1. Introduction
The interaction of the general circulation and terrain of western Mexico and the interior western United States produces diverse spatial and seasonal characteristics of the North American monsoon (Adams and Comrie 1997; Barlow et al. 1998; Douglas et al. 1993). The North American monsoon (NAM) exhibits variability on interannual (e.g., Gutzler 2004), intraseasonal (e.g., Higgins and Shi 2001), and synoptic (e.g., Carleton 1986; Bordoni and Stevens 2006) time scales. Though the focal point of the NAM is primarily centered over western Mexico, monsoonal activity periodically infiltrates northward into the intermountain western United States. The collective influence of this variability across the northern fringe of the NAM results in highly variable warm season precipitation across the southwestern United States (e.g., Higgins et al. 2004).
A large portion of warm season precipitation across the southwestern United States occurs with episodic northward-propagating moisture intrusions, hereafter referred to as monsoon surges, that foster convective outbreaks across the southwestern United States on a variety of spatial scales including both mesoscale, or gulf surges (e.g., Fuller and Stensrud 2000, hereafter FS00), and regional-scale moisture surges (e.g., Higgins et al. 2004). Despite the influence of monsoon surges on localized to regional-scale weather-related hazards in the southwestern United States (e.g., McCollum et al. 1995; Ray et al. 2007; Abatzoglou and Brown 2009), predicting the onset and evolution of monsoon surges remains a forecasting challenge (e.g., Maddox et al. 1995). Previous studies have focused on the mesoscale gulf surges of the NAM, characterized by the northward flux of moist, low-level air from the Gulf of Mexico and Gulf of California into the northern Sonoran Desert (e.g., Adams and Comrie 1997; Bordoni and Stevens 2006). These gulf surges have been attributed to tropical processes including the passage of a tropical easterly wave over western Mexico (e.g., Stensrud et al. 1995, 1997; FS00) and remnants of eastern Pacific tropical cyclones (e.g., Corbosiero et al. 2009). Other studies have stressed the importance of the midlatitude circulation in facilitating a northward flux of moisture into the southwestern United States and organizing widespread vertical motion (e.g., Higgins et al. 2004; Anderson and Roads 2002). Collectively, observational studies demonstrate that low-latitude disturbances aid moisture advection into the southwestern United States; however, they also find that tropical disturbances are not a sufficient condition for monsoon surges and highlight the need to address additional hypotheses regarding the roles played by tropical and midlatitude circulation patterns on transient behavior of the NAM.
Whereas much prior research has focused on gulf surges using observations from Yuma, Arizona, and individual radiosonde sites (e.g., Stensrud et al. 1995, 1997; FS00; Dixon 2005) a conceptual and robust means of diagnosing regional surges of the NAM that have widespread effects across the state of Arizona has not been fully explored. Station-based means to identify gulf surge events were neither designed or intended to capture the regional-scale moisture surges into the southwestern United States (e.g., FS00), yet have been examined in such context (e.g., Higgins et al. 2004). Building upon aforementioned diagnostics to identify gulf surge events and conceptual framework of monsoonal moisture surges by Hales (1972) and Brenner (1974), we suggest a method for diagnosing regional surges of the NAM into the southwestern United States using precipitable water (PW) and integrated water vapor flux (IWVF) from the North American Regional Reanalysis (NARR).
The motivation of this paper is twofold. First, we evaluate the ability of the proposed regional surge method to capture multiday periods of intense and widespread precipitation across the state of Arizona. We hypothesize that a threshold-based diagnostic that includes PW and IWVF from NARR will be better suited to characterize features associated with regional surges than diagnostics specifically designed for gulf surge events. Second, we examine the evolution of both the tropical and extratropical circulation relative to the identified regional surges to further evaluate the roles of tropical and midlatitude circulation in initiating and enabling such events.
2. Data and methods
Although the seasonal window of the NAM in the southwestern United States has been referred to a variety of ways in the literature, we constrain our focus to the core monsoonal season defined as 1 July to 15 September (e.g., Adams and Comrie 1997) for the 28-yr period (1980–2007). We employ three classes of data in our analysis: (i) hourly surface observations for Yuma from the Western Regional Climate Center (http://www.wrcc.dri.edu), (ii) synoptic and mesoscale reanalysis from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR, 4 times daily) reanalysis and the NARR (8 times daily; Mesinger et al. 2006), respectively, and (iii) gridded precipitation from the National Aeronautics and Space Administration (NASA) National Land Data Assimilation System (NLDAS-2, Mitchell et al. 2004; http://ldas.gsfc.nasa.gov/nldas/NLDAS2forcing.php, last accessed 15 June 2011). For (ii) and (iii) above, we calculated daily means and totals ending at 0000 UTC from subdaily and hourly data, respectively, and hereafter use only daily time scales in our analysis.
Prior methods to diagnose moisture surges of the NAM into the southwestern United States have utilized both surface (e.g., Stensrud et al. 1995) and radiosonde observations (Dixon 2005). Building off seminal work on moisture surges by Hales (1972) and Brenner (1974), FS00 developed an objective methodology for identifying gulf surges using surface observations from Yuma. The criteria FS00 used to diagnose an event required sustained maximum daily 2-m dewpoint temperature exceeding 15.6°C (60°F) for several days and at least one reported 10-m wind velocity from the south exceeding 4 m s−1 on the initial day of the event. While the procedure to identify gulf surges additionally classified events as either “strong” (i.e., maximum dewpoint temperature increases during 3-day period after surge onset) or “weak” (i.e., maximum dewpoint temperature decreases during 3-day period after surge onset), we primarily focus on all gulf surge events as identified from FS00. Subsequent studies have argued that the use of a single surface observation from Yuma may not be ideal for identifying regional monsoon surges because of diurnal and geographic biases, lack of upper-air observations, and potential false positives associated with outflow from localized thunderstorms (e.g., Dixon 2005). Dixon (2005) provided evidence that dewpoint temperature on mandatory pressure levels from radiosonde observations could be used to overcome some of those biases and more accurately diagnose widespread surges of moisture and precipitation into the southwestern United States.
Building off the work of FS00 and Dixon (2005), we posit that the regional reanalysis provides an additional means of diagnosing regional surges of the NAM to overcome geographic limitations of station-based approaches. For the purpose of this study, we constrain our focus to regional surges for Arizona and western New Mexico, herein defined as covering 31°–36°N, 108°–114°W (approximated by the North American Monsoon Experiment subregion 2). Following the conceptual models of Hales (1972) and Brenner (1974) we suggest that two quantities efficiently encapsulate the regional northward surge of moisture: precipitable water, or total column integrated water vapor, and the meridional component of the IWVF.
Precipitable water is regarded as a key factor in summertime precipitation events across the Southwest (Reitan 1957; Maddox et al. 1995; Jiang and Lau 2008; Lu et al. 2009) and is often used operationally (staff at NWS Tucson 2007, personal communications). Complimentary to PW, IWVF has been used to examine atmospheric moisture pathways for the NAM (e.g., Schmitz and Mullen 1996; Castro et al. 2001; Jiang and Lau 2008) and has generated recent interest as a forecasting tool for extreme precipitation events (e.g., Neiman et al. 2009). We hypothesize that IWVF is well suited to capture the moist, southerly flow characteristic of monsoon surge events on all scales. Given that the bulk of water vapor in the atmosphere is below 500 hPa, IWVF was calculated from pressure level NARR data by vertically integrating specific humidity and wind velocity from the surface to 500 hPa (Schmitz and Mullen 1996), whereas total atmospheric column PW was acquired as direct output from NARR.
To establish criteria for diagnosing regional surges across the study area, we examined relationships between the primary predictor variables, PW and IWVF, and a target variable, precipitation, by taking areal averages of all variables within the study area. Figure 1 shows a scatterplot of PW and precipitation spatially averaged over the study area with the vector showing the IWVF. For clarity, 3-day averages (totals for precipitation) are shown every third day over the 28-yr period. The strong relationship between PW and accumulated precipitation (daily correlation of r = 0.62, p < 1 × 10−6) reinforces the utility of PW as a predictor for precipitation. Figure 1 also shows that dry periods (daily regional precipitation less than 0.5 mm) generally occur with low PW (<20 mm) and weak eastward to northeastward moisture flux. By contrast, a strong northward IWVF is apparent along with high PW (>32 mm) during wet periods.
Scatterplot of precipitable water and precipitation accumulation spatially averaged over Arizona (31°–36°N, 108°–114°W, boundary denoted by area shaded in inset map) for 1 Jul–15 Sep 1980–2007. The vector component associated with each point corresponds to the observed integrated water vapor flux. For clarity, observations are only plotted every third day and represent the precipitable water and integrative water vapor flux averaged over the previous 72 h and the 72-h precipitation amount.
Citation: Monthly Weather Review 141, 1; 10.1175/MWR-D-12-00037.1
We used a threshold-based criteria for identifying regional surges based on geographically defined percentiles (defined seasonally over the 1 July to 15 September period), rather than absolute values, as a means of overcoming documented biases of NARR (Mo et al. 2005) and allowing for transferability to other subregions of the NAM and other data sources. We diagnose regional surges using a simple threshold-based approach defined as when daily averaged regional PW and the northward IWVF both exceed their 80th percentile (30 mm and 115 kg s−1 m−1, respectively) for at least two consecutive days. Sequences of days meeting the established criteria often occurred over the course of several days, occasionally interrupted by a day or two not meeting criteria. To reduce redundancy, regional surges are defined as the initial date for multiday periods meeting the aforementioned criteria and no regional surge event is considered within four days following the previous detection. It is important to note that such objective methods (i.e., using the 80th percentile) are inherently based off subjective decisions; however, the relationships found in Fig. 1 support the use of the criteria used here.
A universally accepted means to qualify or validate a regional monsoon surge is lacking. In lieu of such a means, we consider significant multiday widespread precipitation event (SMWPE) defined by daily area-averaged precipitation exceeding the 90th percentile for the 1 July–15 September period (4.5 mm day−1) on two or more consecutive days. The 90th percentile threshold was used to capture only the most extreme types of precipitation events that occur infrequently but still have impact. The SMWPE date is qualified as the first of the consecutive days exceeding this threshold. Similar to our qualification of regional surges, we discretized SMWPE by excluding events that occur less than four days from the previous event. A contingency matrix is used to assess the skill of both the monsoon surge diagnostic described herein and that of FS00 to detect SMWPE the day of and up to four days following surge diagnosis. A contingency matrix provides a means for quantifying forecast skill when evaluating dichotomous events (e.g., regional surge with associated regional precipitation). The skill of both methods is measured in success ratio (i.e., identified surge event that did have an accompanying SMWPE within four days divided by the total number of surges) and false alarm ratio (i.e., identified surge event that did not have an accompanying SMWPE within four days divided by the total number of surges).
Finally, we use composite analysis to elucidate synoptic patterns associated with regional surge events. The evolution of synoptic fields are examined up to six days prior to and six days following diagnosed regional surge dates for PW and IWVF using NARR, midtropospheric variables 500-hPa geopotential height, and 700–500-hPa pressure-weighted wind vectors using NCEP–NCAR reanalysis and precipitation using NLDAS-2. We also examine composite anomalies to better identify perturbations relative to the background time-averaged state, defined as 1 July–15 September without removal of the seasonal cycle. The standard t test is used to examine statistical significance for normally distributed values, whereas the nonparametric Wilcoxon–Mann–Whitney rank-sum test is used for precipitation. Hereafter we refer only to results significant at the 95th percentile.
3. Results
A total of 49 regional surges of the NAM were identified over the 28-yr period (1980–2007). An example of a regional surge event detected on 16 August 1983 is shown in Fig. 2. Large-scale ridging at 500 hPa across much of the United States was observed in the days prior to the event with widespread precipitation confined to northern Mexico and the remnants of Hurricane Ismael off the coast of Baja California (Figs. 2a,d). In subsequent days, the ridge breaks down and progresses eastward as a weak trough approaches the west coast of California and Oregon eventually becoming cut off from the jet stream and forming a closed low at 500 hPa, several hundred kilometers off the central California coast (Figs. 2b,e). The configuration of Rossby wave patterns with a closed low upstream of Arizona and eastward displacement of the subtropical ridge allows for the advection of a prolonged fetch of moisture emanating from the Gulf of California and the subtropical eastern Pacific with the remnants of Ismael into the southwestern United States (Fig. 2b). The strong northward moisture flux into the southwestern United States and elevated PW coincide with the regional surge diagnostics and produce widespread precipitation across the Sonoran, Mojave, and Great Basin deserts (Fig. 2c,f).
(a)–(c) Precipitable water (shaded, bold black line denoting 33-mm isoline) and integrated water vapor flux (vectors, largest vector shown is 300 kg s−1 m−1). (d)–(f) Precipitation accumulation (shaded), 500-hPa geopotential height (contours with interval of 20 m, bold line every 60 m), and 700–500-hPa averaged velocity (vectors, largest vector shown is 25 m s−1) for (a),(d) 12–13 Aug 1983; (b),(e) 14–15 Aug 1983; and (c),(f) 16–17 Aug 1983. Areas with less than 1 mm of precipitation are omitted in (d)–(f). The regional surge was diagnosed on 16 Aug 1983.
Citation: Monthly Weather Review 141, 1; 10.1175/MWR-D-12-00037.1
By contrast, approximately 150 gulf surges were identified using the methods of FS00 during the period of 1980–2007, 14% of which were observed within five days following regional surges as diagnosed using the methods of the current study. To evaluate the characteristics associated with the regional surges of the present study, and contextualize this approach relative to the gulf surge identification methods of FS00, we examined a lead–lag composite of precipitation, PW, and meridional IWVF averaged over the study area. A strong increase in PW, northward IWVF, and precipitation were observed for regional surges diagnosed in this study, whereas modest increases in PW and precipitation were observed for gulf surges diagnosed using FS00 (Fig. 3). A subsequent analysis of only strong gulf surges, per the taxonomy of FS00, found that strong gulf surges resulted in a more pronounced increase in PW and precipitation than weak gulf surges; however, the regional increase in PW and precipitation fell far below levels observed for regional surges. This difference is likely tied to the discrepancy in scale between the two surge diagnostics and phenomena of interest. Similar to the results of Higgins et al. (2004), observed precipitation for the FS00 events tended to peak 2–3 days following the initial detection date, whereas the regional schema of the present work showed regional precipitation synchronized with the detection date. This difference in timing of precipitation occurrence is likely due to FS00 identifying the leading edge of the surge moving out of the Gulf of California as detected in Yuma, whereas the current study relies on diagnostics geographically averaged over the region that are strongly coupled to concurrent regional precipitation (i.e., Fig. 1). We note that some of the observed differences could also be due to the number of monsoon surges diagnosed in the present study being approximately one-third of that diagnosed using FS00 and likely a function of scale difference between the surge features identified (regional vs local) and the stricter thresholds of the diagnostics employed in the current work.
Composite of daily (a) precipitation, (b) precipitable water, and (c) southerly IWVF averaged over the study area (31°–36°N, 108°–114°W) relative to regional surge dates diagnosed by this study (solid) and gulf surges following the method of FS00 (dashed).
Citation: Monthly Weather Review 141, 1; 10.1175/MWR-D-12-00037.1
An additional comparison between diagnostic methods was conducted to examine how well methods were a proxy for the 28 SMWPE found during the 28-yr period. More than half of these events (15) occurred on the day of or within two days following regional surges whereas 10 were found to occur within four days following gulf surges. Additionally, regional surges were significantly more accurate predictors of SMWPE as seen through the contingency matrix statistics. During the 1980–2007 study period gulf surges had a higher false-alarm ratio at approximately 93% compared to the regional method that resulted in a false alarm for 69% of the identified regional surges. The regional surge diagnostic also had a higher success ratio of 31% versus the 6.7% success ratio of the gulf surge diagnostic. Collectively, these results suggest that our extension of diagnosing gulf surges by FS00 to regional surges provided added skill in detecting high-impact rainfall events on the regional scale. Methods to identify gulf surge events were not intended to capture regional-scale phenomena (e.g., Stensrud et al. 1997) and key in surges different spatial scales; however, given the widespread usage of gulf surges to broader geographic scales, this comparison is warranted.
Lead–lag composite analysis was performed relative to the 49 regional surge events to better understand associations with tropical and midlatitude circulation patterns. Five to six days prior to regional surges, a strong westward flux of moisture is noted across northern Mexico from the Gulf of Mexico and is corroborated by above-normal precipitation amounts across Mexico (Figs. 4a,d), while an amplified ridge dominates much of the interior western United States (Fig. 4d). Strong radiative heating coincident with the ridge enhances low-level heating and primes the region for instability ahead of the moisture plume. Two to three days prior to the regional surge there is a strong northward flux of subtropical moisture from the Sierra Madre through the Gulf of California and enhanced precipitation across the Sierra Madre Occidental, Baja California, and the Sonoran deserts of California and Arizona (Figs. 4b,e). Coincidentally, an eastward shift in the midlatitude longwave pattern, synchronizes the low-level moisture flux (IWVF) and midtropospheric flow (700–500-hPa steering flow), providing a conduit for subtropical moisture into the southwestern United States. Increased PW and strong northward IWVF into and widespread precipitation across the southwestern United States are seen concurrent to the detection of the regional surge (Figs. 4c,f). The breakdown of the midlatitude ridge enhances lower-to-midtropospheric instability and convective potential due to diffluent flow aloft ahead of the trough and the advection of cooler air into the midlevels (700–400 hPa) following multiple days of enhanced surface heating (e.g., Carleton 1986). Similar analysis for both all and strong gulf surges found that while such events were associated with a relative increase in PW and precipitation following such events, the magnitude of PW and northward flux over the region was well below that seen concurrent with regional surges (not shown).
Composite plots for the 49 regional surge events of (a)–(c) precipitable water (shaded) and IWVF (vectors, largest vector shown is 160 kg s−1 m−1), and (d)–(f) precipitation accumulation (shaded), 500-hPa geopotential height (contours), and 700–500-hPa averaged wind velocity (vectors, largest vector shown is 15 m s−1). The composite evolution of these fields is shown (a),(d) 5–6 days prior to; (b),(e) 2–3 days prior to; and (c),(f) the day of and following the date the regional monsoon surge was detected.
Citation: Monthly Weather Review 141, 1; 10.1175/MWR-D-12-00037.1
Composites of PW, IWVF, and 500-hPa geopotential height anomaly fields were examined to better facilitate the recognition of statistically significant subtropical and midlatitude patterns associated with regional surges. A well-defined westward-moving subtropical wave propagating over the Sierra Madre and into the Gulf of California is apparent in the subtropical IWVF fields (Fig. 5a). The strong westward flux of water vapor results in a significant increase in PW across much of northern Mexico and southwestern United States two to three days prior to the surge, while the enhanced ridge over the western half of the United States shifts northeastward ahead of below-normal height fields off the coast of the Pacific Northwest (Fig. 5b). This same northeastward shift of the midlatitude ridge and weak trough off the west coast of the United States was shown to be a common feature in “wet” and strong subclasses of gulf surges in Higgins et al. (2004) and FS00, respectively, and a feature observed in intraseasonal to interannual variability in monsoonal precipitation over Arizona and New Mexico in Kiladis and Hall-McKim (2004). The commonality of patterns in midlatitude Rossby waves across these studies suggests that the midlatitudes are not a passive contributor to northward intrusions of moisture along the northern fringe of the NAM. As the tropical wave migrates west of the Baja Peninsula, enhanced northward IWVF can be seen from the tip of the Baja peninsula, through the interior western United States and extending well into the upper Midwestern states (Fig. 5c). A composite analysis done specifically on the SMWPE exhibits a signature similar to that for regional surge events (not shown); however, SMWPE not associated with regional surges failed to include a coherent propagating tropical easterly wave and sustained northward moisture transport into the southwestern United States as seen in the regional surge events.
Composite PW anomaly (shaded), IWVF standardized anomaly (vectors, largest vector shown is two standard deviations) and 500-hPa geopotential height standardized anomaly (contours, starting from 0.2σ, contour interval 0.05σ with solid red isolines and dashed blue isolines representing positive and negative anomalies, respectively). The composite evolution of these fields is shown (a) 5–6 days prior to, (b) 2–3 days prior to, and (c) the day of and following the date the regional monsoon surge was detected. Only statistically significant values (p < 0.05) are shown.
Citation: Monthly Weather Review 141, 1; 10.1175/MWR-D-12-00037.1
The characteristics associated with the regional surges identified in this current study align well with the synoptic signatures of wet gulf surges in Higgins et al. (2004). Those wet events were marked by sharp increases in precipitation two to four days after surge detection identified at Yuma, although the patterns shown here are oriented slightly farther west. Likewise, composite 500-hPa geopotential height fields concurrent with regional surges (Fig. 5c) are very similar to those referred to in wet surges of Higgins et al. (2004). These circulation patterns are in stark contrast to those associated with “dry” events where the midlatitude ridge remains fixed over the western United States and dampen the enhanced midlevel southerly flow over the northern extent of the Gulf of California.
Hovmöller plots of 500-hPa geopotential height anomalies latitudinally averaged between 35°–47.5°N and subtropical IWVF averaged between 20°–25°N help illustrate potential synchronization of eastward-propagating midlatitude Rossby waves and westward-propagating tropical easterly waves concurrent with regional surges (Fig. 6). The configuration of Rossby wave patterns over the United States with the ridge axis shifted to the east of the region and trough axis located west of the region may facilitate the advection of subtropical moisture northward into the southwestern United States and mirror the conceptual model for gulf surge onset of FS00 and analysis of Higgins et al. (2004). A complimentary Hovmöller plot of PW anomalies and IVWF anomalies averaged along a south–north transect between 105°–115°W further clarify the signal of northward-surging moisture into the interior western United States over a 5–7-day period associated with an initial anomalous westward flux of moisture that pivots to a northward flux coincident with the surge date.
Hovmöller plot of (a) standardized anomalies of 500-hPa geopotential height latitudinally averaged between 37.5°–47.5°N, (b) standardized anomalies of meridional IWVF latitudinally averaged between 20°–25°N, and (c) standardized anomalies of PW and IWVF longitudinally averaged between 105°–115°W (unit vector of one standard anomaly shown for reference) relative to the 49 regional surges. Statistically significant values in (a) and (b) are denoted by × and IWVF west of 125°W are not included in (b). Vectors of IWVF that are not statistically significant were excluded from (c).
Citation: Monthly Weather Review 141, 1; 10.1175/MWR-D-12-00037.1
Our results suggest that synchronized propagation of subtropical waves westward and midlatitude waves eastward catalyze a northward surge of regional-scale moisture into the southwestern United States, reinforcing the results of previous studies on local to regional scales (e.g., FS00; Higgins et al. 2004). While tropical easterly waves appear prominent in the composite fields, only about 17% of tropical easterly waves, identified following the methods of Ladwig and Stensrud (2009), were followed by a regional surge. This is in agreement with prior studies that show tropical easterly waves as important but not solely sufficient features in driving all NAM surge events (e.g., FS00). Additionally, just over half of regional surges were preceded by a tropical easterly wave within 5 days prior to regional surge identification. Likewise, we did not observe a significant link between east Pacific tropical cyclones (from Corbosiero et al. 2009) and regional surges; however, when our analysis is extended to include the latter half of September and October, land-falling tropical cyclones whose remnants tracked through the study area exhibited a character akin to regional surges and associated widespread precipitation (i.e., Corbosiero et al. 2009; Ritchie et al. 2011).
Finally, while regional surges diagnosed by this study were infrequent, daily precipitation totals spanning one day prior, through four days following regional surges, accounted for between 20%–40% of the 1 July–15 September climatological precipitation (1980–2007) across the interior southwestern United States (Fig. 7a). In addition, we found a statistically significant interannual correlation (Spearman’s rank correlation) between the number of regional surges and the total 1 July–15 September precipitation across much of the Sonoran, Mojave, and Great Basin deserts (Fig. 7b). This highlights the potential importance of extreme events on interannual monsoonal precipitation variability. Either regional surges are preconditioned by low-frequency variability through coupled ocean–atmosphere–terrestrial processes (e.g., antecedent soil moisture, large-scale modes of climate variability), or regional surges are independent of low-frequency climate variability and thus limit our ability to more fully realize potential predictability of the NAM in the southwestern United States.
(a) Percentage of 1 Jul–15 Sep that occurs within three days following regional monsoon surge events, values less than twice climatological normal, or approximately 15%, are omitted. (b) Interannual Spearman’s rank correlation between the number of regional monsoon surge events and 1 Jul–15 Sep precipitation for 1980–2007. Only statistically significant values (p < 0.05) are shown.
Citation: Monthly Weather Review 141, 1; 10.1175/MWR-D-12-00037.1
4. Discussion and conclusions
We demonstrate a regional approach to diagnosing monsoon surges for the southwestern United States that proves a more successful predictor of regional precipitation events compared to methods designed to diagnose gulf surges. The methods described herein on gridded fields allow for transferability to short- to medium-range operational forecasting and research activities using reanalyses and regional climate models. The premise of the methods employed here for Arizona may also be applicable identifying regional surge events in other subregions of the NAM. Likewise, further efforts to understand low-frequency variability of monsoonal precipitation over the southwestern United States may benefit from the consideration of regional monsoon surges given their contribution both to climatology and interannual variability of summer precipitation (i.e., Fig. 6).
The synoptic characteristics associated with regional surges reiterate previous findings and suggest that the passage of a tropical easterly wave across Mexico and into the eastern Pacific provides the subtropical circulation pattern for moisture advection into the southwestern United States (e.g., FS00; Higgins et al. 2004). Similar to previous work, we find that while the likelihood of a regional surge is enhanced by the passage of a tropical easterly wave, such waves are not a sufficient condition for surge occurrence. Additionally, we provide further evidence of the influence of the midlatitude circulation on transient activity of the NAM (e.g., Carleton 1986; FS00; Higgins et al. 2004; Corbosiero et al. 2009). We suggest that the configuration of the midlatitude Rossby wave pattern can be important in facilitating regional surges and widespread precipitation events across the southwestern United States. Regional surges preferentially occurred following the amplification and subsequent breakdown and eastward shift in the ridge over the western United States. Though previous research has shown that the placement of a midlatitude trough upstream adjacent to the NAM core causes a shutdown of surges (e.g., Stensrud et al. 1997; FS00), this analysis suggest that the steering influence of the upstream trough assists the transport of water vapor northward from the core of the NAM into the southwestern United States and enhances large-scale convective midtropospheric instability (e.g., Lang et al. 2007; Anderson and Roads 2002; Carleton 1986). Such findings are consistent with Higgins et al. (2004) who showed that the northeast shift in the monsoonal ridge allows for widespread convective outbreaks over the southwestern United States due to enhanced northward moisture transport into the southwestern United States.
Thus, we hypothesize that the eastward shift of the monsoonal anticyclone and upstream trough provide a conduit for the northward extension of subtropical moisture organized by a tropical disturbance (e.g., easterly wave, tropical cyclone) into the interior southwestern United States. The regional surges that affect the southwestern United States tend to lack the strong midlatitude dynamics of tropical moisture plumes, or atmospheric rivers, that impact the central and western United States (e.g., Dirmeyer and Kinter 2009; Higgins et al. 2011; Ralph et al. 2006). However, we suggest that regional surges associated with the NAM adhere to a subtle, albeit important, coupling of tropical–extratropical dynamics.
Acknowledgments
The authors appreciate the use NLDAS-2 data from acquired as part of the mission of NASA’s Earth Science Division and archived and distributed by the Goddard Earth Sciences (GES) Data and Information Services Center (DISC). They also acknowledge Jim Ashby from the Western Regional Climate Center for providing station data and helpful reviews from Eugene Cordero, Robert Bornstein, and two anonymous reviewers. Funding for this work was provided from NSF Grant ATM-0801474.
REFERENCES
Abatzoglou, J. T., and T. J. Brown, 2009: Influence of the Madden–Julian oscillation on summertime cloud-to-ground lightning activity over the continental United States. Mon. Wea. Rev., 137, 3596–3601.
Adams, D. K., and A. C. Comrie, 1997: The North American monsoon. Bull. Amer. Meteor. Soc., 78, 2197–2213.
Anderson, B. T., and J. O. Roads, 2002: Regional simulation of summertime precipitation over the southwestern United States. J. Climate, 15, 3321–3342.
Barlow, M., S. Nigam, and E. Berbery, 1998: Evolution of the North American monsoon system. J. Climate, 11, 2238–2257.
Bordoni, S., and B. Stevens, 2006: Principal component analysis of the summertime winds over the Gulf of California: A gulf surge index. Mon. Wea. Rev., 134, 3395–3414.
Brenner, I. S., 1974: A surge of maritime tropical air—Gulf of California to the southwestern United States. Mon. Wea. Rev., 102, 375–389.
Carleton, A. M., 1986: Synoptic-dynamic character of ‘bursts’ and ‘breaks’ in the South-West U.S. summer precipitation singularity. J. Climatol., 6, 605–623.
Castro, C. L., T. B. McKee, and R. A. Pielke, 2001: The relationship of the North American monsoon to tropical and North Pacific sea surface temperatures as revealed by observation analyses. J. Climate, 14, 4449–4473.
Corbosiero, K. L., M. J. Dickinson, and L. F. Bosart, 2009: The contribution of eastern North Pacific tropical cyclones to the rainfall climatology of the Southwest United States. Mon. Wea. Rev., 137, 2415–2435.
Dirmeyer, P. A., and J. L. Kinter III, 2009: The “Maya Express”: Floods in the U.S. Midwest. Eos, Trans. Amer. Geophys. Union, 90, 101, doi:10.1029/2009EO120001.
Dixon, P. G., 2005: Using sounding data to detect gulf surges during the North American monsoon. Mon. Wea. Rev., 133, 3047–3052.
Douglas, M. W., R. A. Maddox, K. Howard, and S. Reyes, 1993: The Mexican monsoon. J. Climate, 6, 1665–1677.
Fuller, R. D., and D. J. Stensrud, 2000: The relationship between tropical easterly waves and surges over the Gulf of California during the North American monsoon. Mon. Wea. Rev., 128, 2983–2989.
Gutzler, D. S., 2004: An index of interannual precipitation variability in the core of the North American monsoon region. J. Climate, 17, 4473–4480.
Hales, J. E., 1972: Surges of maritime tropical air northward over the Gulf of California. Mon. Wea. Rev., 100, 298–306.
Higgins, R. W., and W. Shi, 2001: Intercomparison of the principal modes of interannual and intraseasonal variability of the North American Monsoon System. J. Climate, 14, 403–417.
Higgins, R. W., W. Shi, and C. Hain, 2004: Relationships between Gulf of California moisture surges and precipitation in the southwestern United States. J. Climate, 17, 2983–2997.
Higgins, R. W., V. E. Kousky, and P. Xie, 2011: Extreme precipitation events in the south-central United States during May and June 2010: Historical perspective, role of ENSO, and trends. J. Hydrometeor., 12, 1056–1070.
Jiang, X., and N.-C. Lau, 2008: Intraseasonal teleconnection between the North American and western North Pacific monsoons with a 20-day time scale. J. Climate, 21, 2664–2679.
Kiladis, G. N., and E. A. Hall-McKim, 2004: Intraseasonal modulation of precipitation over the North American monsoon region. Preprints, 15th Symp. on Global Change and Climate Variations, Seattle, WA, Amer. Meteor. Soc., 11.4. [Available online at http://ams.confex.com/ams/pdfpapers/72428.pdf.]
Ladwig, W. C., and D. J. Stensrud, 2009: Relationship between tropical easterly waves and precipitation during the North American monsoon. J. Climate, 22, 258–271.
Lang, T. J., D. A. Ahijevych, S. W. Nesbitt, R. E. Carbone, S. A. Rutledge, and R. Cifelli, 2007: Radar-observed characteristics of precipitating systems during NAME 2004. J. Climate, 20, 1713–1733.
Lu, E., X. Zeng, Z. Jiang, Y. Wang, and Q. Zhang, 2009: Precipitation and precipitable water: Their temporal-spatial behaviors and use in determining monsoon onset/retreat and monsoon regions. J. Geophys. Res., 114, D23105, doi:10.1029/2009JD012146.
Maddox, R. A., D. McCollum, and K. Howard, 1995: Large-scale patterns associated with severe summertime thunderstorms over central Arizona. Wea. Forecasting, 10, 763–778.
McCollum, D. M., R. A. Maddox, and K. W. Howard, 1995: Case study of a severe mesoscale convective system in central Arizona. Wea. Forecasting, 10, 643–665.
Mesinger, F., and Coauthors, 2006: North American Regional Reanalysis. Bull. Amer. Meteor. Soc., 87, 343–360.
Mitchell, K. E., and Coauthors, 2004: The multi-institution North American Land Data Assimilation System (NLDAS) project: Utilizing multiple GCIP products and partners in a continental distributed hydrological modeling system. J. Geophys. Res., 109, D07S90, doi:10.1029/2003JD003823.
Mo, K. C., M. Chelliah, M. L. Carrera, R. W. Higgins, and W. Ebisuzaki, 2005: Atmospheric moisture transport over the United States and Mexico as evaluated in the NCEP regional reanalysis. J. Hydrometeor., 6, 710–728.
Neiman, P. J., A. B. White, F. M. Ralph, D. Gottas, and S. I. Gutman, 2009: A water vapor flux tool for precipitation forecasting. Water Manage., 162 (2), 83–94.
Ralph, F. M., P. J. Neiman, G. A. Wick, S. I. Gutman, M. D. Dettinger, D. R. Cayan, and A. B. White, 2006: Flooding on California’s Russian River: Role of atmospheric rivers. Geophys. Res. Lett., 33, L13801, doi:10.1029/2006GL026689.
Ray, A. J., G. M. Garfin, M. Wilder, M. Vásquez-León, M. Lenart, and A. C. Comrie, 2007: Applications of monsoon research: Opportunities to inform decision making and reduce regional vulnerability. J. Climate, 20, 1608–1627.
Reitan, C. H., 1957: The role of precipitable water vapor in Arizona’s summer rains. Tech. Rep. on the Meteorology and Climatology of Arid Regions 2, Institute of Atmospheric Physics, The University of Arizona, Tucson, AZ, 19 pp.
Ritchie, E. A., K. M. Wood, D. S. Gutzler, and S. R. White, 2011: The influence of eastern Pacific tropical cyclone remnants on the southwestern United States. Mon. Wea. Rev., 139, 192–210.
Schmitz, T. J., and S. L. Mullen, 1996: Water vapor transport associated with the summertime North American monsoon as depicted by ECMWF analyses. J. Climate, 9, 1621–1634.
Stensrud, D. J., R. L. Gall, S. L. Mullen, and K. W. Howard, 1995: Model climatology of the Mexican monsoon. J. Climate, 8, 1775–1794.
Stensrud, D. J., R. L. Gall, and M. K. Nordquist, 1997: Surges over the Gulf of California during the Mexican monsoon. Mon. Wea. Rev., 125, 417–437.
