Andrew B. Watkins

Observing and Predicting the 2015/16 El Niño

AbstractThe El Nino of 2015/16 was among the strongest El Nino events observed since 1950 and took place almost two decades after the previous major event in 1997/98. Here, perspectives of the event are shared by scientists from three national meteorological or climate services that issue regular operational updates on the status and prediction of El Nino–Southern Oscillation (ENSO). Public advisories on the unfolding El Nino were issued in the first half of 2015. This was followed by significant growth in sea surface temperature (SST) anomalies, a peak during November 2015–January 2016, subsequent decay, and its demise during May 2016. The life cycle and magnitude of the 2015/16 El Nino was well predicted by most models used by national meteorological services, in contrast to the generally overexuberant model predictions made the previous year. The evolution of multiple atmospheric and oceanic measures demonstrates the rich complexity of ENSO, as a coupled ocean–atmosphere phenomenon with pronounced glob...

Current Trends in Antarctic Sea Ice: The 1990s Impact on a Short Climatology

Abstract Antarctic sea ice extent, open water area, and ice area are shown to have increased significantly during the period 1987–96. These trends are largely attributable to an increase in these parameters during the mid-1990s. It is suggested that these changes may be due to modifications in the position and depth of the circumpolar trough that impact the sea ice directly via the atmosphere and indirectly via the ocean, and could be a response to the unprecedented El Nino–Southern Oscillation event observed from 1990 to 1995, and/or an increase in the meridional temperature gradient. Positive and negative regional trends in seasonal sea ice concentration are shown for the same period. In the western Weddell Sea negative trends in the Scanning Multichannel Microwave Radiometer (1978–87) and Special Sensor Microwave/Imager (1987–96) period sea ice data are consistent with the rates of temperature increases observed over the past 50 yr. Trends in the length of the sea ice season show that for many regions,...

Sensitivity of numerical prognoses to Antarctic sea ice distribution

To examine the effect of Antarctic sea ice concentration upon the atmosphere on synoptic timescales, a general circulation model was used in conjunction with the Australian Bureau of Meteorologys Global Assimilation and Prediction system analyses and the Defense Meteorological Satellite Programs Special Sensor Microwave/Imager sea ice data to perform a suite of 5-day forecasts. As well as using a generic July sea ice concentration, further experiments were conducted with 0, 10, 25, 50, 80, and 100% sea ice concentrations. The 100% forecast was used as the control. Results show that the central pressure, structure and tracks of individual cyclones are sensitive to the “switch on” of different sea ice conditions. Composites of all forecasts made with each concentration showed considerable and statistically significant anomalies in the surface temperatures and turbulent heat fluxes over the sea ice. The magnitudes of these changes varied monotonically with the area of open water. The largest changes were simulated closest to the coast for all concentrations, except for the generic July sea ice run, which displayed maxima over the outer pack. Significant westerly anomalies were also induced over the ice in all cases. Mean sea level pressure (MSLP) anomalies occurred in all forecasts. Again, the generic July sea ice run displayed a distribution of the MSLP anomaly different from all other runs, with maxima occurring in the central to outer pack. All other forecasts displayed maxima at the coast. These results suggest that sea ice concentration induces anomalies in the atmospheric parameters in timescales of less than 5 days. Further, use of a realistic distribution of sea ice concentration produces results distinct from the constant concentration forecasts. Hence it is suggested that real-time Antarctic sea ice data may be of considerable benefit to numerical weather prediction and analysis.

A late spring surge in the open water of the Antarctic sea ice pack

Mean annual cycles of open water area within the Antarctic sea ice pack are presented for 1978–87 and 1987–97, compiled using mean daily data. A minimum of the open water area occurs in mid February, followed by a steady increase until a plateau is established in October, before a sudden surge in early November to a peak of 4.5 × 106 km². As this Antarctic Open water Surge (AOS) occurs on timescales of around one month, its temporal structure only becomes truly clear when using daily means. The AOS is shown to be the result of the relative timings of the sea ice area and extent annual cycles. It is suggested these reflect the late spring crossing of the Antarctic circumpolar trough over the sea ice edge, thus shifting the dominant winds (Ekman forcing) from westerly (northerly) to easterly (southerly).

Ensemble prediction of blocking regime transitions

The skill of ensemble prediction during blocking regime transitions is examined for Northern Hemisphere flows within two atmospheric models. An ensemble prediction scheme based on fast growing perturbations has been implemented for conformal-cubic and spectral general circulation models. The methodology uses a breeding method, based on an implicit linearization of the models, in which perturbations likened to leading Lyapunov vectors are obtained and used to perturb the initial conditions. Detailed comparisons of the skill of ensemble mean forecasts with control forecasts have been carried out for Northern Hemisphere initial conditions in October and November 1979. A particular focus has been the variability of forecast skill during the development, maturation and decay of the large-scale blocking dipoles that occurred in the major blocking regions over Europe, over the Gulf of Alaska, over the North Atlantic and as well over North America. On average, the ensemble mean forecast performs better than the control forecast for forecast times longer than three or four days. Average error growth curves in the two models are quite similar with the conformal-cubic model ensemble generally performing slightly better than for the lower-resolution spectral model. Both ensemble and control forecasts initiated twice daily exhibit considerable variability in forecast skill that is shown to be related to instability regimes of particular synoptic events. At a given forecast lead time, errors tend to be larger for forecasts validating when blocks are developing or decaying and smaller for mature blocks. The spread of ensemble member forecasts has been studied and related to likely forecast skill. Comparison of results from the conformal-cubic and spectral models initialized with two different analysis data sets has allowed the determination of the robustness of our findings relating error growth to instability regimes.

Seasonal Prediction of Climate Extremes for the Pacific: Tropical Cyclones and Extreme Ocean Temperatures

Climate change and climate extremes have a major impact on Australia and Pacific Island countries. Of particular concern are tropical cyclones and extreme ocean temperatures. As a practical response to climate change, through the Pacific Australia Climate Change Science and Adaptation planning Program (PACCSAP), enhanced web-based information tools to provide seasonal forecasts for climatic extremes in the Western Pacific have been developed. Using the dynamical seasonal prediction model POAMA (Predictive Ocean Atmosphere Model for Australia), we aim to improve accuracy of seasonal forecasts of tropical cyclone activity and extreme sea surface temperatures for the Western Pacific. Since the PACCSAP has commenced, encouraging scientific and technological results have been obtained, particularly in the development of web-based information tools to provide climatic extremes forecasts in the Pacific and the Australian regions. Improvements to a statistical model for seasonal tropical cyclone prediction in the Australian region have been made. Additional functionality was added to the Pacific Tropical Cyclone Data Portal, such as enhanced flexibility of spatial and temporal selection. New web-based information tool for sea surface temperature seasonal prediction is also currently under development. Improved knowledge of extreme climatic events, together with the assistance of tailored forecast tools, will help enhance the resilience and adaptive capacity of Australia and Pacific Island Countries under climate change.

Assessing the Climate Response to Major Surface Inundation: Lake Eyre, Australia

A World Meteorological Organization assessment of freshwater resources noted that approximately 1.7 billion people, or one-third of the world’s population, live in countries that are water-stressed (Stockholm Environment Institute 1997) (defined as using more than 20% of their renewable water supply, a commonly used indicator of water stress), and that this number is projected to increase to around 5 billion by 2025. As a result, projects to ‘drought-proof’ regions vulnerable to water stress are clearly of interest to many people and governments.