Papers on Rossby wave breaking
Posted by Ari Jokimäki on August 26, 2010
This is a list of papers on the effect of Rossby wave breaking and its effect to climate and weather. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.
Links between Rossby Wave Breaking and the North Atlantic Oscillation–Arctic Oscillation in Present-Day and Last Glacial Maximum Climate Simulations – Rivière et al. (2010) “Upper-tropospheric Rossby wave–breaking processes are examined in coupled ocean–atmosphere simulations of the Last Glacial Maximum (LGM) and of the modern era. LGM statistics of the Northern Hemisphere in winter, computed from the Paleoclimate Modeling Intercomparison Project Phase II (PMIP2) dataset, are compared with those from preindustrial simulations and from the 40-yr ECMWF Re-Analysis (ERA-40). Particular attention is given to the role of wave-breaking events in the North Atlantic Oscillation (NAO) for each simulation. Anticyclonic (AWB) and cyclonic (CWB) wave-breaking events during LGM are shown to be less and more frequent, respectively, than in the preindustrial climate, especially in the Pacific. This is consistent with the slight equatorward shift of the eddy-driven jets in the LGM runs. The most remarkable feature of the simulated LGM climate is that it presents much weaker latitudinal fluctuations of the eddy-driven jets. This is accompanied by less dispersion in the wave-breaking events. A physical interpretation is provided in terms of the fluctuations of the low-level baroclinicity at the entrance of the storm tracks. The NAO in the preindustrial simulations and in ERA-40 is characterized by strong latitudinal fluctuations of the Atlantic eddy-driven jet as well as by significant changes in the nature of the wave breaking. During the positive phase, the eddy-driven jet moves to the north with more AWB events than usual and is well separated from the subtropical African jet. The negative phase exhibits a more equatorward Atlantic jet and more CWB events. In contrast, the LGM NAO is less well marked by the latitudinal vacillation of the Atlantic jet and for some models this property disappears entirely. The LGM NAO corresponds more to acceleration–deceleration or extension–retraction of the Atlantic jet. The hemispheric point of view of the Arctic Oscillation exhibits similar changes.” Rivière, Gwendal, Alexandre Laîné, Guillaume Lapeyre, David Salas-Mélia, Masa Kageyama, 2010, J. Climate, 23, 2987-3008, doi: 10.1175/2010JCLI3372.1.
On the upper tropospheric formation and occurrence of high and thin cirrus clouds during anticyclonic poleward Rossby wave breaking events – Eixmann et al. (2010) “Ground-based lidar measurements and balloon soundings were employed to examine the dynamical link between anticyclonic Rossby wave breaking and cirrus clouds from 13 to 15 February 2006. For this event, an air mass with low Ertel’s potential vorticity appeared over Central Europe. In the tropopause region, this air mass was accompanied with both an area of extreme cold temperatures placed northeastward, and an area of high specific humidity, located southwestward. ECMWF analyses reveal a strong adiabatic northeastward and upward transport of water vapour within the warm conveyor belt on the western side of the ridge over Mecklenburg, Northern Germany. The backscatter lidar at Kühlungsborn (54.1°N, 11.8°E) clearly identified cirrus clouds at between 9 and 11.4 km height. In the tropopause region high-vertical resolution radiosoundings showed layers of subsaturated water vapour over ice but with a relative humidity over ice >80%. Over Northern Germany radiosondes indicated anticyclonically rotating winds in agreement with backward trajectories of ECMWF analyses in the upper troposphere, which were accompanied by a relatively strong increase of the tropopause height on 14 February. Based on ECMWF data the strong link between the large-scale structure, updraft and ice water content was shown.” RONALD EIXMANN, DIETER H.W. PETERS, CHRISTOPH ZÜLICKE, MICHAEL GERDING, ANDREAS DÖRNBRACK, Tellus A, Volume 62, Issue 3, pages 228–242, May 2010, DOI: 10.1111/j.1600-0870.2010.00437.x. [Full text]
The Role of Rossby Wave Breaking in Shaping the Equilibrium Atmospheric Circulation Response to North Atlantic Boundary Forcing – Strong & Magnusdottir (2010) “The role of Rossby wave breaking (RWB) is explored in the transient response of an atmospheric general circulation model to boundary forcing by sea ice anomalies related to the North Atlantic Oscillation (NAO). When the NCAR Community Climate Model, version 3, was forced by an exaggerated sea ice extent anomaly corresponding to one arising from a positive NAO, a localized baroclinic response developed and evolved into a larger-scale equivalent barotropic pattern resembling the negative polarity of the NAO. The initial baroclinic response shifted the phase speeds of the dominant eddies away from a critical value equal to the background zonal flow speed, resulting in significant changes in the spatial distribution of RWB. The forcing of the background zonal flow by the changes in RWB accounts for 88% of the temporal pattern of the response and 80% of the spatial pattern of the zonally averaged response. Although results here focus on one experiment, this “RWB critical line mechanism” appears to be relevant to understanding the equilibrium response in a broad class of boundary forcing experiments given increasingly clear connections among the northern annular mode, jet latitude shifts, and RWB.” Strong, Courtenay, Gudrun Magnusdottir, 2010, J. Climate, 23, 1269-1276. [Full text]
Planetary Wave Breaking and Tropospheric Forcing as Seen in the Stratospheric Sudden Warming of 2006 – Coy et al. (2009) “The major stratospheric sudden warming (SSW) of January 2006 is examined using meteorological fields from Goddard Earth Observing System version 4 (GEOS-4) analyses and forecast fields from the Navy Operational Global Atmospheric Prediction System–Advanced Level Physics, High Altitude (NOGAPS-ALPHA). The study focuses on the upper tropospheric forcing that led to the major SSW and the vertical structure of the subtropic wave breaking near 10 hPa that moved low tropical values of potential vorticity (PV) to the pole. Results show that an eastward-propagating upper tropospheric ridge over the North Atlantic with its associated cold temperature perturbations (as manifested by high 360-K potential temperature surface perturbations) and large positive local values of meridional heat flux directly forced a change in the stratospheric polar vortex, leading to the stratospheric subtropical wave breaking and warming. Results also show that the anticyclonic development, initiated by the subtropical wave breaking and associated with the poleward advection of the low PV values, occurred over a limited altitude range of approximately 6–10 km. The authors also show that the poleward advection of this localized low-PV anomaly was associated with changes in the Eliassen–Palm (EP) flux from equatorward to poleward, suggesting an important role for Rossby wave reflection in the SSW of January 2006. Similar upper tropospheric forcing and subtropical wave breaking were found to occur prior to the major SSW of January 2003.” Coy, Lawrence, Stephen Eckermann, Karl Hoppel, 2009, J. Atmos. Sci., 66, 495-507. [Full text]
The Role of Tropospheric Rossby Wave Breaking in the Pacific Decadal Oscillation – Strong & Magnusdottir (2009) “The leading pattern of extratropical Pacific sea surface temperature variability [the Pacific decadal oscillation (PDO)] is shown to depend on observed variability in the spatiotemporal distribution of tropospheric Rossby wave breaking (RWB), where RWB is the irreversible overturning of potential vorticity on isentropic surfaces. Composite analyses based on hundreds of RWB cases show that anticyclonic (cyclonic) RWB is associated with a warm, moist (cool, dry) column that extends down to a surface anticyclonic (cyclonic) circulation, and that the moisture and temperature advection associated with the surface circulation patterns force turbulent heat flux anomalies that project onto the spatial pattern of the PDO. The RWB patterns that are relevant to the PDO are closely tied to El Niño–Southern Oscillation, the Pacific–North American pattern, and the northern annular mode. These results explain the free troposphere-to-surface segment of the atmospheric bridge concept wherein El Niño anomalies emerge in summer and modify circulation patterns that act over several months to force sea surface temperature anomalies in the extratropical Pacific during late winter or early spring. Leading patterns of RWB account for a significant fraction of PDO interannual variability for any month of the year. A multilinear model is developed in which the January mean PDO index for 1958–2006 is regressed upon the leading principal components of cyclonic and anticyclonic RWB from the immediately preceding winter and summer months (four indexes in all), accounting for more than two-thirds of the variance.” Strong, Courtenay, Gudrun Magnusdottir, 2009, J. Climate, 22, 1819-1833. [Full text]
Role of Rossby wave breaking in the west Pacific teleconnection – Rivière (2009) “The dynamical link between the west Pacific (WP) teleconnection and Rossby wave breaking (RWB) events is analyzed during winter months using ERA40 reanalysis data from 1957 to 2002. The WP pattern which is characterized by latitudinal fluctuations of the Pacific jet is closely linked to variations in the nature of RWB, similarly to the North Atlantic Oscillation. More anticyclonic (cyclonic) RWBs than usual occur in the Central Pacific during the positive (negative) WP phase when the Pacific jet is more to the north (south) than usual. Time lag daily composites show that before the occurrence of an anticyclonic RWB event, WP anomalies close to the positive phase preexist that are then reinforced during the breaking leading to an increase of the WP index even few days after the peak of the event. Cyclonic RWB events have similar but opposite effects on the WP pattern since they trigger and maintain the negative phase. Finally, a comparison with the RWB anomalies of the Pacific-North American (PNA) teleconnection is provided.” Rivière, G. (2010), Geophys. Res. Lett., 37, L11802, doi:10.1029/2010GL043309.
Modulation of tropical convection by breaking Rossby waves – Allen et al. (2009) “This work discusses observations of both the convective-inhibiting and convective-promoting properties associated with Rossby waves that break in the extratropics and extend into the tropics. Two tropical drought periods—times of reduced tropical cloudiness and rainfall—were observed during mid to late November 2005 over a wide area of north-west Australia, with an observed eruption of a nearby synoptic tropical cloud band in between times. Both convective inhibition and promotion appear to be linked to the descent of dry upper tropospheric air within a series of tropopause folds; convective inhibition was observed within the dry pool itself, whilst convective promotion was observed on the high moisture gradient at the leading edge of an advancing dry slot. A range of satellite images, surface rain gauges, radiosonde and ozonesonde data are used in conjunction with back trajectories and European Centre for Medium-Range Weather Forecasts (ECMWF) analysis fields to investigate the origins and dynamics associated with these convective events, showing each to be ultimately linked to breaking Rossby wave activity on the southern subtropical jet. Together, these observations support a growing number of studies linking midlatitude tropopause-level dynamics with the modulation of tropical deep convection, an influence that is poorly characterized when considering the climatology of tropical cloudiness and rainfall.” G. Allen, G. Vaughan, D. Brunner, P. T. May, W. Heyes, P. Minnis, J. K. Ayers, Quarterly Journal of the Royal Meteorological Society, Volume 135, Issue 638, pages 125–137, January 2009 Part A, DOI: 10.1002/qj.349.
Tropospheric Rossby Wave Breaking and the NAO/NAM – Strong & Magnusdottir (2008) “Objective analysis of several hundred thousand anticyclonic and cyclonic breaking Rossby waves is performed for the Northern Hemisphere (NH) winters of 1958–2006. A winter climatology of both anticyclonic and cyclonic Rossby wave breaking (RWB) frequency and size (zonal extent) is presented for the 350-K isentropic surface over the NH, and the spatial distribution of RWB is shown to agree with theoretical ideas of RWB in shear flow. Composites of the two types of RWB reveal their characteristic sea level pressure anomalies, upper- and lower-tropospheric velocity fields, and forcing of the upper-tropospheric zonal flow. It is shown how these signatures project onto the centers of action and force the velocity patterns associated with the North Atlantic Oscillation (NAO) and Northern Hemisphere annular mode (NAM). Previous studies have presented evidence that anticyclonic (cyclonic) breaking leads to the positive (negative) polarity of the NAO, and this relationship is confirmed for RWB over the midlatitudes centered near 50°N. However, an opposite and statistically significant relationship, in which cyclonic RWB forces the positive NAO and anticyclonic RWB forces the negative NAO, is shown over regions 20° to the north and south, centered at 70° and 30°N, respectively. On a winter mean basis, the frequency of RWB over objectively defined regions covering 12% of the area of the NH accounts for 95% of the NAO index and 92% of the NAM index. A 6-hourly analysis of all the winters indicates that RWB over the objectively defined regions affects the NAO/NAM without a time lag. Details of the objective wave-breaking analysis method are provided in the appendix.” Strong, Courtenay, Gudrun Magnusdottir, 2008, J. Atmos. Sci., 65, 2861-2876, doi: 10.1175/2008JAS2632.1. [Full text]
How Rossby wave breaking over the Pacific forces the North Atlantic Oscillation – Strong & Magnusdottir (2008) “Anticyclonic Rossby wave breaking (RWB) over a well-defined, limited-area region of the east Pacific leads to the positive polarity of the North Atlantic Oscillation (NAO) by locally piling up wave activity where it may be advected downstream, resulting in increased wave activity flux and anticyclonic RWB over the subtropical Atlantic. A composite time series shows that Pacific RWB occurs several days prior to the Atlantic RWB and the peak of the NAO index. Following Pacific RWB, a channel of increased pseudomomentum flux extends from the Pacific wave breaking region, northeastward toward midlatitudes of eastern North America where pseudomomentum density accumulates for several days prior to moving eastward and leading to anticyclonic RWB over the Atlantic.” Strong, C., and G. Magnusdottir (2008), Geophys. Res. Lett., 35, L10706, doi:10.1029/2008GL033578. [Full text]
A New Rossby Wave–Breaking Interpretation of the North Atlantic Oscillation – Woollings et al. (2008) “This paper proposes the hypothesis that the low-frequency variability of the North Atlantic Oscillation (NAO) arises as a result of variations in the occurrence of upper-level Rossby wave–breaking events over the North Atlantic. These events lead to synoptic situations similar to midlatitude blocking that are referred to as high-latitude blocking episodes. A positive NAO is envisaged as being a description of periods in which these episodes are infrequent and can be considered as a basic, unblocked situation. A negative NAO is a description of periods in which episodes occur frequently. A similar, but weaker, relationship exists between wave breaking over the Pacific and the west Pacific pattern. Evidence is given to support this hypothesis by using a two-dimensional potential-vorticity-based index to identify wave breaking at various latitudes. This is applied to Northern Hemisphere winter data from the 40-yr ECMWF Re-Analysis (ERA-40), and the events identified are then related to the NAO. Certain dynamical precursors are identified that appear to increase the likelihood of wave breaking. These suggest mechanisms by which variability in the tropical Pacific, and in the stratosphere, could affect the NAO.” Woollings, Tim, Brian Hoskins, Mike Blackburn, Paul Berrisford, 2008, J. Atmos. Sci., 65, 609-626. [Full text]
Long-term trends of synoptic-scale breaking Rossby waves in the Northern Hemisphere between 1958 and 2001 – Isotta et al. (2008) “Breaking synoptic-scale Rossby waves are frequent features of the upper troposphere and lower stratosphere (UTLS) which affect both global- and regional-scale dynamics. Furthermore, they directly influence ozone distribution through meridional transport of ozone-rich air towards the south and ozone-poor air towards the north. Here, trends in the frequency of these breaking waves are assessed by analysing a 44-year climatology (1958–2002) of potential vorticity (PV) streamers on isentropic surfaces from 310 to 350 K. These streamers are viewed as breaking Rossby waves, whereby stratospheric (tropospheric) streamers indicate southward (northward) breaking waves. Two complementary techniques are used to analyse the trends. First, linear trends are computed using the least-squares regression technique. Statistically significant linear trends are found to vary in location and magnitude between isentropic levels and the four seasons. In winter significant trends are detected in the eastern Pacific between 340 and 350 K. A positive trend of stratospheric streamers in the Tropics is related to an increase of total column ozone, whereas the positive trend of tropospheric streamers in the mid-latitudes is associated with a decrease of total ozone. Secondly, a nonlinear trend analysis is performed using the seasonal-trend decomposition procedure based on Loess (STL). With this technique, the low-frequency variability of the time series is analysed during the 44-year period. For instance, over the eastern Atlantic on 350 K, a phase of decreasing PV streamer frequencies in the 1950s and 1960s is followed by a positive streamer tendency after the 1970s. Additionally, trends of the zonal wind are investigated. One prominent outcome of this analysis is the observation that equatorial easterlies over the Atlantic are weakening. A dynamically meaningful link exists between the trends observed in both wind velocity and PV streamers. “ F. Isotta, O. Martius, M. Sprenger, C. Schwierz, International Journal of Climatology, Volume 28, Issue 12, pages 1551–1562, October 2008, DOI: 10.1002/joc.1647.
Linkage of atmospheric blocks and synoptic-scale Rossby waves: a climatological analysis – Altenhoff et al. (2008) “The link between atmospheric blocking and propagating and breaking synoptic-scale Rossby waves (termed PV streamers) are explored for the climatological period 1958–2002, using the ERA-40 re-analysis data. To this end, potential vorticity (PV) based climatologies of blocking and breaking waves are used, and features of the propagating waves is extracted from Hovmöller diagrams. The analyses cover the Northern Hemisphere during winter, and they are carried out for the Atlantic and Pacific basins separately. The results show statistically significant wave precursor signals, up to 5 d prior to the blocking onset. In the Atlantic, the precursor signal takes the form of a coherent wave train, emanating approximately 110° upstream of the blocking location. In the Pacific, a single long-lived (10 d) northerly velocity signal preludes the blocking onset. A spatial analysis is conducted of the location, frequency and form of breaking synoptic-scale Rossby waves, prior to the onset, during the lifetime and after the blocking decay. It reveals that cyclonic streamers are present to the southwest and anticyclonic streamers to the south and southeast, approximately 43% (36%) of the time in the Atlantic (Pacific) basin, and this is significantly above a climatological distribution.” ADRIAN M. ALTENHOFF, OLIVIA MARTIUS, MISCHA CROCI-MASPOLI, CORNELIA SCHWIERZ, HUW C. DAVIES, Tellus A, Volume 60, Issue 5, pages 1053–1063, October 2008, DOI: 10.1111/j.1600-0870.2008.00354.x.
Blocking and Rossby Wave Breaking on the Dynamical Tropopause in the Southern Hemisphere – Berrisford et al. (2007) “Rossby wave breaking on the dynamical tropopause in the Southern Hemisphere (the −2-PVU surface) is investigated using the ERA-40 dataset. The indication of wave breaking is based on reversal in the meridional gradient of potential temperature, and persistent large-scale wave breaking is taken as a strong indication that blocking may be present. Blocking in the midlatitudes is found to occur predominantly during wintertime in the Pacific and is most vigorous in the east Pacific, while during summertime, the frequency of blocking weakens and its extent becomes confined to the west Pacific. The interannual variability of blocking is found to be high. Wave breaking occurs most frequently on the poleward side of the polar jet and has some, but not all, of the signatures of blocking, so it is referred to as high-latitude blocking. In general, cyclonic wave breaking occurs on the poleward side of the polar jet, otherwise anticyclonic breaking occurs. However, at least in wintertime, wave breaking in the New Zealand/west to mid-Pacific sector between the polar and subtropical jets is a mixture between cyclonic and anticyclonic types. Together, episodes of wave breaking and enhanced westerly flow describe much of the variability in the seasonal Antarctic Oscillation (AnO) index and give a synoptic manifestation of it with a focus on the date line and Indian Ocean that is in agreement with the centers of action for the AnO. During summertime, anticyclonic wave breaking in the upper troposphere is also to be found near 30°S in both the Pacific and Atlantic, and appears to be associated with Rossby waves propagating into the subtropics from the New Zealand region.” Berrisford, P., B. J. Hoskins, E. Tyrlis, 2007, J. Atmos. Sci., 64, 2881-2898. [Full text]
A Seasonal Climatology of Rossby Wave Breaking in the 320–2000-K Layer – Hitchman & Huesmann (2007) “Differential advection in Rossby waves can lead to potential vorticity (PV; P) contours on isentropic surfaces folding over in latitude (Py < 0) in a process called Rossby wave breaking (RWB). Exploring the properties of RWB may shed light on underlying dynamics and enable quantification of irreversible transport. A seasonal climatology of Py and RWB statistics is presented for the 320–850-K layer using NCEP reanalysis data during 1979–2005 and for the 320–2000-K layer using the Met Office (UKMO) data during 1991–2003. A primary goal is to depict the spatial extent and seasonality of RWB maxima. This analysis shows seven distinct RWB regimes: poleward and equatorward of the subtropical westerly jets, poleward and equatorward of the stratospheric polar night jets, flanking the equator in the stratosphere and mesosphere, equatorward of subtropical monsoon anticyclones, and the summertime polar stratosphere. A striking PV gradient maximum exists at the equator throughout the layer 360–2000 K, flanked by subtropical RWB maxima, integral components of the Lagrangian cross-equatorial flow. Strong RWB occurs in the polar night vortex where β is small. Over the summer pole, strong poleward RWB associated with synoptic waves decays into small amplitude motions in the upper stratosphere, where heating gradients cause Py < 0. The seven spatial regimes are linked to three different dynamical causes of reversals: wave breaking associated with westerly jets, a combined barotropic/inertial instability in cross-equatorial flow, and on the periphery of monsoon anticyclones." Hitchman, Matthew H., Amihan S. Huesmann, 2007, J. Atmos. Sci., 64, 1922-1940. [Full text]
Is the North Atlantic Oscillation a Breaking Wave? – Franzke et al. (2004) “Given the recent observational evidence that the positive (negative) phase of the North Atlantic Oscillation (NAO) is the remnant of anticyclonic (cyclonic) wave breaking, this study uses a multilevel primitive equation model to investigate important dynamical attributes of the above wave breaking behavior. For this purpose, a hierarchy of different basic states (two- and three-dimensional) and initial perturbations are used. With the three-dimensional climatological flow as the basic state, it is found that initial perturbations located equatorward (poleward) and upstream of the climatological Atlantic jet lead to wave breaking similar to that of the positive (negative) NAO phase. Consistently, analysis of observational data indeed shows that the Pacific storm track is displaced equatorward (poleward) prior the onset of the positive (negative) NAO phase. This result suggests that the latitudinal position of the Pacific storm track plays an important role for determining the phase of the NAO. Sensitivity experiments show that individual life cycles resemble each other only within the NAO region, but have large case-to-case variability outside of the NAO region. Calculations with zonally symmetric basic states fail to produce wave breaking of the correct spatial and temporal scale, underscoring the dynamical significance of the three-dimensional climatological flow.” Franzke, Christian, Sukyoung Lee, Steven B. Feldstein, 2004, J. Atmos. Sci., 61, 145-160. [Full text]
Synoptic View of the North Atlantic Oscillation – Benedict et al. (2004) “This article investigates the synoptic characteristics of individual North Atlantic Oscillation (NAO) events by examining the daily evolution of the potential temperature field on the nominal tropopause (the 2-PVU surface). This quantity is obtained from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis dataset for the winter season. For both phases, the NAO is found to originate from synoptic-scale waves. As these waves evolve into the low-frequency NAO pattern, they break anticyclonically for the positive phase and cyclonically for the negative phase. The results of this analysis suggest that it is the remnants of these breaking waves that form the physical entity of the NAO. Throughout the NAO events, for both phases, the NAO is maintained by the successive breaking of upstream synoptic-scale waves. When synoptic-scale disturbances are no longer present, mixing processes play an important role in the NAO decay. As in other recent studies of the NAO, it is found that these individual NAO events complete their life cycle in a time period of about two weeks. Additional differences between the wave breaking characteristics of the two NAO phases are found. For the positive NAO phase, anticyclonic wave breaking takes place in two regions: one over the North Atlantic and the other near the North American west coast. For the negative NAO phase, on the other hand, there is a single breaking wave confined to the North Atlantic. An explanation based on kinematics is given to account for this difference.” Benedict, James J., Sukyoung Lee, Steven B. Feldstein, 2004, J. Atmos. Sci., 61, 121-144. [Full text]
A New Perspective on Blocking – Pelly & Hoskins (2003) “It is argued that the essential aspect of atmospheric blocking may be seen in the wave breaking of potential temperature (θ) on a potential vorticity (PV) surface, which may be identified with the tropopause, and the consequent reversal of the usual meridional temperature gradient of θ. A new dynamical blocking index is constructed using a meridional θ difference on a PV surface. Unlike in previous studies, the central blocking latitude about which this difference is constructed is allowed to vary with longitude. At each longitude it is determined by the latitude at which the climatological high-pass transient eddy kinetic energy is a maximum. Based on the blocking index, at each longitude local instantaneous blocking, large-scale blocking, and blocking episodes are defined. For longitudinal sectors, sector blocking and sector blocking episodes are also defined. The 5-yr annual climatologies of the three longitudinally defined blocking event frequencies and the seasonal climatologies of blocking episode frequency are shown. The climatologies all pick out the eastern North Atlantic–Europe and eastern North Pacific–western North America regions. There is evidence that Pacific blocking shifts into the western central Pacific in the summer. Sector blocking episodes of 4 days or more are shown to exhibit different persistence characteristics to shorter events, showing that blocking is not just the long timescale tail end of a distribution. The PV–θ index results for the annual average location of Pacific blocking agree with synoptic studies but disagree with modern quantitative height field–based studies. It is considered that the index used here is to be preferred anyway because of its dynamical basis. However, the longitudinal discrepancy is found to be associated with the use in the height field index studies of a central blocking latitude that is independent of longitude. In particular, the use in the North Pacific of a latitude that is suitable for the eastern North Atlantic leads to spurious categorization of blocking there. Furthermore, the PV–θ index is better able to detect Ω blocking than conventional height field indices.” Pelly, J. L., B. J. Hoskins, 2003, J. Atmos. Sci., 60, 743-755. [Full text]