Moisture Structure of the Quasi-biweekly Mode Revealed by AIRS in Western Pacific

Using Atmospheric Infrared Sounder (AIRS) humidity profiles, rainfall from the Tropical Rainfall Measuring Mission (TRMM) Global Precipitation Index (GPI), Quick Scatterometer (QSCAT) satellite-observed surface winds, and SST from the Advanced Microwave Scanning Radiometer for NASA's Earth Observing System (AMSR_E), we analyzed the structure of the summer quasi-biweekly mode (QBM) over the western Pacific in 2003--2004. We find that the signal of 10--20-day oscillations in the western Pacific originates from the Philippine Sea, and propagates northwestward toward South China. The AIRS data reveal that the boundary-layer moisture provides preconditioning for QBM propagation, and leads the mid-troposphere moisture during the entire QBM cycle. The positive SST anomaly leads or is in-phase with the boundary-layer moistening, and may be a major contributor. Most likely, the 10--20-day SST anomaly positively feeds back to the atmosphere by moistening the boundary layer, destabilizing the troposphere, and leading the QBM to propagate northwestward in the western North Pacific. However, the ECMWF/TOGA (Tropical Ocean and Global Atmosphere) analysis does not display boundary-layer (BL) moisture anomalies leading the mid-troposphere moisture.