Mechanisms of Decadal Climate Predictability

1. Mechanisms of Decadal Climate Predictability Leon Hermanson and Rowan Sutton, Department of Meteorology, University of Reading, UK Second year JJA ensemble mean difference in 1.5m temperature (◦C) overlaid with significance contours at 95%, 97% and 99%. Background A Cold European Summer Initial phase: Pre-industrial to modern day Generation of initial conditions Two initial conditions chosen for case study • In this case, Western European summer temperatures are predictable 30 months ahead (figures from 18 months ahead). • Surface temperature anomalies range from 0.5 – 1.5 ◦C. • Cold sea surface temperatures persist in the East Atlantic. • Anomalous northerly winds blow in over the continent. • An anomalous high pressure sits south of Greenland. • A wave-train is visible in 500mb height. • To what extent does knowledge of initial conditions constrain long-term climate forecasts? • What are the mechanisms that give rise to decadal climate predictability? • This study investigates how predictability arises beyond the limit of seasonal time scales. • Recognising that predictability is likely to be dependent on initial conditions we consider model-based case studies rather than average predictability. Second year JJA ensemble mean difference in SST (◦C) overlaid with 10m winds (m/s). Experiment ensembles (red and purple) are started from two different members of a 20th century transient ensemble (black lines). 1860 1950 ~1980 Second year JJA ensemble mean difference in sea level pressure (hPa) overlaid with 500mb height (m). Mechanism: Persistence and advection of large SST anomalies? Ensemble mean difference in instantaneous initial conditions for 500m ocean heat content (left) and sea surface temperatures (SST, right). Second year DJF ensemble mean difference in precipitation (mm/day) overlaid with significance contours at 95%, 97% and 99%. Second year DJF ensemble mean difference in 1.5m temperature (◦C) overlaid with significance contours at 95%, 97% and 99%. A Cold American Winter A Dry Amazon Wet Season • Here, East US winter temperatures are predictable 12 months ahead. • Anomalies range from 1—3 ◦C. • Alaska and North Canada experience opposite anomalies. • 500mb height shows PNA-like pattern, these temperature anomalies are typical of the PNA. • SST pattern is similar to Pacific Decadal Oscillation (PDO) pattern. • SST anomalies in the Atlantic may also play a role. • Precipitation anomalies over the Amazon are predictable 12 months ahead for these initial conditions. • December, January and February are the beginning of the wet season. • Anomalies range from 0.5—2 mm/day. • The equatorial Pacific is warm in the west and cold in the east. • The tropical Atlantic shows small anomalies of both signs. • 250mb winds and omega show convergence and descent over South America. Second year DJF ensemble mean difference in SST (◦C) overlaid with 250mb winds (m/s). Second year DJF ensemble mean difference in SST (◦C) overlaid with 10m winds (m/s). Second year DJF ensemble mean difference in sea level pressure (hPa) overlaid with 500mb height (m). Second year DJF ensemble mean difference in omega (hPa/s) overlaid with significance contours at 95%, 97% and 99%. Mechanism: The Hadley circulation is perturbed by predictable tropical SST changes in the oceans? Mechanism: Persistence of SST anomalies in the North Pacific and Atlantic? GCEP