5,736 research outputs found
Recommended from our members
Forecasts of near-global sea-surface temperatures using canonical correlation analysis
The skill of global-scale sea surface temperature forecasts using a statistically based linear forecasting technique is investigated. Canonical variates are used to make monthly sea surface temperature anomaly forecasts using evolutionary and steady-state features of antecedent sea surface temperatures as predictors. Levels of forecast skill are investigated over several months' lead time by comparing the model performance with a simple forecast strategy involving the persistence of sea surface temperature anomalies. Forecast skill is investigated over an independent test period of 18 yr (1982/83â1999/2000), for which the model training period was updated after every 3 yr. Forecasts for the equatorial Pacific Ocean are a significant improvement over a strategy of random guessing, and outscore forecasts of persisted anomalies beyond lead times of about one season during the development stages of the El NiñoâSouthern Oscillation phenomenon, but only outscore forecasts of persisted anomalies beyond 6 months' lead time during its most intense phase. Model predictions of the tropical Indian Ocean outscore persistence during the second half of the boreal winter, that is, from about December or January, with maximum skill during the MarchâMay spring season, but poor skill during the autumn months from September to November. Some loss in predictability of the equatorial Pacific and Indian Oceans is evident during the early and mid-1990s, but forecasts appear to have improved in the last few years. The tropical Atlantic Ocean forecast skill has generally been poor. There is little evidence of forecast skill over the midlatitudes in any of the oceans. However, during the spring months significant skill has been found over the Indian Ocean as far south as 20°S and over the southern North Atlantic as far north as 30°N, both of which outscore persistence beyond a lead time of less than about one season
Recommended from our members
Retroactive skill of multi-tiered forecasts of summer rainfall over southern Africa
Seaâsurface temperature (SST) variations of the oceans surrounding southern Africa are associated with seasonal rainfall variability, especially during austral summer when the tropical atmospheric circulation is dominant over the region. Because of instabilities in the linear association between summer rainfall over southern Africa and SSTs of the tropical Indian Ocean, the skilful prediction of seasonal rainfall may best be achieved using physically based models. A twoâtiered retroâactive forecast procedure for the DecemberâFebruary (DJF) season is employed over a 10âyear period starting from 1987/1988. Rainfall forecasts are produced for a number of homogeneous regions over part of southern Africa. Categorized (belowânormal, nearânormal and aboveânormal) statistical DJF rainfall predictions are made for the region to form the baseline skill level that has to be outscored by more elaborate methods involving general circulation models (GCMs). The GCM used here is the Centre for OceanâLandâAtmosphere Studies (COLA) T30, with predicted global SST fields as boundary forcing and initial conditions derived from the National Centres for Environmental Prediction (NCEP) reanalysis data. Biasâcorrected GCM simulations of circulation and moisture at certain standard pressure levels are downscaled to produce rainfall forecasts at the regional level using the perfect prognosis approach.
In the twoâtiered forecasting system, SST predictions for the global oceans are made first. SST anomalies of the equatorial Pacific (NIĂO3.4) and Indian oceans are predicted skilfully at 1â and 3âmonth leadâtimes using a statistical model. These retroâactive SST forecasts are accurate for preâ1990 conditions, but predictability seems to have weakened during the 1990s. Skilful multiâtiered rainfall forecasts are obtained when the amplitudes of large events in the global oceans (such as El Niño and La Niña episodes) are described adequately by the predicted SST fields. GCM simulations using persisted August SST anomalies instead of forecast SSTs produce skill levels similar to those of the baseline for longer leadâtimes. Given highâskill SST forecasts, the scheme has the potential to provide climate forecasts that outscore the baseline skill level substantially
Recommended from our members
Operational long-lead prediction of South African rainfall using canonical correlation analysis
A statistically based technique is used to study the variability and predictability of South African summer rainfall. The country is divided into homogeneous regions on the basis of the interannual rainfall variability. Canonical variates are then used to make 3âmonth aggregate precipitation forecasts for OctoberâNovemberâDecember and JanuaryâFebruaryâMarch for South Africa from globalâscale seaâsurface temperatures. Four consecutive 3âmonth mean periods of seaâsurface temperatures are used to incorporate evolutionary features as well as steadyâstate conditions in the global oceans. Levels and possible origins of forecast skill are investigated for up to 5âmonth leadâtimes. Modest skill (correlation >0.5) is found over mainly the central and western interiors of the country, but the skill is poor over the northâeastern regions. The most important contribution of the prediction skill comes from the equatorial Pacific Ocean, with weaker predictability from the equatorial Indian and Atlantic oceans. Seaâsurface temperatures in the Atlantic and Indian oceans have important influences on the atmospheric circulation and moisture fluxes over southern Africa, and therefore provide useful predictability, at least for the OctoberâDecember rainfall. When forecasting South African rainfall, it is insufficient to consider only the El NiñoâSouthern Oscillation (ENSO) phenomenon, because it does not occur every year and because the seaâsurface temperatures of the adjacent oceans modify the ENSO forcing on South African rainfall. Unfortunately, the predictability during years not associated with the ENSO is weak
The ADHM Construction of Instantons on Noncommutative Spaces
We present an account of the ADHM construction of instantons on Euclidean
space-time from the point of view of noncommutative geometry. We
recall the main ingredients of the classical construction in a coordinate
algebra format, which we then deform using a cocycle twisting procedure to
obtain a method for constructing families of instantons on noncommutative
space-time, parameterised by solutions to an appropriate set of ADHM equations.
We illustrate the noncommutative construction in two special cases: the
Moyal-Groenewold plane and the Connes-Landi plane
.Comment: Latex, 40 page
Recommended from our members
Current Approaches to Seasonal to Interannual Climate Predictions
This review paper presents an assessment of the current state of knowledge and capability in seasonal climate prediction at the end of the 20th century. The discussion covers the full range of issues involved in climate forecasting, including (1) the theory and empirical evidence for predictability; (2) predictions of surface boundary conditions, such as sea surface temperatures (SSTs) that drive the predictable part of the climate; (3) predictions of the climate; and (4) a brief consideration of the application of climate forecasts. Within this context, the research of the coming decades that seeks to address shortcomings in each area is described
Dilemmas and Trade-Offs in Peacemaking: A Framework for Navigating Difficult Decisions
This article focuses on the dilemmas and trade-offs that third parties face when mediating violent political conflicts. Should they ignore human rights violations because pushing the issue could jeopardize relationships with political actors who grant access for humanitarian aid? Will bringing moderates and hardliners together help the peace process or radicalize moderate actors? What should dialogue facilitators do when the act of identifying non-mainstream groups to be included into dialogue increases division and polarization? The activity of peacemaking is inherently characterized by such process and strategy dilemmas where two equally compulsory imperatives seem not to be attainable at the same time. The article proposes a framework to break out of either-or thinking in these situations. We argue that: 1) making oneself aware of how a decision is perceived, and 2) systematically exploring a set of different strategies for creating new unexpected options helps to ease these decisions and avoid rotten compromises. The model reworks and combines existing problem-solving strategies to create a new explorative option generation approach to peacemaking dilemmas and trade-offs. Some of these strategies, such as sequencing and incrementalization, are already well-established in peacemaking. Others, such as compartmentalization and utilization, are rather unconsciously used. All identified strategies, however, are not yet systematically employed to manage third partiesâ own dilemmas and trade-offs. Under the suggested framework, these strategies can act in complement to synthesize creativity and strategic thinking with surprising ease. Using examples from the authorsâ peacemaking activities and observations in Myanmar, Thailand, and Ukraine, the article demonstrates the real-world benefits of the framework in terms of decision assessment and optional thinking
Recommended from our members
Current approaches to season-to-interannual climate prediction
This review paper presents an assessment of the current state of knowledge and capability in seasonal climate prediction at the end of the 20th century. The discussion covers the full range of issues involved in climate forecasting, including (1) the theory and empirical evidence for predictability; (2) predictions of surface boundary conditions, such as sea surface temperatures (SSTs) that drive the predictable part of the climate; (3) predictions of the climate; and (4) a brief consideration of the application of climate forecasts. Within this context, the research of the coming decades that seeks to address shortcomings in each area is described
- âŠ