Negative Southern Annular Mode Research Articles

The Little Ice Age climate of New Zealand reconstructed from Southern Alps cirque glaciers: a synoptic type approach

Little Ice Age (LIA) austral summer temperature anomalies were derived from palaeoequilibrium line altitudes at 22 cirque glacier sites across the Southern Alps of New Zealand. Modern analog seasons with temperature anomalies akin to the LIA reconstructions were selected, and then applied in a sampling of high-resolution gridded New Zealand climate data and global reanalysis data to generate LIA climate composites at local, regional and hemispheric scales. The composite anomaly patterns assist in improving our understanding of atmospheric circulation contributions to the LIA climate state, allow an interrogation of synoptic type frequency changes for the LIA relative to present, and provide a hemispheric context of the past conditions in New Zealand. An LIA summer temperature anomaly of −0.56 °C (±0.29 °C) for the Southern Alps based on palaeo-equilibrium lines compares well with local tree-ring reconstructions of austral summer temperature. Reconstructed geopotential height at 1,000 hPa (z1000) suggests enhanced southwesterly flow across New Zealand occurred during the LIA to generate the terrestrial temperature anomalies. The mean atmospheric circulation pattern for summer resulted from a crucial reduction of the ‘HSE’-blocking synoptic type (highs over and to the west of NZ; largely settled conditions) and increases in both the ‘T’- and ‘SW’-trough synoptic types (lows passing over NZ; enhanced southerly and southwesterly flow) relative to normal. Associated land-based temperature and precipitation anomalies suggest both colder- and wetter-than-normal conditions were a pervasive component of the base climate state across New Zealand during the LIA, as were colder-than-normal Tasman Sea surface temperatures. Proxy temperature and circulation evidence were used to corroborate the spatially heterogeneous Southern Hemisphere composite z1000 and sea surface temperature patterns generated in this study. A comparison of the composites to climate mode archetypes suggests LIA summer climate and atmospheric circulation over New Zealand was driven by increased frequency of weak El Nino-Modoki in the tropical Pacific and negative Southern Annular Mode activity.

Climate-change impact on the 20th-century relationship between the Southern Annular Mode and global mean temperature

The positive phase of the El Niño-Southern Oscillation (ENSO) increases global mean temperature, and contributes to a negative phase of the Southern Annular Mode (SAM), the dominant mode of climate variability in the Southern Hemisphere. This interannual relationship of a high global mean temperature associated with a negative SAM, however, is opposite to the relationship between their trends under greenhouse warming. We show that over much of the 20th century this relationship undergoes multidecadal fluctuations depending on the intensity of ENSO. During the period 1925–1955, subdued ENSO activities weakened the relationship. However, a similar weakening has occurred since the late 1970s despite the strong ENSO. We demonstrate that this recent weakening is induced by climate change in the Southern Hemisphere. Our result highlights a rare situation in which climate change signals emerge against an opposing property of interannual variability, underscoring the robustness of the recent climate change.

A Simple GCM Study on the Relationship between ENSO and the Southern Annular Mode

Abstract This study investigates the relationship between El Niño–Southern Oscillation (ENSO) and southern annular mode (SAM) events with an idealized general circulation model. A series of model calculations are performed to examine why positive (negative) intraseasonal SAM events are observed to occur much more frequently during La Niña (El Niño). Seven different model runs are performed: a control run, three El Niño runs (the first with a zonally symmetric heating field, the second with a zonally asymmetric heating/cooling field, and the third that combines both fields), and three La Niña runs (with heating fields of opposite sign). The model runs with the zonally symmetric and combined heating fields are found to yield the same relationship between the phase of ENSO and the preferred phase for SAM events as is observed in the atmosphere. In contrast, the zonally asymmetric model runs are found to have the opposite SAM–ENSO phase preference characteristics. Since a reduced midlatitude meridional potential vorticity gradient has been linked to a greater frequency of positive-phase SAM events, and vice versa for negative SAM events, the meridional potential vorticity gradient in the various model runs was compared. The results suggest that the phase preference of SAM events during ENSO arises from the impact of the zonal-mean heating on the midlatitude meridional potential vorticity gradient.

Annular mode changes in the CMIP5 simulations

We investigate simulated changes in the annular modes in historical and RCP 4.5 scenario simulations of 37 models from the fifth Coupled Model Intercomparison Project (CMIP5), a much larger ensemble of models than has previously been used to investigate annular mode trends, with improved resolution and forcings. The CMIP5 models on average simulate increases in the Northern Annular Mode (NAM) and Southern Annular Mode (SAM) in every season by 2100, and no CMIP5 model simulates a significant decrease in either the NAM or SAM in any season. No significant increase in the NAM or North Atlantic Oscillation (NAO) is simulated in response to volcanic aerosol, and no significant NAM or NAO response to solar irradiance variations is simulated. The CMIP5 models simulate a significant negative SAM response to volcanic aerosol in MAM and JJA, and a significant positive SAM response to solar irradiance variations in MAM, JJA and DJF.

Synoptic Weather Types for the Ross Sea Region, Antarctica

Abstract Synoptic classifications over the Southern Ocean in the Ross Sea region of Antarctica (50°S–Antarctic coast, 150°E–90°W) have been derived from NCEP reanalysis data (1979–2011), producing a set of six synoptic types for the region. These types describe realistic synoptic conditions for the region and represent the moisture-bearing low pressure systems that circulate around Antarctica. The types are described as follows: low Bellingshausen/Amundsen (L-BA), low (L), zonal (Z), low Ross (L-R), ridge (R), and low Amundsen (L-A). Seasonal frequencies of the synoptic types reflect the seasonal zonal shift of the Amundsen Sea low (ASL) and also correlate well with the Southern Oscillation index (SOI) and the southern annular mode (SAM). Variability in the occurrences of the synoptic types L-R and L-BA indicate a shifting of the position of the ASL farther east (west) toward (away from) the Antarctic Peninsula during La Niña (El Niño) and positive (negative) SAM conditions. A joint linear regression of the SOI and SAM indices show the strongest correlations with the types L-BA and L-R in the spring and quantifies the joint forcing effect of these climate cycles on synoptic variability in the region. As a demonstration of how synoptic classification provides links between large-scale atmospheric circulation and local climate parameters, the synoptic types are related to precipitation and temperature at Roosevelt Island, an ice core site on the Ross Ice Shelf (80°S, 160°W). The synoptic types provide quantification of distinct precipitation and temperature regimes at this site, which allows for more fundamental understanding of the precipitation source regions and transport pathways that drive the variability in snow and ice proxies.

The Southern Annular Mode Seen through Weather Regimes

This article investigates the prominent features of the Southern Hemisphere (south of 20°S) atmospheric circulation when extracted using EOF analysis and a k-means clustering algorithm. The focus is on the southern annular mode (SAM), the nature of its recent trend, and the zonal symmetry of associated spatial patterns. The study uses the NCEP–Department of Energy Atmospheric Model Intercomparison Project II Reanalysis (NCEP-2) (period 1979–2009) to obtain robust patterns over the recent years and the Twentieth Century Reanalysis Project (period 1871–2008) to document decadal changes. Also presented is a comparison of these signals against a station-based reconstruction of the SAM index and a gridded interpolated dataset [Hadley Centre Sea Level Pressure dataset version 2 (HadSLP2)]. Over their common period, both reanalyses are in fair agreement, both in terms of spatial patterns and temporal variability. In particular, both datasets show weather regimes that can be interpreted as the opposite phases of the SAM. At the decadal time scale, the study shows that the trend toward the positive SAM phase (as inferred from the usual EOF-based index) is related more to an increase in the frequency of clusters corresponding to the positive phase, with little changes in the frequency of the negative SAM events. Similarly, the long-term tropospheric warming trend already discussed in the literature is shown to be related more to a decrease in the number of abnormally cold days, with little changes in the number of abnormally warm days. The cluster analysis therefore allows for complement descriptions based on simple indexes or EOF decompositions, highlighting the nonlinear nature of the decadal changes in the Southern Hemisphere atmospheric circulation and temperature.

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.

Interactions of ENSO, the IOD, and the SAM in CMIP3 Models

Abstract Simulations of individual global climate drivers using models from the Coupled Model Intercomparison Project phase 3(CMIP3) have been examined; however, the relationship among them has not been assessed. This is carried out to address several important issues, including the likelihood of the southern annular mode (SAM) forcing Indian Ocean dipole (IOD) events and the possible impact of the IOD on El Niño–Southern Oscillation (ENSO) events. Several conclusions emerge from statistics based on multimodel outputs. First, ENSO signals project strongly onto the SAM, although ENSO-forced signals tend to peak before ENSO. This feature is similar to the situation associated with the IOD. The IOD-induced signal over southern Australia, through stationary equivalent Rossby barotropic wave trains, peak before the IOD itself. Second, there is no control by the SAM on the IOD, in contrast to what has been suggested previously. Indeed, no model produces a SAM–IOD relationship that supports a positive (negative) SAM driving a positive (negative) IOD event. This is the case even in models that do not simulate a statistically significant relationship between ENSO and the IOD. Third, the IOD does have an impact on ENSO. The relationship between ENSO and the IOD in the majority of models is far weaker than the observed. However, the ENSO’s influence on the IOD is boosted by a spurious oceanic teleconnection, whereby ENSO discharge–recharge signals transmit to the Sumatra–Java coast, generating thermocline anomalies and changing IOD properties. Without the spurious oceanic teleconnection, the influence of the IOD on ENSO is comparable to the impact of ENSO on the IOD. Other model deficiencies are discussed.

Interannual modulation of eddy kinetic energy in the southeast Indian Ocean by Southern Annular Mode

[1] Interannual variability in the mesoscale eddy field over the southeast Indian Ocean of 15°S–35°S and 60°E–110°E is investigated on the basis of 16 year satellite altimetry observations. Eddy kinetic energy (EKE) in this region appears stronger in 2000–2004 and weaker in 1993–1996, 1998–2000, and 2007. It is found that this interannual modulation of EKE is mediated by the Southern Annular Mode (SAM), with a positive (negative) SAM corresponding to weak (strong) eddy activity in this region. The interannual modulation of the EKE by the SAM is through modulating the baroclinic instability associated with the surface-intensified South Indian Countercurrent (SICC) and the underlying South Equator Current (SEC) system. In the positive phase of the SAM, the southeastern subtropical Indian Ocean is dominated by an anomalous Ekman upwelling, which slackens the southward tilt of the isotherms and thus reduces the SICC. This shall reduce the vertical velocity shear of the SICC-SEC current system, leading to a weak instability and thus a weak eddy activity.

The Impact of ENSO on Wave Breaking and Southern Annular Mode Events

Abstract This study examines the relationship between intraseasonal southern annular mode (SAM) events and the El Niño–Southern Oscillation (ENSO) using daily 40-yr ECMWF Re-Analysis (ERA-40) data. The data coverage spans the years 1979–2002, for the austral spring and summer seasons. The focus of this study is on the question of why positive SAM events dominate during La Niña and negative SAM events during El Niño. A composite analysis is performed on the zonal-mean zonal wind, Eliassen–Palm fluxes, and two diagnostic variables: the meridional potential vorticity gradient, a zonal-mean quantity that is used to estimate the likelihood of wave breaking, and the wave breaking index (WBI), which is used to evaluate the strength of the wave breaking. The results of this investigation suggest that the background zonal-mean flow associated with La Niña (El Niño) is preconditioned for strong (weak) anticyclonic wave breaking on the equatorward side of the eddy-driven jet, the type of wave breaking that is found to drive positive (negative) SAM events. A probability density function analysis of the WBI, for both La Niña and El Niño, indicates that strong anticyclonic wave breaking takes place much more frequently during La Niña and weak anticyclonic wave breaking during El Niño. It is suggested that these wave breaking characteristics, and their dependency on the background flow, can explain the strong preference for SAM events of one phase during ENSO. The analysis also shows that austral spring SAM events that coincide with ENSO are preceded by strong stratospheric SAM anomalies and then are followed by a prolonged period of wave breaking that lasts for approximately 30 days. These findings suggest that the ENSO background flow also plays a role in the excitation of stratospheric SAM anomalies and that the presence of these stratospheric SAM anomalies in turn excites and then maintains the tropospheric SAM anomalies via a positive eddy feedback.

Biophysical coupling in remotely-sensed wind stress, sea surface temperature, sea ice and chlorophyll concentrations in the South Indian Ocean

Satellite records of chlorophyll, sea-surface temperature, sea-ice concentration and wind-stress curl, together with reanalysis wind fields, have been analysed to identify connections between the physical environment and phytoplankton growth. The study focusses on the BROKE-West survey region in the Indian Ocean sector of the Southern Ocean: 20° to 90°E, 70° to 50°S. Correlation and regression analyses showed that, of the parameters that can be routinely monitored from space, wind stress and sea surface temperature were most strongly correlated with chlorophyll across the BROKE-West survey area during the period of the survey. Each of these factors, as well as sea ice concentration, was found to be strongly correlated with chlorophyll at different locations. Three distinct regions were identified: The continental shelf and slope (max. depth=3000 m), where satellite data were most obstructed by ice and clouds, supported high concentrations of chlorophyll throughout the growth season (October to April), and although the most important factors determining chlorophyll were likely to be those not observable using remote sensing, close to 100% of variability in surface chlorophyll could be predicted by sea-surface temperature and sea ice concentration. Offshore to the west of 45°E, the eastern Weddell Gyre was found to retain sea-ice later into the growth season and support chlorophyll concentrations on the order of 0.1 mg m -3. Wind-driven advection of sea ice to the north and south of the Antarctic Divergence, located at 64°S, produced a strong positive correlation between chlorophyll concentrations along the landward and off-shore edges of the seasonal ice zone and the Southern Annular Mode. Offshore to the east of 45°E, chlorophyll concentrations encountered during the BROKE-West cruise were found to be unusually high. This was attributed to a southerly excursion of the Antarctic Circumpolar Current corresponding to negative Southern Annular Mode indices during the cruise. Significant, regionally varying correlations were found between the physical and biological parameters examined and climatological indices, including the Southern Annular Mode and the tropical El Nino-Southern Oscillation signal. © 2010 Elsevier Ltd.

Descending surface water at the antarctic marginal ice zone and its contribution to intermediate water: An ice-ocean model

This study concerns the unique physical mechanism of Ekman convergence at the marginal ice zone (ECMIZ) produced by the difference between air-ice drag and air-water drag. A coupled ice-ocean model is used to show the strength and distribution of the ECMIZ with respect to Antarctic Intermediate Water (AAIW) formation, which is important for the uptake of carbon dioxide. Strong ECMIZ occurs in the Atlantic and Pacific sectors from July to October, matched in time and space with ice melting, while it is significantly weaker due to strongly divergent background winds in the Indian sector. Transport analysis by artificial tracer experiments reveals the interannual variability of the ECMIZ correlates well with the Southern Annular Mode (SAM). The downward transport of surface water at the MIZ during a positive SAM (2001) is about 1.4 times as large as that during a negative SAM (2000). In particular the transport in the Atlantic sector is twice that in the Pacific sector in both years. Once the downward flux is analyzed in isolation, the contribution from synoptic scale variability is found to increase the volume transport of surface water in the eastern region of the Pacific. Assuming strong isopycnal mixing, we suggest that ECMIZ is an important mechanism supplying surface water to the formation of AAIW, and its zonal variability is responsible for the interbasin differences in AAIW properties. In particular, the increased ECMIZ and surface melt water input in the Atlantic sector would produce AAIW that is colder and fresher than in the Pacific.

Relationship between the southern annular mode and southern hemisphere atmospheric systems

Seasonal relationship between the Southern Annular Mode (SAM) and the spatial distribution of the cyclone systems over Southern Hemisphere is investigated for the period 1980 to 1999. In addition, seasonal frontogenesis and rainfall distribution over South America and South Atlantic Ocean during different SAM phases were also analyzed. It is observed that during negative SAM phases the cyclone trajectories move northward when compared to the positive one, and in the South America and South Atlantic sector there is intense frontogenetic activity and positive anomaly precipitation over the Southeast of the South America. In general, SAM positive phase shows opposite signals.

Characteristics and variability of synoptic features associated with cool season rainfall in southeastern Australia

AbstractCool season rainfall variability in southeastern Australia is investigated via classification and characterization of the predominant types of synoptic systems occurring in the region. These types are frontal systems, cut‐off low systems, and other systems. Rainfall in the region is dominated by cut‐off systems and these systems are the main influence on the interannual variability of rainfall. Both cut‐off systems and frontal systems display an enhancement of thermal (thickness) gradient as rainfall increases, but the mechanisms for intensification differ. Cut‐off systems intensify in the region in association with local increases in baroclinicity and the subtropical jet, whereas frontal systems tend to intensify via a confluence of subtropical and polar jets. Interannual rainfall variability is examined for groupings of years based on both clustering of continental rainfall patterns and on El Nino/Southern Oscillation (ENSO)/Indian Ocean Dipole (IOD) years. Cut‐off systems exhibit consistent enhancements of thermal gradients for groupings of years in which they produce more rainfall. For ENSO/IOD groupings, the cut‐off thermal gradients are consistent with the underlying sea surface temperature (SST) anomalies. Wet years in southeastern Australia are usually produced by cut‐off systems, but can also be produced by frontal systems. In those cases the mid‐tropospheric flow pattern is reminiscent of the negative Southern Annular Mode (SAM) pattern. The positive SAM pattern is also associated with enhanced rainfall in the southeast via local intensification of blocking and cut‐off systems. Copyright © 2008 Royal Meteorological Society

Southern Hemisphere Synoptic Behavior in Extreme Phases of SAM, ENSO, Sea Ice Extent, and Southern Australia Rainfall

Abstract The association between Southern Hemisphere cyclones and anticyclones and the El Niño–Southern Oscillation (ENSO), southern annular mode (SAM), Antarctic sea ice extent (SIE), and rainfall in Perth and Melbourne is explored. Those cities are, respectively, located in the southwestern and southeastern corners of Australia, where substantial decreasing rainfall trends have been observed over the last decades. The need for a more unified understanding of large-scale anomalies in storm indicators associated with the climate features itemized above has motivated this study. The main aim is to identify cyclone-anomalous areas that are potentially important in characterizing continental rainfall anomalies from a hemispheric perspective, focusing on midlatitude Australia. The study covers the “satellite era” from 1979 to 2003 and was conducted for the southern winter when midlatitude rainfall is predominantly baroclinic. The results indicate a well-organized hemispheric cyclone pattern associated with ENSO, SAM, SIE, and rainfall anomalies. There is a moderate large-scale, high-latitude resemblance between La Niña, negative SAM, and reduced SIE in some sectors. In particular, there is a suggestion that SIE anomalies over the Indian Ocean and Western Australia sectors are associated with a large-scale pattern of cyclone/anticyclone anomalies that is more pronounced over the longitudes of Australia and New Zealand. Spatial correlation analysis suggests a robust link between cyclone density over the sectors mentioned above and rainfall in Perth and Melbourne. Statistical analyses of rainfall and SIE show modest correlations for Perth and weak correlations for Melbourne, generally corroborating the above. It is proposed that SAM and SIE are part of a complex physical system that is best understood as a coupled mechanism, and that their impacts on the circulation can be seen as partially independent of ENSO. While SAM and SIE have greater influence on the circulation affecting rainfall in the western side of Australia, ENSO is the dominant influence on the eastern half of the country. A contraction of the sea ice seems to be accompanied by a southward shift of high-latitude cyclones, which is also hypothesized to increase downstream cyclone density at midlatitudes via conservation of mass, similarly to what is observed during the extreme positive phase of the SAM. These associations build on previous developments in the literature. They bring a more unified view on high-latitude climate features, and may also help to explain the declining trends in Australian rainfall.

A Possible Link between the Weddell Polynya and the Southern Annular Mode*

Abstract Shortly after the advent of the first imaging passive microwave sensor on board a research satellite an anomalous climate feature was observed within the Weddell Sea. During the years 1974–1976, a 250 × 103 km2 area within the seasonal sea ice cover was virtually free of winter sea ice. This feature, the Weddell Polynya, was created as sea ice formation was inhibited by ocean convection that injected relatively warm deep water into the surface layer. Though smaller, less persistent polynyas associated with topographically induced upwelling at Maud Rise frequently form in the area, there has not been a reoccurrence of the Weddell Polynya since 1976. Archived observations of the surface layer salinity within the Weddell gyre suggest that the Weddell Polynya may have been induced by a prolonged period of negative Southern Annular Mode (SAM). During negative SAM the Weddell Sea experiences colder and drier atmospheric conditions, making for a saltier surface layer with reduced pycnocline stability. This condition enables Maud Rise upwelling to trigger sustained deep-reaching convection associated with the polynya. Since the late 1970s SAM has been close to neutral or in a positive state, resulting in warmer, wetter conditions over the Weddell Sea, forestalling repeat of the Weddell Polynya. A contributing factor to the Weddell Polynya initiation may have been a La Niña condition, which is associated with increased winter sea ice formation in the polynya area. If the surface layer is made sufficiently salty due to a prolonged negative SAM period, perhaps aided by La Niña, then Maud Rise upwelling meets with positive feedback, triggering convection, and a winter persistent Weddell Polynya.

Mid latitude winter climate variability in the South Indian and southwest Pacific regions since 1300 AD

Mid-latitude winter atmospheric variability in the South Indian Ocean and southwest Pacific Ocean regions of the circum-Antarctic are reconstructed using sea-salt aerosol concentrations measured in the high resolution Law Dome (DSS) ice core from East Antarctica. The sea-salt aerosol concentration data, as sodium (Na), were measured at approximately monthly resolution spanning the past 700 years. Analyses of covariations between Na concentrations in Law Dome ice, and mean sea-level pressure (MSLP) and wind field data were conducted to define the mid-latitude and sub-Antarctic atmospheric circulation patterns associated with variations in Na delivery. High Na concentrations in Law Dome snow are associated with increased meridional aerosol transport from mid-latitude sources. The seasonal average Na concentration for early winter (May, June, July (MJJ)) is strongly correlated to the mid-latitude MSLP field in the South Indian and southwest Pacific Oceans, and southern Australian regions. In addition, the average MJJ Na concentrations display a strong association with the stationary Rossby wave number 3 circulation, and are anti-correlated to the Southern Annular Mode (SAM) index of climate variability: high (low) Na concentrations occurring during negative (positive) SAM phases. This observed relationship is used to derive a proxy record for early-winter MSLP anomalies and the SAM in the South Indian and southwest Pacific Ocean regions over the period 1300–1995 AD. The proxy SAM index from 1300 to 1995 AD shows pronounced decadal-scale variability throughout. The period after 1500 AD is marked by a tendency toward slower variations and a weakly-positive mean SAM (enhanced westerlies in the 50° to 65°S zone) compared to the early part of the record.