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1.
An Ocean-Atmosphere Index (OAI) for ENSO is developed using data on Southern Oscillation Index (SOI) and sea surface temperature
(SST) over eastern equatorial Pacific. Seasonal values of OAI, SOI and SST have been computed for the seasons September-October-November
(SON), December-January-February (DJF), March-April-May (MAM) and June-July-August (JJA). Similarly SON to DJF, DJF to MAM,
MAM to JJA and JJA to SON tendencies have been worked out for SOI, SST and OAI. The relationships between Indian Monsoon Rainfall
(IMR) and SOI/SST/OAI, (i) for the seasons SON, DJF and MAM before and after the monsoon and JJA concurrent with the monsoon
and (ii) for SON to DJF and DJF to MAM tendencies before and after the monsoon, and MAM to JJA tendency concurrent with the
monsoon have been explored. It is found that IMR is more influenced by SOI before the monsoon than it is influenced by SST
before the monsoon and IMR affects SST after monsoon more strongly than it affects SOI after the monsoon. It is also observed
that DJF to MAM tendencies for SOI, SST and OAI before monsoon are significantly related to IMR, among which the relationship
between IMR and DJF to MAM tendency for OAI is the best. 相似文献
2.
Homogeneous Indian Monsoon rainfall: Variability and prediction 总被引：1，自引：0，他引：1
The Indian summer monsoon rainfall is known to have considerable spatial variability, which imposes some limitations on the
all-India mean widely used at present. To prepare a spatially coherent monsoon rainfall series for the largest possible area,
fourteen subdivisions covering the northwestern and central parts of India (about 55% of the total area of the country), having
similar rainfall characteristics and associations with regional/global circulation parameters are merged and their area-weighted
means computed, to form monthly and seasonal Homogeneous Indian Monsoon (HIM) rainfall series for the period 1871–1990. This
paper includes a listing of monthly and seasonal rainfall of HIM region. HIM rainfall series has been statistically analysed
to understand its characteristics, variability and teleconnections for long-range prediction.
HIM rainfall series isfound to be homogeneous, Gaussian distributed and free from persistence. The mean (R) rainfall is 757
mm (87% of annual) and standard deviation (S) 119 mm, with a Coefficient of Variation (CV) of 16%. There were 21 dry (K, -<R S) and 19 wet (R
i R + S) years during 1871–1990. There were clusters of frequent negative departures during 1899–1920 and 1965–1987 and positive
departures during 1942–1961. The recent three decades show very high rainfall variability with 10 dry and 6 wet years. The
decadal averages were alternatively positive and negative for three consecutive decades, viz., 1871–1900 (positive); 1901–1930
(negative); 1931–1960 (positive) and 1961–1990 (negative) respectively. Significant QBO and autocorrelation at 14th lag have
been found in HIM rainfall series.
To delineate the changes in the climatic regime of the Indian summer monsoon, sliding correlation coefficients (CCs) between
HIM rainfall series and (i) Bombay msl pressure, (ii) Darwin msl pressure and (iii) Northern Hemisphere surface air temperature
over the period 1871–1990 have been examined. The 31-year sliding CCs showed the systematic turning points of positive and
negative CCs around the years, 1900 and 1940. In the light of other corroborative evidences, these turning points seem to
delineate ‘meridional’ monsoon regime during 1871–1900 and 1940–1990 and ‘zonal’ monsoon regime during 1901–1940. The monsoon
signal is particularly dominant in many regional and global circulation parameters, during 1951–1990.
Using the teleconnections ofHIM series with 12 regional/global circulation parameters during the recent 36-year period 1951–86 regression models have been
developed for long-range prediction. In the regression equations 3 to 4 parameters were entered, explaining upto 80% of the
variance, depending upon the data period. The parameters that prominently enter the multiple regression equations are (i)
Bombay msl pressure, (ii) April 500 mb Ridge at 75°E, (iii) NH temperature, (iv) Nouvelle minus Agalega msl pressure and (v)
South American msl pressure. Eleven circulation parameters for the period 1951–80 were subjected to Principal Component Analysis
(PCA) and the PC’s were used in the regression model to estimate HIM rainfall. The multiple regression with three PCs explain
72% of variance in HIM rainfall. 相似文献
3.
Indian Monsoon Variability in a Global Warming Scenario 总被引：4，自引：0，他引：4
The Intergovernmental Panel on Climate Change (IPCC) constituted by the World Meteorological Organisation provides expert guidance regarding scientific and technical aspects of the climate problem. Since 1990 IPCC has, at five-yearlyintervals, assessedand reported on the current state of knowledge and understanding of the climate issue. These reports have projected the behaviour of the Asian monsoon in the warming world. While the IPCC Second Assessment Report (IPCC, 1996) on climate model projections of Asian/Indian monsoon stated ``Most climate models produce more rainfall over South Asia in a warmer climate with increasing CO2', the recent IPCC (2001) Third Assessment Report states ``It is likely that the warming associated with increasing greenhouse gas concentrations will cause an increase in Asian summer monsoon variability and changes in monsoon strength.'Climate model projections(IPCC, 2001) also suggest more El Niño – like events in the tropical Pacific, increase in surface temperatures and decrease in the northern hemisphere snow cover. The Indian Monsoon is an important component of the Asian monsoon and its links with the El Niño Southern Oscillation (ENSO) phenomenon, northern hemisphere surface temperature and Eurasian snow are well documented.In the light of the IPCC globalwarming projections on the Asian monsoon, the interannual and decadal variability in summer monsoon rainfall over India and its teleconnections have been examined by using observed data for the 131-year (1871–2001) period. While the interannual variations showyear-to-year random fluctuations, thedecadal variations reveal distinct alternate epochs of above and below normal rainfall. The epochs tend to last for about three decades. There is no clear evidence to suggest that the strength and variability of the Indian Monsoon Rainfall (IMR) nor the epochal changes are affected by the global warming. Though the 1990s have been the warmest decade of the millennium(IPCC, 2001), the IMR variability has decreased drastically.Connections between the ENSO phenomenon, Northern Hemisphere surface temperature and the Eurasian snow with IMR reveal that the correlations are not only weak but have changed signs in the early 1990s suggesting that the IMR has delinked not only with the Pacific but with the Northern Hemisphere/Eurasian continent also. The fact that temperature/snow relationships with IMR are weak further suggests that global warming need not be a cause for the recent ENSO-Monsoon weakening.Observed snow depth over theEurasian continent has been increasing, which could be a result of enhanced precipitation due to the global warming. 相似文献
4.
Temporal distribution of southwest monsoon (June –September) rainfall is very useful for the country’s agriculture and food
grain production. It contributes more than 75% of India’s annual rainfall. In view of this, an attempt has been made here
to understand the performance of the monthly rainfall for June, July, August and September when the seasonal rainfall is reported
as excess, deficient or normal. To know the dependence of seasonal rainfall on monthly rainfall, the probabilities of occurrence
of excess, deficient and normal monsoon when June, July, August and also June + July and August + September rainfall is reported
to be excess or deficient, are worked out using the long homogenous series of 124 years (1871-–1994) data of monthly and seasonal
rainfall of 29 meteorological sub-divisions of the plain regions of India.
In excess monsoon years, the average percentage contribution of each monsoon month to the long term mean (1871–1994) seasonal
rainfall (June –September) is more than that of the normal while in the deficient years it is less than normal. This is noticed
in all 29 meteorological sub-divisions. From the probability analysis, it is seen that there is a rare possibility of occurrence
of seasonal rainfall to be excess/deficient when the monthly rainfall of any month is deficient/excess. 相似文献
5.
Link between convection and meridional gradient of sea surface temperature in the Bay of Bengal 总被引：2，自引：0，他引：2
We use daily satellite estimates of sea surface temperature (SST) and rainfall during 1998–2005 to show that onset of convection
over the central Bay of Bengal (88–92°E, 14–18°N) during the core summer monsoon (mid-May to September) is linked to the meridional
gradient of SST in the bay. The SST gradient was computed between two boxes in the northern (88–92°E, 18–22°N) and southern
(82–88°E, 4–8°N) bay; the latter is the area of the cold tongue in the bay linked to the Summer Monsoon Current. Convection
over central bay followed the SST difference between the northern and southern bay (ΔT) exceeding 0.75°C in 28 cases. There was no instance of ΔT exceeding this threshold without a burst in convection. There were, however, five instances of convection occurring without
this SST gradient. Long rainfall events (events lasting more than a week) were associated with an SST event (ΔT ≥ 0.75°C); rainfall events tended to be short when not associated with an SST event. The SST gradient was important for the
onset of convection, but not for its persistence: convection often persisted for several days even after the SST gradient
weakened. The lag between ΔT exceeding 0.75°C and the onset of convection was 0–18 days, but the lag histogram peaked at one week. In 75% of the 28 cases,
convection occurred within a week of ΔT exceeding the threshold of 0.75°C. The northern bay SST, T
N
, contributed more to ΔT, but it was a weaker criterion for convection than the SST gradient. A sensitivity analysis showed that the corresponding
threshold for T
N
was 29°C. We hypothesise that the excess heating (∼1°C above the threshold for deep convection) required in the northern
bay to trigger convection is because this excess in SST is what is required to establish the critical SST gradient. 相似文献
6.
The predictability of Indian summer monsoon rainfall from pre-season circulation indices is explored from observations during
1939–91. The predictand is the all-India average of June–September precipitation NIR, and the precursors examined are the
latitude position of the 500 mb ridge along 75°E in April (L), the pressure tendency April minus January at Darwin (DPT),
March-April-May temperature at six stations in west central India (T6), the sea surface temperature (SST) anomaly in the northeastern
Arabian Sea in May (ASM), SST anomaly in the Arabian Sea in January (ANJ), northern hemisphere temperature anomaly in January–February
(NHT), and Eurasian snow cover in January (SNOW). Monsoon rainfall tends to be enhanced with a more northerly ridge position,
small Darwin pressure tendency, warmer pre-season conditions, and reduced winter snow cover. However, relationships have varied
considerably over the past half-century, with the strongest associations during 1950–80, and a drastic weakening in the 1980s.
Four prediction models were constructed based on stepwise multiple regression, using as predictors combinations of L, DPT,
T6, ASM, and NHT, with 1939–68 as “dependent” dataset, or training period, and 1969–91 as “independent” dataset or verification
period. For the 1969–80 portion of the verification period calculated and observed NIR values agreed closely, with the models
explaining 74–79% of the variance. By contrast, after 1980 predictions deteriorated drastically, with the explained variance
for the 1969–89 time span dropping to 25–31%. The monsoon rainfall of 1990 and 1991 turned out to be again highly predictable
from models based on stepwise multiple regression and linear discriminant analysis and using as input L + DPT or L + DPT +
NHT, and with this encouragement an experimental real-time forecast was issued of the 1992 monsoon rainfall.
These results underline the need for investigations into decadal-scale changes in the general circulation setting and raise
concern for the continued success of seasonal forecasting. 相似文献
7.
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
8.
The interannual variability of all-India summer monsoon (June to September) rainfall and its teleconnections with the southern
oscillation index (SOI) and sea surface temperature (SST) anomaly of the eastern equatorial Pacific ocean have been examined for the period 1871–1978 for different seasons (i.e.,
winter, spring, summer and autumn). The relationship (correlation coefficient) between all-India summer monsoon rainfall andSOI for different seasons is positive and highly significant. Further examination of 10-, 20- and 30-year sliding window lengths’
correlations, brings out the highly consistent and significant character of the relationships. The relationship between all-India
monsoon rainfall andSST for different seasons is negative and is significant at 1 % level or above. Drought years are characterised by negative anomalies
ofSOI and positive anomalies ofSST and vice versa with flood years. The relationship betweenSOI andSST is negative and significant at 0.1 % level.
The relationships between all-India summer monsoon rainfall,SOI and sst are expected to improve our understanding of the interannual variability of the summer monsoon. 相似文献
9.
Using a historical database (1952–2007) of sea surface temperature (SST) from a subtropical high-controlled area (110°E–140°E,
15°N–35°N) of the west Pacific Ocean and the precipitation over Hunan Province of southeast China, we analyzed time series
variations of precipitation in relation to the East Asian summer monsoon and a global warming setting. The results show that
there has been a significant increase in SST of the subtropical high-controlled area in the recent 50 years. Although the
increase in annual summer monsoon precipitation during the same period has been subtle over Hunan province, seasonal rainfall
distribution has obviously changed, represented by a reduction in May, but a significant increase through June to August,
especially in July. We suggest that the mechanism of seasonal redistribution of monsoon precipitation is primarily due to
the increasing SST that delays the intrusion of the west Pacific Subtropical High, therefore leading to a postponing of migration
of the East Asian summer monsoon rainfall belt inland and northward. 相似文献
10.
Nityanand Singh 《Journal of Earth System Science》1995,104(1):1-36
Large-scale interannual variability of the northern summer southwest monsoon over India is studied by examining its variation
in the dry area during the period 1871–1984. On the mean summer monsoon rainfall (June to September total) chart the 800 mm
isohyet divides the country into two nearly equal halves, named as dry area (monsoon rainfall less than 800 mm) and wet area
(monsoon rainfall greater than 800 mm). The dry area/wet area shows large variations from one year to another, and is considered
as an index for assessing the large-scale performance of the Indian summer monsoon. Statistical and fluctuation characteristics
of the summer monsoon dry area (SMDA) are reported.
To identify possible causes of variation in the Indian summer monsoon, the correlation between the summer monsoon dry area
and eleven regional/global circulation parameters is examined. The northern hemisphere surface air temperature, zonal/hemispheric/global
surface air and upper air temperatures, Southern Oscillation, Quasi-biennial oscillation of the equatorial lower stratosphere,
April 500-mb ridge along 75°E over India, the Indian surface air temperature and the Bombay sea level pressure showed significant
correlation.
A new predictor parameter that is preceding year mean monsoon rainfall of a few selected stations over India has been suggested
in the present study. The stations have been selected by applying the objective technique ‘selecting a subset of few gauges
whose mean monsoon rainfall of the preceding year has shown the highest correlation coefficient (CC) with the SMDA’. Bankura
(Gangetic West Bengal), Cuddalore (Tamil Nadu) and Anupgarh (West Rajasthan) entered the selection showing a CC of 0.724.
Using a dependent sample of 1951–1980 a predictive model (multiple CC = 0.745) has also been developed for the SMDA with preceding
year mean monsoon rainfall of the three selected stations and the sea level pressure tendency at Darwin from Jan–Feb to Mar–May
as independent parameters. 相似文献
11.
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
12.
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
13.
The time series of Indian summer monsoon rainfall for the period 1871–1989 has been analysed using the method of deterministic
chaos. It is found that a strange attractor underlies the time series implying the existence of a prediction function. This
function has been approximated by a second-degree polynomial, involving the rainfalls of the past seven years and the coefficients
have been estimated by least squares fit. The interannual variations of actual and computed rainfalls have been presented
for a comparative study. 相似文献
14.
V. V. S. S. Sarma M. Dileep Kumar M. Gauns M. Madhupratap 《Journal of Earth System Science》2000,109(4):471-479
The variability in partial pressure of carbon dioxide (pCO2) and its control by biological and physical processes in the mixed layer (ML) of the central and eastern Arabian Sea during
inter-monsoon, northeast monsoon, and southwest monsoon seasons were studied. The ML varied from 80–120 m during NE monsoon,
60–80 m and 20–30 m during SW- and inter-monsoon seasons, respectively, and the variability resulted from different physical
processes. Significant seasonal variability was found in pCO2 levels. During SW monsoon, coastal waters contain two contrasting regimes; (a) pCO2 levels of 520–685 μatm were observed in the SW coast of India, the highest found so far from this region, driven by intense
upwelling and (b) low levels of pCO2 (266 μatm) were found associated with monsoonal fresh water influx. It varied in ranges of 416–527 μatm and 375–446 μatm
during inter- and NE monsoon, respectively, in coastal waters with higher values occurring in the north. The central Arabian
Sea pCO2 levels were 351–433, 379–475 and 385–432 μatm during NE-inter and SW monsoon seasons, respectively. The mixed layer pCO2 relations with temperature, oxygen, chlorophylla and primary production revealed that the former is largely regulated by physical processes during SW- and NE monsoon whereas
both physical and biological processes are important in inter-monsoon. Application of Louanchiet al (1996) model revealed that the mixing effect is the dominant during monsoons, however, the biological effect is equally significant
during SW monsoon whereas thermodynamics and fluxes influence during inter-monsoons. 相似文献
15.
Wenguang Yang Hongbo Zheng Ke Wang Xin Xie Guocheng Chen Xi Mei 《Frontiers of Earth Science》2008,2(2):170-176
Sediments with high sedimentation rate at site MD05-2905 in the Northeastern slope of the South China Sea provide unique materials
for a high-resolution study on the paleoenvironment. Based on precise dating of AMS 14C, grain size analysis of terrigenous debris at core MD05-2905 was conducted after organic matter, biological carbonate and
biogenic opal were removed. The results show that 15.5–63.5 μm coarse grain size ingredients may indicate East Asian winter
monsoon changes and that 2–9 μm fine grain size ingredients may be used as a proxy of evolution of the East Asian summer monsoon.
The results of grain size analysis, which suggest East Asian monsoon intensity, reveal that a winter monsoon dominated the
glacial regime and a summer monsoon dominated the Holocene regime. It was also shown that the summer monsoon increased gradually,
experienced several abrupt changes and reached a culmination in the early Holocene (11200–8500 a B.P.) since 36 ka. Controlled
by precession periodicity, it may be related with the amount of solar radiation at the highest stage, which needs further
study.
__________
Translated from Advances in Earth Science, 2007, 22(10): 1012–1018 [译自: 地球科学进展] 相似文献
16.
The impact of Southern Oscillation on thecyclogenesis over the Bay of Bengal duringthe summer monsoon has been investigated.The analysis of correlation coefficients(CCs) between the frequency of monsoondepressions and the Southern OscillationIndex (SOI) reveals that more depressionsform during July and August of El Niñoyears. Due to this, the seasonal frequencyof monsoon depressions remains little higherduring El Niño epochs even though thecorrelations for June and September are notsignificant. The CCs for July and August aresignificant at the 99% level.The El Niño-Southern Oscillation (ENSO)is known to affect Indian MonsoonRainfall (IMR) adversely. The enhancedcyclogenesis over the Bay of Bengal duringJuly and August is an impact of ENSO whichneeds to be examined closely. Increasedcyclogenesis over the Bay of Bengal may bereducing the deficiency in IMR duringEl Niño years by producing more rainfallover the eastern parts of India duringJuly and August. Thus there is a considerablespatial variation in the impact of ENSOon the monsoon rainfall over India and El Niñoneed not necessarily imply a monsoonfailure everywhere in India.The area of formation of monsoon depressionsshifts eastward during El Niño years.Warmer sea surface temperature (SST) anomaliesprevail over northwest and adjoiningwestcentral Bay of Bengal during premonsoon andmonsoon seasons of El Niño years.May minus March SOI can provide useful predictionsof monsoon depression frequencyduring July and August. 相似文献
17.
P. V. Joseph Anu Simon Venu G. Nair Aype Thomas 《Journal of Earth System Science》2004,113(2):139-150
Time series of daily averaged rainfall of about 40 rain gauge stations of south Kerala, situated at the southern-most part
of peninsular India between latitudes about 8‡N and 10‡N were subjected to Wavelet Analysis to study the Intra Seasonal Oscillation
(ISO) in the rainfall and its inter-annual variability. Of the 128 days, 29th May to 3rd October of each of the 95 years 1901-1995
were analysed. We find that the period of ISO does not vary during a monsoon season in most of the years, but it has large
inter-annual variability in the range 23 to 64 days. Period-wise, the years cluster into two groups of ISO, the SHORT consisting
of periods 23, 27 and 32 days and the LONG with a single period of 64 days, both the sets at a significance level of 99%.
During the 95 years at this level of significance there are 44 years with SHORT and 20 years with LONG periods. 11 years have
no ISO even at the 90% level of significance.
We composited NCEP SST anomalies of the summer monsoon season June to September for two groups of years during the period
1965–1993. The first group is of 5 years with a LONG ISO period of 64 days for south Kerala rainfall at significance level
of 99% and the second group is of 12 years with SHORT ISO periods of 23, 27 and 32 days at the same level of significance.
The SST anomaly for the LONG (SHORT) ISO resembles that for an El Nino (La Nina). 相似文献
18.
Analysis of fifty four (1951–2004) years of daily energetics of zonal waves derived from NCEP/NCAR wind (u and υ) data and daily rainfall received over the Indian landmass (real time data) during southwest monsoon season (1 June–30 September)
indicate that energetics (momentum transport and kinetic energy) of lower tropospheric ultra-long waves (waves 1 and 2) of
low latitudes hold a key to intra-seasonal variability of monsoon rainfall over India.
Correlation coefficient between climatology of daily (122 days) energetics of ultra-long waves and climatology of daily rainfall
over Indian landmass is 0.9. The relation is not only significant but also has a predictive potential. The normalised plot
of both the series clearly indicates that the response period of rainfall to the energetics is of 5–10 days during the onset
phase and 4–7 days during the withdrawal phase of monsoon over India. During the established phase of monsoon, both the series
move hand-in-hand. Normalised plot of energetics of ultra-long waves and rainfall for individual year do not show marked deviation
with respect to climatology. These results are first of its kind and are useful for the short range forecast of rainfall over
India. 相似文献
19.
G. S. Bhat 《Journal of Earth System Science》2003,112(2):131-146
This paper describes the near surface characteristics and vertical variations based on the observations made at 17.5‡N and
89‡E from ORV Sagar Kanya in the north Bay of Bengal during the Bay of Bengal Monsoon Experiment (BOBMEX) carried out in July–August
1999. BOBMEX captured both the active and weak phases of convection. SST remained above the convection threshold throughout
the BOBMEX. While the response of the SST to atmospheric forcing was clearly observed, the response of the atmosphere to SST
changes was not clear. SST decreased during periods of large scale precipitation, and increased during a weak phase of convection.
It is shown that the latent heat flux at comparable wind speeds was about 25–50% lower over the Bay during BOBMEX compared
to that over the Indian Ocean during other seasons and tropical west Pacific. On the other hand, the largest variations in
the surface daily net heat flux are observed over the Bay during BOBMEX. SST predicted using observed surface fluxes showed
that 1-D heat balance model works sometime but not always, and horizontal advection is important. The high resolution Vaisala
radiosondes launched during BOBMEX could clearly bring out the changes in the vertical structure of the atmosphere between
active and weak phases of convection. Convective Available Potential Energy of the surface air decreased by 2–3 kJ kg-1 following convection, and recovered in a time period of one or two days. The mid tropospheric relative humidity and water
vapor content, and wind direction show the major changes between the active and weak phases of convection. 相似文献
20.
Some statistical properties of the summer monsoon seasonal rainfall for India during the last 100 years (1881–1980) are presented.
The most recent decade of 1971–1980 shows the lowest value of standard-decadal average monsoon rainfall (86.40 cm) and is
also characterised by the second highest value of coefficient of variation in monsoon rainfall (12.4 %). The combined last
two standard-decadal period of 1961–1980 was the period of the largest coefficient of variation and the lowest average monsoon
rainfall for India.
The possible influence of global climatic variability on the performance of the monsoon is also examined. Analyses of correlation
coefficient show that a statistically significant positive relationship with a time-lag of about six months exists between
monsoon rainfall and northern hemispheric surface air temperature. A cooler northern hemisphere during January/February leads
to a poor monsoon.
All the major drought years during the last 3 decades had much cooler January/February periods over the northern hemisphere—1972
having the coldest January/February with a temperature departure of −0.94°C and the most disastrous monsoon failure. 相似文献