Tuesday, 11 January 2005: 8:45 AM
The direct effect of summertime ENSO conditions on the South Asian monsoons: barotropic and baroclinic teleconnection mechanisms
Two distinct teleconnection mechanisms explaining the interannual covariability of northern hemisphere summertime ENSO conditions and the South Asian monsoons are presented. First, summertime convection over the far eastern Pacific Ocean is shown to excite stationary barotropic Rossby waves that propagate eastward and enter the North African-Asian jet. These disturbances are associated with colder upper tropospheric potential temperatures within the North African-Asian jet, which reduce the upper tropospheric thermal gradient between the Asian landmass and equatorial Indian Ocean. This diminution of the thermal gradient reduces monsoon overturning circulation and associated Indian monsoon rainfall, and explains the long-observed summertime negative correlation between ENSO and the Indian monsoons. Second, east of peninsular India, monsoon rainfall is additionally affected by convection over Indonesia, which increases during La Niņa events. This Indonesian convection produces a baroclinic response within the tropics that increases subsidence and reduces monsoon rainfall over Bangladesh and Myanmar. During El Niņo events, when convection moves east of Indonesia to the central and eastern equatorial Pacific, the baroclinic response induced by this convection does not extend far enough west to produce subsidence and suppression of monsoon rainfall over Bangladesh and Myanmar. Consequently, summer season El Niņo conditions have two effects on the South Asian monsoons east of the Indian peninsula: 1) stationary barotropic Rossby waves reduce monsoon rainfall by diminishing the upper tropospheric land-sea thermal gradient; 2) stationary baroclinic Rossby waves fail to reach Bangladesh and Myanmar and therefore favor monsoon rainfall. These two offsetting effects appear to explain the poor correlation between ENSO and the South Asian monsoons east of India.