The role of potential vorticity anomalies in the Somali Jet on Indian Summer Monsoon Intraseasonal Variability

• Published in: Climate dynamics 2018-06, Vol.50 (11-12), p.4149-4169
• Main Authors: Rai, P., Joshi, M., Dimri, A. P., Turner, A. G.
• Format: Article
• Language: English
• Subjects: Annual variations; Anomalies; Atmospheric boundary layer; Boundary layer winds; Boundary layers; Climate variability; Climatology; Convection; Convection heating; Curl (vectors); Diabatic heating; Earth and Environmental Science; Earth Sciences; Evolution; Feedback; Feedback loops; Geophysics/Geodesy; Heating; Heavy rainfall; Human influences; Monsoon circulation; Monsoon climates; Monsoons; Ocean circulation; Ocean currents; Oceanography; Oceans; Positive feedback; Potential vorticity; Precipitation variability; Predictions; Rain; Rainfall; Sea surface; Sea surface temperature; Somali Jet; South Asian monsoon; Summer; Summer monsoon; Surface temperature; Upwelling; Variability; Vorticity; Water resources; Wind; Wind stress
• ISSN: 0930-7575; 1432-0894
• DOI: 10.1007/s00382-017-3865-9

The climate of the Indian subcontinent is dominated by rainfall arising from the Indian summer monsoon (ISM) during June to September. Intraseasonal variability during the monsoon is characterized by periods of heavy rainfall interspersed by drier periods, known as active and break events respectively. Understanding and predicting such events is of vital importance for forecasting human impacts such as water resources. The Somali Jet is a key regional feature of the monsoon circulation. In the present study, we find that the spatial structure of Somali Jet potential vorticity (PV) anomalies varies considerably during active and break periods. Analysis of these anomalies shows a mechanism whereby sea surface temperature (SST) anomalies propagate north/northwestwards through the Arabian Sea, caused by a positive feedback loop joining anomalies in SST, convection, modification of PV by diabatic heating and mixing in the atmospheric boundary layer, wind-stress curl, and ocean upwelling processes. The feedback mechanism is consistent with observed variability in the coupled ocean–atmosphere system on timescales of approximately 20 days. This research suggests that better understanding and prediction of monsoon intraseasonal variability in the South Asian monsoon may be gained by analysis of the day-to-day dynamical evolution of PV in the Somali Jet.