The neglected Indo‐Gangetic Plains low‐level jet and its importance for moisture transport and precipitation during the peak summer monsoon

Abstract

Accurately simulating the Indo‐Asian monsoon (IAM) using atmospheric general circulation models (AGCMs) is challenging but crucial. This study uses reanalysis products European Centre of Medium‐Range Forecast Interim reanalysis, Japanese Reanalysis year 55, and High Asia Reanalysis to highlight an easterly, low‐level barrier jet along the Indo‐Gangetic Plain (referred from here as IG LLJ), which we identify as the primary moisture transport mechanism for the northeastern branch of the IAM. We show that the NCAR family of AGCMs (Community Atmospheric Model (CAM)) does not capture this circulation until 1/2° or greater spatial horizontal resolution is used. The IG LLJ develops due to a persistent low‐pressure system centered over the Ganges basin and is enhanced by the Himalayas. Using diabatic heating rates and the moist Froude number as diagnostics, we find that in CAM, this branch of the IAM displays two different dynamical regimes as a function of resolution. At low resolution, the atmosphere near the Himalayas is statically unstable, diabatic heating is strong, and the moisture flow is southwesterly from the Arabian Sea and moves over the terrain (unblocked). At high resolution, the moist static stability near the Himalayan Mountains is stable, diabatic heating is weak, and the flow primarily enters easterly from the Bay of Bengal and moves parallel to the terrain (blocked). During the summer season, the low‐resolution CAM is locked into the unblocked mode, which has serious implications for interpreting topography‐monsoon interactions. For a broader context, we demonstrate that more than half of the CMIP5 models do not capture the IG LLJ, which further highlights model‐data mismatch across the IAM region.

Competing Topographic Mechanisms for the Summer Indo‐Asian Monsoon

“…Thus, we find that the vigorous Indo-Gangetic LLJ is only present when the Himalayas are present. This is consistent with the Indo-Gangetic LLJ being driven by orography (Figures 1m and 1q; Acosta & Huber, 2017). The largest difference between the IP and HM case is represented by the various moisture fluxes.…”

“…Monsoon theories—insulation and deviation of subcloud θ e from a critical gradient (Boos & Kuang, ; Nie et al, ; Privé & Plumb, ), latent or sensible heating over elevated regions (Chen et al, ; He et al, ; Wu, Liu, et al, ), and orographic mechanical forcing (Acosta & Huber, ; Park et al, )—have merit for understanding particular aspects of the monsoon, but their interactions can be complex and difficult to parse out. For example, the θ e maximum found along the IGP is associated with the modern precipitation maximum in the Himalayas, and previous studies (Boos & Kuang, , ; Ma et al, ) have linked them causally while dismissing the role of orographic thermal forcings.…”

“…Along the way, we evaluate the three main hypotheses whether topographic thermal forcings He et al, 2018;Wu et al, 2007; or the insulation mechanism (Biasutti et al, 2018;Boos, 2015;Boos & Hurley, 2013;Ma et al, 2014), or mechanical and orographic forcing are necessary or sufficient conditions for generating a modern-like IAM. the models' biases and brings the regional circulation in better agreement with observations is necessary for understanding monsoon-topography interaction (Acosta & Huber, 2017;Johnson et al, 2015;Meehl et al, 2006;Roe et al, 2016;Shields et al, 2016). Additionally, our high-resolution modeling approach enables us to also look at how smaller terrains such as the Western Ghats and Mizoram Mountains impact the regional circulation.…”

“…One limitation of such studies is that they are often difficult to interpret because many factors are changed simultaneously in the name of realism. Another limitation is that such studies are run at resolutions that do not accurately capture monsoon dynamics (Acosta & Huber, 2017) and thermodynamics (Jain et al, 2019;Mishra et al, 2018) due to the necessity of performing long simulations for paleoclimate context. An alternative approach is using idealized modeling methods in which topography is carved out from modern Park et al, 2012;, which has identified mechanisms that are crucial for the IAM.…”

The Western Tibetan Vortex as an Emergent Feature of Near‐Surface Temperature Variations

“…However, the region of correlated near-surface temperatures associated with the WTV also includes regions that experience rapid decline of glaciers, such as Spiti Lahaul (Brun et al, 2017). Precipitation associated with the WTV might shed more light on this issue (Li et al, 2018), but the WTV might not be the main dynamical response to changes in precipitation, as it is for changes in temperature (Acosta & Huber, 2017;Cannon et al, 2015;Cash et al, 2015). Precipitation associated with the WTV might shed more light on this issue (Li et al, 2018), but the WTV might not be the main dynamical response to changes in precipitation, as it is for changes in temperature (Acosta & Huber, 2017;Cannon et al, 2015;Cash et al, 2015).…”