No description available.

none

This thesis investigated the emission and transport of very short–lived halogens (VSLS) over the tropics. VSLS are described as organic halogen gases with lifetimes of less than 6 months. In areas of rapid convective transport they reach the upper troposphere and lower stratosphere where they contribute to total bromine loading (~20 pptv) in the stratosphere that is a cause of ozone (O3) depletion. This thesis investigated speed of transport in the tropics using model age of air (Chapter 3), the strength of VSLS source regions in tropical troposphere (Chapter 4), and quantification of their monthly emission fluxes (Chapter 5). The two most abundant VSLS bromoform (CHBr3) and CH2Br2 were focussed on. A new model age of air calculation was used to describe transport of ocean emissions in the tropical latitudes. Age of air describes how long an air mass has been out of contact with the emission source region. The two most rapid convection regions of the Indian Ocean (InO) and Western Pacific (WPa) showed age of air in the tropical tropopause layer (TTL) to be 25 days. This is similar to the lifetime of CHBr3 (24 days). Using age of air estimated from simulations covering 1989–2013, it was shown how strong El Niño events can increase the age of air over the WPa by 5–7 days in the mid–troposphere, and up to 12 days in the TTL. This increase in age was due to a change in the Walker Circulation, weakening convection in the WPa and increasing convection over the CPa. Over this period, it was shown that age decreases in the tropical circulation system (the Hadley Cell). Decreasing age results from increasing convection, and more rapid transport of VSLS to the upper troposphere, lower stratosphere (UTLS). To study regional emission sources over the WPa, a tagged CHBr3 and CH2Br2 model was developed. It is the first study to quantify how open and coastal emissions contribute separately to the vertical profiles of CHBr3 and CH2Br2 in the WPa. Variability over the WPa is dominated by an open oceanic emission source, with enhanced coastal emissions influencing concentrations in the upper troposphere. Estimations of 3.14 pptv of CHBr3 and CH2Br2 contribution to TTL Bry were in agreement with recent observational studies (3.27 pptv, Navarro et al. (2015)) over the same region. Comparison with aircraft observations showed that the model has a positive bias and this is attributed to over estimation of model emissions. Ground–based observations were used in an inverse model to estimate surface emissions of CHBr3. This method has not been previously used to estimate CHBr3 emissions. The monthly a posteriori emissions had seasonal cycles in the northern and southern hemisphere coastal emissions, and reductions over tropical open oceans. A posteriori emissions were put in to the model and the predicted volume mixing ratios were able to reproduce ground stations observations over the mid–latitude and tropical stations, important for convective transport of VSLS. The model still showed a bias when compared to CAST and CONTRAST aircraft observations over the Western Pacific, but the mean model minus observed residual was reduced by around 0.3 pptv and 0.1 pptv for respective CAST and CONTRAST campaigns from the a priori emissions.

Midlevel clouds are observed globally and impact the general circulation through their influence on the radiation budget and their precipitation production. However, because midlevel clouds occur less frequently than high and low clouds they are relatively understudied. Satellite observations from the MODIS, CALIPSO, and CloudSat instruments onboard the A-Train are combined to study midlevel cloud characteristics in the Tropical Western Pacific (TWP) between January 2007 and December 2010. Characteristic cloud and microphysical properties including cloud top height (CTH), geometric thickness, optical depth, effective radius, and liquid or ice water path (LWP or IWP), and environmental properties, including temperature and specific humidity profiles, are determined for precipitating and non-precipitating midlevel clouds. In the study region, approximately 14% of all cloudy scenes are classified as midlevel clouds (4 km < CTH < 8 km). These are concentrated in areas of deeper convection associated with the Pacific warm pool, ITCZ, and SPCZ. Non-precipitating clouds dominate the region, accounting for approximately 70% of all single and two-layer midlevel clouds scenes. Midlevel clouds occur most frequently in three different scenarios: high over midlevel clouds (~65%), single-layer (~25%), and midlevel over mid- or low-level clouds (~10%). Environmental moisture appears to play a larger role than temperature in determining midlevel cloud distributions due to large variations in moisture between the different cloud scenarios. In all scenes, a trimodal distribution in CTH frequency is found within the midlevel. Two of these peaks have been identified in previous studies; however a third midlevel mode is recognized here. CTHs occur most frequently in peaks between 5-6 km, 6-6.25 km, and 6.5-7.5 km. Although the past studies have only noted two midlevel peaks, this third mode is a robust feature in this dataset. Two types of clouds dominate these peaks: non-precipitating altostratus or altocumulus-like clouds less than 1 km thick and geometrically thick precipitating cumulus congestus clouds. Environmental temperature stable layers and dry maxima are found at each one of these peak frequency heights. Again, moisture seems to play a more dominant role in determining the height of the midlevel clouds due to larger variances between the moisture gradients associated with each peak. Microphysical properties (optical depth, effective radius, and LWP or IWP) are characterized for single-layer clouds. Approximately 30% of all single-layer midlevel clouds are precipitating and these clouds tend to occur on the edges of the deep tropics. In general, precipitating clouds have greater optical depths, effective radii, and water path. This research implies that some past studies at single point locations can be representative of the broader tropics, whereas others are not.

In this study, specimens of Tridacna sp., which are reef dwelling bivalve molluscs and have been shown to live up to 60 years, collected from the Huon Peninsula, Papua New Guinea, were sampled for geochemical profiles. The Huon Peninsula is in the heart of the Western Pacific Warm Pool (WPWP), which plays a key role in ENSO dynamics. The uplifted reef terraces of the Huon Peninsula have been extensively studied, and are well dated, which gives the opportunity to reconstruct the local climate of this region at key intervals during the past. Previous work on Tridacna sp. has shown that they precipitate their aragonite shell in equilibrium with the surrounding seawater, and the δ18O profile of a modern T. gigas from the Huon Peninsula has been shown to correlate with precipitation and temperature anomalies, and the Niño 3.4 temperature anomaly record. Fossil samples from this region are therefore assumed to have the ability to capture changes in δ18O attributable to ENSO. Seasonally resolved δ18O measurements from Tridacna sp. from early Holocene and Marine Isotope Stage 3 (MIS3) reefs were used to reconstruct changes in mean climate, seasonality and inter-annual variability (e.g. ENSO). Reconstructions of the mean state tend to agree with previously published studies of Holocene and MIS3 climate, showing similar temperatures to today during the early Holocene, and an average cooling of 2- 3°C during MIS3. The early Holocene Tridacna sp. samples show a reduction in seasonality, consistent with the reduction in seasonal insolation at this time, while those from MIS3 show variable seasonality between 30-60ka. ENSO appears to have been supressed during the early Holocene by up to 50% compared with the late 20th century, which is consistent with coral data and modelling studies. During MIS3, ENSO appears to have been more variable with some records showing anomalous warm and cool events as strong as those seen in the modern T. gigas, used here as a benchmark. Trace element profiles derived from the Tridacna sp. used in this study show a tentative link with temperature and local productivity, but these relationships are subject to species specific and intra-shell effects.

No description available.