Influence of the Pacific Decadal Oscillation on Arizona Winter Precipitation during Years of Neutral ENSO

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Abstract
Recent studies indicate that correlations between ENSO and winter precipitation in the southwestern United States may vary with the phase of the Pacific decadal oscillation (PDO). In-phase relationships between ENSO and the PDO strengthen the impact of ENSO on winter precipitation, while out-of-phase relationships weaken this impact. It has been suggested that this knowledge of PDO phase can improve long lead winter forecasts. However, all of these studies have focused on the impact of the PDO on both El Niño and La Niña only. Years of neutral ENSO have been neglected even though neutral years occur roughly half the time and coincide with highly variable winter precipitation. It is expected that some of this variability may be caused by the phase of the PDO, although the extent of its relationship with ENSO is not well understood. When years of neutral ENSO from 1925 to 1998 are split by PDO phase, it is found that Arizona winter precipitation and its predictability are strongly influenced. Years of neutral ENSO/cold PDO are nearly as dry as years of La Niña/cold PDO and provide improved predictability for dry winters. While years of neutral ENSO/warm PDO are associated with wet winters and have above-average precipitation, it is of a lesser magnitude than years of El Niño/warm PDO. Because years of neutral ENSO and La Niña are similarly dry and together compose nearly three-quarters of the multidecadal cold phase of the PDO, an increased risk of extended drought for Arizona is implied upon the return of cold PDO.