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1.
The relationship between the Kuroshio volume transport east of Taiwan (~24°N) and the impinging mesoscale eddies is investigated using 8-year reanalysis of a primitive equation ocean model that assimilates satellite altimetry and SST data. The mean and fluctuations of the model Kuroshio transport agree well with the available observations. Analysis of model dynamic heights and velocity fields reveals three dominant eddy modes. The first mode describes a large eddy of ~500 km in diameter, centered at ~22° N. The second mode describes a pair of the north–south counter-rotating eddies of?~?400 km in diameter each, centered at 23° and 20° N, respectively. The third mode describes a pair of the east–west counter-rotating eddies of?~?300 km in diameter each, centered at 21° N. The associated velocity fields indicate eddies extending to 600–700 m in depth with vertical shears concentrated in the upper 400 m. All three modes and the model Kuroshio transport have similar dominant timescales of 70–150 days and generally are coherent. The decreased Kuroshio volume transports typically are associated with the impinging cyclonic eddies and the increased transports with the anticyclonic eddies. Selected drifter trajectories are presented to illustrate the three eddy modes and their correspondence with the varying Kuroshio transports.  相似文献   

2.
Mixed-layer water oscillations in tropical Pacific for ENSO cycle   总被引：2，自引：0，他引：2  
The main modes of interannal variabilities of thermocline and sea surface wind stress in the tropical Pacific and their interactions are investigated,which show the following results.(1) The thermocline anomalies in the tropical Pacific have a zonal dipole pattern with 160°W as its axis and a meridional seesaw pattern with 6-8°N as its transverse axis.The meridional oscillation has a phase lag of about 90° to the zonal oscillation,both oscillations get together to form the El Ni?o/La Ni?a cycle,which be-haves as a mixed layer water oscillates anticlockwise within the tropical Pacific basin between equator and 12°N.(2) There are two main patterns of wind stress anomalies in the tropical Pacific,of which the first component caused by trade wind anomaly is characterized by the zonal wind stress anomalies and its corresponding divergences field in the equatorial Pacific,and the abnormal cross-equatorial flow wind stress and its corresponding divergence field,which has a sign opposite to that of the equatorial region,in the off-equator of the tropical North Pacific,and the second component represents the wind stress anomalies and corresponding divergences caused by the ITCZ anomaly.(3) The trade winds anomaly plays a decisive role in the strength and phase transition of the ENSO cycle,which results in the sea level tilting,provides an initial potential energy to the mixed layer water oscillation,and causes the opposite thermocline displacement between the west side and east side of the equator and also between the equator and 12°N of the North Pacific basin,therefore determines the amplitude and route for ENSO cycle.The ITCZ anomaly has some effects on the phase transition.(4) The thermal anomaly of the tropical western Pacific causes the wind stress anomaly and extends eastward along the equator accompanied with the mixed layer water oscillation in the equatorial Pacific,which causes the trade winds anomaly and produces the anomalous wind stress and the corresponding divergence in favor to conduce the oscillation,which in turn intensifies the oscillation.The coupled system of ocean-atmo-sphere interactions and the inertia gravity of the mixed layer water oscillation provide together a phase-switching mechanism and interannual memory for the ENSO cycle.In conclusion,the ENSO cycle essentially is an inertial oscillation of the mixed layer water induced by both the trade winds anomaly and the coupled ocean-atmosphere interaction in the tropical Pacific basin between the equator and 12°N.When the force produced by the coupled ocean-atmosphere interaction is larger than or equal to the resistance caused by the mixed layer water oscillation,the oscillation will be stronger or maintain as it is,while when the force is less than the resistance,the oscillation will be weaker,even break.  相似文献   

3.
Effects of mesoscale eddies on the marine ecosystem in the Kuroshio Extension (KE) region are investigated using an eddy-resolving coupled physical-biological model. The model captures the seasonal and intra-seasonal variability of chlorophyll distribution associated with the mesoscale eddies, front variability, Kuroshio meanders, and upwelling. The model also reproduces the observed interannual variability of sea surface height anomaly (SSHA) in the KE region along a zonal band of 32–34°N from 2002 to 2006. The distribution of high surface chlorophyll corresponds to low SSHA. Cyclonic eddies are found to detach from the KE jet near 150°E and 158°E and propagate westward. The westward propagating cyclonic eddies lift the nutrient-rich thermocline into the euphotic zone and maintain high levels of chlorophyll in summer. In the subsurface layer, the pattern in chlorophyll is influenced by both lateral and vertical advection. In winter, convection inside the eddy entrains high levels of nutrients into the mixed layer, increasing production, and resulting in high chlorophyll concentration throughout the surface mixed layer. There is significant interannual variability in both the cyclonic eddy activity and the surface phytoplankton bloom south of the KE jet, although whether or not there is a causal link is unclear.  相似文献   

4.
Meterological rocket soundings, launched between 1969–74 at six locations representative of low, middle, and high altitudes, are employed with the use of the statistical theory of diffusion, to determine the zonal and meridional component of eddy diffusivity between 30 and 55 km as a function of season, latitude, and altitude. A comparison is also made between annually-averaged eddy diffusivities above and below 30 km.It is shown that the zonal component of eddy diffusivity is approximately three to five times as large as the meridional component, in most cases. Both components of eddy diffusivity vary greatly with season, latitude, and altitude. Highest eddy diffusivities, found in the vicinity of the winter westerly jet, are approximately one order of magnitude higher than those present during the summer. Tropical eddy diffusivities, however, remain relatively small throughout the year. Annually, a minimum is indicated near 25 km between maximums located at the stratopause and tropopause.  相似文献   

5.
Summary The maintenance of the axisymmetric component of the flow in the atmosphere is investigated by means of a steady-state, quasi-geostrophic formulation of the meteorological equations. It is shown that the meridional variations in the time-averaged axisymmetric variables can be expressed as the sum of three contributions, one being due to the eddy heat transport, another to the eddy momentum transport, and a third to the convective-radiative equilibrium temperature which enters the problem through the specification of a Newtonian form of diabatic heating. The contributions by the large scale eddies are evaluated through the use of observed values for the eddy heat and momentum transports.The contributions from each of the three forcing mechanisms to the temperature and zonal wind fields are invstigated individually and found to be of about equal importance. The sum of the three contributions are also presented for the temperature, the zonal wind, the stream function associated with the mean meridional circulation and the corresponding vertical motion. Although the results fail to reproduce the main observed features of the lower stratosphere, they are found to be in good agreement with observations in the middle latitude troposphere. At any pressure level, for example, the computed mean zonal wind has a jet-like profile and the axis of the jet is found to slope to the south with height, as observed in the atmosphere.Based in part on a thesis submitted by the first author as partial fulfillment of the requirements for the Ph.D. degree at the University of Michigan. — Publication No. 194 from the Department of Meteorology and Oceanography, The University of Michigan.  相似文献   

6.
Mesoscale eddies exist almost everywhere in the ocean and play important roles in the ocean circulation of the world. These eddies may cause sound spread singular regions and bring great influences to the upwater ship and underwater aircraft. Due to the lack of hydrographic survey datasets, study of mesoscale eddies has been greatly restricted. Fortunately, satellite altimeter provided an effective way to study mesoscale eddies. An automatic detection algorithm is introduced to detect mesoscale eddies of specific intensity and spatial/temporal scale based on satellite sea surface height (SSH) data and the algorithm is applied in a strong eddy activity region: the South China Sea and the Northwest Pacific. The algorithm includes four steps. The first step is preprocessing of the SSH image, which includes elimination of error SSH data and interpolation. The second step is to detect suspected mesoscale eddies from preprocessed SSH images by dynamic threshold adjustment and morphological method, and the suspected mesoscale eddy detection includes two procedures: suspected mesoscale eddy core region detection and suspected mesoscale eddy brim extraction. The third step is to pick out mesoscale eddies satisfied with specified criteria from suspected mesoscale eddies. The criteria include three items, that is, intensity criterion, spatial scale, criterion and temporal scale criterion. The last step is algorithm performance analysis and verification. The algorithm has the capability of adaptive parameter adjustment, and can extract mesoscale eddies of interested intensity and spatial/temporal scale. The paper can provide a basis for analyzing space-time characteristics of mesoscale eddy in the South China Sea and the Northwest Pacific.  相似文献   

7.
Using Lagrangian methods, we analyze a 20-year-long estimate of water flux through the Kamchatka Strait in the northern North Pacific based on AVISO velocity field. It sheds new light on the flux pattern and its variability on annual and monthly time scales. Strong seasonality in surface outflow through the strait could be explained by temporal changes in the wind stress over the northern and western Bering Sea slopes. Interannual changes in a surface outflow through the Kamchatka Strait correlate significantly with the Near Strait inflow and Bering Strait outflow. Enhanced westward surface flow of the Alaskan Stream across the 174°E section in the northern North Pacific is accompanied by an increased inflow into the Bering Sea through the Near Strait. In summer, the surface flow pattern in the Kamchatka Strait is determined by the passage of anticyclonic and cyclonic mesoscale eddies. The wind stress over the Bering basin in winter–spring is responsible for eddy generation in the region.  相似文献   

8.
In the framework of the eddy dynamic model developed in two previous papers (Dubovikov, M.S., Dynamical model of mesoscale eddies, Geophys. Astophys. Fluid Dyn., 2003, 97, 311–358; Canuto, V.M. and Dubovikov, M.S., Modeling mesoscale eddies, Ocean Modelling, 2004, 8, 1–30 referred as I–II), we compute the contribution of unresolved mesoscale eddies to the large-scale dynamic equations of the ocean. In isopycnal coordinates, in addition to the bolus velocity discussed in I–II, the mesoscale contribution to the large scale momentum equation is derived. Its form is quite different from the traditional down-gradient parameterization. The model solutions in isopycnal coordinates are transformed to level coordinates to parameterize the eddy contributions to the corresponding large scale density and momentum equations. In the former, the contributions due to the eddy induced velocity and to the residual density flux across mean isopycnals (so called Σ-term) are derived, both contributions being shown to be of the same order. As for the large scale momentum equation, as well as in isopycnal coordinates, the eddy contribution has a form which is quite different from the down-gradient expression.  相似文献   

9.
Mesoscale atmospheric wind, temperature, and density fluctuations are described in terms of their power spectra. In the early 1980s, VanZandt[1] showed that the observed spectrum of the atmospheric fluctuations can result, as in the ocean, from an interna…  相似文献   

10.
The role of data assimilation procedures on representing ocean mesoscale variability is assessed by applying eddy statistics to a state-of-the-art global ocean reanalysis (C-GLORS), a free global ocean simulation (performed with the NEMO system) and an observation-based dataset (ARMOR3D) used as an independent benchmark. Numerical results are computed on a 1/4 ° horizontal grid (ORCA025) and share the same resolution with ARMOR3D dataset. This “eddy-permitting” resolution is sufficient to allow ocean eddies to form. Further to assessing the eddy statistics from three different datasets, a global three-dimensional eddy detection system is implemented in order to bypass the need of regional-dependent definition of thresholds, typical of commonly adopted eddy detection algorithms. It thus provides full three-dimensional eddy statistics segmenting vertical profiles from local rotational velocities. This criterion is crucial for discerning real eddies from transient surface noise that inevitably affects any two-dimensional algorithm. Data assimilation enhances and corrects mesoscale variability on a wide range of features that cannot be well reproduced otherwise. The free simulation fairly reproduces eddies emerging from western boundary currents and deep baroclinic instabilities, while underestimates shallower vortexes that populate the full basin. The ocean reanalysis recovers most of the missing turbulence, shown by satellite products , that is not generated by the model itself and consistently projects surface variability deep into the water column. The comparison with the statistically reconstructed vertical profiles from ARMOR3D show that ocean data assimilation is able to embed variability into the model dynamics, constraining eddies with in situ and altimetry observation and generating them consistently with local environment.  相似文献   

11.
The Bay of Bengal (BoB) is a distinct oceanic region for mesoscale oceanic eddies. The sea level anomaly from the Archiving, Validation, and Interpretation of Satellite Oceanographic (AVISO) help to identify an unusual anti-cyclonic eddy (ACE) over head BoB during May–July 2014. Two Indian moored buoys (BD08 and BD09) located over this region aided to study the subsurface thermohaline structures of the ACE. Compared to no-eddy environment, the temperature and salinity showed an increment of ~?3–4 °C and ~?1–2 PSU, respectively, during the ACE life period. The temperature and depth of the isothermal layer at genesis (peak) stages are increased to ~?30 °C (~?30.7 °C) and ~?20 m (30 m) when compared with no-eddy conditions (28.2 °C and 10 m). The thermocline depth is deepened to 75 m at the peak stage, while it is 50 m in no-eddy condition. A temperature difference of 3 °C between no-eddy and peak stages of ACE is observed up to 50 m. The ocean heat content (OHC) at BD08 (BD09) during genesis and peak stages has increased by ~?72% (~?50%) and ~?247% (~?181%), respectively, when compared with no-eddy conditions. Moreover, the MOHC also shows a similar increment of ~?125% (~?123%) and ~?258% (~?284%), respectively. A noticeable influence is seen in turbulent fluxes and lower atmospheric variables during eddy life. This study highlights the capability of moored buoys in understanding the subsurface thermohaline features of the eddies over northern BoB.  相似文献   

12.
 The circulation of the Southern Ocean is studied in the eddy-resolving model POP (Parallel Ocean Program) by an analysis of zonally integrated balances. The TEM formalism (Transformed Eulerian Mean) is extended to include topography and continental boundaries, thus deviations from a zonally integrated state involve transient and standing eddies. The meridional circulation is presented in terms of the Eulerian, eddy-induced, and residual streamfunctions. It is shown that the splitting of the meridional circulation into Ekman and geostrophic transports and the component induced by subgrid and Reynolds stresses is identical to a particular form of the zonally integrated balance of zonal momentum. In this balance, the eddy-induced streamfunctions represent the interfacial form stresses by transient and standing eddies and the residual streamfunction represents the acceleration of the zonal current by density fluxes in a zonally integrated frame. The latter acceleration term is directly related to the surface flux of density and interior fluxes due to the resolved and unresolved eddies. The eddy-induced circulation is extremely vigorous in POP. In the upper ocean a shallow circulation, reversed in comparison to the Deacon cell and mainly due to standing eddies, appears to the north of Drake Passage latitudes, and in the Drake Passage belt of latitudes a deep-reaching cell is induced by transient eddies. In the resulting residual circulation the Deacon cell is largely cancelled and the residual advection of the zonal mean potential density is balanced by diapycnal eddy and subgrid fluxes which are strong in the upper few hundred meters but small in the ocean interior. The balance of zonal momentum is consistent with other eddy-resolving models; a new aspect is the clear identification of density effects in the zonally integrated balance. We show that the wind stress and the stress induced by the residual circulation drive the eastward current, whereas both eddy species result in a braking. Finally, we extend the Johnson–Bryden model of zonal transport to incorporate all relevant terms from the zonal momentum balance. It is shown that wind stress and induction by the residual circulation carry an eastward transport while bottom form stress and the stress induced by standing eddies yield westward components of transport. Received: 26 June 2001 / Accepted: 2 November 2001  相似文献   

13.
The North Pacific Subtropical Counter Current (STCC) is a weak zonal current comprising of a weak eastward flow near the surface (with speeds of less than 0.1 m/s and a thickness of approximately 50–100 m) and westward flow (the North Equatorial Current) beneath. Previous studies (e.g., Qiu J Phys Oceanogr 29: 2471–2486, 1999) have shown that the STCC is baroclinically unstable. Therefore, despite its weak mean speeds, nonlinear STCC eddies with diameters ~300 km or larger and rotational speeds exceeding the eddy propagation speeds develop (Samelson J Phys Oceanogr 27: 2645–2662, 1997; Chelton et al. Prog Oceanogr 91: 167–216, 2011). In this study, the authors present numerical experiments to describe and explain the instability and eddy-generation processes of the STCC and the seasonal variation. Emphasis is on finite-amplitude eddies which are analyzed based on the parameter of Okubo (Deep-Sea Res 17: 445–454, 1970) and Weiss (Physica D 48: 273–294, 1991). The temperature and salinity distribution in March and April offer the favorable condition for eddies to grow, while September and October are unfavorable seasons for the generation of eddies. STCC is maintained not only by subsurface front but also by the sea surface temperature (SST) front. The seasonal variation of the vertical shear is dominated by the seasonal surface STCC velocity. The SST front enhances the instability and lead to the faster growth of STCC eddies in winter and spring. The near-surface processes are therefore crucial for the STCC system.  相似文献   

14.
A comparison between the modeled NmF2 and hmF2 and NmF2 and hmF2, which were observed by the Kokubunji, Okinawa, Manila, Vanimo, and Darwin ionospheric sounders and by the middle and upper (MU) atmosphere radar, have been used to study the time-dependent response of the low-latitude ionosphere to geomagnetic forcing during a time series of geomagnetic storms from 22 to 26 April 1990. The reasonable agreement between the model results and data requires the modified equatorial meridional E×B plasma drift, the modified HWM90 wind, and the modified NRLMSISE-00 neutral densities. We found that changes in a flux of plasma into the nighttime equatorial F2-region from higher L-shells to lower L-shells caused by the meridional component of the E×B plasma drift lead to enhancements in NmF2 close to the geomagnetic equator. The equatorward wind-induced plasma drift along magnetic field lines, which cross the Earth equatorward of about 20° geomagnetic latitude in the northern hemisphere and about −19° geomagnetic latitude in the southern hemisphere, contributes to the maintenance of the F2-layer close to the geomagnetic equator. The nighttime weakening of the equatorial zonal electric field (in comparison with that produced by the empirical model of Fejer and Scherliess [Fejer, B.G., Scherliess, L., 1997. Empirical models of storm time equatorial zonal electric fields. J. Geophys. Res. 102, 24047–24056] or Scherliess and Fejer [Scherliess, L., Fejer, B.G., 1999. Radar and satellite global equatorial F region vertical drift model. J. Geophys. Res. 104, 6829–6842) in combination with corrected equatorward nighttime wind-induced plasma drift along magnetic field lines in the both geomagnetic hemispheres are found to be the physical mechanism of the nighttime NmF2 enhancement formation close to the geomagnetic equator over Manila during 22–26 April 1990. The model crest-to-trough ratios of the equatorial anomaly are used to study the relative role of the main mechanisms of the equatorial anomaly suppression for the 22–26 April 1990 geomagnetic storms. During the most part of the studied time period, a total contribution from geomagnetic storm disturbances in the neutral temperature and densities to the equatorial anomaly changes is less than that from meridional neutral winds and variations in the E×B plasma drift. It is shown that the latitudinal positions of the crests are determined by the E×B drift velocity and the neutral wind velocity.  相似文献   

15.
Polar and high latitude substorms and solar wind conditions   总被引：1，自引：0，他引：1  
All substorm disturbances observed in polar latitudes can be divided into two types: polar, which are observable at geomagnetic latitudes higher than 70° in the absence of substorms below 70°, and high latitude substorms, which travel from auroral (<70°) to polar (>70°) geomagnetic latitudes. The aim of this study is to compare conditions in the IMF and solar wind, under which these two types of substorms are observable on the basis of data from meridional chain of magnetometers IMAGE and OMNI database for 1995, 2000, and 2006–2011. In total, 105 polar and 55 high latitude substorms were studied. It is shown that polar substorms are observable at a low velocity of solar wind after propagation of a high-speed recurrent stream during the late recovery phase of a magnetic storm. High latitude substorms, in contrast, are observable with a high velocity of solar wind, increased values of the Bz component of the IMF, the Ey component of the electric field, and solar wind temperature and pressure, when a high-speed recurrent stream passes by the Earth.  相似文献   

16.
Decadal variability and trends of the isothermal layer depth (ILD), mixed layer depth (MLD), and barrier layer thickness (BLT) were analyzed for the tropical Pacific during 1979–2015. The decadal variability of ILD, MLD, and BLT shows a close connection with the Pacific Decadal Oscillation (PDO). At PDO positive phase, the eastward shift of precipitation and weakened trade winds result in thinner BLT in western Pacific and thicker BLT in central and eastern Pacific. The situation is reversed at PDO negative phase. The differences in BLT can be up to 9–15 m. The spatial distributions of decadal trends of ILD and MLD are complex, but a thickening of BLT in the western tropical Pacific is clearly present. The raw trends of ILD, MLD, and BLT averaged in the tropical Pacific (30° N–30° S, 120° E–75° W) from 1979 to 2015 are 1.62, 1.20, and 0.51 m per decade, respectively. PDO can explain about 25% of the increasing trends of BLT, while El Niño-Southern Oscillation (ENSO) only explains about 1.7%. Global warming and/or variability at longer time scales is responsible for the remaining increasing trends. The BLT change is related to the warming and freshening of the western Pacific warm pool in recent decades. The ocean-atmosphere interactions about trade winds, wind-driven ocean circulation, temperature, and precipitation/evaporation are discussed.  相似文献   

17.
We present the distribution of226Ra in eight vertical profiles from the eastern Pacific. The profiles are located along a meridional trend near 125°W, from 43°S to 29°N. Surface226Ra concentrations are about 7 dpm/100 kg, except for the two stations south of 30°S where the higher values are due to the Antarctic influence. Deep waters show a distinctive south-to-north increase in the226Ra content, from about 26 to 41 dpm/100 kg near the bottom. Unlike in the Atlantic and Antarctic Oceans, the effect of226Ra injection from bottom sediments is clearly discernible in the area. The presence of this primary226Ra can be traced up to at least 1–1.5 km above the ocean floor, making this part of the sea bed among the strongest source regions for the oceanic226Ra. Numerical solutions of a two-dimensional vertical advection-diffusion model applied to the deep (1.2–4 km)226Ra data give the following set of best fits: upwelling velocity(Vz) = 3.5m/yr, vertical eddy diffusivity(Kz) = 0.6cm2/s, horizontal (north-south) eddy diffusivity(Ky) = 1 × 107cm2/s, and water-column regeneration flux of226Ra(J) = 3.3 × 10?5dpmkg?1yr?1 as an upper limit. These parametric values are in general agreement with one-dimensional (vertical) model fits for the Ra-Ba system. However, consideration of226Ra balance leads us to suspect the appropriateness of describing the vertical exchange processes in the eastern Pacific with constantVz and Kz. If future modeling is attempted, it may be preferable to treat the area as a diffusion-dominant mixing regime with depth-dependent diffusivities.  相似文献   

18.
本文引入3阶主张量分析方法对1993～2008年赤道太平洋地区卫星测高数据进行解析,前两个主张量可有效表征海面变化的经、纬向耦合特征,重构与对比了该时段内6次ENSO事件海面变化的经、纬向演化的空间构型与耦合作用过程.结果表明:海面的经向变化可表征ENSO强度变化,纬向变化表现为受ENSO影响的年周期波动;经、纬向张量...  相似文献   

19.
The zonally averaged UK Meteorological Office (UKMO) zonal mean temperature and zonal winds for the latitudes 8.75°N and 60°N are used to investigate the low-latitude dynamical response to the high latitude sudden stratospheric warming (SSW) events that occurred during winter of the years 1998–1999, 2003–2004 and 2005–2006. The UKMO zonal mean zonal winds at 60°N show a short-term reversal to westward winds in the entire upper stratosphere and lower mesosphere and the low-latitude winds (8.75°N) show enhanced eastward flow in the upper stratosphere and strong westward flow in the lower mesosphere during the major SSW events at high latitudes. The mesosphere and lower thermosphere (MLT) zonal winds acquired by medium frequency (MF) radar at Tirunelveli (8.7°N, 77.8°E) show a change of wind direction from eastward to westward several days before the onset of SSW events and these winds decelerate and weak positive (eastward) winds prevail during the SSW events. The time variation of zonal winds over Tirunelveli is nearly similar to the one reported from high latitudes, except that the latter shows intense eastward winds during the SSW events. Besides, the comparison of daily mean meridional winds over Tirunelveli with those over Collm (52°N, 15°E) show that large equatorial winds are observed over Tirunelveli during the 2005–2006 event and over Collm during the 1998–1999 events. The variable response of MLT dynamics to different SSW events may be explained by the variability of gravity waves.  相似文献   

20.
This study investigates transient eddy activity anomalies in the mid-latitude upper troposphere associated with intensity variability of the wintertime North Pacific subtropical front. Our results show that the meridional gradient of air temperature and baroclinic instability in the mid-latitude atmosphere become stronger as the subtropical front intensifies, and the mid-latitude westerly jet accelerates with barotropic structure. We further divide the mid-latitude atmospheric eddy activities into high-(2–7 days) and low-frequency(10–90 days) eddy activities according to their life periods. We find that, when the oceanic subtropical front intensifies, the high-frequency atmospheric eddy activity in the mid-latitudes strengthens while the low-frequency eddy activity weakens. The stronger high-frequency eddy activity tends to moderate the air temperature gradient and baroclinicity in the mid-latitudes. High-frequency eddy anomalies accelerate the westerly jet on the northern side and downstream of the westerly jet, and enhance the jet with equivalent barotropic structure. In contrast, the weaker low-frequency eddy activity has a negative contribution to zonal wind speed tendency and attenuates the zonal homogenization of the jet. The anomalous thermodynamic forcing of the low-frequency eddy activity helps maintain the meridional gradient of air temperature in the mid-troposphere.  相似文献   

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