Subseasonal forecasting Sources of predictability Simon Mason simoniri
Sub-seasonal forecasting: Sources of predictability Simon Mason simon@iri. columbia. edu Ángel Muñoz agmunoz@iri. columbia. edu The Caribbean Climate Outlook Forum (Cari. COF) 2018 Wet/Hurricane Season Christ Church, Barbados, 24 – 25 May 2018
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Forecast skill 3 Cari. COF 2018 Wet Hurricane Season
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 4 Cari. COF 2018 Wet Hurricane Season
Weather prediction skill temperature 3 -day forecasts 5 Cari. COF 2018 Wet Hurricane Season
Weather prediction skill storm tracks 6 Cari. COF 2018 Wet Hurricane Season
Seasonal prediction skill temperature 7 Cari. COF 2018 Wet Hurricane Season
Seasonal prediction skill rainfall 8 Cari. COF 2018 Wet Hurricane Season
Forecast skill Source of skill: atmospheric initial conditions ECMWF forecast skill } Weekly average rainfall Anomaly correlation for Jun– Aug Source of skill: Madden-Julian Oscillation and Surface boundary conditions Li y Robertson, 2015 9 Cari. COF 2018 Wet Hurricane Season
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Sources of predictability • Tropical-extratropical interactions: heavy tropical rainfall and storms can affect the weather in the mid-latitudes • Persistence of weather regimes: the jet stream can get stuck in one place • Land-surface conditions: dry soil can contribute to persistent heatwaves; snow cover can affect weather patterns • Large-scale weather patterns: some large-scale weather patterns evolve at sub-seasonal scale, especially the Madden-Julian Oscillation (MJO) 11 Cari. COF 2018 Wet Hurricane Season
Large-scale weather patterns MJO Phase Day 1 2 3 4 5 6 7 8 1 12 Cari. COF 2018 Wet Hurricane Season
Conclusions • Sub-seasonal forecasts are forecasts for about 2 – 4 weeks in the future. They may provide valuable information for early alerts and advanced preparedness • Sub-seasonal timescales lie between weather (prediction from current conditions) and seasonal (prediction from sea-surface temperatures) timescales • Some predictability may be provided by both sources • In the tropics, the main source of sub-seasonal predictability is the Madden-Julian Oscillation (MJO) • The MJO has its strongest effects in the Caribbean in the winter months 13 Cari. COF 2018 Wet Hurricane Season
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