Pacific North America Research Articles

Decadal variation of the impact of La Niña on the winter Arctic stratosphere

The impact of La Nina on the winter Arctic stratosphere has thus far been an ambiguous topic of research. Contradictory results have been reported depending on the La Nina events considered. This study shows that this is mainly due to the decadal variation of La Nina’s impact on the winter Arctic stratosphere since the late 1970s. Specifically, during the period 1951–78, the tropospheric La Nina teleconnection exhibits a typical negative Pacific–North America pattern, which strongly inhibits the propagation of the planetary waves from the extratropical troposphere to the stratosphere, and leads to a significantly strengthened stratospheric polar vortex. In contrast, during 1979–2015, the La Nina teleconnection shifts eastwards, with an anomalous high concentrated in the northeastern Pacific. The destructive interference of the La Nina teleconnection with climatological stationary waves seen in the earlier period reduces greatly, which prevents the drastic reduction of planetary wave activities in the extratropical stratosphere. Correspondingly, the stratospheric response shows a less disturbed stratospheric polar vortex in winter.

Changes in atmospheric rivers and moisture transport over the Northeast Pacific and western North America in response to ENSO diversity

The year-to-year changes in atmospheric rivers (ARs) and moisture transport over the northeast Pacific and western North America are investigated during December to February (DJF) from 1979/80 to 2015/16. Changes in AR frequency, intensity, and landfall characteristics are compared between three ENSO phases: central Pacific El Nino (CPEN), eastern Pacific El Nino (EPEN), and La Nina (NINA). During EPEN events, the subtropical jet extends to the south and east with an anomalous cyclonic flow around a deeper Aleutian Low. More moisture is transported towards North America and AR frequency is increased over western North America. In CPEN events, the Aleutian low shifts further southward relative to its position in EPEN, resulting in an increase in the frequency and intensity of landfalling ARs over the southwestern US. In NINA events, the landfalling AR frequency is reduced associated with anomalous anticyclonic circulation over the eastern North Pacific. We diagnose the contribution of multiple factors to the seasonal mean moisture transport using moisture budgets. During the three ENSO phases, the change in low-frequency circulation (dynamical process) is the leading contributor to the seasonal mean moisture flux divergence, while the contributions of the synoptic anomalies and the change in moisture anomaly (thermodynamic process) are not significant along the west coast of North America.

The effects of the Indo-Pacific warm pool on the stratosphere

Sea surface temperature (SST) in the Indo-Pacific warm pool (IPWP) plays a key role in influencing East Asian climate, and even affects global-scale climate change. This study defines IPWP Nino and IPWP Nina events to represent the warm and cold phases of IPWP SST anomalies, respectively, and investigates the effects of these events on stratospheric circulation and temperature. Results from simulations forced by observed SST anomalies during IPWP Nino and Nina events show that the tropical lower stratosphere tends to cool during IPWP Nino events and warm during IPWP Nina events. The responses of the northern and southern polar vortices to IPWP Nino events are fairly symmetric, as both vortices are significantly warmed and weakened. However, the responses of the two polar vortices to IPWP Nina events are of opposite sign: the northern polar vortex is warmed and weakened, but the southern polar vortex is cooled and strengthened. These features are further confirmed by composite analysis using reanalysis data. A possible dynamical mechanism connecting IPWP SST to the stratosphere is suggested, in which IPWP Nino and Nina events excite teleconnections, one similar to the Pacific–North America pattern in the Northern Hemisphere and a Rossby wave train in the Southern Hemisphere, which project onto the climatological wave in the mid–high latitudes, intensifying the upward propagation of planetary waves into the stratosphere and, in turn, affecting the polar vortex.

Circulation characteristics of persistent cold spells in central–eastern North America

The circulation patterns of persistent cold weather spells with durations longer than 10 days in central–eastern North America (United States and Canada; 32°–52°N, 95°–65°W) are investigated by using NCEP reanalysis data from 1948 to 2014. The criteria for the persistent cold spells are: (1) three-day averaged temperature anomalies for the regional average over the central–eastern United States and Canada must be below the 10th percentile, and (2) such extreme cold spells must last at least 10 days. The circulation patterns associated with these cold spells are examined to find the common signals of these events. The circulation anomaly patterns of these cold spells are categorized based on the El Nino–Southern Oscillation, Arctic Oscillation (AO), and other climate indices. The atmospheric circulation patterns that favor the cold spells are identified through composites of geopotential height maps for the cold spells. Negative AO phases favor persistent cold spells. Phases of sea surface temperature (SST) modes that are associated with warm SSTs in the eastern extratropical Pacific also favor persistent cold events in the study region. Stratospheric polar vortex breakdown alone is not a good predictor for the regional extreme cold spells in central–eastern North America. The meridional dispersions of quasi-stationary Rossby waves in the Pacific–North America sector in terms of cut-off zonal wavenumber modulated by background flow are analyzed to provide insight into the difference in evolution of the cold spells under different mean AO phases. The waveguide for AO > 1 is in a narrow latitudinal band centered on 40°N, whereas the waveguide for AO <–1 is in a broader latitudinal band from 40° to 65°N. The circulation patterns and lower boundary conditions favorable for persistent cold spells identified by this study can be a stepping-stone for improving winter subseasonal forecasting in North America.

Upper-ocean thermal variability controlled by ocean dynamics in the Kuroshio-Oyashio Extension region

To understand the upper-ocean thermal variability in the Kuroshio−Oyashio Extension (KOE) region, the upper 400 m heat budget in the western North Pacific is analyzed for the 1981 − 2013 period using outputs from a high resolution (1/12°) ocean general circulation model. Winter heat storage rate on interannual to decadal time scales is mainly determined by oceanic heat advection rather than by net air-sea heat flux. The role of heat advection becomes particularly prominent and widely spread over the entire western North Pacific after the 1990 regime shift in association with the reduced variability of surface heat flux caused by weakened SST variability. The net heat flux acts to dampen temperature anomalies caused by the ocean dynamics. The ocean dynamics causing the upper-ocean heat storage rate is principally associated with the meridional shift of the Oyashio Extension front, which is significantly correlated with both the West Pacific and Pacific-North America teleconnection patterns.

Intra-seasonal variability of extreme boreal stratospheric polar vortex events and their precursors

The dynamical variability of the boreal stratospheric polar vortex has been usually analysed considering the extended winter as a whole or only focusing on December, January and February. Yet recent studies have found intra-seasonal differences in the boreal stratospheric dynamics. In this study, the intra-seasonal variability of anomalous wave activity preceding polar vortex extremes in the Northern Hemisphere is examined using ERA-Interim reanalysis data. Weak (WPV) and strong (SPV) polar vortex events are grouped into early, mid- or late winter sub-periods depending on the onset date. Overall, the strongest (weakest) wave-activity anomalies preceding polar vortex extremes are found in mid- (early) winter. Most of WPV (SPV) events in early winter occur under the influence of east (west) phase of the Quasi-Biennial Oscillation (QBO) and an enhancement (inhibition) of wavenumber-1 wave activity (WN1). Mid- and late winter WPV events are preceded by a strong vortex and an enhancement of WN1 and WN2, but the spatial structure of the anomalous wave activity and the phase of the QBO are different. Prior to mid-winter WPVs the enhancement of WN2 is related to the predominance of La Niña and linked to blockings over Siberia. Mid-winter SPV events show a negative phase of the Pacific-North America pattern that inhibits WN1 injected into the stratosphere. This study suggests that dynamical features preceding extreme polar vortex events in mid-winter should not be generalized to other winter sub-periods.

Snow‐(N)AO relationship revisited over the whole twentieth century

AbstractSeveral studies suggest that the Siberian snow cover in fall is a source of predictability of the Arctic Oscillation (AO) in winter. Although a plausible dynamical mechanism was proposed, the robustness of this relationship was recently challenged. Here we use two atmospheric reanalyses to revisit the snow‐AO relationship and its modulation across the whole twentieth century. While our results support a stratospheric pathway mechanism, they show that the snow‐AO relationship has only emerged in the 1970s and should be rather analyzed as a contrasted multidecadal behavior of the North Atlantic Oscillation (NAO) and Pacific North America pattern. They confirm that the quasi‐biennial oscillation is a plausible candidate for the modulation of the snow‐(N)AO relationship across the twentieth century, but they further show that this modulation might be a purely stochastic effect. Therefore, they emphasize the limitations of any empirical prediction of the (N)AO only based on snow and/or sea ice predictors.

Arctic-Mid-Latitude Linkages in a Nonlinear Quasi-Geostrophic Atmospheric Model

A quasi-geostrophic three-level T63 model of the wintertime atmospheric circulation of the Northern Hemisphere has been applied to investigate the impact of Arctic amplification (increase in surface air temperatures and loss of Arctic sea ice during the last 15 years) on the mid-latitude large-scale atmospheric circulation. The model demonstrates a mid-latitude response to an Arctic diabatic heating anomaly. A clear shift towards a negative phase of the Arctic Oscillation (AO−) during low sea-ice-cover conditions occurs, connected with weakening of mid-latitude westerlies over the Atlantic and colder winters over Northern Eurasia. Compared to reanalysis data, there is no clear model response with respect to the Pacific Ocean and North America.

Magnetic fabrics of arc plutons reveal a significant Late Jurassic to Early Cretaceous change in the relative plate motions of the Pacific Ocean basin and North America

Contrasting magnetic fabrics in five successively emplaced syntectonic plutons reveal temporal and spatial variations in tectonic strain in the oceanic terranes of the Blue Mountains province, northeastern Oregon, during the Late Jurassic to Early Cretaceous. The inferred strain regimes changed from: (1) thrusting and sinistral shearing at ca. 160 Ma, to (2) horizontal stretching at ca. 147 Ma (in the forearc–accretionary wedge Baker terrane), to (3) dextral transpression that started from ca. 140 Ma onward and was associated with progressive anticlockwise rotation of the principal horizontal shortening direction from ca. 130 Ma to ca. 126 Ma (in the Wallowa oceanic arc terrane). These progressive strain reorientations are interpreted in terms of an outboard Wallowa-­Baker terrane collision, lateral extrusion, docking of the amal­gamated Blue Mountains superterrane into a continental-margin reentrant, and onset of oroclinal bending, respectively. The changes in crustal strains are then interpreted as recording a progressive change in relative motions between the Pacific Ocean basin and North America and suggest a transition from Late Jurassic sinistral deformation to Early Cretaceous dextral terrane translations along the paleo-Pacific margin. We speculate that these events may have been linked along large portions of the North American Cordillera, from central California to Blue Mountains, and may have culminated in the onset of accretion in the Franciscan complex and voluminous plutonism in the Sierra Nevada magmatic arc. A similar plate-kinematic change is inferred to have occurred in British Columbia several tens of millions of years later (at ca. 100 Ma), implying that these kinematic transitions may have varied in space and time along the length of the Cordilleran orogen.

A bottom-driven mechanism for distributed faulting in the Gulf of California rift

Observations of active faulting in the continent-ocean transition of the Northern Gulf of California show multiple oblique-slip faults distributed in a 200×70km2 area developed some time after a westward relocation of the plate boundary at ~2Ma. In contrast, north and south of this broad pull-apart structure, major transform faults accommodate Pacific-North America plate motion. Here we propose that the mechanism for distributed brittle deformation results from the boundary conditions present in the Northern Gulf, where basal shear is distributed between the Cerro Prieto strike-slip fault (southernmost fault of the San Andreas fault system) and the Ballenas Transform Fault. We hypothesize that in oblique-extensional settings whether deformation is partitioned in a few dip-slip and strike-slip faults, or in numerous oblique-slip faults may depend on (1) bottom-driven, distributed extension and shear deformation of the lower crust or upper mantle, and (2) the rift obliquity. To test this idea, we explore the effects of bottom-driven shear on the deformation of a brittle elastic-plastic layer with the help of pseudo-three dimensional numerical models that include side forces. Strain localization results when the basal shear abruptly increases in a step-function manner while oblique-slip on numerous faults dominates when basal shear is distributed. We further explore how the style of faulting varies with obliquity and demonstrate that the style of delocalized faulting observed in the Northern Gulf of California is reproduced in models with an obliquity of 0.7 and distributed basal shear boundary conditions, consistent with the interpreted obliquity and boundary conditions of the study area.

Analysis of the complete plastomes of three species of Membranoptera (Ceramiales, Rhodophyta) from Pacific North America.

Next generation sequence data were generated and used to assemble the complete plastomes of the holotype of Membranoptera weeksiae, the neotype (designated here) of M.tenuis, and a specimen examined by Kylin in making the new combination M.platyphylla. The three plastomes were similar in gene content and length and showed high gene synteny to Calliarthron, Grateloupia, Sporolithon, and Vertebrata. Sequence variation in the plastome coding regions were 0.89% between M.weeksiae and M.tenuis, 5.14% between M.weeksiae and M.platyphylla, and 5.18% between M.tenuis and M.platyphylla. We were unable to decipher the complete mitogenomes of the three species due to low coverage and structural problems; however, we assembled and analyzed, the cytochrome oxidase I, II, and III loci and found that M.weeksiae and M.tenuis differed in sequence by 1.3%, M.weeksiae and M.platyphylla by 8.4%, and M.tenuis and M.platyphylla by 8.1%. Evaluation of standard marker genes indicated that sequences from the rbcL, RuBisCO spacer, and CO1 genes closely approximated the pair-wise genetic distances observed between the plastomes of the three species of Membranoptera. A phylogenetic tree based on rbcL sequences showed that M.tenuis and M.weeksiae were sister taxa. Short rbcL sequences were obtained from type specimens of M.dimorpha, M.multiramosa, and M.edentata and confirmed their conspecificity with M.platyphylla. The data support the recognition of three species of Membranoptera occurring south of Alaska: M.platyphylla, M.tenuis, and M.weeksiae.

Linking a sea level pressure anomaly dipole over North America to the central Pacific El Niño

This study demonstrates the close connection between the north–south dipole pattern of sea level pressure anomalies over northeastern North America to the western tropical North Atlantic, referred to as the North American dipole (NAD), and the central Pacific (CP)-type El Nino a year later. In contrast to other ENSO precursors, such as the North Pacific Oscillation (NPO) and Pacific–North America (PNA) pattern, the NAD appears more closely related to the CP-type El Nino than to the eastern Pacific (EP)-type El Nino, indicating that the NAD may serve as a unique precursor for the CP El Nino. The wintertime NAD induces sea surface temperature anomalies in the northern tropical Atlantic (NTA), which subsequently play an important role in developing the CP El Nino-like pattern in the tropical Pacific over the course of the following year. It appears that the NAD influence on CP El Nino involves air–sea interaction over several major basins, including the subtropical/tropical Pacific and the NTA. Additional analysis indicates that the correlation of either the NAD index or the NPO index with the CP El Nino state a year later depends on the status of the other index. When the wintertime NAD index is of the opposite sign to the simultaneous NPO index, the correlation of the NAD or NPO index with the Nino4 index becomes much weaker.

Region-wide and ecotype-specific differences in demographic histories of threespine stickleback populations, estimated from whole genome sequences.

We analysed 81 whole genome sequences of threespine sticklebacks from Pacific North America, Greenland and Northern Europe, representing 16 populations. Principal component analysis of nuclear SNPs grouped populations according to geographical location, with Pacific populations being more divergent from each other relative to European and Greenlandic populations. Analysis of mitogenome sequences showed Northern European populations to represent a single phylogeographical lineage, whereas Greenlandic and particularly Pacific populations showed admixture between lineages. We estimated demographic history using a genomewide coalescence with recombination approach. The Pacific populations showed gradual population expansion starting >100Kya, possibly reflecting persistence in cryptic refuges near the present distributional range, although we do not rule out possible influence of ancient admixture. Sharp population declines ca. 14-15Kya were suggested to reflect founding of freshwater populations by marine ancestors. In Greenland and Northern Europe, demographic expansion started ca. 20-25Kya coinciding with the end of the Last Glacial Maximum. In both regions, marine and freshwater populations started to show different demographic trajectories ca. 8-9Kya, suggesting that this was the time of recolonization. In Northern Europe, this estimate was surprisingly late, but found support in subfossil evidence for presence of several freshwater fish species but not sticklebacks 12Kya. The results demonstrate distinctly different demographic histories across geographical regions with potential consequences for adaptive processes. They also provide empirical support for previous assumptions about freshwater populations being founded independently from large, coherent marine populations, a key element in the Transporter Hypothesis invoked to explain the widespread occurrence of parallel evolution across freshwater stickleback populations.

Fault zone characteristics and basin complexity in the southern Salton Trough, California

Abstract Ongoing oblique slip at the Pacific–North America plate boundary in the Salton Trough produced the Imperial Valley (California, USA), a seismically active area with deformation distributed across a complex network of exposed and buried faults. To better understand the shallow crustal structure in this region and the connectivity of faults and seismicity lineaments, we used data primarily from the Salton Seismic Imaging Project to construct a three-dimensional P-wave velocity model down to 8 km depth and a velocity profile to 15 km depth, both at 1 km grid spacing. A VP = 5.65–5.85 km/s layer of possibly metamorphosed sediments within, and crystalline basement outside, the valley is locally as thick as 5 km, but is thickest and deepest in fault zones and near seismicity lineaments, suggesting a causative relationship between the low velocities and faulting. Both seismicity lineaments and surface faults control the structural architecture of the western part of the larger wedge-shaped basin, where two deep subbasins are located. We estimate basement depths, and show that high velocities at shallow depths and possible basement highs characterize the geothermal areas.

Asymmetry and nonlinearity of the influence of ENSO on the northern winter stratosphere: 2. Model study with WACCM

AbstractLong‐term simulations from the Community Earth System Model with the Whole Atmosphere Community Climate Model (CESM‐WACCM) as its atmospheric component are used to investigate the asymmetry and the nonlinearity of the influences of El Niño–Southern Oscillation (ENSO) on the northern winter stratosphere. As in Part 1 of this study, four different types of ENSO are considered. The composite CESM‐WACCM results first confirm the conclusions drawn from the observations, that the stratospheric polar jet responses to “moderate El Niño” and “strong La Niña” are stronger than those to “strong El Niño” and “moderate La Niña”. In association with the ENSO sea surface temperature (SST) patterns that are reproduced well in the model, the tropical rainfall response centers exhibit an asymmetric east‐west shift between El Niño and La Niña, which directly leads to the nonlinear and asymmetric Pacific‐North America responses in the extratropics. Accordingly, the strengthening (weakening) planetary wave response in the stratosphere during warm (cold) ENSO also exhibits nonlinear and asymmetric features. When the ENSO SST forcing is prescribed to be linear and symmetric in WACCM, the nonlinearity and asymmetry of the stratospheric responses to moderate ENSO reveal the dominant role of the inherent properties of the atmosphere. However, the absence of asymmetry and nonlinearity in the stratospheric responses to strong ENSO in our sensitivity experiments indicates that the asymmetry in SST forcing between strong El Niño and La Niña still plays an important role in the asymmetric and nonlinear influences of ENSO on the extratropics.

High-resolution reconstructions of Pacific–North America plate motion: 20 Ma to present

High-resolution reconstructions of Pacific–North America plate motion: 20 Ma to present

Anisotropy from SKS splitting across the Pacific-North America plate boundary offshore southern California

SKS arrivals from ocean bottom seismometer (OBS) data from an offshore southern California deployment are analysed for shear wave splitting. The project involved 34 OBSs deployed for 12 months in a region extending up to 500 km west of the coastline into the oceanic Pacific plate. The measurement process consisted of removing the effects of anisotropy using a range of values for splitting fast directions and delay times to minimize energy along the transverse seismometer axis. Computed splitting parameters are unexpectedly similar to onland parameters, exhibiting WSW–ENE fast polarization directions and delays between 0.8 and 1.8 s, even for oceanic plate sites. This is the first SKS splitting study to extend across the entire boundary between the North America and Pacific plates, into the oceanic part of the Pacific plate. The splitting results show that the fast direction of anisotropy on the Pacific plate does not align with absolute plate motion (APM), and they extend the trend of anisotropy in southern California an additional 500 km west, well onto the oceanic Pacific plate. We model the finite strain and anisotropy within the asthenosphere associated with density–buoyancy driven mantle flow and the effects of APM. In the absence of plate motion effects, such buoyancy driven mantle flow would be NE-directed beneath the Pacific plate observations. The best-fit patterns of mantle flow are inferred from the tomography-based models that show primary influences from foundering higher-density zones associated with the history of subduction beneath North America. The new offshore SKS measurements, when combined with measurements onshore within the plate boundary zone, indicate that dramatic lateral variations in density-driven upper-mantle flow are required from offshore California into the plate boundary zone in California and western Basin and Range.

Slip history of the La Cruz fault: Development of a late Miocene transform in response to increased rift obliquity in the northern Gulf of California

The Gulf of California rift has accommodated oblique divergence of the Pacific and North America plates in northwestern México since Miocene time. Due to its infancy, its rifted margins preserve a rare onshore record of early continental break-up processes and an opportunity to investigate the role of rift obliquity in strain localization. We map rift-related structures and syn-tectonic basins on southern Isla Tiburón, a proximal onshore exposure of the rifted North America margin. We integrate analysis and geochronology of syn-tectonic sedimentary basins and mapping of crosscutting relationships to characterize the style and timing of fault activity. On southern Isla Tiburón, an early phase of extension initiated between ~19–17Ma and ~12.2Ma. Subsequently, these normal faults and related basins were cut by the La Cruz strike-slip fault and buried by deposits of the La Cruz basin, an elongate, fault-controlled trough coextensive with the La Cruz fault. Crosscutting relationships show that the NW-striking La Cruz fault accrued 5±2km of dextral slip ~8–4Ma. The La Cruz fault and parallel Tiburón transform were kinematically linked to detachment faulting that accommodated latest Miocene to Pliocene oblique opening of the offshore Upper Tiburón pull-apart basin. The onset of strike-slip faulting on Isla Tiburón was synchronous with the ~8–6Ma onset of transform faulting and basin formation along >1000km of the reconstructed Pacific-North America plate boundary. This transition coincides with the commencement of a clockwise azimuthal shift in Pacific-North America relative plate motion that increased the obliquity of the Gulf of California rift and formed the Gulf of California shear zone. The record from the proto-Gulf of California illustrates how highly oblique rift geometries, where transform faults are kinematically linked to pull-apart basins, enhance the ability of continental lithosphere to rupture and, ultimately, hasten the formation of new oceanic rift basins.

First report of the polychaete Polydora hoplura (Annelida: Spionidae) from North and South America and Asian Pacific

The spionid polychaete Polydora hoplura Claparede, 1868, is a harmful shell-borer accidentally transported with objects of aquaculture across the world. It was originally described from the Gulf of Naples, Italy, and later widely reported from European waters, South Africa, the Arabian Gulf, New Zealand, Australia and Tasmania. Here, for the first time, we describe adults and larvae of P. hoplura from Atlantic South America (Brazil), and report this species from Pacific North America (California, USA). We also examined material from Italy, New Zealand, Australia, Chile and Japan, and treat Polydora uncinata Sato-Okoshi, 1998, from Japan, as a junior synonym of P. hoplura. Consequently, we report P. hoplura for the first time in the Asian Pacific, Japan and South Korea, and in Pacific South America (Chile). The new records of P. hoplura from the American continents are cause for concern. The spread of this non-indigenous species may have negative impacts on the farm cultivation of oysters and abalone.

Interdecadal component variation characteristics in heavy winter snow intensity in North-Eastern China and its response to sea surface temperatures

Based on daily precipitation data from 208 weather stations, monthly NCEP/NCAR reanalysis data, and sea surface temperature data reconstructed by NOAA between 1961 and 2012, the heavy winter snow intensity in North-Eastern China was defined; its spatiotemporal variation characteristics were analyzed; the physical mechanisms of the relations between key sea surface temperature (SST) regions and heavy winter snow intensity were studied. Results showed that, in terms of temporal variation characteristics: heavy winter snow intensity in North-Eastern China has been rising, with obvious interdecadal variations during the study interval. In terms of spatial variation characteristics: the first empirical orthogonal function (EOF) mode of the interdecadal component in heavy winter snow intensity showed consistent anomaly characteristics throughout the region; the second mode exhibited opposite variation characteristics between the south and north; and the third mode exhibited opposite variation characteristics between the northwest and southeast. In terms of physical mechanisms underlying the effects of SST on heavy winter snow intensity, the key SST region of the first EOF mode was the Oyashio, the interdecadal component of the autumn Oyashio SST was abnormally high, which corresponded to a higher winter 500hPa height field in the northern section of the North Pacific. This condition led to a blocking situation and was associated with a weaker East Asian winter monsoon (EAMW), which resulted in a stronger interdecadal component of heavy winter snow intensity and vice versa. The second mode was closely related to the Pacific Decadal Oscillation (PDO): the interdecadal component of the autumn PDO was abnormal, which induced the teleconnection pattern of the winter Pacific-North America (PNA) pattern. The PNA pattern may have been related to the second EOF mode through its association with the Mongolian high.