Quasi-biweekly oscillations of the South Asian monsoon and its co-evolution in the upper and lower troposphere

Ortega, Sebastián; Webster, Peter J.; Toma, Violeta E.; Chang, Hai-Ru

doi:10.1007/s00382-016-3503-y

Cited by 30 publications

(45 citation statements)

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“…As is evident in Figures and , a close balance between poleward and equatorward fluxes is indeed observed in ERA‐Interim data, and that this balance is at least in part achieved via Rossby‐wave dynamics can be seen, for instance, in the supplementary movie of Ortega et al . ().…”

Section: Discussionmentioning

confidence: 93%

“…Rossby wave breaking, which occurs when the PV troughs and ridges propagate over and to the west of the westerly duct, appears as a stretching, thinning and tilting of the synoptic PV troughs as they lose their coherence (e.g. Scott et al, ; Scott and Cammas, ; Garny and Randel, ; Ortega et al, ). The advection of eddies over the equatorial easterly winds appears as upper tropospheric PV filaments and eddies, a by‐product of the breaking of the synoptic PV troughs, that are advected westward and slowly northward back toward the midlatitudes (e.g.…”

Section: Pv Variability On the 370 K Isentropementioning

confidence: 99%

“…Over South Asia, the northward displacement of the subtropical westerly jet and the intensification of the equatorial easterly winds, known then as the easterly jet (Koteswaram, 1958), reflect the formation of the upper tropospheric monsoon anticyclone (e.g. Wu et al, 2014) as described by Ortega et al (2017) and as is evident in Figure 1b. A similar, although weaker, transition occurs over North America.…”

Section: Introductionmentioning

confidence: 98%

“…Wu et al, ) as described by Ortega et al . () and as is evident in Figure b. A similar, although weaker, transition occurs over North America.…”

Section: Introductionmentioning

confidence: 99%

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The effect of potential vorticity fluxes on the circulation of the tropical upper troposphere

Ortega

Webster

Toma

et al. 2018

Quart J Royal Meteoro Soc

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With simple analytical arguments, we suggest that the subtropical westerly jets, the equatorial easterly winds, the westerly ducts (i.e. regions of westerly winds over the Equator), and the tropical upper tropospheric troughs, are coupled via potential vorticity (PV) dynamics. We suggest that deep tropical convection leads to advective PV fluxes towards the Poles over the subtropical westerly jets, triggering Rossby waves on the jet, and leading to advective PV fluxes towards the Equator when the Rossby waves break over the westerly ducts. Moreover, we show that the advective PV fluxes towards the Poles closely balance those towards the Equator, and suggest that this close balance is to be expected from the impermeability theorem. We then argue that small imbalances of these advective PV fluxes play an important role in maintaining the upper tropospheric mean flow of the tropical atmosphere. We test our ideas using a shallow‐water model of the atmosphere, where diabatic heating terms are absent, and show how imbalances in the advective PV fluxes of a shallow‐water model maintain its mean flow. We focus on the periodicity of the shallow‐water models to study the effect of transient eddies and relate our results to previous studies.

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Section: Discussionmentioning

confidence: 93%

Section: Pv Variability On the 370 K Isentropementioning

confidence: 99%

Section: Introductionmentioning

confidence: 98%

“…Wu et al, ) as described by Ortega et al . () and as is evident in Figure b. A similar, although weaker, transition occurs over North America.…”

Section: Introductionmentioning

confidence: 99%

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The effect of potential vorticity fluxes on the circulation of the tropical upper troposphere

Ortega

Webster

Toma

et al. 2018

Quart J Royal Meteoro Soc

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“…However, the lead‐lag aspect of these relationships is still unknown. Considering that QBWO features are notable not only in the zonal shift of SAH but also in the rainfall variations of ISM and EASM (Chen et al, ; Krishnamurti & Ardanuy, ; Ortega et al, ; Xu et al, ; Yang et al, ), we further examine these relationships on quasi‐biweekly time scales.…”

Section: Resultsmentioning

confidence: 99%

Quasi‐Biweekly Oscillation of the South Asian High and Its Role in Connecting the Indian and East Asian Summer Rainfalls

Wei

Zhang²,

Yang

et al. 2019

Geophysical Research Letters

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Investigation into the intraseasonal variation of the South Asian high (SAH) and its relationship with the Indian and East Asian summer monsoon rainfalls indicates that the 10-20 day period dominates the subseasonal zonal oscillation of the SAH between the Iranian mode and the Tibetan mode. The eastward shift of SAH lags a rainfall decrease over northern India by about 1 day, but leads a rainfall increase over the Yangtze River valley (YRV) by about 3 days. Interactive processes are also revealed on quasi-biweekly time scales. The latent heating over northern India contributes to the southwestward shift of a pair of cyclone-anticyclone anomalies from the midlatitudes to the SAH region, resulting in the zonal shift of the SAH. An associated upper-level divergence over eastern China leads to the rainfall increase over the YRV 3 days later. The lead-lag relationships revealed may benefit the subseasonal rainfall prediction over the two monsoon regions. Plain Language SummaryThe Indian summer monsoon (ISM) and the East Asian summer monsoon (EASM) are two crucial monsoon systems, affecting more than half of the population in the world. The upper-level high-pressure system, the South Asian high (SAH) plays an important role in connecting the ISM and EASM rainfalls on intraseasonal time scales. The zonal shift of SAH center between the Iranian Plateau and the Tibetan Plateau exhibits a quasi-biweekly oscillation with a peak period about 13 days. The eastward shift of the SAH lags a decrease in rainfall over northern India by about 1 day, but leads an increase in rainfall over the Yangtze River valley by about 3 days. Such lead-lag relationship implies an interaction among the SAH and the ISM and EASM rainfalls. Abundant rainfall over northern India causes the zonal shift of the SAH, which in turn influences the rainfall over East Asia.

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The Tropical Easterly Jet over Africa, its representation in six reanalysis products, and its association with Sahel rainfall

Nicholson

Klotter

2020

Intl Journal of Climatology

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This paper compares the characteristics of the Tropical Easterly Jet (TEJ) and upper-level winds in six reanalysis products, compares them with soundings at seven West African locations, examines the relationship between Sahel rainfall and the TEJ, and examines factors influencing the TEJ. The jet characteristics assessed by MERRA2, NCEP 1, JRA 55, and ERA 5 are similar. CFSR and 20th Century Reanalysis are outliers in nearly every analysis, overestimating wind speeds by as much as 25 to 40% compared to other reanalyses. Over the period 1948 to 2014, the correlation between rainfall and TEJ magnitude is .72. Arguments based on observations and modelling studies provide evidence that on interannual scales changes in the TEJ are not forced by rainfall, that largescale factors drive the TEJ. Potential mechanisms are discussed for a causal relationship such that a strong jet leads to high rainfall. However, further modelling efforts are needed to conclusively determine whether the TEJ/Sahel rainfall link is a result of common forcing factors. The factors that appear to control jet strength include sea-surface temperature (SST) contrast between the central equatorial Pacific and central equatorial Indian Ocean (correlation of −.64), SST contrast between the central equatorial and the southern subtropical Indian Ocean (correlation of −.39), the latitude of the shift between upper-tropospheric easterlies and westerlies in the Southern Hemisphere (correlation of −.84 at 150 hPa), and the intensity of the Southern Hemisphere westerlies (correlation of +.52 at 200 hPa). This suggests considerable control on the TEJ by extra-tropical circulation in the Southern Hemisphere.

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Quasi-biweekly oscillations of the South Asian monsoon and its co-evolution in the upper and lower troposphere

Cited by 30 publications

References 50 publications

The effect of potential vorticity fluxes on the circulation of the tropical upper troposphere

The effect of potential vorticity fluxes on the circulation of the tropical upper troposphere

Quasi‐Biweekly Oscillation of the South Asian High and Its Role in Connecting the Indian and East Asian Summer Rainfalls

The Tropical Easterly Jet over Africa, its representation in six reanalysis products, and its association with Sahel rainfall

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