On the Relationship between Mean Monsoon Precipitation and Low Pressure Systems in Climate Model Simulations

Abstract

The north-northwest-propagating low pressure systems (LPS) are an important component of the Indian summer monsoon (ISM). The objective detection and tracking of LPS in reanalysis products and climate model simulations are challenging because of the weak structure of the LPS compared to tropical cyclones. Therefore, the skill of reanalyses and climate models in simulating the monsoon LPS is unknown. A robust method is presented here to objectively identify and track LPS, which mimics the conventional identification and tracking algorithm based on detecting closed isobars on surface pressure charts. The new LPS tracking technique allows a fair comparison between the observed and simulated LPS. The analysis based on the new tracking algorithm shows that the reanalyses from ERA-Interim and MERRA were able to reproduce the observed climatology and interannual variability of the monsoon LPS with a fair degree of accuracy. Further, the newly developed LPS detection and tracking algorithm is also applied to the climate model simulations of phase 5 of the Coupled Model Intercomparison Project (CMIP5). The CMIP5 models show considerable spread in terms of their skill in LPS simulation. About 60% of the observed total summer monsoon precipitation over east-central India is found to be associated with LPS activities, while in model simulations this ratio varies between 5% and 60%. Those models that simulate synoptic activity realistically are found to have better skill in simulating seasonal mean monsoon precipitation. The model-to-model variability in the simulated synoptic activity is found to be linked to the intermodel spread in zonal wind shear over the Indian region, which is further linked to inadequate representation of the tropical easterly jet in climate models. These findings elucidate the mechanisms behind the model simulation of ISM precipitation, synoptic activity, and their interdependence.

Characteristics of Bay of Bengal Monsoon Depressions in the 21st Century

“…A few GCMs, in particular FGOALS, also simulate eastward moving MDs, but such MDs are nonexistent in Reanalysis data sets ( Figure S3). The linkage between the number of MDs and the amount of summer season precipitation has also been reported in previous studies (Praveen et al, 2015). On average, MDs in the CMIP5 ensemble (Figure 1e, thick black dotted line) spend most of their lifecycle below 20°N while traveling from the BoB to central India, and they only travel across 20°N while traveling west of the CI domain.…”

“…During the historical period , 5.5 (5.4 when 1965-2005 is considered), 5.8, and 5.8 westward moving MDs per year were identified that pass through the BoB in NCEP R1, MERRA2, and ERA-I, respectively, following the criteria described in section 2. It should be noted that the frequency of MDs may increase in Reanalysis data sets when both BoB and land originating MDs are considered over a bigger genesis domain (Praveen et al, 2015). It should be noted that the frequency of MDs may increase in Reanalysis data sets when both BoB and land originating MDs are considered over a bigger genesis domain (Praveen et al, 2015).…”

Structure, characteristics, and simulation of monsoon low‐pressure systems in CFSv2 coupled model

“…The vertical structure of relative vorticity is illustrated in Figure . LPS have a deep barotropic vorticity structure [ Krishnamurthy and Ajayamohan , ; Praveen et al ., ]. The deep structure of relative vorticity is evident in JRA55 with positive vorticity extending up to about 150 hPa over ocean and 200 hPa over land.…”

“…[], Hurley and Boos [], Sørland and Sorteberg [], and Praveen et al . [] to identify depressions/LPS in reanalysis. After investigating the concepts used in the above mentioned studies, an algorithm to track the LPS genesis and propagation has been devised.…”

“…Praveen et al . [] have also studied the simulation of LPS in CMIP5 coupled models where they report considerable spread in model skill at simulating the LPS which stems from the inadequate representation of Tropical Easterly jet in climate models.…”