P V Joseph
Articles written in Journal of Earth System Science
Volume 112 Issue 4 December 2003 pp 529-558
For over a century, the term break has been used for spells in which the rainfall over the Indian monsoon zone is interrupted. The phenomenon of ’break monsoon’ is of great interest because long intense breaks are often associated with poor monsoon seasons. Such breaks have distinct circulation characteristics (heat trough type circulation) and have a large impact on rainfed agriculture. Although interruption of the monsoon rainfall is considered to be the most important feature of the break monsoon, traditionally breaks have been identified on the basis of the surface pressure and wind patterns over the Indian region. We have defined breaks (and active spells) on the basis of rainfall over the monsoon zone. The rainfall criteria are chosen so as to ensure a large overlap with the traditional breaks documented by Ramamurthy (1969) and De
There have been several studies of breaks (and also active spells in several cases) identified on the basis of different criteria over regions differing in spatial scales (e.g., Webster
We have derived the OLR and circulation patterns associated with rainbreaks and active spells and compared them with the patterns associated with breaks/active minus break spells from these studies. Inspite of differences in the patterns over the Indian region, there is one feature which is seen in the OLR anomaly patterns of breaks identified on the basis of different criteria as well as the rainbreaks identified in this paper viz., a quadrapole over the Asia-west Pacific region arising from anomalies opposite (same) in sign to those over the Indian region occurring over the equatorial Indian Ocean and northern tropical (equatorial) parts of the west Pacific. Thus it appears that this quadrapole is a basic feature of weak spells of the intraseasonal variation over the Asia-west Pacific region. Since the rainbreaks are intense weak spells, this basic feature is also seen in the composite patterns of these breaks. We find that rainbreaks (active spells) are also associated with negative
Volume 113 Issue 2 June 2004 pp 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).
Volume 116 Issue 5 October 2007 pp 385-406
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 (𝛥 𝑇) exceeding 0.75°C in 28 cases.There was no instance of 𝛥 𝑇 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 (𝛥 𝑇 ≥ 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 𝛥 𝑇 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 𝛥 𝑇 exceeding the threshold of 0.75°C. The northern bay SST,