Seasonal Forecasting Simon Mason Seasonal Forecasting Using the Climate Predictability Tool Bangkok, Thailand, 12 – 16 January 2015

2 Seasonal Forecasting Using the Climate Predictability Tool Introduction “Are you, Socrates, the one who is called the Expert?” “Not if it is better,” Socrates replied, “to be called the Idiot.” “It would be better to be called the Idiot than the Expert Meteorologist!” Xenophon, Memorabilia

3 Seasonal Forecasting Using the Climate Predictability Tool Forecasting Why is forecasting the weather and climate so difficult?

4 Seasonal Forecasting Using the Climate Predictability Tool Dramatis Personae Pythagoras, the mathematician Daedalus, the inventor Zeus, Father of Gods and men Negotiated deal Temperature atop Etna today will be 1.8°C colder than the square of the temperature yesterday.

5 Seasonal Forecasting Using the Climate Predictability Tool Forecasting a simple (?) system Etna has a mean temperature of about 0.5°C, and standard deviation of about 1.0°C. If Sunday was 0.3°C: Daedalus’s thermometer was off by five one thousandths of a degree (0.005°C)

6 Seasonal Forecasting Using the Climate Predictability Tool Weather prediction What do we need to know to make a good prediction? – The current state (initial conditions) – How the current state will evolve So there are two sources of uncertainty: – The current state (use an ensemble) – How the current state will evolve (use multiple models) How do we know how the current state will evolve? – What causes it to evolve? – How has it evolved in the past?

7 Seasonal Forecasting Using the Climate Predictability Tool Spot the Ball Competition Where is the ball now? Where will the ball be in 20 seconds? Who will win the match?

8 Seasonal Forecasting Using the Climate Predictability Tool Sources of Predictability We can make forecasts at different timescales because there are different reasons why the predictions can work: – days:current weather – months:sea-surface temperatures – years:sub-surface ocean temperatures – decades:atmospheric composition

9 Seasonal Forecasting Using the Climate Predictability Tool El Niño and global climate Sea-surface temperatures, especially in the equatorial Pacific, can affect the frequency and intensity of different weather patterns Typical El Niño impact during December - February

10 Seasonal Forecasting Using the Climate Predictability Tool Thailand MAM rainfall and SSTs Correlations between pre-monsoon (March – May) rainfall in Thailand and global sea-surface temperatures (SSTs), 1961 – 2010.

11 Seasonal Forecasting Using the Climate Predictability Tool Seasonal prediction We make seasonal forecasts by predicting the statistics of weather rather than the actual weather at any specific time. – Based on historical relationships – By looking at the statistics of lots of very long-range weather forecasts, ideally from lots of different models

12 Seasonal Forecasting Using the Climate Predictability Tool Summary To forecast the weather we need to know: – What is the current weather? – How will the weather evolve? Because of errors in answering both questions, we cannot predict the weather beyond a few days. There are influences on the atmosphere that affect what type of weather patterns are more or less likely to occur – primarily sea temperatures for predicting the next few months. We predict the effect of these influences in two possible ways: – Looking at evidence for influences in the past – Modeling the processes that cause the effects