Southern Hemisphere Atmosphere Research Articles

Enhanced Occurrence of Atmospheric Blocking in the Southern Hemisphere by Baroclinic Annular Mode

AbstractAtmospheric blocking is a large‐amplitude, quasi‐stationary, and long‐lasting flow regime in the extratropics. To understand the physical processes governing the occurrence of atmospheric blocking, we identify that the positive phase of Baroclinic Annular Mode (BAM) increases the occurrence of blocking events in the Southern Hemisphere atmosphere. As BAM can translate to regional scales, we identify an enhanced zonal flux of wave activity and reduced dispersiveness associated with high BAM states that are dynamically conducive to the occurrence of atmospheric blocking. Blocking frequency in the high BAM state almost doubles as compared to the climatology, and the enhanced occurrence of blocks is most significant within BAM‐associated wave packets. This finding suggests BAM can be employed as a new source of predictability for atmospheric blocking.

Impacts of Observed Extreme Antarctic Sea Ice Conditions on the Southern Hemisphere Atmosphere

The Antarctic sea ice has undergone dramatic changes in recent years, with the highest recorded sea ice extent in 2014 and the lowest in 2017. We investigated the impacts of the observed changes in these two extremes of Antarctic sea ice conditions on the atmospheric circulation in the Southern Hemisphere. We conducted three numerical simulations with different seasonal cycles of Antarctic sea ice forcings using the Community Atmosphere Model Version 5: the maximum sea ice extent in 2014 (ICE_14), the minimum sea ice extent in 2017 (ICE_17), and the average sea ice extent between 1981 and 2010 (ICE_clm, reference simulation). Our results suggest that the atmospheric response in the Southern Hemisphere showed strong seasonal variations and the atmospheric circulation in winter was more sensitive to the decreased Antarctic sea ice in 2017 than the increased sea ice in 2014. In ICE_14, the westerlies over the polar region were enhanced in summer, but there was no significant change in the zonal-averaged wind in winter. In contrast, in ICE_17, there was a clear equatorward shift in the subtropical jet in winter, but no significant change in summer. The temperature responses were limited to the Antarctic coast, where there were changes in the sea ice in ICE_14 and ICE_17. The warming on the coast of the Amundsen Sea in summer led to a slight increase in precipitation in both simulations.

Australian Fires 2019–2020: Tropospheric and Stratospheric Pollution Throughout the Whole Fire Season

The historically large and severe wildfires in Australia from September 2019 to March 2020 are known to have injected a smoke plume into the stratosphere around New Year, due to pyro-cumulonimbus (pyro-Cb) activity, that was subsequently distributed throughout the Southern Hemisphere (SH). We show with satellite, ground based remote sensing, and in situ observations that the fires before New Year, had already a substantial impact on the SH atmosphere, starting as early as September 2019, with subsequent long-range transport of trace gas plumes in the upper-troposphere. Airborne in situ measurements above Southern Argentina in November 2019 show elevated CO mixing ratios at an altitude of 11 km and can be traced back using FLEXPART trajectories to the Australian fires in mid-November 2019. Ground based solar-FTS (Fourier Transform Spectroscopy) observations of biomass burning tracers CO, HCN and C2H6 at Lauder, South Island, New Zealand show enhanced tropospheric columns already starting in September 2019. In MLS observations averaged over 30°–60°S, enhanced CO mixing ratios compared to previous years become visible in late October 2019 only at and below the 147 hPa pressure level. Peak differences are found with satellite and ground-based observations for all altitude levels in the Southern Hemisphere in January. With still increased aerosol values following the Ulawun eruption in 2019, averaged satellite observations show no clear stratospheric and upper-tropospheric aerosol enhancements from the Australian fires, before the pyro-Cb events at the end of December 2019. However, with the clear enhancement of fire tracers, we suggest the period September to December 2019 (prior to the major pyro-Cb events) should be taken into account in terms of fire pollutant emissions when studying the impact of the Australian fires on the SH atmosphere.

Combined Effects of Global Warming and Ozone Depletion/Recovery on Southern Hemisphere Atmospheric Circulation and Regional Precipitation

AbstractOzone depletion led to a positive trend in the summertime Southern Annular Mode (SAM) during the last decades of the 20th century. During the present century, global warming (GW) is expected to contribute to a positive SAM trend while ozone recovery is expected to act in the opposite direction. Here, Southern Hemisphere (SH) circulation and regional precipitation change are studied with a methodology that separates the effects from GW and ozone depletion/recovery. Our results show that a “tug‐of‐war” between ozone and GW occurs in the summertime stratosphere, propagating to the troposphere where it is manifest in the SAM. However, at the regional scale this “tug‐of‐war” is not as relevant as the combined effects of other remote drivers of circulation change, which force different kinds of precipitation changes in the SH. For regional precipitation changes, the uncertainty in future circulation change is as important as the uncertainty in the GW level.

Dynamics of Southern Hemisphere Atmospheric Circulation Response to Anthropogenic Aerosol Forcing

AbstractAnthropogenic aerosols, concentrated largely in the Northern Hemisphere, not only affect the local climate but also induce pronounced changes in atmospheric circulation that extend into the Southern Hemisphere (SH). In coupled historical single‐forcing simulations, aerosol forcing induces a deceleration of both the subpolar jet (SPJ) and the subtropical jet (STJ) in SH in austral winter. Atmospheric general circulation model experiments indicate that the STJ is weakened by an interhemisphere gradient in the zonal mean sea surface temperature (SST) and an anomalous cross‐equatorial Hadley circulation, while the SPJ response shares similar feedbacks with the greenhouse gas forcing. Specifically, atmospheric eddy adjustments are important for the SPJ change. The atmospheric response unique to anthropogenic aerosol forcing (e.g., cross‐equatorial Hadley cell and the weakened SH STJ) can be exploited for climate change attribution.

Spatial Distributions of Nitric Oxide in the Antarctic Wintertime Middle Atmosphere During Geomagnetic Storms

AbstractEnergetic electron precipitation leads to increased nitric oxide (NO) production in the mesosphere and lower thermosphere. NO distributions in the wintertime, high‐latitude Southern Hemisphere atmosphere during geomagnetic storms are investigated. NO partial columns in the upper mesosphere at altitudes 70–90 km and in the lower thermosphere at 90–110 km have been derived from observations made by the Solar Occultation For Ice Experiment (SOFIE) on board the Aeronomy of Ice in the Mesosphere (AIM) satellite. The SOFIE NO measurements during 17 geomagnetic storms in 2008–2014 have been binned into selected geomagnetic latitude and geographic latitude/longitude ranges. The regions above Antarctica showing the largest instantaneous NO increases coincide with high fluxes of 30–300 keV precipitating electrons from measurements by the second‐generation Space Environment Monitor (SEM‐2) Medium Energy Proton and Electron Detector (MEPED) instrument on the Polar‐orbiting Operational Environmental Satellites (POES). Significant NO increases over the Antarctic Peninsula are likely due to precipitation of >30 keV electrons from the radiation belt slot region. NO transport is estimated using Horizontal Wind Model (HWM14) calculations. In the upper mesosphere strong eastward winds (daily mean zonal wind speed ~20–30 m s−1 at 80 km) during winter transport NO‐enriched air away from source regions 1–3 days following the storms. Mesospheric winds also introduce NO‐poor air into the source regions, quenching initial NO increases. Higher up, in the lower thermosphere, weaker eastward winds (~5–10 m s−1 at 100 km) are less effective at redistributing NO zonally.

Southern Hemisphere Atmospheric Blocking in CMIP5 and Future Changes in the Australia‐New Zealand Sector

AbstractMany general circulation models fail to capture the observed frequency of atmospheric blocking events in the Northern Hemisphere; however, few studies have examined models in the Southern Hemisphere and those studies that have, have often been based on only a few models. To provide a comprehensive view of how the current generation of coupled general circulation models performs in the Southern Hemisphere and how blocking frequency changes under enhanced greenhouse gas forcing, we examine the output of 23 models from the Coupled Model Intercomparison Project Phase 5 (CMIP5). We find that models have differing biases during winter, when blocking occurrence is highest, though models underestimate blocking frequency south of Australia during summer. We show that models generally have a reduction in blocking frequency with future anthropogenic forcing, particularly in the Australia‐New Zealand sector with the number of winter blocked days reduced by about one third by the end of the 21st century.

Effect of Gravity Waves From Small Islands in the Southern Ocean on the Southern Hemisphere Atmospheric Circulation

AbstractThe effect of small islands in the Southern Ocean on the atmospheric circulation in the Southern Hemisphere is considered with a series of simulations using the NASA Goddard Earth Observing System Chemistry‐Climate Model in which the gravity wave stress generated by these islands is increased to resemble observed values. The enhanced gravity wave drag leads to a 2 K warming of the springtime polar stratosphere, partially ameliorating biases in this region. Resolved wave drag declines in the stratospheric region in which the added orographic gravity waves deposit their momentum, such that changes in gravity waves are partially compensated by changes in resolved waves, though resolved wave drag increases further poleward. The orographic drag from these islands has impacts for surface climate, as biases in tropospheric jet position are also partially ameliorated. These results suggest that these small islands are likely contributing to the missing drag near 60°S in the upper stratosphere evident in many data assimilation products.

Response of the Southern Hemisphere Atmosphere to the Stratospheric Equatorial Quasi-Biennial Oscillation (QBO) from Winter to Early Summer

Response of the Southern Hemisphere Atmosphere to the Stratospheric Equatorial Quasi-Biennial Oscillation (QBO) from Winter to Early Summer

The Signature of Southern Hemisphere Atmospheric Circulation Patterns in Antarctic Precipitation.

We provide the first comprehensive analysis of the relationships between large‐scale patterns of Southern Hemisphere climate variability and the detailed structure of Antarctic precipitation. We examine linkages between the high spatial resolution precipitation from a regional atmospheric model and four patterns of large‐scale Southern Hemisphere climate variability: the southern baroclinic annular mode, the southern annular mode, and the two Pacific‐South American teleconnection patterns. Variations in all four patterns influence the spatial configuration of precipitation over Antarctica, consistent with their signatures in high‐latitude meridional moisture fluxes. They impact not only the mean but also the incidence of extreme precipitation events. Current coupled‐climate models are able to reproduce all four patterns of atmospheric variability but struggle to correctly replicate their regional impacts on Antarctic climate. Thus, linking these patterns directly to Antarctic precipitation variability may allow a better estimate of future changes in precipitation than using model output alone.

Improved Low-Cloud Simulation from the Community Atmosphere Model with an Advanced Third-Order Turbulence Closure

Abstract In this study, a simplified intermediately prognostic higher-order turbulence closure (IPHOC) is implemented in the Community Atmosphere Model, version 5 (CAM5), to provide a consistent treatment of subgrid-scale cloud processes, except for deep convection. The planetary boundary layer (PBL) height is prognosticated to better resolve the discontinuity of temperature and moisture above the PBL top. Single-column model tests show that fluxes of liquid water potential temperature and total water, cloud fraction, and liquid water content are improved with this approach. The simplified IPHOC package replaces the boundary layer dry and moist turbulence parameterizations, the shallow convection parameterization, and the liquid-phase part of the cloud macrophysics parameterization in CAM5. CAM5-IPHOC improves the simulation of the low-level clouds off the west coasts of continents and the storm track region in the Southern Hemisphere (SH). The transition from stratocumulus to cumulus clouds is more gradual. There are also improvements on the cloud radiative forcing, especially shortwave, in the subsidence regime. The improvements in the relationships among low cloud amount, surface relative humidity, lower tropospheric stability, and PBL depth are seen in some stratocumulus regions. CAM5-IPHOC, however, produces weaker precipitation at the South Pacific convergence zone than CAM5 because of less energy flux into the SH atmosphere. The more downward surface shortwave radiative cooling and the less top-of-the-atmosphere longwave cloud radiative heating in the SH relative to the Northern Hemisphere explains the anomalous cooling and the lesser energy flux into the SH, which is related to the underestimate of extratropical middle/high clouds in the SH.

Teleconnection between the central tropical Pacific and the Antarctic stratosphere: spatial patterns and time lags

Coupling between the central tropical Pacific (CTP) sea surface temperature (SST) variability and the Antarctic stratosphere known from previous studies is analysed using a lagged correlation method. Monthly mean SST, stratospheric temperatures and zonal winds from the NCEP–NCAR reanalysis and total ozone from the Merged Ozone Data Set over the 1979–2011 period are used. Indices for interannual variability were created and sequential lagged correlations were used to determine the spatial patterns and time lags of the strongest teleconnection. It has been revealed that interannual SST variations in a westward-oriented V-shaped pattern in the CTP region (160–220°E, 30°N–20°S) are associated with statistically significant positive signals in zonally asymmetric region of the Antarctic stratosphere temperature field. Stratospheric pathway dominates in the poleward and downward propagation of the CTP related anomaly in the Southern Hemisphere zonal wind variability. Abrupt modification of the teleconnection patterns in October is accompanied by occurrence of the intense meridional wave train in the lower stratosphere and troposphere. The strongest lagged correlations between the CTP SST index in June and the Southern Hemisphere atmosphere variables in October (r = ±0.7) could be related to the corresponding seasonal peaks in the stationary planetary wave activity. Possible causes of the V-pattern formation and the lagged teleconnection mechanisms are briefly discussed.

Assessment of Modes of Interannual Variability of Southern Hemisphere Atmospheric Circulation in CMIP5 Models

Abstract An assessment is made of the modes of interannual variability in the seasonal mean summer and winter Southern Hemisphere (SH) 500-hPa geopotential height in the twentieth century in models from the Coupled Model Intercomparison Project (CMIP) phase 5 (CMIP5) dataset. Modes of variability of both the slow (signal) and intraseasonal (noise) components in the CMIP5 models are evaluated against those estimated from reanalysis data. There is general improvement in the leading modes of the slow (signal) component in CMIP5 models compared with the CMIP phase 3 (CMIP3) dataset. The largest improvement is in the spatial structures of the modes related to El Niño–Southern Oscillation variability in SH summer. An overall score metric is significantly higher for CMIP5 over CMIP3 in both seasons. The leading modes in the intraseasonal noise component are generally well reproduced in CMIP5 models, and there are few differences from CMIP3. A new total overall score metric is used to rank the CMIP5 models over both seasons. Weighting the seasons by the relative spread of overall scores is shown to be suitable for generating multimodel ensembles for further analysis of interannual variability. In multimodel ensembles, it is found that an ensemble of size 5 or 6 is sufficient in SH summer to reproduce well the dominant modes. In contrast, about 13 models are typically are required in SH winter. It is shown that it is necessary that the selected models individually reproduce well the leading modes of the slow component.

Trends in Southern Hemisphere wind-driven circulation in CMIP5 models over the 21st century: Ozone recovery versus greenhouse forcing

During the late 20th century, Antarctic ozone depletion and increasing greenhouse gases (GHGs) conspired to generate conspicuous atmospheric circulation trends in the Southern Hemisphere (SH), contributing to a poleward intensification of the oceanic supergyre circulation. Forcing of Antarctic ozone depletion dominated the observed trends during the depletion period (1979–2005), but Antarctic ozone is projected to recover by the middle of the 21st century. The recovery provides a mechanism for offsetting the impact from increasing GHG emissions. To what extent will the recovery of ozone mitigate SH atmosphere and ocean circulation trends expected from increasing GHGs? We examine climate model output from the Representative Concentration Pathway 4.5 and 8.5 (RCP4.5 and RCP8.5, respectively) emission scenario experiments, submitted to the Coupled Model Intercomparison Project phase 5. Both scenarios are subject to the effect of ozone recovery. We show that during the recovery period (2006–2045), there is little poleward shift of the supergyre circulation under either RCP scenario in austral summer, due to the dominance of ozone recovery. Further, under RCP8.5 the trend in winter, a season in which ozone recovery has little impact, is greater (more poleward) than in summer, opposite to the seasonality of trends during the depletion period. Under RCP4.5, with the contribution from ozone recovery, the summer poleward shift is projected to stabilize into the postrecovery decades, whereas under RCP8.5, the summer poleward shift accelerates in the postrecovery period, presenting vastly different ocean circulation futures.

Tropospheric Circulation Variability over Central and Southern South America

Combined Empirical Orthogonal Function Analysis of low-level atmospheric circulation after filtering the synoptic scale was performed for the period 1981-2006 over Central and Southern South America. Modes with 40 and near 70 days frequency associated with swings in longitude of the South Pacific and South Atlantic Ocean heights were identified. Their extreme values were related to drought and to high anomalous precipitation associated to floods in South East South America (SESA). These modes were independent of other sources of variability of the Southern Hemisphere atmosphere, namely MJO (Madden-Julian Oscillation), ENSO (El Nino Southern Oscillation) and AAO (Antarctic Oscillation). Mode one, which in its positive phase has a circulation similar to the mean winter, has a trend that explains part of the winter warming observed since 1980’s in Central and Eastern Argentina. Variance was calculated for the intra-annual variability, the one associated to the inter-annual variability including trends and jumps, and that of the annual cycle. The partition of the total variance was roughly 70%, 10% and 20% respectively. This partition implies that predictability of the regional climate is more critically dependent on the predictability of the intra-annual variability than of the inter-annual variability.

Impact of Surface Forcing on Southern Hemisphere Atmospheric Blocking in the Australia–New Zealand Sector

Abstract Characteristics of atmospheric blocking in the Southern Hemisphere (SH) are explored in atmospheric general circulation model (AGCM) simulations with the Community Atmosphere Model, version 3, with a particular focus on the Australia–New Zealand sector. Preferred locations of blocking in SH observations and the associated seasonal cycle are well represented in the AGCM simulations, but the observed magnitude of blocking is underestimated throughout the year, particularly in late winter and spring. This is related to overly zonal flow due to an enhanced meridional pressure gradient in the model, which results in a decreased amplitude of the longwave trough/ridge pattern. A range of AGCM sensitivity experiments explores the effect on SH blocking of tropical heating, midlatitude sea surface temperatures, and land–sea temperature gradients created over the Australian continent during austral winter. The combined effects of tropical heating and extratropical temperature gradients are further explored in a configuration that is favorable for blocking in the Australia–New Zealand sector with warm SST anomalies to the north of Australia, cold to the southwest of Australia, warm to the southeast, and cool Australian land temperatures. The blocking-favorable configuration indicates a significant strengthening of the subtropical jet and a reduction in midlatitude flow, which results from changes in the thermal wind. While these overall changes in mean climate, predominantly forced by the tropical heating, enhance blocking activity, the magnitude of atmospheric blocking compared to observations is still underestimated. The blocking-unfavorable configuration with surface forcing anomalies of opposite sign results in a weakening subtropical jet, enhanced midlatitude flow, and significantly reduced blocking.

Link between the double-Intertropical Convergence Zone problem and cloud biases over the Southern Ocean

The double-Intertropical Convergence Zone (ITCZ) problem, in which excessive precipitation is produced in the Southern Hemisphere tropics, which resembles a Southern Hemisphere counterpart to the strong Northern Hemisphere ITCZ, is perhaps the most significant and most persistent bias of global climate models. In this study, we look to the extratropics for possible causes of the double-ITCZ problem by performing a global energetic analysis with historical simulations from a suite of global climate models and comparing with satellite observations of the Earth's energy budget. Our results show that models with more energy flux into the Southern Hemisphere atmosphere (at the top of the atmosphere and at the surface) tend to have a stronger double-ITCZ bias, consistent with recent theoretical studies that suggest that the ITCZ is drawn toward heating even outside the tropics. In particular, we find that cloud biases over the Southern Ocean explain most of the model-to-model differences in the amount of excessive precipitation in Southern Hemisphere tropics, and are suggested to be responsible for this aspect of the double-ITCZ problem in most global climate models.

Local and remote responses to opposite Ross Sea ice anomalies: a numerical experiment with the CPTEC/INPE AGCM

This work examines the near-surface responses, in the Southern Hemisphere atmosphere, to a reduction and an excess of sea ice cover in the Ross Sea. Large ensembles (60 members) of the Atmospheric General Circulation Model of the Center for Weather Forecast and Climate Studies (AGCM-CPTEC) are used for this purpose. Locally, when the sea ice cover decreases (increases), heat fluxes, temperature, and pressure increase (decreases). A principal component analysis of temperature and geopotential height identifies the potential remote connections. We have found three main results: first, an association between the sea ice cover in the Ross Sea and a wave train that resembles the Pacific South American (PSA) pattern; second, a relationship between temperatures in the Ross Sea and in the southern South America; and third, an apparent correspondence between reduced sea ice cover in the Ross Sea and the subtropical jet over Australia.

The effect of Antarctic sea ice on the Southern Hemisphere atmosphere during the southern summer

This study examines the influence of Antarctic sea ice distribution on the large scale circulation of the Southern Hemisphere using a fully coupled GCM where the sea ice submodel is replaced by a climatology of observed extremes in sea ice concentration. Three 150-year simulations were completed for maximum, minimum and average sea ice concentrations and the results for the austral summer (January–March) were compared using the surface temperatures forced by the sea ice distributions as a filter for creating the composite differences. The results indicate that in the austral summer the polar cell expands (contracts) under minimum (maximum) sea ice conditions with corresponding shifts in the midlatitude Ferrell cell. We suggest that this response occurs because sea ice lies in the margin between the polar and midlatitude cells. The polarity of the Southern Hemisphere Annular (SAM) mode is also influenced such that when sea ice is at a minimum (maximum) the polarity of the SAM tends to be negative (positive).

Extension of the Southern Hemisphere Atmospheric Radiocarbon Curve, 2120–850 Years BP: Results from Tasmanian Huon Pine

Decadal samples of dendrochronologically dated pine (Lagorostrobos franklinii) from the Stanley River basin, Tasmania, have been radiocarbon dated between 2120–850 yr BP. This data set overlaps and extends the current Southern Hemisphere record, which covers the period 110–995 yr BP. There is good agreement between the 2 records between 995–850 yr BP, between sample replicates and with consensus values for standards. As in the younger data set, we find evidence for a distinct but variable offset between the Southern Hemisphere data and IntCal04; although this is likely due to real temporal variability in the interhemispheric offset, further work is planned to rule out possible laboratory or sample preparation differences.