The influences of El Nino–Southern Oscillation (ENSO) on the summer- and wintertime precipitation and circulation over the principal monsoon regions of Asia and Australia have been studied using a suite of 46-yr experiments with a 30-wavenumber, 14-level general circulation model. Observed monthly varying sea surface temperature (SST) anomalies for the 1950–95 period have been prescribed in the tropical Pacific in these experiments. The lower boundary conditions at maritime sites outside the tropical Pacific are either set to climatological values [in the Tropical Ocean Global Atmosphere (TOGA) runs], predicted using a simple 50-m oceanic mixed layer (TOGA-ML runs), or prescribed using observed monthly SST variations. Four independent integrations have been conducted for each of these three forcing scenarios. The essential characteristics of the model climatology for the Asian–Australian sector compare well with the observations. Composites of the simulated precipitation data over the outstanding...

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Impact of ENSO on the Variability of the Asian–Australian Monsoons as Simulated in GCM Experiments

The influences of El Nino–Southern Oscillation (ENSO) events on air–sea interaction in the Indian–western Pacific (IWP) Oceans have been investigated using a general circulation model. Observed monthly sea surface temperature (SST) variations in the deep tropical eastern/central Pacific (DTEP) have been inserted in the lower boundary of this model through the 1950–99 period. At all maritime grid points outside of DTEP, the model atmosphere has been coupled with an oceanic mixed layer model with variable depth. Altogether 16 independent model runs have been conducted. Composite analysis of selected ENSO episodes illustrates that the prescribed SST anomalies in DTEP affect the surface atmospheric circulation and precipitation patterns in IWP through displacements of the near-equatorial Walker circulation and generation of Rossby wave modes in the subtropics. Such atmospheric responses modulate the surface fluxes as well as the oceanic mixed layer depth, and thereby establish a well-defined SST anom...

Atmosphere–Ocean Variations in the Indo-Pacific Sector during ENSO Episodes

Using available instrumental records, this paper examines the variation of precipitation from 1866 to 2006 in the northwestern Himalaya (NWH). The study indicates no trend in the winter precipitation but significant decreasing trend in the monsoon precipitation during the study period. Periodicities on a multi-decadal scale (29–34 years and 58–64 years) obtained in power spectrum analyses point towards epochal behaviour in the precipitation series. Analyses of the temperature data show significant increasing trends in annual temperature in all three stations in the NWH during the data period. Warming effect is particularly noteworthy during the winter season. Negative relationships between mean winter air temperature and snowfall amounts recorded at different meteorological stations in this period reveal strong effect of rising temperatures on the decreasing snowfall component in total winter precipitation, reducing effective duration of winter on the windward side of the Pir Panjal Himalayan Range.



The study also shows influence of global teleconnections [North-Atlantic Oscillation (NAO) during winter months and Southern Oscillation Index (SOI) during the monsoon months] on precipitation fluctuations in the NWH. The teleconnections that appear to exist between the precipitation and the temperature until the late 1960s seem to have weakened considerably in the last three decades. This may be ascribed to the diminishing effect of the natural factors such as quasi-biennial oscillations (QBO), El Nino Southern Oscillations (ENSO), double sunspot cycles (Hale), etc., in this period. Role of increasing concentration of greenhouse gases in the atmosphere cannot be ruled out. Copyright © 2009 Royal Meteorological Society

Climate change and the precipitation variations in the northwestern Himalaya: 1866–2006

Seasonal forecasting of Indian summer monsoon rainfall: A review

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.

Homogeneous Indian Monsoon rainfall: Variability and prediction

The associations between strong to moderate El Nino events and the all-India and subdivisional summer monsoon rainfall is examined for the period 1871 to 1978. The significance of the association is assessed by applying the Chi-square test to the contingency table.

Indian summer monsoon and El Nino.

It is well known that the activity of the summer monsoon, as measured by rainfall, undergoes variations from one month to another. In this study, the monsoon activity has been examined in detail to find if any major climatological discontinuities in the activity exist in the field of the Asian summer monsoon. From amongst the possible criteria for major discontinuity, the criterion based on the lower and the upper quartiles has been selected. The study brings out that major climatological discontinuities do exist over some parts of Asia. These are (i) a marked decrease in activity from July to August, right in the middle of the monsoon season, over the west coast of India, between Bombay and Trivandrum; (ii) a marked decrease from August to September over a small portion of the Indian west coast in the neighbourhood of Mangalore; (iii) a marked decrease from August to September over the Arakan coast; and (iv) a marked increase from August to September over the Viet Nam coast. Possible causes of these discontinuities have been discussed.

D. A. Mooley

Papers

Indian summer monsoon and El Nino.

Major climatological discontinuities in the monthly activity of the summer monsoon over Asia