Positive Trend In Southern Annular Mode Research Articles

The dominant influence of indian ocean dipole-like ocean warming on decreased precipitation over eastern East Antarctica

East Antarctica is undergoing a noticeable decrease in precipitation, significantly impacting ice mass loss. However, there is a lack of research on the underlying factors behind this change. This study highlights that on an interannual timescale, the precipitation variations in Eastern East Antarctica (EEA) are predominantly influenced by the Indian Ocean Dipole mode (IOD) compared to other climate variabilities like the Southern Annular Mode (SAM), El Niño–Southern Oscillation (ENSO), and north Atlantic variability. Through trend analysis of each climate variability, we confirmed that the observed decrease in EEA precipitation can be attributed to positive IOD-like ocean warming. A positive SAM trend also contributed to specific Wilkes Land and Queen Mary Land regions. Despite these influence on long-term trend, the relationship between IOD and EEA precipitation exhibits sporadic changes on interdecadal timescales. Notably, the apparent negative correlation between the two declined to insignificance in the early 2000s, only to re-establish a significant negative correlation by the early 2010s. The primary driver of this change is the inconsistent propagation of waves originating from the Indian Ocean. During periods of high correlation, these waves propagate southeastward, inducing a robust low-pressure anomaly near Victoria Land, ultimately leading to decreased EEA precipitation. However, during periods of low correlation, the waves move eastward and fail to alter the circulation anomalies near East Antarctica.

Positive SAM trend as seen in the Brazilian Earth System Model (BESM) future scenarios.

Polar regions are among the most affected areas by the current global warming. In the Southern Hemisphere (SH), impacts of a warmer climate include decrease in sea-ice extent, changes in oceanic and in atmospheric circulation. Recently, some of these impacts were reinforced by the positive phase of the Southern Annular Mode (SAM). SAM is the dominant mode of variability of the SH extratropical climate and manifests as a "ring-shape" regular pattern of atmospheric mean sea level pressure (MSLP) with opposite sign between mid and high SH latitudes. Over the last three decades, SAM has presented a positive trend, and some studies associate it to stratospheric ozone depletion and to an increase in greenhouse gases concentration. As this debate is still open, climate models constitute useful tools to understand the SH variability in future scenarios. Here we use monthly MSLP outputs from the Brazilian Earth System Model (BESM) to examine SAM temporal and spatial behavior in future climate scenarios compared to the historical period. Our results for the BESM simulations suggest that the mean spatial pattern of SAM does not change with global warming, but an increase in the radiative forcing may reinforce positive SAM values obtained for the historical period.

Hemispheric Asymmetry in Future Wave Power Changes: Seasonality and Physical Mechanisms

AbstractThis study inspects the global changes in ocean wave power (WP) by the end of the 21st century, relative to the 1979–2005 period, focusing on seasonality and physical mechanisms. The multimodel ocean wave simulations from WAVEWATCH‐III, forced with surface winds simulated by seven different CMIP5 global climate models, are particularly examined. These models robustly show a hemispheric asymmetry in WP changes, that is, a decrease in the Northern Hemisphere but an increase in the tropics and Southern Hemisphere, with large seasonal and regional variations. Individual terms of differential WP, which include wave height and wave period changes, are analyzed to isolate the role of each process. Although wave height changes dominate the overall WP changes, wave period changes also play a comparable role in the Southern Hemisphere extratropics especially in the Indian Ocean sector during austral winter (12%). Wave period increase is strikingly higher in austral winter than summer, resembling swell height changes in the Southern Ocean. Intermodel correlation and composite analyses further indicate that the WP increase in the Southern Hemisphere is closely associated with positive Southern Annular Mode (SAM) trend. On the other hand, the WP decrease in the Northern Hemisphere is robustly related with weakened meridional surface temperature gradient during winter, a consequence of Arctic amplification. As a result, models with higher positive SAM trend and larger Arctic warming project stronger hemispheric asymmetries in WP changes.

Seasonal and interannual variability of the wave climate at a wave energy hotspot off the southwestern coast of Australia

Despite Australia having one of the most abundant offshore wave energy resources globally, there remains a lack of understanding of how this offshore resource extends into the coastal zone where most wave energy converters would be deployed. We used the phase-averaged wave model SWAN to simulate 38 years (1980–2017) of wave conditions near Albany, Western Australia, which has been proposed as a future commercial wave energy development site. The nearshore (30 m depth) wave resource varied seasonally and interannually with the wave energy flux and mean wave direction negatively correlated to the phase of the Southern Annular Mode (SAM) and positively correlated to the latitudinal position of the subtropical high-pressure ridge. As a result, the observed positive trend in SAM over recent decades may cause a decrease in nearshore wave energy (including fewer storm events) and an anti-clockwise (more southerly) rotation in wave direction. These changes may facilitate wave energy development and extraction by reducing the number and magnitude of extreme events during which wave energy cannot be extracted and equipment can be damaged. The interannual fluctuations in the wave resource can be significant and should be considered during the site selection for wave energy projects.

Contributions of Greenhouse Gas Forcing and the Southern Annular Mode to Historical Southern Ocean Surface Temperature Trends

AbstractWe examine the 1979–2014 Southern Ocean (SO) sea surface temperature (SST) trends simulated in an ensemble of coupled general circulation models and evaluate possible causes of the models' inability to reproduce the observed 1979–2014 SO cooling. For each model we estimate the response of SO SST to step changes in greenhouse gas (GHG) forcing and in the seasonal indices of the Southern Annular Mode (SAM). Using these step‐response functions, we skillfully reconstruct the models' 1979–2014 SO SST trends. Consistent with the seasonal signature of the Antarctic ozone hole and the seasonality of SO stratification, the summer and fall SAM exert a large impact on the simulated SO SST trends. We further identify conditions that favor multidecadal SO cooling: (1) a weak SO warming response to GHG forcing, (2) a strong multidecadal SO cooling response to a positive SAM trend, and (3) a historical SAM trend as strong as in observations.

Projected effects of declining anthropogenic aerosols on the southern annular mode

Declining emissions of anthropogenic aerosols have been shown to contribute to global warming in climate projections from the Coupled Model Intercomparison Project Phase 5 (CMIP5). This study considers the response of the southern annular mode (SAM) in austral summer to declining aerosols in simulations forced by Representative Concentration Pathway 4.5 (RCP4.5) using CSIRO-Mk3.6, a CMIP5-generation model. A ten-member ensemble forced by RCP4.5 for the period 2006–2100 is compared with another experiment, which is identical except that emissions of anthropogenic aerosols are held fixed at their 2005 values. With fixed aerosol emissions, the model simulates a negative (but statistically insignificant) ensemble-mean SAM trend in austral summer, suggesting that the effects of recovering stratospheric ozone slightly outweigh the effects of increasing long-lived greenhouse gases (GHGs). In contrast, the standard RCP4.5 experiment (including additional warming due to declining aerosols) simulates a positive ensemble-mean SAM trend, and the difference between the two trends is significant at 5%. The response of Southern Hemisphere zonal-mean atmospheric circulation and temperature to declining aerosols resembles the response to increasing GHGs; this suggests that the positive SAM trend due to declining aerosols may be driven by mechanisms that are similar to those that cause the positive SAM trend in response to increasing GHGs.

Autumn Precipitation Trends over Southern Hemisphere Midlatitudes as Simulated by CMIP5 Models

AbstractIn recent decades, Southern Hemisphere midlatitude regions such as southern Africa, southeastern Australia, and southern Chile have experienced a reduction in austral autumn precipitation; the cause of which is poorly understood. This study focuses on the ability of global climate models that form part of the Coupled Model Intercomparison Project phase 5 to simulate these trends, their relationship with extratropical and subtropical processes, and implications for future precipitation changes. Models underestimate both the historical autumn poleward expansion of the subtropical dry zone and the positive southern annular mode (SAM) trend. The multimodel ensemble (MME) is also unable to capture the spatial pattern of observed precipitation trends across semiarid midlatitude regions. However, in temperate regions that are located farther poleward such as southern Chile, the MME simulates observed precipitation declines. The MME shows a strong consensus in twenty-first-century declines in autumn precipitation across southern Chile in both the medium–low and high representative concentration pathway (RCP) scenarios and across southern Africa in the high RCP scenario, but little change across southeastern Australia. Projecting a strong positive SAM trend and continued subtropical dry-zone expansion, the models converge on large SAM and dry-zone-expansion-induced precipitation declines across southern midlatitudes. In these regions, the strength of future precipitation trends is proportional to the strength of modeled trends in these phenomena, suggesting that unabated greenhouse gas–induced climate change will have a large impact on austral autumn precipitation in such midlatitude regions.

Does the Southern Annular Mode contribute to the persistence of the multidecade-long drought over southwest Western Australia?

[1] Record low austral winter rainfall totals over southwest Western Australia (SWWA) in 2010 saw a continuation of the multidecade-long winter drought plaguing the region. During this season, the highest recorded positive Southern Annular Mode (SAM) value was measured, adding weight to an association of a positive SAM with anomalously low SWWA winter rainfall (SWR), and vice-versa. However, such a SAM-SWR teleconnection has been recently questioned. Using observational data in the post-1979 satellite era, it is shown that such a SAM influence does exist. This coherence is confirmed with 1150 years of modelled 20th century SWR anomalies and SAM values from 23 climate models, showing that a nonlinear impact operates: the influence from a negative SAM is greater than that from a positive SAM. This explains why a small positive shift in the SAM can cause a large SWR reduction, as has been observed. A further test shows that models with a greater positive SAM trend systematically produce a greater future SWR reduction. As all climate models project an increase in the SAM these results suggest that as global warming continues unabated, SWWA winter droughts will be more persistent as atmospheric conditions become more unfavourable for drought-breaking rains.

Dendroecological analysis of defoliator outbreaks on Nothofagus pumilio and their relation to climate variability in the Patagonian Andes

In the temperate forests of the southern Andes, Nothofagus pumilio, the dominant species of the most extensive forest type, experiences severe defoliation caused by caterpillars of the Ormiscodes genus (Lepidoptera: Saturniidae). This study uses tree rings to reconstruct the history of Ormiscodes outbreaks for the 1850–2005 period and examines relationships between outbreaks and climate variability. We used local climate records to compare outbreak–climate relationships in the northern Patagonian Andes (c. 41°S) and the cooler southern Patagonian Andes (c. 49°S). We also examined relationships between outbreak events and regional climate variability driven by variability in the Southern Annular Mode (SAM) and the El Niño-Southern Oscillation. Although relationships between Ormiscodes outbreaks and climate proved to be complex, in northern Patagonia defoliation events are associated with drier and warmer than average growing seasons. Warming and drying trends in Patagonia during the latter part of the 20th century have been linked to a positive trend in SAM. During the post-1976 period of accelerated warming in Patagonia, widespread defoliation outbreaks have occurred in both northern and southern Patagonia but the increase in frequency of events has been greater in the south. In southern Patagonia the increases in frequency of outbreaks in the late 20th century appear to be unprecedented over the c. 150 year tree-ring record of reconstructed outbreaks. These results are consistent with the greater magnitude of recent warming in southern Patagonia, and suggest that under predicted warmer and drier climates in the 21st century, defoliator outbreaks may continue to increase in frequency. This study is the first systematic reconstruction of past insect outbreaks in South America and provides a preliminary understanding of how climate variability affects defoliator outbreaks in Patagonian Nothofagus forests.

340 years of atmospheric circulation characteristics reconstructed from an eastern Antarctic Peninsula ice core

[1] Precipitation delivery mechanisms for Dolleman Island (DI), located off the east coast of the Antarctic Peninsula, are investigated using reanalysis and back trajectory data. The Southern Annular Mode (SAM) and ENSO are both shown to influence precipitation delivery and event size. Precipitation delivery variability is compared against the interannual variation of chemical data from two DI ice cores. Nitrate concentration in the cores is strongly linked with the ratio of easterly to westerly back trajectories arriving at DI, as described by a Cross-Peninsula Index (CPI) defined in this paper. This CPI is used subsequently to reconstruct the atmospheric circulation characteristics for the 340-year ice core record. The analysis highlights a period of increased easterlies during 1720–1780 and an increase in westerlies for 1950–1980, the latter concomitant with a positive SAM trend and western Peninsula warming. The reconstruction also reveals periods when polynyas may have been present in the Weddell Sea.