The Influence of the PDO on Winter Precipitation During High- and Low-Index ENSO Conditions in the Eastern United States

Abstract

It is well established that, when high-index ENSO (El Nińo and La Nińa) and the PDO (Pacific Decadal Oscillation) are in the same (different) phase, the impact of the ENSO phase on winter precipitation is enhanced (weakened) in the western United States. During years of low-index (neutral) ENSO conditions, spatial patterns of winter precipitation are similar to those of La Nińa during cold PDO and El Nińo during warm PDO, particularly in the southwestern U.S. Unknown, however, is whether the PDO influence during years of neutral ENSO exists east of the Rocky Mountains. We analyzed the influence of the PDO on winter precipitation during years of high- and low-index ENSO conditions for 260 climate divisions in the eastern U.S. from 1925 to 1998. When the 33 neutral ENSO years are split by cold (12 years) and warm (21 years) PDO, the resulting winter precipitation patterns show wetter (drier) than average conditions in nearly all (80%) of the 260 climate divisions during the warm (cold) phase of the PDO. The two ENSO/PDO combinations with the driest signals for the eastern U.S. are La Nińa/warm PDO and neutral ENSO/cold PDO. The modulation effect of the PDO is significant during La Nińa in northern Texas and parts of New England and during neutral ENSO in southern Texas and the northern Plains. We suggest that seasonal forecasters in these regions may use this knowledge to improve long-lead winter forecasts during those ENSO phases, assuming the current PDO phase is well established.