On the Onset Vortex of the Summer Monsoon

Krishnamurti, T. N.; Ardanuy, Philip E.; Ramanathan, Y.; Pasch, Richard J.

doi:10.1175/1520-0493(1981)109<0344:otovot>2.0.co;2

Cited by 196 publications

(139 citation statements)

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“…If the meridional gradient of insolation is instead taken as the relevant forcing, the annual harmonic becomes even more important relative to the semiannual in the tropics and subtropics, and the third harmonic retains its small amplitude. Using data from a field campaign in 1979, Krishnamurti et al (1981) found that the kinetic energy of the 850 hPa horizontal wind over the Arabian Sea increased from its mean winter to its mean summer value over a single week prior to the start of monsoon rains in central India, which easily qualifies as abrupt by the above definition. The abrupt onset of the southwesterly jet has also been documented in scatterometer data by Halpern and Woiceshyn (1999), who showed that surface wind speeds in this jet increased to their summer mean over a two-week interval in 1997, with peak values exceeding 16 m s −1 .…”

Section: Introductionmentioning

confidence: 58%

Annual intensification of the Somali jet in a quasi‐equilibrium framework: Observational composites

Boos

Emanuel

2009

Quart J Royal Meteoro Soc

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ABSTRACT:The annual intensification of the Somali jet, which accompanies the onset of the Indian summer monsoon, is rapid compared to the evolution of the seasonal insolation forcing. Using observationally-based data sets, the dynamic and thermodynamic changes accompanying the onset of this jet are presented. The abrupt component of jet onset is shown to occur over ocean about 1000 km east of the East African highlands, and is accompanied by increases in both deep convection and baroclinic flow over the off-equatorial Arabian Sea. These abrupt changes are well separated from the core of the cross-equatorial jet, which is located over land adjacent to the East African highlands.The onset of the Somali jet and the associated monsoon are then examined in a convective quasi-equilibrium framework. Such a framework is consistent with the mean summer state, in that peak free-tropospheric temperatures are nearly collocated with peak subcloud layer entropies over the northern Bay of Bengal and adjacent coastal regions. Jet onset is accompanied by a large (O{100 W m −2 }) increase in surface enthalpy flux over the Arabian Sea that is nearly collocated with, and linearly related to, the concurrent increase in deep tropospheric ascent. At the same time, the highest subcloud entropies shift inland slightly from the Bay of Bengal, and the region of warmest free-tropospheric temperatures expands poleward. The consistency of all of these changes with a wind-evaporation feedback and several other hypothesized mechanisms of abrupt monsoon onset is discussed.

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Section: Introductionmentioning

confidence: 58%

Annual intensification of the Somali jet in a quasi‐equilibrium framework: Observational composites

Boos

Emanuel

2009

Quart J Royal Meteoro Soc

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“…Rather the monsoon begins abruptly in a period as short as two weeks, as measured not only by precipitation [e.g., Soman, 1988, 1989;Joseph et al, 2006;Pai and Nair, 2009;Soman and Krishna Kumar, 1993], but also by reversals in both low-level winds [e.g., Boos and Emanuel, 2009;Fieux and Stommel, 1977;Halpern and Woiceshyn, 1999;Krishnamurti et al, 1981] and upper troposphere circulation [e.g., He et al, 1987He et al, , 2003Li and Yanai, 1996;Wu and Zhang, 1998;Yanai and Wu, 2006;Yanai et al, 1992]. Moreover, Goswami et al [1999] showed that the reversal in winds aloft correlates with onsets of monsoons defined by various rainfall indices.…”

Section: Introductionmentioning

confidence: 99%

Signatures of Tibetan Plateau heating on Indian summer monsoon rainfall variability

Rajagopalan

Molnár

2013

JGR Atmospheres

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[1] Despite recent challenges, conventional wisdom has held that heating over the Tibetan Plateau leads to increased Indian summer monsoon rainfall via enhancement of cross-equatorial circulation aloft, and a concurrent strengthening of both the Somali Jet and westerly winds that bring moisture to southern India. We show that such heating, quantified by monthly estimates of moist static energy in the atmosphere just above the surface, correlates with summer monsoon rainfall, but only in the early (20 May to 15 June) and late (September 1 to 15 October) monsoon season. Correlations during the main monsoon season (15 June to 31 August) are small and insignificant. The positive correlations with early and late monsoon season, however, allow for heating over Tibet to modulate as much as 30% of the total rainfall. Furthermore, we demonstrate that heating over Tibet is independent of the El Niño Southern Oscillation, so that together they explain a substantial portion of variability in the early and late season rainfall, providing potential predictability. These links may also explain the wet conditions over India during early Holocene time and provide a quantitative link between a rise of Tibet and stronger Somali Jet.

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“…Many previous studies have described features of the vortex, including the distribution of precipitation and the vortex structure (e.g., Pisharoty and Asnani 1957;Rao and Rajamani 1970;Sharma and Srinivasan 1971). Subsequent studies suggested that barotropic instability associated with horizontal wind shear leads to the formation of the vortex, and that diabatic heat supplies the energy to the vortex required for its development (Krishnamurti 1981(Krishnamurti , 1985Krishnamurti and Ramanathan 1982;Krishnamurti et al 1981). Mak and Kao (1982) found that vertical wind-shear and baroclinic instability play an important role in the development of the vortex.…”

Section: Introductionmentioning

confidence: 99%

Air–sea interaction and formation of the Asian summer monsoon onset vortex over the Bay of Bengal

Guan

Liu

et al. 2011

Clim Dyn

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In spring over the southern Bay of Bengal (BOB), a vortex commonly develops, followed by the Asian summer monsoon onset. An analysis of relevant data and a case study reveals that the BOB monsoon onset vortex is formed as a consequence of air-sea interaction over BOB, which is modulated by Tibetan Plateau forcing and the land-sea thermal contrast over the South Asian area during the spring season. Tibetan Plateau forcing in spring generates a prevailing cold northwesterly over India in the lower troposphere. Strong surface sensible heating is then released, forming a prominent surface cyclone with a strong southwesterly along the coastal ocean in northwestern BOB. This southwesterly induces a local offshore current and upwelling, resulting in cold sea surface temperatures (SSTs). The southwesterly, together with the near-equatorial westerly, also results in a surface anticyclone with descending air over most of BOB and a cyclone with ascending air over the southern part of BOB. In the eastern part of central BOB, where sky is clear, surface wind is weak, and ocean mixed layer is shallow, intense solar radiation and low energy loss due to weak surface latent and sensible heat fluxes act onto a thin ocean layer, resulting in the development of a unique BOB warm pool in spring. Near the surface, water vapor is transferred from northern BOB and other regions to southeastern BOB, where surface sensible heating is relatively high. The atmospheric available potential energy is generated and converted to kinetic energy, thereby resulting in vortex formation. The vortex then intensifies and moves northward, where SST is higher and surface sensible heating is stronger. Meanwhile, the zonal-mean kinetic energy is converted to eddy kinetic energy in the area east of the vortex, and the vortex turns eastward. Eventually, southwesterly sweeps over eastern BOB and merges with the subtropical westerly, leading to the onset of the Asian summer monsoon.

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On the Onset Vortex of the Summer Monsoon

Cited by 196 publications

References 0 publications

Annual intensification of the Somali jet in a quasi‐equilibrium framework: Observational composites

Annual intensification of the Somali jet in a quasi‐equilibrium framework: Observational composites

Signatures of Tibetan Plateau heating on Indian summer monsoon rainfall variability

Air–sea interaction and formation of the Asian summer monsoon onset vortex over the Bay of Bengal

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