Emmanuel et al. (2025) Signatures of MJO in the intra-seasonal variability in occurrence of cirrus and dehydration in the UTLS over Indian region

Identification

• Journal: Climate Dynamics
• Year: 2025
• Date: 2025-12-19
• Authors: Maria Emmanuel, K. Satheesan, Ajil Kottayil
• DOI: 10.1007/s00382-025-07985-1

Research Groups

• Department of Atmospheric Sciences, Cochin University of Science and Technology, Kochi, India
• ACARR, Cochin University of Science and Technology, Kochi, India
• Ministry of Earth Sciences, Prithvi Bhavan, Lodhi Road, New Delhi, India

Short Summary

This study investigates the Madden–Julian oscillation (MJO) signatures in the intraseasonal variability of cirrus cloud occurrence and dehydration in the upper troposphere and lower stratosphere (UTLS) over the Indian region. It finds clear MJO-related variations in deep convection, temperature, cirrus occurrence, and dehydration, particularly during MJO phases 2 and 3, with less prominence during the summer monsoon season.

Objective

• To examine the effect of the Madden–Julian oscillation (MJO) on water vapour dehydration and cirrus clouds in the upper troposphere and lower stratosphere (UTLS) over the Indian region.

Study Configuration

• Spatial Scale: Indian region, including the southern Indian peninsula, equatorial Indian Ocean, and Indo-Pacific region (specifically 10°S–20°N for equatorial Indian region).
• Temporal Scale: Approximately 10 to 20 years of data, covering winter (DJF), pre-monsoon (MAM), summer-monsoon (JJAS), and post-monsoon (ON) seasons. Specific data periods:
  • NOAA OLR: 2000–2020
  • NCEP Reanalysis Temperature: 2000–2020
  • MLS WVMR: 2005–2020
  • CALIOP Cirrus: 2007–2017

Methodology and Data

• Models used:
  • NCEP/NCAR Reanalysis project's global atmospheric spectral model (28 vertical levels, 210 km horizontal resolution).
• Data sources:
  • Satellite:
    • NOAA Climate Data Record (CDR) Outgoing Longwave Radiation (OLR) (from High-Resolution Infrared Radiation Sounder - HIRS).
    • Microwave Limb Sounder (MLS) Level 2 Water Vapour Mixing Ratio (WVMR) profiles.
    • Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) Level 2 Cloud Layer data (onboard CALIPSO).
  • Reanalysis: NCEP reanalysis daily temperature at 100 hPa.
  • Indices: MJO indices from the Australian government Bureau of Meteorology (based on Empirical Orthogonal Functions of zonal winds at 850 hPa and 200 hPa and OLR data).
  • Thresholds: OLR < 200 W/m² for deep convection; WVMR < 3 ppmv for dehydration; Cloud-Aerosol Discrimination (CAD) score of 100 for cirrus clouds; MJO amplitude > 1 for strong MJO events.

Main Results

• Deep convection over the Indian region shows clear intraseasonal variation, peaking during MJO phases 2 and 3.
• A clear cooling is observed at 100 hPa during MJO phases 2, 3, and 4 over the southern Indian peninsula and equatorial Indian Ocean in winter, pre-monsoon, and post-monsoon seasons. This cooling is most prominent in winter.
• A consistent MJO signature is observed in WVMR at 121 hPa and 100 hPa, with a negative anomaly (> 0.5 K) during convective MJO phases (2, 3, and 4) over the equatorial Indian Ocean and Indian landmass, strongest in winter.
• Dehydration (WVMR < 3 ppmv) is confirmed in the Tropical Tropopause Layer (TTL) during MJO phases 2 and 3, with the region of peak occurrence showing an eastward shift with MJO propagation. Occurrence frequency reaches ~15% at 121 hPa and 60–80% at 100 hPa in winter.
• Cirrus cloud occurrence frequency is maximum (up to 60%) in MJO phases 2 and 3 over southern Peninsular India and the adjoining oceanic region in all seasons except summer monsoon.
• The mean thickness of cirrus clouds almost doubles in the convective MJO phases (2 and 3) compared to non-convective phases in winter, pre-monsoon, and post-monsoon seasons.
• A significant connection exists between the occurrence of multilayer cirrus clouds (0–20% frequency) and dehydration (0–20% frequency) within the TTL, suggesting that dehydration is mainly caused by the upper layers of multilayer clouds.
• MJO signatures in cirrus occurrence and dehydration are not clearly discernible or are less prominent during the summer monsoon season due to widespread cirrus and higher tropopause temperatures.

Contributions

• This study provides a comprehensive analysis of the intraseasonal variability of cirrus clouds and water vapour dehydration in the UTLS over the Indian region, specifically linking these processes to the different phases of the Madden–Julian Oscillation (MJO).
• It highlights the significant seasonal dependence of MJO's influence, demonstrating its strong impact during winter, pre-monsoon, and post-monsoon seasons, but a less clear effect during the summer monsoon.
• The research establishes a direct connection between the occurrence of multilayer cirrus clouds and effective dehydration within the Tropical Tropopause Layer (TTL), offering insights into the microphysical processes governing water vapour transport into the stratosphere.

Funding

• DST_SERB NPDF project

Citation

@article{Emmanuel2025Signatures,
  author = {Emmanuel, Maria and Satheesan, K. and Kottayil, Ajil},
  title = {Signatures of MJO in the intra-seasonal variability in occurrence of cirrus and dehydration in the UTLS over Indian region},
  journal = {Climate Dynamics},
  year = {2025},
  doi = {10.1007/s00382-025-07985-1},
  url = {https://doi.org/10.1007/s00382-025-07985-1}
}