The midlatitude warming during 1998-2000

Large scale tropospheric warming of the Northern and Southern Hemisphere extratropics occurred during the period January 1998-January 2000. The anomalies were remarkable for their amplitude, their persistence, their zonal symmetry with warming occurring at all longitudes between 30-50 degrees latitude, and their axial symmetry with respect to the equator. The origin of the anomalies are investigated using atmospheric general circulation models forced by the global sea surface temperatures (SSTs) of this period. Two ensembles, based on different models, reproduce the key observed features indicating that the unusual climate state was forced by the global SSTs. However, the atmospheric states were unrelated to the strong and protracted La Nina of the east equatorial Pacific Ocean during this time. We speculate on the role of the Indo-Pacific warm pool, which itself experienced unprecedented warming in 1998 and 1999.