Occurrence of extremely low cold point tropopause temperature during summer monsoon season: ARMEX campaign and CHAMP and COSMIC satellite observations

[1] Extreme low cold point tropopause (CPT) temperatures (T ≤ 191 K) are often observed during the monsoon season over the Bay of Bengal (BOB) and adjoining areas. This paper reports frequent occurrences of extreme low CPT temperature over the Arabian Sea (AS) and adjoining areas using radiosonde observations during the Arabian Sea Monsoon Experiment (ARMEX) from 24 June to 15 August 2002. Day-to-day variations in temperature at CPT and at the 100 hPa level observed during the ARMEX campaign show modulation by the wave activity with a period of ∼15 days, and it is observed to be closely associated with the Tropical Easterly Jet (TEJ). Characteristics of wave modulating the temperature at the CPT and at the 100 hPa level are brought out and discussed. Spatial and temporal distribution of low CPT temperature over a wide scale is examined using CHAMP and COSMIC satellite temperature data. These observations show occurrences of low CPT temperatures during the early period of the monsoon season over BOB, AS, and adjoining areas, which often extend to Africa's Horn region. An enhanced low CPT temperature occurrence during the early part of the monsoon appears to be due to the modulation of outgoing long wave-radiation (OLR), CPT temperature, and height by intraseasonal oscillation. Modulation of CPT by intraseasonal oscillation suggests that this oscillation could contribute to dehydration of the lower stratosphere. In addition, a close association is noted between the seasonal variations of the latitude of low CPT temperature and low OLR, which is similar to the anticipated seasonal movement of the Intertropical Convergence Zone (ITCZ).