Journal of Southern Hemisphere Earth Systems Science最新文献

Subseasonal to seasonal forecasts of ocean temperatures, including extreme events such as marine heatwaves, have demonstrated utility in informing operational decision-making by marine end users and managing climate risk. Verification is critical for effective communication and uptake of forecast information, together with understanding ocean temperature predictability. The forecast skill of surface and subsurface ocean temperature forecasts from the Bureau of Meteorology’s new ACCESS-S2 seasonal prediction system are assessed here over an extended 38-year hindcast period, from 2 weeks to 6 months into the future. Forecasts of sea surface temperature (SST), heat content down to 300 m (HC300), bottom temperatures on continental shelves, and mixed layer depth are compared to both satellite observations and ocean reanalyses for the globe and the Australian region, using a variety of skill metrics. ACCESS-S2 demonstrates increased SST skill over its predecessor ACCESS-S1 at subseasonal timescales for all variables assessed. Heat content skill is particularly high in the tropics but reduced in subtropical regions especially when compared to persistence. Forecast skill for ocean temperature is higher in the austral summer months than winter at lead times up to 2 months in the Western Pacific region. Mixed layer depth is poorly predicted at all lead times, with only limited areas of skill around Australia and in the south-west Pacific region. Probability of exceedance forecasts for the 90th percentile as an indicator for marine heatwave conditions, shows adequate skill for SST, HC300 and bottom temperatures, especially near shelf regions at shorter lead times. This work will underpin the future development of an operational marine heatwave forecast service, which will provide early warning of these events and thus valuable preparation windows for marine stakeholders.

Hailstorms develop over the La Plata Basin, in south-eastern South America, more often during later winter and early austral spring, between September and October. These systems have significant socioeconomic impacts over the region. Thus, a better understanding of how atmospheric drivers modulate the formation of hailstorms is important to improve the forecast of such phenomena. In this study, we selected a hailstorm event observed over the eastern La Plata Basin during 14–15 July 2016 to evaluate the performance of the Brazilian developments on the Regional Atmospheric Modelling System (BRAMS) model. The ability of the model in simulating cloud microphysical properties was evaluated by comparing simulations driven by different global forcings against in situ and remote sensing observations. The model results showed good skill in capturing the basic characteristics of the thunderstorm, particularly in terms of the spatial distribution of hydrometeors. The simulated spatial distribution of hail covers locations where hail fall was reported. The BRAMS simulations suggest that, despite relatively low values of the convective available potential energy (CAPE) (700–1000 J kg⁻¹), environments with strong 0–8-km bulk shear (60–70 kt, ~30.9–36.0 m s^–1) can promote the formation of ice clouds and hail fall over the eastern La Plata Basin. To be more conclusive, however, further research is needed to understand how different combinations of CAPE and shear affect hail formation over the region.

According to the Australian Bureau of Meteorology, the northern Australian wet season extends through to April, which also formally marks the end of Australia’s tropical cyclone season. Mid-autumn is when the tropical dry season transition period begins, when crop farmers prepare land for annual crops or pasture–fodder harvest, or when beef cattle producers make decisions regarding stock numbers and feed rationing. Potentially knowing if the last rains of the wet season will be later or earlier than normal would be valuable information for northern sectors such as agriculture, infrastructure and tourism. The Bureau of Meteorology provides seasonal forecasts of the Northern Rainfall Onset – the date when a location has accumulated 50 mm of rain from 1 September – yet there is currently no prediction of the rainy season retreat (the Northern Rainfall Retreat, NRR). In this study, we draw on three different NRR definitions and investigate how they vary with the El Niño–Southern Oscillation and the Madden–Julian Oscillation (MJO). In general, retreats occur ~1 week later than normal across the far northern tropics following La Niña events, but little change from normal occurs for El Niño. Although most retreats occur when the MJO is weak, if the MJO is active, retreats are mostly observed in phases 6 and 7, when convection is passing through the western Pacific. Utilising the Bureau of Meteorology’s sub-seasonal to seasonal forecast system, ACCESS-S2, we show that the model has some skill in forecasting the NRR across the far northern regions at a lead time of ~2.5 months, but poor skill in the subtropics and arid locations. Verification of the 2023 NRR forecasts, highlights the challenges of predicting the timing and magnitude of daily rainfall at such a long lead time.

Satellite altimetry was originally intended for oceanographic and geodetic applications. An uncommon application of satellite altimetry data, demonstrated in this paper, is for atmospheric study by utilising the onboard microwave radiometer. The Wet Tropospheric Correction (WTC) data from the Topex/Jason altimetry mission series (Topex/Poseidon, Jason-1, Jason-2/OSTM and Jason-3) are used, which have spanned nearly 30 years, making them sufficient for climate study. Precipitable Water Vapour (PWV) is derived from the WTC and used to study the atmospheric water vapour variability over the tropical Indian Ocean (TIO). Preliminary analysis is performed by comparing the generated PWV data with the PWV from a dedicated meteorological satellite Aqua, which was found to be comparable with a correlation coefficient of 0.94 for the monthly mean data and 0.74 for the anomaly component. Using standard empirical orthogonal function and composite analysis, the interannual variability of the tropospheric water vapour in TIO is thoroughly analysed. The mechanics and impacts of the two leading modes, the El Niño–Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) are characterised. Furthermore, the modulation of the atmospheric circulation cell can be monitored. A distinct characteristic is found for the spurious IOD event in 2017 and 2018, which is the forming of a PWV anomaly meridional gradient in the Indian Ocean during June due to the activity of the Southern Indian Ocean Dipole mode. This showcases the potential of using altimetry satellite data for atmospheric study and opens up the possibility of further utilisation.