Factors influencing sub-seasonal forecast skill of Greenland Blockings

MARISOL OSMAN; GRAMS, CHRISTIAN M.; BEERLI, REMO

Conferencia; S2S Summit; 2023

Greenland blocking (GL) resembles the negative phase of the NAO andfeatures a strong positive Z500 anomaly over Greenland and a zonallyaligned negative anomaly stretching from the eastern North Atlanticinto Northern Europe. The prevailing westerly flow is then deflectedsouthward and extends into the Mediterranean. It causes meltingevents of the Greenland Ice Sheet which can impact global sea-levelrise and has strong downstream impacts on Europe. It occursyear-round, although is more common in winter (11.7%) compared tosummer (9.1%). GL is forecast with good ability by S2S models. Thisskill is driven by the performance in winter, when GL is persistent.In this study, we explore whether the skill of GL blocking can belinked to external meteorological drivers or the prevalence ofspecific meteorological features. Re-forecasts using the EuropeanCentre for Medium-Range Weather Forecasts for the 1999-2019 periodare considered and compared against ERA Interim reanalysis over thesame period. We focus on the factors affecting the skill, as depictedby the Brier Skill Score, from lead times 6 to 10 days, where theskill is 30% to 70% smaller than the skill at lead time 1 day.Resultsshow that most of the GL blocking events associated with low skilloccur in spring. In this season, the model fails in forecasting thetransition from Scandinavian Blocking to Greenland Blocking, inopposition to the rest of the seasons, when this transition is wellpredicted. The analysis of the role of large-scale processes thataffect GL skill reveals that half of the forecasts of GL eventsinitialized up to 30 days after a sudden stratospheric warming showspoor skill. In addition, the forecasts of GL events initialized withan active MJO in phase 6 and 7 present good skill whereas thoseforecast GL events initialized during an active MJO in phase 2 to 4show poor skill. This link between large-scale factors and skilloffers potential guidance in operational forecasting.p { margin-bottom: 0.1in; line-height: 115%; background: transparent }