Early during Titan’s southern fall, images captured by Cassini’s Imaging Science Subsystem (ISS) revealed the formation of a large cloud above Titan’s south pole. Subsequent analysis of Cassini data revealed the cloud contained HCN ice, but the cloud’s evolution has not been examined. We reviewed imagery of Titan’s south pole between 2012 and the end of mission at Ls=93°. We find evidence of cloud formation as early as Ls=32° (April 2012), one terrestrial month earlier than previously reported, after which the cloud persists until Ls=79° (mid 2016). The cloud top altitude descended from about 320 km at Ls=32° to below 230 km by Ls=79°, at which point it became obscured by atmospheric methane absorption. The cloud also grew laterally; initially confined poleward of 81°S, by Ls=75° the cloud extended as far as 64.5°S. These measurements place new constraints on Titan’s polar stratospheric temperature structure and circulation during southern fall.

Previous studies found many climate properties such as northern hemisphere (NH) surface temperature and precipitation respond non-monotonically when CO2 is increased from 1x to 8xCO2 relative to pre-industrial levels. Here, we explore the robustness of the non-monotonicity in NH precipitation response in 11 coupled climate models. Eight models show a decrease of NH precipitation under repeated CO2 doubling beyond a critical level, which varies from 2xCO2 to 8xCO2. These models also show a robust cooling associated with the North Atlantic warming hole, and a prominent weakening in the Atlantic meridional overturning circulation (AMOC) at the same critical CO2 level. The sensitivity of NH precipitation and AMOC to CO2 doublings are positively correlated, especially when the AMOC weakens beyond 10 Sv. Regionally, this decrease in NH precipitation is most prominent over the North Atlantic, Europe and the tropical Pacific.

Examination of historical simulations from CMIP6 models shows substantial pre-industrial to present-day changes in ocean heat (ΔH), salinity (ΔS), oxygen (ΔO2), dissolved inorganic carbon (ΔDIC), chlorofluorocarbon-12 (ΔCFC12), and sulfur hexafluoride (ΔSF6). The spatial structure of the changes and the consistency among models differ among tracers: ΔDIC, ΔCFC12, and ΔSF6 all are largest near the surface, are positive throughout the thermocline with weak changes below, and there is good agreement amongst the models. In contrast, the largest ΔH, ΔS, and ΔO2 are not necessarily at the surface, their sign varies within the thermocline, and there are large differences among models. These differences between the two groups of tracers are linked to climate-driven changes in the ocean transport, with this tracer “redistribution” playing a significant role in changes in ΔH, ΔS, and ΔO2 but not the other tracers. Tracer redistribution is prominent in the southern subtropics, in a region where apparent oxygen utilization and ideal age indicate increased ventilation time scales. The tracer changes are linked to a poleward shift of the peak Southern Hemisphere westerly winds, which causes a similar shift of the subtropical gyres, and anomalous upwelling in the subtropics. This wind - tracer connection is also suggested to be a factor in the large model spread in some tracers, as there is also a large model spread in wind trends. A similar multi-tracer analysis of observations could provide insights into the relative role of the addition and redistribution of tracers in the ocean.

A document by Xiyue Zhang. Click on the document to view its contents.

Southern Hemisphere (SH) Stratospheric Sudden Warmings (SSWs) result in smaller Antarctic ozone holes and are linked to extreme midlatitude weather on subseasonal to seasonal timescales. Therefore, it is of interest how often such events occur and whether we should expect more events in the future. Here, we use a pair of novel multi-millennial simulations with a stratosphere-resolving coupled ocean-atmosphere climate model to show that the frequency of SSWs, such as observed 2002 and 2019, is about one in 22 years for 1990 conditions. In addition, we show that we should expect the frequency of SSWs - and that of more moderate vortex weakening events - to strongly decrease by the end of this century.

The upper-level jet stream impacts surface-level trace gas variability, yet the cause of this relationship remains unclear. We investigate the mechanism(s) responsible for the relationship using idealized tracers with different source regions within a chemical transport model. All tracers’ daily variabilities are correlated with the meridional position of the jet in the mid-latitudes, but tracers emitted south (north) of the jet increase (decrease) in the mid-latitudes when the jet is shifted poleward. The jet stream regulates the near-surface meridional wind, and this coupling together with the meridional tracer gradient robustly predicts where the jet stream and tracers are in and out of phase. Our study elucidates a major driver of trace gas variability and links it to the location of the jet stream and emissions. These results are useful for understanding changes in trace gas variability if the jet stream’s position or major emission source regions change in the future.

We investigate the relationships among summertime ozone (O3), temperature, and humidity on daily timescales across the Northern Hemisphere using observations and model simulations. Temperature and humidity are significantly positively correlated with O3 across continental regions in the mid-latitudes (~35-60N). Over the oceans, the relationships are consistently negative. For continental regions outside the mid-latitudes, the O3-meteorology correlations are mixed in strength and sign but generally weak. Over some high latitude, low latitude, and marine regions, temperature and humidity are significantly anticorrelated with O3. Daily variations in transport patterns linked to the position and meridional movement of the jet stream drive the relationships among O3, temperature, and humidity. Within the latitudinal range of the jet, there is an increase (decrease) in O3, temperature, and humidity over land with poleward (equatorward) movement of the jet, while over the oceans poleward movement of the jet results in decreases of these fields. Beyond the latitudes where the jet traverses, the meridional movement of the jet stream has variable or negligible effects on surface-level O3, temperature, and humidity. The O3-meteorology relationships are largely the product of the jet-induced changes in the surface-level meridional flow acting on the background meridional O3 gradient. Our results underscore the importance of considering the role of the jet stream and surface-level flow for the O3-meteorology relationships, especially in light of expected changes to these features under climate change.

The response of the ventilation of mode and intermediate waters to abrupt changes in the Southern Annular Mode (SAM) is examined by analyzing the ideal age in a global ocean-sea ice model. The age response is shown to differ between the central Pacific Ocean and other basins. In the central Pacific there are large decreases in the age of subtropical mode and intermediate waters associated with a more positive SAM, contrasting only small age changes in the Atlantic and Indian Oceans, except near where intermediate water density surfaces outcrop. These interbasin differences hold for simulations at different horizontal resolutions, and can be explained by the combination of zonal variations in wind stress changes associated with the SAM, and differences in the age response to an increase or shift in the wind stress. These results suggest that the carbon and heat uptake associated with the SAM will likely vary between ocean basins.

The positive trend of the Southern Annular Mode (SAM) will impact the Southern Ocean’s role in Earth’s climate, however the details of the Southern Ocean’s response remain uncertain. We introduce a methodology to examine the influence of SAM on the Southern Ocean, and apply this method to a global ocean–sea-ice model run at three resolutions (1$^{\circ}$, 1/4$^{\circ}$ and 1/10$^{\circ}$). Our methodology drives perturbation simulations with realistic atmospheric forcing of extreme SAM conditions. The thermal response agrees with previous studies; positive SAM perturbations warm the upper ocean north of the windspeed maximum and cool it to the south, with the opposite response for negative SAM. The overturning circulation exhibits a rapid response that increases/decreases for positive/negative SAM perturbations and is insensitive to model resolution. The longer term adjustment of the overturning circulation, however, depends on the representation of eddies, and is faster at higher resolutions.

We introduce a novel wind-derived metric to quantify variability in the Southern Ocean overturning circulation. This metric, which we call the Ekman streamfunction, integrates the Ekman pumping vertical velocity zonally and northwards from the Antarctic coastline to a given latitude. To evaluate the relationship between the Ekman streamfunction and Southern Ocean overturning circulation, we use a global 0.1 ocean–sea-ice model driven with interannual forcing (1958-2018). Throughout much of the Southern Ocean, strong correlations (r>0.9) exist between the Ekman streamfunction and the Southern Ocean overturning circulation on monthly and annual timescales. A regression analysis identifies regions where Ekman streamfunction variability coincides with >4Sv changes in the overturning; one such location is where the wind stress curl changes sign and the Ekman pumping velocity is highly variable. This study offers a new approach to infer recent changes in the Southern Ocean overturning circulation from existing datasets of wind stress.

The Southern Ocean (SO) provides the largest oceanic sink of carbon. Observational datasets highlight decadal-scale changes in SO CO2 uptake, but the processes leading to this decadal-scale variability remain debated. Here, using an eddy-permitting ocean, sea-ice, carbon cycle model, we explore the impact of changes in Southern Hemisphere (SH) westerlies on contemporary (i.e. total), anthropogenic and natural CO2 fluxes using idealised sensitivity experiments as well as an interannually varying forced (IAF) experiment covering the years 1948 to 2007. We find that a strengthening of the SH westerlies reduces the contemporary CO2 uptake by leading to a high southern latitude natural CO2 outgassing. The enhanced SO upwelling and associated increase in Antarctic Bottom Water decrease the carbon content at depth in the SO, and increase the transport of carbon-rich waters to the surface. A poleward shift of the westerlies particularly enhances the CO2 outgassing south of 60S, while inducing an asymmetrical DIC response between high and mid southern latitudes. Changes in the SH westerlies in the 20th century in the IAF experiment lead to decadal-scale variability in both natural and contemporary CO2 fluxes. The ~10% strengthening of the SH westerlies since the 1980s led to a 0.016 GtC/yr^2 decrease in natural CO2 uptake, while the anthropogenic CO2 uptake increased at a similar rate, thus leading to a stagnation of the total SO CO2 uptake. The projected poleward strengthening of the SH westerlies over the coming century will thus reduce the capability of the SO to mitigate the increase in atmospheric CO2.

Many studies have documented the trends in the latitudinal position and strength of the midlatitude westerly jet in the Southern Hemisphere. However, very little attention has been paid to the longitudinal variations of these trends. Here, we specifically focus on the zonal asymmetries in the southern jet trends between 1980-2018. Meteorological reanalyses show a robust strengthening and an equatorward shift of the annual-mean and springtime jet over the Pacific sector, in contrast to a weaker strengthening and poleward shift over the Atlantic and Indian Ocean sectors. The reanalysis trends fall within the ensemble spread for historical climate model simulations, showing that climate models are able to capture the observed trends. Climate model simulations indicate that the differential movement of the jet is a manifestation of internal variability and is not a forced response. Implications of these asymmetries for other components of the climate system are discussed.