THE COUPLING OF THE ANNUAL CYCLE AND ENSO OVER THE TROPICAL PACIFIC

In the tropical Pacific, the annual variation of sea surface temperature (SST) consists of two distinct components with respect to the equator, 1) an antisymmetric extratropical annual cycle and 2) a symmetric equatorial annual cycle (SEAC). The former, explaining about 70% of SST variance on average, is primarily a delayed response to the solar radiation, while the latter, accounting for about 15% of total SST variance, is a result of equatorial ocean-atmosphere interaction. The antisymmetric extratropical annual cycle does not interact directly with the El Nino-Southern Oscillation (ENSO). However, the symmetric equatorial annual cycle is mutually coupled with ENSO evolution, and the artificial separation between the two can result in a distorted description of some ENSO characteristics. The SEAC-ENSO coupling involves at least two modes. One mode represents the equatorial eastern Pacific with an apparent phase evolution corresponding to the annual development of the cold tongue regime. In this mode, about half of the local interannual variance is modulated by the annual cycle, and the most preferable modulation occurs around later winter-early spring. The other mode is a standing SST pattern concentrated in the central Pacific. About one-third of the local interannual variance is directly projected on the mean annual cycle with a maximum strength in later northern summer to early fall.