共查询到20条相似文献,搜索用时 46 毫秒
1.
A typical active–break cycle of the Asian summer monsoon is taken as beginning with maximum SST (pentad 0) over the north
Bay of Bengal when the oceans to its west and east from longitude 40°–160°E, and between latitudes 10° and 25°N (area A) also
has maximum SST. During this pentad the recently found “Cold Pool” of the Bay of Bengal (between latitudes 3°N and 10°N) has
its minimum SST. An area of convection takes genesis over the Bay of Bengal immediately after pentad 0 in the zone of large
SST gradient north of the Cold Pool and it pulls the monsoon Low Level Jetstream (LLJ) through peninsular India. Convection
and the LLJ westerlies then spread to the western Pacific Ocean during pentads 1–4 taken as the active phase of the monsoon
during which convection and LLJ have grown in a positive feed back process. The cyclonic vorticity to the north of the LLJ
axis is hypothesized to act as a flywheel maintaining the convection during the long active phase against the dissipating
effect of atmospheric stabilization by each short spell of deep convection. By the end of pentad 4 the SST over area A has
cooled and the convection weakens there, when the LLJ turns clockwise over the Arabian Sea and flows close to the equator
in the Indian ocean. A band of convection develops at pentad 5 between the equator and latitude 10°S over the Indian ocean
and it is nourished by the cyclonic vorticity of the LLJ now near the equator and the moisture supply through it. This is
taken as the break monsoon phase lasting for about three to four pentads beginning from pentad 5 of a composite active–break
cycle of 40 day duration. With reduced wind and convection over the area A during the break phase, solar radiation and light
winds make the SST there warm rapidly and a new active–break cycle begins. SST, convection, LLJ and the net heat flux at the
ocean surface have important roles in this new way of looking at the active–break cycle as a coupled ocean–atmosphere phenomenon. 相似文献
2.
C. V. Singh 《Theoretical and Applied Climatology》2006,84(4):207-211
Summary In this study, Principal Component Analysis (PCA) has been used to identify the major modes of the outgoing long-wave radiation
data for the period (1979–2002) during the Indian monsoon period (June–September), using seasonal mean values over the Indian
region covering 143 grid points (5° N–35° N and 70° E–95° E at 2.5° Longitude–Latitude intervals. The five principal components
explain up to 98.0% of the total variance. The first principal component explains 60% of the total variance with a pronounced
variation in the outgoing long-wave radiation over the region 10° N to 25° N. It appears that the major reason for the monsoon
variability is the intensity and associated fluctuations in the two major semi-permanent seasonal systems. This is largely
indicative of strong seasonal shift of the major area of cloudiness associated with convergence zone. The second principal
component explaining 20% of the total variance exhibits higher positive component loadings along 25° N and east of 80° E.
The possible reason for this could be the synoptic systems such as monsoon depression/lows over the north bay and trough/vortices
off the west-coast in the Arabian sea. 相似文献
3.
Easterly wave regimes and associated convection over West Africa and tropical Atlantic: results from the NCEP/NCAR and ECMWF reanalyses 总被引:1,自引:0,他引:1
NCEP/NCAR and ECMWF daily reanalyses are used to investigate the synoptic variability of easterly waves over West Africa
and tropical Atlantic at 700 hPa in northern summer between 1979–1995 (1979–1993 for ECMWF). Spectral analysis of the meridional
wind component at 700 hPa highlighted two main periodicity bands, between 3 and 5 days, and 6 and 9 days. The 3–5-day easterly
wave regime has already been widely investigated, but only on shorter datasets. These waves grow both north and south of the
African Easterly Jet (AEJ). The two main tracks, noted over West Africa at 5 °N and 15 °N, converge over the Atlantic on latitude
17.5 °N. These waves are more active in August–September than in June–July. Their average wavelength/phase speed varies from
about 3000 km/8 m s-1 north of the jet to 5000 km/12 m s-1 south of the jet. Rainfall, convection and monsoon flux are significantly modulated by these waves, convection in the Inter-Tropical
Convergence Zone (ITCZ) being enhanced in the trough and ahead of it, with a wide meridional extension. Compared to the 3–5-day
waves, the 6–9-day regime is intermittent and the corresponding wind field pattern has both similar and contrasting characteristics.
The only main track is located north of the AEJ along 17.5 °N both over West Africa and the Atlantic. The mean wavelength
is higher, about 5000 km long, and the average phase speed is about 7 m s-1. Then the wind field perturbation is mostly evident at the AEJ latitude and north of it. The perturbation structure is similar
to that of 3–5-days in the north except that the more developed circulation centers, moving more to the north, lead to a large
modulation of the jet zonal wind component. South of the AEJ, the wind field perturbation is weaker and quite different. The
zonal wind core of the jet appears to be an almost symmetric axis in the 6–9-day wind field pattern, a clockwise circulation
north of the AEJ being associated with a counter-clockwise circulation south of the jet, and vice versa. These 6–9-day easterly
waves also affect significantly rainfall, convection and monsoon flux but in a different way, inducing large zonal convective
bands in the ITCZ, mostly in the trough and behind it. As opposed to the 3–5-day wave regime, these rainfall anomalies are
associated with anomalies of opposite sign over the Guinea coast and the Sahelian regions. Over the continent, these waves
are more active in June–July, and in August–September over the ocean. GATE phase I gave an example of such an active 6–9-day
wave pattern. Considered as a sequence of weak easterly wave activity, this phase was also a sequence of high 6–9-day easterly
wave activity. We suggest that the 6–9-day regime results from an interaction between the 3–5-day easterly wave regime (maintained
by the barotropic/baroclinic instability of the AEJ), and the development of strong anticyclonic circulations, north of the
jet over West Africa, and both north and south of the jet over the Atlantic, significantly affecting the jet zonal wind component.
The permanent subtropical anticyclones (Azores, Libya, St Helena) could help initiation and maintenance of such regime over
West Africa and tropical Atlantic. Based on an a priori period-band criterion, our synoptic classification has enabled us
to point out two statistical and meteorological easterly wave regimes over West Africa and tropical Atlantic. NCEP/NCAR and
ECMWF reanalyses are in good agreement, the main difference being a more developed easterly wave activity in the NCEP/NCAR
reanalyses, especially for the 3–5-day regime over the Atlantic.
Received: 28 May 1998 / Accepted: 2 May 1999 相似文献
4.
The West African monsoon has over the years proven difficult to represent in global coupled models. The current operational seasonal forecasting system of the UK Met Office (GloSea4) has a good representation of monsoon rainfall over West Africa. It reproduces the various stages of the monsoon: a coastal phase in May and June, followed by onset of the Sahelian phase in July when rainfall maxima shift northward of 10N until September; and a secondary coastal rainfall maximum in October. We explore the dynamics of monsoon onset in GloSea4 and compare it to reanalyses. An important difference is the change in the Saharan heat low around the time of Sahelian onset. In Glosea4 the deepening heat low introduces moisture convergence across an east-west Sahelian band, whereas in the reanalyses such an east-west organisation of moisture does not occur and moisture is transported northwards to the Sahara. Lack of observations in the southern Sahara makes it difficult to verify this process in GloSea4 and also suggests that reanalyses may not be strongly constrained by station observations in an area key to Sahelian onset. Timing of monsoon onset has socio-economic importance for many countries in West Africa and we explore onset predictability in GloSea4. We use tercile categories to calculate probabilities for onset occurring before, near and after average in four different onset indicators. Glosea4 has modest skill at 2–3 months’ lead time, with ROC scores of 0.6–0.8. Similar skill is seen in hindcasts with models from the ENSEMBLES project, even in models with large rainfall biases over the Sahel. Forecast skill derives from tropical SST in June and many models capture at least the influence of the tropical Atlantic. This suggests that long-range skill for onset could be present in other seasonal forecasting systems in spite of mean rainfall biases. 相似文献
5.
The horizontal and vertical structure of the 3–5-day and 6–9-day easterly waves over West Africa and tropical Atlantic are
investigated. NCEP/NCAR reanalyses are used for the period 1979–1995 to produce a 17-year climatology of both 3–5-day and
6–9-day easterly waves. Composite patterns of convection, wind, temperature and vertical velocity are analysed with respect
to the following: the modulation by 3–5-day and 6–9-day wave regimes; the contrasts between the ITCZ (5°N–10°N) and the Sahelo-Saharan
band (15°N–20°N); the difference between land and ocean, and seasonal variations. Similarities and differences in the characteristics
of the two wave regimes are identified.
Received: 18 August 1999 / Accepted: 14 March 2001 相似文献
6.
Besides sea surface temperature (SST), soil moisture (SM) exhibits a significant memory and is likely to contribute to atmospheric
predictability at the seasonal timescale. In this respect, West Africa was recently highlighted as a “hot spot” where the
land–atmosphere coupling could play an important role, through the recycling of precipitation and the modulation of the meridional
gradient of moist static energy. Particularly intriguing is the observed relationship between summer monsoon rainfall over
Sahel and the previous second rainy season over the Guinean Coast, suggesting the possibility of a soil moisture memory beyond
the seasonal timescale. The present study is aimed at revisiting this question through a detailed analysis of the instrumental
record and a set of numerical sensitivity experiments. Three ensembles of global atmospheric simulations have been designed
to assess the relative influence of SST and SM boundary conditions on the West African monsoon predictability over the 1986–1995
period. On the one hand, the results indicate that SM contributes to rainfall predictability at the end and just after the
rainy season over the Sahel, through a positive soil-precipitation feedback that is consistent with the “hot spot” hypothesis.
On the other hand, SM memory decreases very rapidly during the dry season and does not contribute to the predictability of
the all-summer monsoon rainfall. Though possibly model dependent, this conclusion is reinforced by the statistical analysis
of the summer monsoon rainfall variability over the Sahel and its link with tropical SSTs. Our results indeed suggest that
the apparent relationship with the previous second rainy season over the Guinean Coast is mainly an artefact of rainfall teleconnections
with tropical modes of SST variability both at interannual and multi-decadal timescales. 相似文献
7.
Summary By analyzing 12-year (1979–1990) 200 hPa wind data from National Centers for Environmental Prediction-National Center for
Atmospheric Research reanalysis, we demonstrate that the intraseasonal time scale (30–60 days) variability of the Tropical
Easterly Jet (TEJ) reported in individual case studies occurs during most years. In the entrance region (east of ∼70° E),
axis of the TEJ at 200 hPa is found along the near equatorial latitudes during monsoon onset/monsoon revivals and propagates
northward as the monsoon advances over India. This axis is found along ∼5° N and ∼15° N during active monsoon and break monsoon
conditions respectively. Examination of the European Centre for Medium Range Weather Forecasts reanalysis wind data also confirms
the northward propagation of the TEJ on intraseasonal time scales.
During the intraseasonal northward propagations, axis of the TEJ is found about 10°–15° latitudes south of the well-known
intraseasonally northward propagating monsoon convective belts. Because of this 10°–15° displacement, axis of the TEJ arrives
over a location about two weeks after the arrival of the monsoon convection. Systematic shifting of the locations by convection,
low level monsoon flow and TEJ in a collective way during different phases of the monsoon suggests that they all may be related. 相似文献
8.
9.
Monthly sea surface temperature anomalies (SSTA) at near-global scale (60 °N–40 °S) and May to October rainfall amounts in
West Africa (16 °N–5 °N; 16 °W–16 °E) are first used to investigate the seasonal and interannual evolutions of their relationship.
It is shown that West African rainfall variability is associated with two types of oceanic changes: (1) a large-scale evolution
involving the two largest SSTA leading eigenmodes (16% of the total variance with stronger loadings in the equatorial and
southern oceans) related to the long-term (multiannual) component of rainfall variability mainly expressed in the Sudan–Sahel
region; and (2) a regional and seasonally coupled evolution of the meridional thermal gradient in the tropical Atlantic due
to the linear combination of the two largest SSTA modes in the Atlantic (11% with strong inverse loadings over the northern
and southern tropics) which is associated with the interannual and quasi-decadal components of regional rainfall in West Africa.
Linear regression and discriminant analyses provide evidence that the main July–September rainfall anomalies in Sudan–Sahel
can be detected with rather good skills using the leading (April–June) or synchronous (July–September) values of the four
main oceanic modes. In particular, the driest conditions over Sahel, more marked since the beginning of the 1970s, are specifically
linked to the warm phases of the two global modes and to cold/warm anomalies in the northern/southern tropical Atlantic. Idealized
but realistic SSTA patterns, obtained from some basic linear combinations of the four main oceanic modes appear sufficient
to generate quickly (from mid-July to the end of August) significant West African rainfall anomalies in model experiments,
consistent with the statistical results. The recent negative impact on West African rainfall exerted by the global oceanic
forcing is primarily due to the generation of subsidence anomalies in the mid-troposphere over West Africa. When an idealized
north to south SSTA gradient is added in the tropical Atlantic, strong north to south height gradients in the middle levels
appear. These limit the northward excursion of the rainbelt in West Africa: the Sahelian area experiences drier conditions
due to the additive effect (subsidence anomalies+latitudinal blocking) while over the Guinea regions wet conditions do not
significantly increase, since the subsidence anomalies and the blocking effect act here in opposite ways.
Received: 26 June 1997 / Accepted: 3 October 1997 相似文献
10.
The climatological summer monsoon onset displays a distinct step wise northeastward movement over the South China Sea and
the western North Pacific (WNP) (110°–160°E, 10°–20°N). Monsoon rain commences over the South China Sea-Philippines region
in mid-May, extends abruptly to the southwestern Philippine Sea in early to mid-June, and finally penetrates to the northeastern
part of the domain around mid-July. In association, three abrupt changes are identified in the atmospheric circulation. Specifically,
the WNP subtropical high displays a sudden eastward retreat or quick northward displacement and the monsoon trough pushes
abruptly eastward or northeastward at the onset of the three stages. The step wise movement of the onset results from the
slow northeastward seasonal evolution of large-scale circulation and the phase-locked intraseasonal oscillation (ISO). The
seasonal evolution establishes a large-scale background for the development of convection and the ISO triggers deep convection.
The ISO over the WNP has a dominant period of about 20–30 days. This determines up the time interval between the consecutive
stages of the monsoon onset. From the atmospheric perspective, the seasonal sea surface temperature (SST) change in the WNP
plays a critical role in the northeastward advance of the onset. The seasonal northeastward march of the warmest SST tongue
(SST exceeding 29.5 °C) favors the northeastward movement of the monsoon trough and the high convective instability region.
The seasonal SST change, in turn, is affected by the monsoon through cloud-radiation and wind-evaporation feedbacks.
Received: 19 October 1999 / Accepted: 5 June 2000 相似文献
11.
Aaron Anthony Boone Isabelle Poccard-Leclercq Yongkang Xue Jinming Feng Patricia de Rosnay 《Climate Dynamics》2010,35(1):127-142
The West African monsoon (WAM) circulation and intensity have been shown to be influenced by the land surface in numerous
numerical studies using regional scale and global scale atmospheric climate models (RCMs and GCMs, respectively) over the
last several decades. The atmosphere–land surface interactions are modulated by the magnitude of the north–south gradient
of the low level moist static energy, which is highly correlated with the steep latitudinal gradients of the vegetation characteristics
and coverage, land use, and soil properties over this zone. The African Multidisciplinary Monsoon Analysis (AMMA) has organised
comprehensive activities in data collection and modelling to further investigate the significance land–atmosphere feedbacks.
Surface energy fluxes simulated by an ensemble of land surface models from AMMA Land-surface Model Intercomparison Project
(ALMIP) have been used as a proxy for the best estimate of the “real world” values in order to evaluate GCM and RCM simulations
under the auspices of the West African Monsoon Modelling Experiment (WAMME) project, since such large-scale observations do
not exist. The ALMIP models have been forced in off-line mode using forcing based on a mixture of satellite, observational,
and numerical weather prediction data. The ALMIP models were found to agree well over the region where land–atmosphere coupling
is deemed to be most important (notably the Sahel), with a high signal to noise ratio (generally from 0.7 to 0.9) in the ensemble
and a inter-model coefficient of variation between 5 and 15%. Most of the WAMME models simulated spatially averaged net radiation
values over West Africa which were consistent with the ALMIP estimates, however, the partitioning of this energy between sensible
and latent heat fluxes was significantly different: WAMME models tended to simulate larger (by nearly a factor of two) monthly
latent heat fluxes than ALMIP. This results due to a positive precipitation bias in the WAMME models and a northward displacement
of the monsoon in most of the GCMs and RCMs. Another key feature not found in the WAMME models is peak seasonal latent heat
fluxes during the monsoon retreat (approximately a month after the peak precipitation rates) from soil water stores. This
is likely related to the WAMME northward bias of the latent heat flux gradient during the WAM onset. 相似文献
12.
Based on calculations of data from FGGE Level III b, a discussion is made of the energy balance in the 40-50 day periodic oscillation over the Asian monsoon region during the 1979 summer. It is found that the main source of 40-50 day periodic perturbation is the monsoon region extending from central South Asia to Southeast Asia. In the upper layer over the North Pacific subtropical area (10-20oN, 150oE-150oW) pres-sure work turns into kinetic energy that maintains 40-50 day periodic perturbation associated with the variation in position and intensity of the mid-Pacific trough. The mean energy budget in the three-dimensional space (0-30oE, 30oE-150oW, 100-1000 hPa) indicates that the 40-50 day periodic perturbation transports kinetic energy to a seasonal mean and a transient perturbation wind field. 相似文献
13.
利用ERA-Interim再分析资料分析了夏秋季西北太平洋季风槽的气候特征以及季节和年际变化特征及其对西北太平洋热带气旋和台风(TCs)生成大尺度环境因子的影响。研究结果表明了西北太平洋季风槽有很明显的季节变化,在6~7月,季风槽和强对流活动区在5°N~15°N的南海和西北太平洋西侧上空,并逐渐东伸;到了8~9月,季风槽和强对流活动区向北移动、并向东扩展,一般位于10°N~20°N的南海和西北太平洋西侧、中部上空,有的年份可东伸到西北太平洋东侧,强度加强;到了10~11月,季风槽迅速减弱,并成为涡旋,强对流活动区也向南移和向西收缩。同时,研究还表明了西北太平洋季风槽有明显的年际变化。在季风槽强的年份,季风槽和强对流活动区可以从南海经西北太平洋西侧和中部东伸到西北太平洋的东侧上空;而在季风槽弱的年份,季风槽和强对流活动区主要位于南海和西北太平洋西侧和中部上空,季风槽强度的年际变化对它的季节变化也有重要影响。此外,研究还表明了随着季风槽的季节和年际变化,西北太平洋TCs生成的大尺度环境因子分布也发生很明显的变化。 相似文献
14.
E. A. Afiesimama 《Theoretical and Applied Climatology》2007,90(1-2):103-111
Summary The paper examines the annual cycle of the mid-tropospheric easterly jet (MTJ) over West Africa against the background of
many reviews indicating different locations and characteristics of the jet and considering it as a summer feature. NCEP–NCAR
reanalysis zonal wind datasets for the period 1971–2000 and upper air datasets over the region are used. The results exhibit
realistic spatial structure of the easterly jet. The long-term mean of the datasets suggests that the jet over West Africa
is not only a summer feature but can also be found in winter with the same order of magnitude in the wind velocity at the
core. The jet axis is located at about lat. 2° N close to the Guinean Coast in winter and at lat. 14° N in summer. The meridional
oscillation of the jet suggests that as it advances northward, it maintains an altitude of 700 hPa in winter and transits
in mid-spring to 650 hPa and reaches 600 hPa in summer. In the retreat, it displaces to 650 hPa at the end of September rather
sharply to reach 700 hPa in October. The jet’s core has been observed to have a northeast–southwest orientation from season
to season, covering a longitude of 29° from its southernmost to the northernmost positions. 相似文献
15.
Tropical cyclone genesis frequency over the western North Pacific simulated in medium-resolution coupled general circulation models 总被引:4,自引:0,他引:4
This study examines the tropical cyclone (TC) genesis frequency over the western North Pacific simulated in atmosphere–ocean
coupled general circulation models from the World Climate Research Programme’s Coupled Model Intercomparison Project phase
3. We first evaluate performances of eight models with atmospheric horizontal resolution of T63 or T106 by analyzing their
daily-mean atmospheric outputs of twentieth-century climate simulations available from the Program for Climate Model Diagnosis
and Intercomparison database. The genesis frequency is validated against the best-track data issued by the Japan Meteorological
Agency. Five of the eight models reproduce realistic horizontal distribution of the TC genesis with a large fraction over
the 10°–20°N, 120°–150°E area. These five high-performance models also realistically simulate the summer–winter contrast of
the frequency. However, detailed seasonal march is slightly unrealistic; four of the models overestimate the frequency in
the early season (May–June) while all of them underestimate the frequency in the mature season (July–September). Reasons for
these biases in the seasonal march for the five high-performance models are discussed using the TC genesis potential (GP)
index proposed by Emanuel and Nolan (in Am Meteor Soc, pp 240–241, 2004). The simulated GP has seasonal biases consistent with those of the TC genesis frequency. For all five models, the seasonal
biases in GP are consistent with those in environmental lower-tropospheric vorticity, vertical wind shear, and relative humidity,
which can be attributed to the simulated behavior of monsoon trough. The observed trough migrates northward from the equatorial
region to reach the 10°–20°N latitudinal band during the mature season and contributes to the TC frequency maximum, whereas
the simulated trough migrates northward too rapidly and reaches this latitude band in the early season, leading to the overestimation
of the TC genesis frequency. In the mature season, the simulated trough reaches as far as 15°–25°N, accompanied by a strong
vertical shear south of the trough, providing an unfavorable condition for TC genesis. It is concluded that an adequate simulation
of the monsoon trough behavior is essential for a better reproduction of the TC frequency seasonal march. 相似文献
16.
Some annual variation characteristics for the Northern Hemispheric monthly mean precipitation fields 总被引:2,自引:0,他引:2
Tang Maocang 《大气科学进展》1989,6(2):186-201
By the utilization of monthly precipitation data from all stations in the Northern Hemisphere annexed to the “World Survey of climatology, Vol. 1-15, the distributions of the maximum precipitation months (MPM), the annual relative precipitation (ARP) and the monthly relative precipitation (percent of annual) in January and July are respectively mapped. Moreover the distributions of intermonthly relative precipitation variabilities from January to December are plotted as well. From these figures, the precipitation in the Northern Hemisphere may be classified into three types (continental, oceanic and transitional types) and 17 regions. The precipitation regime may also be divided into two patterns, the global and regional patterns. The global pattern consists of planetary front system and ITCZ and its inter-monthly variation shows the north-and-south shift of the rain belt; the regional pattern consists of the sea-land monsoon and plateau monsoon regime, in which the inter-monthly variation of rain belt shows a east- and-west shift. 相似文献
17.
A nine-member ensemble of simulations with a state-of-the-art atmospheric model forced only by the observed record of sea
surface temperature (SST) over 1930–2000 is shown to capture the dominant patterns of variability of boreal summer African
rainfall. One pattern represents variability along the Gulf of Guinea, between the equator and 10°N. It connects rainfall
over Africa to the Atlantic marine Intertropical Convergence Zone, is controlled by local, i.e., eastern equatorial Atlantic,
SSTs, and is interannual in time scale. The other represents variability in the semi-arid Sahel, between 10°N and 20°N. It
is a continental pattern, capturing the essence of the African summer monsoon, while at the same time displaying high sensitivity
to SSTs in the global tropics. A land–atmosphere feedback associated with this pattern translates precipitation anomalies
into coherent surface temperature and evaporation anomalies, as highlighted by a simulation where soil moisture is held fixed
to climatology. As a consequence of such feedback, it is shown that the recent positive trend in surface temperature is consistent
with the ocean-forced negative trend in precipitation, without the need to invoke the direct effect of the observed increase
in anthropogenic greenhouse gases. We advance plausible mechanisms by which the balance between land–ocean temperature contrast
and moisture availability that defines the monsoon could have been altered in recent decades, resulting in persistent drought.
This discussion also serves to illustrate ways in which the monsoon may be perturbed, or may already have been perturbed,
by anthropogenic climate change. 相似文献
18.
Asian summer monsoon anomalies induced by aerosol direct forcing: the role of the Tibetan Plateau 总被引:20,自引:2,他引:20
In this paper we present results of a numerical study using the NASA finite-volume GCM to elucidate a plausible mechanism for aerosol impact on the Asian summer monsoon involving interaction with physical processes over the Tibetan Plateau (TP). During the pre-monsoon season of March–April, dusts from the deserts of western China, Afghanistan/Pakistan, and the Middle East are transported into and stacked up against the northern and southern slopes of the TP. The absorption of solar radiation by dust heats up the elevated surface air over the slopes. On the southern slopes, the atmospheric heating is reinforced by black carbon from local emission. The heated air rises via dry convection, creating a positive temperature anomaly in the mid-to-upper troposphere over the TP relative to the region to the south. In May through early June in a manner akin to an “elevated heat pump”, the rising hot air forced by the increasing heating in the upper troposphere, draws in warm and moist air over the Indian subcontinent, setting the stage for the onset of the South Asia summer monsoon. Our results suggest that increased dust loading coupled with black carbon emission from local sources in northern India during late spring may lead to an advance of the rainy periods and subsequently an intensification of the Indian summer monsoon. The enhanced rainfall over India is associated with the development of an aerosol-induced large-scale sea level pressure anomaly pattern, which causes the East Asia (Mei-yu) rain belt to shift northwestward, suppressing rainfall over East Asia and the adjacent oceanic regions. 相似文献
19.
H. Evangelista J. Maldonado R. H. M. Godoi E. B. Pereira D. Koch K. Tanizaki-Fonseca R. Van Grieken M. Sampaio A. Setzer A. Alencar S. C. Gonçalves 《Journal of Atmospheric Chemistry》2007,56(3):225-238
The total extent of the atmospheric impacts associated to the aerosol black carbon (BC) emissions from South America is not
completed described. This work presents results of BC monitored during three scientific expeditions (2002, 2003 and 2004)
on board of a Brazilian oceanographic vessel Ary Rongel that covered the South–West Atlantic coast between 22–62°S. This latitudinal
band encloses major urban regions of South America and the outflow region of the SACZ (South Atlantic Convergent Zone), which
is an important mechanism of advective transport of heat, moisture, minor gases and aerosols from the South America continental
land to the Southern Atlantic Ocean. Our results showed that aerosol BC enhanced concentrations from urban/industrial origin
can be transported to the South–West Atlantic Ocean due to the migration of sub-polar fronts that frequently reach tropical/subtropical
regions. Despite the decrease of aerosol BC concentrations southwards (from ∼1,200 ng m−3 at latitude 22°S to ∼10 ng m−3 at latitude 62°S), several observed peak events were attributed to regional urban activities. Most of such events could be
explained by the use of air mass back trajectories analysis. In addition, a global model simulation is presented (Goddard
Institute for Space Studies – GISS GCM BC simulation) to explore the origins of aerosol BC in the South–West Atlantic. The
model allowed isolating the biomass emissions from South America and Africa and industrial (non-biomass) pollution from other
regions of the globe. This model suggests that the apportionment of about half of the aerosol BC at the South–West Atlantic
may derive from South American biomass burning. 相似文献
20.
A. Yu. Mikhailov A. N. Zolotokrylin T. B. Titkova 《Russian Meteorology and Hydrology》2012,37(2):85-89
The geographical position of the climatological polar front over the Russian plains in summer of 1948–1960, 1961–1990, and
1991–2007 is detailed. The location of the polar front is derived using the frequency of cyclonic centers and the module of
horizontal temperature gradient computed from the reanalysis data of UEA CRU 2.5° × 2.5°. The East European and Asian branches
of the polar front are reliably distinguished by all indicators. The geographical position of branches differs from S.P. Khromov’s
polar front map in July in details only. The present-day southward shift of the polar front over the East European Plain and
the northward shift over the West Siberian Plain are revealed relative to the period of 1948–1990. 相似文献