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11.
Summary Intraseasonal variation of tropical convergence zones (TCZ) is studied focussing on the three major features of the TCZ over the Indian longitudes during the summer monsoon viz. (i) the oscillation between active and weak spells, (ii) the occurrence of two favourable zones — one over the equatorial oceans and another over the heated continent and (iii) poleward propagations of the oceanic TCZ onto the heated continent. An observational study of the intraseasonal variation over different parts of the tropics has shown that the first feature may be an ubiquitous feature of the TCZ variations, the second occurs only over the Asian summer and winter monsoon zones, and the third only over the Asian summer monsoon. Analysis of a simple monsoon model has revealed that poleward propagation occurs in the presence of a meridional surface temperature gradient because the convective heating is asymmetric, with more heating on the poleward side. Preliminary analysis of the T-21 version of the ECMWF model has shown that it is capable of simulating the three major features of the intraseasonal variation of the TCZ over the Indian longitudes during the summer monsoon.With 16 Figures 相似文献
12.
Monsoon precipitation in the AMIP runs 总被引:4,自引:1,他引:4
We present an analysis of the seasonal precipitation associated with the African, Indian and the Australian-Indonesian monsoon
and the interannual variation of the Indian monsoon simulated by 30 atmospheric general circulation models undertaken as a
special diagnostic subproject of the Atmospheric Model Intercomparison Project (AMIP). The seasonal migration of the major
rainbelt observed over the African region, is reasonably well simulated by almost all the models. The Asia West Pacific region
is more complex because of the presence of warm oceans equatorward of heated continents. Whereas some models simulate the
observed seasonal migration of the primary rainbelt, in several others this rainbelt remains over the equatorial oceans in
all seasons. Thus, the models fall into two distinct classes on the basis of the seasonal variation of the major rainbelt
over the Asia West Pacific sector, the first (class I) are models with a realistic simulation of the seasonal migration and
the major rainbelt over the continent in the boreal summer; and the second (class II) are models with a smaller amplitude
of seasonal migration than observed. The mean rainfall pattern over the Indian region for July-August (the peak monsoon months)
is even more complex because, in addition to the primary rainbelt over the Indian monsoon zone (the monsoon rainbelt) and
the secondary one over the equatorial Indian ocean, another zone with significant rainfall occurs over the foothills of Himalayas
just north of the monsoon zone. Eleven models simulate the monsoon rainbelt reasonably realistically. Of these, in the simulations
of five belonging to class I, the monsoon rainbelt over India in the summer is a manifestation of the seasonal migration of
the planetary scale system. However in those belonging to class II it is associated with a more localised system. In several
models, the oceanic rainbelt dominates the continental one. On the whole, the skill in simulation of excess/deficit summer
monsoon rainfall over the Indian region is found to be much larger for models of class I than II, particularly for the ENSO
associated seasons. Thus, the classification based on seasonal mean patterns is found to be useful for interpreting the simulation
of interannual variation. The mean rainfall pattern of models of class I is closer to the observed and has a higher pattern
correlation coefficient than that of class II. This supports Sperber and Palmer’s (1996) result of the association of better
simulation of interannual variability with better simulation of the mean rainfall pattern. The hypothesis, that the skill
of simulation of the interannual variation of the all-India monsoon rainfall in association with ENSO depends upon the skill
of simulation of the seasonal variation over the Asia West Pacific sector, is supported by a case in which we have two versions
of the model where NCEP1 is in class II and NCEP2 is in class I. The simulation of the interannual variation of the local
response over the central Pacific as well as the all-India monsoon rainfall are good for NCEP2 and poor for NCEP1. Our results
suggest that when the model climatology is reasonably close to observations, to achieve a realistic simulation of the interannual
variation of all-India monsoon rainfall associated with ENSO, the focus should be on improvement of the simulation of the
seasonal variation over the Asia West Pacific sector rather than further improvement of the simulation of the mean rainfall
pattern over the Indian region.
Received: 2 June 1997 / Accepted: 8 January 1998 相似文献
13.
Sulochana Gadgil 《Journal of Earth System Science》2018,127(1):1
For well over 300 years, the monsoon has been considered to be a gigantic land–sea breeze driven by the land–ocean contrast in surface temperature. In this paper, this hypothesis and its implications for the variability of the monsoon are discussed and it is shown that the observations of monsoon variability do not support this popular theory of the monsoon. An alternative hypothesis (whose origins can be traced to Blanford’s (1886) remarkably perceptive analysis) in which the basic system responsible for the Indian summer monsoon is considered to be the Intertropical Convergence Zone (ITCZ) or the equatorial trough, is then examined and shown to be consistent with the observations. The implications of considering the monsoon as a manifestation of the seasonal migration of the ITCZ for the variability of the Indian summer monsoon and for identification of the monsoonal regions of the world are briefly discussed. 相似文献
14.
On breaks of the Indian monsoon 总被引:1,自引:0,他引:1
For over a century, the term break has been used for spells in which the rainfall over the Indian monsoon zone is interrupted.
The phenomenon of ’break monsoon’ is of great interest because long intense breaks are often associated with poor monsoon
seasons. Such breaks have distinct circulation characteristics (heat trough type circulation) and have a large impact on rainfed
agriculture. Although interruption of the monsoon rainfall is considered to be the most important feature of the break monsoon,
traditionally breaks have been identified on the basis of the surface pressure and wind patterns over the Indian region. We
have defined breaks (and active spells) on the basis of rainfall over the monsoon zone. The rainfall criteria are chosen so
as to ensure a large overlap with the traditional breaks documented by Ramamurthy (1969) and Deet al (1998). We have identified these rainbreaks for 1901-89. We have also identified active spells on the basis of rainfall over
the Indian monsoon zone. We have shown that the all-India summer monsoon rainfall is significantly negatively correlated with
the number of rainbreak days (correlation coefficient -0.56) and significantly positively correlated with the number of active
days (correlation coefficient 0.47). Thus the interannual variation of the all-India summer monsoon rainfall is shown to be
related to the number of days of rainbreaks and active spells identified here.
There have been several studies of breaks (and also active spells in several cases) identified on the basis of different criteria
over regions differing in spatial scales (e.g., Websteret al 1998; Krishnanet al it 2000; Goswami and Mohan 2000; and Annamalai and Slingo 2001). We find that there is considerable overlap between the rainbreaks
we have identified and breaks based on the traditional definition. There is some overlap with the breaks identified by Krishnanet al (2000) but little overlap with breaks identified by Websteret al (1998). Further, there are three or four active-break cycles in a season according to Websteret al (1998) which implies a time scale of about 40 days for which Goswami and Mohan (2000), and Annamalai and Slingo (2001) have
studied breaks and active minus break fluctuations. On the other hand, neither the traditional breaks (Ramamurthy 1969; and
Deet al 1998) nor the rainbreaks occur every year. This suggests that the `breaks’ in these studies are weak spells of the intraseasonal
variation of the monsoon, which occur every year.
We have derived the OLR and circulation patterns associated with rainbreaks and active spells and compared them with the patterns
associated with breaks/active minus break spells from these studies. Inspite of differences in the patterns over the Indian
region, there is one feature which is seen in the OLR anomaly patterns of breaks identified on the basis of different criteria
as well as the rainbreaks identified in this paper viz., a quadrapole over the Asia-west Pacific region arising from anomalies
opposite (same) in sign to those over the Indian region occurring over the equatorial Indian Ocean and northern tropical (equatorial)
parts of the west Pacific. Thus it appears that this quadrapole is a basic feature of weak spells of the intraseasonal variation
over the Asia-west Pacific region. Since the rainbreaks are intense weak spells, this basic feature is also seen in the composite
patterns of these breaks. We find that rainbreaks (active spells) are also associated with negative 相似文献
15.
Marlon E. Pierce Geoffrey C. Fox Mehmet S. Aktas Galip Aydin Harshawardhan Gadgil Zhigang Qi Ahmet Sayar 《Pure and Applied Geophysics》2008,165(3-4):635-651
We describe our distributed systems research efforts to build the “cyberinfrastructure” components that constitute a geophysical Grid, or more accurately, a Grid of Grids. Service-oriented computing principles are used to build a distributed infrastructure of Web accessible components for accessing data and scientific applications. Our data services fall into two major categories: Archival, database-backed services based around Geographical Information System (GIS) standards from the Open Geospatial Consortium, and streaming services that can be used to filter and route real-time data sources such as Global Positioning System data streams. Execution support services include application execution management services and services for transferring remote files. These data and execution service families are bound together through metadata information and workflow services for service orchestration. Users may access the system through the QuakeSim scientific Web portal, which is built using a portlet component approach. 相似文献
16.
Generally average rainfall over meteorological subdivisions is used for assessment of the variability of monsoon rainfall.
It is shown here that variations of seasonal rainfall over the meteorological subdivisions of interior Karnataka are not coherent.
A methodology for delineating coherent rainfall zones is developed in this paper and applied to derive such zones for the
State of Karnataka. 相似文献
17.
R. Nijagunappa Sulochana Shekhar B. Gurugnanam P. L. N. Raju Prabir De 《Journal of the Indian Society of Remote Sensing》2007,35(3):267-274
In the present study, three categories of network analysis problems were solved using mostly ArcView Network Analyst extension
module. They are Network Tracing; Path Analysis; and Tour Analysis. The methodology provides a means to incorporate existing
data sources, integrate the data in a useful environment and visualize results. 相似文献