Cays on the Great Barrier Reef (GBR) are dynamic low-lying carbonate sedimentary landforms that adopt various morphologies in response to the complex interplay between physical processes and sediments. This study presents a preliminary analysis of the drivers which most strongly influence cay morphodynamic activity. Focusing on two contrasting examples from the GBR, a small unvegetated cay (Taylor Cay) and a large, vegetated cay (Masthead Island), we assessed cay morphodynamic activity against geomorphic variables and variables associated with exposure to energy. We found significant (p < 0.05) negative correlation between cay activity and significant wave height and maximum annual days of cyclone exposure, and positive correlation with geomorphic predictors (average cay area, length, width and volume). The outcomes indicate that cays are most geomorphologically active when exposed to greater hydrodynamic energy over the tidal cycle. Geomorphic characteristics including larger size can buffer against shoreline perturbations driven by hydrodynamic energy exposure. Thus, more vulnerable cays can be identified and prioritized for management using the quantitative spectrum of drivers of cay morphodynamic activity.

Intraspecific trait variation (ITV) enhances the precision of applying functional trait approaches in plant ecology. Despite its benefits, ITV is rarely considered in functional trait-based seagrass research. The goal of our research is to measure ITV in the tropical seagrass species Halodule uninervis and assess the environmental factors associated with its variation. We measured eight traits of H. uninervis collected along the Queensland coast. Statistical analyses were conducted to identify the range of ITV, determine the relationship between environmental factors and trait variation, and estimate the potential effect of ITV on ecosystem services supported by these meadows. H. uninervis exhibits a distinct dimorphic pattern of ITV, particularly evident in leaf width and rhizome diameter, with air exposure and mean sediment grain size associated with variation in these traits. These findings highlight the importance of accounting for ITV in seagrass field surveys and suggest that variation within species may influence their ecological responses and functional roles in meadows.

Cays are low-lying reef islands which support a variety of high-value ecological and cultural functions. The morphology of cays worldwide is governed by the complex interplay between sediments and hydrodynamic processes modified by the reef platform geomorphology. This study aims to identify the strongest variables influencing cay geomorphic activity across 16 cays selected as representative of the range of cay morphologies present within the Great Barrier Reef (GBR), northeastern Australia. Cay geomorphic activity was assessed using the Reef Island Geomorphic Activity Assessment metric of maximum historical overlap. Cay geomorphic activity ranged from highly dynamic (no stable core over an eight-year period) to relatively stable (78.1% of the cay remained in place over 8 years). Forty-seven variables potentially linked to cay dynamics were investigated using a principal component analysis, followed by Spearman's Rank linear correlations to quantify the strength of relationships between cay geomorphic activity and these variables. The ratio of cay area to reef area (R = 0.81) and minimum significant wave heights (R = −0.69) were significantly (p < 0.05) correlated with maximum historical overlap; cays occupying a larger proportion of their reef platform and exposed to smaller wave heights are associated with lower cay geomorphic activity. The identification of key predictors of cay geomorphic activity within the GBR facilitates the prioritisation of more detailed studies and management intervention for vulnerable cays to preserve ecological and cultural functions.

The water vapor reserve over the Tibetan Plateau, known as the "atmospheric water tower," significantly influences the spatial patterns and trends of surface water resources in the region and surroundings. However, the reserve of the "atmospheric water tower" has not been precisely quantified, and how climate forcings from three oceans (i.e., the North Atlantic, Indian, and Pacific Oceans) influence variations in the reserve remain unclear. Here, we investigate interannual variations in the reserve over the Tibetan Plateau during 1951-2022 using ERA5 data. The results show that the climatological mean reserve of the "atmospheric water tower" is (25.76 +/- 3.83) x 106 kg s-1 and the reserve gradually decreased from 1951 to 1973, but gradually increased after 1973. A major shift of the Atlantic Multidecadal Oscillation (AMO) around 1973 coincides with this trend of the reserve. We further demonstrate that climatic forcings including AMO, Indian Ocean Basin-Wide (IOBW), and El Nino-Southern Oscillation (ENSO) jointly drive the increasing trend in the reserve after 1973; however, the effect of the AMO on the reserve became relatively weaker after 1994 as the influence of ENSO became stronger. Clearly, the dominant climatic forcing for the increasing trend of the reserve has gradually changed from the North Atlantic Ocean to the Pacific Ocean over time. Our findings reveal a previously unrecognized transition in the dominant climate forcings for the reserve, moving beyond a static view to a dynamic one. We provide a new framework for predictions of regional climate and water resource changes under global warming.

Microbial communities play a significant role in maintaining the health of Great Barrier Reef (GBR) ecosystems, however, the influence of sediment composition and other environmental factors such as temperature and wave regime on microbial communities are largely unknown. Here we show how sediment composition and exposure influences bacterial communities across the inner section of the GBR (Cleveland Bay, Halifax Bay and Dunk Island) between 2016 and 2018. Sediment traps were installed and routinely deployed ( every 3 months) at eight sites in the inshore GBR and analysed for water chemistry, sediment geochemistry and organic characteristics and associated bacterial communities. Results showed a significant variation in water turbidity, sediment collection rate and geochemistry across the trap sites. Bacterial communities also significantly varied along the inner GBR, with the shift in relative abundance of Actinobacteria, Acidobacteria, Planctomycete, Verrucomicrobia and Chloroflexi being the main cause of the bacterial community dynamics. The variation in spatial patterns of bacterial communities was highly correlated with water turbidity and the geochemical characteristics of associated sediments (e.g., K, Fe, Mn, Co, Al, Cr, Ca) collected across the marine trap sites. Our findings indicate that sediment composition and collection rate (and linked water turbidity) can change the spatial patterns of bacterial communities by creating environmental gradients along the inner section of the GBR.

Recent observations from the ExoMars Trace Gas Orbiter (TGO) have revealed the presence of hydrogen chloride (HCl) in the martian atmosphere. HCl shows strong seasonality, primarily appearing during Mars' perihelion period before decreasing faster than projected from photolysis and gas‐phase chemistry. HCl profiles also display local anti‐correlation with water ice aerosol. One candidate explanation is heterogeneous chemistry. We present the first results from a heterogeneous chlorine chemistry scheme incorporated into a Mars global climate model (GCM), with atmospheric dust/water ice parameterized as an HCl source/sink respectively. Results were compared against a Mars GCM with gas‐phase only chlorine chemistry and observations from TGO's Atmospheric Chemistry Suite. We found that the heterogeneous scheme significantly improved the modeled HCl seasonal, latitudinal, and vertical distribution, supporting a crucial role for heterogeneous chemistry in Mars' chlorine cycle. Remaining discrepancies show that further work is needed to characterize the exact aerosol reactions involved.

Because of its rotation period of 243 days, Venus is considered a slowly rotating planet. However, its persistent superrotating atmospheric jets, which increase in speed from surface to cloud tops, effectively set a faster rotation speed than the surface rotation. Using the Venus Planetary Climate Model and wind measurements taken by the Pioneer Venus entry probes, we show that the Rossby radius of deformation of the atmosphere varies with height. The atmosphere falls into three circulation regimes: (1) from the surface to 20 km, the Rossby radius of deformation exceeds the planetary radius and no Rossby waves form; (2) from 20 to 50 km, the tropical Rossby radius becomes smaller than the planetary radius, and a circulation regime characterized by a superrotating equatorial jet and mid-latitude Rossby gyres appears; (3) from 50 to 70 km, the extratropical Rossby radius becomes smaller than the planetary radius, the jet develops mid-latitude maxima, and the Rossby gyres shift to high latitudes. Studies of exoplanetary circulation regimes as a function of rotation period have repeatedly shown a similar progression. While observing the circulations of exoplanets to confirm these predictions is not currently possible, the presence of different circulation regimes on Venus and their dependence on altitude could be tested by observing campaigns. Such evidence would be the first observational support for the theory connecting differences in planetary rotation periods to circulation regime transitions and would ground predictions of exoplanet circulations in a validated framework.

Study region: Great Barrier Reef catchment, north-eastern Australia Study Focus: The Great Barrier Reef (GBR) catchment of north-eastern Australia contains 35 separate river basins comprising an area of 423,000 km2. This study compiled flow data for 49 gauging stations to elucidate drivers of hydrologic patterns across the GBR catchment. We compare different methods to upscale annual flow volumes recorded at individual gauging stations to total end of system volumes for the GBR catchment. Accurate estimates of total basin discharge are essential for pinpointing sources of pollution and focussing land management strategies within the GBR. New hydrological insights for the region: Our spatial analysis revealed distinct north-south gradients in the discharge data largely related to rainfall/climate variability. The northern basins generally have higher stream discharge per unit area, lower coefficients of variation for both inter- and intra-annual discharge, higher runoff to rainfall ratios, higher baseflow contributions and fewer zero-flow days relative to basins in the south. River basins deviated from the north-south gradient due to location-specific factors such as basin size, rainfall variability and anthropogenic modification to flow regimes. We provide recommendations on the most appropriate factors to use when scaling up the gauged discharge data to represent total basin discharge. Our study systematically assesses the spatial variability in river discharge statistics across a north-south latitudinal gradient and provides insights on key drivers of hydrological processes.

Identifying possible fluvial activity during the past and near present on Mars is a key area of martian research, as the occurrence of stable surface water can be an indicator of potentially habitable locations which are of astrobiological interest. Lyot crater is an early Amazonian-aged impact crater in the northern lowlands of Mars. It has a microenvironment due to the low elevation of its interior, where fluvial features and ice-rich landforms have been identified today. We modelled the mesoscale atmospheric conditions in Lyot over the past 20 million years, as Mars’s orbital parameters changed, to identify periods in time and space where conditions were suitable for stable surface water. As well as investigating surface temperature and pressure, an assessment was also made of the impact of relative humidity and evaporation on stable surface water, and the effect of solar insolation and shadowing on the longevity of ice-rich meltwater source materials. At lower obliquities the modelled atmospheric states identify conditions when stable surface water could flow in the summer over much of the crater for a few hours per sol. At higher obliquities modelled conditions suitable for stable water were restricted to the lowest elevation areas of the crater interior. These are areas where the longest channels have been identified and are also areas where standing bodies of water may have formed as these long channels drained into topographic lows. The conditions suitable for supporting stable liquid water at the surface only occurred in the model for short periods of time, hence, an assessment was also made as to the possible behaviour of surface water during the longer intervals when conditions were not suitable. During these times surface water would have boiled or frozen, although if the depth of surface water was sufficiently deep, then a layer of ice on the surface of the water could have protected the liquid water underneath. It would then flow again when conditions were suitable and the ice cover melted or if conditions remained unsuitable the covered water feature would stagnate and disappear over time. This research supports the conclusion drawn from geomorphological analysis that stable surface water occurred in specific locations such as Lyot crater in the recent past on Mars.

Clarifying relationships between stable oxygen isotope ratios in precipitation (delta 18Op) and atmospheric circulations including the Indian Ocean Dipole (IOD) and western Pacific subtropical high (WPSH) forms the basis of paleocirculation reconstructions. However, whether the IOD and WPSH modulate interannual variations of delta 18Op remains unclear. Here, we reveal the links between the IOD/WPSH and the annual delta 18Op in southern East Asia. We found that the IOD strongly influenced annual delta 18Op before 1999 by changes in moisture supply from different transport pathways and convection. However, the link became decoupled after 1999, resulting from the transition of the IOD from a symmetric to an asymmetric pattern. In contrast, significantly enhanced WPSH emerges as an important influence on annual delta 18Op after 1999. Therefore, the IOD and WPSH alternately influence interannual variation of delta 18Op around 1999. Our findings imply that signals of IOD and WPSH should be considered in different periods to better interpret paleoclimate records.

This manuscript describes the collation of available water quality data from the freshwater reaches of surface streams within the Great Barrier Reef catchment area, northeastern Australia. This compilation represents one of the most comprehensive online datasets for historical tropical and subtropical freshwater quality around the world. We document the criteria for selection of the data and associated publications as well as the processes of data cleaning used to produce a qualitative assessment of the datasets. The final compilation includes 41 individual datasets that collectively report 466 sites and contain over 26,000 discrete water quality sample records totaling more than 350,000 unique water quality results. Finally, we outline the nuances of the data that end users need to take into account when combining them for spatial and temporal analyses. The dataset ensures that these valuable water quality data collected over the past four decades are preserved for the next generations of researchers, practitioners, management agencies and policy makers.

Introduction: The spatial and temporal variation of ozone (O3) is key to understanding ongoing atmospheric processes and transport in the martian atmosphere[1]. As a photochemically active species, O3 has been used to validate the photochemistry of short lived species and, by extension, the water vapour cycle [2,3,4,5] and as a tracer to track the global circulation[6].The Ultraviolet and Visible Spectrometer (UVIS) instrument[7], a channel of the NOMAD spectrometer suite[8], has been in orbit around Mars for over two years with near continuous nadir observations through the latter half of Mars Year(MY) 34 through to MY36. The UVIS observations provide high resolution spatial and temporal maps of O3 column abundance and provides observations at different local times.A retrieval procedure was developed, using spectral measurements over the Hartley band to obtain the O3 column abundances. The radiative transfer simulation is performed using the discrete ordinates DISORT package[9] and we employ the ‘front-end’ routines (DISORT_MULTI) developed by Mike Wolff, for studies of the martian atmosphere [1,10,11]Results: In this work we present the geographic and seasonal distribution of O3 in the martian atmosphere as measured by UVIS between LS = 148 and 360° in Mars Year 34 (April 2018 to March 2019). Seasonally the O3 distribution is consistent, with low O3 abundances in equatorial regions and higher O3 abundances at higher latitudes in the winter season. As the martian atmosphere cools through northern spring between Ls = 350°(MY34) and Ls = 90°(MY35), due to the reduced solar insolation, we observed a steady increase in equatorial O3, coinciding with the onset of the aphelion cloud belt. The equatorial O3 peaks near the northern winter solstice Ls = 90°, associated with the cooler and dryer atmosphere of the martian aphelion season before reducing again as the atmosphere warms and more water vapour enters the atmosphere.Ozone entrapment[1] is observed in large impact basins, such as the Hellas Basin and to a lesser extent Argyre Planitia. Ozone abundances measured within Hellas can be an order of magnitude higher than the surrounding regions with abundances of 20 µm-atm observed within Hellas compared to 2-5 µm-atm in the surrounding area at aphelion. Ozone entrapment is still observed during the perihelion season, with abundances within Hellas being 5-10 µm-atm.The UVIS dataset provides comprehensive seasonal coverage between latitudes ~74° N and 74° S and samples multiple local times at a giving location enabling unprecedented detail in the O3 diurnal cycle.References:[1] Clancy, R. T, et al. Icarus 266 (2016). [2] Lefèvre, F, et al. Nature 454.7207 (2008). [3] Montmessin, F and Lefèvre, F. Nature Geoscience 6.11 (2013). [4] Holmes, J,A. et al. Icarus 302 (2018). [5] Daerden, F., et al. Icarus 326 (2019). [6] Holmes, J,A. et al. Icarus 282 (2017). [7] Patel, M, R., et al. Applied Optics 56.10 (2017). [8] Neefs, Eddy, et al. Applied Optics 54.28 (2015). [9] Stamnes, Knut, et al. Applied Optics 27.12 (1988). [10] Wolff, M, J., et al. Icarus 208.1 (2010). [11] Wolff, M, J., et al. Icarus 332 (2019).

Introduction: Mars’ winter atmosphere is characterized by a polar vortex of low temperatures around the winter pole, circumscribed by a strong westerly jet [e.g. 1]. These vortices are a key part of the atmospheric circulation and impact heavily on dust and volatile transport. In particular, they have a complex and asymmetrical (north/south) relationship with atmospheric dust loading [1]. Regional and global dust events have been shown to cause rapid vortex displacement [2,3] in the northern vortex, while the southern vortex appears more robust.Suspended atmospheric dust aerosol is a crucial active component of Mars’ atmosphere, with significant radiative-dynamical effects through its scattering and absorption of radiation [5]. The exact nature of these effects depends on a variety of factors: aerosol optical depth is important, as are the specific radiative properties of the aerosol particles [6,7], and the vertical distribution of the dust itself [8].Mars Global Dust Storms (GDS) are spectacular, planet-spanning events which dramatically increase atmospheric dust loading. The 2018 GDS was observed through its lifecycle by the Mars Climate Sounder (MCS) instrument aboard the Mars Reconnaissance Orbiter [9]; using data assimilation [10] to integrate MCS retrievals [11] with the LMD-UK Mars Global Circulation Model (MGCM) [12] therefore offers an opportunity to examine the effects of the GDS on the polar vortices, and the interplay between the factors described above. The reanalysis contains the MGCM’s best possible representation of the GDS geographical, temporal, and in particular vertical structure.Model and assimilation scheme: We use the LMD-UK Mars Global Circulation Model (MGCM), which solves the meteorological primitive equations of fluid dynamics, radiative and other parameterised physics to calculate the state of the martian atmosphere [3,8]. The UK version of the MGCM possesses a spectral dynamical core and semi-Lagrangian advection scheme [13], and is a collaboration between the Laboratoire de Météorologie Dynamique, The Open University, the University of Oxford, and the Instituto de Astrofisica de Andalucia. The model was run at spectral spatial resolution T42 and a vertical resolution of 50 levels, the latter spaced non-linearly. The assimilation scheme used was a modified version of the Analysis Correction scheme developed at the Met Office, adapted for use on Mars [6].Retrievals used: The retrievals used in this study are from the Mars Climate Sounder (MCS) instrument aboard the Mars Reconnaissance Orbiter (MRO) [4], which now has amassed over five full martian years’ worth of data. For this study, the assimilated MCS variables were temperatures, derived column dust optical depth (CDOD), and dust profiles. Temperature profiles extend from the surface to approximately 100 km, and dust profiles from as low as 10 km above the surface up to a maximum height of approximately 50 km. The retrieval version used is 5.2, a re-processing using updated 2D geometry [7]. This results in improved retrievals, especially in the polar regions.Results: The 2018 GDS had large and asymmetric impacts on dynamics at both poles. This will be presented via changes in zonal winds and polar vorticity at both poles relative to a clear martian year, MY 30. The GDS provided a natural laboratory for testing the effects of equinoctial high dust loading on polar dynamics, allowing investigation of both how the polar atmosphere behaves in a clear year and under the case of extreme dust loading at this time of year. We present results on the effects of the GDS on both southern and northern polar dynamics, with implications for tracer transport.Discussion: The 2018 GDS dataset allows the opportunity for investigation of the polar dynamical effects of that specific event, the first fully observed by MCS. The polar vortices and associated zonal jets act as a barrier for cross-vortex tracer transport; their weakening can therefore allow dust to be transported onto the seasonal CO2 ice caps. Understanding how these barriers work is therefore important for understanding the evolution of Mars’ past climate: the Mars’ ice caps contain a record of past dust deposition [e.g. 8].Upcoming retrievals from the ExoMars 2016 Trace Gas Orbiter and its NOMAD spectrometer suite [9] will allow for further investigation of tracer transport and an opportunity to both cross-validate and jointly assimilate NOMAD and MCS data, including over a range of martian local times, which will enable investigation of the diurnal cycles of tracer transport and atmospheric dynamics at the poles.Acknowledgements: PMS acknowledges support from the UK Science and Technology Facilities Council under STFC grant ST/N50421X/1 and The Open University in the form of a PhD studentship. SRL, MRP and JAH also acknowledge the support of the UK Space Agency and STFC under grants ST/R001405/1, ST/S00145X/1 and ST/P001262/1 and STFC under ST/P000657/1. The authors are particularly grateful for ongoing collaborations with Dan McCleese, David Kass and the MCS team (NASA-JPL) and with Peter Read (Oxford) and François Forget and colleagues (LMD/CNRS Paris).References: [1] Waugh, D. W. et al (2016) J. Geophys. Res. Planets, 121, 1770-1785. [2] Guzewich, S. D. et al (2016) Icarus, 278, 100-118. [3] Mitchell, D. M. et al (2015) Q.J.R. Meteorol. Soc., 141, 550-562. [4] McCleese D. J. et al (2010) J. Geophys. Res., 115(E12016). [5] Gierasch P. J. and Goody R. M. (1972) J. Atmos. Sci., 29(2), 400-402. [6] Turco R. P. et al (1984) Scientific American, 251(2), 33-43. [7] Madeleine J.-B. et al (2011) JGR (Planets), 116 (E11010). [8] Tanaka, K. L. (2000), Icarus, 144(2), 254-266. [9] Patel, M. R. et al (2017), Appl. Opt., 56(10), 2771-2782.

Against the traditional view, a recently published theory argued that isotope ratios are higher in convective precipitation but lower in stratiform precipitation and proposed that isotope ratios reflect rain type proportions. This theory has been widely cited despite some early reservations. Whether the theory represents a faithful reflection of signals of water isotope ratios remains unclear. Here, we reassess its validity from different timescales and broader observations from the pantropics. Unexpectedly, our findings contradict the theory on daily, monthly, and even annual timescales. Pantropical precipitation isotope ratios remain strongly correlated to convection intensity but are independent of rain type proportions because stratiform precipitation isotope ratios cover a large range of values. We find that the theory has many serious weaknesses related to preferential data selection and suggest that new theories need to be validated at more locations on different timescales before gaining widespread acceptance.

The wealth of observations now available from multiple spacecraft in orbit around Mars and rovers/landers on the surface provides information on several aspects of the atmosphere, although they are restricted in space and time. Most of the observational datasets are largely complementary, so an efficient method to combine them in a physically consistent way will lead to more constrained studies of the evolution of the global martian atmosphere. Data assimilation is one such method, combining multiple retrievals with a Mars Global Circulation Model (GCM) while accounting for errors in both sources of information and producing an optimal representation of the evolving martian surface and atmosphere. Data assimilation is a powerful tool in that multiple parameters each observed independently by different instruments (e.g. water vapour, ozone, carbon monoxide, dust opacity, temperature) are all realistically constrained and physically consistent at the same time, and unobserved parameters can also be influenced by assimilated data (e.g. water vapour assimilation will impact on the water ice distribution). It also allows for study of atmospheric features that change significantly between observations and identifying processes that lead to the observed changes. Data assimilation studies are prevalent on Earth and are becoming more mainstream for Mars, with several different Mars GCMs now capable of assimilating retrievals using different assimilation schemes. The Open University (OU) ExoMars modelling group Mars GCM has been combined with several retrieval datasets via data assimilation to study features of the ozone, carbon monoxide, water and dust cycles alongside dynamical features such as the polar vortices, surface warming during a global dust storm and planetary waves. OpenMARS (Open access to Mars Assimilated Remote Soundings), a publicly available global reanalysis dataset from 1999-2015, was also created using the OU assimilation system. This talk will give a brief overview of the benefits and limitations of data assimilation for Mars, and will demonstrate how combining retrievals of different atmospheric parameters with a Mars GCM via data assimilation leads to a better constrained analysis of the martian atmosphere than is possible with retrievals or GCMs alone.

Aboriginal manufacture and use of pottery was unknown in Australia prior to European settlement, despite wellknown ceramic-making traditions in southern Papua New Guinea, eastern Indonesia, and the western Pacific. The absence of ancient pottery manufacture in mainland Australia has long puzzled researchers given other documented deep time Aboriginal exchange networks across the continent and the close proximity of potterybearing Lapita and post-Lapita maritime communities in the western Pacific with ocean-going watercraft and sophisticated navigation abilities. We report the oldest securely dated ceramics found in Australia from archaeological excavations on Jiigurru (Lizard Island Group) on the Great Barrier Reef, northeast Australia. Comprehensive radiocarbon dating and Bayesian modelling constrains ceramic deposition to between 2950-2545 cal BP and 1970-1815 cal BP. This timing overlaps with late Lapita and post-Lapita ceramic traditions of southern Papua New Guinea. Geological characterisation of the sherds strongly suggests local manufacture as the vessels belong to three temper and clay groups locally sourced to northeast Australia, and most likely to Jiigurru. The oldest occupation layers date to 6510-5790 cal BP, making Jiigurru the earliest offshore island occupied on the northern Great Barrier Reef. The results demonstrate that northeast Australian First Nations communities had sophisticated canoe voyaging technology and open-sea navigational skills and were intimately engaged in ancient maritime networks, connecting them with peoples, knowledges, and technologies across the Coral Sea region.

Optically thick cloud decks shroud the deep atmosphere of Venus from the view of most observing instruments. Limited nightside spectral windows, however, have allowed detection of trace gas species like carbon monoxide (CO) and sulphur dioxide (SO2). The distribution of these species offers clues to the general circulation below the clouds. CO is formed above the clouds by photolysis of CO2 and reaches the deep atmosphere through downwelling at high latitudes associated with a cloud deck Hadley-like cell. SO2, on the other hand, is believed to originate from the surface, potentially from volcanic processes, and varies in abundance above the cloud deck on both short-term and long-term timescales due to a combination of photochemical and dynamical processes. Using the Venus Planetary Climate Model (VPCM), a 3-D global climate model of Venus, we performed two age of air simulations to study how the surface, deep atmosphere, cloud decks, and upper atmosphere are connected by dynamical transport processes. In the experiments, a source region was initialised with a passive tracer whose abundance increased linearly with simulation time. The VPCM’s tracer routines transported the tracer from the source region around the model atmosphere. We then used the tracer abundance in each model gridbox to calculate the time since the air in that gridbox was last in the source region, a quantity known as the “mean age of air.” In one experiment (Surface), the tracer source region was the planetary surface, and in the second (Cloud Deck), the source region was the model level just beneath the lowest cloud deck.Figure 1: Zonal mean age of air for the Surface and Cloud Deck tracer experiments.In the Surface experiment, air took around 25 Earth years to reach the region below the cloud decks, while in the Cloud Deck experiment, it took 1.5 Earth years to reach the upper atmosphere above the clouds. To put these timescales in context, we define the troposphere-to-stratosphere turnover ratio, a ratio of the transport time through the troposphere to the transport time through the stratosphere, for Venus in comparison to Earth and Mars: Planet Venus Earth Mars Turnover ratio (equator) 0.06 14.5 1 Turnover ratio (poles) 0.075 21.9 0.5 (seasonal) Source Our work Krol et al. (2018) [1] Waugh et al. (2019) [2] While on Earth, air moves through the troposphere much more quickly than through the stratosphere, this is reversed for Venus, where the upper atmosphere sees much faster turnover. However, we also found a latitudinal gradient in the age of air which varies with altitude, with significantly younger air in polar regions at altitudes from 25-60 km, and older air at the poles both below and above this altitude range.Figure 2: Six snapshots of the age of air in the Surface simulation at the south pole. The outermost circle is 60° and each radial gridline represents 5°.To explain this latitudinal gradient, we investigated the circulation between 25-60 km and found that this region of the deep atmosphere generates planetary-scale Rossby waves with a zonal wave number of one. These waves are characterised by a pair of high-latitude cyclonic/anticyclonic structures or gyres in each hemisphere which collect the age of air tracers, leading to a younger age of air near the poles. The waves propagate around the planet with a period of 36 Earth days, as can be seen from the change in longitudinal location of the age of air minimum in Figure 2. Figure 3 shows the general circulation at two altitudes within the wave region. Fig. 3a) contains large gyres in the top and bottom left of the panel with an area of upwelling at their western edges. The gyres are less evident in Fig. 3c) because they have become confined to very high latitudes as the superrotating equatorial jet comes to dominate most of the planet. However, time series analysis of the vertical wind at the gyre latitudes found that the waves continue to exist at these altitudes and remain in phase throughout the 25-60 km altitude range.Traveling high-latitude Rossby waves could be a source of the observed variability in the trace gas distributions below the cloud deck and contribute to the complex, as yet poorly understood dynamics of the Venusian polar regions. The predicted existence of these waves could be tested by long-term or appropriately timed observations of the trace gas species near one or both poles to determine if there is a regular periodic variation in abundance.Figure 3: Horizonal and vertical wind in the deep atmosphere at 25 km and 47 km altitude, 12 days apart.References:[1] Krol, M., de Bruine, M., Killaars, L., Ouwersloot, H., Pozzer, A., Yin, Y., Chevallier, F., Bousquet, P., Patra, P., Belikov, D., Maksyutov, S., Dhomse, S., Feng, W., and Chipperfield, M. P.: Age of air as a diagnostic for transport timescales in global models, Geosci. Model Dev., 11, 3109–3130, https://doi.org/10.5194/gmd-11-3109-2018, 2018.[2] Waugh, D.W., Toigi, A.D., Guzewich, S.D.: Age of martian air: timescales for martian atmospheric transport, Icarus, 317, 148-157, https://doi.org/10.1016/j.icarus.2018.08.002

Observations of the vertical distribution of water vapour provide a unique snapshot of the vertical transport processes that contribute to the global martian hydrological cycle. While previous datasets have largely been seasonally and spatially sparse, vertical profiles of water retrieved from the Nadir and Occultation for MArs Discovery (NOMAD) and Atmospheric Chemistry Suite (ACS) instruments on the ExoMars Trace Gas Orbiter (TGO) provide the most complete dataset so far. These data are now capable of providing robust constraints on the 4-D distribution of water, especially when also combined with retrievals of additional atmospheric properties (e.g. temperature profiles, dust column) that exert an influence on the evolving global water distribution. A key limitation though is the fact that observations of water profiles are still relatively limited in coverage, in the global sense, and the vertical distribution of water at latitudes and times not regularly probed by NOMAD and ACS remains poorly understood. To address this, we have created a global reference climatology of water vertical distribution for Mars Year (MY) 34 through a multi-spacecraft data assimilation combining several retrieval datasets with a Mars Global Circulation Model. Retrievals of dust column and temperature profiles from Mars Climate Sounder on the Mars Reconnaissance Orbiter and water vapour and temperature profiles from multiple instruments on the ExoMars TGO during the primary science phase covering the latter half of MY34 are combined through assimilation to create one unified physically consistent global dataset. The vertical water vapour distribution is investigated globally. During the initial coverage of TGO observation that covers the dusty season in MY34, northern polar latitudes are largely absent of water vapour below 20 km with variations in abundance above this altitude throughout the dusty season linked to transport from mid-latitudes during a global dust storm, perihelion season and the intense MY34 C storm. The atmosphere is in a supersaturated state above 60 km for most of the time period investigated, with lower altitudes showing more diurnal variation in the saturation state of the atmosphere. A key benefit of the data assimilation technique is that constraints on dynamical transport imposed by the assimilated water vapour and temperature profiles leads to improvements in the simulated water ice distribution even though it is not altered directly by the assimilation process. The climatology created, which will become publicly available for wider use by the martian scientific community, has also been independently validated against water vapour profiles from the SPICAM instrument.

Introduction: Suspended atmospheric aerosols are key components of the martian atmosphere, and their vertical distribution has long been a subject of investigation with orbital observations and modelling. The aerosols found in Mars' atmosphere are mineral dust, water ice, and CO2 ice, and each have distinct spatiotemporal distributions and radiative effects.Of particular interest for this study is the vertical distribution of atmospheric aerosols. In recent years, dust has been observed to have a more complex vertical distribution structure than previously thought, with the detection of detached dust layers [1] and large plume-like structures during Global Dust Storms (GDS) [2].Water ice distribution is tied to the seasonal behaviour of its associated cloud formations, with seasonally recurring features including the aphelion cloud belt (ACB) [3] and polar hood clouds [4] at tropospheric altitudes, as well as higher altitude mesospheric (>40 km) clouds during Mars’ perihelion season [5] as well as during GDS [6,7].Mars’ low atmospheric temperatures also enable the formation of CO2 ice clouds, which have been detected at mesospheric altitudes over the tropics/subtropics and generally during the colder aphelion season [5,8]. These are thought to be more ephemeral than their water ice counterparts, with lifetimes as low as minutes [9]. More persistent and optically thicker CO2 ice clouds have been detected at tropospheric altitudes in the polar night [10]. The Ultraviolet and Visible (UVIS) Spectrometer [11], part of the Nadir and Occultation for MArs Discovery (NOMAD) spectrometer suite aboard the ExoMars Trace Gas Orbiter (TGO) [12], has now observed the martian atmospheric limb via solar occultations for over 1.5 martian years. This period covers the 2018/Mars Year (MY) 34 GDS and regional dust storm, as well as the entirety of the more typical MY 35. As such, UVIS solar occultation data provides a great opportunity to examine Mars’ vertical aerosol structure.Results: We present a new UVIS occultation opacity profile dataset, openly available for use by the community. We also discuss particular features of interest in the dataset, and interpret these features by reference to previous published work and by comparison with the MGCM. In particular, we focus on notable mesospheric water ice cloud phenomena observed in both MY 34 and MY 35. We describe the spatiotemporal distribution of these features, and the link between specific water ice features and strong atmospheric dust activity from global and regional storms. The MGCM temperature and aerosol opacity fields provide valuable points of comparison with the UVIS dataset, for the purposes of both explanation and validation of the MGCM’s existing parametrizations. The UVIS dataset offers opportunities for further research into the vertical aerosol structure of the martian atmosphere, and improvement of how this is represented in numerical models.References: [1] Heavens, N. G. et al (2011) JGR (Planets), 116(E4), E04003. [2] Heavens, N. G. et al (2019) GRL, 124(11), 2863-2892. [3] Smith M. D. (2008) Annu. Rev. Earth Planet Sci, 26, 191-219. [4] Wang, H. & Ingersoll, A. P. (2002) JGR (Planets), 107(E10), 8-1-8-16. [5] Clancy, R. T. et al (2019) Icarus, 328, 246-273. [6] Liuzzi G. et al (2020) JGR (Planets), 125(4). [7] Stcherbinine, A. et al (2020) JGR (Planets), 125(3). [8] Aoki, S. et al (2018) Icarus, 302, 175-190. [9] Listowski, C. et al (2014) Icarus, 237, 239-261. [10] Hayne, P. O. et al (2012) JGR (Planets), 117(E8). [11] Patel, M. R. et al (2017) Appl. Opt., 56(10), 2771-2782. [12] Vandaele, A. C. et al (2015) Planet. Space Sci., 119, 233-249.