Decadal Variability of Boreal Spring Surface Air Temperature Over India

ABSTRACT This study examines the decadal variability in the boreal spring surface air temperature (SAT) over India and reports the underlying mechanisms. The decadal variability of spring SAT over India exhibits close coherence with the Inter‐decadal Pacific Oscillation (IPO). Anomalous upper‐level wind convergence over India, influenced through changes in the Walker circulation modulated by the negative IPO, has played a primary role in maintaining warm SAT over India. The causal relationship further confirms the lead–lag relationship between SAT over India and IPO. The strong anomalous upper‐level (200 hPa) anticyclonic circulation, evidenced by strong positive geopotential height (GPH) anomalies southeast of the Caspian Sea extending to northern parts of India, provoked by mid‐latitude Rossby waves, is the second leading mechanism for the decadal variability of SAT over India. The source of the Rossby wave is found to be determined by the absolute vortex stretching over the north Pacific, Canada, Atlantic and Eastern Europe. It induces a wave train propagating along the mid‐latitudes leading to an anticyclonic anomaly marked by significant positive GPH anomalies in the southeast of the Caspian Sea. This convergence accumulates mass and increases GPH over northern parts of India, induces adiabatic compression and downward motion helping to sustain clear sky conditions. The local forcing such as downward short‐wave radiation, soil temperature and sensible heat flux is found to be the third leading contributor for the decadal SAT variability. Low cloud cover, soil moisture, latent heat flux and relative humidity and their feedback sustain the decadal spring SAT warming over India.