Factors controlling the interannual variation of 30–60-day boreal summer intraseasonal oscillation over the Asian summer monsoon region

• Published in: Climate dynamics Vol. 52; no. 3-4; pp. 1651 - 1672
• Main Authors: Li, Jianying, Mao, Jiangyu
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.02.2019; Springer; Springer Nature B.V
• Subjects: Analysis; Annual variations; Climatology; Earth and Environmental Science; Earth Sciences; El Nino; El Nino phenomena; El Nino-Southern Oscillation event; Empirical analysis; Geophysics/Geodesy; Interannual variability; Interannual variations; Intraseasonal oscillation; La Nina; Levels; Monsoons; Ocean currents; Oceanography; Oceans; Orthogonal functions; Propagation; Sea surface; Sea surface temperature; Sea surface temperature anomalies; Southern Oscillation; Spatial data; Summer; Summer monsoon; Surface temperature; Temperature anomalies; Tropical climate; Variability; Variation; Westerlies; Asia; Indian Ocean; Boreal summer intraseasonal oscillation; Multi-scale interaction; Interannual variation
• ISSN: 0930-7575; 1432-0894
• DOI: 10.1007/s00382-018-4216-1

The 30–60-day boreal summer intraseasonal oscillation (BSISO) is a dominant variability of the Asian summer monsoon (ASM), with its intensity being quantified by intraseasonal standard deviations based on OLR data. The spatial and interannual variations of the BSISO intensity are identified via empirical orthogonal function (EOF) analysis for the period 1981–2014. The first EOF mode (EOF1) shows a spatially coherent enhancement or suppression of BSISO activity over the entire ASM region, and the interannual variability of this mode is related to the sea surface temperature anomaly (SSTA) contrast between the central–eastern North Pacific (CNP) and tropical Indian Ocean. In contrast, the second mode (EOF2) exhibits a seesaw pattern between the southeastern equatorial Indian Ocean (EIO) and equatorial western Pacific (EWP), with the interannual fluctuation linked with developing ENSO events. During strong years of EOF1 mode, the enhanced low-level westerlies induced by the summer-mean SSTA contrast between the warmer CNP and cooler tropical Indian Ocean tend to form a wetter moisture background over the eastern EIO, which interacts with intraseasonal low-level convergent flows, leading to stronger equatorial eastward propagation. The intensified easterly shear favors stronger northward propagation over the South Asian and Eastern Asian/Western North Pacific sectors, respectively. Opposite situation is for weak years. For interannual variations of EOF2 mode, the seesaw patterns with enhanced BSISO activity over the southeastern EIO while weakened activity over the EWP mostly occur in the La Niña developing summers, but inverse patterns appear in the El Niño developing summers.