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In this study, week-1 to week-4 forecasts of tropical cyclones (TCs) in the western North Pacific are evaluated. The CWB TC Tracking System 2.0 (Lo et al. 2021) is used to objectively detect TCs in the 46-day ECMWF ensemble (ENS) forecasts in the 2021 season and also the reforecasts during 2001-2020. Preliminary evaluations of the probabilistic TC activity forecasts in the 20-year reforecasts show promising forecast skills. The reliability diagrams indicate slight over-forecasting bias in the weeks 1-4 forecasts, and the AUCs (Area Under Curves) are ranging from 0.91 (week-1) to 0.80 (week-4). The relationship between the TC activity forecast skill and the western North Pacific summer monsoon (WNPSM) is also investigated. The WNP monsoon index (WNPMI) proposed by Wang et al. (2001) is computed to provide a measure for the summer monsoon, and the TC forecast skills are evaluated under different levels of the WNPMI. To identify the potential false alarms, a spatial-temporal track clustering technique (Tsai et al. 2019) is implemented to objectively group similar vortex tracks in the 51-member forecasts. The corresponding ensemble mean track for each cluster is then used for performing the event-based verifications after the end of season. More details about the TC forecast verifications in weeks 1-4 using the ECMWF monthly ensemble will be presented in the meeting.

本研究使用ERA5再分析資料，針對氣象局一步法(1-tier)全球模式在台灣地區及東亞地區之季節尺度地面（SKT）與海表面（SST）溫度變異預報進行校驗及誤差修正。針對2000-2020年，以ERA5與預報之逐月與逐季平均變異之標準差比值為修正係數，修正模式預報結果。計算模式在修正前後地表溫度之機率密度函數（PDF）、累積機率分佈函數（CDF），與修正前後之連續分級機率評分（CRPS）評估改進後之結果。結果顯示東亞地區逐月尺度下SKT與SST變異，預報誤差各發生在±2°與±1°之間，累進誤差則呈現暖變異時低估，冷變異高估的情況。經過校正程序修正模式輸出後，不論逐月或季節尺度，SKT變異皆以首月預報之改善成果較佳，SST變異則是預報後期，改善效果較好。透過霍夫莫勒圖（Hovmöller diagram）分析SST預報改善前後，在西北太平洋地區隨時間變異的演化情況，結果顯示不但能提升模式掌握此變數時空分布的演變過程，對於其冷暖變異的幅度，亦更接近於真實變化的情況。本結果可提供未來修正大氣模式初始資料場，進行次季節至季節尺度預報之重要依據。 This study aims to improve seasonal scale forecast in surface temperature anomaly from the 1-tier model (1T1) developed by the Central Weather Bureau during (CWB) 2010-2020. To improve the forecasts of skin (SKT) and sea surface (SST) temperature anomalies, the ratio of standard deviations derived from ERA5 and 1T1 is considered as a factor to correct the biases generated by the original forecast. After corrected, the results indicate that this method efficiently reduces the biases in ranges of ±2° and ±1° originally found in the probability forecasts of SKT and SST anomalies, respectively. The systematic bias associated with overestimation (underestimation) occurring in warm (clod) anomaly is also eliminated. It is also found that the model is overall significantly improved in SKT at the first forecast month, as well as in SST at the second and the following forecast months on seasonal time-scale. It should be expected that the proposed method might be deployed to the further CWB operations and improves the seasonal forecast as well. &nbsp

Despite evidence from a number of Earth systems that abrupt temporal changes known as regime shifts are important, their nature, scale and mechanisms remain poorly documented and understood. Applying principal component analysis, change-point analysis and a sequential t-test analysis of regime shifts to 72 time series, we confirm that the 1980s regime shift represented a major change in the Earth's biophysical systems from the upper atmosphere to the depths of the ocean and from the Arctic to the Antarctic, and occurred at slightly different times around the world. Using historical climate model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) and statistical modelling of historical temperatures, we then demonstrate that this event was triggered by rapid global warming from anthropogenic plus natural forcing, the latter associated with the recovery from the El Chichón volcanic eruption. The shift in temperature that occurred at this time is hypothesized as the main forcing for a cascade of abrupt environmental changes. Within the context of the last century or more, the 1980s event was unique in terms of its global scope and scale; our observed consequences imply that if unavoidable natural events such as major volcanic eruptions interact with anthropogenic warming unforeseen multiplier effects may occur. published

Widespread abrupt warming in the extratropical Northern Hemisphere (NH) occurred in the late 1980s. This warming was associated with a change in the relative influence of the Pacific Decadal Oscillation (PDO)-like pattern and the Arctic Oscillation (AO)-like pattern. The AO-like pattern has had a dominant influence on the NH-mean temperature since the late 1980s, whereas the influence of the PDO has weakened. The AO-like mode appears as part of natural variability in the pre-industrial simulations of the CMIP3/IPCC climate models. However, its emergence in the late 1980s was not simulated by most models with or without the observed increasing greenhouse effect in the 20th century. Copyright © 2010 Royal Meteorological Society

A summer climate regime shift in temperature in Taiwan and East Asia during the early 1950s was identified in this study. The event was characterized by a cooling land-warming ocean dipole in East Asia and the western North Pacific, marking the decreasing land–sea thermal contrast from the 1940s to the 1950s. The corresponding sea surface temperature (SST) anomaly was characterized by the sign flipping of a north–south dipole in the western North Pacific from north/cool-south/warm to north/warm-south/cool, indicating a weakening north–south SST gradient in the area. The corresponding change in mean sea level pressure was characterized by the rising pressure in continental East Asia and the Philippine Sea, and the falling pressure over the extratropical western North Pacific to the east of Japan. This change was the reflection of a weakening thermal low in the continental East Asia, a weakening monsoon trough in the tropical western North Pacific, a strengthening and southwestward-expanding ridge in the subtropical western North Pacific, and a deepening mid-latitude trough over eastern China and Japan. The phase reversal of the SST anomaly in the western North Pacific exhibited the characteristics of the Pacific Decadal Oscillation (PDO), but lagged behind the phase reversal of the PDO in the extratropical North Pacific by several years. The connection with the PDO is speculated, although the mechanism is not understood.