High-latitude Southern Hemisphere Research Articles

Potential Vorticity Circulation across the Boundary of the Northern Troposphere and Its Link with East Asian Summer Monsoon in July

Abstract Based on the potential vorticity (PV) theory and employing statistics and diagnostics, this study investigated the characteristics of the PV substance (PVS) and PV circulation (PVC) and the relationship between the PVC across the equator and tropopause in the Northern Hemisphere (NH) and the East Asian summer monsoon (EASM) in July. The results indicate that the elevation of the thermal tropopause produces a prominent downward slope in the midlatitudes, with stronger absolute PVS in the stratosphere. The wind-shear-related PVC penetrates the tropopause downward over the NH high latitudes, travels southward in general to the Southern Hemisphere (SH), penetrates the tropopause upward over the SH high latitudes, and finally travels northward to the northern stratosphere. The cross-equatorial PVC exhibits a layered structure from the surface to the tropopause depending on the vertical shear of the zonal wind. The cross-tropopause PVC has strong meridional gradients over the northern Tibetan Plateau (TP). The combined effects of the PVC across the tropopause and equator can trigger the adjustment of the tropospheric internal PVC, resulting in PVS and circulation redistribution within the NH. Abnormal temperature advection, moisture convergence, and vertical motion were generated, forming a meridional seesaw of precipitation over the East Asian summer monsoon area. Attribution diagnoses revealed that the tropical sea surface temperature anomaly contributed significantly to the cross-equatorial PVC, whereas the meridional gradient of the PVS over the northwestern TP contributes more to the cross-tropopause PVC. Their joint effects generate a precipitation seesaw in the EASM in July.

Landscape burning facilitated Aboriginal migration into Lutruwita/Tasmania 41,600 years ago.

The establishment of Tasmanian Palawa/Pakana communities ~40 thousand years ago (ka) was achieved by the earliest and farthest human migrations from Africa and necessitated migration into high-latitude Southern Hemisphere environments. The scarcity of high-resolution paleoecological records during this period, however, limits our understanding of the environmental effects of this pivotal event, particularly the importance of using fire as a tool for habitat modification. We use two paleoecological records from the Bass Strait islands to identify the initiation of anthropogenic landscape transformation associated with ancestral Palawa/Pakana land use. People were living on the Tasmanian/Lutruwitan peninsula by ~41.6 ka using fire to penetrate and manipulate forests, an approach possibly used in the first migrations across the last glacial landscape of Sahul.

Spurious Trends in High Latitude Southern Hemisphere Precipitation Observations

AbstractThe high latitude Southern Hemisphere (SH) is an important region for Earth's climate. Ocean heat content, cryosphere interactions, Antarctic bottom water development and the cloud‐albedo feedbacks need to be understood to form a complete picture of the climate system. However, the high latitude SH is one of the most under‐observed regions due to its remoteness. The advent of satellites and reanalyses have improved our monitoring of this region. Some previous studies observed an increase in precipitation over the SH high latitudes, however we argue that some of the trends in commonly used data sets may be artifacts. We use regression analysis of trends in precipitation and the Southern Annular Mode to contrast these relationships in satellite and reanalysis products, and to evaluate precipitation over the SH. We suggest that sensor changes and the lack of in situ data available for calibration may be responsible for unusual precipitation patterns especially around 65°S.

Southern Hemisphere atmospheric history of carbon monoxide over the late Holocene reconstructed from multiple Antarctic ice archives

Abstract. Carbon monoxide (CO) is a naturally occurring atmospheric trace gas, a regulated pollutant, and one of the main components determining the oxidative capacity of the atmosphere. Evaluating climate–chemistry models under different conditions than today and constraining past CO sources requires a reliable record of atmospheric CO mixing ratios ([CO]) that includes data since preindustrial times. Here, we report the first continuous record of atmospheric [CO] for Southern Hemisphere (SH) high latitudes over the past 3 millennia. Our continuous record is a composite of three high-resolution Antarctic ice core gas records and firn air measurements from seven Antarctic locations. The ice core gas [CO] records were measured by continuous flow analysis (CFA), using an optical feedback cavity-enhanced absorption spectrometer (OF-CEAS), achieving excellent external precision (2.8–8.8 ppb; 2σ) and consistently low blanks (ranging from 4.1±1.2 to 7.4±1.4 ppb), thus enabling paleo-atmospheric interpretations. Six new firn air [CO] Antarctic datasets collected between 1993 and 2016 CE at the DE08-2, DSSW19K, DSSW20K, South Pole, Aurora Basin North (ABN), and Lock-In sites (and one previously published firn CO dataset at Berkner) were used to reconstruct the atmospheric history of CO from ∼1897 CE, using inverse modeling that incorporates the influence of gas transport in firn. Excellent consistency was observed between the youngest ice core gas [CO] and the [CO] from the base of the firn and between the recent firn [CO] and atmospheric [CO] measurements at Mawson station (eastern Antarctica), yielding a consistent and contiguous record of CO across these different archives. Our Antarctic [CO] record is relatively stable from −835 to 1500 CE, with mixing ratios within a 30–45 ppb range (2σ). There is a ∼5 ppb decrease in [CO] to a minimum at around 1700 CE during the Little Ice Age. CO mixing ratios then increase over time to reach a maximum of ∼54 ppb by ∼1985 CE. Most of the industrial period [CO] growth occurred between about 1940 to 1985 CE, after which there was an overall [CO] decrease, as observed in Greenland firn air and later at atmospheric monitoring sites and attributed partly to reduced CO emissions from combustion sources. Our Antarctic ice core gas CO observations differ from previously published records in two key aspects. First, our mixing ratios are significantly lower than reported previously, suggesting that previous studies underestimated blank contributions. Second, our new CO record does not show a maximum in the late 1800s. The absence of a [CO] peak around the turn of the century argues against there being a peak in Southern Hemisphere biomass burning at this time, which is in agreement with (i) other paleofire proxies such as ethane or acetylene and (ii) conclusions reached by paleofire modeling. The combined ice core and firn air [CO] history, spanning −835 to 1992 CE, extended to the present by the Mawson atmospheric record, provides a useful benchmark for future atmospheric chemistry modeling studies.

Limited change in silicate chemical weathering intensity during the Permian–Triassic transition indicates ineffective climate regulation by weathering feedbacks

Vast emissions of volcanogenic greenhouse gases resulted in rapid global warming during the latest Permian mass extinction (LPME, ∼252 Ma), the impact of which on continental chemical weathering conditions was complex. Here, we compile Chemical Index of Alteration (CIA) records from 25 terrestrial and 12 marine sections with a near-global paleogeographic distribution to reconstruct the evolution of chemical weathering intensity of silicates during the LPME. This compilation shows that the CIA profiles, except those from the Australian Sydney and Bowen basins of the high-latitude Southern Hemisphere, exhibit either indiscernible changes or decreases through the LPME interval, contrary to previous inferences of increased silicate chemical weathering intensity. The lack of major changes in silicate chemical weathering intensity is further supported by the near uniformity of clay mineral assemblages through the LPME interval, with only minor increases of illite and chlorite and, if present, decreases in kaolinite content in some terrestrial sections. We infer a shift from dominantly transport-limited to kinetic-limited weathering regimes, likely caused by environmental climate factors such as widespread devegetation, river-flow changes (i.e., toward flashier and more intense discharge), and aridification during the earliest Triassic that effectively reduced water availability on land for water/rock reactions. Combined with limited changes in chemical weathering fluxes from previous estimates, our data imply possibly ineffective climate regulation by silicate chemical weathering at a (near-)global scale, despite high erosion rates, amid the intense global warming that followed the LPME.

Periodicity of the Southern Annular Mode in Southern Patagonia, insight from the Lago Argentino varve record

Climatic variability across a large fraction of the Southern Hemisphere is controlled by the Southern Annular Mode and associated latitudinal shifts in the Southern Westerly Wind belt. In Patagonia, these changes control the large-scale temperature and precipitation trends – and resulting glacier surface mass balance. Our understanding of recent changes in this climatic oscillation is, however, limited by the number of paleo-environmental records in the mid to high-latitude Southern Hemisphere. Here, we first use a synthetic proxy record to demonstrate that periodicity may be preserved in a wider range of records than can be used for quantitative paleoclimatic reconstructions. We then analyze a 5000-year-long sedimentation record derived from Lago Argentino, a 1500 km2 ice-contact lake in Southern Patagonia. We extract a mass accumulation rate and greyscale pixel intensity record from 28 cores across all of Lago Argentino's main depositional environments. We align the mass accumulation rate and pixel intensity records to a common time axis through multivariate dynamic-time-warping, and investigate their spectral properties using the multi-taper Lomb Scargle periodogram. We find statistically significant spectral peaks at 200 ± 20, 150 ± 16, and 85 ± 9 years in two thirds of mass accumulation rate and one third of the pixel intensity records. These periodicities reveal the centennial periodicity of the Southern Annular Mode, which is the key climatic driver of sedimentation at Lago Argentino.

Analysis of Temperature Semi-Annual Oscillations (SAO) in the Middle Atmosphere

The middle atmosphere plays an important role in the research of various dynamical and energy processes. Microwave Limb Sounder (MLS), reanalyses and model simulations with NCAR’s Whole Atmosphere Community Climate Model (WACCM) data in the range between 100 and 0.1 hPa from 2005 to 2020 have been analyzed with a focus on the temperature semi-annual oscillations (SAO). Significant SAO of temperature is prominent in the tropical region (20°S–20°N) around 1–3 hPa, which is consistent with previous studies. We also found significant SAO in the northern hemisphere (NH) high latitudes between 8 and 0.3 hPa and southern hemisphere (SH) high latitudes between 0.5 and 0.1 hPa, which has been of less concern in previous studies. The thermal budget based on MERRA2 and simulations is used to explain the mechanism of SAO in the middle atmosphere. In the tropics, the two temperature peaks are mainly determined by radiative processes. In the NH high latitudes of the stratosphere, the temperature peak in January is mainly related to dynamical processes, while the temperature peak in July is determined by a combination of dynamical and radiative processes. In the NH high latitudes of the lower mesosphere, the first peak in June is primarily associated with dynamical and radiative processes, while the second peak in December is primarily associated with the dynamical processes. In the SH high latitudes of the lower mesosphere, the first temperature peak in July is mainly due to dynamical processes while the second temperature peak in December is mainly due to radiative processes. Various features are present in the SH and NH high latitude SAO in the lower mesosphere. Furthermore, we performed model simulations with and without SAO in sea surface temperatures (SST-SAO) to study the connection between SST and temperature SAO. WACCM6 results indicate that the SAO in the middle atmosphere is partially affected by the existence of an SST-SAO. By removing SAO in SST, the PSD magnitude of the SAO decreases in the tropical region and increases in the polar region. The amplitudes of total heating rates are also modified. The WACCM experiment confirms the relationship between SST-SAO and temperature SAO in the middle atmosphere.

Hydrochemical and isotopic baselines for understanding hydrological processes across Macquarie Island.

Isotopic and hydrochemical data from lakes provide direct information on catchment response to changing rainfall, evaporation, nutrient cycling, and the health of ecosystems. These techniques have not been widely applied to lakes in the Southern Hemisphere high latitudes, including Southern Ocean Islands (SOIs) experiencing rapid, significant shifts in climate. Historical work has highlighted the localised nature of geochemical drivers in controlling the hydrochemical evolution of lakes, such as geology, sea spray contribution, vegetation, geographical location, and ice cover extent. The role of groundwater in lake hydrology and hydrochemistry has not been identified until now, and its omission will have major implications for interpreting soil-water-air processes affecting lakes. Here we present the first comprehensive, island-wide hydrochemical and isotopic survey of lakes on a SOI. Forty lakes were examined across Macquarie Island, using comparable methods to identify key environmental processes and their geochemical drivers. Methods include stable carbon (δ13CDOC: dissolved organic carbon and δ13CDIC: dissolved inorganic carbon), oxygen (δ18O), hydrogen (δ2H) and strontium isotopic ratios (87Sr/86Sr) in water. These provide essential baseline data for hydrological, biological, and geochemical lake processes. Lakes on the western side of the island are influenced by sea spray aerosols. In general, it was found that lakes at higher elevations are dilute and those located in lower elevation catchments have experienced more water-rock interactions. The hydrochemical and isotopic tracers suggest that lakes in lower elevations contain more terrestrial sourced ions that may be contributed from groundwater. Increasing temperatures and changing rainfall patterns predicted for the region will lead to shifts in nutrient cycles, and impact the island's unique ecosystems. Future research will focus on long-term monitoring to understand seasonal, annual, and long-term variability to test fundamental hypotheses concerning ecosystem function and the consequences of environmental change on SOIs.

Impacts of Aeolus horizontal Line‐Of‐Sight (HLOS) wind assimilation on the Korean integrated model (KIM) forecast system

AbstractThe Korean Integrated Model (KIM) forecast system, based on a hybrid four‐dimensional ensemble‐variational method, was extended to assimilate Horizontal Line‐Of‐Sight (HLOS) wind observations from the Atmospheric Laser Doppler Instrument (ALADIN) on board the Atmospheric Dynamic Mission Aeolus satellite. In a global cycling experiment, assimilation of Aeolus HLOS wind observations led to reductions in the average root‐mean‐square error of 0.8 and 0.5% for the zonal and meridional wind analyses when compared against European Center for Medium‐Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS) analyses. Even though the observed variable is wind, there was also an overall beneficial impact on analyses of the mass variables. In the Southern Hemisphere (SH), the reduced analysis errors led to forecast skill improvements out to 72 h. In contrast, in the Northern Hemisphere (NH) there was relatively little reduction of analysis errors, but wind forecasts were nevertheless improved, and these positive impacts lasted longer – out to 120 h rather than 72 h. Experiments suggest that the relatively poor long‐range performance in the SH high latitudes was due to problems with the mass increments derived from Aeolus wind increments via the ensemble‐based part of the hybrid background error covariance (B), which eventually led to adverse effects on the wind variables as forecasts progressed in the SH. This study shows that it is necessary to estimate the ensemble B in the Antarctic region with its high elevation more accurately in order to effectively use the Aeolus observation information.

Meridional Propagation of Carbon Dioxide (CO2) Growth Rate and Flux Anomalies From the Tropics Due to ENSO

AbstractEl Niño Southern Oscillation (ENSO) influence on carbon dioxide (CO2) growth rates are not spatially uniform or simultaneous around the globe. Using atmospheric CO2 observations and atmospheric chemistry‐transport model (ACTM), we show that anomalies in CO2 fluxes and growth rate originate in the tropics, as an effect of ENSO. The CO2 anomalies are then propagated from the equator toward the poles with asymmetric delays. A maximum delay of about 8 and 4 months are found at the northern hemisphere (NH) and southern hemisphere (SH) high latitudes, respectively. This asymmetric time delay is because of the CO2 flux anomaly mainly originate in the tropical SH land and transport of SH air into the NH is slower than that for NH air into the SH. The poleward increase of time delay is more homogeneous in the upper troposphere, as per the ACTM simulations, but observational evidence suffers from gaps in long‐term measurements.

Responses of East Asian Winter Monsoon‐Australian Summer Monsoon to Local and Remote Orbital Forcing During Holocene

AbstractHere, we perform a set of sensitivity experiments in a climate model to quantify the impacts of local and remote insolation changes on the East Asian winter monsoon (EASM) and Australian summer monsoon (AusSM) at early and middle Holocene relative to the present. We show that the EAWM during Holocene is enhanced, driven overwhelmingly by the local insolation change in the low to mid‐latitude Northern Hemisphere (NH) through its impacts on land‐sea thermal contrast, with the NH high latitude forcing playing a negligible role. In contrast, the AusSM is also enhanced, opposite to the local insolation forcing, because the local forcing impact is overwhelmed by that from the remote forcing of both NH low latitude and Southern Hemisphere high latitude, with the latter dominating in the mid‐Holocene.

Can current reanalyses accurately portray changes in Southern Annular Mode structure prior to 1979?

Early reanalyses are less than optimal for investigating the regional effects of ozone depletion on Southern Hemisphere (SH) high-latitude climate because the availability of satellite sounder data from 1979 significantly improved their accuracy in data sparse regions, leading to a coincident inhomogeneity. To determine whether current reanalyses are better at SH high-latitudes in the pre-satellite era, here we examine the capabilities of the European Centre for Medium-range Weather Forecasts (ECMWF) fifth generation reanalysis (ERA5), the Twentieth Century Reanalysis version 3 (20CRv3), and the Japanese Meteorological Agency (JMA) 55-year reanalysis (JRA-55) to reproduce and help explain the pronounced change in the relationship between the Southern Annular Mode (SAM) and Antarctic near-surface air temperatures (SAT) between 1950 and 1979 (EARLY period) and 1980–2020 (LATE period). We find that ERA5 best reproduces Antarctic SAT in the EARLY period and is also the most homogeneous reanalysis across the EARLY and LATE periods. ERA5 and 20CRv3 provide a good representation of SAM in both periods with JRA-55 only similarly skilful in the LATE period. Nevertheless, all three reanalyses show the marked change in Antarctic SAM-SAT relationships between the two periods. In particular, ERA5 and 20CRv3 demonstrate the observed switch in the sign of the SAM-SAT relationship in the Antarctic Peninsula: analysis of changes in SAM structure and associated meridional wind anomalies reveal that in these reanalyses positive SAM is linked to cold southerly winds during the EARLY period and warm northerly winds in the LATE period, thus providing a simple explanation for the regional SAM-SAT relationship reversal.

Spatial Distribution of Aerosol Characteristics over the South Atlantic and Southern Ocean Using Multiyear (2004–2021) Measurements from Russian Antarctic Expeditions

Since 2004, we have carried out yearly measurements of physicochemical aerosol characteristics onboard research vessels at Southern Hemisphere high latitudes (34–72° S; 45° W–110° E). In this work, we statistically generalize the results from multiyear (2004–2021) measurements in this area of the aerosol optical depth (AOD) of the atmosphere, concentrations of aerosol and equivalent black carbon (EBC), as well as the ionic composition of aerosol. A common regularity was that the aerosol characteristics decreased with increasing latitude up to the Antarctic coast, where the aerosol content corresponded to the global background level. Between Africa and Antarctica, AOD decreased from 0.07 to 0.024, the particle volume decreased from 5.5 to 0.55 µm3/cm3, EBC decreased from 68.1 to 17.4 ng/m3, and the summed ion concentration decreased from 24.5 to 2.5 µg/m3. Against the background of the common tendency of the latitude decrease in aerosol characteristics, we discerned a secondary maximum (AOD and ion concentrations) or a plateau (aerosol and EBC concentrations). The obtained spatial distribution of aerosol characteristics qualitatively agreed with the model-based MERRA-2 reanalysis data, but showed quantitative differences: the model AOD values were overestimated (by 0.015, on average); while the EBC concentrations were underestimated (by 21.7 ng/m3). An interesting feature was found in the aerosol spatial distribution in the region of Antarctic islands: at a distance of 300 km from the islands, the concentrations of EBC decreased on average by 29%, while the aerosol content increased by a factor of 2.5.

Enhanced hydrological cycle during Oceanic Anoxic Event 2 at southern high latitudes: New insights from IODP Site U1516

The Oceanic Anoxic Event 2 (OAE 2) represents one of the most profound global environmental disturbances of the Mesozoic Era, which was associated with a positive carbon isotope excursion due to widespread organic carbon burial. However, the character, evolutionary history, and exact driving mechanisms of OAE 2 are still much debated, exacerbated by the lack of records from climate-sensitive southern high latitudes. Here we present a multi-proxy record of a stratigraphically continuous succession spanning the late Cenomanian to early Turonian at the International Ocean Discovery Program Site U1516 in the Mentelle Basin off southwest Australia, which was located at southern high latitudes (~ 60–62°S) during the Late Cretaceous. Sedimentary records at Site U1516 allow the first detailed insights into source-to-sink processes in the Mentelle Basin and associated paleoenvironmental reconstruction in the southern Indian Ocean during OAE 2. Rare earth element abundances and clay-bound neodymium isotopes of the siliciclastic sediment fractions indicate that southwest Australia was the main detrital source of sediment at Site U1516 during OAE 2, with volcanogenic sediments representing only a minor component. A major provenance shift is observed across OAE 2, indicating an increased sediment contribution from distant sources (i.e., Yilgarn Craton and Albany-Fraser Orogen) relative to proximal sources (i.e., Perth Basin and Leeuwin Block) in southwest Australia, interpreted as reflecting a major reorganization of the drainage system due to enhanced fluvial runoff. Based on these findings, we infer that the OAE 2 interval showing the most prominent δ13C excursion was associated with an intensification of the hydrological cycle in the high-latitude southern hemisphere, with potential impacts on regional ocean chemistry and marine productivity. In particular, we suggest that enhanced terrigenous input and riverine nutrient export likely played a key role in controlling siliceous productivity at nearby ocean margins. As a consequence, the intense burial of organic matter (i.e., black shale) at Site U1516, which occurred episodically throughout OAE 2, possibly resulted from the combination of enhanced riverine-driven marine productivity and increased burial efficiency associated with the development of anoxic/euxinic bottom water conditions.

Zonal wave 3 pattern in the Southern Hemisphere generated by tropical convection

A distinctive feature of the Southern Hemisphere extratropical atmospheric circulation is the quasi-stationary zonal wave 3 pattern. This pattern is present in both the mean atmospheric circulation and its variability on daily, seasonal and interannual timescales. While the zonal wave 3 pattern has substantial impacts on meridional heat transport and Antarctic sea ice extent, the reason for its existence remains uncertain, although it has long been assumed to be linked to the presence of three major landmasses in the Southern Hemisphere extratropics. Here we use an atmospheric general circulation model to show that the stationary zonal wave 3 pattern is instead driven by zonally asymmetric deep convection in the tropics, with little influence from extratropical orography or landmasses. Localized regions of deep convection in the tropics form a local Hadley cell, which in turn creates a wave source in the subtropics that excites a poleward- and eastward-propagating wave train, forming quasi-stationary waves in the Southern Hemisphere high latitudes. Our findings suggest that changes in tropical deep convection, either due to natural variability or climate change, fundamentally control the zonal wave 3 pattern, with implications for southern high-latitude climate, ocean circulation and sea ice. The zonal wave 3 circulation pattern in the Southern Hemisphere is driven by tropical convection, according to results from an atmospheric general circulation model.

Changes in extreme ocean wave heights under 1.5 °C, 2 °C, and 3 °C global warming

This study inspects the global changes in seasonal extreme ocean wave heights under different levels of global warming (1.5 °C, 2 °C, and 3 °C) based on statistical wave projections derived from CMIP5 multi-model simulations. The results show robust increases in wave extremes up to 15% (∼1 m) over Southern Hemisphere high latitudes and tropical Pacific, particularly at 3 °C warming. Strong seasonality is observed, especially for the North Pacific. Under higher warming, stronger increases are identified in both amplitude and area of extreme wave heights. The change in magnitude translates into shorter return intervals of extreme wave events in a warmer world, particularly at 3 °C warming. Differences between 1.5 °C and 2 °C worlds reveal potential benefits of limiting global warming over large regions of global ocean. Strong inter-model relationships indicate that wave height increases are associated with intensified climate mode variability, particularly the Southern Annular Mode and El Niño–Southern Oscillation, in a warmer world. An important implication is the potential impact of increased wave extremes on West Antarctic ice shelves with respect to calving and associated loss of buttressing, which would facilitate sea level rise in a warmer world.

Generation of the Amundsen Sea Low by Antarctic Orography

AbstractThe Amundsen Sea Low (ASL) is a distinctive feature of the Southern Hemisphere high latitude atmospheric circulation, regulating regional Antarctic climate, meridional heat transport, ocean circulation, and sea‐ice in the Amundsen‐Bellingshausen Seas. Most previous research on the ASL has focused on its variability with only a few studies attempting to understand why the climatological ASL exists. These studies have proposed different hypotheses to explain the presence of the ASL, however, a clear understanding of the mechanisms responsible for the generation of the ASL remains uncertain. Here we use an atmospheric general circulation model to show that the ASL is a consequence of the interaction between Antarctic topography and the westerly wind jet, with negligible influence from low‐latitude teleconnections. A nonrotating fluid flow simulation further suggests that the ASL can be explained by flow separation resulting from the interaction of westerly winds with the topography of Antarctica.

Polar Phasing and Cross‐Equatorial Heat Transfer Following a Simulated Abrupt NH Warming of a Glacial Climate

AbstractA 150‐ to 220‐year lag between abrupt Greenland warming and maximum Antarctic warming characterizes past glacial Dansgaard‐Oeschger events. In a modeling study, we investigate how the cross‐equatorial oceanic heat transport (COHT) might drive this phasing during an abrupt Northern Hemisphere (NH) warming. We use the MITgcm in an idealized continental configuration with two ocean basins, one wider, one narrower, under glacial‐like conditions with sea ice reaching midlatitudes. An exaggerated eccentricity‐related solar radiation anomaly is imposed over 100 years to trigger an abrupt NH warming and sea‐ice melting. The Hadley circulation shifts northward in response, weakening the NH trade winds, subtropical cells, and COHT in both ocean basins. This induces heat convergence in the Southern Hemisphere (SH) ocean subsurface, from where upward heat release melts sea ice and warms SH high latitudes. Although the small‐basin meridional overturning circulation also weakens, driven by NH ice melting, it contributes at most one‐third to the total COHT anomaly, hence playing a subsidiary role in the SH and NH initial warming. Switching off the forcing cools the NH; yet heat release continues from the SH ocean subsurface via isopycnal advection‐diffusion and vertical mixing, driving further sea ice melting and high latitude warming for ~50–70 more years. A phasing in polar temperatures resembling reconstructions thus emerges, linked to changes in the subtropical cells’ COHT, and SH ocean heat storage and surface fluxes. Our results highlight the potential role of the atmosphere circulation and wind‐driven global ocean circulation in the NH–SH phasing seen in DO events.

Changes in the high-latitude Southern Hemisphere through the Eocene–Oligocene transition: a model–data comparison

Abstract. The global and regional climate changed dramatically with the expansion of the Antarctic Ice Sheet at the Eocene–Oligocene transition (EOT). These large-scale changes are generally linked to declining atmospheric pCO2 levels and/or changes in Southern Ocean gateways such as the Drake Passage around this time. To better understand the Southern Hemisphere regional climatic changes and the impact of glaciation on the Earth's oceans and atmosphere at the EOT, we compiled a database of 10 ocean and 4 land-surface temperature reconstructions from a range of proxy records and compared this with a series of fully coupled, low-resolution climate model simulations from two models (HadCM3BL and FOAM). Regional patterns in the proxy records of temperature show that cooling across the EOT was less at high latitudes and greater at mid-latitudes. While certain climate model simulations show moderate–good performance at recreating the temperature patterns shown in the data before and after the EOT, in general the model simulations do not capture the absolute latitudinal temperature gradient shown by the data, being too cold, particularly at high latitudes. When taking into account the absolute temperature before and after the EOT, as well as the change in temperature across it, simulations with a closed Drake Passage before and after the EOT or with an opening of the Drake Passage across the EOT perform poorly, whereas simulations with a drop in atmospheric pCO2 in combination with ice growth generally perform better. This provides further support for previous research that changes in atmospheric pCO2 are more likely to have been the driver of the EOT climatic changes, as opposed to the opening of the Drake Passage.

Stratospheric Ozone-induced Cloud Radiative Effects on Antarctic Sea Ice

Recent studies demonstrate that the Antarctic Ozone Hole has important influences on Antarctic sea ice. While most of these works have focused on effects associated with atmospheric and oceanic dynamic processes caused by stratospheric ozone changes, here we show that stratospheric ozone-induced cloud radiative effects also play important roles in causing changes in Antarctic sea ice. Our simulations demonstrate that the recovery of the Antarctic Ozone Hole causes decreases in clouds over Southern Hemisphere (SH) high latitudes and increases in clouds over the SH extratropics. The decrease in clouds leads to a reduction in downward infrared radiation, especially in austral autumn. This results in cooling of the Southern Ocean surface and increasing Antarctic sea ice. Surface cooling also involves ice-albedo feedback. Increasing sea ice reflects solar radiation and causes further cooling and more increases in Antarctic sea ice.