AMIP II Diagnostic Subproject No. 6:
Methodology and Validation
A realistic simulation of seasonal mean, the intraseasonal variability (ISV) and interannual variability (IAV) of Asian summer monsoon is a prerequiste for Atmospheric General Circulation Models (AGCMs) if they are to be used to predict or to understand the tropical climate and its variability (WCRP, 1992, 1993). The aims of AMIP are (i) identification of systematic errors in AGCMs and (ii) improvements in model formulations. The physical processes that govern the mean and variability of the monsoon are complex and interactive. In the present proposal, we focus only on two main themes:
- (i) Role of ENSO vs Tropospheric
biennial oscillation (TBO) variability
(ii) Relationship between intraseasonal and interannual variability
More than 70 years ago, Walker (1924) recognised that there might be a simultaneous relationship between ENSO and the monsoon. However, recent studies indicate that the influence of ENSO on the monsoon is modest and possibly not the dominating factor (Wesbter and Yang, 1992, Soman and Slingo, 1997, Annamalai and Goswami, 1997). At interannual time scales, the observed monsoon precipitation over India and China is dominated by quasi-biennial variability associated with the TBO rather than the lower frequency ENSO, which has a time scale of 4-5 years (Annamalai, 1995, Shen and Lau, 1995). The TBO is gradually emerging as an important mode of variability at interannual timescales. It is increasingly evident that the Asian summer monsoon plays an active, rather than passive role in both the TBO and in El Nino. A specific requirement for CLIVAR-GOALS is to understand the connection between these two major players in the global circulation.
During the monsoon season, there are two preferred locations of the tropical convergence zone (TCZ), one over the continent and the other over the warm waters of the equatorial Indian Ocean (Sikka and Gadgil, 1980). These two TCZs fluctuate at intraseasonal time scales of 30-50 days. Also, the continental TCZ oscillates with a time scale of 10-20 days (Krishnamurti and Bhalme, 1976). Due to the low frequency nature of these variations, it may be expected that their statistics (frequency, mean amplitude etc) would influence the seasonal mean and hence the IAV. Some recent modelling studies support this perspective where the spatial patterns of intraseasonal and interannual variability of the simulated monsoon are very similar (Ferranti et al. 1997).
Focusing on these two broad themes, the present proposal will explore the role of ENSO vs TBO variability on the monsoon, and the relationship between ISV and IAV of the monsoon. Recognizing that the regional characteristics of monsoon variability are complex (e.g. inverse relationship of Indian vs. South East Asian rainfall with ENSO), this subproject will primarily focus on the Asian summer monsoon system as a whole. Where appropriate specific aspects of the Indian monsoon will also be studied.
- 1) Assess the simulation of mean,
interannual and intraseasonal variability of the Asian monsoon during the
northern summer (May-September).
2) Quantify the variance accounted
for by the TBO and ENSO variability over the Asian monsoon domain. Assess
the phase relationship between ENSO and TBO, and their nonlinear interaction
in modulating the monsoon.
3) Investigate the models' ability to simulate the precursory signals during the pre-monsoon seasons.
4) Examine the relationship between
the dominant modes of intraseasonal and interannual variability.
5) Investigate the role of intraseasonal variability in modulating the seasonal mean monsoon.
The mean evolution of the Asian summer monsoon will be described using the monthly means of different variables at selected pressure levels available in the AMIP II standard output. The diagnostics will concentrate on the evolution of lower and upper tropospheric flow, behaviour of the quasi-permanent monsoon trough, the ascending and descending branches of the Hadley circulation. The models' ability to simulate the two preferred locations of the TCZ will be investigated.
The interannual variability will be assessed using a range of indices which identify the regional and largescale behaviour. Traditionally All India Rainfall has been used but it is now recognised that this is a regional quantity and may not be representative of the larger scale. Recently a rainfall index for a more extensive monsoon core region has been developed based on the precipitation data of Xie and Arkin (1996). Specifically the following indices for monsoon variability will be computed:
- (1) All India rainfall (Parthasarathy
et al., 1991)
(2) Monsoon core region rainfall
(3) Dynamical index based on vertical shear of zonal wind (Webster and Yang, 1992).
(4) Local Hadley circulation index (Goswami et al., 1997).
The teleconnection between these indices and the SST distribution will be computed for each month.
The largescale atmospheric and land surface characteristics during the pre-monsoon seasons contain valuable predictive information on the performance of the ensuing monsoon (Yang et al., 1996). The coherent characteristics of the precursory signals will be examined using composites of strong minus weak monsoons.
For intraseasonal variability, the models' winds at 850hPa and OLR variance at different time scales will be examined. The ability of the models to simulate the two preferred positions of the TCZ will be extracted through EOF analysis. The principal component time series will be examined to understand the active/break cycles of monsoon. Composites of active/break cycles will be constructed and their relationship with the composites of strong/weak monsoon years will be examined. The statistics of the ISV (mean amplitude, frequency of occurence etc) will be explored with specific reference to their role in determining the seasonal mean monsoon and its IAV.
NCEP, ECMWF reanalysis (ERA), satellite and observed precipitation data will be used to assess the ability of the models to simulate the mean, intraseasonal and interannual variability of the Asian summer monsoon. The very high resolution (T106) ERA data will shed light on the synoptic and ISV. The observed monthly precipitation (Xie and Arkin, 1996), OLR from AVHRR and daily and seasonal All-India rainfall series will be examined to validate the models' ability to simulate the mean and variability of the monsoon.
This subproject will build on the parallel research being undertaken through the EC funded project, Studies of Hydrology, Influence and Variability of Asian summer monsoon (SHIVA). SHIVA involves a similar intercomparison of European climate models and includes the development of an extensive monsoon climatology based on ERA and satellite data which will be used to validate the AMIP models.
Upper-air and single-level low frequency (monthly mean, Tables 1 and 2) standard output will be extensively used to understand the roles of ENSO vs TBO variability on the monsoon. High frequency (6 hourly, Table 3) output will be used to understand the relationship between ISV and IAV of the monsoon.
The variables required from upper-air monthly mean (Table 1) are:
The northward and eastward winds, geopotential height, specific humidity, air temperature, relative humidity and vertical motion at 1000, 850, 700, 500, 300 and 200 hPa levels.
The variables required from single-level monthly mean (Table 2) are:
Surface air temperature, mean sea-level pressure, total precipitation rate, total soil water content, snow depth, snow cover, and outgoing longwave radiation.
The variables required from the the high-frequency (6-hourly) output (Table 3) are:
Northward and eastward wind (850 and 200 hPa), outgoing longwave radiation, total precipitation rate and mean sea-level pressure.
Annamalai, H., 1995: Intrinsic problems in the seasonal prediction of Indian summer monsoon, Met. Atmos. Phys., 55, 61-76
Annamalai, H and B.N. Goswami, 1997: Interaction between Planetary and regional scale circulations and interannual variability of Indian monsoon, J. Climate (submitted).
Charney, J. G and J. Shukla, 1981: Predictability of monsoons. Chapter in 'Monsoon Dynamics', J. Lighthill and R.P. Pearce (Eds), Cambridge University Press, 99-110.
Ferranti, L, J.M. Slingo, T.N. Palmer and B.J. Hoskins, 1997: Relations between interannual and intraseasonal monsoon variability as diagnosed from AMIP intergrations, Q.J.R. Meteorol. Soc., (Accepted).
Goswami, B.N, H. Annamalai and V.Krishnamurthy, 1997: Asian summer monsoon Hadley circulation: A new broad scale circulation index of Indian summer monsoon. (submitted).
Krishnamurthi, T.N and H.N. Bhalme, 1976: Oscillations of a monsoon system. Part I. Observational aspects. J. Atmos. Sci., 33, 1937-1954.
Meehl, G.A., 1994: Coupled land-ocean-atmosphere processes and south Asian monsoon variability, Science, 265, 263-167.
Palmer, T.N., 1994: Chaos and predictability in forecasting the monsoons. Proc. Indian Nat. Sci. Acad., 60A, 57-66.
Parthasarathy, B, K. Rupakumar and A.A. Munot, 1991: Evidence of secular variations in Indian monsoon rainfall-circulation relationships. J. Climate, 4, 927-938.
Shen, S and K.M. Lau, 1995: Biennial oscillation associated with the East Asian summer monsoon and tropical sea surface temperatures. J. Meteor. Soc. Japan, 73, 105-124.
Sikka, D.R. and S. Gadgil, 1980: On the maximum cloud zone and the ITCZ over Indian longitudes during the southwest monsoon. Mon. Wea. Rev., 108, 1840-1853.
Soman, M.K and J.M. Slingo, 1997: Sensitivity of the Asian summer monsoon to aspects of the sea surface temperature anomalies in the tropical Pacific Ocean. Q.J.R. Meteorol. Soc., (Accepted).
Sperber, K.R and T.N. Palmer, 1996:
Interannual tropical rainfall variability in General circulation model
simulations associated with the Atmospheric model
intercomparison project. J. Climate (Accepted).
Walker, G.T., 1924: Correlation in seasonal variations of weather III: The local distribution of monsoon rainfall. Mem. Indian. Met. Dept., 23, 23-40.
WCRP, 1992: Simulation of interannual and intraseasonal monsoon variability. WMO/TD-No. 470, 185pp
WCRP, 1993: Simulation and prediction of monsoons: Recent results. WMO/TD-No. 546, 73pp.
Webster, P.J and S. Yang, 1992: Monsoon and ENSO: Selectively interactive systems. Q.J.R. Meteorol. Soc., 118, 877-926.
Xie, P.P and P.A. Arkin, 1996: Analyses of global monthly precipitation using gauge observations, satellite estimates, and numerical model predictions. J. Climate, 9, 840-858.
Yang, S, K.M. Lau and M. S. Rao, 1996: Precursory signals associated with the interannual variability of the Asian summer monsoon. J. Climate, 9, 949-964.
For further information, contact H. Annamalai (firstname.lastname@example.org) or the AMIP Project Office (email@example.com).
Last update: 25 September 1997. This page is maintained by firstname.lastname@example.org