Two distinctive ENSO cycles and associated extratropical atmospheric anomalies

This study examines the time evolution of tropical ENSO sea surface temperature (SST) anomalies and associated monthly mean extratropical atmospheric circulation over the North Pacific and North America, using the NCEP-NCAR reanalysis data (1948-2004). Composites of the ENSO SSTs indicate that there are two distinctive ENSO cycles, in which the Nino 3.4 SST anomalies peak in October/November (early) and December/January (late), respectively. The two ENSO cycles also display different spatial evolutions. The early case is characterized by the emergence of SST anomalies in the eastern tropical Pacific during spring and a westward spread in the following summer and fall. In the late case the SST anomalies emerge in the central Pacific during summer and reach the maximum value in the same region during winter. The related extratropical circulation shows clearly a timing of the extratropical response to tropical forcing. The Pacific-North American (PNA) circulation pattern is established in November and December in the early case but in January through March in the late case. The circulation anomalies are much stronger in the late case than in the early case. The differences are due to the sensitivity of the extratropical response to the location of the ENSO SST anomalies, as well as the seasonal changes of the extratropical mean flow. The corresponding ENSO cycles and related atmospheric circulation in the global climate model simulations for the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) are also examined. Comparisons between the 20th century and 21st century simulations are made to demonstrate the impact of CO2 increase on the tropical ENSO cycle and the related extratropical atmospheric circulation.