Abstract
Based on the vertical velocity field of reanalysis datasets, this study defines a new dynamical index for the India-Burma trough and supports this index’s advantages by analyzing reanalysis and observational datasets. For a convenient understanding, the vertical velocities of 5 levels ranging from 700 hPa to 500 hPa within the area of 15.625°N–24.375°N and 90.625°E–100.625°E are multiplied by −1 and summed up into a time series involving each year from 1979 to 2012. The standardized value of the time series is defined as the index of India-Burma trough (IIBT). IIBT can reflect the characteristics of the annual strength and the interdecadal variation of the India-Burma trough. IIBT can also well reveal the relationship between the India-Burma trough and its upstream teleconnection. What is more, through a correlation analysis on the grid point precipitation field, respectively, with the IIBT and the India-Burma trough indices defined with vorticity and geopotential height, over southern Asia the correlation pattern between the IIBT and the precipitation field is found to nearly be the sum of the correlation patterns of the latter 2 indices with the precipitation. To the south of the TP, the correlation field between the IIBT and the grid point precipitation shows dipolar distribution, which is consistent with the correlation patterns of the IIBT with the vertical velocity, specific humidity, and the mid-level geopotential height in the same spatial location. IIBT is beneficial for more accurate study of the impact of the India-Burma trough on the associated weather and climate.
Highlights
The TP splits and merges the tropospheric westerlies
From the perspective of climate, Suo and Ding [3] studied the structure and evolution characteristics of the subtropical south branch trough in the winter half year and pointed out that the India-Burma trough is the semipermanent trough of low pressure, appearing in the subtropical south branch westerly to the south of the plateau, in more detail which is over BOB in the winter half year
The monthly Modern-Era Retrospective Analysis for Research and Applications (MERRA) atmospheric circulation data provided by Goddard Earth Sciences Data and Information Sciences Center (GES DISC) and Global Modeling and Assimilation Office (GMAO) is used
Summary
The TP splits and merges the tropospheric westerlies. When westerlies meet the TP, the anticyclonically curved flow to the north side forms the dynamic high pressure ridge, whereas the other branch flow to the south cyclonically shapes the dynamic low pressure trough, which is named as the southern branch trough or the India-Burma trough [1]. Zhang et al [11] defined the India-Burma index with the averaged geopotential height in the area of 15∘N–27.5∘N and 80∘E–100∘E at 500 hPa and pointed out that the IndiaBurma trough was relatively strong in the winter half year from the 1950s to the 1970s. Wang et al [12] defined the India-Burma index with the averaged vorticity in the area of 15∘N–25∘N and 80∘E–100∘E at 700 hPa and pointed out that the index defined with vorticity is better than that defined by geopotential height They pointed out the increasing tendency of the India-Burma trough strength after 1978 and further studied the differences of the IndiaBurma trough’s impacts on the climate of South and East Asia between 1949–1977 and 1978–2010.
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