Mean Sea Level Pressure and Wind Climatology over the North Indian Ocean: Quality control, Validation and Biases M. Rajeevan and S.K.Dikshit India Meteorological Department Pune. India
Introduction India is a WMO responsible country for the Marine Climatological work. So far, IMD had published: annual summaries for the period 1961-1970 decadal summaries for the period 1971-1980. Recently, IMD completed a Marine Climatological Atlas of Indian Ocean using marine data of 1961-1900.
List of parameters Air Temperature Sea surface temperature Air-Sea surface temp difference Dew Point Temperature MSL Pressure Wind Direction Wind Speed Gale Wind Low Cloud amount Total Cloud amount Mean Wave Height Wave Height > 4.0 m Maximum Wave Height Mean Wave Period Visibility
Data Sources The data used to prepare the Atlas were obtained from two sources: Observations recorded in the meteorological log books by Indian Voluntary Observing Fleet. Observations made by other ships in the Indian area of responsibility and sent to IMD by other WMO members in the IMMPC/IMMT formats. All the observations available with IMD for the period 1961-1990 have been used for the preparation.
Quality Control Data were scrutinized to eliminate instrumental, positional and coding errors. Data were further subjected to quality control as per the WMO guidelines Duplicate observations were eliminated. Internal consistency checks were done. Extreme value checks were done.
Analysis Methods The area of responsibility is divided into boxes with constant grid spacing of 2.5 degree in latitude and longitude. All available quality controlled observations were averaged in each boxes for each month during the 30 year period. The raw monthly mean fields were then objectively analyzed to filter out spatial noise.
Analysis Methods The objective analysis scheme is an iterative differencecorrection scheme with a weight function. The Barnes weight function is defined as W= exp ( -4 r 2 R -2 ) for r < R Where r is the distance between the grid point and analysis grid point. R is the radius of influence. It was decreased with each pass in order to analyze smaller scale features with each successive iteration.
Analysis Method Resultant Climatology of MSL Pressure and winds was compared with other data sets. The data sets considered for comparison are: NCEP/NCAR reanalysis UK Met office GMSLP SSMI wind speed
NO. Records 1000000 900000 800000 700000 600000 500000 400000 300000 200000 100000 0 Korea 73 Brazil 294 Uganda 372 Tanzania 576 Denmark 971 Thailand 1012 Country Ireland Kenya South Africa Croatia New Zealand Sweden Norway Malaysia Belgium Portugal Canada Singapore GDR Pakistan Israel Australia Hong Kong Yogoslovia 1305 1667 1875 2391 2875 4055 5107 5503 5999 14437 15619 16367 19554 20723 25001 25211 27549 28371 Number of Observations contributed by WMO Members Poland 56432 Russia 84195 Japan 110149 France 233794 U.S.A 252530 India 333434 Germany 418131 Netherlands 575575 United Kingdom 652142
Data Records 160000 140000 120000 100000 80000 60000 40000 20000 0 1961 1962 1963 1964 1965 1966 1967 1968 1969 Year 1970 1971 1972 1973 1974 1975 1976 1977 1978 Number of Data Records 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990
MSL Pressure
MSL Pressure : Winter
MSL Pressure : Monsoon Season
MSL Pressure : Monsoon Season
Comparison of MCLIM and GMSLP DATA 1025 1020 y = 0.834x + 168.39 R 2 = 0.8955 Bias = -0.7 hpa GMLS Pressure ( hpa) 1015 1010 1005 1000 995 995 1000 1005 1010 1015 1020 1025 MCLIM Pressure ( hpa)
Wind
Wind Speed : Monsoon Season
Wind Speed : Annual
Wind Speed: MCLIM VS NCEP West Arabian Sea Wind Speed (50-60E, 5-12.5N) MCLIM Vs NCEP 16.0 14.0 MCLIM NCEP 12.0 Wind Speed (m/sec) 10.0 8.0 6.0 4.0 2.0 0.0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month
Zonal Wind Stress : Monsoon Season
Meridional Wind Stress : Monsoon Season
Wind Speed : MCLIM Vs SSMI
Indian Ocean Warming and Associated Changes
SST Difference : 1961/75 1976/90
Wind Speed Difference : 1961/75 1976/90
Total Cloud Difference : 1961/75 1976/90
Conclusions MCLIM Pressure Gradient was stronger than the pressure gradient in NCEP reanalysis. Absolute differences are of the order of 0.5 to 1 hpa. MCLIM pressures are systematically smaller than the GMSL pressure by about 1 hpa. Differences in wind speed are consistent with the pressure differences. Somali Jet is stronger in NCEP data compared to MCLIM data. Somali Jet is stronger in SSMI data compared to MCLIM data. Interesting and physically consistent changes have been observed in wind speed and cloud associated with the Indian Ocean warming in Mid 70s.