Abstract
Abstract. Forecasts of Pacific jet variability are used to predict stratosphere-to-troposphere transport (STT) and tropical-to-extratropical moisture export (TME) during boreal spring over the Pacific–North American region. A retrospective analysis first documents the regionality of STT and TME for different Pacific jet patterns. Using these results as a guide, Pacific jet hindcasts, based on zonal-wind forecasts from the European Centre for Medium-Range Weather Forecasting Integrated Forecasting System, are utilized to test whether STT and TME over specific geographic regions may be predictable for subseasonal forecast leads (3–6 weeks ahead of time). Large anomalies in STT to the mid-troposphere over the North Pacific, TME to the west coast of the United States, and TME over Japan are found to have the best potential for subseasonal predictability using upper-level wind forecasts. STT to the planetary boundary layer over the intermountain west of the United States is also potentially predictable for subseasonal leads but likely only in the context of shifts in the probability of extreme events. While STT and TME forecasts match verifications quite well in terms of spatial structure and anomaly sign, the number of anomalous transport days is underestimated compared to observations. The underestimation of the number of anomalous transport days exhibits a strong seasonal cycle, which becomes steadily worse as spring progresses into summer.
Highlights
The first three empirical orthogonal functions (EOFs) patterns of the 200 hPa zonal wind all exhibit anomalies that correspond to some amount of extension or retraction and/or latitudinal shifting of the Pacific jet compared to climatology (Fig. 2)
Because the patterns of the stratosphere-to-troposphere transport (STT) and to-extratropical moisture export (TME) anomaly composites are so similar for both EOF phases, we show only the negative EOF pattern; see Figs
IFS Pacific jet forecasts for four Pacific–North American subregions are associated with significant shifts in the probability of anomalous transport, including the following: STT into the free troposphere over the North Pacific (Fig. 10a); STT into the planetary boundary layer over the intermountain-western United States (Fig. 10b); TME over the west coast of the United States (Fig. 10c); and TME to Japan and far eastern Asia (Fig. 10d)
Summary
Mass transport is important to many aspects of Pacific–North American climate, including the following: stratosphere-totroposphere transport (STT) of ozone to the planetary boundary layer, which has negative impacts on human health (Fiore et al, 2003; U.S EPA, 2006; Langford et al, 2009; Lefohn et al, 2011); STT to the free troposphere, which is needed to estimate the North American background distribution of ozone (Fiore et al, 2014; Cooper et al, 2015; Young et al, 2018); and water vapor transport, which contributes to precipitation variability (Ralph and Dettinger, 2011; Mahoney et al, 2016; Guan and Waliser, 2015; Gershunov et al, 2017). Sometime between early March and late April, the Pacific jet undergoes a transition – which typically occurs very abruptly – from being strong and largely zonally contiguous between Asia and North America to being weak, with a discontinuity in the jet that spans most of the Pacific basin (Nakamura, 1992; Newman and Sardeshmukh, 1998; Hoskins and Hodges, 2019; Breeden et al, 2021) The characteristics of this transition, and its relationship to forms of low-frequency variability that might be predictable on subseasonal timescales (e.g., ENSO), have been explored in the context of the STT of mass and ozone. For both the retrospective and hindcast analyses, STT and TME are taken from the ETH-Zürich feature-based climatology database (available for years 1979–2016; Sprenger et al, 2017), which allows us to apply a single, self-consistent measure of transport for both the retrospective (1979–2016) and hindcast (1997– 2016) analysis periods
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