scholarly journals Large-Scale Influences on the Evolution of Winter Subtropical Maritime Cyclones Affecting Australia’s East Coast

2013 ◽  
Vol 141 (7) ◽  
pp. 2416-2431 ◽  
Author(s):  
Stuart A. Browning ◽  
Ian D. Goodwin
Keyword(s):  
Indian Ocean ◽  
Large Scale ◽  
East Coast ◽  
Southern Oscillation ◽  
Southern Annular Mode ◽  
Winter Period ◽  
Evolutionary Trajectory ◽  
Late Autumn ◽  
Tasman Sea ◽  
Eastern Seaboard

Abstract Subtropical maritime low pressure systems frequently impact Australia’s eastern seaboard. Closed circulation lows in the Tasman Sea region are termed East Coast Cyclones (ECC); they can evolve in a range of climatic environments and have proven most destructive during the late autumn–winter period. Using criteria based on pressure gradients, inferred wind field, and duration, an objectively determined database of ECC occurrences is established to explore large-scale influences on ECC evolution. Subclassification based on evolutionary trajectory reveals two dominant storm types during late autumn–winter: easterly trough lows (ETL) and southern secondary lows (SSL). Synoptic composites are used to investigate the climatological evolution of each storm type. ETL cyclogenesis occurs along the eastern seaboard at the confluence of warm moist subtropical easterlies and cool air over the continent that is advected from higher latitudes. SSL develop when a cold extratropical cyclone moves equatorward and interacts with warm moist conditions in the Tasman Sea. At seasonal time scales, a complex interplay of tropical and extratropical influences contributes to high-frequency storm seasons. ETL are more frequent during neutral or positive phases of the El Niño–Southern Oscillation, cool sea surface temperature anomalies (SSTAs) in the tropical Indian Ocean, and neutral to positive southern annular mode phases. SSL are more frequent during years with warm SSTAs in the eastern Indian Ocean, warm SSTAs in the western Pacific, and high-latitude blocking.

A Monsoon-Like Southwest Australian Circulation and Its Relation with Rainfall in Southwest Western Australia

2010 ◽  
Vol 23 (6) ◽  
pp. 1334-1353 ◽  
Author(s):  
Juan Feng ◽  
Jianping Li ◽  
Yun Li
Keyword(s):  
Indian Ocean ◽  
Western Australia ◽  
Large Scale ◽  
Southern Oscillation ◽  
Southern Annular Mode ◽  
Late Winter ◽  
Early Winter ◽  
Winter Rainfall ◽  
Enso Modoki ◽  
Distinct Features

Abstract Using the NCEP–NCAR reanalysis, the 40-yr ECMWF Re-Analysis (ERA-40), and precipitation data from the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) and the Australian Bureau of Meteorology, the variability and circulation features influencing southwest Western Australia (SWWA) winter rainfall are investigated. It is found that the climate of southwest Australia bears a strong seasonality in the annual cycle and exhibits a monsoon-like atmospheric circulation, which is called the southwest Australian circulation (SWAC) because of its several distinct features characterizing a monsoonal circulation: the seasonal reversal of winds, alternate wet and dry seasons, and an evident land–sea thermal contrast. The seasonal march of the SWAC in extended winter (May–October) is demonstrated by pentad data. An index based on the dynamics’ normalized seasonality was introduced to describe the behavior and variation of the winter SWAC. It is found that the winter rainfall over SWWA has a significant positive correlation with the SWAC index in both early (May–July) and late (August–October) winter. In weaker winter SWAC years, there is an anticyclonic anomaly over the southern Indian Ocean resulting in weaker westerlies and northerlies, which are not favorable for more rainfall over SWWA, and the opposite combination is true in the stronger winter SWAC years. The SWAC explains not only a large portion of the interannual variability of SWWA rainfall in both early and late winter but also the long-term drying trend over SWWA in early winter. The well-coupled SWAC–SWWA rainfall relationship seems to be largely independent of the well-known effects of large-scale atmospheric circulations such as the southern annular mode (SAM), El Niño–Southern Oscillation (ENSO), Indian Ocean dipole (IOD), and ENSO Modoki (EM). The result offers qualified support for the argument that the monsoon-like circulation may contribute to the rainfall decline in early winter over SWWA. The external forcing of the SWAC is also explored in this study.

scholarly journals Large-scale drivers of Australian east coast cyclones since 1851

2016 ◽  
Vol 66 (2) ◽  
pp. 125
Author(s):  
Stuart A. Browning ◽  
Ian D. Goodwin
Keyword(s):  
19Th Century ◽  
Ocean Circulation ◽  
Large Scale ◽  
East Coast ◽  
Early Years ◽  
Winter Period ◽  
Storm Activity ◽  
Data Scarcity ◽  
Tasman Sea ◽  
The 19Th Century

Subtropical maritime low-pressure systems are one of the most complex and destructive storm types to impact Australia’s eastern seaboard. This family of storms, commonly referred to as East Coast Cyclones (ECC), is most active during the late autumn and early winter period when baroclinicity increases in the Tasman Sea region. ECC have proven challenging to forecast at both event and seasonal timescales. Storm activity datasets, objectively determined from reanalyses using cyclone detection algorithms, have improved understanding of the drivers of ECC over the era of satellite data coverage. In this study we attempt to extend these datasets back to 1851 using the Twentieth Century Reanalysis version 2c (20CRv2c). However, uncertainty in the 20CRv2c increases back through time due to observational data scarcity, and individual cyclones counts tend to be underestimated during the 19th century. An alternative approach is explored whereby storm activity is estimated from seasonal atmosphere-ocean circulation patterns. Seasonal ECC frequency over the 1955 to 2014 period is significantly correlated to regional sea-level pressure and sea surface temperature (SST) patterns. These patterns are used to downscale the 20CRv2c during early years when individual events are not well simulated. The stormiest periods since 1851 appear to have been 1870 to the early 1890s, and 1950 to the early 1970s. Total storm activity has been below the long-term average for most winters since 1976. Conditions conducive to frequent ECC events tend to occur during periods of relatively warm SST in the southwest Pacific typical of negative Interdecadal Pacific Oscillation (IPO-ve). Extratropical cyclogenesis is associated with negative Southern Annular Mode (SAM-ve) and blocking in the southern Tasman Sea. Subtropical cyclogenesis is associated with SAM+ve and blocking in the central Tasman Sea. While the downscaling approach shows some skill at estimating seasonal storm activity from the large-scale circulation, it cannot overcome data scarcity based uncertainties in the 19th century when the 20CRv2c is effectively unconstrained throughout most of the southern hemisphere. Storm frequency estimates during the 19th century are difficult to verify and should be interpreted cautiously and with reference to available documentary evidence.

Impact of the El Niño–Southern Oscillation, Indian Ocean Dipole, and Southern Annular Mode on Daily to Subdaily Rainfall Characteristics in East Australia

2012 ◽  
Vol 140 (5) ◽  
pp. 1665-1682 ◽  
Author(s):  
Alexander Pui ◽  
Ashish Sharma ◽  
Agus Santoso ◽  
Seth Westra
Keyword(s):  
Indian Ocean ◽  
Indian Ocean Dipole ◽  
Large Scale ◽  
El Nino ◽  
Southern Oscillation ◽  
Southern Annular Mode ◽  
El Nino Southern Oscillation ◽  
Rainfall Events ◽  
Annular Mode ◽  
Climate Modes

Abstract The relationship between seasonal aggregate rainfall and large-scale climate modes, particularly the El Niño–Southern Oscillation (ENSO), has been the subject of a significant and ongoing research effort. However, relatively little is known about how the character of individual rainfall events varies as a function of each of these climate modes. This study investigates the change in rainfall occurrence, intensity, and storm interevent time at both daily and subdaily time scales in east Australia, as a function of indices for ENSO, the Indian Ocean dipole (IOD), and the southern annular mode (SAM), with a focus on the cool season months. Long-record datasets have been used to sample a large variety of climate events for better statistical significance. Results using both the daily and subdaily rainfall datasets consistently show that it is the occurrence of rainfall events, rather than the average intensity of rainfall during the events, which is most strongly influenced by each of the climate modes. This is shown to be most likely associated with changes to the time between wet spells. Furthermore, it is found that despite the recent attention in the research literature on other climate modes, ENSO remains the leading driver of rainfall variability over east Australia, particularly farther inland during the winter and spring seasons. These results have important implications for how water resources are managed, as well as how the implications of large-scale climate modes are included in rainfall models to best capture interannual and longer-scale variability.

scholarly journals Relationships between Southeast Australian Temperature Anomalies and Large-Scale Climate Drivers

2014 ◽  
Vol 27 (4) ◽  
pp. 1395-1412 ◽  
Author(s):  
Alexandre O. Fierro ◽  
Lance M. Leslie
Keyword(s):  
Large Scale ◽  
Southern Oscillation ◽  
Power Spectra ◽  
Warm Season ◽  
Principal Component ◽  
Temperature Variability ◽  
Maximum Temperature ◽  
Past Century ◽  
Tasman Sea ◽  
Australian Continent

Abstract Over the past century, particularly after the 1960s, observations of mean maximum temperatures reveal an increasing trend over the southeastern quadrant of the Australian continent. Correlation analysis of seasonally averaged mean maximum temperature anomaly data for the period 1958–2012 is carried out for a representative group of 10 stations in southeast Australia (SEAUS). For the warm season (November–April) there is a positive relationship with the El Niño–Southern Oscillation (ENSO) and the Pacific decadal oscillation (PDO) and an inverse relationship with the Antarctic Oscillation (AAO) for most stations. For the cool season (May–October), most stations exhibit similar relationships with the AAO, positive correlations with the dipole mode index (DMI), and marginal inverse relationships with the Southern Oscillation index (SOI) and the PDO. However, for both seasons, the blocking index (BI, as defined by M. Pook and T. Gibson) in the Tasman Sea (160°E) clearly is the dominant climate mode affecting maximum temperature variability in SEAUS with negative correlations in the range from r = −0.30 to −0.65. These strong negative correlations arise from the usual definition of BI, which is positive when blocking high pressure systems occur over the Tasman Sea (near 45°S, 160°E), favoring the advection of modified cooler, higher-latitude maritime air over SEAUS. A point-by-point correlation with global sea surface temperatures (SSTs), principal component analysis, and wavelet power spectra support the relationships with ENSO and DMI. Notably, the analysis reveals that the maximum temperature variability of one group of stations is explained primarily by local factors (warmer near-coastal SSTs), rather than teleconnections with large-scale drivers.

scholarly journals Triggering the Indian Ocean Dipole from the Southern Hemisphere

2021 ◽  
Author(s):  
Lian-Yi Zhang ◽  
Yan Du ◽  
Wenju Cai ◽  
Zesheng Chen ◽  
Tomoki Tozuka ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Southern Hemisphere ◽  
Indian Ocean Dipole ◽  
Southern Oscillation ◽  
Southern Annular Mode ◽  
Triggering Mechanism ◽  
Phase Development ◽  
The Indian Ocean ◽  
High System ◽  
The Tropics

<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 Niño/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>

Large-Scale Atmospheric Drivers of Snowfall on Thwaites Glacier

2020 ◽  
Author(s):  
Michelle Maclennan ◽  
Jan Lenaerts
Keyword(s):  
Temporal Variability ◽  
Large Scale ◽  
Climate Model ◽  
Southern Oscillation ◽  
Regional Climate ◽  
Southern Annular Mode ◽  
Spatial And Temporal Variability ◽  
Atmospheric Conditions ◽  
Amundsen Sea ◽  
Weather Patterns

<p>High snowfall events on Thwaites Glacier are a key influencer of its ice mass change. In this study, we diagnose the mechanisms for orographic precipitation on Thwaites Glacier by analyzing the atmospheric conditions that lead to high snowfall events. A high-resolution regional climate model, RACMO2, is used in conjunction with MERRA-2 and ERA5 reanalysis to map snowfall and associated atmospheric conditions over the Amundsen Sea Embayment. We examine these conditions during high snowfall events over Thwaites Glacier to characterize the drivers of the precipitation and their spatial and temporal variability. Then we examine the seasonal differences in the associated weather patterns and their correlations with El Nino Southern Oscillation and the Southern Annular Mode. Understanding the large-scale atmospheric drivers of snowfall events allows us to recognize how these atmospheric drivers and consequent snowfall climatology will change in the future, which will ultimately improve predictions of accumulation on Thwaites Glacier.</p>

scholarly journals Seasonal Contrast in Precipitation Mechanisms over Nepal Deduced from Relationship with the Large-Scale Climate Patterns

2013 ◽  
Vol 13 (1) ◽  
pp. 115-123 ◽  
Author(s):  
Madan Sigdel ◽  
Motoyoshi Ikeda
Keyword(s):  
Indian Ocean ◽  
Large Scale ◽  
Southern Oscillation ◽  
Summer Precipitation ◽  
Walker Circulation ◽  
Moisture Flux ◽  
Rain Gauge ◽  
Western Region ◽  
Eastern Region ◽  
The Relationship

Summer precipitation dominates over winter one for the annual total in south Asia, while the winter condition is still important for agricultural productions. Rain gauge data over Nepal were analyzed with large-scale atmospheric patterns such as El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). In the period of June to September, summer monsoon rainfall over Nepal (SMRN) is generally higher in the eastern region along with a peak in the central region associated with the local orography. Its interannual variability was found to be correlated with the southern oscillation index (SOI): i.e., when La Niña occurs, eastward moisture flux is blocked over Bay of Bengal (BOB) by the anomalous Walker circulation extending from the Pacific. The local-scale condition for higher SMRN is implied by a main moisture route along the eastern arm of the low pressure in northeastern India, as proved by a significant correlation between SMRN and the northward moisture flux. In winter (DJFM), precipitation occurs more in the western region. The higher winter precipitation over Nepal (WPN) was correlated almost equally with positive Dipole Mode Index (DMI) over the Indian Ocean and also SOI, while the relationship with SOI is reversed from summer. A clear linkage was suggested with moisture flux from the Arabian Sea and the further western region. Thus, possible impacts of anomalous precipitation have to be predicted under the relationship with the large-scale indices depending on seasons. Nepal Journal of Science and Technology Vol. 13, No. 1 (2012) 115-123 DOI: http://dx.doi.org/10.3126/njst.v13i1.7450

scholarly journals Using Indicators of ENSO, IOD, and SAM to Improve Lead Time and Accuracy of Tropical Cyclone Outlooks for Australia

2020 ◽  
Vol 59 (11) ◽  
pp. 1901-1917
Author(s):  
Andrew D. Magee ◽  
Anthony S. Kiem
Keyword(s):  
Tropical Cyclone ◽  
Indian Ocean ◽  
Lead Time ◽  
Southern Oscillation ◽  
Pearson Correlation ◽  
Southern Annular Mode ◽  
Sea Surface ◽  
Selection Algorithm ◽  
Statistical Framework ◽  
The Indian Ocean

AbstractCatastrophic impacts associated with tropical cyclone (TC) activity mean that the accurate and timely provision of TC outlooks are important to people, places, and numerous sectors in Australia and beyond. In this study, we apply a Poisson regression statistical framework to predict TC counts in the Australian region (AR; 5°–40°S, 90°–160°E) and its four subregions. We test 10 unique covariate models, each using different representations of the influence of El Niño–Southern Oscillation (ENSO), Indian Ocean dipole (IOD), and southern annular mode (SAM) and use an automated covariate selection algorithm to select the optimum combination of predictors. The performance of preseason TC count outlooks generated between April and October for the AR TC season (November–April) and in-season TC count outlooks generated between November and January for the remaining AR TC season are tested. Results demonstrate that skillful TC count outlooks can be generated in April (i.e., 7 months prior to the start of the AR TC season), with Pearson correlation coefficient values between r = 0.59 and 0.78 and covariates explaining between 35% and 60% of the variance in TC counts. The dependence of models on indices representing Indian Ocean sea surface temperature highlights the importance of the Indian Ocean for TC occurrence in this region. Importantly, generating rolling monthly preseason and in-season outlooks for the AR TC season enables the continuous refinement of expected TC counts in a given season.

scholarly journals Skill in Simulating Australian Precipitation at the Tropical Edge*

2016 ◽  
Vol 29 (4) ◽  
pp. 1477-1496 ◽  
Author(s):  
Penelope Maher ◽  
Steven C. Sherwood
Keyword(s):  
Large Scale ◽  
Climate Models ◽  
Linear Independence ◽  
Southern Oscillation ◽  
Coupled Model ◽  
Southern Annular Mode ◽  
Reanalysis Data ◽  
Hadley Cell ◽  
Model Errors ◽  
Skill Scores

Abstract Expansion of the tropics will likely affect subtropical precipitation, but observed and modeled precipitation trends disagree with each other. Moreover, the dynamic processes at the tropical edge and their interactions with precipitation are not well understood. This study assesses the skill of climate models to reproduce observed Australian precipitation variability at the tropical edge. A multivariate linear independence approach distinguishes between direct (causal) and indirect (circumstantial) precipitation drivers that facilitate clearer attribution of model errors and skill. This approach is applied to observed precipitation and ERA-Interim reanalysis data and a representative subset of four models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) and their CMIP3 counterparts. The drivers considered are El Niño–Southern Oscillation, southern annular mode, Indian Ocean dipole, blocking, and four tropical edge metrics (position and intensity of the subtropical ridge and subtropical jet). These models are skillful in representing the covariability of drivers and their influence on precipitation. However, skill scores have not improved in the CMIP5 subset relative to CMIP3 in either respect. The Australian precipitation response to a poleward-located Hadley cell edge remains uncertain, as opposing drying and moistening mechanisms complicate the net response. Higher skill in simulating driver covariability is not consistently mirrored by higher precipitation skill. This provides further evidence that modeled precipitation does not respond correctly to large-scale flow patterns; further improvements in parameterized moist physics are needed before the subtropical precipitation responses can be fully trusted. The multivariate linear independence approach could be applied more widely for practical model evaluation.

scholarly journals Regional aspects of streamflow droughts in the Andean rivers of Patagonia, Argentina. Links with large-scale climatic oscillations

2017 ◽  
Vol 49 (1) ◽  
pp. 134-149 ◽  
Author(s):  
Juan Antonio Rivera ◽  
Diego C. Araneo ◽  
Olga C. Penalba ◽  
Ricardo Villalba
Keyword(s):  
Time Series ◽  
El Niño ◽  
Large Scale ◽  
El Nino ◽  
Southern Oscillation ◽  
Southern Annular Mode ◽  
Drought Duration ◽  
Variable Threshold ◽  
Climatic Oscillations ◽  
Drought Variability

Abstract Under the current global warming trend, droughts are expected to increase, with serious implications for water resources management. This study analyzed the regional aspects of droughts in terms of streamflow deficiencies over the Andean rivers of Patagonia, Argentina. Based on the variable threshold level method, the main characteristics of streamflow droughts were obtained for the hydrological years 1962/63–2014/15, considering three different severity levels over 11 representative basins. Two distinct regional behaviors were identified in terms of temporal variations of streamflow drought duration and its cumulative deficit volume, dividing the study area into North and Central Patagonia. The effects of the Southern Annular Mode (SAM), the El Niño–Southern Oscillation (ENSO), and the Pacific Decadal Oscillation (PDO) on the interannual and interdecadal variability of streamflow droughts were assessed through an empirical decomposition applied to the regional time series. These large-scale climatic oscillations have a distinct regional and temporal behavior in terms of the modulation of streamflow drought variability. Considering the interannual streamflow drought variability, the El Niño signal is more consistent and contributes with humid conditions, especially over North Patagonia. The multi-decadal component of the streamflow drought time series is linked to the upward trend in SAM, particularly over Central Patagonia.

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