Interdecadal Modulation of ENSO Teleconnections

The existence of quasi-decadal variability in the Pacific sector climate (e.g., Latif and Barnett 1994, 1996; Mantua et al. 1997; Minobe 1997) has important implications for long-range climate prediction in North America. The spatial manifestations in climate variables around the North Pacific sector associated with the North Pacific oscillation (NPO) are qualitatively similar to those of ENSO. Characterizing the high NPO phase are the anomalously deep Aleutian low, cold western and central North Pacific, and warm eastern Pacific coastal waters, as well as anomalous warming in the central and eastern tropical Pacific. Reversed climatic conditions characterize the low NPO phase. The response of the Pacific sector climate to ENSO is qualitatively similar but relatively stronger in the Tropics and weaker in the midlatitudes (Mantua et al. 1997; Zhang et al. 1997). This similarity of climatic responses to ENSO and NPO suggests that the more slowly evolving NPO can modulate ENSOrelated climate predictability over North America, especially in the west. The expected form of this modulation is a stronger climate response to El Niño (La Niña) during those epochs marked by a high (low) phase of the NPO. We hypothesize that this would correspond to a synergetic (constructive) match of NPO and ENSO phases (warm ENSO–high NPO or cold ENSO–low NPO), while the opposite (destructive) match would tend to weaken the effects of either mode’s influence on the North American climate. The purpose of this paper is to show that this is indeed the case. ENSO signals in the North American climate have been documented by many researchers (e.g., Ropelewski and Halpert 1986, 1996; Kiladis and Diaz 1989; Gershunov and Barnett 1998). The large inter– El Niño or inter–La Niña variability characteristic of these seasonal anomalies has also attracted attention (Kumar and Hoerling 1997; Livezey et al. 1997; Gershunov 1998). This type of interevent variability Interdecadal Modulation of ENSO Teleconnections