2019 ECMWF Seminar A perspective on predictability on sub-seasonal to seasonal time-scales Brian Hoskins Grantham Institute for Climate Change, Imperial College London Department of Meteorology, University of Reading, UK 2011: 12th IMO Lecture at the quadrennial WMO Assembly 2013: Q. J. R. Meteorol. Soc., 138, 573-584 Dynamics can give chaos & predictability Lorenz model: chaos The Quasi-Biennial Oscillation in the equatorial stratosphere Pressure Pressure (hPa) Weather & Climate Prediction Focus 1980-2005 1day 1week 1month 1season 1year 1 decade 1 century The Seamless Prediction Problem 1day 1week 1month 1season 1year 1 decade 1 century Fronts Cyclones MJO ENSO PDO Convective NAO AMO systems Blocks QBO The Seamless Prediction Problem Ice sheets atmospheric chemistry land moisture vegetation ocean skin upper full atmosphere region global 1day 1week 1month 1season 1year 1 decade 1 century Fronts Cyclones MJO ENSO PDO Convec NAO AMO systems Blocks QBO The Prediction Problem Time-scale of interest t shorter longer slave phenomena free external Mid-latitude weather systems growing on a zonal flow MSLP T967 Blocking 20 November 1993 12 UTC on the dynamical Z on 250 hPa tropopause (2 PVU) Tyrlis & Hoskins JAS 2008 Rossby waves High PV y + phase group Low PV u + - Stationary waves possible Influence to the east On the sphere and influenced by the ambient jets Strong jets can act as waveguides Autumn 2000: record rain in the UK Pakistan floods summer 2010/Indian floods 2013 250 hPa winds 24-30 July 2010 200 hPa winds 14-18 June 2013 The link between the Mediterranean summer climate and the Indian Summer Monsoon Rodwell & H 1995 Vertical motion in the lower troposphere Observed Simple model Idealised S Asian heating no mountains Summer 2002 Blackburn & Hoskins (2006) Flooding in Central Europe Drought in India Teleconnections - Wallace & Gutzler 1981: Maximum negative correlations in monthly pressure-like fields MSLP Z500 North Atlantic Oscillation in DJF: Zonal wind over the Atlantic (0-60W) Ambaum et al (2001) NAO- NAO+ Subtropical jet Eddy-driven jet Greenland blocking = NAO-? Woollings et al 2008 DJF global circulation anomalies associated with an MJO cycle Matthews et al 2004 45-day cycle from 2 EOFs of 20-200 day filtered OLR: heavy contours Regressed ψ200: colours Regressions on OLR 60-100E, 0-20N, JJA 2009, Lag = 0 150hPa ξ, V OLR 950hPa V Regressions on OLR 60-100E, 0-20N, JJA 2009, Lag = ±3d Lag -3 Lag +3 Tropical-extratropical interaction in DJF 21 Jan2008 370K PV & winds 16 Jan2008 350K PV & winds 250hPa winds in the N Atlantic region 18-20 Jan 2008 21-23 Jan 2008 ms-1 The stratospheric connection Baldwin & Dunkerton 2001 Composites of annular behaviour for weak and strong 10 hPa vortex events Anomalies in underlying land & ocean Land - moisture, snow cover Sea Ice SST – tropics – ENSO etc mid-lats Cold Land Warm Ocean 2019 ECMWF Seminar A perspective on predictability on sub- seasonal to seasonal time-scales Concluding Comments •Context: the seamless weather-climate prediction problem •On all time-scales natural phenomena and external conditions give promise of some possible predictability •The extent to which the possible skill on various time-scales may be useful is not yet clear •We need to observe, model and understand to realise the potential skill •We need to learn to recognise how much music there is in what may seem like noise! Theta on PV2 18-25 Jan 2008 Forecasts for Pakistan floods of 2010 ECMWF Rainfall 28 & 29 July : Observed 4-day forecast Forecast lead time Niklas Boers et al 2019 Nature Daily TRMM rainfall data 1998 – 2016 Extreme Rainfall Events (EREs) – greater than 95th %ile of wet days Complex network methodology used to look at connections. Time series at each node compared with those at the other 575,999. Synchronisation taken as within 10 days (results not sensitive). Significant if P < 0.005. Rossby wave-guides in DJF H & Ambrizzi, 1993.