INTERNATIONAL JOURNAL OF CLIMATOLOGY, VOL. 16, 1019-1031 (1996)

SYNOPTIC CONTROLS ON PRECIPITATION IN THE VALLEY OF OAXACA, MEXICO

MAXX DILLEY Disaster Management Center; 432 North Luke Street, University of Wisconsin, Madison, Wisconsin 53 706-1 498, USA

email: mdilley@usaid.gov

Received I May 1995 Accepted 3 Januav 1996

ABSTRACT

The valley of Oaxaca is an important agricultural centre in the highlands of southern Mexico. Precipitation is a major control on rain-fed agriculture. Daily, monthly, and annual precipitation in the valley of Oaxaca are linked synoptically to distinct meso- and large-scale patterns of atmospheric pressure and temperature. During the summer rainy season, daily precipitation in the valley coincides with lower atmospheric pressures over Mexico. Anomalous lows increase convergence and draw maritime air landward. From June through to September, monthly average 850 hPa level temperatures in the MexiccdJSA border region are consistently higher during the valley's wettest years compared with dry years. This link with atmospheric temperatures in the border region suggests that precipitation in the valley of Oaxaca is affected by variations in the Mexico monsoon, a tongue of maritime air that is drawn up the Mexican west coast and into the south-west USA during summer. Precipitation in the valley of Oaxaca also varies according to the frequency and position of tropical storms and hurricanes in the Gulf of Mexico and Pacific Ocean. Significant correlations between precipitation in the valley and storm frequency occur during August and September. Finally, El NiibSouthem Oscillation (ENSO) warm events are associated with dry conditions in the valley owing to a southward displacement of the Inter Tropical Convergence Zone (ITCZ) during the low phase of the Southern Oscillation and diminished storm frequency in the Gulf of Mexico.

KEY WORDS: Oaxaca; southern Mexico; synoptic climatology; Mexican monsoon; El Niiio-Southern Oscillation (ENSO); rainfall

INTRODUCTION

Previous investigations into controls on precipitation in southern Mexico have tended to be regional, examining the effects of a single climatic phenomenon over spatially extensive areas. A summer monsoon was identified as a source of moisture transport and precipitation in Mexico by Hales ( 1 974) and further documented by Garcia and Mosiiio (1990) and Douglas et al. (1993). The impact of the extremes of the Southern Oscillation (El Niiio and non- El Niiio events) on the climate of southern Mexico have been assessed by Cavazos and Hastenrath (1990), Rogers (1988), Aceituno (1988), and Kiladis and Diaz (1993).

This study uses a synoptic approach to examine the interaction of several meso- and large-scale atmospheric controls on precipitation at a single location. The valley of Oaxaca is located at the centre of the state of Oaxaca in the highlands of southern Mexico. At approximately 17"N and 97"W, the valley is situated in the transition belt between the tropics and extratropics. Precipitation seasonality and interannual variation in the valley are modulated by monsoonal effects from the North American land-mass, tropical storms in the Gulf of Mexico and Pacific Ocean, and El Niiio-Southern Oscillation (ENSO) events. High altitude ( 1 500 m above sea-level at the valley floor) and

CCC 0899-8418/96/09-1019-13 0 1996 by the Royal Meteorological Society

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rugged topography also influence precipitation, the latter through rain-shadow effects from mountain ranges to the north-east, south-west and south, rising to over 3000 m above sea-level (Figure 1).

The largest expanse of flat land in Mexico's southern highlands, the valley of Oaxaca is an important agricultural centre with a history spanning over 3000 years. Interannual variations in summer growing-season precipitation greatly affect agriculture. Most cropland is rain-fed and insufficient rainfall causes harvest failure one year in four (Kirkby, 1973). In this study, seasonal and interannual variations in precipitation are explained by examining their synoptic controls. A moderately strong ENS0 precipitation teleconnection may ultimately offer some predictive capabilities for agricultural planning, through advances in ENS0 forecasting (see Cane et al; 1986).

DATA

Gridded pressure and temperature data were obtained from the United States National Meteorological Center (NMC). The grid consists of 1977 equally spaced points on a solar stereographic projection centred over the North Pole and extending down as far as 9"N latitude, south of Mexico (University of Washington, 1990). Daily and monthly values of sea-level pressure (SLP) and 850, 700, and 500 hPa geopotential heights are computed from NMC final analysis tapes for each grid-point. The SLP values for stations above sea-level, such as those in Oaxaca and throughout much of Mexico, are calculated based on the assumed characteristics of a hypothetical column of air extending from the station elevation to sea-level. The resulting SLPs provide a standardized surface-pressure grid, with the effect of elevation removed, for Mexico and surrounding ocean areas. The SLP and 500 hPa data were

Figure 1. Orographic effects on annual rainfall (mm) in Mexico and the valley of Oaxaca, from Arbingast et al. (1 975, p. 17) and Kirkby (1973, p. 19)

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available from 1946 to 1988,850 and 700 hPa from 1963. A window around Mexico extending from 40"N to1O"N latitude and 130" W to 60"W longitude was selected from the larger grid.

Precipitation data for the valley of Oaxaca came from two sources. From the Mexican National Meteorological Service, monthly precipitation data were available from 1878 through to 1990, albeit with some interruptions, for Oaxaca de Juirez, the state capital located in the centre of the valley. In addition to the long-term record from the Oaxaca de Juarez station, a shorter record of daily precipitation for three other stations was obtained from the records of the Secretariat of Agriculture and Water Resources in Oaxaca de Juarez. These stations (Etla, Ocotlh, and Tlacolula), represent each of the three arms of the valley (see Figure 1). Monthly totals from these stations were averaged together with monthly total precipitation from the Oaxaca de Juhrez airport to obtain a monthly precipitation average for the entire valley from 1978 to 1990. Because this average includes one station from each of the valley arms, it captures a degree of spatial precipitation variation, an important consideration in an area where it can be raining at one location but not raining at another several kilometres away. The valley-average precipitation is affected by missing data for some stations, especially from 1986 to 1990.

Tropical storm and hurricane tracks in both the Atlantic and Pacific Oceans were digitized on to a 5" x 5" grid for the years 1956 to 1990 from the Mariners Weather Log ( U S Department of Commerce, 1956-1990). Additional storm data was provided by the Mexican National Secretariat of Agriculture and Water Resources (SARH, 1988).

SYNOPTIC CONTROLS ON AVERAGE, SEASONAL PRECIPITATION

The rainy season in the valley of Oaxaca begins during April and May and peaks during June (Figure 2). In July and August a mid-season dry period occurs. This period, during which the Canis Mujor constellation appears in the morning sky, is known as the canicula. In September, rainfall rises again to a second, lesser peak before falling off rapidly during October. Rainfall during the winter dry season is negligible, caused by occasional cold fronts which push down into southern Mexico from the extratropics (Dimego et al; 1976, p. 71 1). The six months from May through to October receive 90 per cent of average annual precipitation, and 72 per cent of average annual precipitation falls between June and September. As indicated by monthly precipitation standard deviations in Figure 2, there is considerable interannual variation around monthly averages. Peak temperatures occur during April and May (22"C), after which they drop because of evaporative cooling and enhanced cloud cover during the rainy season.

Stations in all three arms of the valley conform to these general seasonal patterns, although total annual rainfall differs among them due to orographic effects (visible in Figure 1 inset). Most of the valley has a steppe climate (BS

Mean T= 20.6 C, Mean Total P= 685.1 rnm 250 I - - 125

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- Temp. Std. Dev. + Precip. Std. Dev. I Figure 2. Average annual temperature and precipitation at the Oaxaca de Juhrez weather station, 1878-1990 ('Oaxaca' in Figure 1 inset)

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in the Koppen classification), with the exception of the southern arm, where Ocotlin is located, which is Cw (temperate, dry winter). Some areas in the south almost qualify as Aw (tropical wet-and-dry, dry winter) except for a shortage of summer rainfall. The north Etla and central Oaxaca de Juirez areas are closer to Cw climates than the drier eastern (Tlacolula) arm (Lorenzo, 1960, pp. 53-54).

The noon sun is directly overhead in Oaxaca twice a year: on 8 May and again on 6 August. The June and September seasonal precipitation maxima therefore each occur 1 month after a solar zenith. Because the latitudinal position of the thermal equator and ITCZ tends to lag behind the latitude of declination by a month, it is tempting to explain the June and September precipitation maxima as no more than the product of convection within the zone of maximum atmospheric heating. This simple explanation, however, must be supplemented with an examination of the roles played by convergence, moisture transport, and tropical disturbances in determining Oaxaca’s precipitation seasonality and interannual variability.

Oaxaca is influenced by circulatory regimes that bring precipitable moisture on to the continent from both the Gulf of Mexico and the Pacific Ocean. Were it not for the mountains rising from the coastal plain north-east of the state, easterly flow from the Gulf would be easily the dominant atmospheric control on precipitation seasonality in the valley. During winter, when dry conditions prevail in Oaxaca, the upper air flow is westerly and surface high pressure over the USA sends cold, dry continental air from there down across Mexico. By June the westerly flow abates aloft over southern Mexico and the surface flow over most of the country becomes decidedly easterly. The Bermuda high-pressure system becomes established over the South Atlantic (Figure 3(a)). The Bermuda High pushes moist air from the Gulf into a thermal surface trough over Mexico, providing a moisture source for Oaxaca precipitation. Much of the water vapour carried at lower levels, however, rains out before reaching the valley, on the

Figure 3. June mean SLP (a) and July mean 500 hPa geopotential heights (b), 1946-1988

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eastern slopes of the Sierra Madre de Oaxaca mountains rising from the Gulf coastal plain in Veracruz to the north- east (see isohyets in Figure 1). Only moisture at levels above these mountains penetrates into the plateau region where the valley is located. During winter, a thermal inversion prevents virtually all upper-air moisture transport and contributes to winter dryness on the plateau. In summer, the inversion disappears (Gutnick, 1958, p. 41 1) and water evaporated over the Gulf can be carried further aloft. Heating of the plateau surface often produces temperatures exceeding those over the Gulf at the same altitude, a phenomenon known as the elevated heat source effect (Mosiiio and Garcia, 1974, p. 353). When the incoming Gulf air passing over the plateau encounters the elevated heat-source, convective lifting causes precipitation. The elevated heat-source effect and predominant easterly flow combine during June to produce the first seasonal precipitation maximum in the valley of Oaxaca. The passage of easterly waves in the 700 hPa wind-field provides a trigger of atmospheric instability, lifting the air at their crests and producing precipitation in their wake (Jackson, 1989, p. 27) with an approximate 5-day periodicity.

In July and August, during the cuniculu, average monthly precipitation decreases from June levels, producing a mid-season dry period. During July the subtropical high-pressure system becomes spatially extensive at the 500 hPa level and splits into two centres, one over Bermuda and the other over Texas separated by a trough stretching from Florida to the Yucatan peninsula (Figure 3(b)). In July, easterly flow from the southern Atlantic swings slightly northward following the curve of the isobars, reducing moisture transport and the frequency of easterly waves over Mexico (Mosiiio and Garcia, 1974, p. 377). At the same time, relatively high pressure aloft over the border region circulates dry, subsiding continental air southward over the eastern half of the country (Mosiiio and Reyna-Trujillo, 1989). Weather stations on the east side of Mexico, for which the moisture supply comes predominantly from the Gulf, experience a decline in rainfall during July and August (Garcia et ul., 1989). The valley of Oaxaca is among the locations that experiences an average precipitation decline during these months. With the diversion of easterly waves, moisture from the Pacific side becomes increasingly important for precipitation in Oaxaca. Reaching its apex in July and August, a tongue of moist maritime air intrudes up the Mexican west coast (Garcia and Mosiiio, 1990), the effects of which on annual precipitation are visible in Figure 1. This tongue is drawn northward into the thermal low over the USA-Mexico border in monsoon fashion (Hales, 1974; Douglas et ul., 1993).

Both easterly flow and the Mexican monsoon abate during September as winter conditions gradually return. Nonetheless, a secondary maximum occurs in Oaxaca’s precipitation record during that month. Accumulated summer warming of tropical ocean waters provides energy for increased storm formation. Tropical storms and hurricanes provide mid-tropospheric disturbances that trigger atmospheric instability. These disturbances can cause rainfall to occur over the Mexican plateau even when the storm remains far from the coast. The fact that cyclones and hurricanes affecting the Mexican coast are more than twice as frequent during September than in any other month (SAW, 1988) partially explains the second, September, precipitation maximum.

Within the valley of Oaxaca itself, orographic influences affect the spatial distribution of precipitation. For example, the Tlacolula valley arm is drier than areas to the west because it is in the rain shadow of the Sierra Madre de Oaxaca mountains (Figure 1 inset).

Local convection can result in heavy rains in one part of the valley while the sun shines a short distance away. Spatial variability of rainfall within the valley has important implications for long-term climate analyses in the valley. A long-term record exists only for a single station at Oaxaca de Juirez. The valley’s rainfall spatial variability is better accounted for in recent years, when an average precipitation figure for four valley stations can be computed.

SYNOPTIC CONTROLS ON PRECIPITATION MTERANNUAL VARIATION

The average seasonal distribution of Oaxaca’s monthly precipitation masks considerable interannual variation. Seasonal differences between groups of years can be illustrated by classifying them according to the similarity of their seasonal precipitation profiles based on a cluster analysis using Ward’s method. The input to the analysis consisted of the 12 monthly rainfall totals for each year in the record from 1878 through to 1990 at the Oaxaca de Juarez station. Years with similar seasonal rainfall patterns, particularly during May through to October when rainfall variation is greatest, cluster together. Seven clusters were judged optimal, based on the ratio of within- to between-cluster variation. Average monthly precipitation for each cluster, or rainy season type, is shown in Figure 4. Three of the types occurred frequently, accounting for 76 per cent of the years (Figure 4(a)). The others accounted for the remaining 24 per cent (Figure 4(b)). The three frequently occurring types resemble the average seasonal

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precipitation profile. The four infrequent types represent years when the usual bimodal pattern of June and September maxima was disrupted, probably due to shifts in timing of the seasonal atmospheric controls. The sequencing of rain-season types shows that the more frequent types 1 to 3 often persist over several years (Figure 5) . Interestingly, infrequent types 4 to 7 have become more frequent since 1960. There is less persistence of types over multiple years since then as well. This confirms reports by older valley residents that rainfall is less 'reliable' in recent years than it was in their youth (Dilley, 1993).

Precipitation variability in Oaxaca is related to changes in meso- and large-scale atmospheric pressure, which are in turn linked to moisture convergence, as discussed previously. Composite pressure anomalies comparing 358 rainy versus 706 dry days in the valley show that SLP and 500 hPa geopotential heights tend to be lower over the north- central Mexican plateau and in the Gulf on days when precipitation falls in the valley (Figure 6). Locally, in a comparison of two years-I98 1, which was very wet in Oaxaca, versus 1982, which was extremely dry-average SLP and 500 hPa geopotential heights for the four NMC grid-points closest to the valley show a similar relationship to precipitation (Figure 7). The SLP values are generally higher during the drought year of 1982 throughout most months of the rainy season, as are 500 hPa geopotential heights. Note that in months where the the pressure was

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0 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

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Figure 4. Oaxaca de Juice2 station monthly average precipitation for frequently occurring (a) and infrequently occurring (b) rainy season types

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higher in 198 1, as occurred at the 500 hPa level during June, the rainfall in 198 1 was correspondingly lower (Figure

Interannual variations in Oaxaca's monthly precipitation coincide with variations in atmospheric temperatures over the USA-Mexico border region. This relationship is visible in composites comparing monthly 850 hPa atmospheric temperatures from the wettest and driest quartiles of years from 1962 through to 1988 (based on monthly precipitation at Oaxaca de Juiuez). A similar pattern distinguishes wet from dry years: the air at 850 hPa is 1-3°C warmer over the north-central plateau during wet years in Oaxaca during June through to September (Figure

703)).

Figure 6. Composite gridded SLP (a) and 500 hPa geopotential height (b) anomalies for days with spatially extensive precipitation in the valley of Oaxaca compared with days without precipitation from 1978 to 1990. Days with spatially extensive precipitation are those during which more than 1 cm of precipitation at all four stations (Etla, Oaxaca, Ocotlh, and Tlacolula). Dry days are those during which no precipitation was

recorded at any of the four stations. Each composite shows the average grid-point values for wet days minus those for dry days.

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Month

5900 350

Apr May Jun Jut Aug Sep oct Month

1981 Rain - 1981 500 mcm 1982 Rain -- 1982 500 mb I Figure 7. Oaxaca Valley monthly average SLP (a), 500 hPa geopotential height (b) and precipitation for a wet (1981) versus a dry year (1982)

8). The difference is especially pronounced and spatially extensive during August, when the Mexican monsoon reaches its peak. Further work needs to be undertaken, however, to determine specifically how 850 hPa border- region temperatures relate to the development and strength of the Mexican monsoon.

Variation in frequency, duration, and position of tropical storms and hurricanes also affects precipitation in the valley of Oaxaca. The fiequency of tropical storms digitized on to a 5" x 5" grid for the years 1956-1 990 was used to predict monthly precipitation at the Oaxaca de Juhrez station in a regression analysis. Storm positions are recorded at 12-h intervals, and the number of 12-h segments of storm track occumng in each grid-cell each month was used as the independent variable. Storm frequency correlations with Oaxaca precipitation from 1956 to 1990 are strongest during August (Figure 9(a)). During this month a belt of significantly positive correlations materializes between 10" and 25"N latitude. This belt corresponds to the passage of easterly waves, which both stimulate storm formation and trigger convection in Oaxaca. Significant positive correlations occur in the Pacific as well. In September, the highest correlations are found in the grid-cells on the Mexican coast adjacent to Oaxaca (Figure 9(b)). As easterly flow

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Figure 8. Composite June (a), July (b), August (c), and September (d) gridded 850 hPa geopotential height temperature composites comparing the six wettest years (quartile) with the six driest in the valley of Oaxaca from 1962 to 1988. Each composite shows the average grid-point values

for wet-quartile months minus those for dry-quartile months.

abates during September, easterly waves become less influential and those storms that are proximate to Oaxaca are the ones with the greatest effect on precipitation.

Precipitation in the valley of Oaxaca is also influenced by ENSO. Low values of the Southern Oscillation Index (S0I)-SLP in Tahiti minus that in Darwin, Australia-tend to be associated with dry conditions in the valley.

When the SO1 is high, July-September precipitation is greater in southern Mexico than it is during the low phase (Rogers, 1988, p. 173). Cavazos and Hastenrath (1990) note that May-October precipitation, averaged over several stations in the state of Oaxaca from 1966 to 1987, correlates with the July-August average of the SO1 at 0.38. A survey of droughts reported during El Niiio years reveals a tendency towards drought in Central America, occasionally affecting southern Mexico (Glantz et al., 1987). Drier than average conditions over southern Mexico occur during years when the SO1 changes sign from positive to negative and when central and eastern equatorial Pacific SST anomalies become strongly positive (Kiladis and Diaz, 1993).

In the valley of Oaxaca, ENSO warm events during the twentieth century, as identified by Kiladis and Diaz (1 989), are associated with higher than normal May precipitation, but with lower than normal precipitation during August and September (Figure 10). For the period 1978-1 990, when average precipitation for four valley stations is available, the correlation between MayyOctober SO1 and precipitation over that period is 0.74. The correlation with the single Oaxaca de Juhrez station over the same period is lower, at 0.4 1 . For 29 years classified as strong or

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Figure 8. Continued.

moderate El Niiios by Quinn et al. (1987), from 1889 to 1990,41 per cent of El Niiio years fell into the driest total annual precipitation quartile from Oaxaca de Juarez, significant at the 5 per cent level.

El Niiio years are characterized by a decrease in western Atlantic hurricane activity (Gray, 1984). Humcane influence on Oaxaca precipitation is highest during August and September, so less frequent humcanes associated with ENSO warm events helps explain decreased precipitation in Oaxaca during these months. In contrast, northward displacement of the ITCZ during high SO1 years (Cavazos and Hastenrath, 1990) extends the period during which Oaxaca is under its influence, contributing to higher precipitation in the valley in non-El Niiio years. The Mexican monsoon is not affected appreciably by ENSO (Kiladis and Diaz, 1993).

SUMMARY AND CONCLUSIONS

Precipitation seasonality and interannual variation in the valley of Oaxaca are influenced by several synoptic controls. Although one control may dominate during a given month, all influence precipitation to some degree throughout the summer rainy season.

The seasonal average precipitation maximum in June is primarily a function of convergence of maritime air into southern Mexico, initiated by a thermal trough that develops over north-central Mexico and the south-west USA

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Figure 9. Correlation between gridded 12-h storm frequencies and both August (a) and September (b) rainfall in the valley of Oaxaca, 1956- 1990 (*indicates significance at 0.05 level)

during May and June. During July and August the Mexican monsoon reaches its maximum extent but precipitation in Oaxaca declines, perhaps because the border-region thermal trough is slightly weaker in these months. A secondary seasonal average precipitation maximum occurs in September, when both the monsoon and easterly flow are diminished, owing to the high number of tropical cyclones and hurricanes affecting the Mexican coast. These storms are associated with atmospheric disturbances that trigger instability and precipitation in Oaxaca.

Precipitation interannual variability coincides with fluctuations in temperatures at the 850 hPa level over the USA-Mexican border. These temperatures are consistently several degrees higher during the wettest quartile months in Oaxaca than they are during the months in the driest quartile. Although the position of the centre of 850 hPa heating over the border-region suggests that this phenomenon is related to the Mexican monsoon, a complete physical explanation is currently lacking. Tropical cyclone and hurricane frequency in both the Gulf of Mexico and the Pacific Ocean correlate positively with Oaxaca precipitation, primarily during August and September. The reduction in the frequency of Gulf hurricanes during ENS0 warm events suggests why these events tend to be associated with dry conditions in the valley during the latter part of the rainy season. In contrast, the ITCZ is displaced northward during high SOI-ENSO cold phases, which contributes to more prolonged convection over Oaxaca and increased rainfall during non-El Niiio years.

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I I 1

U I -20’0 I 1 I I I I I 9 I I I ! 1 1 I I I I 1 I I I J

J F M A M J J A S O N D J F M A M J J A S O N D Month

- Warm SST years -- Cold SST years

Figure 10. Precipitation anomalies at Oaxaca de Juhrez for twentieth century warm and cold SST-ENS0 event years and year + 1. The monthly anomalies for both warm and cold SST years were smoothed with a 3-month running mean according to the methodology presented in Bradley

et al. (1987; in Kiladis and Dim, 1993).

ACKNOWLEDGEMENTS

The author wishes to thank Diana M. Liverman for her generous support of this research through her US National Science Foundation grant for the study of drought and agriculture in Mexico. Tereza Cavazos and Henry Diaz made many helpful suggestions that improved an earlier draft. Michael W. Douglas and an anonymous reviewer provided thoughtful criticisms and comments during the final revision. Any errors or misinterpretations of the data are the responsibility of the author.

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