Abstract
The inter-annual variability of boreal summer intra-seasonal oscillation (BSISO) propagation from the Indian Ocean (IO) to the western Pacific (WP) is investigated for the boreal summers (May to September) of 1979–2018. It is shown that the interannual variability of BSISO mainly happens in its evolution, not in its strength over the IO. Here, we classify four distinctive modes for inter-annual variability of BSISO propagation: (i) northeast mode, propagating from the IO to the western equatorial Pacific (WEP) and the western North Pacific (WNP); (ii) north-only mode, only propagating to the WNP; (iii) east-only mode, only propagating to the WEP; and (iv) stationary mode, propagating to neither the WEP nor the WNP. It is found that the Maritime Continent (MC) and WEP are two key regions determining these four modes concerning mean state moisture and vertical motion. Associated with central equatorial Pacific cooling, the BSISO of northeast and north-only modes can reach the WP by passing over the MC due to positive mean moisture anomalies and upward mean motion anomalies over the MC. The strong negative mean moisture anomalies and downward mean motion anomalies over the WEP, related to strong central Pacific cooling, prevents the development of BSISO there, resulting in north-only mode. For the east-only and stationary modes associated with the central Pacific warming, their BSISO can hardly pass the MC due to negative mean moisture anomalies and downward mean motion anomalies. The positive mean moisture anomalies and upward mean motion anomalies over the WEP related to strong central Pacific warming, however, will reinitiate the BSISO in the WEP for the east-only mode.
Highlights
Whether the boreal summer intra-seasonal oscillation (BSISO) originating from the Indian Ocean (IO) can propagate to the western Pacific (WP) in a certain year is important for improving sub-seasonal predictability of the East Asian summer monsoon system
The inter-annual variation of northwestward propagation of the BSISO in the western North Pacific (WNP) was found to be affected by the El Niño Southern Oscillation (ENSO) through changing the mean circulation, while the northward propagation over the Indian summer monsoon region is insensitive to the ENSO [18]
Since we focus on inter-annual variability, we only have one mode represented by the summer averaged BSISO evolution for each year
Summary
Whether the boreal summer intra-seasonal oscillation (BSISO) originating from the Indian Ocean (IO) can propagate to the western Pacific (WP) in a certain year is important for improving sub-seasonal predictability of the East Asian summer monsoon system. Its pronounced characteristic is eastward propagation from the equatorial IO to the WP, with its main convection located to the south of the equator. An independent northward mode, which is not associated with eastward equatorial propagation, was found in the boreal summer from May to October [11]. This type of ISO, characterized by Atmosphere 2019, 10, 596; doi:10.3390/atmos10100596 www.mdpi.com/journal/atmosphere. The inter-annual variation of northwestward propagation of the BSISO in the WNP was found to be affected by the ENSO through changing the mean circulation, while the northward propagation over the Indian summer monsoon region is insensitive to the ENSO [18].
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