Climate Dynamics https://doi.org/10.1007/s00382-019-04782-5 Atmospheric blocking and intercomparison of objective detection methods: fow feld characteristics M. C. Pinheiro1 · P. A. Ullrich1,2 · R. Grotjahn1 Received: 6 August 2018 / Accepted: 17 April 2019 © The Author(s) 2019 Abstract Objective methods for identifying and quantifying atmospheric blocking have been developed over recent decades, primarily targeting North Atlantic blocks. Diferences arise from these methods, leading to changes in the resultant blocking clima- tology. To understand these diferences, and better inform future assessments built on quantitative detection of blocks, this paper examines blocking properties produced by three diferent objective detection algorithms over the global extratropics. Z∗ Blocking criteria examined include 500 hPa geopotential height anomaly ( ), column-averaged potential vorticity anomaly ∗ ( PV ), and 500 hPa geopotential height gradient (AGP). Results are analyzed for blocking climatologies and for instantane- ous blocking patterns, as well as distributions of block size, speed, duration, and distance traveled. The results emphasize ∗ ∗ physical characteristics of the fow feld and the subsequent blocking regions that emerge; overall, PV and Z blocked regions often have higher pattern correlation and spatial similarity, though these two methods also display high agreement ∗ ∗ with AGP in some instances.Z fnds the largest (and greatest number of) blocked regions, while PV -detected regions are ∗ smallest in all instances except Southern Hemisphere winter. In some cases, PV tracks a nearby jet streak, leading to difer- ences with height-based algorithms. All three algorithms detect some questionable low-latitude blocks that are stationary and persist but do not impair zonal fow, although at diferent times. Therefore, careful consideration of the algorithm biases is important in future blocking studies. For example, linking extreme weather to detected blocking could vary substantially depending on the algorithm used. Keywords Blocking · Objective detection · Climate variability · Climatology 1 Introduction (Glickman 2000) uses three criteria for classifying a fow pattern as blocked: Atmospheric blocking is a synoptic-scale weather phenom- enon with important social and ecological impacts that arise 1. persistent obstruction of the normal west-to-east fow due to its correlation with many kinds of extreme weather, pattern, such as heat waves (Pfahl and Wernli 2012; Grotjahn 2011; 2. pronounced meridional fow in the upper levels, and Lee and Grotjahn 2015), cold spells (Sillmann et al. 2011; 3. anticyclonic circulation at high latitudes accompanying Grotjahn and Faure 2008), and foods (Houze et al. 2011; cyclonic circulation at low latitudes. Hong et al. 2011). Yet it is a phenomenon that is not fully understood in terms of the underlying physics. The Ameri- The onset of a blocking feature results in temporary redi- can Meteorological Society (AMS) defnition of blocking rection of the jet stream, which is in turn responsible for the aforementioned anomalous weather conditions. Blocks take several forms, including the well-known omega block, * M. C. Pinheiro a poleward high co-located between two equatorward lows; [email protected] high/low dipoles; and persistent ridges, and often take on more than one form during their life cycles. All of these 1 Department of Land, Air, and Water Resources, University varieties nonetheless satisfy the criteria established by the of California, Davis, 1 Shields Ave, Davis, CA, USA AMS defnition. 2 Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, Berkeley, CA, USA Vol.:(0123456789)1 3 M. C. Pinheiro et al. Blocking has been studied for decades, but the first methods produced similar results for the omega block, but attempts at fnding blocks required visual inspection of fow displayed notable diference in both the size and center loca- patterns and were thus limited in scope (Rex 1950). Such tions of the blocked areas for the ridge. The authors attribute subjective assessment leads diferent individuals to poten- the diferences to both the choice of the variable (Z500 versus tially diferent conclusions about a fow pattern. The devel- PV) and the use of an anomaly versus total feld. opment of an objective procedure for identifying blocks was This study expands upon previous intercomparison thus important for a number of reasons: eforts; blocking is assessed in terms of distinct blocking fea- tures rather than per gridpoint, and each algorithm is applied 1. Consistency While the development of objective meth- across the full latitude-longitude (2D) range of the study ods requires some preliminary human judgment in the regions, which include both the Northern (NH) and South- choice of parameters, an algorithmic defnition of block- ern (SH) Hemisphere midlatitudes. Assessing algorithmic ing removes the human subjectivity from the rest of the diferences via individual blocking events allows determi- procedure and thus produces results that are internally nation of block characteristics beyond blocking frequency: consistent for a given dataset. here, we consider the size, duration, distance traveled, and 2. Efciency These methods can be automated, making zonal speed of each block as determined by each algorithm. computation and comparison of blocking climatologies We choose to utilize 2D rather than 1D blocking indices in possible across very large volumes of high-resolution, order to more fully examine regional variations in blocking; multi-decadal data. most notably, low-latitude blocking is often missed by 1D 3. Improved scientifc understanding Algorithms are based methods. Furthermore, it demonstrates how these algorithms on current concepts of block formation and mainte- perform in regional climatologies outside of those for which nance. Objective detection methods can allow for these they were developed. In particular, attention is paid to the concepts to be rigorously tested and improved as more underlying fow patterns that lead to diferences in objective information is gathered. blocking climatologies. Our analysis shows that each of the assessed algorithms only capture a subset of meteorological Objective methods based on a variety of felds and techniques patterns defned by the AMS defnition of blocking, and the have been developed over the years (see Figure 1 of Barriope- level of agreement between algorithms is highly dependent dro et al. 2010 for an overview), and intercomparison studies on region and block type. This an important point to con- such as Barnes et al. (2012) have explored whether these meth- sider when attempting to assess current and future blocking ods produce consistent blocking climatologies. Barnes et al. trends and the impacts of corresponding extreme weather. (2012) compares three longitudinally-varying (1D) methods A further beneft of this study is that the metrics and algo- (i.e. calculated about a single time-varying central latitude): rithms developed through this work may be leveraged for Pelly and Hoskins 2003 (potential temperature ( ) on a con- evaluation of global climate datasets, either from individual stant potential vorticity surface), Tibaldi and Molteni 1990 model runs or from coordinate intercomparison eforts (for (500 hPa geopotential height (Z500) gradient over a latitudinal details, see “Appendix B”). range), and Scaife et al. 2010 (zonal wind over a latitudinal Section 2 outlines the three objective detection algorithms range). The analysis, which was performed on 43 years of and the analysis framework, which was developed with the Northern Hemisphere data, concludes that these methods yield goal of standardizing the detection methodology as much as similar results in terms of calculated blocking frequency and possible across the algorithms. Section 3 compares results duration across the time and longitude axes. However, these between the three algorithms in terms of both the averaged are only two possible metrics under which objective methods and instantaneous blocking patterns, as well as some of the can be examined, and other papers have noted inherent dif- characteristics of the detected blocks. Section 4 explains ferences in the methods due to both the data and the chosen some of the meteorological factors which infuence the algo- method. For example, Davini et al. (2012) notes that there are rithms’ results, and Section 5 summarizes and discusses the distinct regional diferences in both the geopotential height implications of diferences between algorithm results. felds and the resultant characteristics of detected blocks. Over Greenland, blocks principally correspond to cyclonic Rossby wave breaking with a dipole structure, and split-fow blocking 2 Data and methodology generally happen in the midlatitudes over central Europe. The structure of a block impacts the efectiveness of the detection 2.1 Data method; Scherrer et al. (2006) compared detection of an omega block versus a persistent ridge, using the aforementioned Z500 Our dataset is the ERA-Interim reanalysis from the Euro- gradient method of Tibaldi and Molteni (1990) as well as two pean Center for Medium-Range Weather Forecasts (Dee potential vorticity (PV)-based metrics. All three detection et al. 2011). Temperature, meridional and zonal wind, and 1 3 Atmospheric blocking and intercomparison of objective detection methods: fow feld… ◦ geopotential variables are 6-h at 1 spatial resolution in of block center latitude/longitude coordinates) and size (in the time period