eastern Indian Ocean, the Kimberley region off north-western Australia is characterised by a wide continental shelf, complex coastal topography and very large tides (>10 m). Diversity of fishes is high although little is known about the distribution of their larvae in this dynamic region. This study investigated spatial variation in ichthyoplankton assemblages along four coastal-oceanic transects and related the results to tidal cycles and other environmental variables. Larvae representing 92 neritic and 21 mesopelagic teleost families were collected; this far exceeds the diversity described for other comparable tropical continental shelf and oceanic systems. Distinct ichthyoplankton assemblages characterised inner shelf (Engraulidae and Gobiidae were dominant families), outer shelf / slope (mixed neritic and mesopelagic taxa) and oceanic waters (mesopelagic taxa, particularly Myctophidae, Gonostomatidae and Phosichthyidae), and were correlated with changes in environmental variables (water depth, seawater density, mixed layer depth and zooplankton). The spring-neap tidal cycle and subsequent tides (same stations sampled 12 h apart) had no significant influence on larval fish assemblages. Coastal waters were delineated by a density front and larval fish assemblages were significantly different from offshore assemblages. This study provides new insights into the pelagic ecosystem in the tropical eastern Indian Ocean and contributes to the understanding of the role of environmental variables and physical forcing in the structuring of larval fish assemblages

Extreme ocean surface wave heights significantly affect coastal structures and offshore activities and impact many vulnerable populations of low-lying islands. Therefore, better understanding of ocean wave height variability plays an important role in potentially reducing risk in such regions. In this study, global impacts of natural climate variability such as El Niño–Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), and Pacific decadal oscillation (PDO) on extreme significant wave height (SWH) are analyzed using ERA-Interim (1980–2014) and ECMWF twentieth-century reanalysis (ERA-20C; 1952–2010) datasets for December–February (DJF). The nonstationary generalized extreme value (GEV) analysis is used to determine the influence of natural climate variability on DJF maxima of SWH (Hmax), wind speed (Wmax), and mean sea level pressure gradient amplitude (Gmax). The major ENSO influence on Hmax is found over the northeastern North Pacific (NP), with increases during El Niño and decreases during La Niña, and its counter responses are observed in coastal regions of the western NP, which are consistently observed in both Wmax and Gmax responses. The Hmax response to the PDO occurs over similar regions in the NP as those associated with ENSO but with much weaker amplitude. Composite analysis of different ENSO and PDO phase combinations reveals stronger (weaker) influences when both variability modes are of the same (opposite) phase. Furthermore, significant NAO influence on Hmax, Wmax, and Gmax is observed throughout Icelandic and Azores regions in relation to changes in atmospheric circulation patterns. Overall, the response of extreme SWH to natural climate variability modes is consistent with seasonal mean responses.

The diurnal cycle of precipitation in the Tropics is represented poorly in general circulation models (GCMs), which is primarily attributed to the representation of moist convection. Nonetheless, in areas where precipitation is driven by the diurnal cycle in the synoptic‐scale flow, GCMs may represent that circulation–rainfall relationship well. Over northwest Australia there is a tendency for precipitation to peak overnight where the diurnal cycle of the heat low circulation leads to the development of strong convergence after local sunset. In order to assess the heat low–precipitation relationship in more detail, a case‐study approach is used to investigate the actual ‘weather’ that is responsible for night‐time precipitation. The study shows that, where there is sufficient moisture, precipitation typically forms along convergence zones that coincide with boundaries between relatively moist and dry air masses (termed a ‘dryline’). A convergence line detection algorithm is then used to identify the fraction of observed nocturnal rainfall that is associated with any convergence zones. The same evaluation is then undertaken for a relatively high‐resolution (MetUM) and low‐resolution (ACCESS1.0) GCM, which simulate rainfall‐generation processes similar to the observations. Finally, the convergence line detection/precipitation algorithm is run on other GCM data (from CMIP5) to see whether the same processes occur despite different model configurations (i.e. physics), which appears to be the case."This is the pre-peer reviewed version of the following article: [Ackerley, D. , Birch, C. E., Garcia‐Carreras, L. , Lavender, S. L. and Weller, E. (2018), The physical processes causing nocturnal rainfall over northwest Australia and their representation in high‐ and low‐resolution models with parametrized convection. Q.J.R. Meteorol. Soc., 144: 511-528. doi:10.1002/qj.3223], which has been published in final form. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions."

Phase locked to austral winter and spring, canonical positive Indian Ocean Dipoles (pIODs) generally peak in spring. In recent decades, there has been an increase in unseasonable pIODs which, different from canonical pIODs, peak and decay by September. Distinguishing unseasonable pIODs from canonical pIODs is important, as conditions leading to more frequent unseasonable events are projected to persist in a warming climate. Here using superimposition of the first two seasonally evolving dominant modes of tropical Indian Ocean rainfall variability, we differentiate these types of pIODs. The first mode reflects characteristics of canonical pIODs, in which anomalies intensify with seasonal evolution. However, the second mode, with cool and dry anomalies extending from the eastern pole, reverses from winter to spring, signifying the demise of unseasonable pIODs. Processes embedded in the second mode reflect timing of propagation in equatorial Kelvin waves and their relative importance to the first mode, in generating different pIODs.

Extreme positive-Indian-Ocean-dipole events cause devastating floods in eastern tropical Africa and severe droughts in Asia; increasing greenhouse gas emissions will make these dipole events about three times more frequent in the twenty-first century. Countries in the southern tropical Indian Ocean region are prone to extensive flooding and droughts in years when the Indian Ocean dipole (IOD) climate cycle is in an extreme positive phase. In these bad years, such as 1961, 1994 and 1997, warm waters appear in the western part of the basin and precipitation increases, whereas in the east cooler waters predominate and precipitation decreases. Here Wenju Cai et al. assess climate model projections in a scenario of high greenhouse gas emissions and find that the frequency of extreme positive IODs is likely to increase from one event approximately every 17.3 years through the twentieth century to one event every 6.3 years during the twenty-first century. The Indian Ocean dipole is a prominent mode of coupled ocean–atmosphere variability1,2,3,4, affecting the lives of millions of people in Indian Ocean rim countries5,6,7,8,9,10,11,12,13,14,15. In its positive phase, sea surface temperatures are lower than normal off the Sumatra–Java coast, but higher in the western tropical Indian Ocean. During the extreme positive-IOD (pIOD) events of 1961, 1994 and 1997, the eastern cooling strengthened and extended westward along the equatorial Indian Ocean through strong reversal of both the mean westerly winds and the associated eastward-flowing upper ocean currents1,2. This created anomalously dry conditions from the eastern to the central Indian Ocean along the Equator and atmospheric convergence farther west, leading to catastrophic floods in eastern tropical African countries13,14 but devastating droughts in eastern Indian Ocean rim countries8,9,10,16,17. Despite these serious consequences, the response of pIOD events to greenhouse warming is unknown. Here, using an ensemble of climate models forced by a scenario of high greenhouse gas emissions (Representative Concentration Pathway 8.5), we project that the frequency of extreme pIOD events will increase by almost a factor of three, from one event every 17.3 years over the twentieth century to one event every 6.3 years over the twenty-first century. We find that a mean state change—with weakening of both equatorial westerly winds and eastward oceanic currents in association with a faster warming in the western than the eastern equatorial Indian Ocean—facilitates more frequent occurrences of wind and oceanic current reversal. This leads to more frequent extreme pIOD events, suggesting an increasing frequency of extreme climate and weather events in regions affected by the pIOD.

The Australian decade-long “Millennium Drought” broke in the summer of 2010/11 and was considered the most severe drought since instrumental records began in the 1900s. A crucial question is whether climate change played a role in inducing the rainfall deficit. The climate modes in question include the Indian Ocean dipole (IOD), affecting southern Australia in winter and spring; the southern annular mode (SAM) with an opposing influence on southern Australia in winter to that in spring; and El Niño–Southern Oscillation, affecting northern and eastern Australia in most seasons and southeastern Australia in spring through its coherence with the IOD. Furthermore, the poleward edge of the Southern Hemisphere Hadley cell, which indicates the position of the subtropical dry zone, has possible implications for recent rainfall declines in autumn. Using observations and simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), it is shown that the drought over southwest Western Australia is partly attributable to a long-term upward SAM trend, which contributed to half of the winter rainfall reduction in this region. For southeast Australia, models simulate weak trends in the pertinent climate modes. In particular, they severely underestimate the observed poleward expansion of the subtropical dry zone and associated impacts. Thus, although climate models generally suggest that Australia’s Millennium Drought was mostly due to multidecadal variability, some late-twentieth-century changes in climate modes that influence regional rainfall are partially attributable to anthropogenic greenhouse warming.

An assessment of how well climate models simulate the Indian Ocean dipole (IOD) is undertaken using 20 coupled models that have partaken in phase 5 of the Coupled Model Intercomparison Project (CMIP5). Compared with models in phase 3 (CMIP3), no substantial improvement is evident in the simulation of the IOD pattern and/or amplitude during austral spring [September–November (SON)]. The majority of models in CMIP5 generate a larger variance of sea surface temperature (SST) in the Sumatra–Java upwelling region and an IOD amplitude that is far greater than is observed. Although the relationship between precipitation and tropical Indian Ocean SSTs is well simulated, future projections of SON rainfall changes over IOD-influenced regions are intrinsically linked to the IOD amplitude and its rainfall teleconnection in the model present-day climate. The diversity of the simulated IOD amplitudes in models in CMIP5 (and CMIP3), which tend to be overly large, results in a wide range of future modeled SON rainfall trends over IOD-influenced regions. The results herein highlight the importance of realistically simulating the present-day IOD properties and suggest that caution should be exercised in interpreting climate projections in the IOD-affected regions.

Recent studies have shown that the impact of the Indian Ocean dipole (IOD) on southern Australia occurs via equivalent barotropic Rossby wave trains triggered by convective heating in the tropical Indian Ocean. Furthermore, the El Niño–Southern Oscillation (ENSO) influence on southern Australian climate is exerted through the same pathway during austral spring. It is also noted that positive phase [positive IOD (pIOD) and El Niño] events have a much larger impact associated with their respective skewness. These phenomena play a significant role in the region's rainfall reduction in recent decades, and it is essential that climate models used for future projections simulate these features. Here, the authors demonstrate that climate models do indeed simulate a greater climatic impact on Australia for pIOD events than for negative IOD (nIOD) events, but this asymmetric impact is distorted by an exaggerated influence of La Niña emanating from the Pacific. The distortion results from biases in the Pacific in two respects. First, the tropical and extratropical response to La Niña is situated unrealistically too far westward and hence too close to Australia, leading to an overly strong impact on southeast Australia that shows up through the nIOD–La Niña coherence. Second, the majority of models simulate a positive sea surface temperature skewness in the eastern Pacific that is too weak, overestimating the impact of La Niña relative to that of El Niño. As such, the impact of the positive asymmetry in the IOD only becomes apparent when the impact of ENSO is removed. This model bias needs to be taken into account when analyzing projections of regional Australian climate change.

The recent observed expansion of the Indo-Pacific warm pool is robustly attributed to anthropogenic greenhouse gas increases., The Indo-Pacific warm pool (IPWP) has warmed and grown substantially during the past century. The IPWP is Earth’s largest region of warm sea surface temperatures (SSTs), has the highest rainfall, and is fundamental to global atmospheric circulation and hydrological cycle. The region has also experienced the world’s highest rates of sea-level rise in recent decades, indicating large increases in ocean heat content and leading to substantial impacts on small island states in the region. Previous studies have considered mechanisms for the basin-scale ocean warming, but not the causes of the observed IPWP expansion, where expansion in the Indian Ocean has far exceeded that in the Pacific Ocean. We identify human and natural contributions to the observed IPWP changes since the 1950s by comparing observations with climate model simulations using an optimal fingerprinting technique. Greenhouse gas forcing is found to be the dominant cause of the observed increases in IPWP intensity and size, whereas natural fluctuations associated with the Pacific Decadal Oscillation have played a smaller yet significant role. Further, we show that the shape and impact of human-induced IPWP growth could be asymmetric between the Indian and Pacific basins, the causes of which remain uncertain. Human-induced changes in the IPWP have important implications for understanding and projecting related changes in monsoonal rainfall, and frequency or intensity of tropical storms, which have profound socioeconomic consequences.

Both air-sea heat exchanges and changes in ocean advection have contributed to observed upper-ocean warming most evident in the late-twentieth century. However, it is predominantly via changes in air-sea heat fluxes that human-induced climate forcings, such as increasing greenhouse gases, and other natural factors such as volcanic aerosols, have influenced global ocean heat content. The present study builds on previous work using two different indicators of upper-ocean temperature changes for the detection of both anthropogenic and natural external climate forcings. Using simulations from phase 5 of the Coupled Model Intercomparison Project, we compare mean temperatures above a fixed isotherm with the more widely adopted approach of using a fixed depth. We present the first multi-model ensemble detection and attribution analysis using the fixed isotherm approach to robustly detect both anthropogenic and natural external influences on upper-ocean temperatures. Although contributions from multidecadal natural variability cannot be fully removed, both the large multi-model ensemble size and properties of the isotherm analysis reduce internal variability of the ocean, resulting in better observation-model comparison of temperature changes since the 1950s. We further show that the high temporal resolution afforded by the isotherm analysis is required to detect natural external influences such as volcanic cooling events in the upper-ocean because the radiative effect of volcanic forcings is short-lived.

A continental shelf scale survey from 22°S to 34°S along the Western Australia coast provides the first detailed synoptic examination of the structure, circulation and modification of the southward flowing Leeuwin Current (LC) during the late austral autumn-early winter (May-June 2007). At lower latitudes (22°S-25°S), the LC was masked within a broad expanse of warm ambient surface water, which extended across the shelf and offshore before becoming constrained at the shelf break and attaining its maximum velocity of ~1.0ms-1 at 28°S. The temperature and salinity signature of the LC experienced substantial modification as it flowed poleward; surface temperature of the LC decreased by ~5.25°C while surface salinity increased by ~0.72, consistent with climatology estimates and smaller (larger) for temperature (salinity) than those found during summer. Subsequently, LC water was denser by ~2σT in the south compared to the north, and the surface mixed layer of the LC revealed only a small deepening trend along its poleward trajectory. Modification of the LC resulted from a combination of mixing due to geostrophic inflow and entrainment of cooler, more saline surrounding subtropical waters, and convective mixing driven by large heat loss to the atmosphere. Air-sea heat fluxes accounted for 50% of the heat lost from the LC in the south, whilst only accounting for 25% in the north, where large geostrophic inflow occurred and the LC displayed its maximum flow. The onshore transport was characterised by distinct jet-like structures, enhanced in the upper 200m of the water column, and the presence of eddies in the vicinity of the shelf break generated offshore transport.

An individual-based model, incorporating outputs of a data-assimilating hydrodynamic model, was developed to investigate the role of ocean circulation in the recruitment processes of western rock lobster ( Panulirus cygnus ) during its 9- to 11-month larval phase off the west coast of Australia. During austral summer, strong northward alongshore winds aid the offshore movement of early-stage model larvae from midshelf hatching sites into open ocean; during austral winter, eastward flows that feed the enhanced Leeuwin Current facilitate onshore movement of late-stage larvae towards nearshore habitats. Stokes drift induced by swells from the Southern Ocean is critical to retain larvae off the west coast. Diurnal migration and temperature-dependent growth are also important. Model larvae hatched in late spring – early summer grow faster because of longer exposure to warm summer temperature, which allows them to be transported towards the coast by the strong onshore flows in winter and reduces their natural mortality. Preliminary source–sink relationship indicates that the population was well mixed off the coast, with higher likelihood of settlement success from hatching sites in the north, mostly due to higher surface temperature. Weighted with the breeding stock distribution, the area between 27.5°S and 29.5°S, including the Abrolhos Islands, is the most important hatching area to the success of settlement.

A regional-scale estimate of the surface heat budget of the Great Barrier Reef and Coral Sea (10°–26°S, 142°–155°E) has been developed for the period 1995–2005 in the hope of understanding the trends of sea surface temperatures and the surface heat balance. This report describes the methodology to acquire input parameters from satellite observations, the resultant individual components of the surface heat budget, and their validation with existing datasets and surface measurements. The accuracy of individual flux components of the heat budget were analyzed with an array of surface measurements. Derived monthly averaged latent and sensible heat flux estimates show RMS errors of approximately 25.2 and 3.4 W m−2, respectively. Monthly averaged longwave and shortwave radiation flux estimates show RMS errors of approximately 6.7 and 13.3 W m−2, respectively. These improved estimates allow a higher confidence in studies that examine recent sea surface temperature (SST) trends and observed mass coral bleaching for the region. It is proposed that the greatest uptake of heat occurs over the spring/summer period in the central and southern regions of the Great Barrier Reef, agreeing well with areas where anomalously high sea surface temperatures are observed and where the most significant coral bleaching has occurred, and not in the most northern, more tropical region, as might be expected. The surface heat budget climatology was used to examine the mass bleaching episode that occurred in 2002. Results show that areas of maximum and minimum bleaching are better discriminated by the anomaly from mean seasonal values in the net surface heat flux (QNET), with accuracy of 86% and 79%, respectively, than by absolute QNET, absolute SST, or SST anomaly. Possible reasons for this are discussed.

A 10-year solar radiation climatology is developed for the Great Barrier Reef region using data from the Geostationary Meteorological Satellite (GMS). The method uses a look-up table based on the STREAMER radiative transfer model for a tropical atmosphere. One outstanding feature appears in the summer and autumn climatologies which exhibit distinct maxima in the southern end of the study area, between latitudes 18° S and 26° S. There is a small but statistically significant increase in solar radiation over the 10-year study period, with maximum values of just under 1% per decade. The radiation climatology was used to examine previous coral bleaching episodes, and in particular the largest mass bleaching episode that occurred in 2002. Results show that the area of maximum bleaching corresponded closely to the area of maximum solar insolation. Sea surface temperature and solar radiation were used in a variance analysis to discriminate bleached from unbleached reefs. Solar radiation is a better discriminator for inshore areas, but temperature is better at discriminating offshore reefs. Possible reasons for this difference are discussed.

This study assessed the anthropogenic contribution to the 2015 record-breaking high sea surface temperatures (SSTs) observed in the central equatorial Pacific and tropical Indian Ocean. Considering a close link between extreme warm events in these regions, we conducted a joint attribution analysis using a fraction of attributable risk approach. Probability of occurrence of such extreme anomalies and long-term trends for the two oceanic regions were compared between CMIP5 multi-model simulations with and without anthropogenic forcing. Results show that the excessive warming in both regions is well beyond the range of natural variability and robustly attributable to human activities due to greenhouse gas increase. We further explored associated mechanisms including the Bjerknes feedback and background anthropogenic warming. It is concluded that background warming was the main contribution to the 2015 extreme SST event over the central equatorial Pacific Ocean on a developing El Nino condition, which in turn induced the extreme SST event over the tropical Indian Ocean through the atmospheric bridge effect.

The Intertropical Convergence Zone (ITCZ) in the tropical eastern Indian Ocean exhibits strong interannual variability, often co-occurring with positive Indian Ocean Dipole (pIOD) events. During what we identify as an extreme ITCZ event, a drastic northward shift of atmospheric convection coincides with an anomalously strong north-minus-south sea surface temperature (SST) gradient over the eastern equatorial Indian Ocean. Such shifts lead to severe droughts over the maritime continent and surrounding islands but also devastating floods in southern parts of the Indian subcontinent. Understanding future changes of the ITCZ is therefore of major scientific and socioeconomic interest. Here we find a more-than-doubling in the frequency of extreme ITCZ events under greenhouse warming, estimated from climate models participating in the Coupled Model Intercomparison Project phase 5 that are able to simulate such events. The increase is due to a mean state change with an enhanced north-minus-south SST gradient and a weakened Walker Circulation, facilitating smaller perturbations to shift the ITCZ northwards.

The Indian Ocean Dipole Mode (IODM) impacts many surrounding and remote regions of the Indian Ocean, with devastating floods over East Africa but severe droughts in countries surrounding Indonesia during a positive IODM event. Understanding the dynamics is important for seasonal prediction and climate projections, but the role of meridional temperature and circulation anomalies remains unclear. Here, we show that in combination with the zonal structure of temperature and rainfall anomalies, northward contraction of the warm water pool over the eastern equatorial Indian Ocean region (EEIO) also generates an anomalous meridional cross-equatorial temperature gradient in the east. This meridional temperature gradient controls northward retreat of the atmospheric convection in association with northward cross-equatorial winds, and hence declining rainfall over the EEIO. Our results have important implications for the mean state change under greenhouse warming.

International audience; Natural modes of variability centred in the tropics, such as the El Nino/Southern Oscillation and the Indian Ocean Dipole, are a significant source of interannual climate variability across the globe. Future climate warming could alter these modes of variability. For example, with the warming projected for the end of the twenty-first century, the mean climate of the tropical Indian Ocean is expected to change considerably. These changes have the potential to affect the Indian Ocean Dipole, currently characterized by an alternation of anomalous cooling in the eastern tropical Indian Ocean and warming in the west in a positive dipole event, and the reverse pattern for negative events. The amplitude of positive events is generally greater than that of negative events. Mean climate warming in austral spring is expected to lead to stronger easterly winds just south of the Equator, faster warming of sea surface temperatures in the western Indian Ocean compared with the eastern basin, and a shoaling equatorial thermocline. The mean climate conditions that result from these changes more closely resemble a positive dipole state. However, defined relative to the mean state at any given time, the overall frequency of events is not projected to change [mdash] but we expect a reduction in the difference in amplitude between positive and negative dipole events.

Multi-decadal weakening trend of the equatorial Pacific easterly winds since 1960 has reversed after 1993. The trend reversal has induced cooling (shallow thermocline) trend in the equatorial western Pacific before 1993, followed by a warming (deep thermocline) trend from 1993 to the present. All available atmospheric reanalysis products corroborate the trend reversal during the two multi-decadal periods. The magnitudes of the multi-decadal trends of the easterly winds, however, differ among the reanalysis products. The trend reversals of regional ocean circulations are assessed using linear regressions between wind and transport anomalies in an eddy-permitting numerical model, suggesting that since 1993 the Indonesian Throughflow and the Leeuwin Current transports have also reversed their multi-decadal weakening trends. Key Points: - There have been reversals of the multi-decadal weakening trends of trade winds - Different reanalysis products capture different trends in trade winds