Trough Axis Research Articles

The Lago Sofia Conglomerate : Debris Flow to Hyperconcentrated Flow Deposits in a Cretaceous Submarine Channel, Southern Chile

The Lago Sofia conglomerates encased in the Cretaceous Cerro Toro Formation, southern Chile, represent a gigantic submarine channel system developed along a foredeep trough. The channel system consists of several tributaries along the trough margin and a trunk channel along the trough axis. Voluminous debris flows were generated ubiquitously along the tract of the submarine channel mainly by the failure of nearby channel banks or slopes. The flows transformed immediately into multiphase flows and resulted in very thick-bedded mass-flow deposits with a peculiar structure sequence. The mass-flow deposits commonly overlie fluted or grooved surfaces and consist of a lower division of clast-supported and imbricated pebble-cobble conglomerate with common basal inverse grading, and an upper division of clast- to matrix-supported and disorganized pebble conglomerate or pebbly mudstone with abundant intraformational clasts. The structure sequence suggests a temporal succession of a turbidity current, a bipartite hyperconcentrapted flow with active clast collisions near the flow base, and a cohesive debris flow probably with a rigid plug. The multiphase flow is interpreted to have resulted from transformation of clast-rich but cohesive debris flows. Cohesive debris flows appear to transform more easily into dilute flow types in subaqueous environments because they are apt to hydroplane. This is in contrast to the flow transitions in subaerial environments where noncohesive debris flows are dominant and difficult to hydroplane.

Deformable backstop as seaward end of coseismic slip in the Nankai Trough seismogenic zone

Wide-angle seismic surveys performed in the last decade have clarified the 3-D crustal structure along the Nankai Trough. The geometry and velocity structure of the southwestern Japan subduction zone are now well constrained. Comparing these observations with the rupture distribution of historic great thrust earthquakes, it appears that the coseismic rupture occurred along plate boundaries deeper than the wedge/backstop boundary (the boundary between the Neogene–Quaternary accretionary wedge and the crust forming the backstop). From the view of spatial relationship, both rupture distributions of the last two large events and the crust forming the backstop are considerably retreated from the trough axis in the west and east off the Kii Peninsula. In both areas, seamount or ridge subduction is apparent in seismic results, geomorphological data and geomagnetic data. The landward indentation of the deformable backstop, which corresponds to the crustal block of old accreted sediments, may be formed by seamount subduction according to published results of sandbox modeling. In particular, the subducted seamount may be a structural factor affecting the recession of the crustal block forming the backstop.

Drainage evolution in the Margaritifer Sinus region, Mars

Margaritifer Sinus, Mars, records a complex history of water transport, storage, and release extending from the late Noachian into at least the mid‐Hesperian. Collection, transport, and discharge of water were accomplished by systems of differing character flanking opposite sides of the Chryse Trough. Drainage on the western side of the trough was via the segmented Uzboi‐Ladon‐Margaritifer mesoscale outflow system that heads in Argyre Basin and incises and fills as it crosses ancient multiringed impact basins. By contrast, Samara and Parana‐Loire Valles, two of the largest and best integrated valley systems on Mars, dominated drainage on the eastern trough flank. Valley morphometry suggests formation was due to precipitation‐recharged groundwater sapping. All systems discharged into Margaritifer Basin, located along the Chryse Trough axis, and caused ponding that persisted into the early Hesperian. The Uzboi‐Ladon‐Margaritifer system dominated discharge that was coincident with widespread geomorphic activity on Mars. As channel and valley formation ended, some water in Margaritifer Basin infiltrated the subsurface. Collapse and release of water began shortly thereafter and persisted into mid‐Hesperian times, thereby forming Margaritifer and Iani Chaos and incising Ares Vallis.

On the mechanism of winter cyclogenesis in relation to vertical axis tilt

¶This work is concerned with the behavior of the vertical trough axis tilt during the development of a Mediterranean cyclone. The period covering stages of the development of the depression is from 1200 UTC 18 January to 1200 UTC 25 January 1981. Two procedures have been used to illustrate the behavior of the vertical axis tilt with the development of our case study. In the first direct procedure we trace the centers of the cyclone for each level for each day and determine its longitude. In the second indirect procedure we compute the values and signs of the product of the perturbation of temperature and meridional velocity over the computational domain. The energetics of the depression is studied and the partition of available potential energy and kinetic energy into zonal and eddy component is adopted. The energy contents and their changes in different atmospheric levels are discussed in the course of the cyclone’s development.

The slip distribution of the 1946 Nankai earthquake estimated from tsunami inversion using a new plate model

Abstract A new model for the shape of the Philippine Sea Plate (PSP) subducting beneath southwest Japan was constructed by combining marine seismic survey results and seismicity data. This resulted in a plate model that is generally 3–5 km shallower than previous estimates, and contains new details of plate boundary structure near the Nankai Trough axis. We estimated the slip distribution of the 1946 Nankai earthquake by inverting tsunami waveforms using the new plate boundary model, and found that the large slip off Cape Muroto and seismic moment were reduced by about 10% from the previous estimate of Tanioka and Satake (2001a) . However, we also found reason to suspect the tide gauge closest to the epicenter (at Shimotsu), and this resulted in a much more substantial reduction in the amount of slip in the eastern segment of the rupture area. We also find that our new rupture model correlates well with features of the subducting plate boundary, such as a large subducting seamount off Cape Muroto ( Kodaira et al., 2000a ). This suggests that such features have an important effect on either earthquake rupture propagation or the interseismic accumulation of elastic strain energy, or both.

Splay fault branching along the Nankai subduction zone.

Seismic reflection profiles reveal steeply landward-dipping splay faults in the rupture area of the magnitude (M) 8.1 Tonankai earthquake in the Nankai subduction zone. These splay faults branch upward from the plate-boundary interface (that is, the subduction zone) at a depth of approximately 10 kilometers, approximately 50 to 55 kilometers landward of the trough axis, breaking through the upper crustal plate. Slip on the active splay fault may be an important mechanism that accommodates the elastic strain caused by relative plate motion.

Deep crustal structure of the eastern Nankai Trough and Zenisu Ridge by dense airgun–OBS seismic profiling

An unprecedentedly extensive seismic refraction and wide-angle reflection survey using 65 ocean bottom seismographs revealed detailed crustal structure around the eastern Nankai Trough. A previously published crustal model shows an abrupt offset of the Moho at the south of the Zenisu Ridge, a prominent topographic high along the oceanward slope of the Nankai Trough. Our crustal model indicates that this offset of the Moho extends southwestward continuously to 138°E, decreasing its gap. The survey area experienced the last two great earthquakes in 1854 and 1944. However, the northeastern part of the survey area seems to have remained unruptured since the 1854 event. Factors controlling the size of the rupture area for great earthquakes are still a matter of debate. There are several candidates for these factors in the survey area: hypothetical tectonic boundaries that may or may not be oceanward prolongation of major on-land tectonic lines, estimated locations of slab disruption, and the extent of Moho offset along the strike of the Zenisu Ridge. The main purpose of this survey is to clarify the relation between the crustal structure and these geophysical and geological features bounding the rupture area. Our crustal model from the trough axis to the continental slope is characterized by a well-developed sedimentary wedge bounded by island arc crustal blocks, consisting of upper and lower crust, to the northwest. Furthermore, the subducting oceanic crust, which can be traced down to 25 km depth, shows that the down-dip angle steepens at 55 km landward from the trough axis. On the basis of compilation of our crustal model with previously published models around the eastern Nankai Trough, we derived an image of the entire subducting plate geometry for depths shallower than 20 km, which is still poorly constrained by the land observation of microearthquakes. Significant lateral variations of the crustal structure and the slab geometry are recognized along one prominent canyon, and the offset of the Moho at the south of the Zenisu Ridge disappears to the southwest of the canyon. Moreover, it seems that the slab disruption recognized at a depth greater than 20 km is connected to this canyon. Therefore, the lateral variation of the crustal structure along the canyon may be one of the causes to stop rupture propagation of great earthquakes. Furthermore, the crustal variation may also form a tectonic boundary that distinguishes the subduction pattern of the Philippine Sea plate, including the influence of the Izu–Ogasawara collision, in the eastern Nankai Trough from the simple subduction pattern of the western Nankai Trough.

Faults configuration around the eastern Nankai trough deduced by multichannel seismic profiling

The Japan Marine Science and Technology Center (JAMSTEC), the Ocean Research Institute, University of Tokyo and the Laboratoire de Géologie, Ecole Normale Supérieure at Paris carried out in collaboration a 120-channel reflection seismic survey using an airgun array with total 4200 cubic inch capacity around the eastern Nankai trough in 1996. Data obtained by R/V Hakuho, University of Tokyo were primarily processed by JAMSTEC and were shared by the above three main bodies of this program to complete geoscientific interpretation. The fault configuration around the eastern Nankai trough is very complicated due to the collision of the Izu Peninsula against central Japan. The major remarkable faults around the eastern Nankai area are the Zenisu, the Kodaiba and the Tokai thrusts. The main objective of this paper is to clarify these fault configurations. The Zenisu thrust dips gently northward from the southern slope of the Zenisu ridge, and reaches the oceanic Moho. The location of the Tokai thrust may jump at the Tenryu canyon, and appears to reach the top of the subducting oceanic plate. The Kodaiba fault penetrates a 5.5–5.6-km/s layer, which is the crust of the central Japan forearc. A large fault in the trough axis cuts the entire oceanic crust and has a flower structure, which indicates that it has strike-slip component. Its strike is NW–SE, parallel to the plate motion and perpendicular to the strike of the Zenisu ridge. The top of the subducting oceanic crust of the Philippine Sea plate was defined beneath the continental slope of the central Japan forearc. The angle of subduction becomes abruptly steeper 50 km north of the deformation front. We also observed at the southern roof of the Zenis thrust a drape fold structure which is proof of subduction. We suggest a subduction on the southern side of the Zenisu ridge, strongly deformed oceanic crust at the trough axis and some major faults in the forearc region reaching subducting oceanic crust.

Morphology of Europan bands at high resolution: A mid‐ocean ridge‐type rift mechanism

We utilize imaging data from the Galileo spacecraft to investigate band formation on one of Jupiter's moons, Europa. Bands are polygonal features first observed in Voyager data close to Europa's anti‐Jovian point and represent areas where preexisting terrain has been pulled apart, allowing new material to move up into the gap. We examine the detailed morphology of several bands imaged at different resolutions and lighting geometries. We identify several distinct morphological characteristics, including central troughs, hummocky textures, and ridge and trough terrains, some of which are common among the bands studied. In many cases, bands have initiated along segments of one or more preexisting double ridges, ubiquitous within Europa's ridged plains. Distinctive morphological features and high standing topography imply that the bands formed from compositionally or thermally buoyant ice, rather than liquid water. Comparisons between Europan band morphologies and features found on terrestrial mid‐ocean ridges reveal several similarities, including axial troughs, subcircular hummocks, normal faults, and indications of symmetrical spreading. We conclude that terrestrial mid‐ocean ridge rifting is a good analogy for Europan band formation. If a terrestrial seafloor‐spreading model is applicable to Europan bands, we speculate that band morphologies might be related to the relative rate of spreading of each band. Bands may have contributed significantly to the resurfacing of Europa. Europan bands we examine predate (but do not postdate) lenticulae and related features, implying that the style of resurfacing on Europa has changed over recent geological time in these regions.

Determination of Paleocurrent Direction from Oblique Sections of Trough Cross-Stratification--A Precise Approach

The study of paleoflow patterns is an essential part of basin analysis. Cross-stratification, both planar and trough, is the principal directional structure used for paleocurrent analysis. Trough cross-stratification is especially characteristic of fluvial deposits, where it is typically formed by the primary current and correlates strongly with the flow direction (Harms and Fahnestock 1965; Wermund 1965; Williams 1968; Barrett 1970; McGowen and Garner 1970; Dott 1973; Michelson and Dott 1973; High and Picard 1974). The only exception, reported by Meckel (1967), is an artifact of an erroneous sampling technique (High and Picard 1974). However, the paucity of three-dimensional exposures of trough cross-strata that are suitable for direct measurement of trough axis orientations quite often hinders the systematic collection of paleocurrent data. With a view to overcoming this problem, attempts have been made to estimate paleoflow directions from two-dimensional exposures of trough cross-strata (Slingerland and Williams 1979; DeCelles et al. 1983), because they are much more common. Slingerland and Williams (1979) proposed two specific ways of determining the orientations of trough axes from two-dimensional exposures. According to their first method, a single strike measured on a trough side can be corrected to give the trough axial azimuth, with reference to the plunge of the trough measured from a quasi-three-dimensional exposure (Slingerland and Williams 1979, p. 731). The plunge direction itself (after necessary tilt correction), however, indicates the flow direction, and if it can be measured beyond doubt no further measurement of any other part of the trough is required. This kind of exposure is also not qualitatively very different from a three-dimensional one. The second method proposed by Slingerland and Williams (1979) involves estimation of the trough axial azimuth by measuring, on perfectly two-dimensional vertical exposures, the face azimuths and two …

The 1998 Ice Storm—Analysis of a Planetary-Scale Event

Abstract The ice storm of 5–9 January 1998, affecting the northeastern United States and the eastern Canadian provinces, was characterized by freezing rain amounts greater than 100 mm in some areas. The event was associated with a 1000–500-hPa positive (warm) thickness anomaly, whose 5-day mean exceeded +30 dam (+15°C) over much of New York and Pennsylvania. The region of maximum precipitation occurred in a deformation zone between an anomalously cold surface anticyclone to the north and a surface trough axis extending from the Gulf of Mexico into the Great Lakes. The thermodynamic impact of this unprecedented event was studied with the use of a four-dimensional data assimilation spanning an 18-day period ending at 0000 UTC 9 January 1998. A moisture budget for the precipitation region reveals the bulk of the precipitation to be associated with the convergence of water vapor transport throughout the precipitation period. The ice storm consisted of two primary synoptic-scale cyclonic events. The first even...

Wavelet analysis and the governing dynamics of a large‐amplitude mesoscale gravity‐wave event along the East Coast of the United States

AbstractDetailed diagnostic analyses are performed upon a mesoscale numerical simulation of a well‐observed gravity‐wave event that occurred on 4 January 1994 along the East Coast of the United States. The value of using wavelet analysis to investigate the evolving gravity‐wave structure and of using potential vorticity (PV) inversion to study the nature of the flow imbalance in the wave generation region is demonstrated. The cross‐stream Lagrangian Rossby number, the residual in the nonlinear balance equation, and the unbalanced geopotential‐height field obtained from PV inversion are each evaluated for their usefulness in diagnosing the flow imbalance. All of these fields showed clear evidence of strong imbalance associated with a middle‐to‐upper tropospheric jet streak, and tropopause fold upstream of the large‐amplitude gravity wave several hours before the wave became apparent at the surface.Analysis indicates that a train of gravity waves was continuously generated by geostrophic adjustment in the exit region of the unbalanced upper‐level jet streak as it approached the inflection axis in the height field immediately downstream of the maximum imbalance associated with the tropopause fold. A split front in the middle troposphere, characterized by the advance of the dry conveyor belt above the warm front, was overtaken by one of these propagating waves. During this merger process, a resonant interaction resulted, which promoted the rapid amplification and scale contraction of both the incipient wave (nonlinear wave development) and the split front (frontogenesis). The gravity wave and front aloft became inseparable following this merger. The situation became even more complex within a few hours as the vertical motion enhanced by this front‐wave interaction acted upon a saturated, potentially unstable layer to produce elevated moist convection. An analysis of the temporal changes in the vertical profile of wave energy flux suggests that moist convective downdraughts efficiently transported the wave energy from the midlevels downward beneath the warm‐front surface, where the wave became ducted. However, pure ducting was not sufficient for maintaining and amplifying the waves; rather, wave‐CISK (Conditional Instability of the Second Kind) was crucial.This complex sequence of nonlinear interactions produced a long‐lived, large‐amplitude gravity wave that created hazardous winter weather and disrupted society over a broad and highly populated area. Although gravity waves with similar appearance to this large‐amplitude wave of depression occasionally have been seen in other strong cyclogenesis cases involving a jet streak ahead of the upper‐level trough axis, it is unknown whether other such events share this same sequence of interactions.

Wavelet analysis and the governing dynamics of a large-amplitude mesoscale gravity-wave event along the East Coast of the United States

Detailed diagnostic analyses are performed upon a mesoscale numerical simulation of a well-observed gravity-wave event that occurred on 4 January 1994 along the East Coast of the United States. The value of using wavelet analysis to investigate the evolving gravity-wave structure and of using potential vorticity (PV) inversion to study the nature of the flow imbalance in the wave generation region is demonstrated. The cross-stream Lagrangian Rossby number, the residual in the nonlinear balance equation, and the unbalanced geopotential-height field obtained from PV inversion are each evaluated for their usefulness in diagnosing the flow imbalance. All of these fields showed clear evidence of strong imbalance associated with a middle-to-upper tropospheric jet streak, and tropopause fold upstream of the large-amplitude gravity wave several hours before the wave became apparent at the surface. Analysis indicates that a train of gravity waves was continuously generated by geostrophic adjustment in the exit region of the unbalanced upper-level jet streak as it approached the inflection axis in the height field immediately downstream of the maximum imbalance associated with the tropopause fold. A split front in the middle troposphere, characterized by the advance of the dry conveyor belt above the warm front, was overtaken by one of these propagating waves. During this merger process, a resonant interaction resulted, which promoted the rapid amplification and scale contraction of both the incipient wave (nonlinear wave development) and the split front (frontogenesis). The gravity wave and front aloft became inseparable following this merger. The situation became even more complex within a few hours as the vertical motion enhanced by this front-wave interaction acted upon a saturated, potentially unstable layer to produce elevated moist convection. An analysis of the temporal changes in the vertical profile of wave energy flux suggests that moist convective downdraughts efficiently transported the wave energy from the midlevels downward beneath the warm-front surface, where the wave became ducted. However, pure ducting was not sufficient for maintaining and amplifying the waves; rather, wave-CISK (Conditional Instability of the Second Kind) was crucial. This complex sequence of nonlinear interactions produced a long-lived, large-amplitude gravity wave that created hazardous winter weather and disrupted society over a broad and highly populated area. Although gravity waves with similar appearance to this large-amplitude wave of depression occasionally have been seen in other strong cyclogenesis cases involving a jet streak ahead of the upper-level trough axis, it is unknown whether other such events share this same sequence of interactions.

Splay fault and megathrust earthquake slip in the Nankai Trough

We consider whether splay fault slip may be a factor influencing the behavior of megathrust earthquakes in the Nankai Trough. Consideration of tsunami inversion results from other studies indicates that slip on one or more splay faults may be particularly important for the segment of the Nankai Trough offshore western Shikoku. These results suggest that during at least two megathrust earthquakes substantial slip may have occurred on one or more splay faults which cut the island arc crust over 100 km landward of the trench axis. In contrast to smaller subsidiary thrusts in the semi-consolidated and unconsolidated sediments very near the trough axis, the crustal faults considered here could accumulate and release considerable tectonic stress. Through a simple finite element calculation we demonstrate that slip on such faults can release some of the shear stress on the megathrust accumulated through plate subduction, and therefore may have some influence on the behavior of megathrust earthquakes.

Lateral variations of effective elastic thickness of the subducting Philippine Sea plate along the Nankai trough

We investigated lateral variations of effective elastic thickness of the subducting Philippine Sea plate along the Nankai trough. For this purpose, we applied two-dimensional thin elastic plate bending theory to several profiles almost perpendicular to the trough axis, correcting the effect of thick marine sediments in the Shikoku Basin. The 15 topographic profiles were selected from 61 profiles and were fit to respective theoretical deflection curves by minimization of the L1 norm. Classifying the Shikoku Basin into three regions, namely, the western, the middle, and the eastern regions, we obtained the results that the effective elastic plate thickness tends to decrease gradually from the east of the Ryukyu-Palau ridge to the Kinan seamount chain. This is consistent with heat flow data, seafloor age, formation process of the Shikoku Basin, length of the slab subducted from the Nankai trough, and spatial distribution of subcrustal earthquakes associated with subduction of the Philippine Sea plate.

Extensional faulting and caldera collapse in the axial region of fast spreading ridges: Analog modeling

The axial high of the East Pacific Rise (EPR) is bounded by ridge‐parallel lateral grabens that develop 2–8 km off‐axis. These troughs appear to lengthen away from the ridge crest, suggesting that tectonics is active at least 10 km away from the axis. Along 15–20% of the length of the ridge the axial high is notched by a summit trough 500 to 1800 m wide. These large axial troughs represent elongated collapse calderas that form when the EPR magma reservoir (comprising the melt lens and the underlying crystal mush zone) deforms and compacts during periods of waning magma supply under continuous stretching. We report the results of analog experiments performed in order to constrain the tectonic‐magmatic evolution of the crestal region of fast spreading ridges and more particularly the possible link that may exist between the development of axial caldera and the creation of lateral grabens along the crestal region. We used inflatable elongated balloons filled with water as an analog of the magma reservoir. The balloon is capped with a silicone layer representing hot rocks below the brittle‐ductile transition and is covered by a sand layer representing the brittle crust. The sand surface was given a dome shape that approximates the morphology of a fast spreading ridge. Mobile walls activated by a stepping motor allowed us to conduct the deflation experiments during continuous extension. Combination of deflation and extension leads to the creation of two lateral depressions and one axial trough. The lateral depressions are controlled by normal faults, while the central trough is delineated by reverse faults. During balloon deflation, tectonic extension is accommodated away from the axis due to the presence of the ductile silicone layer, whereas the deflation of the axial reservoir is accommodated by the collapse of the overlaying brittle crust. This accounts (1) for the development of axial troughs as collapse calderas, and (2) for the ubiquitous formation of lateral grabens of the flanking tectonic province in response to the deformation of the crystal mush.

Bathymetry and magnetic anomalies in the Havre Trough and southern Lau Basin: from rifting to spreading in back-arc basins

The Havre Trough and Lau Basin are active back-arc basins related to Pacific-Australian plate convergence. These back-arc basins provide insight into the evolution of rifting through various stages, including a final stage of oceanic spreading. We carried out a swath bathymetry and magnetic anomaly survey focused on the less-studied Havre Trough and southern Lau Basin. The western portion of the whole Havre Trough is deeper and more thickly sedimented whereas shallow bathymetry and volcanic edifices are dominant in the east. We suggest that crustal rifting progresses asymmetrically with respect to the trough axis. The active tectono-magmatic zone migrates trenchward during ongoing widening. Rifting along normal faults trending in the longitudinal trough strike occurs in the southern Havre Trough. Volcanism is restricted to a line along the normal faults. Circular or elongated knolls in the northern Havre Trough indicate that en echelon type segmented rifting of short strike length and successive volcanic intrusion takes place. Volcanism in this area is dominated by diapir-like magma intrusion into an extended and thinned arc crust. The southernmost tip of the spreading center of Lau Basin is defined by the deep graben, striking at N20°E, located at 24°00′S, 177°10′W.

Intraseasonal variation of monsoon activities associated with the rainfall over Bangladesh during the 1995 summer monsoon season

The rainfall over Bangladesh during the 1995 summer monsoon season has been investigated in terms of the intraseasonal variation of monsoon activities. The rainfall over Bangladesh is basically dominated by the north‐south oscillation of the monsoon trough. The rainfall increases when the monsoon trough is located at the foot of the Himalayas, because synoptic‐scale convective activity is much more vigorous to the south of the monsoon trough axis than to the north of it. In addition, the strong southwesterly wind to the south of the monsoon trough intensifies local convective activity owing to the effects of the orography to the north and east of the country. It is also found that the monsoon rainfall over Bangladesh in 1995 varies with a periodicity of ∼20 days, and this rainfall variation is closely associated with synoptic‐scale monsoon activities spreading over South and Southeast Asia. The active/break cycle of the rainfall variation during the 1995 summer monsoon season can be mostly explained by the northward propagation of what is called the 10–20 day variation of monsoon activities.

Possible splay fault slip during the 1946 Nankai earthquake

This paper considers seafloor deformation associated with the 1946 Nankai earthquake, which occurred in the Nankai Trough subduction zone off the coast of southwest Japan. We note that recent high‐resolution determinations of the slip distribution, assumed to be confined to the plate boundary, result in a pronounced maximum in seafloor displacement midway between the coast of Shikoku and the trough axis. We show that this feature could be due to slip on a splay fault rather than the plate boundary. Such a model for the western part of the slip distribution is able to account for the main features of the tsunamigenic seafloor deformation obtained in previous studies, with a maximum slip consistent with the plate convergent rate and history of earthquake recurrence at the Nankai Trough.

An empirical classification of weather types in the Mediterranean Basin and their interrelation with rainfall

This paper presents a classification of weather types in the Mediterranean Basin based on cluster analysis of the daily occurrences of several surface pressure centers and the subjective identification of 500 hPa trough axis positions (1992–1996). The procedure results in 20 types that explain 69% of overall pressure center variance and which are consistent with the seasonal succession of regional circulation. The development of weather types in winter is primarily controlled by the eastward propagation of barotropic waves while departures from the zonal flow pattern in summer tend to be linked to blocked stationary pools. H1-types with anticyclonic circulation in the Western Mediterranean and cyclonic flow in the eastern part are well interrelated with zonal and anticyclonic general weather types in Central Europe. H2-types featuring a weak Azores Anticyclone interrelate with a variety of meridional circulation types after the Hess and Brezowski (1969) classification. The 20 types explain rainfall variance in the core Mediterranean regions (as defined by principal components) to a high degree while rainfall variance in marginal regions is influenced by circulation patterns not being typical for the Mediterranean Basin.