Abstract
Intra-Seasonal circulation regimes are identified from a cluster analysis of 5-day mean anomaly fields of 850 hPa horizontal winds from the ERA-Interim reanalysis for the boreal summer season (June–Sept. for 1979–2018) over the region (50°–100° E; 5° S–35° N). The k-means method was applied to the leading 6 principal components yielding k clusters. The degree of clustering is significant compared to synthetic data sets for any value of $$k > 3$$ k > 3 . The circulation is most likely to stay in the same cluster from one pentad to the next; significant transitions (with 95% confidence level) form a cycle. The similarity between the cycle depicted from 4 or 5 clusters and the active-break cycle, as well as the 45-day oscillation, is established by composites of 850 hPa winds, 200 hPa divergence, 500 hPa vorticity and vertical pressure velocity, precipitable water, diabatic heating and rainfall over India: Strong convection over the subtropical Indian Ocean moves to the central Bay of Bengal and central India, subsequently to the northern Bay of Bengal and west Bengal, and then further north into the Himalayas. We also find preferred transitions in which the convection moves equatorward from central India. The number of complete cycles found in 40 summers is 7 in the 4-cluster analysis. The number of times the system undergoes four (three) consecutive legs of the cycle is 16 (31). For 5 clusters only 3 complete cycles are found. sequences of five, four and three consecutive legs occur 10, 11 and 28 times, respectively.