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Prediction of the Arctic Oscillation in boreal winter by dynamical seasonal forecasting systems
Kang, Daehyun,Lee, Myong-In,Im, Jungho,Kim, Daehyun,Kim, Hye-Mi,Kang, Hyun-Suk,Schubert, Siegfried D.,Arribas, Alberto,MacLachlan, Craig American Geophysical Union 2014 Geophysical research letters Vol.41 No.10
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 2months 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).
Why does the MJO’s Southward Detouring of Indonesia Occur Only during the Australian Monsoon Deason?
Daehyun Kang,Daehyun Kim 한국기상학회 2021 한국기상학회 학술대회 논문집 Vol.2021 No.10
The Madden-Julian Oscillation (MJO) is the dominant mode of intraseasonal variability of eastward propagating disturbances in the tropical atmosphere. The MJO’s eastward propagation exhibits a peculiar behavior across the Indo-Pacific Maritime Continent (MC): the major convective envelop propagates through the south of the MC rather than the equator, so-called MJO’s southward detouring. Despite that local precipitation anomaly and remote impact of the detouring MJO differ from those of the MJO without detouring, dynamical processes associated with the detouring is not fully understood. One of the remaining questions is why the southward detouring occur only during boreal winter (December-March; DJFM), which the current study aims to address. Our results strongly suggest that the southward detouring in DJFM is primarily contributed by i) the moisture-precipitation relationship and ii) the wind-evaporation feedback. Even though the MJO-related variance of column-integrated moisture anomalies in DJFM are comparable with those in the seasons before (October-November; ON) and after (April-May; AM), moisture sensitivity of precipitation is higher in DJFM than that in the other seasons. The larger mean precipitation in DJFM corresponds to the smaller convective moisture adjustment timescale, resulting in the more efficient conversion of anomalous moisture to anomalous precipitation. Additionally, with the lower tropospheric westerly mean wind in DJFM distinguished from other seasons, the larger surface latent heat flux is positively correlated with the westerly wind anomalies on convection anomalies. This stronger wind-evaporation feedback is positively contributes to the maintenance of intraseasonal convection anomalies. In summary, the seasonal cycle of the mean state factors important for moisture-precipitation coupling and wind-evaporation feedback closely follows that of Australian monsoon active exclusively in DJFM, resulting in the seasonal locking of the MJO"s southward detouring in boreal winter.