The analysis of corrected ship reports [sea level pressure (SLP), sea surface temperature (SST), air temperature (AT)] and corrected land data (SLP, AT, rainfall) in the Indian sector reveals the existence of two low-frequency modes of monsoon variability during the 1900–1970 period. • A definite biennial (B) mode exists on the SLP fields. This B oscillation is unambiguously linked with a southwest–northeast SLP anomaly gradient. During the summer monsoon, the B SLP pattern can be interpreted as an expansion/contraction of the monsoon activity since this mode is strongly coupled with rainfall variations over peninsular India. • A strong low-frequency (LF) mode with period spanning 4–6 years is also seen on SLP fields over the Indian Ocean and subcontinent. The variance associated with this band is typically more important than the one observed for the B mode, and its spatial mark is also strikingly different since it is linked with a global pattern of variation. This mode has also a strong influence on the Indian summer rainfall fluctuations, particularly on the Ghats and in the Indo-Gangetic plains. The amplitude of these oscillations varies widely during the 1900–1970 period. The LF mode is well defined during 1900–1923 and 1947–1970. There is a tendency for the energy associated with the B mode to decrease on the land while it increases over the Indian Ocean during the whole 1900–1970 interval. Although these two timescales exist also on SST fields, cross-spectral analysis shows that ocean–atmosphere interactions are much stronger at the B timescale. This result stresses the B nature of the monsoon system. The existence of these interannual signals in the Indian areas where the annual cycle is so strong raises difficult problems: How can climatic anomalies persist for several years in spite of strong seasonality? Or, still more intriguing, how can be explained the persistence of climatic anomalies during one year and the appearance of opposite sign climatic anomalies in the following year for the B mode? A composite analysis has suggested an explanation for this last problem: SST anomalies produced by exceptional summer monsoons can indeed persist during the whole winter and exert an influence on the next summer monsoon.