Interannual Variability of the North American Warm Season Precipitation Regime

VOLUME 12 JOURNAL OF CLIMATE MARCH 1999

R. W. HIGGINS Climate Prediction Center, NOAA/NWS/NCEP, Washington, D.C.

Y. C HEN Research and Data Systems Corporation, Greenbelt, Maryland

A. V. DOUGLAS Department of Atmospheric Sciences, Creighton University, Omaha, Nebraska

(Manuscript received 17 October 1997, in ®nal form 13 March 1998)

ABSTRACT Interannual variability of the North American warm season±precipitation regime is examined in three regions of the United States and Mexico: Arizona±New Mexico, northwest Mexico, and southwest Mexico. Daily observed precipitation over the United States and Mexico for a 26-yr (1963±88) period and various ®elds from the National Centers for Environmental Prediction±National Center for Atmospheric Research Reanalysis are used to compare and contrast hydrologic conditions and atmospheric circulation features associated with early, late, wet, and dry monsoons in each region. Relationships between anomalous monsoon behavior and the El NinÄo±Southern Oscillation phenomenon are examined. Some factors associated with the atmosphere's lower boundary conditions that might in¯uence the interannual variability of the warm season precipitation regime are discussed. The mean seasonal evolution of the North American monsoon system is characterized by the regular northward progression of heavy precipitation from southern Mexico by early June to the southwestern United States by early July. While the seasonal normal rainfall and its variability are largest in southwest Mexico, the mean seasonal percent departure from normal is largest in Arizona±New Mexico. Wet (dry) monsoons in southwest Mexico tend to occur during La NinÄa (El NinÄo). This association is attributed, in part, to the impact of local sea surface temperature anomalies on the land±sea thermal contrast, hence the strength of the monsoon. There is also a weak association between dry monsoons in Arizona±New Mexico (northwest Mexico) and La NinÄa (El NinÄo). Wet summer monsoons in Arizona±New Mexico tend to follow winters characterized by dry conditions in the southwestern United States and vice versa. Although the onset and duration of the monsoon are quite regular in each region, the precise date of onset in a given region is highly variable and likely to be unrelated to the date of onset in the other regions. Early monsoons in Arizona±New Mexico tend to have heavy seasonal rainfall while late monsoons in northwest Mexico tend to have de®cient seasonal rainfall. The onset date in southwest Mexico is not related to seasonal rainfall. However, interannual ¯uctuations in rainfall over the entire monsoon region for the 2-month period after onset in southwest Mexico are highly correlated, suggesting that knowledge of the starting date in southwest Mexico may be useful for analyzing, understanding, and possibly predicting these ¯uctuations.

1. Introduction reversals of the circulation and precipitation regimes (e.g., Ramage 1971) with many links to weather and Monsoon circulation systems, which develop over climate ¯uctuations (e.g., Kiladis and van Loon 1988; low latitude continental regions in response to thermal contrast between the continent and adjacent oceanic re- Webster and Yang 1992). Much of North America is gions, are a major component of continental warm sea- characterized by such a monsoon system (hereafter re- son precipitation regimes (e.g., see the review by Web- ferred to as the North American monsoon system or ster 1987). These systems are characterized by seasonal NAMS). This system provides a useful framework for describing and diagnosing warm season climate controls and the nature and causes of year-to-year variability (e.g., Higgins et al. 1997b). This system displays many Corresponding author address: Dr. R. W. Higgins, Climate Pre- similarities (as well as differences) with its Asian coun- diction Center, NWS/NCEP W/NP52, 5200 Auth Road, Room 605, Washington, DC 20233-9910. terpart (e.g., Tang and Reiter 1984). While the NAMS E-mail: [email protected] is less impressive than its Asian sister on a global scale,

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Unauthenticated | Downloaded 09/25/21 07:54 PM UTC 654 JOURNAL OF CLIMATE VOLUME 12 it still has a tremendous impact on local climate. It is region; criteria used to identify these monsoon char- a speci®c goal of this study to improve our understand- acteristics are discussed in detail in sections 2b, 5, and ing of the interannual variability of the warm season 6. In addition, relationships between El NinÄo±Southern precipitation regime over North America as it relates to Oscillation (ENSO) and these monsoon characteristics the evolution of the NAMS. are explored. Of signi®cance for the understanding of the warm To advance the seasonal prediction of warm season season precipitation regime of North America is the fact precipitation over North America requires a better un- that the NAMS affects much of the United States and derstanding of the physical processes that govern the Mexico (e.g., Higgins et al. 1997b). Over the United time-dependent behavior of the monsoon system. To this States there is evidence of a continental-scale mode in end, this study also identi®es some factors that in¯uence the warm season precipitation pattern consisting of an the interannual variability of the NAMS. In the case of out of phase relationship between the Southwest and the the Asian summer monsoon, it is well known that var- Great Plains and an in phase relationship between the iations in land±sea temperature contrast exert a strong Southwest and the East Coast (e.g., Higgins et al. 1997b; control on the strength of the monsoon circulation, Mo et al. 1997). Okabe (1995) has shown that phase hence the start of the monsoon. A number of factors reversals in this pattern are related to the development have been linked to this contrast, including external con- and decay of the monsoon. A detailed description of the ditions, such as the impact of snowcover from the pre- life cycle of the NAMS in terms of development, mature vious winter on albedo and slowly varying boundary and decay phases, and a literature review of the major forcing, such as soil moisture or sea surface temperature elements of the NAMS are given in Higgins et al. (e.g., Meehl 1994). The relationships highlighted here (1997b). will be used to investigate the mechanisms of interan- Knowledge of the start or ``onset'' of the NAMS is nual variability of the NAMS in follow on studies. of considerable importance since, for example, it rep- Section 2 describes the datasets and the methodology. resents one key to the timing of the planting of crops. Section 3 reviews key features of the warm season pre- The onset is usually sudden, with the weather in the cipitation regime. The interannual variability of warm monsoon region changing abruptly from relatively hot, season precipitation over North America is discussed in dry conditions to cool, rainy ones. The interannual var- section 4. Hydrologic conditions and atmospheric cir- iability of the monsoon modulates the annual cycle to culation features associated with early, late, wet, and occasionally produce years with ¯ood or drought. While dry monsoons are discussed in sections 5 and 6. A sum- the interannual variability of the NAMS is important, mary and discussion are given in section 7. the difference between the monsoon ¯ood and drought years is smaller than the difference between the weather in the preonset and the postonset period (e.g., Webster 2. Data analysis 1987). In spite of the highly periodic nature of the a. Datasets NAMS, there are also large variations in the circulation and rainfall within the monsoon season, often referred In order to study the interannual variability of warm to as ``active'' and ``break'' periods. The intensity of season precipitation we employ a set of gridded daily the seasonal mean monsoon is in¯uenced by the nature precipitation analyses over the conterminous United of variability within the monsoon season. Previous at- States and Mexico. The analyses over the United States tempts to relate rainfall anomalies for the monsoon sea- were developed from hourly observations for approxi- son to the date of onset of the Indian monsoon (e.g., mately 2500 stations obtained from the National Weath- Dhar et al. 1980) have generally shown little relationship er Service-Techniques Development Laboratory (Hig- indicating that the intraseasonal variability of monsoon gins et al. 1996). The analyses over Mexico were de- rainfall is quite large. veloped from long-term daily observations for 161 sta- We extend our earlier work by diagnosing the inter- tions archived at the National Climatic Data Center, annual variability of the North American warm season Asheville, North Carolina. The combined United States precipitation in three regions of Mexico and the United and Mexico dataset covers the period 1 January 1963 States: Arizona±New Mexico (AZNM), northwest Mex- through 31 December 1988. The analyses were gridded ico (NWMEX), and southwest Mexico (SWMEX). The to a horizontal resolution of 2Њ lat ϫ 2.5Њ long (Fig. 1) variability of monsoon rainfall in each region is studied using a Cressman (1959) scheme with modi®cations using observed daily precipitation over the United States (Glahn et al. 1985; Charba et al. 1992). For convenience, and Mexico for a 26-yr (1963±88) period. Composites we will refer to these analyses as the US±MEXICO of observed precipitation and various ®elds from the precipitation dataset. We note that in this study the term National Centers for Environmental Prediction ``rainfall'' is equivalent to measurable precipitation. (NCEP)±National Center for Atmospheric Research The primary dataset used to study atmospheric cir- (NCAR) Reanalysis are used to compare and contrast culation features is the NCEP±NCAR Reanalysis (cur- hydrologic conditions and atmospheric circulation fea- rently underway at NCEP in cooperation with NCAR; tures for early, late, wet, and dry monsoons in each Kalnay et al. 1996). The reanalysis project has provided

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FIG. 1. Typical station distribution for hourly reporting stations in the United States and daily reporting stations in Mexico used in the US±MEXICO merged precipitation dataset. Gridlines represent the (33 ϫ 26) grid to which the station data have been analyzed. The topography of the region is also included, courtesy of the United States National Geophysical Data Center. The resolution of the data is 5 min (0.083Њ). The topography data are available online at http://www:ngdc.noaa.gov/mgg/mggd.html. Shading intervals are at 500, 1000, 2000, and 3000 m. more than 50 years (1948±98ϩ) of global gridded ®elds July. Histograms of the mean (1963±94) daily rainfall produced with a ®xed state-of-the-art analysis system (and the 5-day running mean) during summer at each and large input database (including data available after grid point over Mexico and the southwestern United the operational cutoff time). The NCEP±NCAR assim- States (Fig. 2) show the timing of the northward pro- ilation system consists of the NCEP Medium-Range gression of the monsoon. Further examination of Fig. Forecast (MRF) spectral model and the operational 2 shows that the mean daily precipitation amounts also NCEP Spectral Statistical Interpolation (SSI; Parrish decrease rapidly toward the north. The southern and and Derber 1992) with the latest improvements (Kalnay southwestern coasts of Mexico display a peak in rainfall et al. 1996). The assimilation is performed at a hori- during June followed by a relative minimum in July± zontal resolution of T62 and 28 sigma levels in the August and a secondary peak in September (not shown, vertical with seven levels below 850 hPa. In this study but see section 3 Fig. 9g). The midsummer relaxation we utilize the reanalysis winds, which are instantaneous in precipitation is not observed further north (also see ®elds available every 6 h. section 3 Fig. 9f). The northern edge of the monsoon In section 6 we explore relationships between wet extends into Arizona and New Mexico (e.g., Douglas (dry) monsoons and ENSO using sea surface temper- et al. 1993), but the rainfall is much lighter and more ature (SST) data obtained from the historical reconstruc- directly in¯uenced by midlatitude effects. Based on dif- tion of Smith et al. (1996) and gridded to a horizontal ferences in the characteristics of warm season precipi- resolution of 2Њ lat ϫ 2Њ long for the period 1950±95. tation and in the onset date of the monsoon (also see section 3 Figs. 5, 10, and 12) in Arizona±New Mexico, northwest Mexico, and southwest Mexico, we selected b. Identifying the onset date three regions (indicated by the ``#,'' ``*,'' and ``ϩ'' The onset of the Mexican Monsoon (Douglas et al. symbols on Fig. 2) to study the interannual variability 1993; Stensrud et al. 1995) is characterized by heavy of the warm season precipitation regime; hereafter we rainfall over southern Mexico, which quickly spreads refer to these regions as AZNM, NWMEX, and northward along the western slopes of the Sierra Madre SWMEX, respectively. Occidental and into Arizona and New Mexico by early In section 5 we will use the date of onset of the

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FIG. 2. Histograms of the mean (1963±88) daily and 5-day running mean precipitation (units: mm dayϪ1) during May±August at grid points in the southwestern United States and Mexico from the merged US±MEXICO precipitation database. Grid points used for the AZNM, NWMEX, and SWMEX precipitation indices (de®ned in section 2b) are indicated by the ``#,'' ``*,'' and ``ϩ'' symbols, respectively. The grid points at which the data are valid are located at the center of each box. The topography of the region is also included, as described in Fig. 1.

Unauthenticated | Downloaded 09/25/21 07:54 PM UTC MARCH 1999 HIGGINS ET AL. 657 monsoon in each region to classify monsoons as early dependent samples, i is the lag number, N is the total or late. The procedure for identifying the onset of the number of lags, n is the sample length, and ␳a and ␳b summertime rains in each region is as follows. In each are lagged autocorrelations for time series a and b. There region a precipitation index is obtained by averaging are a total of 26-yearly values in the study period, and daily accumulations of observed precipitation at each autocorrelations up to lag 13 (i.e., one-half of all sam- grid point of the appropriate region (the shaded regions ples in time) were computed (i.e., n ϭ 26 and N ϭ 13). on Fig. 2). Care is used in choosing the grid points for Each time series was then ``thinned'' according to the each precipitation index; for example, Arizona exhibits value of T and the correlation coef®cient was computed a ``pure'' monsoon signal, that is, a sudden onset of from only the independent data. The critical value of monsoon rains, while eastern New Mexico has a more the correlation coef®cient was evaluated statistically us- gradual increase due to mixed in¯uences of the mon- ing a t test (with a null hypothesis of zero correlation); soon, the dryline, and the Great Plains low-level jet (see in each case statistical signi®cance was assessed relative Fig. 4 of Higgins et al. 1997b). The mean daily (and 5- to the 95% con®dence level. day running mean) area averaged precipitation for the three regions (Fig. 3) clearly show the northward pro- 3. Key features of the summer gression of the monsoon. precipitation regime The onset date in each region is determined using the resulting time series and a threshold crossing procedure. Rainfall associated with the NAMS is clearly evident Because the rainfall amounts in each region are different in Fig. 5a, which shows seasonal mean (1963±88) pre- (see Fig. 2), it is necessary to use different threshold cipitation for July±September, usually the three rainiest criteria to de®ne monsoon onset. The magnitude and summer months. The largest values of seasonal mean duration criteria used are 0.5 mm dayϪ1 and 3 days for rainfall (Fig. 5a), in excess of 800 mm, occur along the AZNM, 1.0 mm dayϪ1 and 5 days for NWMEX, and southeast coast of Mexico in the vicinity of the Bay of 2.0 mm dayϪ1 and 5 days for SWMEX. In each region, Campeche. For this region, orography appears to play the start of the monsoon occurs when the selection cri- an important role in determining the seasonal mean rain- teria are ®rst satis®ed after 1 May. Composite evolution fall. Heavy precipitation is also observed to the west of ®elds for 1963±88 are obtained by averaging over all the Sierra Madre Occidental along the west coast of of the monsoons relative to the day when the precipi- Mexico. Rainfall in excess of 200 mm is found in the tation in a given region ®rst satis®es the threshold cri- United States from the Great Plains to the East Coast, teria; this day is designated as the onset day, or day 0. with an area exceeding 400 mm over Florida. Exami- Note that by realigning the time series in this way we nation of the mean monthly rainfall for June, July, Au- are not performing a simple average based on calendar gust, and September (not shown) reveals that, in general, day. The composite evolution of each precipitation in- July and August are comparable but larger than that for dex (Fig. 4) shows the onset of the monsoon rains. The June or September. Along the west coast of Mexico and average calendar date of onset (day 0 on each panel) is in Arizona±New Mexico, the heaviest rainfall occurs 7 June, 17 June, and 7 July in SWMEX, NWMEX, and during the month of August. AZNM, respectively. It is important to note that the The extension of the monsoon rainfall into the south- compositing scheme makes the monsoon onset appear western United States is evident in Fig. 5b, which shows to be abrupt because it is keyed to synoptic as well as the ratio (expressed in percent) of rain falling during climate variability, as evidenced by the overshoot on the 3-month period July±September to the annual mean each panel of Fig. 4. However, our choice of threshold precipitation. The highest values (exceeding 60%) are criteria minimizes the overshoot immediately after on- found along the west coast of Mexico; similar results set. Statistics for the onset date of the monsoon and a were found by Douglas et al. (1993). The maximum classi®cation of early and late monsoons in each region values extend northward along the axis of the Sierra are discussed in section 5. Madre Occidental and then northeastward across the Rio Grande valley in New Mexico and into the high plains of southeastern Colorado and western Kansas. As found c. Signi®cance of correlations by Douglas et al. (1993), southwest New Mexico ap- To assess the signi®cance of correlations presented pears to be the region most affected by the monsoon in in sections 4±6, we followed the approach in Janowiak the United States. et al. (1998). The effective time between independent The contribution of the summer monsoon rainfall to samples (Livezey 1995) was ®rst computed according the annual total does not reveal the month-to-month to variations in rainfall. To put the summer monsoon precipitation in context within the annual cycle, the con- N i tributions to the annual total precipitation for each T ϭ 1 ϩ 21Ϫ ␳␳, ͸ n ab month of the year are displayed in Fig. 6. The bulk of iϭ1 ΂΃ the annual rainfall over much of Mexico occurs during where T is the effective number of years between in- the 4-month period (June±September). Other striking

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FIG. 3. Mean (1963±88) daily and 5-day running mean area averaged precipitation (units: mm dayϪ1) for the (a) AZNM, (b) NWMEX, and (c) SWMEX regions.

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FIG. 4. Evolution of the composite mean (1963±88) daily precipitation (mm dayϪ1) for the (a) AZNM, (b) NWMEX, and (c) SWMEX regions relative to monsoon onset.

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FIG. 5. (a) Mean (1963±88) seasonal precipitation (units: mm) for July±September from the US±MEXICO merged analysis. The contours are 50, 100, 200, 400, 600, 800, and 1000 mm and values greater than 100 mm are shaded. (b) Contribution of the precipitation during July±September to the annual total, expressed in percent, from the US±MEXICO merged precipitation analysis. The contour interval is 5% and values greater than 40% are shaded.

Unauthenticated | Downloaded 09/25/21 07:54 PM UTC MARCH 1999 HIGGINS ET AL. 661 and Mexico for each month from the US±MEXICO merged analysis. The contour interval is 5% and areas with values exceeding 15% are shaded. . 6. Analysis of the contribution of the mean (1963±88) monthly precipitation to the annual mean (units: percent) over the conterminous United States IG F

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FIG. 7. Mean (1968±88) monthly 925-hPa vector wind (m sϪ1), 200-hPa streamlines, and US±MEXICO precipitation (shading) for (a) May, (b) June, (c) July, and (d) August. Circulation data are from the NCEP±NCAR Reanalysis. A topography mask has been applied to the 925-hPa winds. Precipitation amounts are in mm dayϪ1 and values greater than 1 mm dayϪ1 are shaded. The characteristic vector length is 10 m sϪ1. features include 1) the rapid increase in values along and the warm season Bolivian High over South America the west coast of Mexico and over southeastern Arizona (e.g., Johnson 1976). During this 4-month period the and New Mexico from June to July; 2) the sharp gradient large-scale 200-hPa ¯ow (streamlines on Fig. 7) is char- in values over northern Baja California during July and acterized by the northward migration of the monsoon August; 3) the rapid decrease in contributions along the anticyclone along the west coast of Mexico to a position west coast of Mexico from September to October; and over northwestern Mexico by July. Increases in precip- 4) the increased values in September over northeast itation over the southwestern United States coincide Mexico, which are likely due to increases in the fre- with the arrival of the monsoon anticyclone in July (e.g., quency of land falling tropical storms. Okabe 1995; Higgins et al. 1997b). The precipitable Throughout the warm season the low-level ¯ow over water (not shown) indicates abundant moisture over the the southern United States and Mexico is strongly in- tropical eastern Paci®c, Gulf of California, Baja Cali- ¯uenced by the subtropical anticyclones (vectors on Fig. fornia, western Mexico, and the eastern half of the Unit- 7), with brisk southerlies over the southern Great Plains ed States (see Higgins et al. 1997b). (re¯ecting the Great Plains low-level jet) and northwes- Climatological aspects of the onset of the warm sea- terlies west of Baja California (re¯ecting the Baja jet); son precipitation regime over Mexico and the United areas with no vectors indicate where the surface is above States can be viewed from maps of the mean rainfall 925 hPa. The 200-hPa circulation center (streamlines in and circulation difference between consecutive months Fig. 7) is located over the western and southern United (Fig. 8). The April±May period is characterized by a States during July and August and is likely related to transition from the cold season circulation regime to the enhanced atmospheric heating over the elevated terrain warm season one (Fig. 8a). This is accompanied by a of the western United States. The resulting middle- and decrease in upper-level westerlies over the continent, by upper-tropospheric ``monsoon high'' is analogous to the an increase in precipitation over southern Mexico in Tibetan High over Asia (e.g., Tang and Reiter 1984) response to the developing NAMS, and by an increase

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FIG. 8. Mean (1968±88) monthly 925-hPa vector wind (m sϪ1), 200-hPa streamlines, and US±MEXICO precipitation (shading) represented as a difference between consecutive months for (a) May±April, (b) June±May, (c) July±June, and (d) August±July. A topography mask has been applied to the 925-hPa winds. The characteristic vector length is2msϪ1 and precipitation differences Ͼ0.5 mm dayϪ1 (ϽϪ0.5 mm dayϪ1) are shaded dark (light). in precipitation over the central and southern Great southwestern North America and in the Southeast. Plains in response to increases in the amplitude of the Changes in the upper-tropospheric wind and divergence diurnal cycle of precipitation (e.g., Wallace 1975; Hig- ®elds (mean vertical motion) are broadly consistent with gins et al. 1996) and in the frequency of occurrence of the evolution of this precipitation pattern (e.g., Higgins the Great Plains low-level jet (e.g., Bonner 1968; Hel- et al. 1997b). Previous studies have linked the onset of fand and Schubert 1995; Higgins et al. 1997a). From summer rains over northern Mexico and the south- May to June precipitation increases over most of Mex- western United States to a decrease of rainfall over the ico; the largest increases in rainfall over the continent Great Plains (e.g., Higgins et al. 1997b; Mock 1996; for any consecutive two-month period occur over south- Tang and Reiter 1984; Douglas et al. 1993) and to an eastern Mexico during this period. Over the western increase of rainfall along the East Coast (Tang and Reiter United States there are notable increases in height (as 1984). From July to August, there are no signi®cant re¯ected in the 200-hPa winds) and an increased south- changes in the large-scale precipitation pattern over the erly component in the low-level (925 hPa) ¯ow off the conterminous United States, consistent with the fact that west coast of Mexico, consistent with the increased pre- the NAMS is in its mature phase. There is a tendency cipitation there. The largest monthly variation in rainfall for the monsoon anticyclone to begin its southward trek for the southwestern United States occurs between June as indicated by the upper-level anticyclonic circulation and July (Fig. 8c) when increases exceeding 1 mm dayϪ1 over west central Mexico and the broad cyclonic cir- are found over much of southeastern Arizona and south- culation over the northwestern United States in the dif- western New Mexico. During this period the precipi- ference map (Fig. 8d). tation regime is characterized by an out-of-phase rela- Histograms of the mean monthly precipitation at var- tionship between precipitation over southwestern North ious locations around the conterminous United States America and the U.S. Great Plains/Northern Tier and and Mexico reveal other aspects of regional relation- an in-phase relationship between precipitation over ships in precipitation (Fig. 9). Over Arizona (Fig. 9a)

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FIG. 9. Histograms of mean (1963±94) monthly precipitation (mm day Ϫ1) from selected grid points over the conterminous United States and Mexico: (a) (34ЊN, 110ЊW) in Arizona, (b) (30ЊN, 97.5ЊW) in Texas, (c) (36ЊN, 95ЊW) in Oklahoma, (d) (46ЊN, 110ЊW) in Montana, (e) (28ЊN, 110ЊW) in Sonora, (f) (20ЊN, 105ЊW) in Jalisco, and (g) (16ЊN, 97.5ЊW) in Oaxaca.

Unauthenticated | Downloaded 09/25/21 07:54 PM UTC MARCH 1999 HIGGINS ET AL. 665 the maximum precipitation occurs in August during the the precipitation database and Southern Oscillation In- peak of the monsoon. Over Texas (Fig. 9b) and dex (SOI) data, which is commonly used as an indicator Oklahoma (Fig. 9c) there are two peaks (May and Sep- of the state of ENSO. Five-month running mean SOI tember) with a relative minimum in rainfall during July data were used to identify summer (June±September) and August. Similar behavior is found over Montana seasons that experienced mature ``cold'' and ``warm'' (Fig. 9d) though the September maximum is much episode conditions during the 1963±88 period. Summer weaker. This out-of-phase relationship is consistent with seasons with mature warm episode conditions occurred changes in the large-scale circulation (as discussed in 1965, 1969, 1972, 1977, 1982, and 1987. Summer above). Rainfall over Sonora (northwest Mexico) (Fig. seasons with mature cold episode conditions occurred 9e) and Jalisco (west central Mexico) (Fig. 9f) exhibits in 1964, 1971, 1973, 1975, and 1988. Warm and cold a single peak during July±August in concert with the episodes are indicated by ``W'' and ``C,'' respectively, precipitation maximum in Arizona. Of the three Mex- on Fig. 11c. Correlations between the SOI and the sea- ican states shown, Oaxaca (southern Mexico) (Fig. 9g) sonal percent departure from normal in AZNM, is closest to convection associated with the intertropical NWMEX, and SWMEX for the 26-yr period are Ϫ0.15, convergence zone (ITCZ); this region displays a typical 0.18, and 0.48, respectively; when the 1982±83 event double peak in summer precipitation (June and Septem- is removed the correlations are roughly the same. The ber) with a relative minimum during July and August. correlation between the SOI and the seasonal percent Additional features of the tropospheric mean climate departure in SWMEX is statistically signi®cant at the during the warm season are discussed in Kousky and 95% level, implying that wet (dry) summer monsoons Ropelewski (1997). in SWMEX tend to be associated with La NinÄa (El NinÄo). This is consistent with evidence presented in section 6b that local SST in¯uences on the land±sea 4. Interannual variability of warm thermal contrast are probably an important factor for season precipitation monsoon strength in SWMEX. Maps of the standard deviation of seasonal mean rain- The composite seasonal percent departure for the El fall and the standard deviation of the seasonal percent NinÄo (La NinÄa) years is 0.6%, Ϫ10.3%, and Ϫ7.2% departure from normal based on 26 yr (1963±88) of data (Ϫ8.5%, Ϫ2.2%, and 6.5%) for AZNM, NWMEX, and are shown in Figs. 10a,b, respectively. The standard SWMEX, respectively. Thus, in the mean, El NinÄo deviation of seasonal mean rainfall (Fig. 10a) is large events are associated with de®cient monsoons in over the southern half of Mexico, where the seasonal NWMEX and SWMEX while La NinÄa events are as- normal precipitation is large. The standard deviation of sociated with heavy monsoons in SWMEX and de®cient the seasonal percent departure (Fig. 10b) is large in the monsoons in AZNM. Note that there is a reversal in the southwestern United States where the amount of rainfall departures from SWMEX to AZNM, suggesting that is small but where large changes can occur from year local boundary forcing and other midlatitude factors to year. may dominate SST effects in AZNM during ENSO (see The seasonal (June±September) percent departure section 6). Table 1 shows that during mature warm ep- from normal rainfall for each year in AZNM, NWMEX, isode conditions, negative seasonal departures are ob- and SWMEX based on 26 yr of data is shown in Fig. served in four of six cases (three of six cases) in 11. The correlations among these time series (after ac- NWMEX (SWMEX); one additional weak negative de- counting for the effective time between independent parture is observed in each region. During mature cold yearly samples as described in section 2c) are 0.54, 0.02, episode conditions, negative seasonal departures are ob- and 0.26 for (AZNM, NWMEX), (AZNM, SWMEX) served in four of ®ve cases in AZNM. and (NWMEX, SWMEX), respectively; only the ®rst coef®cient is statistically signi®cant at the 95% level. While AZNM and NWMEX are not statistically inde- 5. Characteristics of early and late monsoons pendent by this measure, differences in the mean sea- a. Interannual variability of the onset date sonal precipitation (Fig. 5a), in the standard deviation of the mean seasonal precipitation (Fig. 10a), and in the The daily precipitation indices for AZNM, NWMEX, onset date of the monsoon (Fig. 12) seem to justify our and SWMEX (see section 2b) were used to determine choice of regions. In section 6 we will show that mon- the starting date of the summer monsoon for each year soons with heavy or de®cient rainfall over one of the during the period 1963±88. Statistics associated with regions are not necessarily (and often are not) accom- the onset date in each region are given in Table 2. The panied by anomalies of the same sign (or magnitude) table shows that the mean and median dates for the start in one of the other regions, further supporting this point. of the summer monsoon are very close in each region. Research has shown that variations in seasonal pre- The largest variability is found in NWMEX, where the cipitation in some parts of North America are linked to time span between the earliest and latest start dates is the ENSO phenomenon (e.g., Ropelewski and Halpert 51 days. In AZNM the range between the earliest and 1986, 1996). In this study we examine this concept using latest start dates is closer to one month. The mean onset

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FIG. 10. Standard deviation of (a) the seasonal (June±September) mean precipitation (mm) and (b) the seasonal (June± September) percent departure from normal (units: percent) based on 26 yr (1963±88) of data. In (a) the contours are 50, 100, and 200 mm and values greater than 50 mm are shaded. In (b) the contour interval is 10% and values greater than 20% are shaded. In each case, areas where the seasonal precipitation is less than 30 mm are masked.

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month variability of monsoon rainfall in this region is quite large. Recall that this part of Mexico normally experiences a relative minimum in monsoon precipitation during July and August. The anomalies for the monsoon season in NWMEX are also not signi®cantly correlated with the onset date (the correlation between the time series is Ϫ0.32). However, the correlation between rainfall anomalies for the monsoon season in AZNM and the onset date is 0.52, which is statistically signif- icant at the 95% level. Thus, early (late) monsoons in AZNM tend to have heavy (de®cient) seasonal precipitation. Correlations between the June SOI and the onset date in each region are not statistically signi®cant at the 95% level (Ϫ0.26, 0.27, and Ϫ0.13 for AZNM, NWMEX, and SWMEX, respectively). When this calculation is repeated using the seasonal June±September SOI, similar results are obtained.

b. Classi®cation If we use the time series in Fig. 13 to classify early (late) monsoons as those whose onset date is at least one standard deviation below (above) the mean onset date, then we ®nd AZNM: Early: 1967, 1977, 1978, 1981, 1984, FIG. 11. Seasonal (June±September) percent departure from normal 1986, 1988 precipitation (%) for (a) AZNM, (b) NWMEX, and (c) SWMEX based Late: 1963, 1971, 1972, 1979, 1982, on 26 yr (1963±88) of data. Anomalies are computed with respect 1985, 1987 to the apropriate area mean for the 26-yr period. On panel (c) the years with mature cold or warm episode conditions are indicated by NWMEX: Early: 1964, 1972, 1977, 1983, 1986, a C or a W, respectively. 1987 Late: 1963, 1969, 1974, 1975, 1979, 1982 date for each grid point used in the precipitation indices SWMEX: Early: 1964, 1966, 1967, 1968, 1972, is shown in Fig. 12. Correlations among the time series 1974, 1986 of the onset date (Fig. 13) are 0.18, 0.33, and 0.22 for Late: 1969, 1970, 1973, 1975, 1980, (AZNM, NWMEX), (AZNM, SWMEX), and 1982, 1983, 1988. (NWMEX, SWMEX), respectively. These correlations are not statistically signi®cant suggesting that the onset Very few of the years are in common between the re- date of the monsoon in each region is more or less gions, reinforcing the point that there is little relation- independent. In other words, early (late) onset in one ship between the onset date of the monsoon in each region does not necessarily imply early (late) onset in region. This classi®cation produces a reasonable sepa- the other regions. The low correlation between time ration between the average calendar date of early (late) series of the onset date in each region (Figs. 13a±c) is monsoons in each region; 22 May (21 June), 28 May somewhat surprising since, in a mean sense, the mon- (2 July), and 23 June (17 July) for SWMEX, NWMEX, soon progresses northward in a rapid, orderly fashion and AZNM, respectively. The time span between the (Fig. 12). average date of onset of early and late monsoons is The possible association between the dates of onset largest in NWMEX (35 days), consistent with our pre- of the monsoon in a given region and the seasonal vious ®ndings on the variability of the onset date (see (June±September) precipitation anomalies in that region Table 2). The composite seasonal (JJAS) rainfall anom- can be determined by correlating the time series of rain- alies (percent departure from normal) for early (late) fall departure (Fig. 11) with the time series of onset date monsoons are 21.5%, 0.8%, and 0.2% (Ϫ3.0%, (Fig. 13). Despite the fact that the date of onset can Ϫ10.1%, and 0.3%) in AZNM, NWMEX, and SWMEX, ¯uctuate by more than a month, we ®nd the interesting respectively. Thus, the strongest relationships are be- but counterintuitive result that the rainfall anomalies for tween early monsoons in AZNM and heavy rainfall and the monsoon season in SWMEX are not related to the between late monsoons in NWMEX and de®cient rain- date of onset in SWMEX (the correlation between the fall; comparison of the years of early monsoons in time series is Ϫ0.03). This implies that the month-to- AZNM (listed above) to the years of wet monsoons in

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FIG. 12. Mean (1963±88) calendar date of onset for the summer monsoon at each grid point used in the AZNM, NWMEX, and SWMEX precipitation indices.

TABLE 1. Seasonal (June±September) percent departure from normal seasonal precipitation (units: percent) in AZNM, NWMEX, and SWMEX for mature-warm and -cold episode conditions. Warm and TABLE 2. Statistics associated with the onset of the summer mon- cold episode conditions were identi®ed using the 5-month running soon in Arizona±New Mexico, northwest Mexico, and southwest mean Southern Oscillation index (SOI). Mexico for 1963±88. Both Julian dates in the calendar year and calendar dates are shown. Year Event type AZNM NWMEX SWMEX Standard 1965 WARM Ϫ14.7 Ϫ8.1 5.9 deviation 1969 WARM Ϫ16.2 Ϫ20.3 Ϫ7.3 Monsoon start date of the on- 1972 WARM 8.3 4.5 Ϫ2.9 set date Range of the 1977 WARM 22.0 Ϫ1.6 Ϫ14.5 Region Mean Median (days) onset date 1982 WARM 8.5 Ϫ15.2 Ϫ35.2 AZNM 188 190 9.2 169±202 1987 WARM Ϫ4.5 Ϫ21.0 10.9 (7 Jul) (9 Jul) (18 Jun±21 Jul) 1964 COLD Ϫ9.9 8.6 Ϫ4.3 NWMEX 168 170 13.2 142±193 1971 COLD Ϫ5.3 Ϫ0.5 8.1 (17 Jun) (19 Jun) (22 May±12 Jul) 1973 COLD Ϫ42.9 Ϫ9.4 14.3 1975 COLD Ϫ7.6 Ϫ5.2 Ϫ0.8 SWMEX 158 159 11.6 135±178 1988 COLD 23.2 Ϫ4.5 15.3 (7 Jun) (8 Jun) (15 May±27 Jun)

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FIG. 13. Time series of the date of onset for the summer monsoon in (a) AZNM, (b) NWMEX, and (c) SWMEX for the period 1963± 88. The mean onset date for each region is indicated by a solid line and one standard deviation is indicated by dashed lines. All dates are Julian dates in the calendar year (e.g., day 182 ϭ 1 July, etc.).

AZNM (see section 6a) shows ®ve out of seven years in common. Reasons for the reverse relationship between early (late) monsoons and heavy (de®cient) rainfall in these regions compared to SWMEX are likely related to differences in the importance of SST anomalies, soil-moisture anomalies, and internal dynamics in each region; these differences will be explored in our follow-on studies. The composite evolution of monthly precipitation for early, late, and all (1963±88) monsoons in AZNM, NWMEX, and SWMEX is shown in Figs. 14a, b, and FIG. 14. Composite evolution of monthly precipitation (mm dayϪ1) c, respectively. In both early and late years the onset of over (a) AZNM, (b) NWMEX, and (c) SWMEX for early (dashed the monsoon rains is clearly evident in each region, just line), late (dash-dot line), and all (1963±88) monsoons (thick solid as it is in the composite based on all of the years. Early line). monsoons in AZNM are characterized by several months of above normal precipitation, starting in June c. Monsoon monitoring and potential predictability and extending through the end of the year (Fig. 14a), consistent with large seasonal anomalies (see section While the onset of the monsoon in one region is more 6b). Late monsoons in NWMEX have several months or less independent of the onset in another region, a of below normal precipitation, though the temporal co- different picture emerges if the onset date in a particular herence is not as high as in AZNM. Late monsoons in region is used to accumulate rainfall in all of the regions. AZNM, early monsoons in NWMEX, and both early Figure 15a shows time series of precipitation anomalies and late monsoons in SWMEX have more intraseasonal [departures from the mean (1963±88) daily precipitation ¯uctuations, which accounts for the weaker seasonal in each region for the 60-day period after monsoon onset signal. in SWMEX]. Correlations between the three time series

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NWMEX), (AZNM, SWMEX), and (NWMEX, SWMEX), respectively] and not statistically signi®cant. The correlation between rainfall in SWMEX and rainfall over Mexico and the conterminous United States for the 60-day period after monsoon onset in SWMEX (Fig. 16a) is signi®cant at the 95% level along the west coast of Mexico and over southern Arizona. The most signi®cant out of phase relationship is with precipitation over the northern and central Great Plains. These correlations do not imply, however, that the objective procedure is ineffective in choosing the onset date, because wet (dry) conditions for a 60-day period after onset in SWMEX do not guarantee a wet (dry) monsoon in the other regions, which usually have later onset dates. Thus, knowing the onset date of the monsoon in SWMEX may be useful for characterizing precipitation not only in SWMEX, NWMEX, and AZNM but also over other portions of the continental United States, at least for the 2-month period after onset. Signi®cant correlations with monsoon rainfall in NWMEX and AZNM (Figs. 16b,c) are more regionally con®ned than they are with monsoon rainfall in SWMEX, suggesting that knowledge of the onset date in these regions is likely to be less useful for characterizing rainfall elsewhere. In summary, these results suggest that the onset date FIG. 15. Time series of accumulated precipitation anomalies [de- of the monsoon in SWMEX is potentially useful for partures from mean (1963±88) daily values] in SWMEX (dash-dot characterizing variations in warm season precipitation line), NWMEX (dashed line), and AZNM (solid line) for the 60-day period after monsoon onset in (a) SWMEX, (b) NWMEX, and (c) over the monsoon region after onset. Maps such as those AZNM. in Fig. 16 could be constructed for any period after onset and then used to characterize rainfall for that period. There are a large number of factors that produce vari- in Fig. 15a are all statistically signi®cant at the 95% ability within a monsoon season, including synoptic- level [0.80, 0.47, and 0.70 for (AZNM, NWMEX), scale disturbances (lows, tropical storms), bursts and (AZNM, SWMEX), and (NWMEX, SWMEX), respec- breaks (e.g., Carleton 1986), monsoon troughs, quasi- tively]. In addition, the time series of the onset date in periodic oscillations, and midlatitude effects. Each one SWMEX (Fig. 13c) is signi®cantly correlated with each of these factors needs to be isolated and investigated time series in Fig. 15a (e.g., the correlation between within the context of the NAMS before we can fully SWMEX onset dates and SWMEX precipitation anom- appreciate how the onset of the monsoon is related to alies is 0.75) implying that early (late) monsoons in intraseasonal ¯uctuations in precipitation. SWMEX favor less (more) rainfall over all three regions for two months after onset. A possible physical expla- nation is that early monsoons in SWMEX tend to occur 6. Characteristics of wet and dry monsoons when the land±sea thermal contrast is weaker than at normal onset time (due to weaker heating over land), a. Classi®cation which might account for the lighter precipitation, and The results shown in Fig. 11 may be used to classify vice versa. We note that the correlations are lower when individual monsoons as wet or dry (relative to normal) precipitation is accumulated for longer periods after on- in each region; the terms wet (dry) refer to the amount set in SWMEX (e.g., 90±120 days) and that the seasonal of precipitation during the monsoon season relative to (JJAS) rainfall is uncorrelated with the onset date as normal and are synonymous with the terms heavy (de- reported in section 5a. ®cient). If we choose years when seasonal anomalies When this calculation is repeated using onset dates (percent departure from normal) are greater than or in NWMEX (Fig. 15b), the correlations are somewhat equal to 0.5 (less than or equal to Ϫ0.5) standard de- lower [0.75, 0.26, and 0.53 for (AZNM, NWMEX), viations, then (AZNM SWMEX), and (NWMEX, SWMEX), respectively]; the ®rst and third coef®cients are statistically AZNM: Wet: 1967, 1977, 1983, 1984, 1986, signi®cant at the 95% level. When it is repeated using 1988 onset dates in AZNM (Fig. 15c) the correlations are Dry: 1965, 1969, 1973, 1974, 1978, much lower [0.31, Ϫ0.32, and 0.15 for (AZNM, 1979, 1980

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FIG. 16. Correlation between rainfall for the 60-day period after monsoon onset in (a) SWMEX, (b) NWMEX, and (c) AZNM and rainfall over Mexico and the United States. The contour interval is 10%, the zero contour is omitted for clarity. Regions where the correlation is locally signi®cant at the 95% con®dence level are shaded.

NWMEX: Wet: 1964, 1966, 1967, 1968, 1978, monsoons in NWMEX and SWMEX, consistent with 1984, 1986 the fact that the de®cient events show a stronger rela- Dry: 1965, 1969, 1973, 1979, 1982, tionship with El NinÄo. 1987 SWMEX: Wet: 1965, 1967, 1970, 1971, 1973, b. Precipitation and tropospheric circulation 1978, 1987, 1988 Dry: 1969, 1976, 1977, 1979, 1982, The composite evolution of monthly precipitation for 1986. wet, dry, and all (1963±88) monsoons in AZNM, NWMEX, and SWMEX is shown in Figs. 17a, b, and It can be seen that each region has a similar number of c, respectively. In both wet and dry years the onset of wet and dry monsoons. The composite seasonal (JJAS) the monsoon is clearly evident in each region, just as percent departure from normal for wet (dry) monsoons it is in the composites based on all of the years. Wet in each region is 34.3%, 14.7%, and 10.6% (Ϫ24.5%, (dry) monsoons are characterized by several consecutive Ϫ16.5%, Ϫ14.6%) for AZNM, NWMEX, and months of above (below) normal rainfall in each region SWMEX, respectively. (i.e., high temporal coherence in a composite sense) It is of interest to note that during wet (dry) mon- generally starting in June. Figure 17a also shows that soons, the individual months also show heavy (de®cient) wet (dry) summer monsoons in AZNM are preceded by rain. Table 3 gives the rainfall departure from normal dry (wet) conditions during the preceding winter. Hig- for the season and for the individual months of June, gins et al. (1998) showed that wet (dry) summer mon- July, August, and September during wet and dry monsoons in AZNM tend to follow winters characterized by soons in each region. For most of the years, at least dry (wet) conditions in the southwestern United States three of the four months have departures of the same and wet (dry) conditions in the Paci®c Northwest. They sign. This indicates that, in spite of large month-to- attributed this association, in part, to the ``memory'' month variability, particular seasons of heavy and de- imparted to the atmosphere by the accompanying Paci®c ®cient rain have signi®cant temporal and spatial coher- sea surface temperature anomalies (SSTA) in the pre- ence. The temporal coherence is higher for the de®cient ceding seasons.

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TABLE 3. Seasonal (JJAS) and monthly percent departure from El NinÄo (La NinÄa), though the signal decreases toward normal precipitation (%) for wet and dry monsoon years in AZNM, the south presumably because of the latitudinal depen- NWMEX, and SWMEX. In each case the anomalies are departures from the appropriate mean (1963±88) seasonal or mean monthly val- dence of the extended (retracted) Paci®c jet on precip- ues. itation. Maps of the composite seasonal (JJAS) precipitation Year Season Jun Jul Aug Sep anomalies [departures from the mean (1963±88) sea- AZNM (Wet monsoon) sonal values] for wet and dry events are shown in Fig. 1967 26.4 29.5 15.3 20.2 48.3 18; similar composites for El NinÄo and La NinÄa events 1977 22.0 24.6 66.1 5.6 Ϫ10.3 are shown in Fig. 19. In each case, seasonal departures 1983 23.9 Ϫ47.0 Ϫ20.1 14.1 116.5 1984 72.7 168.5 58.9 104.5 12.8 from normal exceeding 10% of the mean are shaded. 1986 37.9 170.3 19.1 37.5 19.7 The maps in Figs. 18 and 19 were compared using pat- 1988 23.2 57.1 4.5 64.8 Ϫ25.0 tern correlation. It was found that the best agreement AZNM (Dry monsoon) was between SWMEX wet and La NinÄa (r ϭ 0.67) and 1965 Ϫ14.7 33.9 Ϫ4.4 Ϫ51.1 10.0 between SWMEX dry and El NinÄo (r ϭ 0.71). The 1969 Ϫ16.2 Ϫ51.3 Ϫ5.4 Ϫ6.2 Ϫ32.8 SWMEX dry, NWMEX dry, and AZNM wet compos- 1973 Ϫ42.9 16.3 Ϫ15.7 Ϫ52.4 Ϫ82.3 ites all show some evidence of a Great Basin signal 1974 Ϫ22.6 Ϫ87.1 18.7 Ϫ45.9 Ϫ19.9 1978 Ϫ37.2 Ϫ5.0 Ϫ46.7 Ϫ35.0 Ϫ38.8 (seasonal percent departures in the 10%±25% range), 1979 Ϫ24.2 54.7 Ϫ47.1 Ϫ0.0 Ϫ56.2 which Ropelewski and Halpert (1986) identi®ed during 1980 Ϫ14.4 Ϫ89.3 Ϫ16.9 Ϫ7.1 2.0 the warm season of El NinÄo events. We note that when NWMEX (Wet monsoon) the 1982±83 event is removed there is little change in 1964 8.6 1.2 6.4 Ϫ7.1 40.4 the qualitative nature of the composites and quantitative 1966 20.0 63.9 Ϫ7.1 47.9 Ϫ1.0 changes are minimal. 1967 8.6 81.7 Ϫ9.5 16.7 Ϫ13.5 During wet and dry monsoons the large-scale upper- 1968 6.3 Ϫ22.7 33.5 10.8 Ϫ30.3 level ¯ow shows dramatic seasonal JJAS departures 1978 9.6 Ϫ17.0 Ϫ0.8 4.4 49.9 1984 36.0 172.7 26.0 33.8 Ϫ19.7 from normal (Fig. 20) consistent with departures in the 1986 13.8 55.0 18.8 6.8 Ϫ6.9 continental-scale precipitation pattern (Fig. 18). Wet NWMEX (Dry monsoon) (dry) monsoons in AZNM (Figs. 20a,b) feature an en- 1965 Ϫ8.1 Ϫ37.8 Ϫ15.4 0.6 7.4 hanced (suppressed) monsoon anticyclone consistent 1969 Ϫ20.3 Ϫ77.7 11.9 Ϫ24.2 Ϫ37.0 with the precipitation anomalies over the southwestern 1973 Ϫ9.4 Ϫ32.5 Ϫ10.0 1.5 Ϫ12.4 United States. As shown by Higgins et al. (1998), wet 1979 Ϫ25.1 Ϫ28.5 Ϫ16.4 Ϫ30.7 Ϫ29.4 (dry) monsoons in this region are associated with a sup- 1982 Ϫ15.2 Ϫ69.2 0.3 Ϫ24.6 2.7 1987 Ϫ21.0 Ϫ44.3 Ϫ19.8 Ϫ8.5 Ϫ29.1 pressed (enhanced) local Hadley circulation during the spring and summer consistent with the patterns of trop- SWMEX (Wet monsoon) ical precipitation anomalies in the vicinity of the ITCZ. 1965 5.9 Ϫ2.3 2.2 18.4 1.8 1967 11.9 13.7 Ϫ20.0 16.5 38.9 Higgins et al. (1998) also showed that anomalies in 1970 12.6 21.8 11.6 Ϫ3.1 24.8 ITCZ precipitation and in the local Hadley circulation 1971 8.1 8.5 Ϫ8.6 6.6 27.2 are most pronounced during the spring preceding the 1973 14.3 Ϫ4.4 17.1 20.6 17.8 monsoon and that these changes are accompanied by 1978 5.5 3.0 6.4 Ϫ9.3 22.8 consistent and coherent changes in the SST and the 1987 10.9 Ϫ31.9 14.0 16.7 32.2 1988 15.3 37.9 7.7 17.8 4.0 subsurface thermal structure in the vicinity of the eastern Paci®c cold tongue. SWMEX (Dry monsoon) Composites of the seasonal (JJAS) 200-hPa wind and 1969 Ϫ7.3 Ϫ51.4 Ϫ18.2 32.3 Ϫ8.3 1976 Ϫ9.4 Ϫ7.7 4.2 Ϫ15.7 Ϫ17.8 streamfunction anomalies [departures from the mean 1977 Ϫ14.5 2.6 Ϫ25.5 Ϫ8.6 Ϫ22.1 (1968±88) seasonal values] for wet monsoons in 1979 Ϫ16.3 Ϫ46.1 Ϫ8.3 Ϫ13.1 Ϫ6.7 SWMEX (Fig. 20e) and for La NinÄa (Fig. 21b) both 1982 Ϫ35.2 Ϫ49.0 Ϫ8.3 Ϫ47.1 Ϫ40.1 feature a cyclonic couplet straddling the equator (e.g., 1986 Ϫ5.0 13.0 Ϫ14.6 0.6 Ϫ14.3 Arkin 1982) and 200-hPa westerly (925-hPa easterly) anomalies along the equator, typical of mature cold episode conditions (Fig. 21a). Of course the anomalies are The composite evolution of monthly precipitation in stronger in the La NinÄa composite, but all of the basic each region for El NinÄo and La NinÄa (Figs. 17d±f) years features are present in each case. Alternately, dry mon- shows how the monthly departures contribute to the soons in SWMEX feature an anticyclonic couplet strad- seasonal departures reported in section 4. The similarity dling the equator and 200-hPa easterly (925-hPa west- between the SWMEX wet (dry) and La NinÄa (El NinÄo) erly) anomalies along the equator (Fig. 20f), typical of composites during the warm season is evident, though mature warm episode conditions. The SWMEX wet the difference between wet and dry is almost twice the (dry) patterns also have easterly (westerly) departures difference between cold and warm. During the late fall± of the winds in the Northern Hemisphere subtropics early winter each region is relatively wet (dry) during consistent with a retracted (extended) North Paci®c jet.

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FIG. 17. Composite evolution of monthly precipitation (mm dayϪ1) over (a) AZNM, (b) NWMEX, and (c) SWMEX for wet (dashed line), dry (dash-dot line), and all (1963±88) monsoons (thick solid line). Composite evolution of monthly precipitation (mm dayϪ1) over (d) AZNM, (e) NWMEX, and (f) SWMEX for La NinÄa (dashed line) and El NinÄo (dash-dot) years, and all (1963±88) years (thick solid line).

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FIG. 18. Maps of the composite seasonal (JJAS) precipitation anomalies (mm dayϪ1) for (a) wet monsoons in AZNM, (b) dry monsoons in AZNM, (c) wet monsoons in NWMEX, (d) dry monsoons in NWMEX, (e) wet monsoons in SWMEX, and (f) dry monsoons in SWMEX. In each case the anomalies are departures from the mean (1963±88) seasonal values. The contour interval is 0.25 mm dayϪ1. Dark (light) shading indicates areas where seasonal precipitation anomalies are greater than 10% of the mean and positive (negative). Areas where the seasonal precipitation is less than 30 mm are masked.

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FIG. 19. Same as Fig. 18, except for (a) El NinÄo and (b) La NinÄa.

We note that the NWMEX dry pattern (Fig. 20d) is quite SSTA represented as the difference between wet and similar to the SWMEX dry pattern, consistent with the dry monsoons in AZNM, NWMEX, and SWMEX, re- possible association between El NinÄo and de®cient mon- spectively. Comparison of these ®gures to the composite soons in NWMEX. seasonal SSTA represented as the difference between The association between ENSO and anomalous mon- La NinÄa and El NinÄo (Fig. 22d) shows a good degree soons in SWMEX is attributed, in part, to the impact of similarity between SWMEX and ENSO. Both com- of local sea surface temperature anomalies on land±sea posites imply that, for a given boundary forcing over thermal contrast, hence the strength of the monsoon. land, negative SSTA would tend to enhance the land± Composites of seasonal (June±September) SSTA (de- sea thermal contrast, hence the strength of the monsoon partures from mean 1963±88 seasonal values) for wet and vice versa. We note that about half of the years with and dry monsoons in each region show a high degree mature El NinÄo (La NinÄa) events (see the classi®cation of antisymmetry for each pair of composites. For this in section 4) are also years with dry (wet) monsoons in reason, Figs. 22a±c show composites of the seasonal SWMEX. In AZNM and NWMEX the comparison be-

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FIG. 20. Composite seasonal (JJAS) 200-hPa wind anomalies, 200-hPa streamfunction anomalies, and US±MEXICO precipitation anomalies for (a) wet monsoons in AZNM, (b) dry monsoons in AZNM, (c) wet monsoons in NWMEX, (d) dry monsoons in NWMEX, (e) wet monsoons in SWMEX, and (f) dry monsoons in SWMEX. All ®elds are represented as departures from mean (1968±88) seasonal values. The contour interval is 1ϫ106m2 sϪ1 and the standard vetor length is 10 m sϪ1. Precipitation anomalies Ͼ0.25 mm dayϪ1 (ϽϪ0.25 mm dayϪ1) are shaded dark (light).

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FIG. 21. Same as Fig. 20, except for (a) El NinÄo and (b) La NinÄa.

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FIG. 22. Composite seasonal (JJAS) sea surface temperature anomalies [departures from mean (1963±88) seasonal values] represented as the difference between wet and dry monsoons in (a) AZNM, (b) NWMEX, and (c) SWMEX, and (d) as the difference between El NinÄo and (b) La NinÄa. In each panel the dark (light) shading indicates positive (negative) anomalies signi®cant at the 90% con®dence level. The contour interval is 0.2ЊC. tween the monsoon composites and ENSO is not as good used to compare and contrast hydrologic conditions and and, in particular, along the west coast of North America atmospheric circulation features associated with early, the SSTA are in the opposite sense. In these regions late, wet, and dry monsoons in three regions (AZNM, other factors (such as local feedbacks due to slowly NWMEX, and SWMEX) that exhibit many different varying land surface forcing or internal dynamics) might characteristics of monsoon precipitation. dominate. Again we note that the SSTA pattern is pre- While the seasonal normal rainfall and its variability served when we remove the 1982±83 event from the are largest in SWMEX, the mean seasonal percent de- composites, though there are slight changes in ampli- parture from normal is largest in AZNM. There is con- tude. siderable intraseasonal variability during the monsoon as the standard deviation of monthly mean departures is roughly twice that of the seasonal departures. 7. Summary and discussion Wet (dry) monsoons in each region are characterized The North American warm season is characterized by by several consecutive months of above (below) normal a monsoon system that provides a useful framework for rainfall. Wet (dry) summer monsoons in AZNM tend to describing and diagnosing the interannual variability of follow winters characterized by dry (wet) conditions in the warm season precipitation regime. Daily observed the southwestern United States and wet (dry) conditions precipitation over the United States and Mexico and in the Paci®c Northwest. The strongest association be- various ®elds from the NCEP±NCAR Reanalysis were tween the monsoon and ENSO was found in southwest

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Mexico, where wet (dry) monsoons often occur during iability of the warm season precipitation regime over La NinÄa (El NinÄo). This association was attributed, in North America. Our analyses also highlight the critical part, to the impact of local sea surface temperature need for adequate in situ precipitation data and upper- anomalies on land±sea thermal contrast, hence the air analyses at suf®ciently high temporal resolution and strength of the monsoon. Roughly half of the wet and in a form convenient for analysis. In general, ®ner res- dry monsoons in SWMEX occurred during ENSO olution models and long-term observations will be re- events, implying that other factors are also important quired to understand regional-scale impacts of the mon- for determining monsoon strength in this region. soon. Ideally, this could be accomplished via a multiyear While the mean seasonal evolution of the North regional (North American) reanalysis similar in philos- American monsoon system (NAMS) is characterized by ophy to the multiyear NCEP±NCAR global reanalysis. the regular northward progression of heavy precipitation from southern Mexico by early June to the southwestern Acknowledgments. We wish to thank the NCEP± United States by early July, the precise date of onset in NCAR Reanalysis Team for the Reanalysis data and a given region is highly variable and likely to be un- David Easterling of the National Climatic Data Center, related to the date of onset in the other regions. The Asheville, for the Mexican precipitation data. The au- date of onset is correlated with seasonal rainfall anom- thors are also indebted to John Janowiak, Gerry Bell, alies such that early monsoons in AZNM tend to be Vern Kousky, Gene Rasmusson, Chet Ropelewski, Ants heavy and late monsoons in NWMEX tend to be de®- Leetmaa, and Kingtse Mo for insightful discussions. cient. It is notable that the classi®cations of early and This work was partially supported by the NOAA Of®ce wet monsoons in AZNM have ®ve events in common. of Global Programs under the Pan American Climate No relationship exists between the SWMEX onset date Studies (PACS) project and the GEWEX Continental- and seasonal rainfall anomalies, which is somewhat Scale International Project (GCIP) and by Interagency counterintuitive given the association with ENSO. 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