The thermal response of the tropical atmosphere to variations in equatorial Pacific sea surface temperature

Abstract

For the period 1966–1982, monthly mean temperature anomalies were determined for 30 pressure levels from 1000 mbar (0 km altitude) to 15 mbar (28 km) at five tropical radiosonde stations, four in the western Pacific Ocean (Yap, Majuro, Wake Island, and Pago Pago) and one in the Caribbean (Curacao). Long‐term trends in the data were removed, and the detrended time series were cross correlated with an index of nonseasonal Pacific sea surface temperature (SST) anomalies with lags ranging up to ±30 months. At each station the correlation diagrams show a clear division of the atmosphere into four distinct regions, each having a different response to SST variations. In the main troposphere (850–150 mbar; 2–14 km) the correlation is positive, with a maximum at a lag of 1–4 months, in general agreement with earlier studies. In the lower stratosphere (100–40 mbar; 16–22 km) the correlation is negative, with a similar delay, while the surface layer (1000–850 mbar; 0–2 km) and the upper troposphere (150–100 mbar; 14–16 km) show responses that vary from station to station. The correlation drops essentially to zero above 40 mbar (22 km). Examination of the lags at the different stations indicate that the tropospheric inphase and stratospheric out‐of‐phase responses appear to propagate zonally from an apparent point of origin in the east central equatorial Pacific with a speed of the order of 1 m s−1. The possible mechanisms underlying this propagation are discussed, and it is concluded that simple advection of air from centers of convective activity is unlikely to explain both the tropospheric and the stratospheric responses. An alternative explanation involving variations in the rate of tropospheric subsidence and stratospheric upwelling accompanying variations in convective activity is suggested. The long‐term trends in atmospheric temperature are shown to have a variation with height that parallels the variation in the SST correlation. This may indicate that the atmosphere is responding to a corresponding long‐term trend in Pacific sea surface temperatures.