Cross‐equatorial influences of submonthly scale southerly surges over the eastern Indian Ocean during Southern Hemisphere winter

2019

JGR Atmospheres

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The structure and characteristics of tropical synoptic‐scale wave disturbances over the Maritime Continent (MC) and northern Australia during the Southern Hemisphere summer are analyzed. The tropical synoptic‐scale waves are identified by an extended empirical orthogonal function analysis on 2‐ to 8‐day‐filtered daily 850‐hPa meridional wind anomalies during December–February for the period 1979–2016. Extended empirical orthogonal function‐based composite analyses of various fields reveal a synoptic‐scale wave train, with a well‐organized structure, which is coupled with deep convection anomalies. The composite waves are considered to be tropical depression‐type waves that develop along a strong meridional gradient of the mean monsoon westerly flow. These waves propagate westward at approximately 8 m/s, with zonal wavelengths of about 3,000–4,000 km. Eastward amplification of wave troughs and ridges due to group propagation occurs within the synoptic‐scale wave train. Wave activity diagnostics confirm that downstream wave energy propagation facilitates the development of the synoptic‐scale wave train along the monsoon westerly flow. Rainfall is enhanced over the MC and the ocean in the vicinity of the north coast of Australia, which is associated with the passage of the wave trough. The development of this synoptic‐scale wave is found to play an important role in day‐to‐day weather variation over the MC. Extratropical forcing of the tropical synoptic‐scale waves is also explored. Southerly surges that originated in the midlatitude Indian Ocean could intensify the tropical synoptic‐scale wave trough. The southerly surges are induced by the development of midlatitude synoptic‐scale baroclinic disturbances over the eastern Indian Ocean.

Section: Summary and Discussionsupporting

confidence: 57%

Tropical Synoptic‐Scale Waves Propagating Across the Maritime Continent and Northern Australia

2019

JGR Atmospheres

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“…Despite the fact that the timescale of the analyzed waves overlaps that of the lowerfrequency MRG waves near a 10-day period, it is still unclear whether the interpretation based on the MRG-wave dynamics is appropriate or not. Another possible interpretation is that the equatorial gyre is a kind of ER waves modified from their theoretical forms as discussed by Fukutomi and Yasunari (2009). The mechanism applicable to the present case still remains unclear.…”

Section: Summary and Discussionmentioning

confidence: 79%

Structure and characteristics of submonthly-scale waves along the Indian Ocean ITCZ

Yasunari

2012

Clim Dyn

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This study examines wave disturbances on submonthly (6-30-day) timescales over the tropical Indian Ocean during Southern Hemisphere summer using Japanese Reanalysis (JRA25-JCDAS) products and National Oceanic and Atmospheric Administration outgoing longwave radiation data. The analysis period is December-February for the 29 years from 1979/1980 through 2007/2008. An extended empirical orthogonal function (EEOF) analysis of daily 850-hPa meridional wind anomalies reveals a well-organized wave-train pattern as a dominant mode of variability over the tropical Indian Ocean. Daily lagged composite analyses for various atmospheric variables based on the EEOF result show the structure and evolution of a wave train consisting of meridionally elongated troughs and ridges along the Indian Ocean Intertropical Convergence Zone (ITCZ). The wave train is oriented in a northeast-southwest direction from Sumatra toward Madagascar. The waves have zonal wavelengths of about 3,000-5,000 km and exhibit westward and southwestward phase propagation. Individual troughs and ridges as part of the wave train sequentially travel westward and southwestward from the west of Sumatra into Madagascar. Meanwhile, eastward and northeastward amplification of the wave train occurs associated with the successive growth of new troughs and ridges over the equatorial eastern Indian Ocean. This could be induced by eastward and northeastward wave energy dispersion from the southwestern to eastern Indian Ocean along the mean monsoon westerly flow. In addition, the waves modulate the ITCZ convection. Correlation statistics show the average behavior of the wave disturbances over the tropical Indian Ocean. These statistics and other diagnostic measures are used to characterize the waves obtained from the composite analysis. The waves appear to be connected to the monsoon westerly flow. The waves tend to propagate through a band of the large meridional gradient of absolute vorticity produced by the mean monsoon westerly flow. This suggests that the monsoon westerly flow provides favorable background conditions for the propagation and maintenance of the waves and acts as a waveguide over the tropical Indian Ocean. The horizontal structure of the wave train may be interpreted as that of a mixture of equatorial Rossby waves and mixed Rossbygravity wavelike gyres.

“…The other effect consists of low‐level meridional wind surges produced by midlatitude waves. As described by Fukutomi and Yasunari [], transient cross‐equatorial flows induced by the meridional wind surges exert important lateral forcing on tropical disturbances, such as westerly wind bursts, convectively coupled equatorial waves, and tropical cyclones over the western Pacific [e.g., Love , ; Chu , ; Kiladis et al , ; Meehl et al , ; Yu and Rienecker , ; Compo et al , ]. Various extratropical‐tropical interaction phenomena over the Indian Ocean have also been reported previously, although by comparatively fewer studies.…”

Section: Introductionmentioning

confidence: 99%

“…Various extratropical‐tropical interaction phenomena over the Indian Ocean have also been reported previously, although by comparatively fewer studies. Fukutomi and Yasunari [, ] presented evidence that low‐level southerly surges caused by Southern Hemisphere (SH) midlatitude wave propagation excite an equatorial convective disturbance over the eastern Indian Ocean during austral winter. Fauchereau et al [], Cretat et al [], and Hart et al [] deduced that the northeastward propagation of midlatitude waves from the Atlantic toward the southwest Indian Ocean (SWIO) is linked to the development of tropical‐temperate troughs (TTTs) during austral summer.…”

Section: Introductionmentioning

confidence: 99%

Extratropical forcing of tropical wave disturbances along the Indian Ocean ITCZ

Yasunari

2014

JGR Atmospheres

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The role of extratropical waves in the excitation of tropical waves on submonthly timescales is explored along the Indian Ocean Intertropical Convergence Zone (ITCZ) during the austral summer using Japanese Reanalysis products and NOAA outgoing longwave radiation data. The analysis period is December-February of 1979/1980to 2008. The submonthly tropical waves are regarded as a type of Rossby wave propagating along the mean monsoon westerly flow. They play an important role in modulating the Indian Ocean ITCZ convection. The linkage between the tropical and extratropical waves, which is responsible for the formation and strengthening of tropical waves, is examined. Composite analysis results linked to the tropical wave train development show that the midlatitude Rossby wave train progresses eastward and northeastward from the South Atlantic into the subtropical Indian Ocean. As a trough and ridge that form part of the midlatitude wave train approach the southern Africa-southwest Indian Ocean (SWIO) region, a southwest-northeast-oriented wave train is subsequently established, originating from this feature, and is strengthened across the tropical Indian Ocean. The midlatitude wave propagation toward the subtropics induces the growth of the trough and ridge over the SWIO. Wave activity flux diagnostics indicate that the amplified trough and ridge over the SWIO act as an energy source for northeastward amplification of the tropical waves through the wave energy dispersion process. The results suggest that the propagation of the midlatitude wave toward the SWIO is the fundamental mechanism behind the development of the tropical waves along the Indian Ocean ITCZ.