Daehyun Kang

Prediction of the Arctic Oscillation in boreal winter by dynamical seasonal forecasting systems

This study assesses the skill of boreal winter Arctic Oscillation (AO) predictions with state-of-the-art dynamical ensemble prediction systems (EPSs): GloSea4, CFSv2, GEOS-5, CanCM3, CanCM4, and CM2.1. Long-term reforecasts with the EPSs are used to evaluate how well they represent the AO and to assess the skill of both deterministic and probabilistic forecasts of the AO. The reforecasts reproduce the observed changes in the large-scale patterns of the Northern Hemispheric surface temperature, upper level wind, and precipitation associated with the different phases of the AO. The results demonstrate that most EPSs improve upon persistence skill scores for lead times up to 2 months in boreal winter, suggesting some potential for skillful prediction of the AO and its associated climate anomalies at seasonal time scales. It is also found that the skill of AO forecasts during the recent period (1997–2010) is higher than that of the earlier period (1983–1996).

Increase in the potential predictability of the Arctic Oscillation via intensified teleconnection with ENSO after the mid-1990s

This study examines why the seasonal prediction skill of the Arctic Oscillation (AO) has increased significantly since the mid-1990s in state-of-the-art seasonal forecasting systems in operation. This skill increase is primarily attributed to variability over the North Atlantic with an enhanced connection between the AO and the El Niño and Southern Oscillation (ENSO). The relationship between ENSO and AO depends primarily on low-frequency variability in the North Pacific driven by the North Pacific Gyre Oscillation, as represented by the Hawaiian sea level pressure (SLPHI) index. When the sign of the SLPHI index and that of the NINO3.4 index are out-of-phase (in-phase) with the variability center of ENSO shifted to the central Pacific (eastern Pacific), more intense (weaker) ENSO-AO teleconnection results. Linear barotropic model experiments with prescribed ENSO forcing and differing phase and intensity of SLPHI support the observed relationship in La Niña years, highlighting the important and independent role of the SLPHI variability as a modulator of the ENSO teleconnection to higher latitudes.

Validation of the experimental hindcasts produced by the GloSea4 seasonal prediction system

Using 14 year (1996–2009) ensemble hindcast runs produced with the Global Seasonal Forecasting System version 4 (GloSea4), this study evaluates the spatial and temporal structure of the hindcast climatology and the prediction skill of major climate variability. A special focus is on the fidelity of the system to reproduce and to forecast phenomena that are closely related to the East Asian climate. Overall the GloSea4 system exhibits realistic representations of the basic climate even though a few model deficiencies are identified in the sea surface temperature and precipitation. In particular, the capability of GloSea4 to capture the seasonal migration of rain belt associated with Changma implies a good potential for the Asian summer monsoon prediction. It is found that GloSea4 is as skillful as other state-of-the-art seasonal prediction systems in forecasting climate variability including the El-Nino/southern oscillation (ENSO), the East Asian summer monsoon, the Arctic Oscillation (AO), and the Madden-Julian Oscillation (MJO). The results presented in this study will provide benchmark evaluation for next seasonal prediction systems to be developed at the Korea Meteorological Administration.