Impacts of subtropical pacific variability on snow precipitation fraction (SPF) in the Upper Colorado River Basin

Abstract The Upper Colorado River Basin (UCRB) provides a critical freshwater supply for over ten million people in the southwestern United States, with snowpack serving as a natural reservoir that regulates streamflow throughout the year. Understanding the partitioning of total precipitation into snowfall and rainfall is essential for assessing hydrologic responses to climate variability and long-term change. This study investigates the spatiotemporal variability of the snowfall-to-total-precipitation fraction (SPF) in the UCRB during the cold season (November–March) for the 1980–2022 period. Results reveal a distinct north–south gradient in SPF, with snowfall remaining the dominant component of total precipitation except in the southern and central regions. The empirical orthogonal function analysis shows that the leading mode explains 50.3% of the total variance of SPF. This mode is strongly linked to the Pacific Meridional Mode (PMM), indicating a significant connection between SPF variability and large-scale ocean–atmosphere interactions. A statistically significant negative correlation between the PMM index and SPF is identified, suggesting that positive PMM phases are associated with warmer surface air temperatures and circulation anomalies (e.g. 500 hPa geopotential height patterns) that suppress snow formation, resulting in lower SPF values. Overall, the findings highlight the strong influence of large-scale ocean–atmosphere variability on precipitation phase partitioning in the UCRB, with important implications for regional water resource management and climate adaptation planning.