Characteristics of the Tropical Easterly Jet: Long‐term trends and their features during active and break monsoon phases

Abstract

[1] Using high-vertical resolution Indian mesosphere-stratosphere-troposphere (MST) radar and GPS sonde observations from Gadanki (13.5°N, 79.2°E), an attempt has been made for the first time to delineate the characteristics of Indian summer monsoon winds with special emphasis on the Tropical Easterly Jet stream (TEJ), its day-to-day, monthly, and interannual variability using observations available for more than a decade (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008). For comparison, data available during 1996-2008 from routine radiosonde observations from nearby India Meteorological Department (IMD) observatory located at Chennai (13.0°N, 80.1°E) and National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis data sets have been considered. Relatively poor vertical resolution data from IMD lead to underestimation of the jet characteristics like jet width, vertical shear above and below the jet, and the wind reversal height. Although reanalysis data sets represented well the spatial variability, they underestimate the peak zonal winds by about 5 m/s when compared to Indian MST radar and GPS sonde observations. Large day-to-day variability of TEJ characteristics (peak jet speed, jet height, jet width, zonal wind reversal height, and vertical shear of zonal wind) over Gadanki is noticed. No significant trend is observed in peak jet speed using reanalysis data sets either over Gadanki grid or over jet core region in Indian summer monsoon months. However, MST radar and GPS sonde observations show increasing trend ($1 m/s/yr) in peak jet speed particularly from the year 2000. Characteristics of the TEJ during active and break phases showed quite different features. Thus, statistics generated using the long-term database from Indian MST radar and GPS sonde provide valuable information for studying the TEJ characteristics and will be useful to incorporate in the future reanalysis data sets.

Effect of changing tropical easterly jet, low level jet and quasi‐biennial oscillation phases on Indian summer monsoon

“…Various studies hint that the size of TEJ has reduced in the recent years (Pattanaik and Satyan, 2000;Rao et al, 2004). Decrease in strength of TEJ in the recent years has been reported by Sathiyamoorthy (2005) but an increase of 1 m s −1 year −1 in strength of TEJ from 2000 onwards has been shown by Roja Raman et al (2009) and Venkat Ratnam et al (2013). Different physical mechanisms accountable for the maintenance of the TEJ are found in literature.…”

Is the trend in TEJ reversing over the Indian subcontinent?

“…During the break phase, the jet speed was slightly greater with greater jet width. Shear below (13-14 km) the jet core is greater in the active phase but shear above (17-18 km) the jet core is greater in the break phase [Raman et al, 2009]. Large sub-daily changes in the TEJ characteristics are also observed [Ratnam et al, 2011] in addition to day-to-day changes.…”

“…Interestingly, this feature is not noticed clearly in the reanalysis data though some signal of increasing trend is seen in the recent years particularly in the NECP/NCAR reanalysis data. This delayed response in the reanalysis data sets could be mainly due to the underestimation of mean winds as reported in Raman et al [2009] or partly due to selection of larger grid in these data sets.…”

“…It is to be noted that these reanalysis data sets underestimate the TEJ strength on an average by~5 m/s when compared to the MST radar and radiosonde data over Gadanki. Due to these differences, the onset of TEJ will be delayed and lasts for about a week in the months of June and September, respectively, over a given location by employing WMO criteria [Raman et al, 2009]. Nevertheless, several studies have proven that these data can rightly be used for investigation of the long-term trends [Sathiyamoorthy, 2005;Bansod et al, 2012].…”

“…[5] High resolution observations available over Gadanki (13.5 N,79.2 E) since 1996 using mesosphere-stratospheretroposphere (MST) radar [Rao et al, 1995] and radiosonde have proven to be a potential source for investigating the detailed characteristics of TEJ due to their high vertical resolutions [Narayana Rao et al, 2000;Raman et al, 2009;Ratnam et al, 2011]. Note that we have used most of the data from MST radar which works on the principle of Doppler shift, and stability will not come into picture for measuring winds.…”