Joachim Ribbe

Near‐global impact of the Madden‐Julian Oscillation on rainfall

The accuracy of synoptic-based weather forecasting deteriorates rapidly after five days and is not routinely available beyond 10 days. Conversely, climate forecasts are generally not feasible for periods of less than 3 months, resulting in a weather-climate gap. The tropical atmospheric phenomenon known as the Madden-Julian Oscillation (MJO) has a return interval of 30 to 80 days that might partly fill this gap. Our near-global analysis demonstrates that the MJO is a significant phenomenon that can influence daily rainfall patterns, even at higher latitudes, via teleconnections with broadscale mean sea level pressure (MSLP) patterns. These weather states provide a mechanistic basis for an MJO-based forecasting capacity that bridges the weather-climate divide. Knowledge of these tropical and extra-tropical MJO-associated weather states can significantly improve the tactical management of climate-sensitive systems such as agriculture.

Simulation of the Middle Miocene Climate Optimum

[1] Proxy data constraining land and ocean surface paleotemperatures indicate that the Middle Miocene Climate Optimum (MMCO), a global warming event at 15 Ma, had a global annual mean surface temperature of 18.4C, about 3C higher than present and equivalent to the warming predicted for the next century. We apply the latest National Center for Atmospheric Research (NCAR) Community Atmosphere Model CAM3.1 and Land Model CLM3.0 coupled to a slab ocean to examine sensitivity of MMCO climate to varying ocean heat fluxes derived from paleo sea surface temperatures (SSTs) and atmospheric carbon dioxide concentrations, using detailed reconstructions of Middle Miocene boundary conditions including paleogeography, elevation, vegetation and surface temperatures. Our model suggests that to maintain MMCO warmth consistent with proxy data, the required atmospheric CO2 concentration is about 460–580 ppmv, narrowed from the most recent estimate of 300–600 ppmv. Citation: You, Y., M. Huber,

Variability and Trend of North West Australia Rainfall: Observations and Coupled Climate Modeling

Abstract Since 1950, there has been an increase in rainfall over North West Australia (NWA), occurring mainly during the Southern Hemisphere (SH) summer season. A recent study using twentieth-century multimember ensemble simulations in a global climate model forced with and without increasing anthropogenic aerosols suggests that the rainfall increase is attributable to increasing Northern Hemisphere aerosols. The present study investigates the dynamics of the observed trend toward increased rainfall and compares the observed trend with that generated in the model forced with increasing aerosols. It is found that the observed positive trend in rainfall is projected onto two modes of variability. The first mode is associated with an anomalously low mean sea level pressure (MSLP) off NWA instigated by the enhanced sea surface temperature (SST) gradients toward the coast. The associated cyclonic flows bring high-moisture air to northern Australia, leading to an increase in rainfall. The second mode is associa...

A model of suspended sediment transport by internal tides

[Abstract]: The ability of internal tides to resuspend and advect sediment over continental shelfs and slope regions is investigated by applying an internal wave and sediment transport model. Numerical experiments are carried out, firstly, with the ratio of bathymetry and internal waves characteristics creating critical, subcritical, and supercritical conditions, and secondly, for an observed section of the Australian North West Shelf. In the former cases, the model is forced with an internal tide propagating through the model domain. The latter application involves forcing by a barotropic tide which in turn generates internal waves at the shelf slope. Internal wave generated bottom layer shear stresses are large enough to resuspend sediment. The application of a turbulence closure scheme results in the creation and maintenance of a thin nepheloid layer. The thickness of the suspended sediment layer is controlled by vertical diffusion which is large within the bottom boundary layer, but very small outside. The residual velocity and the asymmetry associated with the velocity field, result in both down- and upslope net suspended sediment fluxes, and deposition of resuspended material onto the shelf. These suspended sediment fluxes are largest for critical bottom slopes. The parting point between down- and upslope net sediment flux is found to be sensitive to the formulation of vertical mixing with the parting point moving downslope for increased mixing. At the Australian North West Shelf, near the shelf break and upper slope, the net flux of resuspended material is influenced by both the barotropic and internal tide. The phase relationship in bottom layer shear stresses generated from those two tides causes regions of both enhancement and reduction in the resuspension rates and net suspended sediment fluxes.

Multidecadal variability in the transmission of ENSO signals to the Indian Ocean

Since 1980, transmission of El Nino-Southern Oscillation (ENSO) signals into the Indian Ocean involves an equatorial, and a subtropical North Pacific (NP) Rossby wave pathway. We examine the robustness of the amount of energy that leaves the Pacific via each of the pathway using the Simple Ocean Data Assimilation with the Parallel Ocean Program (SODA-POP) reanalysis and a multi-century coupled model control experiment. We find that in the pre-1980 period, little ENSO signal is transmitted to the Indian Ocean and does not involve the subtropical NP pathway. Such multidecadal variability is periodically produced by the climate model. Examinations reveal that when ENSO is weak as determined by Nino3.4, their meridional extent is narrow, the associated discharge-recharge does not involve the subtropical NP pathway; further, weak ENSO events have a low signal-to-noise ratio, making the transmission hard to detect. The dynamics of multidecadal variability in ENSO strength awaits further investigation.

Assessing water renewal time scales for marine environments from three-dimensional modelling: A case study for Hervey Bay, Australia

We apply the three-dimensional COupled Hydrodynamical Ecological model for REgioNal Shelf seas (COHERENS) to compute water renewal time scales for Hervey Bay, a large coastal embayment situated off the central eastern coast of Australia. Water renewal time scales are not directly observable but are derived indirectly from computational studies. Improved knowledge of these time scales assists in evaluating the water quality of coastal environments and can be utilised in sustainable marine resource management. Results from simulations with climatological September forcing are presented and compared to cruise data reported by Ribbe (2006. A study into the export of saline water from Hervey Bay, Australia. Estuarine Coastal and Shelf Science 66, 550-558). A series of simulations using idealised forcing provides detailed insight into water renewal pathways and regional differences in renewal timescales. We find that more than 85% of the coastal embayments water is fully renewed within about 50-80 days. The eastern and western shallow coastal regions are ventilated more rapidly than the central, deeper part of the domain. The climatological simulation yields temperature and salinity patterns that are consistent with the observed situation and water renewal time scales in the range of those derived from idealised model studies. While the reported simulations involve many simplifications, the global assessment of the renewal time scale is in the range of a previous estimate derived for this coastal embayment from a simpler model and observational data.

Intermediate water mass production controlled by southern hemisphere winds

It is demonstrated that the production of intermediate water in a coarse resolution ocean general circulation model is controlled by Southern Hemisphere winds. Results from four equilibrium experiments using simplified topography and surface forcing are presented. The first experiment was carried out with no wind forcing, subsequent experiments employed annual mean surface stresses, which were amplified using factors of 0.5, 1, and 2.0 south of 30°S. In all experiments, the salinity minimum characteristic for intermediate water is reproduced. Volume transports are directly proportional to the applied Southern Hemisphere surface stresses. These force an increased export of intermediate water and heat into the South Pacific Ocean northward across 30°S and through Drake Passage into the South Atlantic Ocean. It results in a warming of the South Pacific Ocean, which is at a maximum in the intermediate water density range.

Are Anthropogenic Aerosols Responsible for the Northwest Australia Summer Rainfall Increase? A CMIP3 Perspective and Implications

AbstractSevere rainfall deficiencies have plagued southern and eastern Australian regions over the past decades, where the long-term rainfall is projected to decrease. By contrast, there has been an increase over northwest Australia (NWA) in austral summer, which, if it continues, could be an important future water resource. If increasing anthropogenic aerosols contribute to the observed increase in summer rainfall, then, as anthropogenic aerosols are projected to decrease, what will the likely impact over NWA be? This study uses output from 24 climate models submitted to phase 3 of the Coupled Model Intercomparison Project (CMIP3) with a total of 75 experiments to provide a multimodel perspective. The authors find that none of the ensemble averages, either with both the direct and indirect anthropogenic aerosol effect (10 models, 32 experiments) or with the direct effect only (14 models, 43 experiments), simulate the observed NWA rainfall increase. Given this, it follows that a projected rainfall reducti...

Oceanography: The southern supplier

Physical processes in the Southern Ocean largely control nutrient distribution in the global marine environment, a finding that further highlights the influence of this oceanic region on Earths climate.

Hervey Bay and Its Estuaries

Hervey Bay and its estuaries are located along the east coast of Australia just to the south of the Great Barrier Reef Marine Park. The region has long been recognised as one of Australia’s most biodiverse marine environments and including the Great Sandy Strait in the south of the Bay, it is referred to as the Great Sandy Biosphere. The United Nations Educational, Scientific and Cultural Organisation (UNESCO) included the area in its list of 580 designated biospheres located in 114 countries. Although widely recognised for its exceptional biodiversity, little is known about the physical processes and climate characteristics that shape its natural marine environment. Only recently, the Bay has been classified as a large low inflow and predominantly hypersaline system. River runoff and discharge from its many estuaries is very small. It has almost been absent during the Australian Millennium Drought lasting the first decade of the twenty-first century. During other times, freshwater inflow is only significant following flooding as a result of tropical/subtropical depressions, which often is amplified during La Nina events. A positive freshwater balance leads to a net loss of water establishing hypersaline conditions. This appears to prevail throughout the year and is re-established shortly after storm events. Hydrodynamic modelling suggests that predominant southeasterly to easterly trade winds establish a cyclonic water renewal pathway, with Hervey Bay water exiting along the western shoreline. Hypersalinity and the characteristics of an inverse estuarine circulation are evident from observations and modelling. This chapter reviews our understanding of the environmental processes that shape Hervey Bay and its estuaries in the context of its climate. Future changes in the regional freshwater balance indicate a continued trend toward drier and warmer conditions. It leads to an intensification of hypersaline and possible inverse circulation states of the Bay. Insight into the environmental forces shaping Hervey Bay, its estuaries, and a unique and biodiverse environment, informs continued sustainable natural resource management and policy development. It is anticipated that over the next few decades physical processes associated with climatic trends and variability are likely to impact more dramatically upon the natural environment of the region than direct human activities such as fishing, aquaculture, tourism and continued local population and urbanisation trends.