Atmospheric circulation associated with extreme generalized frosts persistence in central-southern South America

Generalized frosts (GF) in central-southern South America have a strong impact due to their spatial extension, and they are especially important when they become persistent. This paper aims at identifying the atmospheric circulation features that determine the extreme GF persistence, i.e. very persistent and without persistence, and the differences between them, during the 1961–1990 winters. Since the GF without persistence group outnumbers the other one, two subgroups are composed with events selected from winters with maximum and minimum frequency of GF occurrence, respectively. Additionally, the individual event of July 1988 within the very persistent GF group is analyzed due to its exceptional persistence. GF persistence is mainly conditioned by two large-scale dynamic factors. One is the Rossby wave train propagation across the Pacific Ocean, and the other one is the location with respect to the continent and the magnitude of the confluence in the jet entrance region in subtropical latitudes. A predominantly meridional Rossby wave train propagation with a confluence region to the west of the continent prior to the event favors GF with intermediate (null) persistence depending on the greater (lesser) jet acceleration. This is conditioned by the magnitude of the confluence, which, in turn, depends on the disposition of the wave train propagation pattern. Instead, an essentially zonal propagation with a confluence region to the east of the continent favors the GF persistence for several days, yet if there is no confluence the event does not persist. The greatest persistence of an event combines the confluence/diffluence of the jet entrance/exit region, which depends on the disposition with respect to the continent of the zonally propagating Rossby wave trains.     

冬半年南支西风中Rossby波传播及其与南支槽形成的关系

利用1948—2005年的10月—次年5月NCEP/NCAR再分析资料对西风急流中Rossby波的传播及其与冬半年副热带南支西风槽形成的关系进行研究, 结果表明:冬半年亚洲副热带西风急流下方存在3个南支波动, 分别位于阿拉伯海、孟加拉湾和我国华南地区, 其中孟加拉湾南支槽变率最小, 是半永久性低压槽; 从北非、阿拉伯海到孟加拉湾的“-+-”遥相关波列表明南支槽与北非槽呈正相关, 与阿拉伯海槽呈反相关。在西风波动从北非东传到孟加拉湾的过程中, 往往会在阿拉伯海有所停顿, 这种由西向东的传播过程约20 d一次, 具有明显的低频振荡特征; 源自北非的定常Rossby波能量沿着急流波导传播到孟加拉湾, 可能是南支槽明显增幅的一个主要机制; 另外, 从青藏高原东西两侧南下的冷空气活动也是南支槽加深发展的一个重要因素。     

Rossby wave propagation in a barotropic atmosphere

The ideas of ray tracing from geometrical optics and wave propagation in a slowly varying medium are applied to Rossby waves propagating in a barotropic atmosphere.The propagation of low-frequency Rossby waves in a zonally symmetric basic state is compared with that for stationary waves presented by Hoskins and Karoly (1981). These ideas are then used to study the propagation of Rossby waves in a basic state with zonally varying middle latitude or low latitude jets. Conditions which allow cross-equatorial wave propagation are presented. For a zonally varying middle latitude jet, there is weak wave convergence in regions of decreasing jet speed, However, this is not sufficient to explain the enhanced wave amplitude found in numerical-model experiments using a zonally varying basic state.     

Teleconnections associated with Northern Hemisphere summer monsoon intraseasonal oscillation

The boreal summer intraseasonal oscillation (BSISO) has strong convective activity centers in Indian (I), Western North Pacific (WNP), and North American (NA) summer monsoon (SM) regions. The present study attempts to reveal BSISO teleconnection patterns associated with these dominant intraseasonal variability centers. During the active phase of ISM, a zonally elongated band of enhanced convection extends from India via the Bay of Bengal and Philippine Sea to tropical central Pacific with suppressed convection over the eastern Pacific near Mexico. The corresponding extratropical circulation anomalies occur along the waveguides generated by the North African-Asian jet and North Atlantic-North European jet. When the tropical convection strengthens over the WNPSM sector, a distinct great circle-like Rossby wave train emanates from the WNP to the western coast of United States (US) with an eastward shift of enhanced meridional circulation. In the active phase of NASM, large anticyclonic anomalies anchor over the western coast of US and eastern Canada and the global teleconnection pattern is similar to that during a break phase of the ISM. Examination of the evolution of the BSISO teleconnection reveals quasi-stationary patterns with preferred centers of teleconnection located at Europe, Russia, central Asia, East Asia, western US, and eastern US and Canada, respectively. Most centers are embedded in the waveguide along the westerly jet stream, but the centers at Europe and Russia occur to the north of the jet-induced waveguide. Eastward propagation of the ISO teleconnection is evident over the Pacific-North America sector. The rainfall anomalies over the elongated band near the monsoon domain over the Indo-western Pacific sector have an opposite tendency with that over the central and southern China, Mexico and southern US, providing a source of intraseasonal predictability to extratropical regions. The BSISO teleconnection along and to the north of the subtropical jet provides a good indication of the surface sir temperature anomalies in the NH extratropics.     

Mean atmospheric circulation leading to generalized frosts in central southern South America

Summary The purpose of this study is to analyze the atmospheric circulation patterns associated to extreme frost episodes which affect the extensive region known as the Wet Pampas in the center-south of South America during the 1961–1990 period. The years with an extreme frequency of generalized frost are identified by selecting the cases beyond one standard deviation above and below the average of the corresponding period. Two groups were formed: one for the years with events above the average (+s) and the other with those below the average (−s). The years of generalized frosts were separated into the periods from May to September, June to August and individual months.From the comparison between the composite of the two groups (+s and −s), it was possible to determine that the anomaly fields are opposite in almost all the periods studied. In the seasonal composites of generalized frosts below average, the circulation field presented a barotropic structure with an anomalous cyclonic persistence over the south-west of South America and a weak subtropical jet over the continent. On the other hand, the structure and evolution of the systems that produce generalized frosts in extremely cold winters were analyzed by calculating the composites of the daily fields from these episodes. The results showed that the incursion of cold air in the lower levels begins with a migratory anticyclone moving from the south-east Pacific to the south of the continent and a deepening of an anomalous low pressure center over the south-east of the Argentine Atlantic coast. Both systems cause cold, dry air advection from the south, with a reduction in temperature over the center and eastern parts of the country. The development of a mid-latitude wave was observed at higher levels with a large-amplitude trough over South America extending to tropical latitudes and a progressive amplification from another trough located upstream in the Pacific Ocean.The analysis of the different periods confirmed the relationship between the intensification of the subtropical jet in South America and the higher frequency of generalized frosts. The intensification of the jet may be related to an amplification of the pressure gradient in the region due to the increase in Rossby wave activity.     

Influence of the intraseasonal variability on heat waves in subtropical South America

The influence of the intraseasonal variability on heat wave development over subtropical South America during austral summer is analyzed. The role of the South Atlantic Convergence Zone (SACZ) on this development is documented. Results show that intraseasonal variability can explain on average at least 32% of summer temperature variance. Moreover, 73% of the heat waves in subtropical South America develop in association with an active SACZ. The analysis of pentad maps shows that warm conditions in the region under study develop in association with the strengthening of an anticyclonic anomaly, which is discernible over the subtropical regions at least 15?days before temperature peak occurrence. That circulation anomaly is embedded in a large-scale Rossby wave train extending along the South Pacific Ocean that is linked to convection anomalies at the equatorial western and central Pacific Ocean. In addition, the development of the anticyclonic circulation over subtropical South America appears to be strengthened by the subsidence conditions promoted by the active SACZ, which result in temperature rise in the subtropical region under relatively dry conditions. On the other hand, during the last 2?days of evolution, SACZ activity weakens and the progressive temperature rise in the region is dominated by warmer and moister air being anomalously advected from the north. Results confirm the important role that SACZ activity on intraseasonal time scales has in inducing persistent circulation anomalies at the subtropical regions that can result in the development of persistent heat waves, and very extreme daily temperature.     

Advances in research on atmospheric energy propagation and the interactions between different latitudes

Early theoretical analyses indicated that the tropics and extratropics are relatively independent due to the existence of critical latitudes. However, considerable observational evidence has shown that a clear dynamical link exists between the tropics and midlatitudes. To better understand such atmospheric teleconnection, several theories of wave energy propagation are reviewed in this paper: (1) great circle theory, which reveals the characteristics of Rossby waves propagating in the spherical atmosphere; (2) westerly duct theory, which suggests a “corridor” through which the midlatitude disturbances in one hemisphere can propagate into the other hemisphere; (3) energy accumulation-wave emanation theory, which proposes processes through which tropical disturbances can affect the atmospheric motion in higher latitudes; (4) equatorial wave expansion theory, which further explains the physical mechanisms involved in the interaction between the tropics and extratropics; and (5) meridional basic flow theory, which argues that stationary waves can propagate across the tropical easterlies under certain conditions. In addition, the progress made in diagnosing wave-flow interaction, particularly for Rossby waves, inertial-gravity waves, and Kelvin waves, is also reviewed. The meridional propagation of atmospheric energy exhibits significant annual and interannual variations, closely related to ENSO and variation in the westerly jets and tropical upper-tropospheric troughs, amongst others.     

Stratospheric Polar Vortex Splitting in December 2009

Abstract

The 2009–10 Arctic stratospheric winter, in comparison with other recent winters, is mainly characterized by a major Sudden Stratospheric Warming (SSW) in late January associated with planetary wavenumber 1. This event led to a large increase in the temperature of the polar stratosphere and to the reversal of the zonal wind. Unlike other major SSW events in recent winters, after the major SSW in January 2010 the westerlies and polar vortex did not recover to their pre-SSW strength until the springtime transition. As a result, the depletion of the ozone layer inside the polar vortex over the entire winter was relatively small over the past 20 years. The other distinguishing feature of the 2010 winter was the splitting of the stratospheric polar vortex into two lobes in December. The vortex splitting was accompanied by an increase in the temperature of the polar stratosphere and a weakening of the westerlies but with no reversal. The splitting occurred when, in addition to the high-pressure system over northeastern Eurasia and the northern Pacific Ocean, the tropospheric anticyclone over Europe amplified and extended to the lower stratosphere. Analysis of wave activity in the extratropical troposphere revealed that two Rossby wave trains propagated eastward to the North Atlantic several days prior to the vortex splitting. The first wave train propagated from the subtropics and mid-latitudes of the eastern Pacific Ocean over North America and the second one propagated from the northern Pacific Ocean. These wave trains contributed to an intensification of the tropospheric anticyclone over Europe and to the splitting of the stratospheric polar vortex.     

The Impact of Mid- and High-Latitude Rossby Wave Activities on the Medium-Range Evolution of the EAP Pattern During the Pre-Rainy Period of South China

Based on the NCEP DOE AMIP II daily reanalysis data (1979–2005), the evolution of the East Asia/Pacific (EAP) teleconnection pattern during the pre-rainy period of South China is studied on the medium-range time scale. It is found that positive and negative EAP patterns share a similar generation process. In the middle and upper troposphere, Rossby wave packets emanating from the northeast Atlantic or Europe prop-agate toward East Asia along the Eurasian continent waveguide and finally give rise to the three anomaly centers of the EAP pattern over East Asia. Among the three anomaly centers, the western Pacific subtropical center appears the latest. Rossby wave packets propagate from the high latitude anomaly center toward the mid-latitude and the subtropical ones. The enhancement and maintenance of the subtropical anomaly center is closely associated with the subtropical jet waveguide and the incoming Rossby wave packets from the upstream. In the lower troposphere, Rossby wave packets emanate from the subtropical Asia toward East Asia. Positive and negative EAP patterns could not be regarded as "mirrors" to each other with simply reversed phase. For the positive pattern, the positive height anomaly center around the Scandinavia Peninsula keeps its strength and position during the mature period, and the Rossby wave packets thus propagate persistently toward East Asia, facilitating a longer mature time of the positive pattern. As for the formation of the negative EAP pattern, however, the incoming Rossby wave energy from the upstream contributes to both the enhancement and southeastward movement of the negative anomaly belt from the Yenisei River to the Bering Strait and the positive anomaly center around Mongolia. At the peak time, the two anomlous circulations are evolved into the Northeast Asia and the mid-latitude anomaly centers of the negative pat- tern, respectively. The energy dispersion of Rossby wave packets is relatively fast due to the predominant zonal circulation in the extratropics, causing a shorter mature period of the negative pattern. During the pre-rainy period of South China, the prevalence of the EAP pattern significantly affects the rainfall over the region south of the Yangtze River. The positive (negative) EAP pattern tends to cause positive (negative) precipitation anomalies in that region. This is different from the earlier research findings based on monthly mean data.     

东亚高空急流协同变化研究新进展

东亚高空急流是东亚大气环流系统的重要组成部分,对东亚地区的天气和气候具有重要影响。以往对东亚高空急流的研究多关注副热带急流及其对天气气候的影响,近年来,学者们在明确区分东亚副热带急流和极锋急流的基础上,从东亚副热带急流和极锋急流协同变化的视角,对东亚高空急流的变化规律和机理及其对我国气候异常的影响,开展了一系列研究,揭示出副热带急流和极锋急流强度的反位相协同变化是以副热带急流强（弱）伴随着极锋急流弱（强）为其主要配置形式和模态,并对应着特定的大气环流形势以及相应的气温和降水异常分布,与冬季冷空气活动、梅雨期降水、极端事件、冬季风等具有密切关系。本文聚焦东亚高空急流协同变化方面的最新研究成果,从东亚高空急流协同变化规律、高空急流协同变化的热力和动力学影响机理、高空急流协同变化气候效应、高空急流与中高纬低频遥相关型的联系等方面进行较为全面的总结,以加深东亚高空急流活动基本特征和变化规律的认识。     

Role of the subtropical westerly jet waveguide in a southern China heavy rainstorm in December 2013

An extreme rainstorm hit southern China during 13–17 December 2013, with a record-breaking daily rainfall rate, large spatial extent, and unusually long persistence. We examined what induced this heavy rainfall process, based on observed rainfall data and NCEP–NCAR reanalysis data through composite and diagnostic methods. The results showed that a Rossby waveguide within the subtropical westerly jet caused the event. The Rossby wave originated from strong cold air intrusion into the subtropical westerly jet over the eastern Mediterranean. With the enhancement and northward shift of the Middle East westerly jet, the Rossby wave propagated slowly eastward and deepened the India–Burma trough, which transported a large amount of moisture from the Bay of Bengal and South China Sea to southern China. Strong divergence in the upper troposphere, caused by the enhancement of the East Asian westerly jet, also favored the heavy rainfall process over Southeast China. In addition, the Rossby wave was associated with a negative-to-positive phase shift and enhancement of the North Atlantic Oscillation, but convergence in the eastern Mediterranean played the key role in the eastward propagation of the Rossby wave within the subtropical westerly jet.     

Impacts of Sea Surface Temperature on the Interannual Variability of Winter Haze Days in Guangdong Province

The impact of sea surface temperature (SST) on winter haze in Guangdong province (WHDGD) was analyzed on the interannual scale. It was pointed out that the northern Indian Ocean and the northwest Pacific SST play a leading role in the variation of WHDGD. Cold (warm) SST anomalies over the northern Indian Ocean and the Northwest Pacific stimulate the eastward propagation of cold (warm) Kelvin waves through the Gill forced response, causing Ekman convergence (divergence) in the western Pacific, inducing abnormal cyclonic (anticyclonic) circulation. It excites the positive (negative) Western Pacific teleconnection pattern (WP), which results in the temperature and the precipitation decrease (increase) in Guangdong and forms the meteorological variables conditions that are conducive (not conducive) to the formation of haze. ENSO has an asymmetric influence on WHDGD. In El Ni?o (La Ni?a) winters, there are strong (weak) coordinated variations between the northern Indian Ocean, the northwest Pacific, and the eastern Pacific, which stimulate the negative (positive) phase of WP teleconnection. In El Ni?o winters, the enhanced moisture is attributed to the joint effects of the horizontal advection from the surrounding ocean, vertical advection from the moisture convergence, and the increased atmospheric apparent moisture sink (Q2) from soil evaporation. The weakening of the atmospheric apparent heat source (Q1) in the upper layer is not conducive to the formation of inversion stratification. In contrast, in La Ni?a winters, the reduced moisture is attributed to the reduced upward water vapor transport and Q2 loss. Due to the Q1 increase in the upper layer, the temperature inversion forms and suppresses the diffusion of haze.     

THE INTERLINKAGE OF LARGE-SCALE SYSTEMS BETWEEN HIGH AND LOW LATITUDES RELATED TO PROPAGATION OF THE PLANETARY WAVE AROUND EAST ASIA IN SUMMER

This study has reviewed recent studies about impact of the Rossby wave propagation on theweather of East Asia.Then,we have focused on the northward progression of the subtropical highaffected by the propagations and their interaction.The following results are obtained:(1)The twokinds of meridional Rossby wave propagations often affect the East Asian weather in summer,which propagate in two conflicting directions(northwest-southeastward and southwest-northeast-ward).This is considered as the interaction of the large-scale systems between high and lowlatitudes with wave propagation,especially to the circulation systems around East Asia.(2)Thenorthward progression of the subtropical high is largely restrained,aecompanied by the southwardshift of the convective activity in middle latitudes,under the strong and persistent Rossby wavepropagation from high to low latitudes.And the opposite role could be found in the Rossby wavepropagation from low to high latitudes.     

South Indian Ocean Rossby Waves

ABSTRACT

South Indian Ocean Rossby waves (SIO-RW) are identified in the Global Ocean Data Assimilation System (GODAS) 1.5–7?yr filtered sea surface height (SSH) time series. There is a persistent three-year oscillation in the 5°–15°S latitude band from 55° to 85°E. Field correlations show little coupling at 90°E, but as the SIO-RW undulates westward at approximately 0.19?m?s^?1 across the mid-basin, a northwest–southeast axis of warm sea surface temperatures (SSTs) and deep convection forms. Many teleconnections in earlier work are confirmed: interannual pulses of zonal wind in the eastern basin trigger the SIO-RW via anticyclonic wind stress curl. New insights derive from an understanding of links with the upper troposphere. As the SIO-RWs move westward with the onset of an El Niño in the Pacific, increased convection over the north Indian Ocean corresponds to reduced evaporation and SST warming. Mid-tropospheric heating T′?>?2°C over the northwest Indian Ocean accelerates the southern sub-tropical jet to greater than 10?m?s^?1 over the southeast Indian Ocean, reinforcing the anticyclonic vorticity. The downstream acceleration of the jet generates upper-level divergence and moist convection over the western basin, anchoring an atmospheric Rossby wave in a northwest–southeast alignment underpinned by differential propagation of the SIO-RW. As the ocean Rossby wave reaches Africa, the coupling fades and transitions. What distinguishes Indian Ocean from Pacific Ocean Rossby waves are their southern latitude and higher frequency. The tropical mid-tropospheric heating that accelerates the southern sub-tropical jet shifts westward in tandem with the SIO-RW.     

THE INTERLINKAGE OF LARGE-SCALE SYSTEMS BETWEEN HIGH AND LOW LATITUDES RELATED TO PROPAGATION OF THE PLANETARY WAVE AROUND EAST ASIA IN SUMMER*

This study has reviewed recent studies about impact of the Rossby wave propagation on the weather of East Asia.Then,we have focused on the northward progression of the subtropical high affected by the propagations and their interaction.The following results are obtained:(1) The two kinds of meridional Rossby wave propagations often affect the East Asian weather in summer,which propagate in two conflicting directions(northwest-southeastward and southwest-northeast-ward).This is considered as the interaction of the large-scale systems between high and low latitudes with wave propagation,especially to the circulation systems around East Asia.(2) The northward progression of the subtropical high is largely restrained,aecompanied by the southward shift of the convective activity in middle latitudes,under the strong and persistent Rossby wave propagation from high to low latitudes.And the opposite role could be found in the Rossby wave propagation from low to high latitudes.     

High and low latitude types of the downstream influences of the North Atlantic Oscillation

Using reanalysis data, we find that the downstream-propagating quasi-stationary Rossby wave train associated with the North Atlantic Oscillation (NAO) generally propagates along a high (low)-latitude pathway during warm (cold) El Niño-Southern Oscillation (ENSO) boreal winters. Consistent with the different propagation directions of the NAO-related downstream wave train, during the warm (cold) ENSO winters, the NAO is associated with significant 300 hPa geopotential height anomalies over eastern Siberia (the Arabian Sea, the east coast of Asia at around 40°N, and the North Pacific), and the near-surface air temperature perturbations associated with the NAO over the high latitudes of Asia are relatively strong (weak). Based on these differences, we argue that the NAO has two distinct types of downstream influence: a high-latitude type and a low-latitude type. Furthermore, we argue that the two types of NAO’s downstream influence are modulated by the intensity of the subtropical potential vorticity (PV) meridional gradient over Africa. When this gradient is weak (strong), as in the warm (cold) ENSO winters, the NAO’s downstream influence tends to be of the high (low)-latitude type. These results are further supported by analysis of intraseasonal NAO events. We separate NAO events into two categories in terms of the intensity of the subtropical PV gradient over Africa. Composites of the NAO events accompanied by a weak (strong) subtropical PV gradient show that the NAO-related downstream wave train tends to propagate along a high (low)-latitude pathway.     

Variability patterns of Rossby wave source

Rossby waves (RW) propagation due to a local forcing is one of the mechanisms responsible for wave trains configurations known as teleconnections. The term teleconnection refers to anomalies patterns that are correlated in several regions of the world, causing large-scale changes in atmospheric waves patterns and temperature and precipitation regimes. The aim of teleconnections studies is to provide a better understanding of atmospheric variability and their mechanisms of action in order to identify patterns that can be tracked and predicted. The purpose of this study was to characterize seasonal and spatial variability of atmospheric RW sources. The RW source at 200?hPa was calculated for the four seasons with reanalysis data of zonal and meridional wind. In the Northern Hemisphere (NH), the RW sources were located on East Asia, North America, North Atlantic and Pacific. The main RW sources in the Southern Hemisphere (SH) were located over Intertropical, South Pacific, South Atlantic and South Indian Convergence Zones. Extratropical regions were also identified, mainly to the south of Australia. The vortex stretching term (S1) and the advection of absolute vorticity by the divergent wind (S2) were analyzed to discuss the physical mechanisms for RW generation. In the NH, the source at East Asia in DJF changed to a sink in JJA, related to the dominance of S1 term in DJF and S2 term in JJA. In the SH, the vortex stretching term had the dominant contribution for RW source located to the south of Australia. The main forcing for RW sources at east of Australia was the advection of absolute vorticity by divergent flow. Over South America, both terms contributed to the source in DJF. The main modes of RW source variability were discussed by using empirical orthogonal functions analysis. RW variability was characterized by wave trains configurations in both hemispheres over regions of jet streams and storm tracks, associated with favorable and unfavorable areas for RW generation.     

东亚2005年和2006年冬季风异常及其与准定常行星波活动的关系

利用NCEP/NCAR再分析资料系统地分析了2005年与2006年冬季欧亚大陆的气温和东亚冬季风的差别及其与北半球准定常波活动的关系。分析结果表明:2005年冬季欧亚大陆中、高纬度地区气温偏低,东亚冬季风偏强;而2006年冬季欧亚大陆中、高纬度地区气温偏高,出现暖冬,东亚冬季风偏弱。分析结果还表明,这两年冬季东亚冬季风的差别不仅是由于西伯利亚高压和阿留申低压的变异所造成,而且是由于北极涛动(北半球环状模)的变化所造成。并且,作者还从这两年冬季北半球准定常行星波活动的差异,在动力理论上进一步讨论了这两年冬季北半球气候和东亚冬季风差异的机理。结果表明:2005年冬季北半球行星波活动为低指数,准定常行星波在高纬度往平流层传播加强,而往低纬度对流层上层传播减弱,造成了行星波E-P通量在高纬度地区对流层中、上层辐合加强,而在副热带地区对流层中、上层辐散加强,引起了北半球高纬度地区极锋急流减弱,而副热带急流加强,这有利于西伯利亚高压的发展,从而引起了东亚冬季风增强;相反,2006年冬季北半球行星波活动为高指数,准定常行星波在高纬度往平流层传播减弱,而往低纬度对流层上层传播加强,造成了行星波E-P通量在高纬度地区对流层中、上层辐散加强,而在副热带地区对流层中、上层辐合加强,引起了北半球高纬度地区极锋急流加强,而副热带急流减弱,这不利于西伯利亚高压的发展,从而引起了东亚冬季风减弱。     

Planetary geostrophic equations for the atmosphere with evolution of the barotropic flow

Atmospheric phenomena such as the quasi-stationary Rossby waves, teleconnection patterns, ultralong persistent blockings and the polar/subtropical jet are characterized by planetary spatial scales, i.e. scales of the order of the earth’s radius. This motivates our interest in the relevant physical processes acting on the planetary scales. Using an asymptotic approach, we systematically derive reduced model equations valid for atmospheric motions with planetary spatial scales and a temporal scale of the order of about 1 week. We assume variations of the background potential temperature comparable in magnitude with those adopted in the classical quasi-geostrophic theory. At leading order, the resulting equations include the planetary geostrophic balance. In order to apply these equations to the atmosphere, one has to prescribe a closure for the vertically averaged pressure. We present an evolution equation for this component of the pressure which was derived in a systematic way from the asymptotic analysis. Relative to the prognostic closures adopted in existing reduced-complexity planetary models, this new dynamical closure may provide for more realistic increased large-scale, long-time variability in future implementations.     

冬季500hPa候平均高度场的遥相关

早在本世纪初,人们就已发现大气环流存在遥相关现象。Wallace等从北半球冬季500hPa月平均高度场总结出5种遥相关型。理论研究和数值模拟表明遥相关型是正压不稳定及Rossby波在球面上频散的反映。由于Rossby波频散是定性估计大型环流中期变化趋势的基本思路及500hPa候平均图是中期预报的基本工具,因此,分析500kPa候平均高度场的遥相关是有意义的。