Atmospheric responses to the sea surface temperature anomalies (SSTAs) composited by Rasmusson and Carpenter (1982) for the mature phase of El Nino are studied with a global general circulation model. The model is a five layer tropospheric model with a horizontal resolution of 5° by 4°in the longitudinal and latitudinal directions, respectively. Experiments are performed under the perpetual January condition; one without the SSTAs (case CNTRL), other with the SSTAs (case SSTA1) and the other with the doubled SSTAs (case SSTA2). Time integration is made for 210 days in each case.
In the tropics, precipitation increases over warm SSTAs, and the forced equatorial anomalies are formed in the vicinity of the equator, although they are not so simple as the ones obtained by Gill (1980),
etc.. Anomalies in the east-west circulation closely reflect warm SSTA distributions. Atmospheric responses increase almost linearly with the increase of SSTAs.
In the extratropics of the Northern Hemisphere, response patterns in the time averaged stream function field are elongated in the zonal direction and the so-called Pacific/North-American (PNA) pattern is missing. The amplitude of responses does not increase with the increase of the equatorial SSTAs. In case SSTA2, wavy anomaly patterns appear in the Northern Hemisphere. However, they are shifted both eastwards and equatorwards from the PNA positions. These results differ from those presented so far (Shukla and Wallace, 1983; Blackmon
et al., 1983; Geisler
et al., 1985).
An additional run (case SSTA1 ∗) is made to interpret those differences. The SSTAs westward of 140°E are eliminated in this case. Elsewhere, the adopted SSTAs are the same as those of SSTA1. This run reveals that the warm SSTAs in the vicinity of the maritime continent and the South China Sea have large impacts not only on the extratropical atmosphere (especially of the Northern Hemisphere) as suggested by Simmons et al. (1983) and Branstator (1985) but also on the equatorial atmosphere. In SSTA1 and SSTA2, equatorial forced Rossby type response has narrower extents in both the longitudinal and latitudinal directions than those in SSTA1∗, especially in the Northern Hemisphere, due to the suppression of upward motions in SSTA1 and SSTA2 over the equatorial central Pacific Ocean by the warm SSTAs westward of 140°E. This change is considered to be an important factor in causing large differences in the extratropics by changing a propagation path of a Rossby wave-train. Wavy response similar to the PNA pattern appears in SSTA1∗ in the extratropics of the Northern Hemisphere.
Characteristics of long-term variations in each case are studied with the use of the empirical orthogonal function (EOF) analysis. Dominant patterns of long-term variations change with the changes of equatorial SSTAs. In case CNTRL, the first EOF mode describes basically the north-south seesaw and resembles the one obtained from the observed data by Lau (1981) and Tokioka and Chiba (1986). On the other hand, the PNA-like pattern is obtained as the first and the second modesin case SSTA1 and SSTA2, respectively. This mode oscillates slowly with a period of about 100 days. This type of mode is also found in cases CNTRL and SSTA1*. However, it is only the fourth mode in both cases. The warm SSTAs westward of 140°E seem to enhance the PNA-like standing oscillations in the mature phase of E1 Nino.
View full abstract