Climatologies of blocking highs detected by a unified Eddy-ABS approach
2020
Liu, Ping
Typical blocking indices in longitude or both longitude and latitude are compared to demonstrate: (1) the central blocking latitude (CBL) should be any of 40–80° N; (2) reversals of meridional gradients in absolute fields (ABS) about specified CBLs or in combination with time anomalies exceeding a threshold missed blocking patterns or misidentified non-blocking structures. Blocking highs are identified by a new Eddy-ABS index. A large-scale high is represented by a local maximum of zonal eddy anomalies, its immediately surrounding points and subsequent contiguous grids with decreasing anomalies but exceeding the 75th percentile. A high with any ABS reversal is an instantaneous blocking (IB). Maximum overlapping IBs become quasi-stationary when moving less than 10° longitudes per day. Four and more quasi-stationary IBs form a blocking event (BE). This unified index has identified IBs and BEs at vertical levels in the Northern (NH) and Southern Hemispheres (SH) during different seasons. In the NH, BEs exhibit strong seasonality, geographical preference and quasi-barotropicity. The lifetime, maximum intensity, impact area, and moving speed have a log-linear, log-normal, log-normal, and normal distribution, respectively. These features occur similarly in the SH, except for very small occurrences at 200 hPa in JJA and a normal distribution for the maximum intensity. Properties other than the moving speed significantly correlate with the maximum intensity in both hemispheres. The missing and misidentification issues are reasonably well resolved by this new index.
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