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Toshio Yamagata - One of the best experts on this subject based on the ideXlab platform.
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Opposite response of strong and moderate positive Indian Ocean Dipole to global warming
Nature Climate Change, 2020Co-Authors: Wenju Cai, Kai Yang, Gang Huang, Agus Santoso, Guojian Wang, Toshio YamagataAbstract:A strong positive Indian Ocean Dipole (pIOD) induces weather extremes such as the 2019 Australian bushfires and African floods. The impact is influenced by sea surface temperature (SST), yet models disagree on how pIOD SST may respond to greenhouse warming. Here we find increased SST variability of strong pIOD events, with strong equatorial eastern Indian Ocean cool anomalies, but decreased variability of moderate pIOD events, dominated by western warm anomalies. This opposite response is detected in the Coupled Model Inter-comparison Project (CMIP5 and CMIP6) climate models that simulate the two pIOD regimes. Under greenhouse warming, the lower troposphere warms faster than the surface, limiting Ekman pumping that drives the moderate pIOD warm anomalies; however, faster surface warming in the equatorial western region favours atmospheric convection in the west, strengthening equatorial nonlinear advection that forces the strong pIOD cool anomalies. Climate extremes seen in 2019 are therefore likely to occur more frequently under greenhouse warming. The strength of a positive Indian Ocean Dipole (pIOD) is set by sea surface temperature gradient across the equatorial Indian Ocean. Modelling shows warming will increase strong pIODs but decrease moderate pIODs, as faster surface warming in the west sets up conducive conditions for the strong events.
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stabilised frequency of extreme positive Indian Ocean Dipole under 1 5 c warming
Nature Communications, 2018Co-Authors: Wenju Cai, Agus Santoso, Guojian Wang, Bolan Gan, Xiaopei Lin, Zhaohui Chen, Fan Jia, Toshio YamagataAbstract:Extreme positive Indian Ocean Dipole (pIOD) affects weather, agriculture, ecosystems, and public health worldwide, particularly when exacerbated by an extreme El Nino. The Paris Agreement aims to limit warming below 2 °C and ideally below 1.5 °C in global mean temperature (GMT), but how extreme pIOD will respond to this target is unclear. Here we show that the frequency increases linearly as the warming proceeds, and doubles at 1.5 °C warming from the pre-industrial level (statistically significant above the 90% confidence level), underscored by a strong intermodel agreement with 11 out of 13 models producing an increase. However, in sharp contrast to a continuous increase in extreme El Nino frequency long after GMT stabilisation, the extreme pIOD frequency peaks as the GMT stabilises. The contrasting response corresponds to a 50% reduction in frequency of an extreme El Nino preceded by an extreme pIOD from that projected under a business-as-usual scenario.
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Improved Prediction of the Indian Ocean Dipole Mode by Use of Subsurface Ocean Observations
Journal of Climate, 2017Co-Authors: Takeshi Doi, Swadhin K Behera, Andrea Storto, Antonio Navarra, Toshio YamagataAbstract:AbstractThe numerical seasonal prediction system using the Scale Interaction Experiment–Frontier version 1 (SINTEX-F) Ocean–atmosphere coupled model has so far demonstrated a good performance for prediction of the Indian Ocean Dipole mode (IOD) despite the fact that the system adopts a relatively simple initialization scheme based on nudging only the sea surface temperature (SST). However, it is to be expected that the system is not sufficient to capture in detail the subsurface Oceanic precondition. Therefore, the authors have introduced a new three-dimensional variational Ocean data assimilation (3DVAR) method that takes three-dimensional observed Ocean temperature and salinity into account. Since the new system has successfully improved IOD predictions, the present study is showing that the Ocean observational efforts in the tropical Indian Ocean are decisive for improvement of the IOD predictions and may have a large impact on important socioeconomic activities, particularly in the Indian Ocean rim co...
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Anomalous Walker circulations associated with two flavors of the Indian Ocean Dipole
Geophysical Research Letters, 2016Co-Authors: Tomoki Tozuka, Satoru Endo, Toshio YamagataAbstract:The Walker circulation is the key component of the atmospheric zonal circulation in the tropics. In this study, it is shown that anomalous Walker circulations associated with two types of the Indian Ocean Dipole (IOD) are remarkably different. During a positive canonical IOD with negative (positive) sea surface temperature (SST) anomalies in the eastern (central to western) tropical Indian Ocean, a single-cell anomalous Walker circulation forms over the Indian Ocean. On the other hand, a double-cell anomalous Walker circulation with a rising branch in the central Indian Ocean is formed during a positive IOD Modoki, which is associated with positive (negative) SST anomalies over the central (eastern and western) tropical Indian Ocean. The above anomalous Walker circulations are found to develop as part of positive Ocean-atmosphere feedback. Furthermore, the above difference in the anomalous Walker circulation may affect the biennial tendency of the IOD.
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Increased frequency of extreme Indian Ocean Dipole events due to greenhouse warming
Nature, 2014Co-Authors: Wenju Cai, Evan Weller, Yukio Masumoto, Karumuri Ashok, Agus Santoso, Guojian Wang, Toshio YamagataAbstract: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.
Wenju Cai - One of the best experts on this subject based on the ideXlab platform.
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Thermocline warming induced extreme Indian Ocean Dipole in 2019
2021Co-Authors: Yuhong Zhang, Tomoki Tozuka, Lianyi Zhang, Wenju CaiAbstract:<p>The 2019 positive Indian Ocean Dipole (IOD) was the strongest event since the 1960s which developed independently without coinciding El Ni&#241;o. The dynamics is not fully understood. Here we show that in March-May, westward propagating Oceanic Rossby waves, a remnant consequence of the weak 2018 Pacific warm condition, led to anomalous sea surface temperature warming in the southwest tropical Indian Ocean (TIO), inducing deep convection and anomalous easterly winds along the equator, which triggered the initial cooling in the east. In June-August, the easterly wind anomalies continued to evolve through Ocean-atmosphere coupling involving Bjerknes feedback and equatorial nonlinear Ocean advection, until its maturity in September-November. This study clarifies the contribution of Oceanic Rossby waves in the south TIO in different dynamic settings and reveals a new triggering mechanism for extreme IOD events that will help to understand IOD diversity.</p>
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Triggering the Indian Ocean Dipole from the Southern Hemisphere
2021Co-Authors: Lianyi Zhang, Wenju Cai, Zesheng Chen, Tomoki TozukaAbstract:<p>This study identifies a new triggering mechanism of the Indian Ocean Dipole (IOD) from the Southern Hemisphere. This mechanism is independent from the El Ni&#241;o/Southern Oscillation (ENSO) and tends to induce the IOD before its canonical peak season. The joint effects of this mechanism and ENSO may explain different lifetimes and strengths of the IOD. During its positive phase, development of sea surface temperature cold anomalies commences in the southern Indian Ocean, accompanied by an anomalous subtropical high system and anomalous southeasterly winds. The eastward movement of these anomalies enhances the monsoon off Sumatra-Java during May-August, leading to an early positive IOD onset. The pressure variability in the subtropical area is related with the Southern Annular Mode, suggesting a teleconnection between high-latitude and mid-latitude climate that can further affect the tropics. To include the subtropical signals may help model prediction of the IOD event.</p>
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Opposite response of strong and moderate positive Indian Ocean Dipole to global warming
Nature Climate Change, 2020Co-Authors: Wenju Cai, Kai Yang, Gang Huang, Agus Santoso, Guojian Wang, Toshio YamagataAbstract:A strong positive Indian Ocean Dipole (pIOD) induces weather extremes such as the 2019 Australian bushfires and African floods. The impact is influenced by sea surface temperature (SST), yet models disagree on how pIOD SST may respond to greenhouse warming. Here we find increased SST variability of strong pIOD events, with strong equatorial eastern Indian Ocean cool anomalies, but decreased variability of moderate pIOD events, dominated by western warm anomalies. This opposite response is detected in the Coupled Model Inter-comparison Project (CMIP5 and CMIP6) climate models that simulate the two pIOD regimes. Under greenhouse warming, the lower troposphere warms faster than the surface, limiting Ekman pumping that drives the moderate pIOD warm anomalies; however, faster surface warming in the equatorial western region favours atmospheric convection in the west, strengthening equatorial nonlinear advection that forces the strong pIOD cool anomalies. Climate extremes seen in 2019 are therefore likely to occur more frequently under greenhouse warming. The strength of a positive Indian Ocean Dipole (pIOD) is set by sea surface temperature gradient across the equatorial Indian Ocean. Modelling shows warming will increase strong pIODs but decrease moderate pIODs, as faster surface warming in the west sets up conducive conditions for the strong events.
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Influence of internal climate variability on Indian Ocean Dipole properties.
Scientific reports, 2018Co-Authors: Wenju Cai, Tim CowanAbstract:The Indian Ocean Dipole (IOD) is the dominant mode of interannual variability over the tropical Indian Ocean (IO) and its future changes are projected to impact the climate and weather of Australia, East Africa, and Indonesia. Understanding the response of the IOD to a warmer climate has been largely limited to studies of individual coupled general circulation models or multi-model ensembles. This has provided valuable insight into the IOD’s projected response to increasing greenhouse gases but has limitations in accounting for the role of internal climate variability. Using the Community Earth System Model Large Ensemble (CESM-LE), the IOD is examined in thirty-five present-day and future simulations to determine how internal variability influences properties of the IOD and their response to a warmer climate. Despite small perturbations in the initial conditions as the only difference between ensemble members, significant relationships between the mean state of the IO and the IOD arise, leading to a spread in the projected IOD responses to increasing greenhouse gases. This is driven by the positive Bjerknes feedback, where small differences in mean thermocline depth, which are caused by internal climate variability, generate significant variations in IOD amplitude, skewness, and the climatological zonal sea surface temperature gradient.
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a spurious positive Indian Ocean Dipole in 2017
Chinese Science Bulletin, 2018Co-Authors: Lianyi Zhang, Wenju CaiAbstract:The Indian Ocean Dipole (IOD) is an intrinsic Ocean-atmosphere coupled mode in the tropical Indian Ocean (IO). It features a sea surface temperature (SST) anomaly gradient, defined by the Dipole mode index (DMI), the difference of area-averaged SSTanomaly between the western tropical IO (50°–70°E, 10°N–10°S) and the southeastern tropical IO (90°–110°E, Eq.-10°S). The IOD event tends to develop and mature in the boreal summer and fall seasons. When a positive IOD (pIOD) occurs, SST warms up in the west and cools down in the east, which sets up an east-west pressure gradient. The atmosphere hence responds to the anomalous SST gradient. Anomalous easterly winds prevail along the equator driven by the zonal pressure gradient, causing low sea level anomaly (SLA) in the eastern tropical IO. Rainless weather dominates over the Maritime Continent due to a weakened convection. Anomalies of an opposing polarity are generated during a negative IOD event.
Swadhin K Behera - One of the best experts on this subject based on the ideXlab platform.
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Improved Prediction of the Indian Ocean Dipole Mode by Use of Subsurface Ocean Observations
Journal of Climate, 2017Co-Authors: Takeshi Doi, Swadhin K Behera, Andrea Storto, Antonio Navarra, Toshio YamagataAbstract:AbstractThe numerical seasonal prediction system using the Scale Interaction Experiment–Frontier version 1 (SINTEX-F) Ocean–atmosphere coupled model has so far demonstrated a good performance for prediction of the Indian Ocean Dipole mode (IOD) despite the fact that the system adopts a relatively simple initialization scheme based on nudging only the sea surface temperature (SST). However, it is to be expected that the system is not sufficient to capture in detail the subsurface Oceanic precondition. Therefore, the authors have introduced a new three-dimensional variational Ocean data assimilation (3DVAR) method that takes three-dimensional observed Ocean temperature and salinity into account. Since the new system has successfully improved IOD predictions, the present study is showing that the Ocean observational efforts in the tropical Indian Ocean are decisive for improvement of the IOD predictions and may have a large impact on important socioeconomic activities, particularly in the Indian Ocean rim co...
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Influence of the state of the Indian Ocean Dipole on the following year’s El Niño
Nature Geoscience, 2010Co-Authors: Takeshi Izumo, Jerome Vialard, Matthieu Lengaigne, Swadhin K Behera, Jingjia Luo, Clément De Boyer Montégut, Sophie Cravatte, Sébastien Masson, Toshio YamagataAbstract:El Nino-Southern Oscillation (ENSO) consists of irregular episodes of warm El Nino and cold La Nina conditions in the tropical Pacific Ocean(1), with significant global socio-economic and environmental impacts(1). Nevertheless, forecasting ENSO at lead times longer than a few months remains a challenge(2,3). Like the Pacific Ocean, the Indian Ocean also shows interannual climate fluctuations, which are known as the Indian Ocean Dipole(4,5). Positive phases of the Indian Ocean Dipole tend to co-occur with El Nino, and negative phases with La Nina(6-9). Here we show using a simple forecast model that in addition to this link, a negative phase of the Indian Ocean Dipole anomaly is an efficient predictor of El Nino 14 months before its peak, and similarly, a positive phase in the Indian Ocean Dipole often precedes La Nina. Observations and model analyses suggest that the Indian Ocean Dipole modulates the strength of the Walker circulation in autumn. The quick demise of the Indian Ocean Dipole anomaly in November-December then induces a sudden collapse of anomalous zonal winds over the Pacific Ocean, which leads to the development of El Nino/La Nina. Our study suggests that improvements in the observing system in the Indian Ocean region and better simulations of its interannual climate variability will benefit ENSO forecasts.
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interaction between el nino and extreme Indian Ocean Dipole
Journal of Climate, 2010Co-Authors: Jingjia Luo, Fei-fei Jin, Swadhin K Behera, Ruochao Zhang, Yukio Masumoto, Roger Lukas, Toshio YamagataAbstract:Abstract Climate variability in the tropical Indo-Pacific sector has undergone dramatic changes under global Ocean warming. Extreme Indian Ocean Dipole (IOD) events occurred repeatedly in recent decades with an unprecedented series of three consecutive episodes during 2006–08, causing vast climate and socioeconomic effects worldwide and weakening the historic El Nino–Indian monsoon relationship. Major attention has been paid to the El Nino influence on the Indian Ocean, but how the IOD influences El Nino and its predictability remained an important issue to be understood. On the basis of various forecast experiments activating and suppressing air–sea coupling in the individual tropical Ocean basins using a state-of-the-art coupled Ocean–atmosphere model with demonstrated predictive capability, the present study shows that the extreme IOD plays a key role in driving the 1994 pseudo–El Nino, in contrast with traditional El Nino theory. The pseudo–El Nino is more frequently observed in recent decades, coinci...
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Influence of the state of the Indian Ocean Dipole on the following year's El Niño
Nature Geoscience, 2010Co-Authors: Takeshi Izumo, Jerome Vialard, Matthieu Lengaigne, Swadhin K Behera, Jingjia Luo, Clément De Boyer Montégut, Sophie Cravatte, Sébastien Masson, Toshio YamagataAbstract:El Niño-Southern Oscillation (ENSO) consists of irregular episodes of warm El Niño and cold La Niña conditions in the tropical Pacific Ocean(1), with significant global socio-economic and environmental impacts(1). Nevertheless, forecasting ENSO at lead times longer than a few months remains a challenge(2,3). Like the Pacific Ocean, the Indian Ocean also shows interannual climate fluctuations, which are known as the Indian Ocean Dipole(4,5). Positive phases of the Indian Ocean Dipole tend to co-occur with El Niño, and negative phases with La Niña(6-9). Here we show using a simple forecast model that in addition to this link, a negative phase of the Indian Ocean Dipole anomaly is an efficient predictor of El Niño 14 months before its peak, and similarly, a positive phase in the Indian Ocean Dipole often precedes La Niña. Observations and model analyses suggest that the Indian Ocean Dipole modulates the strength of the Walker circulation in autumn. The quick demise of the Indian Ocean Dipole anomaly in November-December then induces a sudden collapse of anomalous zonal winds over the Pacific Ocean, which leads to the development of El Niño/La Niña. Our study suggests that improvements in the observing system in the Indian Ocean region and better simulations of its interannual climate variability will benefit ENSO forecasts.
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Indian Ocean Dipole influence on South American rainfall
Geophysical Research Letters, 2008Co-Authors: Steven Chan, Swadhin K Behera, Toshio YamagataAbstract:[1] The rainfall anomalies over South America are found to be influenced by the Indian Ocean Dipole (IOD). Between subtropical La Plata Basin and central Brazil, the IOD excites a dipolar pattern in rainfall anomalies; rainfall is reduced (enhanced) over latter (former) during austral-spring, when IOD reaches its peak phase. A Rossby wave train extends from the subtropical south Indian Ocean to the subtropical South Atlantic. The associated anomaly in surface circulation suggests an intensification of the South Atlantic High. The anomalous anticyclone in the lower troposphere causes anomalous divergence (convergence) of moisture over central Brazil (subtropical La Plata Basin). These results based on the University of Delaware precipitation analysis and the NCEP-NCAR reanalysis data are corroborated by that of the Scale Interaction Experiment-Frontier version 1 (SINTEX-F1) coupled general circulation model.
Tomoki Tozuka - One of the best experts on this subject based on the ideXlab platform.
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Triggering the Indian Ocean Dipole from the Southern Hemisphere
2021Co-Authors: Lianyi Zhang, Wenju Cai, Zesheng Chen, Tomoki TozukaAbstract:<p>This study identifies a new triggering mechanism of the Indian Ocean Dipole (IOD) from the Southern Hemisphere. This mechanism is independent from the El Ni&#241;o/Southern Oscillation (ENSO) and tends to induce the IOD before its canonical peak season. The joint effects of this mechanism and ENSO may explain different lifetimes and strengths of the IOD. During its positive phase, development of sea surface temperature cold anomalies commences in the southern Indian Ocean, accompanied by an anomalous subtropical high system and anomalous southeasterly winds. The eastward movement of these anomalies enhances the monsoon off Sumatra-Java during May-August, leading to an early positive IOD onset. The pressure variability in the subtropical area is related with the Southern Annular Mode, suggesting a teleconnection between high-latitude and mid-latitude climate that can further affect the tropics. To include the subtropical signals may help model prediction of the IOD event.</p>
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Thermocline warming induced extreme Indian Ocean Dipole in 2019
2021Co-Authors: Yuhong Zhang, Tomoki Tozuka, Lianyi Zhang, Wenju CaiAbstract:<p>The 2019 positive Indian Ocean Dipole (IOD) was the strongest event since the 1960s which developed independently without coinciding El Ni&#241;o. The dynamics is not fully understood. Here we show that in March-May, westward propagating Oceanic Rossby waves, a remnant consequence of the weak 2018 Pacific warm condition, led to anomalous sea surface temperature warming in the southwest tropical Indian Ocean (TIO), inducing deep convection and anomalous easterly winds along the equator, which triggered the initial cooling in the east. In June-August, the easterly wind anomalies continued to evolve through Ocean-atmosphere coupling involving Bjerknes feedback and equatorial nonlinear Ocean advection, until its maturity in September-November. This study clarifies the contribution of Oceanic Rossby waves in the south TIO in different dynamic settings and reveals a new triggering mechanism for extreme IOD events that will help to understand IOD diversity.</p>
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Two flavors of the Indian Ocean Dipole
Climate Dynamics, 2016Co-Authors: Satoru Endo, Tomoki TozukaAbstract:The Indian Ocean Dipole (IOD) is known as a climate mode in the tropical Indian Ocean accompanied by negative (positive) sea surface temperature (SST) anomalies over the eastern (western) pole during its positive phase. However, the western pole of the IOD is not always covered totally by positive SST anomalies. For this reason, the IOD is further classified into two types in this study based on SST anomalies in the western pole. The first type (hereafter “canonical IOD”) is associated with negative (positive) SST anomalies in the eastern (central to western) tropical Indian Ocean. The second type (hereafter “IOD Modoki”), on the other hand, is associated with negative SST anomalies in the eastern and western tropical Indian Ocean and positive SST anomalies in the central tropical Indian Ocean. Based on composite analyses, it is found that easterly wind anomalies cover the whole equatorial Indian Ocean in the canonical IOD, and as a result, positive rainfall anomalies are observed over East Africa. Also, due to the basin-wide easterly wind anomalies, the canonical IOD is accompanied by strong sea surface height (SSH) anomalies. In contrast, zonal wind anomalies converge in the central tropical Indian Ocean in the IOD Modoki, and no significant precipitation anomalies are found over East Africa. Also, only weak SSH anomalies are seen, because equatorial downwelling anomalies induced by westerly wind anomalies in the west are counteracted by equatorial upwelling anomalies caused by easterly wind anomalies in the east.
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Anomalous Walker circulations associated with two flavors of the Indian Ocean Dipole
Geophysical Research Letters, 2016Co-Authors: Tomoki Tozuka, Satoru Endo, Toshio YamagataAbstract:The Walker circulation is the key component of the atmospheric zonal circulation in the tropics. In this study, it is shown that anomalous Walker circulations associated with two types of the Indian Ocean Dipole (IOD) are remarkably different. During a positive canonical IOD with negative (positive) sea surface temperature (SST) anomalies in the eastern (central to western) tropical Indian Ocean, a single-cell anomalous Walker circulation forms over the Indian Ocean. On the other hand, a double-cell anomalous Walker circulation with a rising branch in the central Indian Ocean is formed during a positive IOD Modoki, which is associated with positive (negative) SST anomalies over the central (eastern and western) tropical Indian Ocean. The above anomalous Walker circulations are found to develop as part of positive Ocean-atmosphere feedback. Furthermore, the above difference in the anomalous Walker circulation may affect the biennial tendency of the IOD.
Tim Cowan - One of the best experts on this subject based on the ideXlab platform.
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Influence of internal climate variability on Indian Ocean Dipole properties.
Scientific reports, 2018Co-Authors: Wenju Cai, Tim CowanAbstract:The Indian Ocean Dipole (IOD) is the dominant mode of interannual variability over the tropical Indian Ocean (IO) and its future changes are projected to impact the climate and weather of Australia, East Africa, and Indonesia. Understanding the response of the IOD to a warmer climate has been largely limited to studies of individual coupled general circulation models or multi-model ensembles. This has provided valuable insight into the IOD’s projected response to increasing greenhouse gases but has limitations in accounting for the role of internal climate variability. Using the Community Earth System Model Large Ensemble (CESM-LE), the IOD is examined in thirty-five present-day and future simulations to determine how internal variability influences properties of the IOD and their response to a warmer climate. Despite small perturbations in the initial conditions as the only difference between ensemble members, significant relationships between the mean state of the IO and the IOD arise, leading to a spread in the projected IOD responses to increasing greenhouse gases. This is driven by the positive Bjerknes feedback, where small differences in mean thermocline depth, which are caused by internal climate variability, generate significant variations in IOD amplitude, skewness, and the climatological zonal sea surface temperature gradient.
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Projected response of the Indian Ocean Dipole to greenhouse warming
Nature Geoscience, 2013Co-Authors: Wenju Cai, Matthieu Lengaigne, Evan Weller, Xiao-tong Zheng, Mat Collins, Tim Cowan, Toshio YamagataAbstract:The Indian Ocean Dipole is a key mode of interannual climate variability influencing much of Asia and Australia. A Review suggests that in response to greenhouse warming, mean conditions of the Indian Ocean will shift toward a positive Dipole state, but with no overall shift in the frequency of positive and negative events as defined relative to the mean climate state.
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Projected response of the Indian Ocean Dipole to greenhouse warming
Nature Geoscience, 2013Co-Authors: Wenju Cai, Matthieu Lengaigne, Evan Weller, Xiao-tong Zheng, Mat Collins, Tim Cowan, Toshio YamagataAbstract: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.
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Positive Indian Ocean Dipole events precondition southeast Australia bushfires
Geophysical Research Letters, 2009Co-Authors: Wenju Cai, Tim Cowan, Michael R. RaupachAbstract:[1] The devastating “Black Saturday” bushfire inferno in the southeast Australian state of Victoria in early February 2009 and the “Ash Wednesday” bushfires in February 1983 were both preceded by a positive Indian Ocean Dipole (pIOD) event. Is there a systematic pIOD linkage beyond these two natural disasters? We show that out of 21 significant bushfires seasons since 1950, 11 were preceded by a pIOD. During Victoria's wet season, particularly spring, a pIOD contributes to lower rainfall and higher temperatures exacerbating the dry conditions and increasing the fuel load leading into summer. Consequently, pIODs are effective in preconditioning Victoria for bushfires, more so than El Nino events, as seen in the impact on soil moisture on interannual time scales and in multi-decadal changes since the 1950s. Given that the recent increase in pIOD occurrences is consistent with what is expected from global warming, an increased bushfire risk in the future is likely across southeast Australia.
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recent unprecedented skewness towards positive Indian Ocean Dipole occurrences and its impact on australian rainfall
Geophysical Research Letters, 2009Co-Authors: Wenju Cai, Tim Cowan, Arnold SullivanAbstract:[1] Is the recent high frequency of positive Indian Ocean Dipole (pIOD) events a consequence of global warming? Using available observations and reanalyses, we show that the pIOD occurrences increase from about four per 30 years early in the 20th century to about 10 over the last 30 years; by contrast, the number of negative Indian Ocean Dipole (nIOD) events decreases from about 10 to two over the same periods, respectively. A skewness measure, defined as the difference in occurrences of pIODs and nIODs, illustrates a systematic trend in this parameter commencing early in the 20th century. After 1950, there are more pIODs than nIODs, with consistent mean circulation changes in the pIOD-prevalent seasons. Over southeastern Australia (SEA), these changes potentially account for much of the observed austral winter and spring rainfall reduction since 1950. These features are consistent with projected future climate change and hence with what is expected from global warming.
