EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Cross-equatorial dry intrusions and their impact on Indian summer monsoon-associated water cycle

Deepika Rai and Shira Raveh-Rubin
Deepika Rai and Shira Raveh-Rubin
  • Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot, Israel

Dry intrusion (DI) is the slantwise descent of dry air from the extratropical upper troposphere to the mid/lower troposphere of the lower latitudes. When reaching the tropical regions, DIs substantially change the overall amount of available moisture, ocean surface fluxes into the atmosphere, as well as the atmospheric stability to vertical motion and the 3-dimensional flow and associated dynamics. However, the occurrence of such events has not been quantified systematically. Here, we quantify the climatological occurrence of DIs that extend from the extratropics to tropical regions. Specifically, we focus on events that host subsequent cross-equatorial flow. Using 6-hourly ERA-Interim reanalysis data with a Lagrangian approach, we show that during the summer monsoon season (June to September) DIs enter the tropical region from the southern hemisphere with peaks that exceed 10 % frequency in time. DI arrival into the tropics is associated with dry and cold lower-tropospheric anomalies, and consequently induced ocean evaporation and sensible heat flux into the atmosphere. Although cross-equatorial DIs are rare, a hotspot of such DIs is evident in the Indian Ocean, having a potential role for Indian summer monsoon (ISM) water cycle. The dominance of the ISM for the annual rainfall over India implies that small changes in the evaporation and moisture pathways may influence the ISM precipitation downstream significantly. Indeed, we demonstrate the connection between ISM rainfall and the preceding water-cycle interaction under DI conditions, and further show that DIs entering the Indian subcontinent modify the low-level jets. 

How to cite: Rai, D. and Raveh-Rubin, S.: Cross-equatorial dry intrusions and their impact on Indian summer monsoon-associated water cycle , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4545,, 2020

Comments on the presentation

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Presentation version 1 – uploaded on 03 May 2020
  • CC1: Comment on EGU2020-4545, Dipanjan Dey, 04 May 2020

    Thank you for the nice poster. I have few questions for you.  What is the starting point for the Lagrangian trajectories and how long they ran before stopping? Recently a paper came out in climate dynamics by Swathi et al and they found intraseasonal modulation of the low level jet is strongly affected by the convection from remote sources for example northwest pacific, is this something related to that? What is the dynamics behind the DI into the tropics from the extra-topics?

    • AC1: Reply to CC1, Deepika Rai, 31 May 2020

      The calculation of trajectories is carried out for every 6-h time step between 0000 UTC 4 January 1979 and 1800 UTC 31 December 2018. At each starting time, DI trajectories initiated with a descent of at least 400 hPa during initial 48 hours are selected and are further extended, 48 h backward and 72 h further forward in time, yielding a total DI trajectory length of 7 days. A detailed discussion on the selection of DIs are available in Raveh-Rubin (2017).

      Thanks for suggesting the interesting article by Swathi et al. (2020). This article shows how the intraseasonal convective activities along with remote influences from the Northwest and East tropical Pacific jointly explains the major variations in the low-level jets associated with Indian summer monsoon (ISM). However, the present study is focused on the impact of southern-hemisphere DIs on the ISM properties which is not related directly to tropical Pacific.   

      The investigation of dynamical mechanisms governing DIs from extra-tropical regions to tropics are under process. Preliminary results suggest a trough in the upper-tropospheric waveguide in the south Indian Ocean to be related to the initiation of the DIs. This issue is currently under further investigation.