Abstract
The summer surface air temperature (SAT) in the Humid Subtropical Climate Zone in India, exhibits a significant cooling trend (~−3 °C/40 yrs.) in CRU observational data during 1961–2000. Here we investigate the contribution of internal and external factors, which are driving this cooling trend. Using the Community Earth System Model-Large Ensemble (CESM-LE), we analyze the historical climate change in presence of internal climate variability. Most of the model ensemble members could reproduce this amplified cooling (<−3 °C) as shown from CRU data. Further analyses reveals that external forcing displays a strong cooling effect over this region, while internal variability displays mixed cooling (in most cases) and warming signals. The signal to noise ratio i.e. the ratio of external forcings and internal climatic variability is less than 1, which indicates that internal climatic variability dominates over the forced response. Furthermore, to quantify the role of different external forcing factors we used the CCSM4 single forcing simulations. The simulation results from CESM-LE and CCSM4 suggest that the cooling trend over the region is primarily due to the combined influence of internal variability (~73%) and partly due to aerosol (~10%) and ozone only forcing, which strongly mask the warming effect of GHG and solar forcing.
Highlights
The Humid Subtropical Climatic Zone (HSTC) in India, which primarily comprises the Indo-Gangetic Plain (IGP) region of Central India are highly vulnerable to the climatic extremities
It is known that the Indian Summer Monsoon (ISM) is initiated by unequal surface solar heating of the Indian subcontinent and the Indian Ocean in the pre-monsoon and monsoon season
A multidecadal cooling trend is predominant over the HSTC zone, in the IGP region
Summary
The Humid Subtropical Climatic Zone (HSTC) in India, which primarily comprises the Indo-Gangetic Plain (IGP) region of Central India are highly vulnerable to the climatic extremities. This region is coming under the direct influence of the Indian summer monsoon (ISM). At regional scale the signature of anthropogenic climate change might have masked by the internal variability[6,7,10,11,12]. For isolating the relative contribution of anthropogenic climate change and the internally generated climatic variability, it requires a specific climate model with ensembles of simulations and each member is subjected to the identical external forcing
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