Eddy flow characteristics and mean flow interactions in the North Pacific

Abstract A climatology of large-scale, persistent cyclonic flow anomalies over the North Pacific was constructed using the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) global reanalysis data for the cold season (November–March) for 1977–2003. These large-scale cyclone (LSC) events were identified as those periods for which the filtered geopotential height anomaly at a given analysis point was at least 100 m below its average for the date for at least 10 days. This study identifies a region of maximum frequency of LSC events at 45°N, 160°W [key point 1 (KP1)] for the entire period. This point is somewhat to the east of regions of maximum height variability noted in previous studies. A second key point (37.5°N, 162.5°W) was defined as the maximum in LSC frequency for the period after November 1988. The authors show that the difference in location of maximum LSC frequency is linked to a climate regime shift at about that time. LSC events occur with a maximum frequency in the period from November through January. A composite 500-hPa synoptic evolution, constructed relative to the event onset, suggests that the upper-tropospheric precursor for LSC events emerges from a quasi-stationary long-wave trough positioned off the east coast of Asia. In the middle and lower troposphere, the events are accompanied by cold thickness advection from a thermal trough over northeastern Asia. The composite mean sea level evolution reveals a cyclone that deepens while moving from the coast of Asia into the central Pacific. As the cyclone amplifies, it slows down in the central Pacific and becomes nearly stationary within a day of onset. Following onset, at 500 hPa, a stationary wave pattern, resembling the Pacific–North American teleconnection pattern, emerges with a ridge immediately downstream (over western North America) and a trough farther downstream (from the southeast coast of the United States into the western North Atlantic). The implications for the resulting sensible weather and predictability of the flow are discussed. An adjoint-derived sensitivity study was conducted for one of the KP1 cases identified in the climatology. The results provide dynamical confirmation of the LSC precursor identification for the events. The upper-tropospheric precursor is seen to play a key role not only in the onset of the lower-tropospheric height falls and concomitant circulation increases, but also in the eastward extension of the polar jet across the Pacific. The evolution of the forecast sensitivities suggest that LSC events are not a manifestation of a modal instability of the time mean flow, but rather the growth of a favorably configured perturbation on the flow.

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Understanding the Basin Asymmetry in Surface Response to Sudden Stratospheric Warmings from an Ocean-Atmosphere Coupled Perspective

Journal of Climate ◽

10.1175/jcli-d-21-0314.1 ◽

2021 ◽

pp. 1-54

Author(s):

Ying Dai ◽

Peter Hitchcock

Keyword(s):

North Pacific ◽

Mean Flow ◽

Seasonal Forecasts ◽

Stratospheric Polar Vortex ◽

Near Surface ◽

Surface Response ◽

The North ◽

The Pacific ◽

Ocean Atmosphere ◽

The North Pacific

AbstractThe canonical tropospheric response to a weakening of the stratospheric vortex—an equatorward shift of the eddy-driven jet—is mostly limited to the North Atlantic following sudden stratospheric warmings (SSWs). A coherent change in the Pacific eddy-driven jet is notably absent. Why is this so? Using daily reanalysis data, we show that air-sea interactions over the North Pacific are responsible for the basin-asymmetric response to SSWs. Prior to the onset of some SSWs, their tropospheric precursors produce a dipolar SST pattern in the North Pacific, which then persists as the stratospheric polar vortex breaks down following the onset of the SSW. By reinforcing the lower tropospheric baroclinicity, the dipolar SST pattern helps sustain the generation of baroclinic eddies, strengthening the near-surface Pacific eddy-driven jet and maintaining its near-climatological-mean state. This prevents the jet from being perturbed by the downward influence of the stratospheric anomalies. As a result, these SSWs exhibit a highly basin-asymmetric surface response with only the Atlantic eddy-driven jet shifted equatorward. For SSWs occurring without the atmospheric precursors in the North Pacific troposphere, the dipolar SST pattern is absent due to the lack of the atmospheric forcing. In the absence of the dipolar SST pattern and the resultant eddy-mean flow feedbacks, these SSWs exhibit a basin-symmetric surface response with both the Atlantic and the Pacific eddy-driven jets shifted equatorward. Our results provide an ocean-atmosphere coupled perspective on stratosphere-troposphere interaction following SSW events and have potential for improving subseasonal to seasonal forecasts for surface weather and climate.

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Intraseasonal Modulation of the North Pacific Storm Track by Tropical Convection in Boreal Winter

Journal of Climate ◽

10.1175/2010jcli3676.1 ◽

2011 ◽

Vol 24 (4) ◽

pp. 1122-1137 ◽

Cited By ~ 30

Author(s):

Yi Deng ◽

Tianyu Jiang

Keyword(s):

North Pacific ◽

Storm Track ◽

Tropical Convection ◽

Boreal Winter ◽

Mean Flow ◽

Central Pacific ◽

Synoptic Eddy ◽

The North ◽

The North Pacific ◽

North Pacific Storm Track

Abstract The modulation of the North Pacific storm track by tropical convection on intraseasonal time scales (30–90 days) in boreal winter (December–March) is investigated using the NCEP–NCAR reanalysis and NOAA satellite outgoing longwave radiation (OLR) data. Multivariate empirical orthogonal function (MEOF) analysis and case compositing based upon the principal components (PCs) of the EOFs reveal substantial changes in the structure and intensity of the Pacific storm track quantified by vertically (925–200 mb) averaged synoptic eddy kinetic energy (SEKE) during the course of a typical Madden–Julian oscillation (MJO) event. The storm-track response is characterized by an amplitude-varying dipole propagating northeastward as the center of the anomalous tropical convection moves eastward across the eastern Indian Ocean and the western-central Pacific. A diagnosis of the SEKE budget indicates that the storm-track anomaly is induced primarily by changes in the convergence of energy flux, baroclinic conversion, and energy generation due to the interaction between synoptic eddies and intraseasonal flow anomalies. This demonstrates the important roles played by eddy–mean flow interaction and eddy–eddy interaction in the development of the extratropical response to MJO variability. The feedback of synoptic eddy to intraseasonal flow anomalies is pronounced: when the center of the enhanced tropical convection is located over the Maritime Continent (western Pacific), the anomalous synoptic eddy forcing partly drives an upper-tropospheric anticyclonic (cyclonic) and, to its south, a cyclonic (anticyclonic) circulation anomaly over the North Pacific. Associated with the storm-track anomaly, a three-band (dry–wet–dry) anomaly in both precipitable water and surface precipitation propagates poleward over the eastern North Pacific and induces intraseasonal variations in the winter hydroclimate over western North America.

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Trends and Variability of North Pacific Polar Lows

Advances in Meteorology ◽

10.1155/2013/170387 ◽

2013 ◽

Vol 2013 ◽

pp. 1-11 ◽

Cited By ~ 8

Author(s):

Fei Chen ◽

Hans von Storch

Keyword(s):

Spatial Distribution ◽

North Pacific ◽

Canonical Correlation ◽

Decadal Variability ◽

Observational Evidence ◽

Positive Trend ◽

Mean Flow ◽

Polar Lows ◽

The North ◽

The North Pacific

The 6-hourly 1948–2010 NCEP 1 reanalyses have been dynamically downscaled for the region of the North Pacific. With a detecting-and-tracking algorithm, the climatology of North Pacific Polar Lows has been constructed. This derived climatology is consistent with the limited observational evidence in terms of frequency and spatial distribution. The climatology exhibits strong year-to-year variability but weak decadal variability and a small positive trend. A canonical correlation analysis describes the conditioning of the formation of Polar Lows by characteristic seasonal mean flow regimes, which favor, or limit, cold air outbreaks and upper air troughs.

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Estimating Bolus Velocities from Data—How Large Must They Be?

Journal of Physical Oceanography ◽

10.1175/2008jpo3905.1 ◽

2009 ◽

Vol 39 (1) ◽

pp. 70-88 ◽

Cited By ~ 1

Author(s):

Peter D. Killworth

Keyword(s):

North Pacific ◽

Density Level ◽

Eddy Flux ◽

Mean Flow ◽

Flux Divergence ◽

The North ◽

Resolution Model ◽

Eddy Fluxes ◽

The Mean ◽

The North Pacific

Abstract This paper examines the representation of eddy fluxes by bolus velocities. In particular, it asks the following: 1) Can an arbitrary eddy flux divergence of density be represented accurately by a nondivergent bolus flux that satisfies the condition of no normal flow at boundaries? 2) If not, how close can such a representation come? 3) If such a representation can exist in some circumstances, what is the size of the smallest bolus velocity that fits the data? The author finds, in agreement with earlier authors, that the answer to the first question is no, although under certain conditions, which include a modification to the eddy flux divergence, a bolus representation becomes possible. One such condition is when the eddy flux divergence is required to balance the time-mean flux divergence. The smallest bolus flow is easily found by solving a thickness-weighted Poisson equation on each density level. This problem is solved for the North Pacific using time-mean data from an eddy-permitting model. The minimum bolus flow is found to be very small at depth but larger than is usually assumed near the surface. The magnitude of this minimum flow is of order one-tenth of the mean flow. Similar but larger results are found for a coarse-resolution model.

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Tropospheric Precursors of Anomalous Northern Hemisphere Stratospheric Polar Vortices

Journal of Climate ◽

10.1175/2010jcli3010.1 ◽

2010 ◽

Vol 23 (12) ◽

pp. 3282-3299 ◽

Cited By ~ 177

Author(s):

Chaim I. Garfinkel ◽

Dennis L. Hartmann ◽

Fabrizio Sassi

Keyword(s):

Eastern Europe ◽

Northern Hemisphere ◽

North Pacific ◽

Climate Model ◽

Polar Vortex ◽

Mean Flow ◽

Quasi Biennial Oscillation ◽

Stratospheric Polar Vortex ◽

The North ◽

The North Pacific

Abstract Regional extratropical tropospheric variability in the North Pacific and eastern Europe is well correlated with variability in the Northern Hemisphere wintertime stratospheric polar vortex in both the ECMWF reanalysis record and in the Whole Atmosphere Community Climate Model. To explain this correlation, the link between stratospheric vertical Eliassen–Palm flux variability and tropospheric variability is analyzed. Simple reasoning shows that variability in the North Pacific and eastern Europe can deepen or flatten the wintertime tropospheric stationary waves, and in particular its wavenumber-1 and -2 components, thus providing a physical explanation for the correlation between these regions and vortex weakening. These two pathways begin to weaken the upper stratospheric vortex nearly immediately, with a peak influence apparent after a lag of some 20 days. The influence then appears to propagate downward in time, as expected from wave–mean flow interaction theory. These patterns are influenced by ENSO and October Eurasian snow cover. Perturbations in the vortex induced by the two regions add linearly. These two patterns and the quasi-biennial oscillation (QBO) are linearly related to 40% of polar vortex variability during winter in the reanalysis record.

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The Nonlinear Transient Atmospheric Response to Tropical Forcing

Journal of Climate ◽

10.1175/2007jcli1383.1 ◽

2007 ◽

Vol 20 (22) ◽

pp. 5642-5665 ◽

Cited By ~ 32

Author(s):

Hai Lin ◽

Jacques Derome ◽

Gilbert Brunet

Keyword(s):

North Atlantic ◽

El Niño ◽

North Pacific ◽

El Nino ◽

La Niña ◽

Mean Flow ◽

The North ◽

La Nina ◽

The North Atlantic ◽

The North Pacific

Abstract Ensemble integrations using a primitive-equation dry atmospheric model were performed to investigate the atmospheric transient response to tropical thermal forcings that resemble El Niño and La Niña. The response develops in the North Pacific within 1 week after the integration. The signal in the North Atlantic and Europe is established by the end of the second week. Significant asymmetry was found between the responses in El Niño and La Niña that is similar to the observations, that is, one feature is that the 550-hPa positive height response in the North Pacific of the La Niña run is located about 30° west of the negative response of the El Niño run; another feature is that the responses in the North Atlantic and Europe for the La Niña and El Niño cases have similar patterns with the same polarity. The first feature is established within 2 weeks of the integration, while the second feature develops starting from the end of the second week. Several factors contribute to this nonlinearity of the response. In the Tropics, the shape of the Rossby wave response and the zonal extent of the Kelvin wave are not symmetric between El Niño and La Niña, which seems to be associated with the dependence of the wave property on the modified zonal mean flow. This is especially important in the equatorial region to the west of the forcing, which is likely responsible for the phase shift of the major extratropical response in the North Pacific. The transient eddy activity in the extratropics feeds back to the response and helps to maintain the nonlinearity.

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Energetics of Eddy–Mean Flow Interactions along the Western Boundary Currents in the North Pacific

Journal of Physical Oceanography ◽

10.1175/jpo-d-18-0201.1 ◽

2019 ◽

Vol 49 (3) ◽

pp. 789-810 ◽

Cited By ~ 2

Author(s):

Xiaomei Yan ◽

Dujuan Kang ◽

Enrique N. Curchitser ◽

Chongguang Pang

Keyword(s):

Kinetic Energy ◽

Western Boundary ◽

Mean Flow ◽

Boundary Currents ◽

The North ◽

Flow Interactions ◽

The Mean ◽

Eddy Field ◽

The Kuroshio ◽

The North Pacific

AbstractThe energetics of eddy–mean flow interactions along two western boundary currents of the North Pacific, the Kuroshio and Ryukyu Currents, are systematically investigated using 22 years of numerical data from the Ocean General Circulation Model for the Earth Simulator (OFES). For the time-mean and time-varying flow fields, all the energy components and conversions exhibit inhomogeneous spatial distributions. In the two currents, complex cross-stream and along-stream variations are seen in the eddy–mean flow energy conversions. East of Taiwan, the kinetic energy is mainly transferred from the mean flow to the eddy field through barotropic instability, whereas the baroclinic energy conversions form a meridional dipole structure caused by the topographic constraint. In the northern area, particularly, the eddy field drains 2.25 × 108 W of kinetic energy and releases 2.82 × 108 W of available potential energy when interacting with the mean flow, indicating that mesoscale eddies impinging on the Kuroshio decay with baroclinic inverse energy cascades. In the Ryukyu Current, inverse energy conversions from the eddy field to the mean flow also dominate the power transfer in the subsurface layer. The eddy field transfers 0.16 × 108 W of kinetic energy and 1.89 × 108 W of available potential energy to the mean flow, suggesting that meososcale eddies play an important role in maintaining the velocity and hydrographic structure of the current. In other areas, both barotropic and baroclinic instabilities contribute to the generation of eddy kinetic energy with the latter one providing more than 3 times as much power as the former one.

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