Added-value from initialization in skilful predictions of North Atlantic multi-decadal variability

Transcription

1 Added-value from initialization in skilful predictions of North Atlantic multi-decadal variability J. García-Serrano #, V. Guemas &, F. J. Doblas-Reyes * Climate Forecasting Unit (CFU) at Institut Català de Ciències del Clima (IC3), Barcelona, Spain # currently at LOCEAN-IPSL, UPMC, Paris, France & also at CNRM-GAME, Météo-France, Toulouse, France * also at ICREA, Barcelona, Spain

2 - Decadal prediction bridges seasonal forecast and climate-change projection - The decadal timescale is a key planning horizon for adaptation - Decadal prediction not only deals with anthropogenically forced variability, but also with natural (unforced) internal variability - Decadal prediction opens the possibility of achieving skill in multi-year timescales, and in both regional and large-scale domains Meehl et al. (2009, BAMS)

3 Atlantic Multi-decadal Oscillation (AMO) SST anomalies in the North Atlantic [EQ-60N/80W-0E] minus global SST anomalies [60S-60N] Trenberth and Shea (2006, GRL) Due to the limited length in the observational record, a more proper denomination for the AMO-like variability is Atlantic Multi-decadal Variability (AMV) Ting et al. 2011, GRL) (i.e.

4 CMIP5 (initialized) hindcasts + historical&rcp4.5: 10-year long ensemble size & start date frequency: variable multi-model: 11 systems (5-yr) / 6 systems (1-yr) over the period

5 - Verification upon 4-year forecast averages, in order to retain low-frequency (interannual-to-decadal) predictability and remove most of the interannual variability that is unpredictable beyond one year in dynamical forecasting (e.g. ENSO) García-Serrano and Doblas-Reyes (2012, Clim Dyn) Goddard et al. (2013, Clim Dyn) - Verification over ERSST as observational reference clear skill improvement in INIT compared to NoINIT Doblas-Reyes et al. (2013, Nature Comms)

6

7

8 Correlation at each grid-point between multi-model ensemble-mean detrended SST and observed (ERSST) AMV index for INIT [left] and NoINIT [middle]

9 Correlation at each grid-point between multi-model ensemble-mean detrended SST and observed (ERSST) AMV index for INIT [left] and NoINIT [middle]

10 - CMIP5-Init capture the variations in the AMV index: the cold AMV phase spanning from late-1960s to mid-1990s and the warming over the last decade. These episodes appear to be driven by subpolar SST changes (Terray 2012) and attributable to internal variability (Ting et al. 2009; DelSole et al. 2011; Terray 2012). - CMIP5-NoInit show a minor, positive trend over these last decades. - Added multi-year skill in CMIP5-Init relies on initializing the internal component of the AMV.

11 summary - The CMIP5 initialized decadal hindcasts outperform damped persistence and historical forcing runs in reproducing the observed AMV fluctuations. The forced component provides a flat positive trend. - The radiatively-forced signal, including natural and anthropogenic forcings, appears to have an impact on the central North Atlantic midlatitudes and at both sides of Greenland in relation to the observed AMV. But, it fails to predict the horseshoe-like AMV signature. - The CMIP5 initialized decadal hindcasts have skill in forecasting the horseshoe-like AMV signature, particularly over the subpolar gyre region, the Labrador Sea, and the eastern subtropical North Atlantic. - The impact of the start date frequency is described, showing that the standard of five-year interval between start dates yields the main features of the AMV skill that are robustly detected in hindcasts with yearly start date sampling.

12 Added-value from initialization in skilful predictions of North Atlantic multi-decadal variability thank you!! Guemas et al. Decadal prediction in the Mediterranean region Poster S4-65