Orbital Forcing Strongly Influences Trend in the Spring Himalayan Jet Latitude over the Past Millennium
Abstract
The subtropical jet (STJ) controls the climate of the Himalayan region during boreal winter and spring by guiding the mid-tropospheric storms, which are the primary source of water for nearly one billion Asian people during the dry seasons. Changes in the behavior of the STJ over the Himalayas therefore have drastic consequences for regional water supply and management. However, studies on the impacts of long-term climatic changes on jet properties over the Himalayas are rare. In this work, we used Community Earth System Model-Last Millennium Ensemble simulations to make the first attempt to diagnose forced trends in the latitudinal position of the STJ over the Himalayas (Himalayan jet latitude or HJL) spanning the past millennium (850-2005). We show, during the pre-industrial period (850-1849), compared to the weak equatorward trend in winter HJL, the spring HJL has relatively larger and significant poleward trend in its mean position. The spring HJL also displayed positive trends in variance and frequency of poleward/equatorward excursions. During the 20th century (1850-2005), the weak equatorward trend in winter HJL continues, while the spring HJL does not exhibit any significant trend. We demonstrate, during the past millennium, that orbitally-driven changes in boreal spring insolation reduced the thermal gradient between tropics and subtropical Asia, shifting the spring Himalayan jet towards the pole. The pre-industrial poleward trend in the orbital-only spring HJL continues until 2005, but that trend is masked by the 20th century greenhouse gas forcing. These findings suggest it is essential that climate models properly simulate potential nonlinear interactions between orbital forcing and anthropogenic factors in order to correctly project the low frequency (centennial-millennial) Himalayan jet variability and the associated weather.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFMPP52A..08T