Role of the Asian Monsoon on the Interannual Variability of the Global Climate System

Abstract

The role of the Asian summer monsoon on the interannual variability of the global climate system particularly relevant to the ENSO time scales is discussed, by examining the statistical and dynamical links between the Asian summer monsoon, the atmosphere/ocean system in the tropics and the westerly flow regimes in the extratropics.The Asian monsoon, the ocean and the atmosphere in the tropical Pacific are tightly linked together as one climate system, named here as the MAOS (Monsoon and the Coupled Atmosphere/Ocean System). The MAOS prominently shows the biennial oscillatory nature which tends to have anomalous states starting in the northern summer monsoon season and persisting for about one year (Yasunari, 1990a(Yasunari, : 1991.The anomalous state of the MAOS produces the anomalous atmospheric circulation over the subtropics and the extratropics of the north Pacific during summer through the early winter, through the modulation of the subtropical high and the stationary Rossby wave propagation mechanism. In the mid winter, this anomalous circulation over the north Pacific is evolved to the hemispheric winter anomalous circulation with wavenumber-one and/or-two structure.The anomalous circulation over Eurasia associated with this hemispheric anomalous flow regime seems to provide a favorable condition for the extensive (or diminished) snow cover area over central Asia, which in turn is responsible for the reversed anomalous state of the next Asian summer monsoon and the MAOS. That is, the biennial nature of the climate system in the northern hemisphere may be due, at least partly, to this two-way interactions between the tropics and the extratropics. In these processes, the Asian monsoon plays a key role as a transmitter of climate signals between the tropics and the extratropics through the land/atmosphere/ocean interaction in the seasonal cycle.In addition, it is strongly suggested that the North Atlantic Oscillation (NAO), in reality, plays a crucial role in the timing of the occurrence of the ENSO event, by stochastically amplifying or damping the biennial oscillation of this coupled climate system. That is, the more or less irregular ENSO cycle may result from this interaction between the MAOS and the NAO, where the former seems to have the nature of an almost-intransitive climate system, while the latter seems to represent the more chaotic nature of the westerly flow regime.

Spring northward retreat of Eurasian snow cover relevant to seasonal and interannual variations of atmospheric circulation

“…El Niño-southern oscillation (ENSO) has a vigorous impact on the change of mid-latitude westerly regimes (Yasunari and Seki, 1992). If the interannual variation of the snow-cover extent is closely linked to the tropospheric circulation, the modulation of the westerly jet due to the ENSO phenomenon may change the summer monsoon activity through land-atmosphere interaction.…”

Lagged teleconnections between snow depth in northern Eurasia, rainfall in Southeast Asia and sea‐surface temperatures over the tropical Pacific Ocean

“…It was suggested that the relationship has weakened since the 1980s and one explanation was that increasing mid latitude continental temperatures have favoured strong monsoons (Kumar et al, 1999). Another possibility may be related to decadal variations of the NAO which is found to be a significant mechanism affecting Asian monsoon and El Niñ o relationships (Yasunari and Seki, 1992).…”

“…Studies by Yang (1996) and Yang and Lau (1998) suggest that El Niñ o plays a more significant role in Indian monsoon rainfall anomalies than Eurasian winter snow cover and related soil moisture. Yasunari and Seki (1992) proposed that the interactions between Asian summer monsoon and El Niñ o are strongly modified by the North Atlantic Oscillation (NAO) which stochastically amplifies or dampens the biannual nature of the monsoon and the coupled tropical atmosphere/ocean system. In summary, it seems that (i) snow volume (or depth) has a more significant connection to summer monsoon rainfall than snow coverage; (ii) the teleconnections between winter snow and summer rainfall are geographically dependent; and (iii) abnormal SSTs over the eastern tropical Pacific interact with Eurasian snowfall and monsoon rainfall through extratropical atmospheric circulation.…”

2.8 million years of phytoplankton history in Lake Baikal recorded by the residual photosynthetic pigments in its sediment core.

“…This also suggests the contribution of diatoms to TOC was less and that from other phytoplankton was significant before 1.2 Ma ago. The upheaval of the Himalayas around 1.2 Ma ago has been considered to cause a big climate change (Yasunari and Seki, 1992), and the second cooling event in the Baikal record occurred 1.8 Ma ago . Such big climate changes may correspond to the diminution of dinoflagellate and green algae.…”