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1.
In this study, we elucidate the temporal characteristics of the onset and withdrawal of the Indian southwest monsoon, making
use of the model integration and daily analyses of the National Centre for Medium Range Weather Forecasting, India. The onset
of the Indian southwest monsoon over the Bay of Bengal is discernable by a gradual increase in the adiabatic generation of
kinetic energy, while over the Arabian Sea it is first noticeable by a steep and abrupt increase of generation. The horizontal
transport of heat indicates a convergence regime over the Bay of Bengal prior to onset, while over the Arabian Sea a convergence
regime is indicated by a change from the divergence to the convergence regime. The withdrawal of the southwest monsoon is
characterized by the horizontal transport of heat and moisture that evince a transition from the convergence to divergence
regime; similarly, diabatic heating noticed during the active period changes to cooling. The withdrawal over the Arabian Sea
is characterized by the divergence regime of the horizontal transport of moisture. This change precedes even the circulation
changes over northwest India, which may be regarded as a precursor. The withdrawal is further supported by a monotonic decrease
in the net tropospheric moisture over the Arabian Sea, followed by a similar change at land locations. 相似文献
2.
In this article, the interannual variability of certain dynamic and thermodynamic characteristics of various sectors in the
Asian summer monsoon domain was examined during the onset phase over the south Indian peninsula (Kerala Coast). Daily average
(0000 and 1200 UTC) reanalysis data sets of the National Centre for Environmental Prediction/National Centre for Atmospheric
Research (NCEP/NCAR) for the period 1948–1999 were used. Based on 52 years onset date of the Indian summer monsoon, we categorized
the pre-onset, onset, and post-onset periods (each an average of 5 days) to investigate the interannual variability of significant
budget terms over the Arabian Sea, Bay of Bengal, and the Indian peninsula. A higher difference was noticed in low-level kinetic
energy (850 hPa) and the vertically integrated generation of kinetic energy over the Arabian Sea from the pre-onset, onset,
and post-onset periods. Also, significant changes were noticed in the net tropospheric moisture and diabatic heating over
the Arabian Sea and Indian peninsula from the pre-onset to the post-onset period. It appears that attaining the magnitude
of 40 m2 s−2 and then a sharp rise in kinetic energy at 850 hPa is an appropriate time to declare the onset of the summer monsoon over
India. In addition to a sufficient level of net tropospheric moisture (40 mm), a minimum strength of low-level flow is needed
to trigger convective activity over the Arabian Sea and the Bay of Bengal. An attempt was also made to develop a location-specific
prediction of onset dates of the summer monsoon over India based on energetics and basic meteorological parameters using multivariate
statistical techniques. The regression technique was developed with the data of May and June for 42 years (1948–1989) and
validated with 10 years NCEP reanalysis from 1990 to 1999. It was found that the predicted onset dates from the regression
model are fairly in agreement with the observed onset dates obtained from the Indian Meteorology Department. 相似文献
3.
Analysis of monthly momentum transport of zonal waves at 850 hPa for the period 1979 to 1993, between ‡S and ‡N for January
to April, using zonal (u) and meridional (v) components of wind taken from the ECMWF reanalysis field, shows a positive correlation (.1% level of significance) between
the Indian summer monsoon rainfall (June through September) and the momentum transport of wave zero TM(0) over latitudinal
belt between 25‡S and 5‡N (LB) during March. Northward (Southward) TM(0) observed in March over LB subsequently leads to a
good (drought) monsoon season over India which is found to be true even when the year is marked with the El-Nino event. Similarly
a strong westerly zone in the Indian Ocean during March, indicates a good monsoon season for the country, even if the year
is marked with El-Nino. The study thus suggests two predictors, TM(0) over LB and the strength of westerly zone in the Indian
Ocean during March. 相似文献
4.
The second campaign of the Arabian Sea Monsoon Experiment (ARMEX-II) was conducted in two phases viz., March–April and May–June
2003. In the present work, the buoy and ocean research vessel data collected during the second phase of ARMEX-II have been
analysed to bring out the characteristic features of monsoon onset. The results have shown that the thermodynamical features
such as build up of lower tropospheric instability and increased height of zero degree isotherm occurred about a week before
the monsoon onset over Kerala and adjoining southeast Arabian Sea. There was a sharp fall in the temperature difference between
850 and 500 hPa, and the height of zero degree isotherm about 2–3 days before the monsoon onset. The flux of sensible heat
was positive (sea to air) over south Arabian Sea during the onset phase. Over the Bay of Bengal higher negative (air to sea)
values of sensible flux prevailed before the monsoon onset which became less negative with the advance of monsoon over that
region.
The pre-onset period was characterized by large sea surface temperature (SST) gradient over the Arabian Sea with rapid decrease
towards north of the warm pool region. The buoy observations have shown that SST remained close to 30.5°C in the warm pool
region during the pre-onset period in 2003 but only 2–3 degrees away (north of this region) SSTs were as low as 28.5–29°C.
An interesting aspect of sea level pressure (SLP) variability over the Indian seas during the onset phase of summer monsoon
2003 was undoubtedly, the highest SLP in the warm pool region inspite of very high SSTs. 相似文献
5.
A. Suryachandra Rao Y. Sadhuram V. V. Gopala Krishna 《Journal of Earth System Science》1995,104(4):607-611
We estimate, from the moisture budget the bulk aerodynamic coefficient for latent heat flux (C
e) during the monsoon season over the central Arabian Sea. The average value ofC
eunder active monsoon conditions was found to be 2.25 × 10−3 which is nearly 60% higher than those previously used. 相似文献
6.
Weakening of lower tropospheric temperature gradient between Indian landmass and neighbouring oceans and its impact on Indian monsoon 总被引:2,自引:0,他引:2
S. M. Bawiskar 《Journal of Earth System Science》2009,118(4):273-280
The study shows that in the scenario of global warming temperature gradient (TG) between Indian landmass and Arabian Sea/Bay
of Bengal is significantly decreasing in the lower troposphere with maxima around 850 hPa. TG during pre-monsoon (March to
May) is reducing at a significant rate of 0.036°/year (Arabian Sea) and 0.030°/year (Bay of Bengal). The above alarming results
are based on sixty years (1948–2007) of daily temperature and wind data extracted from CDAS-NCEP/NCAR reanalysis datasets.
TG based on ERA-40 data also indicates a decreasing trend of 0.0229°/year and 0.0397°/year for Arabian Sea and Bay of Bengal
respectively. As TG is not governed by any type of significant oscillation, there is a possibility of TG tending to zero.
It is further observed that the rate of warming over the oceans is more than that over the land which has resulted into the
weakening of TG. Pre-monsoon TG has significant correlations with
Except AISMR, the decreasing trends observed in all the above parameters are significant. All India rainfall for July and
August together shows a significant decreasing trend of 0.995mm/year. Reducing number of depressions and cyclonic storms and
increasing number of break days during monsoon over India are the reflections of the weakening of TG. 相似文献
• | All India Seasonal Monsoon Rainfall (AISMR) |
• | kinetic energy of waves 1 and 2 at 850 hPa |
• | kinetic energy, and |
• | stream function at 850 hPa over Indian landmass during monsoon season. |
7.
T. K. Manual Ateef Khan Y. Nazeer Ahammed R. S. Tanwar R. S. Parmar K. S. Zalpuri Prabhat K. Gupta S. L. Jain Risal Singh A. P. Mitra S. C. Garg A. Suryanarayana V. S. N. Murty M. Dileep Kumar Andrew J. Shepherd 《Journal of Earth System Science》2006,115(4):473-484
Characteristics of trace gases (O3, CO, CO2, CH4 and N2O) and aerosols (particle size of 2.5 micron) were studied over the Arabian Sea, equatorial Indian Ocean and southwest part
of the Bay of Bengal during the monsoon transition period (October–November, 2004). Flow of pollutants is expected from south
and southeast Asia during the monsoonal transition period due to the patterns of wind flow which are different from the monsoon
period. This is the first detailed report on aerosols and trace gases during the sampled period as the earlier Bay of Bengal
Experiment (BOBMEX), Arabian Sea Monsoon Experiment (ARMEX) and Indian Ocean Experiments (INDOEX) were during monsoon seasons.
The significant observations during the transition period include: (i) low ozone concentration of the order of 5 ppbv around
the equator, (ii) high concentrations of CO2, CH4 and N2O and (iii) variations in PM2.5 of 5–20μg/m3. 相似文献
8.
An analysis of the meteorological data collected by the research vessel ORV Sagarkanya for the mean latent and sensible heat
fluxes over the Arabian Sea has indicated appreciable changes between active and weak phases of the southwest monsoon of 1986.
We suggest that: (a) the presence of a core of low level winds associated with the Somali jet and its southward shift during
the season, along with (b) a ridge in surface pressure over the central Arabian Sea could be responsible for the deficit in
monsoon rainfall along the west coast of India in 1986. 相似文献
9.
M. R. Ramesh Kumar S. Sathyendranath N. K. Viswambharan L. V. Gangadhara Rao 《Journal of Earth System Science》1986,95(3):435-446
Using the satellite derived sea surface temperature (SST) data for 1979 (bad monsoon) and 1983 (good monsoon), the SST variability for two contrasting monsoon seasons is studied. The study indicates that large negative anomalies off the Somali and Arabian coasts are associated with good monsoon rainfall over India. The strong monsoonal cooling in these regions can be attributed to strong low level winds and intense upwelling. The reappearance of 27°C isotherm off Somali coast in May/June coincides with the onset of southwest monsoon over India. Further, the influence of zonal anomaly of SST off Somalia Coast (SCZASST) and Central Indian Ocean Zonal Anomaly of SST (CIOZASST) with monsoon rainfall over India is brought out. The former is negatively related to the monsoon rainfall over western and central parts of India, whilst CIOZASST is positively related. 相似文献
10.
In order to investigate how monsoons influence biogeochemical fluxes in the ocean, twelve time-series sediment traps were
deployed at six locations in the northern Indian Ocean. In this paper we present particle flux data collected during May 1986
to November 1991 and November 1987 to November 1992 in the Arabian Sea and Bay of Bengal respectively. Particle fluxes were
high during both the SW and NE monsoons in the Arabian Sea as well as in the Bay of Bengal. The mechanisms of particle production
and transport, however, differ in both the regions.
In the Arabian Sea, average annual fluxes are over 50gm-2y-1 in the western Arabian Sea and less than 27gm-2 y-1 in the central part. Biogenic matter is dominant at sites located near upwelling centers, and is less degraded during peak
flux periods. High particle fluxes in the offshore areas of the Arabian Sea are caused by injection of nutrients into the
euphotic zone due to wind-induced mixed layer deepening. In the Bay of Bengal, average annual fluxes are highest in the central
Bay of Bengal (over
50gm-2y-1) and are least in the southern part of the Bay (37gm-2y-1). Particle flux patterns coincide with freshwater discharge patterns of the Ganges-Brahmaputra river system. Opal/carbonate
and organic carbon/carbonate carbon ratios increase during the SW monsoon due to variations in salinity and productivity patterns
in the surface waters as a result of increased freshwater and nutrient input from rivers.
Comparison of S years data show that fluxes of biogenic and lithogenic particulate matter are higher in the Bay of Bengal
even though the Arabian Sea is considered to be more productive. Our results indicate that in the northern Indian Ocean interannual
variability in organic carbon flux is directly related to the strength and intensity of the SW monsoon while its transfer
from the upper layers to the deep sea is partly controlled by input of lithogenic matter from adjacent continents. 相似文献
11.
Monthly mean anomaly fields of various parameters like sea surface temperature, air temperature, wind stress, effective radiation
at the surface, heat gain over the ocean and the total heat loss between a good and bad monsoon composite and the evaporation
rates over the Arabian Sea and southern hemisphere have been studied over the tropical Indian Ocean. The mean rates of evaporation
on a seasonal scale over the Arabian Sea during a good and bad monsoon composites were equal (about 2·48 × 1010 tons/day). The evaporation rates over the southern hemisphere were greater during all the months. The mean evaporation rates
over the southern hemisphere on a seasonal scale for the good and bad monsoon composites were 4·4 × 1010 and 4·6 × 1010 tons/day respectively. The maximum evaporation rates over the southern hemisphere were observed in August. The anomalies
of wind stress, effective radiation at the surface and the heat gain over the ocean also exhibit large variations in August,
as compared to other monsoon months. 相似文献
12.
In this paper Regional Pressure Index (RPI) over the Indian region (20‡N—40‡N and 70‡0E—85‡E) has been constructed for 101
years (1899-1999) on a monthly scale. The relationship of these indices was carried out with the Indian Summer Monsoon Rainfall
(June–September) (ISMR) over the various homogeneous regions, for all the time scales. From the analysis it has been seen
that RPI in the month of May is significantly associated with ISMR over various regions on all the scales. The relationship
is statistically significant at 1% level. The study reveals that RPI in the month of May and January will be a new precursor
for the long range forecasting of ISMR on the smaller spatial scale. On the decadal and climatological scale, winter and spring
time RPI show a significant inverse relationship with the rainfall over the regions Peninsular India (PI) and North West India
(NWI), while the association is direct with Central North East India (CNEI) and North East India (NEI). The relationship is
significant at 0.1 and 1% level respectively. 相似文献
13.
SAVITA PATWARDHAN ASHWINI KULKARNI K KRISHNA KUMAR 《Journal of Earth System Science》2012,121(1):203-210
A state-of-the-art regional climate modelling system, known as PRECIS (Providing REgional Climates for Impacts Studies) developed
by the Hadley Centre for Climate Prediction and Research, UK is applied over the Indian domain to investigate the impact of
global warming on the cyclonic disturbances such as depressions and storms. The PRECIS simulations at 50 × 50 km horizontal
resolution are made for two time slices, present (1961–1990) and the future (2071–2100), for two socioeconomic scenarios A2
and B2. The model simulations under the scenarios of increasing greenhouse gas concentrations and sulphate aerosols are analysed
to study the likely changes in the frequency, intensity and the tracks of cyclonic disturbances forming over north Indian
Ocean (Bay of Bengal and Arabian Sea) and the Indian landmass during monsoon season. The model overestimates the frequency
of cyclonic disturbances over the Indian subcontinent in baseline simulations (1961–1990). The change is evaluated towards
the end of present century (2071–2100) with respect to the baseline climate. The present study indicates that the storm tracks
simulated by the model are southwards as compared to the observed tracks during the monsoon season, especially for the two
main monsoon months, viz., July and August. The analysis suggests that the frequency of cyclonic disturbances forming over
north Indian Ocean is likely to reduce by 9% towards the end of the present century in response to the global warming. However,
the intensity of cyclonic disturbances is likely to increase by about 11% compared to the present. 相似文献
14.
The role of low-frequency intraseasonal oscillations in the anomalous Indian summer monsoon rainfall of 2002 总被引:1,自引:0,他引:1
We analyze the dynamical features and responsible factors of the low-frequency intraseasonal time scales which influenced
the nature of onset, intensity and duration of active/break phases and withdrawal of the monsoon during the anomalous Indian
summer monsoon of 2002 — the most severe drought recorded in recent times. During that season, persistent warm sea surface
temperature anomalies over the equatorial Indian Ocean played a significant role in modulating the strength of the monsoon
Hadley circulation. This in turn affected the onset and intense break spells especially the long break during the peak monsoon
month of July. Strong low-frequency intraseasonal modulations with significant impact on the onset and active/break phases
occurred in 2002 which were manifested as a good association between low-frequency intraseasonal oscillations and the onset
and active/break spells. Further, SST anomalies over the equatorial Indo-Pacific region on low-frequency intraseasonal time
scales were found to affect the equatorial eastward and thereby off-equatorial northward propagations of enhanced convection
over the Indian region. These propagations in turn modulated the active/break cycle deciding the consequent severity of the
2002 drought. 相似文献
15.
S. S. C. Shenoi N. Nasnodkar G. Rajesh K. Jossia Joseph I. Suresh A. M. Almeida 《Journal of Earth System Science》2009,118(5):483-496
This paper describes the variability in the diurnal range of SST in the north Indian Ocean using in situ measurements and tests the suitability of simple regression models in estimating the diurnal range. SST measurements obtained
from 1556 drifting and 25 moored buoys were used to determine the diurnal range of SSTs. The magnitude of diurnal range of
SST was highest in spring and lowest in summer monsoon. Except in spring, nearly 75–80% of the observations reported diurnal
range below 0.5°C. The distributions of the magnitudes of diurnal warming across the three basins of north Indian Ocean (Arabian
Sea, Bay of Bengal and Equatorial Indian Ocean) were similar except for the differences between the Arabian Sea and the other
two basins during November–February (winter monsoon) and May. The magnitude of diurnal warming that depended on the location
of temperature sensor below the water level varied with seasons. In spring, the magnitude of diurnal warming diminished drastically
with the increase in the depth of temperature sensor. The diurnal range estimated using the drifting buoy data was higher
than the diurnal range estimated using moored buoys fitted with temperature sensors at greater depths.
A simple regression model based on the peak solar radiation and average wind speed was good enough to estimate the diurnal
range of SST at ∼1.0 m in the north Indian Ocean during most of the seasons except under low wind-high solar radiation conditions
that occur mostly during spring. The additional information on the rate of precipitation is found to be redundant for the
estimation of the magnitude of diurnal warming at those depths. 相似文献
16.
U. R. Rao P. S. Desai P. C. Joshi P. C. Pandey B. S. Gohil B. Simon 《Journal of Earth System Science》1998,107(1):33-43
Detailed analysis of the surface winds over the Indian Ocean derived from ERS-1 scatterometer data during the years 1993 and
1994 has been used to understand and unambiguously identify the onset phase of south-west monsoon. Five day (pentad) averaged
wind vectors for the period April to June during both years have been examined to study the exact reversal of wind direction
as well as the increase in wind speed over the Arabian Sea in relation to the onset of monsoon over the Indian west coast
(Kerala). The related upper level humidity available from other satellites has also been analysed.
The results of our analysis clearly show a consistent dramatic reversal in wind direction over the western Arabian Sea three
weeks in advance of the onset of monsoon. The wind speed shows a large increase coinciding with the onset of monsoon. These
findings together show the dominant role of sea surface winds in establishing the monsoon circulation. The study confirms
that the cross equatorial current phenomenon becomes more important after the onset of monsoon. 相似文献
17.
DHRUVA KUMAR PANDEY SHAILENDRA RAI A K SAHAI S ABHILASH N K SHAHI 《Journal of Earth System Science》2016,125(1):29-45
This study investigates the forecast skill and predictability of various indices of south Asian monsoon as well as the subdivisions of the Indian subcontinent during JJAS season for the time domain of 2001–2013 using NCEP CFSv2 output. It has been observed that the daily mean climatology of precipitation over the land points of India is underestimated in the model forecast as compared to observation. The monthly model bias of precipitation shows the dry bias over the land points of India and also over the Bay of Bengal, whereas the Himalayan and Arabian Sea regions show the wet bias. We have divided the Indian landmass into five subdivisions namely central India, southern India, Western Ghat, northeast and southern Bay of Bengal regions based on the spatial variation of observed mean precipitation in JJAS season. The underestimation over the land points of India during mature phase was originated from the central India, southern Bay of Bengal, southern India and Western Ghat regions. The error growth in June forecast is slower as compared to July forecast in all the regions. The predictability error also grows slowly in June forecast as compared to July forecast in most of the regions. The doubling time of predictability error was estimated to be in the range of 3–5 days for all the regions. Southern India and Western Ghats are more predictable in the July forecast as compared to June forecast, whereas IMR, northeast, central India and southern Bay of Bengal regions have the opposite nature. 相似文献
18.
Characteristics of certain surface meteorological parameters in relation to the interannual variability of Indian summer monsoon 总被引:1,自引:0,他引:1
With an objective to understand the influence of surface marine meteorological parameters in relation to the extreme monsoon activity over the Indian sub-continent leading to flood/drought, a detailed analysis of the sea level pressure over the Southern Hemisphere and various surface meteorological parameters over the Indian seas is carried out. The present study using the long term data sets (Southern Hemispheric Sea Level Pressure Analysis; Comprehensive Ocean Atmospheric Data Set over the Indian Seas; Surface Station Climatology Data) clearly indicates that the sea surface temperature changes over the south eastern Pacific (El Ninõ/La Niña) have only a moderate impact (not exceeding 50% reliability) on the Indian summer monsoon activity. On the other hand, the sea level pressure anomaly (SOI) over Australia and the south Pacific has a reasonably high degree of significance (more than 70%) with the monsoon activity over India. However, these two parameters (SLP and SST) do not show any significant variability over the Indian seas in relation to the summer monsoon activity. Over the Indian seas, the parameters which are mainly associated with the convective activity such as cloud cover, relative humidity and the surface wind were found to have a strong association with the extreme monsoon activity (flood/drought) and thus the net oceanic heat loss over the Indian seas provides a strong positive feed-back for the monsoon activity over India. 相似文献
19.
Anomalous behaviour of the Indian summer monsoon 2009 总被引:1,自引:0,他引:1
The Indian subcontinent witnessed a severe monsoon drought in the year 2009. India as a whole received 77% of its long period
average during summer monsoon season (1 June to 30 September) of 2009, which is the third highest deficient all India monsoon
season rainfall year during the period 1901–2009. Therefore, an attempt is made in this paper to study the characteristic
features of summer monsoon rainfall of 2009 over the country and to investigate some of the possible causes behind the anomalous
behaviour of the monsoon. 相似文献
20.
南亚季风降水的双极振荡* 总被引:1,自引:2,他引:1
文章利用气象资料揭示在印度半岛南部和北部,南亚季风降水变化在10年尺度以上呈翘翘板变化形式;利用更长的季风降水资料,即300年的喜马拉雅山达索普冰芯降水记录和印度半岛南部石笋降水记录,发现印度南部和喜马拉雅山季风降水呈双极振荡行为。自1700年以来,喜马拉雅山,即印度北部(或印度半岛南部)季风降水经历了1700~1764年期间的减小(或增加)趋势,1764~1876年期间的增大(或减小)趋势,1876~2000年期间的减小(或增加)趋势。同时,发现印度半岛南部的季风降水同北半球温度变化具有相同的变化特征,而喜马拉雅山季风降水同北半球温度变化具有相反的变化特征。南亚季风降水的这种南北翘翘板变化形式,与跨赤道气流有密切的联系。 相似文献