LINEAR REGRESSION ANALYSIS OF THE INFLUENCE OF WESTERN NORTH PACIFIC TROPICAL CYCLONES ON THEIR LARGE‐SCALE ENVIRONMENT

Influences of the western North Pacific (WNP) tropical cyclones (TCs) on their large-scale environment are investigated by lag regressing several physical variables, including 850 hPa relative vorticity, surface zonal wind, total column water vapor, outgoing longwave radiation (OLR), precipitation and sea surface temperature (SST), on an index of TC activity accumulated cyclone energy (ACE) on a monthly time scale. Differences between these variables on the monthly and weekly time scales are discussed. The results show that: (1) The zonal wind regressed against ACE on the monthly time scale is more significant than that on the weekly time scale for both its intensity and scope. The high-intensity and long-lifetime TCs near the equatorial region have a positive effect on triggering or strengthening surface westerly anomalies in the equatorial region. The long-lived and intense equatorial surface westerly anomalies can cause westerly wind bursts, which is believed to be a major factor during the occurrence and development of ENSO events. (2) More variables such as OLR, precipitation and SST exhibit a feature corresponding to El Nino on a monthly time scale compared to that on a weekly time scale, which suggests that TC activities on a monthly time scale are more closely associated with the ENSO cycle. (3) One to two months after the genesis of TCs, the column water vapor decreases while OLR increases at the TC genesis region. This could reduce the potential for TC genesis. Although the SST decrease caused by TCs only occurs at a small spatial scale, it can be spread to a large scale via oceanic and atmospheric propagation exhibiting feedback effect. In addition, the feedback does not occur instantaneously but with a certain lag, which explains why the signal of TC influences on large-scale environment is more pronounced on the monthly time scale.