scholarly journals The Association of Tropical and Extratropical Climate Modes to Atmospheric Blocking across Southeastern Australia

2013 ◽  
Vol 26 (19) ◽  
pp. 7555-7569 ◽  
Author(s):  
Tim Cowan ◽  
Peter van Rensch ◽  
Ariaan Purich ◽  
Wenju Cai
Keyword(s):  
Indian Ocean ◽  
Southern Oscillation ◽  
Low Pressure ◽  
Atmospheric Blocking ◽  
High Pressure Cell ◽  
Austral Spring ◽  
Climate Modes ◽  
Pressure Cell ◽  
Tropical Moisture ◽  
Austral Autumn

Abstract Relationships of the Indian Ocean dipole (IOD), El Niño–Southern Oscillation (ENSO), and the southern annular mode (SAM) with atmospheric blocking are investigated using a linear framework over the austral autumn–spring (cool) seasons for southeast Australia (SEA). Positive blocking events occurring at 130°–140°E increase the likelihood of cutoff low pressure systems developing that generate significant rainfall totals across SEA. In mid to late austral autumn (April–May), blocking is coherent with negative IOD events. During this season, a negative IOD event and blocking are associated with warm sea surface temperature anomalies in the eastern tropical Indian Ocean and a blocking high pressure cell south of Australia. An anomalous cyclonic pressure center over southern Australia directs tropical moisture flux anomalies to the region. Despite this, only a small portion of a negative IOD's impact on SEA rainfall comes through blocking events. During austral winter, ENSO is coherent with blocking; anomalous tropical moisture fluxes from the western Pacific during a La Niña merge with anomalous cyclonic flows centered over SEA, delivering enhanced rainfall via cutoff lows. The low pressure cell constitutes a center of the Southern Oscillation associated with ENSO. This ENSO-blocking coherence is considerably weaker in austral spring, whereby circulation anomalies associated with blocking resemble a SAM-like pattern. As such, a large portion of the SAM's impact on SEA spring rainfall occurs in conjunction with blocking events. The relative importance of associations between the dominant climate modes and blocking in generating the drought-breaking cool season precipitation in 2010 across SEA is discussed.

Influence of Global-Scale Variability on the Subtropical Ridge over Southeast Australia

2011 ◽  
Vol 24 (23) ◽  
pp. 6035-6053 ◽  
Author(s):  
Wenju Cai ◽  
Peter van Rensch ◽  
Tim Cowan
Keyword(s):  
Indian Ocean ◽  
Southern Oscillation ◽  
Climate Modes ◽  
Southeast Australia ◽  
The Indian Ocean ◽  
Wave Trains ◽  
The Impact ◽  
Austral Autumn ◽  
Subtropical Ridge

Abstract In recent decades, southeast Australia (SEA) has experienced a severe rainfall decline, with a maximum reduction in the austral autumn season. The cause(s) of this decline remain unclear. This study examines the interaction between remote large-scale climate modes and an atmospheric phenomenon known as the subtropical ridge (STR) at the local scale. A focus is placed on the utility of using the STR as a bridge for understanding how these remote climate drivers influence SEA rainfall through a response in local atmospheric conditions. Using observational data since 1979, it is found that a strong seasonality exists in the impact of the STR on SEA rainfall. In austral autumn, because SEA rainfall is poorly correlated with the STR intensity (STRI) and STR position (STRP) on an interannual basis, it follows that most of the autumn rainfall reduction cannot be explained by the STRI changes in this season. There is also no clear relationship between the autumn STR and known remote modes of variability. Reductions in SEA rainfall have occurred in the austral winter and spring seasons; however, neither is significant. During winter, although El Niño–Southern Oscillation (ENSO) has little impact on the STR, there is a significant influence from the Indian Ocean dipole (IOD) and the southern annular mode (SAM). The IOD impact is conducted through equivalent-barotropic Rossby wave trains stemming from the eastern Indian Ocean in response to the IOD-induced anomalous convection and divergence. These wave trains modify the intensity and position of the ridge over SEA. The impact from the SAM is similarly projected onto the STRI and STRP. The STR trend accounts for the entire observed decline in SEA winter rainfall, 80% of which is contributed by the upward trend of the IOD; the SAM exhibits virtually no trend over the 30-yr period in this season. In spring, SEA rainfall shows strong interannual variability and is well correlated with the STRI; the ridge itself is influenced by the IOD and ENSO but not by the SAM. The Indian Ocean is a major pathway for ENSO’s impact on SEA rainfall in this season, which is conducted by two wave trains emanating from the east and west poles of the IOD. These wave train patterns share an anomalously high surface pressure center south of Australia, which does not align with the STR over SEA. As such, only a small portion of the STRI variance is accounted for by fluctuations in ENSO and the IOD. Long-term changes in the STRI account for about 90% of the observed decline in SEA spring rainfall, all of which are due to a recent increased frequency in the number of positive IOD events (upward IOD trend); ENSO shows no long-term trend over the 30-yr period. In summary, variability and change in winter and spring rainfall across SEA can be understood through the impact of remote climate modes, such as ENSO, the IOD, and the SAM, on the STR. This approach, however, offers no utility for understanding what drives the long-term SEA autumn rainfall decline, the dynamics of which remain elusive.

scholarly journals El Niño-Southern Oscillation and Indian Ocean Dipole Modes: Their Effects on South American Rainfall during Austral Spring

Atmosphere ◽  
2021 ◽  
Vol 12 (11) ◽  
pp. 1437
Author(s):  
Mary T. Kayano ◽  
Wilmar L. Cerón ◽  
Rita V. Andreoli ◽  
Rodrigo A. F. Souza ◽  
Itamara P. Souza ◽  
...  
Keyword(s):  
Indian Ocean ◽  
El Niño ◽  
Indian Ocean Dipole ◽  
El Nino ◽  
Southern Oscillation ◽  
Tropical Indian Ocean ◽  
El Niño Southern Oscillation ◽  
El Nino Southern Oscillation ◽  
Austral Spring ◽  
Precipitation Anomalies

This paper examines the effects of the tropical Pacific Ocean (TPO) and Indian Ocean Dipole (IOD) modes in the interannual variations of austral spring rainfall over South America (SA). The TPO mode refers to the El Niño-Southern Oscillation (ENSO). The isolated effects between IOD and TPO were estimated, events were chosen from the residual TPO (R-TPO) or residual IOD (R-IOD), and the IOD (TPO) effects for the R-TPO (R-IOD) composites were removed from the variables. One relevant result was the nonlinear precipitation response to R-TPO and R-IOD. This feature was accentuated for the R-IOD composites. The positive R-IOD composite showed significant negative precipitation anomalies along equatorial SA east of 55° W and in subtropical western SA, and showed positive anomalies in northwestern SA and central Brazil. The negative R-IOD composite indicated significant positive precipitation anomalies in northwestern Amazon, central–eastern Brazil north of 20° S, and western subtropical SA, and negative anomalies were found in western SA south of 30° S. This nonlinearity was likely due to the distinct atmospheric circulation responses to the anomalous heating sources located in longitudinally distinct regions: the western tropical Indian Ocean and areas neighboring Indonesia. The results obtained in this study might be relevant for climate monitoring and modeling studies.

scholarly journals Indian Ocean Dipole and El Niño/Southern Oscillation impacts on regional chlorophyll anomalies in the Indian Ocean

2013 ◽  
Vol 10 (10) ◽  
pp. 6677-6698 ◽  
Author(s):  
J. C. Currie ◽  
M. Lengaigne ◽  
J. Vialard ◽  
D. M. Kaplan ◽  
O. Aumont ◽  
...  
Keyword(s):  
Indian Ocean ◽  
El Niño ◽  
Indian Ocean Dipole ◽  
El Nino ◽  
Southern Oscillation ◽  
El Niño Southern Oscillation ◽  
El Nino Southern Oscillation ◽  
Near Surface ◽  
Climate Modes ◽  
The Indian Ocean

Abstract. The Indian Ocean Dipole (IOD) and the El Niño/Southern Oscillation (ENSO) are independent climate modes, which frequently co-occur, driving significant interannual changes within the Indian Ocean. We use a four-decade hindcast from a coupled biophysical ocean general circulation model, to disentangle patterns of chlorophyll anomalies driven by these two climate modes. Comparisons with remotely sensed records show that the simulation competently reproduces the chlorophyll seasonal cycle, as well as open-ocean anomalies during the 1997/1998 ENSO and IOD event. Results suggest that anomalous surface and euphotic-layer chlorophyll blooms in the eastern equatorial Indian Ocean in fall, and southern Bay of Bengal in winter, are primarily related to IOD forcing. A negative influence of IOD on chlorophyll concentrations is shown in a region around the southern tip of India in fall. IOD also depresses depth-integrated chlorophyll in the 5–10° S thermocline ridge region, yet the signal is negligible in surface chlorophyll. The only investigated region where ENSO has a greater influence on chlorophyll than does IOD, is in the Somalia upwelling region, where it causes a decrease in fall and winter chlorophyll by reducing local upwelling winds. Yet unlike most other regions examined, the combined explanatory power of IOD and ENSO in predicting depth-integrated chlorophyll anomalies is relatively low in this region, suggestive that other drivers are important there. We show that the chlorophyll impact of climate indices is frequently asymmetric, with a general tendency for larger positive than negative chlorophyll anomalies. Our results suggest that ENSO and IOD cause significant and predictable regional re-organisation of chlorophyll via their influence on near-surface oceanography. Resolving the details of these effects should improve our understanding, and eventually gain predictability, of interannual changes in Indian Ocean productivity, fisheries, ecosystems and carbon budgets.

scholarly journals Late Quaternary climate variability at Mfabeni peatland, eastern South Africa

2019 ◽  
Vol 15 (3) ◽  
pp. 1153-1170 ◽  
Author(s):  
Charlotte Miller ◽  
Jemma Finch ◽  
Trevor Hill ◽  
Francien Peterse ◽  
Marc Humphries ◽  
...  
Keyword(s):  
South Africa ◽  
High Pressure ◽  
Indian Ocean ◽  
Late Quaternary ◽  
High Pressure Cell ◽  
Sea Ice Extent ◽  
Pressure Cell ◽  
The Past ◽  
Antarctic Sea Ice ◽  
Quaternary Climate

Abstract. The scarcity of continuous, terrestrial, palaeoenvironmental records in eastern South Africa leaves the evolution of late Quaternary climate and its driving mechanisms uncertain. Here we use a ∼7 m long core from Mfabeni peatland (KwaZulu-Natal, South Africa) to reconstruct climate variability for the last 32 000 years (cal ka BP). We infer past vegetation and hydrological variability using stable carbon (δ13Cwax) and hydrogen isotopes (δDwax) of plant-wax n-alkanes and use Paq to reconstruct water table changes. Our results indicate that late Quaternary climate in eastern South Africa did not respond directly to orbital forcing or to changes in sea-surface temperatures (SSTs) in the western Indian Ocean. We attribute the arid conditions evidenced at Mfabeni during the Last Glacial Maximum (LGM) to low SSTs and an equatorward displacement of (i) the Southern Hemisphere westerlies, (ii) the subtropical high-pressure cell, and (iii) the South Indian Ocean Convergence Zone (SIOCZ), which we infer was linked to increased Antarctic sea-ice extent. The northerly location of the high-pressure cell and the SIOCZ inhibited moisture advection inland and pushed the rain-bearing cloud band north of Mfabeni, respectively. The increased humidity at Mfabeni between 19 and 14 cal kyr BP likely resulted from a southward retreat of the westerlies, the high-pressure cell, and the SIOCZ, consistent with a decrease in Antarctic sea-ice extent. Between 14 and 5 cal kyr BP, when the westerlies, the high-pressure cell, and the SIOCZ were in their southernmost position, local insolation became the dominant control, leading to stronger atmospheric convection and an enhanced tropical easterly monsoon. Generally drier conditions persisted during the past ca. 5 cal ka BP, probably resulting from an equatorward return of the westerlies, the high-pressure cell, and the SIOCZ. Higher SSTs and heightened El Niño–Southern Oscillation (ENSO) activity may have played a role in enhancing climatic variability during the past ca. 5 cal ka BP. Our findings highlight the influence of the latitudinal position of the westerlies, the high-pressure cell, and the SIOCZ in driving climatological and environmental changes in eastern South Africa.

scholarly journals An Asymmetry in the IOD and ENSO Teleconnection Pathway and Its Impact on Australian Climate

2012 ◽  
Vol 25 (18) ◽  
pp. 6318-6329 ◽  
Author(s):  
Wenju Cai ◽  
Peter van Rensch ◽  
Tim Cowan ◽  
Harry H. Hendon
Keyword(s):  
Indian Ocean ◽  
El Niño ◽  
El Nino ◽  
Southern Oscillation ◽  
Wave Train ◽  
Austral Spring ◽  
Southeast Australia ◽  
Unit Change ◽  
The Impact ◽  
Sst Anomalies

Abstract Recent research has shown that the climatic impact from El Niño–Southern Oscillation (ENSO) on middle latitudes west of the western Pacific (e.g., southeast Australia) during austral spring (September–November) is conducted via the tropical Indian Ocean (TIO). However, it is not clear whether this impact pathway is symmetric about the positive and negative phases of ENSO and the Indian Ocean dipole (IOD). It is shown that a strong asymmetry does exist. For ENSO, only the impact from El Niño is conducted through the TIO pathway; the impact from La Niña is delivered through the Pacific–South America pattern. For the IOD, a greater convection anomaly and wave train response occurs during positive IOD (pIOD) events than during negative IOD (nIOD) events. This “impact asymmetry” is consistent with the positive skewness of the IOD, principally due to a negative skewness of sea surface temperature (SST) anomalies in the east IOD (IODE) pole. In the IODE region, convection anomalies are more sensitive to a per unit change of cold SST anomalies than to the same unit change of warm SST anomalies. This study shows that the IOD skewness occurs despite the greater damping, rather than due to a breakdown of this damping as suggested by previous studies. This IOD impact asymmetry provides an explanation for much of the reduction in spring rainfall over southeast Australia during the 2000s. Key to this rainfall reduction is the increased occurrences of pIOD events, more so than the lack of nIOD events.

Antarctic Sea Ice Response to Weather and Climate Modes of Variability*

2016 ◽  
Vol 29 (2) ◽  
pp. 721-741 ◽  
Author(s):  
Tsubasa Kohyama ◽  
Dennis L. Hartmann
Keyword(s):  
Indian Ocean ◽  
Sea Ice ◽  
Time Scale ◽  
Ross Sea ◽  
Southern Oscillation ◽  
Large Fraction ◽  
Sea Ice Extent ◽  
Climate Modes ◽  
Antarctic Sea Ice ◽  
The Indian Ocean

Abstract The relationship between climate modes and Antarctic sea ice is explored by separating the variability into intraseasonal, interannual, and decadal time scales. Cross-spectral analysis shows that geopotential height and Antarctic sea ice extent are most coherent at periods between about 20 and 40 days (the intraseasonal time scale). In this period range, where the atmospheric circulation and the sea ice extent are most tightly coupled, sea ice variability responds strongly to Rossby waves with the structure of the Pacific–South American (PSA) pattern. The PSA pattern in this time scale is not directly related to El Niño–Southern Oscillation (ENSO) or the southern annular mode (SAM), which have received much attention for explaining Antarctic sea ice variability. On the interannual time scale, ENSO and SAM are important, but a large fraction of sea ice variance can also be explained by Rossby wave–like structures in the Drake Passage region. After regressing out the sea ice extent variability associated with ENSO, the observed positive sea ice trends in Ross Sea and Indian Ocean during the satellite era become statistically insignificant. Regressing out SAM makes the sea ice trend in the Indian Ocean insignificant. Thus, the positive trends in sea ice in the Ross Sea and the Indian Ocean sectors may be explained by the variability and decadal trends of known interannual climate modes.

scholarly journals Spatial and seasonal responses of precipitation in the Ganges and Brahmaputra river basins to ENSO and Indian Ocean dipole modes: implications for flooding and drought

2014 ◽  
Vol 2 (2) ◽  
pp. 1671-1692
Author(s):  
M. S. Pervez ◽  
G. M. Henebry
Keyword(s):  
Indian Ocean ◽  
El Niño ◽  
El Nino ◽  
Southern Oscillation ◽  
Climate Impacts ◽  
Climate Modes ◽  
Ganges Basin ◽  
Brahmaputra Basin ◽  
Precipitation Records ◽  
The Ganges

Abstract. We evaluated the spatial and temporal responses of precipitation in the basins as modulated by the El Niño Southern Oscillation (ENSO) and Indian Ocean (IO) dipole modes using observed precipitation records at 43 stations across the Ganges and Brahmaputra basins from 1982 to 2010. Daily observed precipitation records were extracted from Global Surface Summary of the Day dataset and spatial and monthly anomalies were computed. The anomalies were averaged for the years influenced by climate modes combinations. Occurrences of El Niño alone significantly reduced (60% and 88% of baseline in the Ganges and Brahmaputra basins, respectively) precipitation during the monsoon months in the northwestern and central Ganges basin and across the Brahmaputra basin. In contrast, co-occurrence of La Niña and a positive IO dipole mode significantly enhanced (135% and 160% of baseline, respectively) precipitation across both basins. During the co-occurrence of neutral phases in both climate modes (occurring 13 out of 28 yr), precipitation remained below average to average in the agriculturally extensive areas of Haryana, Uttar Pradesh, Bihar, eastern Nepal, and the Rajshahi district in Bangladesh in the Ganges basin and northern Bangladesh, Meghalaya, Assam, and Arunachal Pradesh in the Brahmaputra basin. This pattern implies that a regular water deficit is likely in these areas with implications for the agriculture sector due to its reliance on consistent rainfall for successful production. Major flooding and drought occurred as a consequence of the interactive effects of the ENSO and IO dipole modes, with the sole exception of extreme precipitation and flooding during El Niño events. This observational analysis will facilitate well informed decision making in minimizing natural hazard risks and climate impacts on agriculture, and supports development of strategies ensuring optimized use of water resources in best management practice under changing climate.

scholarly journals Dynamical Link of Peat Fires in South Sumatra and the Climate Modes in the Indo-Pacific Region

2019 ◽  
Vol 51 (1) ◽  
pp. 18
Author(s):  
Raden Putra ◽  
Deni Okta Lestari ◽  
Edy Sutriyono ◽  
Sabaruddin Sabaruddin ◽  
Iskhaq Iskandar
Keyword(s):  
Indian Ocean ◽  
El Niño ◽  
El Nino ◽  
Southern Oscillation ◽  
Pacific Region ◽  
Climate Anomaly ◽  
Climate Modes ◽  
Environmental Disasters ◽  
Peat Fire ◽  
Peat Fires

Peat fire is one of the environmental disasters occurring widespread during the dry season in South Sumatra. The region has long been recognized to have extensive peatland, hence it is considered as the vulnerable areas to fire. This study employs spatial analysis to evaluate the likely linked factors causing peat fire in the study area. Two interannual climate modes such as the El Niño – Southern Oscillation and Indian Ocean Dipole were considered to have affected the area with respect to climate anomaly at the 1995-2016 periods. This phenomenon was followed by the peat fire in many areas. There appears a close linkage between the occurrence of peat fires and climate anomaly. A number of hotspots tend to occur annually during the drought season. A significant number of hotspots took place during the 2006 pIOD and 2015 El Niño events due to a significant decrease in rainfall intensities.

scholarly journals Corrigendum to: ACCESS-S1: The new Bureau of Meteorology multi-week to seasonal prediction system

2020 ◽  
Vol 70 (1) ◽  
pp. 393
Author(s):  
Debra Hudson ◽  
Oscar Alves ◽  
Harry H. Hendon ◽  
Eun-Pa Lim ◽  
Guoqiang Liu ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Land Surface ◽  
Southern Oscillation ◽  
Seasonal Prediction ◽  
Maximum Temperature ◽  
Prediction System ◽  
Seasonal Forecasts ◽  
Austral Spring ◽  
Eastern Australia ◽  
The Indian Ocean

ACCESS-S1 will be the next version of the Australian Bureau of Meteorology's seasonal prediction system, due to become operational in early 2018. The multiweek and seasonal performance of ACCESS-S1 has been evaluated based on a 23-year hindcast set and compared to the current operational system, POAMA. The system has considerable enhancements compared to POAMA, including higher vertical and horizontal resolution of the component models and state-ofthe-art physics parameterisation schemes. ACCESS-S1 is based on the UK Met Office GloSea5-GC2 seasonal prediction system, but has enhancements to the ensemble generation strategy to make it appropriate for multi-week forecasting, and a larger ensemble size.ACCESS-S1 has markedly reduced biases in the mean state of the climate, both globally and over Australia, compared to POAMA. ACCESS-S1 also better predicts the early stages of the development of the El Niño Southern Oscillation (through the predictability barrier) and the Indian Ocean Dipole, as well as multi-week variations of the Southern Annular Mode and the Madden-Julian Oscillation — all important drivers of Australian climate variability. There is an overall improvement in the skill of the forecasts of rainfall, maximum temperature (Tmax) and minimum temperature (Tmin) over Australia on multi-week timescales compared to POAMA. On seasonal timescales the differences between the two systems are generally less marked. ACCESS-S1 has improved seasonal forecasts over Australia for the austral spring season compared to POAMA, with particularly good forecast reliability for rainfall and Tmax. However, forecasts of seasonal mean Tmax are noticeably less skilful over eastern Australia for forecasts of late autumn and winter compared to POAMA.The study has identified scope for improvement of ACCESS-S in the future, particularly 1) reducing rainfall errors in the Indian Ocean and Maritime Continent regions, and 2) initialising the land surface with realistic soil moisture rather than climatology. The latter impacts negatively on the skill of the temperature forecasts over eastern Australia and is being addressed in the next version of the system, ACCESS-S2.

scholarly journals ACCESS-S1 The new Bureau of Meteorology multi-week to seasonal prediction system

2017 ◽  
Vol 67 (3) ◽  
pp. 132 ◽  
Author(s):  
Debra Hudson ◽  
Oscar Alves ◽  
Harry H. Hendon ◽  
Eun-Pa Lim ◽  
Guoqiang Liu ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Land Surface ◽  
Southern Oscillation ◽  
Seasonal Prediction ◽  
Maximum Temperature ◽  
Prediction System ◽  
Seasonal Forecasts ◽  
Austral Spring ◽  
Eastern Australia ◽  
The Indian Ocean

ACCESS-S1 will be the next version of the Australian Bureau of Meteorology's seasonal prediction system, due to become operational in early 2018. The multiweek and seasonal performance of ACCESS-S1 has been evaluated based on a 23-year hindcast set and compared to the current operational system, POAMA. The system has considerable enhancements compared to POAMA, including higher vertical and horizontal resolution of the component models and state-ofthe-art physics parameterisation schemes. ACCESS-S1 is based on the UK Met Office GloSea5-GC2 seasonal prediction system, but has enhancements to the ensemble generation strategy to make it appropriate for multi-week forecasting, and a larger ensemble size.ACCESS-S1 has markedly reduced biases in the mean state of the climate, both globally and over Australia, compared to POAMA. ACCESS-S1 also better predicts the early stages of the development of the El Niño Southern Oscillation (through the predictability barrier) and the Indian Ocean Dipole, as well as multi-week variations of the Southern Annular Mode and the Madden-Julian Oscillation — all important drivers of Australian climate variability. There is an overall improvement in the skill of the forecasts of rainfall, maximum temperature (Tmax) and minimum temperature (Tmin) over Australia on multi-week timescales compared to POAMA. On seasonal timescales the differences between the two systems are generally less marked. ACCESS-S1 has improved seasonal forecasts over Australia for the austral spring season compared to POAMA, with particularly good forecast reliability for rainfall and Tmax. However, forecasts of seasonal mean Tmax are noticeably less skilful over eastern Australia for forecasts of late autumn and winter compared to POAMA.The study has identified scope for improvement of ACCESS-S in the future, particularly 1) reducing rainfall errors in the Indian Ocean and Maritime Continent regions, and 2) initialising the land surface with realistic soil moisture rather than climatology. The latter impacts negatively on the skill of the temperature forecasts over eastern Australia and is being addressed in the next version of the system, ACCESS-S2.

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