The 10-25 day modes and their modulation of the Northern Hemisphere monsoon systems

The intraseasonal modes in the submonthly frequency range are examined during the Northern Hemisphere summer season. The focus is on the tropical extratropical interactions and the influence of the Asian monsoon on rainfall over the United States.

The leading mode of outgoing long wave radiation anomalies (OLRAs) in the 10-30 day band is a three-cell pattern with positive loadings centered at the equator over the Indian Ocean, negative loadings over the South China Sea and another weak positive center located at Eq, 170 oE. The time-space evolution shows westward propagation of OLRA from the western Pacific through the South China Sea to Burma and northward propagation of OLRA from the Indian Ocean to the Indian continent. The period is about 20 days. These are important features of the 10-25 day mode modulating the Indian monsoon. The atmospheric responses to different phases of the mode are the Rossby wave trains extending from the convective region to North America. When positive anomalies arrive at the southwestern United States, rainfall starts there. The rainfall anomalies over the United States associated with this mode show eastward propagation from the west coast of California through the central United States to Florida. The three cell precipitation anomaly pattern and the phase reversal of rainfall anomalies between the Southwest and the central United States are signatures of the 20-25 day mode based on the Arizona and New Mexico rainfall index (Mo 2000).

The third mode has a shorter time scale of 16-20 days. It is associated with rainfall over the South China Sea. The large impact on rainfall over North America can be seem over Mexico and the southern United States.