Propagation of Mid-Latitudinal Rossby waves along the Jetstream waveguides and their Role in Summer Temperature Intraseasonal Oscillations and Extremes over the Indian Region
Authors: Saradambal Lekshmi , Chattopadhyay Rajib 
Published: February 2023

Extratropical Rossby waves intrude over tropical region as well as the Indian region and exert significant influence on the weather features. Over Indian region, the pre-monsoon is a dry summer season. During this season, several studies have identified drivers of heatwaves based on different aspects such as the synoptic-scale systems, regional factors, and large-scale teleconnection patterns around the globe (Perkins 2015). Essentially these drivers identified for the Indian region do not describe the heatwave events as the intensification of some modes. Midlatitude heatwaves, on the other hand, are identified as the extreme phase of Rossby Wave mode amplification. However, over the Indian region studies do not explicitly point out the existence of temperature intraseasonal modes during April-May over the Indian region, and it is not clear if some of the drivers of heatwaves can also explain the April-May temperature variations during heatwaves as derivatives (or amplification) of some subseasonal modes. This study identifies the dominant pair of the intrinsic mode of temperature intraseasonal oscillations (ISO) related to subtropical and extratropical Rossby waves, which can also explain the heatwave spikes.&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; The ISO modes are derived using the empirical orthogonal function analysis of the detrended surface temperature and further regression analysis demonstrates the dynamical origin of these spatial modes. It is found that both the modes are driven by the mid-latitudinal Rossby waves which propagate towards the Indian region following the &#8216;preferred teleconnection pathways&#8217; (Ambrizzi and Hoskins 1997). The dominant mode is related to the subtropical westerly jet waveguide, and the second mode is induced by the extratropical to European eddy-driven jet which follows the Europe-Middle East-Indian Ocean pathway. From the different phases of the oscillation obtained from these modes, two phases are favorable for the extreme temperature events and these two phases account for more than 50% of the extreme event occurred over the Indian region.&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; Global warming is however steering these two inherent modes of ISOs in surface temperature with the first mode having a significant decreasing trend and the second mode showing an increasing trend. The modal difference in trend is likely to be related to the weakening of the subtropical jetstream waveguide and the strengthening of the extratropical jetstream in a warming scenario (Archer and Caldeira 2008). The usefulness of this study is that the ISOs defined in this study could explain the maximum number of extreme temperature events occurring over the Indian region as a projection on two temperature modes. The modal trend could also account for the regional asymmetry of warming over the Indian region in the global warming scenario, and is related to the trend in jetstream waveguide those steers these modes towards the Indian region.ReferencesAmbrizzi T, Hoskins BJ (1997) Stationary rossby-wave propagation in a baroclinic atmosphere. Q J R Meteorol Soc 123:919&#8211;928. https://doi.org/https://doi.org/10.1002/qj.49712354007Archer CL, Caldeira K (2008) Historical trends in the jet streams. Geophys Res Lett 35:. https://doi.org/https://doi.org/10.1029/2008GL033614Perkins SE (2015) A review on the scientific understanding of heatwaves&#8212;Their measurement, driving mechanisms, and changes at the global scale. Atmos Res 164&#8211;165:242&#8211;267. https://doi.org/https://doi.org/10.1016/j.atmosres.2015.05.014