Why do Similar Patterns of Tropical Convection yield Extratropical Circulation Anomalies of Opposite Sign?

Tropical precipitation anomalies associated with El Niño and Madden-Julian Oscillation (MJO) phase 1 (La Niña and MJO phase 5) are characterized by a tripole, with positive (negative) centers over the Indian Ocean and central Pacific, and a negative (positive) center over the Warm Pool region. However, their midlatitude circulation responses over the North Pacific and North America tend to be of opposite sign. To investigate these differences in the extratropical response to tropical convection, the dynamical core of a climate model is used, with boreal winter climatology as the initial flow. The model is run using the full heating field for the above four cases, as well as with heating restricted to each of seven small domains located near or over the equator, to investigate which convective anomalies may be responsible for the different extratropical responses. An analogous observational study is also performed.

For both studies, it is found that despite having a similar tropical convective anomaly spatial pattern, the extratropical response to El Niño and MJO phase 1 (La Niña and MJO phase 5) is quite different. Most notably, responses with opposite-signed upper tropospheric geopotential height anomalies are found over the eastern North Pacific, northwestern North America, and the southeastern United States. The extratropical response for each convective case most closely resembles that for the domain associated with the largest amplitude precipitation anomaly—the central equatorial Pacific for El Niño and La Niña, and the Warm Pool region for MJO phases 1 and 5.