The JASMINE Pilot Study

Abstract

The methods and initial results of an extensive pilot study, the Joint Air-Sea Monsoon Interaction Experiment (JASMINE) held in the Indian Ocean during the summer of 1999, are described. The experimental design was based on the precept that the monsoon sways back and forth from active to inactive (or break) phases and that these intraseasonal oscillations are coupled ocean-atmosphere phenomena that are important components of the monsoon system. JASMINE is the first comprehensive study of the coupled ocean-atmosphere system in the eastern Indian Ocean and the southern Bay of Bengal. Two research vessels, the NOAA ship Ronald H. Brown and the Australian research vessel Franklin, totaled 52 days of surveillance in April?June and September, with 388 conductivity-temperature-depth (CTD) casts and 272 radiosonde ascents. In addition, both ships carried identical flux systems to measure the ocean-atmosphere interaction. The Brown had five radar systems and profilers, including a cloud radar and a Doppler C-band rain radar. Active and break periods of the monsoon, and the transitions between these phases, and the onset of the 1999 South Asian summer monsoon occurred during JASMINE. The undisturbed and disturbed periods had vast differences in the net heating of the ocean, ranging from daily averages of +150 W m?2 during the former to ?100 W m?2 in the latter. Accompanying these changes in the monsoon phase were distinct states of the upper ocean and the atmosphere, including complete reversals of the near-equatorial currents on the timescales of weeks. Diurnal variability occurred in both phases of the monsoon, particularly in near-surface thermodynamical quantities in undisturbed periods and in convection when conditions were disturbed. The JASMINE observations and analyses are compared with those from other tropical regions. Differences in the surface fluxes between disturbed and undisturbed periods appear to be greater in the monsoon than in the western Pacific Ocean. However, in both regions, it is argued that the configuration of convection and vertical wind shear acts as a positive feedback to accelerate low-level westerly winds. Outstanding questions and tentative plans for the future are also discussed.