scholarly journals Rainfall Changes over Southwestern Australia and Their Relationship to the Southern Annular Mode and ENSO

2014 ◽  
Vol 27 (15) ◽  
pp. 5801-5814 ◽  
Author(s):  
Bhupendra A. Raut ◽  
Christian Jakob ◽  
Michael J. Reeder
Keyword(s):  
Southern Oscillation ◽  
Summer Rainfall ◽  
Southern Annular Mode ◽  
Winter Rainfall ◽  
Sea Level Pressure ◽  
Annular Mode ◽  
Southwestern Australia ◽  
Synoptic Conditions ◽  
The Mean ◽  
Rainfall Patterns

Abstract Since the 1970s, winter rainfall over coastal southwestern Australia (SWA) has decreased by 10%–20%, while summer rainfall has been increased by 40%–50% in the semiarid inland area. In this paper, a K-means algorithm is used to cluster rainfall patterns directly as opposed to the more conventional approach of clustering synoptic conditions (usually the mean sea level pressure) and inferring the associated rainfall. It is shown that the reduction in the coastal rainfall during winter is mainly due to fewer westerly fronts in June and July. The reduction in the frequency of strong fronts in June is responsible for half of the decreased rainfall in June–August (JJA), whereas the reduction in the frequency of weaker fronts in June and July accounts for a third of the total decrease. The increase in rainfall inland in December–February (DJF) is due to an increased frequency of easterly troughs in December and February. These rainfall patterns are linked to the southern annular mode (SAM) index and Southern Oscillation index (SOI). The reduction in coastal rainfall and the increase in rainfall inland are both related to the predominantly positive phase of SAM, especially when the phase of ENSO is neutral.

scholarly journals Trends in CMIP5 Rainfall Patterns over Southwestern Australia

2017 ◽  
Vol 30 (5) ◽  
pp. 1779-1788 ◽  
Author(s):  
Bhupendra A. Raut ◽  
Michael J. Reeder ◽  
Christian Jakob
Keyword(s):  
Summer Rainfall ◽  
First Century ◽  
Cmip5 Models ◽  
Historical Period ◽  
Gradual Reduction ◽  
Winter Rainfall ◽  
Light Rain ◽  
Southwestern Australia ◽  
Rainfall Patterns ◽  
Twenty First Century

Abstract Previous work has shown that the sharp fall in winter rainfall over coastal southwestern Australia in the 1970s was mainly due to a fall in the frequency of fronts; the gradual reduction in rainfall since the late 1990s was due to a reduction in the number of light-rain days; and the increased inland summer rainfall in the 1970s was due to an increased number of easterly troughs. The current paper extends this earlier work by identifying the rainfall patterns in the region in 14 CMIP5 models for the period 1980–2005 and by calculating how these patterns are projected to change in the twenty-first century. The patterns are identified using k-means clustering of the rainfall, which are validated against observed rainfall clusters. Although the agreement between the models and the observation is generally good, the models underestimate the frequency of raining fronts. In both representative concentration pathway 4.5 and 8.5 (RCP4.5 and RCP8.5) scenarios the number of dry days increases significantly at the expense of light-rain days and frontal rainfall. However, these trends are twice as large in the RCP8.5 scenario as in the RCP4.5 scenario. The reduction in the rainfall from the historical period to the second half of the twenty-first century is produced mainly by a reduction in both the frequency and intensity of light rain and a reduction in the frequency of fronts in the westerlies.

Change in the Relationship between the Southern Subtropical Dipole Modes and the Southern Annular Mode in the Mid-1980s

2015 ◽  
Vol 28 (23) ◽  
pp. 9235-9249
Author(s):  
Li Yan ◽  
Gen Li
Keyword(s):  
Southern Oscillation ◽  
Austral Summer ◽  
Summer Rainfall ◽  
Southern Annular Mode ◽  
Minor Role ◽  
Annular Mode ◽  
Decadal Shift ◽  
Subtropical Highs ◽  
A Minor ◽  
The Relationship

Abstract The southern subtropical dipole modes (SSDMs) and southern annular mode (SAM) are important climate modes, which are dominant in the southern middle and high latitudes, respectively, with considerable regional climatic impacts. However, the relationship between the two modes remains unclear. A close inspection reveals that the SAM was significantly correlated with the SSDMs during the austral summer before the mid-1980s. However, the correlations have degraded since then. This decadal shift in the relationship between these two southern dominant modes is due to a weakened connection between the SAM and the subtropical highs that control the SSDMs. This decadal change could be traced back to a poleward shift in the southern westerly belt. El Niño–Southern Oscillation (ENSO) typically plays a moderate role in influencing the precipitation in Australia and a minor role in influencing the precipitation in Africa and South America. Nevertheless, the two southern modes could still affect the austral summer rainfall in the midlatitudes, even though the ENSO signal is absent. All these links between the two southern modes and southern land precipitation may be attributable to the associated transport of moisture in the lower-level circulation.

Differential Credibility of Climate Modes in CMIP6

2021 ◽  
pp. 1
Author(s):  
Jacob Coburn ◽  
S.C. Pryor
Keyword(s):  
Southern Oscillation ◽  
Probability Distributions ◽  
Critical Role ◽  
Regional Scale ◽  
Southern Annular Mode ◽  
Climate Projections ◽  
Annular Mode ◽  
Climate Modes ◽  
First Order ◽  
Skill Scores

AbstractThis work quantitatively evaluates the fidelity with which the Northern Annular Mode (NAM), Southern Annular Mode (SAM), Pacific-North American pattern (PNA), El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) and the first-order mode interactions are represented in Earth System Model (ESM) output from the CMIP6 archive. Several skill metrics are used as part of a differential credibility assessment (DCA) of both spatial and temporal characteristics of the modes across ESMs, ESM families and specific ESM realizations relative to ERA5. The spatial patterns and probability distributions are generally well represented but skill scores that measure the degree to which the frequencies of maximum variance are captured are consistently lower for most ESMs and climate modes. Substantial variability in skill scores manifests across realizations from individual ESMs for the PNA and oceanic modes. Further, the ESMs consistently overestimate the strength of the NAM-PNA first-order interaction and underestimate the NAM-AMO connection. These results suggest that the choice of ESM and ESM realizations will continue to play a critical role in determining climate projections at the global and regional scale at least in the near-term.

scholarly journals Teleconnections and relationship between the El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) in reconstructions and models over the past millennium

2020 ◽  
Vol 16 (2) ◽  
pp. 743-756 ◽  
Author(s):  
Christoph Dätwyler ◽  
Martin Grosjean ◽  
Nathan J. Steiger ◽  
Raphael Neukom
Keyword(s):  
Climate Model ◽  
El Nino ◽  
Southern Oscillation ◽  
Southern Annular Mode ◽  
Last Millennium ◽  
Annular Mode ◽  
Model Simulations ◽  
The Past ◽  
Climate Model Simulations ◽  
Past Millennium

Abstract. The climate of the Southern Hemisphere (SH) is strongly influenced by variations in the El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). Because of the limited length of instrumental records in most parts of the SH, very little is known about the relationship between these two key modes of variability over time. Using proxy-based reconstructions and last-millennium climate model simulations, we find that ENSO and SAM indices are mostly negatively correlated over the past millennium. Pseudo-proxy experiments indicate that currently available proxy records are able to reliably capture ENSO–SAM relationships back to at least 1600 CE. Palaeoclimate reconstructions show mostly negative correlations back to about 1400 CE. An ensemble of last-millennium climate model simulations confirms this negative correlation, showing a stable correlation of approximately −0.3. Despite this generally negative relationship we do find intermittent periods of positive ENSO–SAM correlations in individual model simulations and in the palaeoclimate reconstructions. We do not find evidence that these relationship fluctuations are caused by exogenous forcing nor by a consistent climate pattern. However, we do find evidence that strong negative correlations are associated with strong positive (negative) anomalies in the Interdecadal Pacific Oscillation and the Amundsen Sea Low during periods when SAM and ENSO indices are of opposite (equal) sign.

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 Decadal Variability of the ENSO Teleconnection to the High-Latitude South Pacific Governed by Coupling with the Southern Annular Mode*

2006 ◽  
Vol 19 (6) ◽  
pp. 979-997 ◽  
Author(s):  
Ryan L. Fogt ◽  
David H. Bromwich
Keyword(s):  
High Latitude ◽  
Decadal Variability ◽  
Southern Oscillation ◽  
Phase Relationship ◽  
South Pacific ◽  
Southern Annular Mode ◽  
Height Anomaly ◽  
Austral Spring ◽  
Annular Mode ◽  
The Pacific

Abstract Decadal variability of the El Niño–Southern Oscillation (ENSO) teleconnection to the high-latitude South Pacific is examined by correlating the European Centre for Medium-Range Weather Forecasts (ECMWF) 40-yr Re-Analysis (ERA-40) and observations with the Southern Oscillation index (SOI) over the last two decades. There is a distinct annual contrast between the 1980s and the 1990s, with the strong teleconnection in the 1990s being explained by an enhanced response during austral spring. Geopotential height anomaly composites constructed during the peak ENSO seasons also demonstrate the decadal variability. Empirical orthogonal function (EOF) analysis reveals that the 1980s September–November (SON) teleconnection is weak due to the interference between the Pacific–South American (PSA) pattern associated with ENSO and the Southern Annular Mode (SAM). An in-phase relationship between these two modes during SON in the 1990s amplifies the height and pressure anomalies in the South Pacific, producing the strong teleconnections seen in the correlation and composite analyses. The in-phase relationship between the tropical and high-latitude forcing also exists in December–February (DJF) during the 1980s and 1990s. These results suggest that natural climate variability plays an important role in the variability of SAM, in agreement with a growing body of literature. Additionally, the significantly positive correlation between ENSO and SAM only during times of strong teleconnection suggests that both the Tropics and the high latitudes need to work together in order for ENSO to strongly influence Antarctic climate.

Is There a Relationship between the SAM and Southwest Western Australian Winter Rainfall?

2010 ◽  
Vol 23 (22) ◽  
pp. 6082-6089 ◽  
Author(s):  
Juan Feng ◽  
Jianping Li ◽  
Yun Li
Keyword(s):  
Western Australia ◽  
Inverse Relationship ◽  
Southern Annular Mode ◽  
High Latitudes ◽  
Upward Trend ◽  
Winter Rainfall ◽  
Annular Mode ◽  
Apparent Relationship ◽  
Circulation Anomalies ◽  
Western Australian

Abstract Previous studies have raised the possibility that the recent decline in winter rainfall over southwest Western Australia (SWWA) is related to the concurrent upward trend in the southern annular mode (SAM). On the basis of an analysis of 60-yr (1948–2007) reanalysis and observed data, the authors suggest that the apparent inverse relationship between the SAM and SWWA winter rainfall (SWR) is caused by a single extreme year—1964. It is shown that both the negative and positive phases of the SAM have little impact on SWR in the case that data for 1964 are excluded from the analysis. In addition, for periods prior to and after 1964 in the case that data for 1964 are excluded, the apparent relationship between the SAM and SWR becomes insignificant, and the circulation anomalies with respect to SWR appear to be an SAM-like pattern for which the anomalies at high latitudes are not significant. The result indicates that the SAM does not significantly influence the winter rainfall over SWWA. Instead, the variation of SWR would be more closely linked to the variability in regional circulations.

scholarly journals Quantifying the Eddy–Jet Feedback Strength of the Annular Mode in an Idealized GCM and Reanalysis Data

2017 ◽  
Vol 74 (2) ◽  
pp. 393-407 ◽  
Author(s):  
Ding Ma ◽  
Pedram Hassanzadeh ◽  
Zhiming Kuang
Keyword(s):  
Statistical Methods ◽  
Low Frequency ◽  
Southern Annular Mode ◽  
Reanalysis Data ◽  
New Method ◽  
Annular Mode ◽  
Feedback Strength ◽  
Low Pass ◽  
The Mean ◽  
Mean State

Abstract A linear response function (LRF) that relates the temporal tendency of zonal-mean temperature and zonal wind to their anomalies and external forcing is used to accurately quantify the strength of the eddy–jet feedback associated with the annular mode in an idealized GCM. Following a simple feedback model, the results confirm the presence of a positive eddy–jet feedback in the annular mode dynamics, with a feedback strength of 0.137 day−1 in the idealized GCM. Statistical methods proposed by earlier studies to quantify the feedback strength are evaluated against results from the LRF. It is argued that the mean-state-independent eddy forcing reduces the accuracy of these statistical methods because of the quasi-oscillatory nature of the eddy forcing. Assuming the mean-state-independent eddy forcing is sufficiently weak at the low-frequency limit, a new method is proposed to approximate the feedback strength as the regression coefficient of low-pass-filtered eddy forcing onto the low-pass-filtered annular mode index. When time scales longer than 200 days are used for the low-pass filtering, the new method produces accurate results in the idealized GCM compared to the value calculated from the LRF. The estimated feedback strength in the southern annular mode converges to 0.121 day−1 in reanalysis data using the new method. This work also highlights the significant contribution of medium-scale waves, which have periods less than 2 days, to the annular mode dynamics. Such waves are filtered out if eddy forcing is calculated from daily mean data. The present study provides a framework to quantify the eddy–jet feedback strength in GCMs and reanalysis data.

scholarly journals The Connection between the Southern Annular Mode and a Feature-Based Perspective on Southern Hemisphere Midlatitude Winter Variability

2020 ◽  
Vol 33 (1) ◽  
pp. 115-129 ◽  
Author(s):  
Clemens Spensberger ◽  
Michael J. Reeder ◽  
Thomas Spengler ◽  
Matthew Patterson
Keyword(s):  
Storm Track ◽  
Principal Component ◽  
Southern Annular Mode ◽  
Strong Correlations ◽  
Sea Level Pressure ◽  
Annular Mode ◽  
Rossby Wave Breaking ◽  
The Pacific ◽  
Feature Based ◽  
Consistent Picture

AbstractThis article provides a reconciling perspective on the two main, but contradictory, interpretations of the southern annular mode (SAM). SAM was originally thought to characterize meridional shifts in the storm track across the entire hemisphere. This perspective was later questioned, and SAM was interpreted as a statistical artifact depending on the choice of base region for the principal component analysis. Neither perspective, however, fully describes SAM. We show that SAM cannot be interpreted in terms of midlatitude variability, as SAM merely modulates the most poleward part of the cyclone tracks and only marginally influences the distribution of other weather-related features of the storm track (e.g., position of jet axes and Rossby wave breaking). Instead, SAM emerges as the leading pattern of geopotential variability due to strong correlations of sea level pressure around the Antarctic continent. As SAM correlates strongly both with the pan-Antarctic mean temperature and the meridional heat flux through 65°S, we hypothesize that SAM can be interpreted as a measure of the degree of the (de)coupling between Antarctica and the southern midlatitudes. As an alternative way of characterizing southern midlatitude variability, we seek domains in which the leading EOF patterns of both the geopotential and storm-track features yield a dynamically consistent picture. This approach is successful for the South Pacific. Here the leading variability patterns are closely related to the Pacific–South America pattern and point toward an NAO-like variability.

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