Monsoon Variability and Extreme Weather Events

Transcription

1 Monsoon Variability and Extreme Weather Events M Rajeevan National Climate Centre India Meteorological Department Pune rajeevan@imdpune.gov.in Outline of the presentation Monsoon rainfall Variability and long term trends Extreme rainfall events Links to global warming? Surface air temperature Variability and long term trends Temperature Extreme events Heat waves Tropical cyclones Severe Droughts 1

2 Precipitation Rain-gauge network INDIA has a rich network of raingauge stations, reporting rainfall. Some of the observations are from mid-1880s. Almost 1400 stations have more or less continuous rainfall data of at least 100 years ( ). The network shows large bias. Density is high over northern plains and extreme south peninsula. Over east central India, network density is very low. 2

3 Annual Rainfall Monsoon Season Rainfall About 70-90% of annual rainfall is contributed by monsoon season (June to September) rainfall. Rainfall pattern shows more rainfall along the west coast and Northeast India. West Rajasthan and extreme Southeast parts of south peninsula get the lowest rainfall. Monsoon Rainfall Time Series All India South-west Monsoon Rainfall (JUNE-SEPTEMBER) ( ) RAINFALL ( % DEPARTURE ) S.Dev -1 S.Dev YEAR All-India monsoon season rainfall time series shows NO long term trends. It is marked by large year to year variations. There is a tendency of occurrence of more droughts in some epochs (for example, , ). 3

4 Multi-decadal Variations 31-Year Moving Average Indian Summer Monsoon Rainfall Rainfall ( % Dep) Year Indian monsoon rainfall is known to exhibit significant multi-decadal variations with epochs of below and above normal rainfall. Monsoon Season Annual Trends of Sub-divisional rainfall : It shows significant reduction of annual rainfall over east central India and Kerala and increase over Maharashtra. Guhathakurta and Rajeevan

5 July August Changes / trends in contribution of July and August Rainfall in annual rainfall. There is a change in rainfall distribution. Contribution of rainfall in July has decreased and August contribution has increased. Trends of annual rainfall using 104 years of high resolution (100 km) gridded rainfall data. It shows significant reduction of annual rainfall over east central India and Kerala. 5

6 Trends of monsoon depressions Monsoon Depressions are important synoptic systems during the monsoon season, which contribute to widespread rainfall over the country. The frequency of monsoon depressions show substantial decrease during the 1980s and 1990s. This decrease was seen in spite of an increase in Sea Surface Temperature over Bay of Bengal, but possibly due to other dynamical parameters. This decrease in frequency of monsoon depressions may be the cause of rainfall over east central India. Decrease in rainfall over Kerala is not fully explained. Trends of Total Cloud cover over India Analysis of mean cloud amount shows significant decreasing trends of total cloud cover over the east central India. 6

7 26 July 2005 Mumbai Deluge Trends of Intense Precipitation One of the most significant consequences of global warming due to increase in greenhouse gases would be an increase in magnitude and frequency of extreme precipitation events. These increased extreme precipitation events can be attributed to increase in moisture levels, thunderstorm activities and large scale storm activity. Network considered to study extreme precipitation events over India U.R. Joshi and M. Rajeevan,, 2006, NCC Research Report, IMD 7

8 Trends of rainfall indices highest one day total RX1day (Left) and highest five day total RX5day (right) during the Southwest Monsoon Season (June to September). Red (green) colour indicates increasing (decreasing) trends. The above results suggest increase in intense rainfall events along the west coast and Maharashtra U.R. Joshi and M. Rajeevan,, 2006, NCC Research Report, IMD Trends of rainfall due to moderate wet days (R75ptot) (left) and Very wet days (R95ptot) (right) during the Southwest Monsoon Season (June to September). The above results suggest increase in intense rainfall events along the west coast and Maharashtra. U.R. Joshi and M. Rajeevan,, 2006, NCC Research Report, IMD 8

9 Extreme rainfall events Goswami et al. (2006, Science) using high resolution gridded rainfall data of IMD examined long term trends of extreme rainfall events over India. They found that frequency of intense rainfall events (more than 15 cm) has increased over a large area of Central India. However, frequency of moderate rainfall events (less than 15 cm) had decreased, so that the annual rainfall does not show any significant trend. Goswami et al. 2006, Science JUNE-SEPT SST ANOMALIES AND HEAVY RAINFALL FREQUENCY OVER CENTRAL INDIA ( ) ANO MALY( o C) SST ANOMALY HEAVY R/F FREQUENCY SST AREA : 20 o S-20 o N, 50 o E- 100 o E FREQ UENCY YEAR Frequency of intense rainfall events (Green) shows an increasing trend. Similar trend has been noted for the Sea surface Temperatures (Red) over the tropical Indian Ocean. The 21-year smoothed curves (blue) show a similar pattern, suggesting that the increase in intense rainfall events may be linked to the increase in SST over the tropical Indian Ocean 9

10 Correlation between the frequency of rainfall events over central India during the monsoon season and global sea surface temperatures. The spatial pattern suggests significant correlation over the tropical Indian Ocean, suggesting the tropical Indian ocean may be responsible for the observed increase in intense rainfall events. Trends over the North Indian Ocean There are appreciable changes over the north Indian Ocean also. SSTs have increased, Specific humidity and Latent heat flux (evaporation) have increased. These changes may be responsible for the increasing trend of intense precipitation events over India. More studies are required for linking the changes in the extreme events to global warming. Wind Speed (Meter/Sec) Wind Speed - Indian Ocean between 50oE to 100oE Year 10Sx10N Linear (10Sx10N) Sp. Humidity Anomaly - Indian Ocean between 50 o E to 100 o E Late nt H ea t Flux ( W /m 2 ) gm/kgm Sx10N Linear (10Sx10N) Latent Heat Flux Anomaly - Indian Ocean between 50 o E to 100 o E Sx10N Linear (10Sx10N) 10

11 Monsoon Circulation Variation of Mean Monsoon Wind Strength Wind Speed (m/sec) Year There is a small decreasing trend of strength of monsoon circulation. Strength of monsoon circulation is determined as the area averaged wind speed at 1.5 Km over southern parts of India/north Indian Ocean. This could be due to artifact of the reanalysis data, may not indicate the real change in monsoon circulation Monsoon Breaks 11

12 45 NUMBER OF BREAK DAYS FOR JUNE-SEPT ( ) C.C. WITH ISMR = BREAK DAYS YEAR During the monsoon season, there are significant variations with suppressed rainfall activity (breaks) and enhanced rainfall activity (active). The analysis of time series of break days for the period shows there is no appreciable trend in the number of break days, confirming that there is no significant trend in the monsoon circulation. Temperature 12

13 IMD has developed a long term data set of monthly temperature over India ( ) and this data set is updated every year operationally. The analysis of past data suggests temperatures over India have increased uniformly, except over small regions of Rajasthan and Bihar. Most of the warming over India has caused due to increase in Maximum Temperatures. However, during the recent years, minimum temperatures showed some rapid warming suggesting an influence of increase in greenhouse gases. 13

14 9 POINT BINOMIAL FILTER TREND= C/100 YEARS TEMP. ANOMALIES ( 0 C) Y E A R S The time series of annual mean temperature averaged over India for the period The year 2002 was the warmest year and 2006 was the second warmest was the fourth warmest year on record. HEAT WAVES Heat waves are extreme weather events having adverse impact on human health. In 2003, a severe heat wave occurred over Central Europe killing thousands of people. It exposed the serious problems in health care in developed countries like France. Later modeling studies suggested this heat wave could be a part of increasing global warming caused by human activities 14

15 Frequency of Heat Wave Days During Pre monsoon Season over Central and NW India Frequency of Days with Max Temp More than 45C Over Central and NW India Average Days Average Days Year Year Along with the increase in annual temperatures, frequency of heat waves over central India and NW India also has increased. Left: Time series of frequency of heat wave days over central and NW India has increased significantly over the period Right: The time series of frequency of days with maximum temperatures more than 45C has increased significantly. Analysis of past data of Tropical cyclones over the north Indian Ocean shows a small decreasing trend. The time series is however marked by significant multi-decadal variations. 15

16 In India, there is no study showing the trend of aridity/drought. The analysis of Dai et al (2004) (right) shows an increase in severity of droughts over most of northern parts of India. This is due to increase in annual temperatures and decrease in monsoon rainfall. More detailed analysis is required to establish this result. An increase in vegetation cover? Normalized Difference Vegetation Index (NDVI) =(CH2-CH1)/ (CH2+CH1) CH1 Visible Radiation CH2 Near IR Radiation NOAA satellite derived NDVI shows a large increase in NDVI suggesting increase in vegetation over north peninsula and NW India around Rajasthan. 16

17 IPCC Projections of future climate Regional model projections indicate likely increase in precipitation during June-July-August. The models also suggest an increase in extreme precipitation over India during the same period. 17

18 With increased greenhouse gases, surface air temperatures are likely to increase, more in winter season. With the increased global warming, precipitation is likely to increase, especially during summer monsoon season. Most of the models predict an increase in precipitation over India Models are predicting a late withdrawal of monsoon from India, or prolonged monsoon. 18

19 Most of the climate models participated in climate projections show good skill in reproducing the annual cycle of temperature and precipitation over India, thus giving more confidence in the climate model projections. Conclusions Monsoon rainfall over the country has not shown any long term trends, but shown significant multi-decadal variations. However, rainfall over some specific regions (like east central India and Kerala) ) showed decreasing trend. This decreasing trend is also supported by similar changes in cloud cover. This observed trend over east central India may be due to decrease in monsoon depressions forming over Bay of Bengal. 19

20 Conclusions contd Studies have shown increase in extreme rainfall events over central India and along the west coast. The observed increase in extreme events may be caused due to increase in Sea surface temperatures (global warming?) and associated changes in evaporative fluxes over the north Indian ocean. There is no significant trend in monsoon circulation and also frequency of break days during the monsoon season. Conclusions Annual mean surface air temperatures over India have shown an increasing trend (0.5 C for 100 years). Most of the warming is caused due to increase in maximum temperatures. However, during the recent years, minimum temperatures also shown a rapid warming. In association with the increase in surface air temperatures over India, frequency of extreme temperature events (heat wave) has increased over central and NW India. Frequency of tropical cyclones over the north Indian Ocean has not n shown any significant trends. There is no study on long term trends of severity of droughts over India. A recent study has however shown an increase in the severity of droughts over India. 20

21 Conclusions: Climate change projections Climate models suggest an increase in mean surface air temperatures over India. The warming is more dominant during winter season. Models suggest fewer colder nights in future, as minimum temperatures are likely to increase more. Climate models also suggest an increase in precipitation and extreme precipitation events over India during summer monsoon season. This means more floods to come. Extreme rainfall and winds associated with tropical cyclones are likely to increase. Thank you 21