The role of the Southern Hemisphere semiannual oscillation in the development of a precursor to central and eastern Pacific Southern Oscillation warm events

The semiannual oscillation (SAO) is a twice‐yearly northward movement (in May‐June‐July (MJJ) and November‐December‐January (NDJ)) of the circumpolar trough of sea level pressure (SLP) in the Southern Hemisphere with effects throughout the troposphere. During MJJ the second harmonic of SLP, describing the SAO, has low values of SLP north of 50°S in the subtropical South Pacific, while the first harmonic, which is dominant over the Australian sector, increases to its peak. This once‐a‐year peak in negative SLP gradients (decreasing to the east) between Australia and the ocean to its east extends to the equatorial Pacific. Southern Oscillation warm events since 1950, with an intensification of this seasonal cycle, have larger‐amplitude SST anomalies in the eastern equatorial Pacific in MJJ and during the following mature phase in NDJ. Weak amplification of the seasonal cycle in MJJ tends to be followed by larger‐amplitude SST anomalies in the central equatorial Pacific during NDJ. Key Points Variations in the Southern Hemisphere semiannual oscillation can affect El Niño development A large‐amplitude semiannual oscillation can contribute to the development of a strong eastern Pacific El Niño event A small‐amplitude semiannual oscillation can contribute to the development of a relatively weak central Pacific El Niño event Plain Language Summary Variations in the seasonal cycle in the Australian‐Pacific region can affect where and how El Niño events develop.