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How does climate change affect seasonal weather?
Over the past four decades there has been a gradual trend toward higher sea surface temperatures and surface land temperatures across the globe. Increasing greenhouse gas concentrations have resulted in regional temperature biases in the order of +0.8°C (1.4°F) over the UK and Ireland compared to the 1961 - 1990 average temperatures. As temperatures rise further, the climate impacts are likely to become more pronounced across the nation.
The warm bias incorporated into our seasonal weather prediction model has the following impacts:
The inter-annual variability in winter temperature over the British Isles is around ±1.5°C (2.7°F) - so the odds of seasonal temperatures being "above average" are now ~34% (one third) greater than during the 1961 - 1990 base period. Currently around 1 in every 3 seasons are below the long term average - a value that is expected to decrease to 1 in every 8 by the year 2050
Rainfall intensity is now 6 to 7% greater. With a fixed relative humidity, a warmer atmosphere can hold more water vapour, so there is a greater potential for heavier rainfall events than in the past or when the seasonal prediction indicates average or below average UK temperatures
Flooding due to sporadic rainfall events (thundery showers) are now much more frequent and extremes of wet and dry weather are now more common - an impact that can either be dampened or amplified on a regional scale by a strong blocking pattern in the upper atmosphere. Examples of extreme weather patterns induced by strong jet stream deflections include the droughts in Russia and flooding in Pakistan during summer 2010
The number of rainfall days has remained largely constant, however there is a steady trend of wetter summers in the north and hotter and drier summer in the south of the British Isles. This is due to a gradual northward shift of the polar jet stream and sub-tropical highs as the world warms. As a long term average, there is generally the same cloud fraction as in the past. Clouds are located at higher altitudes and are more likely to carry a tiny bit more water or ice particles that can fall as precipitation
Heat waves are between 5 and 30% more frequent during the summer seasons. The higher temperatures dramatically increase the average cost of air conditioning bills by 40 to 60% for home and business*
The number of days of snowfall in the UK have decreased by 15 to 25%
The energy demand for household heating in winter has decreased by 10 to 20%*
*These figures assume that customer behaviour in energy usage and energy efficiency has remained unchanged. In general there has been a steady trend toward a higher temperature comfort zone, which has been offset by improved household loft and roof insulation and through the increased use of double glazing. Air conditioning is now used in households much more frequently and is also considered a desired component of the work place
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How does Weather Logistics UK calculate climate impacts?
The model at Weather Logistics UK, is based on deviations from the 1961 - 1990 temperature averages. The temperature anomalies (T_predict - T_climate) for each region are linearly combined with climate information by a logistic regression scheme that considers: the effects of the El Niño Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO index and forecast), the solar cycle and the global temperature trends. The sea surface temperature climatology over the North Atlantic Ocean is perturbed, using National Oceanic and Atmospheric Adminstration (NOAA) data, so that air tracking toward the British Isles is either warmer or cooler depending upon the current anomaly. The laws of thermodynamics are used to perturb the rainfall totals, in line with increases in the saturated vapour pressure of the atmosphere as the air warms or cools. Gaussian statistics are used to predict the liklihood of atmospheric parameters, such as temperature and rainfall, exceeding prescribed boundary intervals.
