Extratropical−Tropical Teleconnection during the Summer Monsoon Season over Indian Region

1. Introduction

The atmospheric circulation process is strongly influenced by (a) direct local interactions or transport (mass, energy and momentum exchange) and (b) teleconnection between different global patterns. The direct transport enables local mass and energy balance and teleconnection interaction enables long distance information exchange over the globe and thereby relating two non-neighboring circulation pattern over the globe. An example of bidirectional teleconnection process is the interaction between the tropical to extratropical (T2E) region and extratropical to tropical (E2T) regions (Stan et al., 2017). The E2T teleconnection mediated by Eddy momentum and heat exchange during the Indian summer monsoon season is an interesting example for several reasons.

Impact of extratropical circulation on monsoon breaks in subseasonal scale has been shown decades back by Ramaswamy et al. 1962. Impacts of natural climate modes of variability such as NAO (North Atlantic Oscillation), PNA (Pacific North American Pattern) induced by E2T interaction give rise to interannual to decadal scale impacts over globe have been well documented by Walker & Bliss, 1932 and Wallace & Gutzler, 1981. Chart analysis suggests high index circulation over midlatitude are associated with active spells over Indian region while low index circulation over midlatitude is closely associated with break conditions (Ramaswamy, 1962 and Ramamurthy K, 1969). Recently over Indian region, by Chattopadhyay et al., 2015 have shown E2T interaction accruing through SST change and its impact on seasonal monsoon variability. Similarly, extratropical lower stratospheric intrusion to the tropical upper tropospheric region impacts the monsoon rainfall as shown by Fadnavis & Chattopadhyay, 2017. Spectral analysis of different Pacific station data suggesting that eddies with 4 days periodicity over equatorial region as well as 10 and 6 days periodicity over the subtropical region playing a significant role (Nitta, 1970b)in E2T teleconnection.

The current study explores the interannual variability of the horizontal and vertical structures of the E2T subseasonal eddy transport during the summer monsoon season and explores its possible role in monsoonal E2T teleconnection.

2.Data and Method

The study is based on daily data from NCEP/NCAR reanalysis (Kalnay et al. 1996) and daily rainfall data at a high spatial resolution (0.25° × 0.25° Pai et al 2014). First we have observed power spectra of u and v wind component near jet stream core. It shows two distinct spectral peaks at near 60 day and near 15 days. Then we extend the result of Nita et al. (1970) from 1950 to 2016 over Indian region based on co-spectra analysis.Co-spectra anlysis gives a measure of amplitude direction of Eddy transport.

3.Results and Conclusion

For most of the years selected, the momentum flux transport (Figure 1a-d) by all the subseasonally filtered bands are associated with poleward (positive red shading) transport and maximum amplitude is confined to 300-200hPa level. Few cases (around 7 year) are associated with equatorward (negative blue shading) transport of eddy flux in the 20-40 day & 40-60 day band. It is also seen that in the 5-10 day & 10-20 day band, the upper troposphere (100hPa and above) is transporting eddy flux equatorward in most of the cases .

The interannual variability of heat flux transport can be seen in all the spectral bands (Figure 2a-d). The major feature is at upper troposphere(above ~100hPa), it shows equatorward transport and for middle troposphere (~500-300hPa) it shows poleward transport in the entire band except in the 5-10 day period as in this band the eddy transport gets reversed for almost all the levels and for all the years.

The study further explored the interconnection between the observed eddy variability over the Indian region with the large scale variability (such as ENSO, PDO etc) of the global climate system and indicates that PDO actively modify E2T Eddy transport during the monsoon season.

3.References

Chattopadhyay, R., Phani, R., Sabeerali, C. T., Dhakate, A. R., Salunke, K. D., Mahapatra, S., et al. (2015). Quarterly Journal of the Royal Meteorological Society, 141(692), 2760–2775. https://doi.org/10.1002/qj.2562

Fadnavis, S., & Chattopadhyay, R. (2017). Journal of Climate, 30(13), 5083–5095. https://doi.org/10.1175/JCLI-D-16-0463.1

Kalnay, E., & Co authors (1996). Bull Amer Meteor Soc, 77 SRC-, 437–470.

Nitta, T. (1970). Journal of the Meteorological Society of Japan, 48(4), 348–359.

Pai, D. S., Sridhar, L., Badwaik, M. R., & Rajeevan, M. (2015). Analysis of the daily rainfall events over India using a new long period (1901–2010) high resolution (0.25° × 0.25°) gridded rainfall data set. Climate Dynamics, 45(3–4), 755–776. https://doi.org/10.1007/s00382-014-2307-1

Ramamurthy K. (1969). Forecasting Manual India Meteorological Department, Poona, India, IV 18.3(Iv), 1–57.

RAMASWAMY, C. (1962).Tellus, 14(3), 337–349. https://doi.org/10.1111/j.2153-3490.1962.tb01346.x

Stan, C., Straus, D. M., Frederiksen, J. S., Lin, H., Maloney, E. D., & Schumacher, C. (2017). Reviews of Geophysics, 55(4), 902–937. https://doi.org/10.1002/2016RG000538

Walker, G. T., & Bliss, E. W. (1932). https://doi.org/10.1002/qj.49705422601

Wallace, J. M., & Gutzler, D. S. (1981). Monthly Weather Review. https://doi.org/10.1175/1520-0493(1981)109<0784:TITGHF>2.0.CO;2

Figure Caption:

Figure 1. Interannual variations of momentum cospectra (C_f(uv), red color (positive) shading represent poleward transport & blue color (negative) shading represent equatorward transport) analysis from 1950 to 2016 year of JJAS months classified in different bands (averaged over 25°N-35°N/ 65°E-100°E), (a)5 to 10 days, (b)10-20 days, (c)20-40 days and (d)40-60 days period eddies.

Figure 2. Interannual variations of heat cospectra (C_f(Tv), red color (positive) shading represent poleward transport & blue color (negative) shading represent equatorward transport) analysis from 1950 to 2016 year of JJAS months classified in different bands (averaged over 25°N to 35°N/ 65°E to 100°E), (a)5 to 10 days, (b)10 to 20 days, (c)20 to 40 days and (d) 40 to 60 days period eddies.