Glossary
Accumulated Cyclone Energy (ACE) - A measure of a named storm’s potential for wind and storm surge destruction defined as the sum of the square of a named storm’s maximum wind speed (in 104 knots2) for each 6-hour period of its existence. The 1950-2000 average value of this parameter is 96 for the Atlantic basin.
Accuracy - The average distance between a set of measurements and the ‘true’ value of the object being measured. For a prediction, this can be defined as the average distance between a set of forecasts and an estimate of the observational reference.
Anomalies - Departures of specific measurements and/or forecasts from their long-term climatological values. Anomalies describe how much a specific variable differs from its normal state.
Atlantic Multi-Decadal Oscillation (AMO) - A mode of natural variability that occurs in the North Atlantic Ocean and evidencing itself in fluctuations in sea surface temperature and sea level pressure fields. The AMO has been associated with fluctuations in the strength of the oceanic thermohaline circulation and tropospheric aerosols.
Atlantic Basin - The area including the entire North Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico.
Bias - Average difference between the values of the forecasts and the observations on a long- term basis. While accuracy is always positive, the bias could be either positive or negative depending on the situation.
Climate - Climate in a narrow sense is usually defined as the average weather, or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. The classical period for averaging these variables is 30 years, as defined by the World Meteorological Organization. The relevant quantities are most often surface variables such as temperature, precipitation and wind. Climate in a wider sense is the state, including a statistical description, of the climate system.
Climate Model Simulations - Numerical solutions of a set of equations that represent the most relevant processes describing the climate system. Climate models can be of very different levels of complexity but the most elaborate ones appear to be able to realistically reproduce key meteorological and climatological phenomena.
Climate Variability - Climate variability refers to variations in the mean state and other statistics (such as standard deviations, the occurrence of extremes, etc.) of the climate on all spatial and temporal scales beyond that of individual weather events. Variability may be due to natural internal processes within the climate system (internal variability), or to variations in natural or anthropogenic external forcing (external variability).
Downscaling - Downscaling is a method that derives local- to regional-scale (10 to 100 km) information from larger-scale models or data analyses. Two main methods exist: dynamical downscaling and empirical/statistical downscaling. The dynamical method uses the output of regional climate models, global models with variable spatial resolution or high-resolution global models. The empirical/statistical methods develop statistical relationships that link the large-scale atmospheric variables with local/regional climate variables. In all cases, the quality of the driving model remains an important limitation on the quality of the downscaled information.
Dynamical Forecast - Forecast performed with a global climate model (GCM), wherein the forecasted hurricane activity is based on that simulated by the model. It relies on finding and tracking the tropical cyclones directly in that simulation and calibrating the simulated statistics based on past hindcasts.
El Niño Southern Oscillation (ENSO) - The term El Niño was initially used to describe a warm-water current that periodically flows along the coast of Ecuador and Peru, disrupting the local fishery. It has since become identified with a basin-wide warming of the tropical Pacific Ocean east of the dateline. This oceanic event is associated with a fluctuation of a global-scale tropical and subtropical surface pressure pattern called the Southern Oscillation. This coupled atmosphere-ocean phenomenon, with preferred time scales of 3 to about 7 years, is collectively known as the El Niño-Southern Oscillation. It is often measured by the surface pressure anomaly difference between Darwin and Tahiti and the sea surface temperatures in the central and eastern equatorial Pacific. During an ENSO event, the prevailing trade winds weaken, reducing upwelling and altering ocean currents such that the sea surface temperatures warm, further weakening the trade winds. This event has a great impact on the wind, sea surface temperature, and precipitation patterns in the tropical Pacific. It has climatic effects throughout the Pacific region and in many other parts of the world, through global teleconnections. Moderate or strong El Niño events occur irregularly, about once every 3-7 years on average. The cold phase of ENSO is called La Niña.
Ensemble - A collection of model simulations characterizing a climate prediction (or projection). Differences in the initial conditions and model formulations result in different evolution of the modelled systems and may give information on uncertainty associated with model error, error in the initial conditions and with the internally generated climate variability.
Forecast time - The time elapsed since the beginning of the forecast. This can be a range of time (e.g. months 2-4).
Global Climate Models or General Circulation Models (GCMs) - Computer code used to solve a set of mathematical equations describing the laws of physics relevant to the atmospheric and oceanic circulation, the distribution of heat and the interaction between electromagnetic radiation and atmospheric gases and aerosols. Climate models represent an implementation of our theoretical knowledge of the climate system, describing interconnections between processes. They consist of different modules describing the atmosphere, ocean, sea-ice/snow and the land surface, and represent the world in terms of boxes stacked next to and on top of each other. The values for temperature, motion and mass are solved in each of these boxes, based on well known physical laws.
Hindcast - A forecast made for a period in the past that leaves the year out of the prediction model for which you are currently forecasting. A set of hindcasts can be used to bias-correct and/or calibrate the forecast and/or provide a measure of the skill.
Hurricane (H) - A tropical cyclone with 1-min sustained low-level winds of 119 km/h (33 m/s, 74 mph or 64 knots) or greater.
Hybrid Forecast - A forecast which relies on both global climate models and past statistical relationships between the climate and hurricane activity. Usually the climate model is used to derived the large-scale conditions that will prevail during the hurricane season. The hurricane predictions are then derived from known historical relationships between the predicted large-scale conditions and hurricane activity.
Likelihood - A probabilistic estimate of the occurrence of a single event or of an outcome, for example, a climate parameter, observed trend, or projected change lying in a given range. Likelihood may be based on statistical or modeling analyses, elicitation of expert views, or other quantitative analyses.
Main Development Region (MDR) - An area in the tropical Atlantic where a majority of major hurricanes form, which we define as 7.5-22.5°N, 20-75°W.
Major Hurricane (MH) - A hurricane which reaches 1-min sustained low-level wind of at least 178 km/h (50 m/s, 111 mph or 96 knots) at some point in its lifetime.
Mean Square Skill Score (MSSS) - A measure to evaluate the skill of hurricane forecasts:
MSSS = 1 - MSEfor / MSEref
where MSEfor is the mean square error of the forecast and MSEref is the mean square error of a baseline forecast. MSSS = 1 shows a perfect forecast and MSSS=0, a forecast with no improvement over the baseline. It is important to keep in mind that a high MSSS does not necessarily mean a good forecast, but simply a large improvement compared to what was previously available before (the reference).
Multivariate ENSO Index (MEI) - An index defining ENSO that takes into account tropical Pacific sea surface temperatures, sea level pressures, zonal and meridional winds and cloudiness.
Named Storm (NS) - A hurricane, a tropical storm or a sub-tropical storm.
Predictability - The extent to which future states of a system may be predicted based on knowledge of current and past states of the system. Since knowledge of the climate system’s past and current states is generally imperfect, as are the models that utilise this knowledge to produce a climate prediction, and since the climate system is inherently nonlinear and chaotic, predictability of the climate system is inherently limited.
Saffir/Simpson Hurricane Wind Scale - A measurement scale ranging from 1 to 5 of hurricane wind intensity. One is a weak hurricane; whereas, five is the most intense hurricane. Hurricanes of any intensity can cause devastating levels of damage.
Southern Oscillation Index (SOI) - A normalized measure of the surface pressure difference between Tahiti and Darwin. Low values typically indicate El Niño conditions.
Skill - Measures of the success of a prediction against observationally-based information. No single measure can summarize all aspects of forecast quality, and therefore a suite of metrics is typically considered.
Skill Score - A relative measure of the quality of the forecasting system compared to the benchmark or reference forecast (e.g. climatology, persistence, etc.).
Statistical Forecast - Forecast based on past (observed) relationships between climate and tropical cyclone activity. The forecast is generally based on atmospheric/oceanic conditions observed a few months ahead of or at the beginning of the hurricane season.
Statistical Significance - Describes the likelihood of an observation or a result being due to pure chance. It is often used in connection with a null-hypothesis (an alternative explanation, usually such as there is no correlation or no causal relationship), and gives the odds that the null-hypothesis is correct.
Thermohaline Circulation (THC) - A large scale circulation in the Atlantic Ocean that is driven by fluctuations in salinity and temperature. When the THC is stronger than normal, the AMO tends to be in its warm (or positive) phase, and more Atlantic hurricanes typically form.
Tropical Cyclone (TC) - A large-scale circular flow occurring within the tropics and subtropics which has its strongest winds at low levels; including hurricanes, tropical storms and other weaker rotating vortices.
Tropical Storm (TS) - A tropical cyclone with maximum sustained winds between 63 km/h (18 m/s, 39 mph or 34 knots) and 118 km/h (32 m/s, 73 mph or 63 knots).
Uncertainty - Means lack of precision or that the exact value for a given time is not predictable, but it does not usually imply lack of knowledge. Often, the future state of a process may not be predictable, such as a roll with dice, but the probability of finding it in a certain state may be well known (the probability of rolling a six is 1/6, and flipping tails with a coin is 1/2). In climate science, the dice may be loaded, and we may refer to uncertainties even with perfect knowledge of the odds.
Vertical Wind Shear - The difference in horizontal wind, typically measured between 200 mb (approximately 12 km) and 850 mb (approximately 1.6 km)
