Pacific Hemisphere Research Articles

Structure of the Earth's crust and upper mantle and problems of global geodynamics

This work presents findings of deep geological and geophysical studies, which have revealed new and often unexpected features of the structure of the Earth's crust and the upper mantle of the continents and oceans, not easily explainable by current geodynamic concepts. For example, deepwater drilling has shown the old age of the oceanic crust and its inhomogeneous composition. Seismic studies of long-range profiles with nuclear explosions and laboratory studies of mantle xenoliths at high pressures and temperatures have revealed that the advection of energy-intensive and geochemically active deep fluids is central to the formation of the upper mantle structure. A comprehensive analysis of these data provides a geodynamic model for the natural transformation of the upper mantle substance influenced by deep fluids and the formation of continents and oceans as a result of unevenly distributed degassing of the Earth. The global variability of the deep fluid advection intensity led to formation of two hemispheres: the Indo Atlantic Hemisphere where almost all continents are located and the Pacific Hemisphere that consists of oceans. This formation of the hemispheres can be explained by the specific features of the Earth's rotation in the Earth-Moon-Sun system. This work provides a review and analysis of these data and then offers a possible explanation based on an integrated geodynamic model.

Global-scale constraints on light-absorbing anthropogenic iron oxide aerosols

Anthropogenic iron oxide aerosols (FeOx) have been identified as a climatically significant atmospheric light absorber, and as a contributor of free iron to the oceans. Here we provide global-scale constraints on their atmospheric abundance with measurements over the remote Pacific and Atlantic Oceans from aircraft campaigns spanning 10 years. We find FeOx-like aerosols are transported far from source regions with similar efficiency as black carbon particles. Strong contrast in concentrations was observed between the Northern and Southern Hemisphere Pacific. We provide observational constraints in remote regions on the ambient ratios of FeOx relative to BC from fossil fuel burning. Comparison with a global aerosol model tuned to recent observations in East-Asian source regions confirm an upward revision of emissions based on model/observation comparison over the Pacific receptor region. We find that anthropogenic FeOx-like particles generate global-scale shortwave atmospheric heating 0.3–26% of that of black carbon in remote regions where concentrations of both aerosols are very low.

Recent geomagnetic variations and the force balance in Earth’s core

SUMMARY The nature of the force balance that governs the geodynamo is debated. Recent theoretical analyses and numerical simulations support a quasigeotrophic (QG), magneto-Archimedes-Coriolis (MAC) balance in Earth’s core, where the Coriolis and pressure forces equilibrate at leading order in amplitude, and where the buoyancy, Lorentz and ageostrophic Coriolis forces equilibrate at the next order. In contrast, earlier theoretical expectations have favoured a magnetostrophic regime where the Lorentz force would reach leading order at the system scale. The dominant driver (buoyant or magnetic) for the general circulation in Earth’s core is equally debated. In this study, these questions are explored in the light of the high-quality geomagnetic data recently acquired by satellites and at magnetic ground observatories. The analysis involves inverse geodynamo modelling, a method that uses multivariate statistics extracted from a numerical geodynamo model to infer the state of Earth’s core from a geomagnetic field model interpretation of the main field and secular variation data. To test the QG-MAC dynamic hypothesis against the data, the framework is extended in order to explicitly prescribe this force balance into the inverse problem solved at the core surface. The resulting inverse solutions achieve a quantitatively adequate fit to the data while ensuring deviations from the QG-MAC balance (which amount to an inertial driving of the flow) lower than each of the leading forces. The general circulation imaged within the core over the past two decades confirms the existence of a planetary-scale, eccentric, axially columnar gyre that comprises an intense, equatorially symmetric jet at high latitudes in the Pacific hemisphere. The dominant driver of this circulation is shown to be of buoyant nature, through a thermal wind balance with a longitudinally hemispheric buoyancy anomaly distribution. Geomagnetic forecasts initiated with the inverted core states are systematically more accurate against the true interannual geomagnetic field evolution when enforcing the QG-MAC constraint. This force balance is therefore consistent with the geomagnetic data at the large scales of Earth’s core that can be imaged by the method.

Identifying the signature of sea-surface properties in dinocyst assemblages: Implications for quantitative palaeoceanographical reconstructions by transfer functions and analogue techniques

Abstract Dinoflagellate cyst (=dinocyst) assemblages are widely used for the reconstruction of multiple oceanographic variables through the application of transfer functions. There is evidence that the number and kind of variables driving compositional changes in dinocyst assemblages vary regionally and that the selection of driving factors and the evaluation of transfer function performances are method-specific and complicated by spatial autocorrelation. Here, we used two new modern datasets from the Northern Hemisphere Atlantic-Arctic and the Northern Hemisphere Pacific oceans to re-evaluate the impact of sea-surface properties in dinocyst assemblages. We determined the dimensionality of the dinocyst ecological response and identified the main drivers for both regions. We calibrated and evaluated transfer function methods for the prediction of these variables and estimated their performances considering spatial autocorrelation. In both datasets, multiple environmental variables mutually and independently affect assemblage compositions, but the number and kind of these variables differ between datasets. We detected spatial autocorrelation, which was often due to environmental similarity, but some variables appeared to reflect geographical closeness, implying that spatial independence between sample sites depends upon the variables. We identified different primary drivers in both areas, highlighting the merit of regional calibrations and the necessity to carry out variable selection for each region separately. The multiple gradients identified imply that potentially multiple parameters could be reconstructed from the same fossil dinocyst assemblages. However, as the multiple gradients reflect geographical structuring, we propose that regional calibrations, even at the expense of generalisation, could improve the reliability and interpretation of transfer function reconstructions.

A reduced stochastic model of core surface dynamics based on geodynamo simulations

SUMMARYWe make use of recent geodynamo simulations to propose a reduced stochastic model of the dynamics at the surface of Earth’s core. On decadal and longer periods, this model replicates the most energetic eigen directions of the geodynamo computation. Towards shorter timescales, it proposes a compensation for weaknesses of these simulations. This model furthermore accounts for the signature, in the geomagnetic secular variation, of errors of representativeness associated with unresolved processes. We incorporate the reduced stochastic model into a geomagnetic data assimilation algorithm—an augmented state ensemble Kalman filter—and apply it to re-analyse magnetic field changes over the period 1880–2015. Errors of representativeness appear to be responsible for an important fraction of the observed changes in the secular variation, as it is the case in the dynamo simulation.Recovered core surface motions are primarily symmetric with respect to the equator. We observe the persistence of the eccentric westward gyre over the whole studied era and vortices that partly follow isocontours of the radial magnetic field at the core surface. Our flow models provide a good fit to decadal changes in the length-of-day and predict its interannual variations over the period 1940–2005. The largest core flow acceleration patterns are found in an equatorial belt below 10° in latitude and are associated with non-axisymmetric features. No systematic longitudinal drift of acceleration patterns is found, even over the past decades where satellite data are available. The acceleration of the high-latitude westward jet in the Pacific hemisphere is, during the satellite era, a factor 5 smaller than previously reported and its structure shows some evidence for equatorial asymmetry. The era of continuous satellite records provides enhanced contrast on the rapid core flow variations. The proposed assimilation algorithm offers the prospect of evaluating Earth-likeness of geodynamo simulations.

Unsolved problems of the global tectonics and possibility of their solutions

The deep geological and geophysical studies of the continents and oceans have revealed a number of well-defined new regularities in the structure of the crust and upper mantle that do not find a clear explanation in the modern geodynamic concepts. The regularities are the following. The Earth is divided into two hemispheres with different structure of the lithosphere: the Pacific and Indo-Atlantic hemisphere. The Pacific hemisphere is surrounded by a ring of the tectonically active zones with high seismicity (Benioff Zones). The system of the mid-ocean ridges with approximately equal distances between them, 90°, is symmetrical relatively to the South Pole. The crust in the oceans is different in age and composition, it identified the remnants of an ancient (Archaean) crust and large areas of subcontinental crust. The continents are characterized by the large thickness of the lithosphere (more than 200 km), composed of the lower density depleted matter. Experimental data on petrophysical properties of the crust and upper mantle matter at high pressure and temperature, the data on deep xenoliths, geochemical studies of natural gases have showen a large role of deep energy-intensive fluids in the formation of the sialic crust and depleted mantle rocks. These data give possibility to explain the continents and oceans origin. The irregular in space the Earth degassing results in different lithosphere types formation: the thick granite-gneiss crust and the lower density depleted mantle of the continents were created in the areas of the higher deep fluids flows; in the areas of the lower fluids advection the primary oceanic crust was preserved and only some separate spots of the transition crust appeared. The lower density of the continental «roots» was the main factor in the formation of continents: the lower density lithosphere led to its emerging in respect to the oceanic lithosphere. Two hemispheres with different lithosphere structure were formed may be due to the elliptical form of the orbit, causing periodic changes of the planet accelerations. The structural symmetry of the global rift system relative to the planet poles gives possibility to explain its origin by the expansion of the planet.

On the geomagnetic secular variation in the Pacific region

It is widely believed that in the Pacific region the geomagnetic secular variation is low and that the non-dipole field is weak. This is also known as hemispherical asymmetry between the Pacific and the Atlantic hemispheres, representing the low secular variation in the Pacific hemisphere. With this characteristic feature, a question has been left unsolved whether it is a permanent feature or a transient one.Reexamination of the geomagnetic field for the past 400 years indicates that in the early 17th century the non-dipole field was not weak, nor the secular variation was low in the Pacific region, implying that the hemispherical asymmetry is not a permanent phenomenon. This paper examines the secular variation for the past 400 years from viewpoint of drifting and standing field model. It has been found that the low secular variation in the Pacific region is mainly caused by the drift of the drifting field. At the Earth’s surface the drifting field is dominated by spherical harmonics Y22(θ,ϕ) with two pairs of ridges and troughs with their axes aligned in the meridional direction. The two pairs are not equal; one pair is stronger than the other. Movements of the different magnitude of the pairs are considered to have caused the hemispherical asymmetry of the secular variation for recent years, giving rise to the non-weak field in the Pacific region in the early 17th century.

Two new species of deep-water Calcigorgia gorgonians (Anthozoa: Octocorallia) from the Kurile Islands, Sea of Okhotsk, with a review of distinctive characters of the known species of the genus

This paper provides descriptions of Calcigorgia gorgonians collected from the Sea of Okhotsk and shores of the Kurile Islands between 3 Aug. 1984 and 20 Aug. 1987. New species described are the deep-water gorgonians Calcigorgia matua sp. nov. and C. simushiri sp. nov., belonging to the temperate fauna of the North Pacific hemisphere. Specimens belonging to all species of the genus were examined and the distinctive characters for each one are provided and summarized in a table. A modified diagnosis of the genus is proposed. The finding of a previously undescribed species emphasizes the need of further surveys, particularly in shelf and deeper waters, in order to improve our knowledge of this neglected fauna in Far East seas.

Measuring the seismic velocity in the top 15 km of Earth’s inner core

We present seismic observations of the uppermost layer of the inner core. This was formed most recently, thus its seismic features are related to current solidification processes. Previous studies have only constrained the east-west hemispherical seismic velocity structure in the Earth’s inner core at depths greater than 15 km below the inner core boundary. The properties of shallower structure have not yet been determined, because the seismic waves PKIKP and PKiKP used for differential travel time analysis arrive close together and start to interfere. Here, we present a method to make differential travel time measurements for waves that turn in the top 15 km of the inner core, and measure the corresponding seismic velocity anomalies. We achieve this by generating synthetic seismograms to model the overlapping signals of the inner core phase PKIKP and the inner core boundary phase PKiKP. We then use a waveform comparison to attribute different parts of the signal to each phase. By measuring the same parts of the signal in both observed and synthetic data, we are able to calculate differential travel time residuals. We apply our method to data with ray paths which traverse the Pacific hemisphere boundary. We generate a velocity model for this region, finding lower velocity for deeper, more easterly ray paths. Forward modelling suggests that this region contains either a high velocity upper layer, or variation in the location of the hemisphere boundary with depth and/or latitude. Our study presents the first direct seismic observation of the uppermost 15 km of the inner core, opening new possibilities for further investigating the inner core boundary region.

Fine-scale structure of the mid-mantle characterised by global stacks of PP precursors

Subduction zones are likely a major source of compositional heterogeneities in the mantle, which may preserve a record of the subduction history and mantle convection processes. The fine-scale structure associated with mantle heterogeneities can be studied using the scattered seismic wavefield that arrives as coda to or as energy preceding many body wave arrivals. In this study we analyse precursors to PP by creating stacks recorded at globally distributed stations. We create stacks aligned on the PP arrival in 5° distance bins (with range 70–120°) from 600 earthquakes recorded at 193 stations stacking a total of 7320 seismic records. As the energy trailing the direct P arrival, the P coda, interferes with the PP precursors, we suppress the P coda by subtracting a best fitting exponential curve to this energy. The resultant stacks show that PP precursors related to scattering from heterogeneities in the mantle are present for all distances. Lateral variations are explored by producing two regional stacks across the Atlantic and Pacific hemispheres, but we find only negligible differences in the precursory signature between these two regions. The similarity of these two regions suggests that well mixed subducted material can survive at upper and mid-mantle depth. To describe the scattered wavefield in the mantle, we compare the global stacks to synthetic seismograms generated using a Monte Carlo phonon scattering technique. We propose a best-fitting layered heterogeneity model, BRT2017, characterised by a three layer mantle with a background heterogeneity strength (ϵ=0.8%) and a depth-interval of increased heterogeneity strength (ϵ=1%) between 1000 km and 1800 km. The scalelength of heterogeneity is found to be 8 km throughout the mantle. Since mantle heterogeneity of 8 km scale may be linked to subducted oceanic crust, the detection of increased heterogeneity at mid-mantle depths could be associated with stalled slabs due to increases in viscosity, supporting recent observations of mantle viscosity increases due to the iron spin transition at depths of ∼1000 km.

The Pan American Imagination: Contested Visions of the Hemisphere in Twentieth-Century LiteratureThe Geopoetics of ModernismThe Black Pacific Narrative: Geographic Imaginings of Race and Empire between the World Wars

The Pan American Imagination: Contested Visions of the Hemisphere in Twentieth-Century LiteratureThe Geopoetics of ModernismThe Black Pacific Narrative: Geographic Imaginings of Race and Empire between the World Wars

Ridge subduction sparked reorganization of the Pacific plate‐mantle system 60–50 million years ago

AbstractA reorganization centered on the Pacific plate occurred ~53–47 million years ago. A “top‐down” plate tectonic mechanism, complete subduction of the Izanagi plate, as opposed to a “bottom‐up” mantle flow mechanism, has been proposed as the main driver. Verification based on marine geophysical observations is impossible as most ocean crust recording this event has been subducted. Using a forward modeling approach, which assimilates surface plate velocities and shallow thermal structure of slabs into mantle flow models, we show that complete Izanagi plate subduction and margin‐wide slab detachment induced a major change in sub‐Pacific mantle flow, from dominantly southward before 60 Ma to north‐northeastward after 50 Ma. Our results agree with onshore geology, mantle tomography, and the inferred motion of the Hawaiian hot spot and are consistent with a plate tectonic process driving the rapid plate‐mantle reorganization in the Pacific hemisphere between 60 and 50 Ma. This reorganization is reflected in tectonic changes in the Pacific and surrounding ocean basins.

The Major Stratospheric Sudden Warming of January 2013: Analyses and Forecasts in the GEOS-5 Data Assimilation System

Abstract The major stratospheric sudden warming (SSW) of 6 January 2013 is examined using output from the NASA Global Modeling and Assimilation Office (GMAO) Goddard Earth Observing System version 5 (GEOS-5) near-real-time data assimilation system (DAS). GEOS-5 analyses showed that the SSW of January 2013 was a major warming by 1200 UTC 6 January, with a wave-2 vortex-splitting pattern. Upward wave activity flux from the upper troposphere (~23 December 2012) displaced the ~10-hPa polar vortex off the pole in a wave-1 pattern, enabling the poleward advection of subtropical values of Ertel potential vorticity (EPV) into a developing anticyclonic circulation region. While the polar vortex subsequently split (wave-2 pattern) the wave-2 forcing [upward Eliassen–Palm (EP) flux] was smaller than what was found in recent wave-2, SSW events, with most of the forcing located in the Pacific hemisphere. Investigation of a rapidly developing tropospheric weather system over the North Atlantic on 28–29 December 2012 showed strong transient upward wave activity flux from the storm with influences up to 10 hPa; however, the Pacific hemisphere wave forcing remained dominate at this time. Results from the GEOS-5 five-day forecasts showed that the forecasts accurately predicted the major SSW of January 2013. The overall success of the 5-day forecasts provides motivation to produce regular 10-day forecasts with GEOS-5, to better support studies of stratosphere–troposphere interaction.

Satellite observations of peroxyacetyl nitrate from the Aura Tropospheric Emission Spectrometer

Abstract. We present a description of the algorithm used to retrieve peroxyacetyl nitrate (PAN) concentrations from the Aura Tropospheric Emission Spectrometer (TES). We describe the spectral microwindows, error analysis, and the utilization of a priori and initial guess information provided by the GEOS-Chem global chemical transport model. The TES PAN retrievals contain up to one degree of freedom for signal. In general, the retrievals are most sensitive to PAN in the mid-troposphere. Estimated single-measurement uncertainties are on the order of 30 to 50%. The detection limit for a single TES measurement is dependent on the atmospheric and surface conditions as well as on the instrument noise. For observations where the cloud optical depth is less than 0.5, we find that the TES detection limit for PAN is in the region of 200 to 300 pptv. We show that PAN retrievals capture plumes associated with boreal burning. Retrievals over the Northern Hemisphere Pacific in springtime show spatial features that are qualitatively consistent with the expected distribution of PAN in outflow of Asian pollution.

A Reanalysis of Long-Term Surface Air Temperature Trends in New Zealand

Detecting trends in climate is important in assessments of global change based on regional long-term data. Equally important is the reliability of the results that are widely used as a major input for a large number of societal design and planning purposes. New Zealand provides a rare long temperature time series in the Southern Hemisphere, and it is one of the longest continuous climate series available in the Southern Hemisphere Pacific. It is therefore important that this temperature dataset meets the highest quality control standards. New Zealand’s national record for the period 1909 to 2009 is analysed and the data homogenized. Current New Zealand century-long climatology based on 1981 methods produces a trend of 0.91 °C per century. Our analysis, which uses updated measurement techniques and corrects for shelter-contaminated data, produces a trend of 0.28 °C per century.

Variability modes in core flows inverted from geomagnetic field models

The flow of liquid metal inside the Earth's core produces the geomagnetic field and its time variations. Understanding the variability of those deep currents is crucial to improve the forecast of geomagnetic field variations, which affect human spacial and aeronautic activities. Moreover, it may provide relevant information on the core dynamics. The main goal of this study is to extract and characterize the leading variability modes of core flows over centennial periods, and to assess their statistical robustness. To this end, we use flows that we invert from two geomagnetic field models (gufm1 and COV-OBS), and apply Principal Component Analysis and Singular Value Decomposition of coupled fields. The quasi geostrophic (QG) flows inverted from both geomagnetic field models show similar features. However, COV-OBS has a less energetic mean and larger time variability. The statistical significance of flow components is tested from analyses performed on subareas of the whole domain. Bootstrapping methods are also used to extract significant flow features required by both gufm1 and COV-OBS. Three main empirical circulation modes emerge, simultaneously constrained by both geomagnetic field models and expected to be robust against the particular a priori used to build them (large scale QG dynamics). Mode 1 exhibits three large vortices at medium/high latitudes, with opposite circulation under the Atlantic and the Pacific hemispheres. Mode 2 interestingly accounts for most of the variations of the Earth's core angular momentum. In this mode, the regions close to the tangent cylinder and to the equator are correlated, and oscillate with a period between 80 and 90 years. Each of these two modes is energetic enough to alter the mean flow, sometimes reinforcing the eccentric gyre, and other times breaking it up into smaller circulations. The three main circulation modes added together to the mean flow account for about 70% of the flows variability, 90% of the root mean square total velocities, and 95% of the secular variation induced by the total flows. Direct physical interpretation of the computed modes is not straightforward. Nonethe-less, similarities found between the two first modes and time/spatial features identified in different studies of core dynamics, suggest that our approach can help to pinpoint the relevant physical processes inside the core on centennial timescales.

Bottom-up control of geomagnetic secular variation by the Earth’s inner core

Temporal changes in the Earth's magnetic field, known as geomagnetic secular variation, occur most prominently at low latitudes in the Atlantic hemisphere (that is, from -90 degrees east to 90 degrees east), whereas in the Pacific hemisphere there is comparatively little activity. This is a consequence of the geographical localization of intense, westward drifting, equatorial magnetic flux patches at the core surface. Despite successes in explaining the morphology of the geomagnetic field, numerical models of the geodynamo have so far failed to account systematically for this striking pattern of geomagnetic secular variation. Here we show that it can be reproduced provided that two mechanisms relying on the inner core are jointly considered. First, gravitational coupling aligns the inner core with the mantle, forcing the flow of liquid metal in the outer core into a giant, westward drifting, sheet-like gyre. The resulting shear concentrates azimuthal magnetic flux at low latitudes close to the core-mantle boundary, where it is expelled by core convection and subsequently transported westward. Second, differential inner-core growth, fastest below Indonesia, causes an asymmetric buoyancy release in the outer core which in turn distorts the gyre, forcing it to become eccentric, in agreement with recent core flow inversions. This bottom-up heterogeneous driving of core convection dominates top-down driving from mantle thermal heterogeneities, and localizes magnetic variations in a longitudinal sector centred beneath the Atlantic, where the eccentric gyre reaches the core surface. To match the observed pattern of geomagnetic secular variation, the solid material forming the inner core must now be in a state of differential growth rather than one of growth and melting induced by convective translation.

CMIP5 기후모델에서 나타나는 열대저기압 생성빈도의 연진동과 경년변동성: 잠재생성지수의 이용

The potential for tropical cyclogenesis in a given oceanic and atmospheric environments can be represented by genesis potential index (GPI). Using the 18 Coupled Model Inter Comparison Project phase 5 (CMIP5) models, the annual cycle of GPI and interannual variability of GPI are analyzed in this study. In comparison, the annual cycle of GPI calculated from reanalysis data is revisited. In particular, GPI differences between CMIP5 models and reanalysis data are compared, and the possible reasons for the GPI differences are discussed. ENSO (El Nino and Southern Oscillation) has a tropical phenomenon, which affects tropical cyclone genesis and its passages. Some dynamical interpretations of tropical cyclogenesis are suggested by using the fact that GPI is a function of four large-scale parameters. The GPI anomalies in El Nino or La Nina years are discussed and the most contributable factors are identified in this study. In addition, possible dynamics of tropical cyclogenesis in the Northern Hemisphere Pacific region are discussed using the large-scale factors.

Core surface magnetic field evolution 2000-2010

SUMMARY We present new dedicated core surface field models spanning the decade from 2000.0 to 2010.0. These models, called gufm-sat, are based on CHAMP, Orsted and SAC-C satellite observations along with annual differences of processed observatory monthly means. A spatial parametrization of spherical harmonics up to degree and order 24 and a temporal parametrization of sixth-order B-splines with 0.25 yr knot spacing is employed. Models were constructed by minimizing an absolute deviation measure of misfit along with measures of spatial and temporal complexity at the core surface. We investigate traditional quadratic or maximum entropy regularization in space, and second or third time derivative regularization in time. Entropy regularization allows the construction of models with approximately constant spectral slope at the core surface, avoiding both the divergence characteristic of the crustal field and the unrealistic rapid decay typical of quadratic regularization at degrees above 12. We describe in detail aspects of the models that are relevant to core dynamics. Secular variation and secular acceleration are found to be of lower amplitude under the Pacific hemisphere where the core field is weaker. Rapid field evolution is observed under the eastern Indian Ocean associated with the growth and drift of an intense low latitude flux patch. We also find that the present axial dipole decay arises from a combination of subtle changes in the southern hemisphere field morphology.

A Synoptic Climatology of Episodic, Subseasonal Retractions of the Pacific Jet

Abstract Twenty-eight years of NCEP–NCAR reanalysis data are employed in a composite analysis of the structure and evolution of the large-scale circulation associated with rapid, subseasonal, westward retractions of the Northern Hemisphere Pacific jet. Nineteen Pacific jet retractions are identified in the dataset. The salient characteristics of these transitions are presented, emphasizing the structure and evolution of anomalies in the zonal wind, upper-tropospheric geopotential height, sea level pressure (SLP), and tropopause potential vorticity (PV). The composite analysis demonstrates that as the jet retracts, a transition from a dominant negative anomaly to a dominant positive anomaly across the North Pacific in both the 500-hPa geopotential height and SLP fields occurs in ~10 days. The resulting anticyclonic anomalies are sprawling and intense with SLP anomalies of nearly 20 hPa and 500-hPa geopotential height anomalies of more than 200 m. The vertical superposition of these upper- and lower-level anomalies indicates that these composite structures are equivalent barotropic. These results are also consistent with the composite 200–250-hPa Ertel PV anomaly field, in which a jet retraction event is characterized by the formation of a zonally elongated, meridionally oriented PV anomaly couplet in the central North Pacific that becomes increasingly isotropic in the jet exit region simultaneous with the retraction of the jet. Because of the large spatial scale of these events, the results are also discussed in the context of storm-track variability, the East Asian winter monsoon, and tropical diabatic heating anomalies.