15 FEBRUARY 2010 N A Z E M O S ADAT AND GHAEDAMINI 887

On the Relationships between the Madden–Julian Oscillation and Precipitation Variability in Southern and the Arabian Peninsula: Atmospheric Circulation Analysis

M. J. NAZEMOSADAT AND H. GHAEDAMINI Water Engineering Department, College of Agriculture, Climate Research Center, University, Shiraz, Iran

(Manuscript received 9 July 2007, in final form 28 August 2009)

ABSTRACT

The influence of the Madden–Julian oscillation (MJO) on daily, monthly, and seasonal precipitation was investigated for southern Iran and the Arabian Peninsula using November–April data for the period of 1979– 2005. The positive MJO phase is considered to be the periods for which the enhanced convection center was placed over the south Indonesian–north Australian region. On the other hand, the convection center shifts over the western Indian Ocean tropics and most of the study area as the negative MJO phase prevails. Seasonal precipitation and the frequency of wet events were significantly increased during the negative phase. The ratios of the precipitation amount during the negative phase to the corresponding values during the positive phase were about 1.75–2.75 and 2.75–4.00 for the southwestern and southeastern parts of Iran, respectively. This ratio reached to about 3.00 for Riyadh, 4.20 and 5.50 for Masqat and Doha, 2.10 for Kuwait, and 1.20 for Bahrain. The results of the seasonal and monthly analysis were generally found to be consistent, although because of the smaller sample size the outcomes of the monthly investigations were less statistically significant. While the negative MJO phase does not have a consistent effect on March precipitation over some parts of southern Iran, it has consistently enhanced precipitation over the eastern and southern coasts of the peninsula in Oman, Yemen, and Saudi Arabia. During the negative MJO phase, while enhanced low-level southerly winds transfer a substantial amount of moisture to the study area, upward motion increases in the middle layers of the atmosphere. Synchronized with the prevalence of these rain-bearing southerly winds, the existence of a strong horizontal wind speed gradient at the exit region of the North Africa–Arabian jet enhances precipitation. The jet exit, which was mostly located over Egypt in November, moved westward into the study area in Iran and Saudi Arabia during the rainy period of January–March. The direction of near-surface wind anomalies changed from mostly southeasterly in November to southwesterly in March and April, influencing precipitation pattern during various months of the rainy season. In contrast to the negative phase, an enhanced low-level dry northerly wind and suppressed horizontal wind speed gradient at the jet exits are the main characteristics of atmospheric circulation over the study area during the positive MJO phase. Furthermore, an increased downward air motion at the middle levels of the atmosphere and a significant shortage in precipitation are the other climatic components of the southwest Asian region during such a period.

1. Introduction ciated period ranging from 30 to 60 days. In its active stage, the MJO is associated with increased convective The Madden–Julian oscillation (MJO) is known as the activity over the equatorial eastern Indian and western primary mode of large-scale intraseasonal variability in Pacific Oceans. Trailing the active center is a region of tropical regions (Madden and Julian 1994). Donald et al. suppressed convective activity and near-surface-level (2004) characterized the MJO as a 40-day wave that westerly wind. develops over the tropical Indian Ocean and then travels Since the discovery of the oscillation, many studies east across the tropics at 5–10 m s21. The phenomenon have shown that MJO also affects extratropical weather has a frequency of 6–12 events per year, with an asso- and climate systems (Liebmann and Hartmann 1984; Krishnamurti et al. 1997). Moreover, it has been shown that the MJO strongly influences the monsoon-related Corresponding author address: Mohamad J. Nazemosadat, Water Engineering Department, College of Agriculture, Climate precipitation patterns in Asia and Australia, and mod- Research Center, Shiraz University, Shiraz 71441-65186, Iran. erately influences precipitation in North and South E-mail: [email protected] America (Lau and Chan 1986; Mo 2000; Paegle et al.

DOI: 10.1175/2009JCLI2141.1

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2000; Higgins and Shi 2001; Carvalho et al. 2004; Donald characterized by low amounts and high variability of et al. 2006). The MJO has also been linked with en- precipitation as well as a high potential for evapotrans- hanced precipitation and increased incidence of floods piration (Sadeghi et al. 2002; Sormana and Abdulrazzak in northwestern parts of the United States, although its 1995). The average annual precipitation in southern Iran influence was found to be substantially different dur- varies from about 600 mm in western and central areas ing early versus late winter (Bond and Vecchi 2003). to less than 100 mm in the east, with precipitation gen- Hendon et al. (2000) found that forecasts in the tropics erally occurring between November and April. The in- and midlatitudes of the Northern Hemisphere during tensity and frequency of hydrometeorological disasters boreal winter have less skill when they are initialized are generally greater for this part of the country than either during or prior to periods of active MJO as opposed other areas. For the accessible stations in the Arabian to quiescent episodes of the oscillation. Donald et al. Peninsula, annual precipitation varied from about (2006) have provided a mechanistic basis for an MJO- 105 mm in Riyadh to 20 mm in Thumrait (Fig. 1). Re- based forecasting capacity that bridges the weather– liable prediction of seasonal and intraseasonal pre- climate divide for tropical and extratropical weather cipitation in these regions is considered a challenge for states. Maloney and Hartmann (1998) and Maloney and climate researchers. Kiehl (2002) introduced an MJO index for characteriz- Recent studies have shown that major teleconnec- ing the intensity and state of the MJO. After this, tion patterns, including ENSO and sea surface temper- Wheeler and Hendon (2004) developed another index atures over both the Persian Gulf and Caspian Sea, have for monitoring and predicting the MJO. a significant impact on rainfall in Iran (Nazemosadat Barlow et al. (2005) analyzed the impact of the MJO and Cordery 2000; Nazemosadat 2001; Nazemosadat on daily precipitation in southwest Asia for the period and Mousavi 2001; Nazemosadat and Ghasemi 2004; from 1979 to 2002. They showed that the MJO signals Nazemosadat et al. 2006). Chakraborty et al. (2006) can modulate regional precipitation with strength com- have analyzed the influences of the Indian Ocean parable to interannual variability. Furthermore, the MJO- dipole (IOD) and ENSO on tropospheric moisture associated winds aloft, which are largest poleward and over Saudi Arabia. They reported an increased atmo- to the west of the primary tropical rainfall anomalies, spheric moisture flux during El Nin˜ oandthepositive are shown to have a clear influence on the local jet struc- IOD phase. ture in southwest Asia. They have suggested further Given the predictability of the MJO out to at least research by including several key areas with more ob- 4 weeks in combination with its strong influence on re- servational data of precipitation and consistent report- gional precipitation (Wheeler and Hendon 2004; Donald ing, for the verification of their results based on outgoing et al. 2006), the outcome of the study could potentially longwave radiation (OLR) data. Also, Mariotti (2007) improve subseasonal forecasting of dry and wet periods has indicated that the enhanced precipitation in south- over the study regions. The applied analysis, hopefully, west central Asia during El Nin˜o–Southern Oscillation bridges the gap between synoptic and subseasonal fore- (ENSO) events results from an anomalous southwesterly casting, which is an essential component for risk man- moisture flux coming from the Arabian Sea and tropical agement in west Asian areas. The goals of the study are, Africa, which is generated along the northwestern flank therefore, to examine the effects of the MJO extreme of the high pressure anomaly over the Indian and western phases on precipitation and the frequency of dry or wet Pacific Oceans, which is part of the canonical ENSO sea- events in southern Iran and the Arabian Peninsula. saw pressure anomalies. Moreover, the impact of these phases on atmospheric An assessment of the effects of the MJO with respect circulations over the study region is analyzed. to atmospheric circulation features, including moisture transport, was not a major focus of Barlow et al. (2005). Also, the study of rainfall variability and atmospheric 2. Data circulation over Iran and the Arabian Peninsula that is a. The MJO indices and their relationships the main concern of the present study was not the theme of their investigation. In addition to a seasonal analysis, Based on the first two principal components (PC1 and the present study also examines the effects of the MJO PC2) of the bandpass-filtered 850-hPa zonal wind data extreme phases on precipitation and atmospheric cir- over the equatorial (58N–58S) ocean waters (globally), culation on the monthly time scale, which was not the Maloney and Hartmann (1998) and Maloney and Kiehl focus of earlier studies. (2002) introduced an MJO index by adding PC1 to the Almost all of the Arabian Peninsula and the southern value of PC2 12 days later (hereafter the MH index). regions of Iran (Fig. 1) are situated in a region that is Their PC1 time series is most closely associated with

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FIG. 1. The geographical location of the study stations in southern Iran and the Arabian Peninsula. The part of Iran for which precipitation data were analyzed is delineated (shaded area). For the stations located over the Arabian Peninsula the station names are presented on the map. Corresponding to the given numbers in the map of southern Iran, the stations names are presented in Table 1.

zonal wind over the Indian Ocean and eastern Pacific, to those of RMM1. The MH and WH indices are mostly while PC2 emphasizes variance over the western Pacific. positive (or negative) when the enhanced (or suppressed) Daily time series of the MH index was gratefully sup- precipitation anomalies are propagating from equatorial plied by E. Maloney (2006, personal communication) for parts of the eastern Indian Ocean region to the western the of period 1979–2005. parts of the Pacific Ocean. Negative values of these in- In addition to the MH index, Wheeler and Hendon dices suggest a weaker positive precipitation anomaly (2004) developed a pair of principal component time over the westernmost Indian Ocean and central Pacific. series, called the real-time ultivariate MJO series 1 More details about the MH or WH indices are given on

(RMM1) and series 2 (RMM2), as the oscillation indices. the referred references. These indices, which use satellite-based outgoing long- The correlation coefficients between the daily series wave radiation and near-equatorial 850- and 200-hPa of these two indices were examined for all 27 yr (with zonal winds as input, have subsequently been widely 365 records in each analysis) and were found to be used for monitoring and predicting the state of the MJO. around 0.70 for each analysis (significant at the 99% The two multivariate series were most highly correlated level). As an example, the coefficients were found to be when RMM2 lags RMM1 by 10–15 days. The time series 0.69, 0.66, 0.75, and 0.75 for 1981, 1990, 1997, and 2002, of RMM1 (hereafter the WH index) were also used respectively. In addition to the concurrent correlations, as another MJO index to assess the oscillation impact the lag-1–5-day correlations were also examined, but on precipitation variability in southern Iran and the the best results were found for the contemporaneous

Arabian Peninsula. The RMM2 series was also exam- analysis. The resampling bootstrap analysis (with re- ined but did not provide meaningful results compared placement) with 1000 synthetic pairs of samples was

Unauthenticated | Downloaded 10/07/21 04:00 AM UTC 890 JOURNAL OF CLIMATE VOLUME 23 then performed to examine the approximation of the phase). Furthermore, it was found that both indices were confidence intervals around the computed correla- simultaneously positive or negative for 1807 and 1714 tion coefficients (r) using observed data (Efron and days (37% and 35% of the study periods), respectively. Tibshirani 1993; Wilks 1995; Wood 2004). Each com- This means that the sign of WH and MH indices was puted (r) was significant (at the 95% level) if, among the different for about 28% [100% 2 (37% 1 35%)] of the 1000 sorted series of the simulated coefficients, it does considered period. not fall within the 25 greatest or 25 lowest of the series. Figures 2a,b shows the anomaly maps of OLR and the In addition to yearly analysis, the correlation co- 850-hPa vector wind fields for the days on which both efficients between these two indices were also examined indices were concurrently positive and negative (1807 for each month of all 27 yr. Again, the results were and 1714 days, respectively). For the purpose of this generally found to be around 0.6–0.7. study, the given conditions in Figs. 2a,b were corre- In spite of these strong relationships that indicate spondingly referred to as the positive and negative MJO that the variance in each of the considered time series phase. As indicated, during the MJO positive phase accounts for about 50% of the variance in the other an enhanced convection center (with OLR anomalies index, some discrepancies exist in the captured phases around 212 W m22) trailing a near-surface westerly by these two indices. For finding the reasons of the ob- wind (with maximum anomalies more than 1.5 m s21) served discrepancies, the concurrent variations of daily was extended over the south Indonesian–north Austra- and monthly series of the MH and WH indices were lian regions. For such conditions, the OLR anomalies compared (not shown). According to the given results, over most parts of the study regions were more than for the episodes in which the MJO amplitudes were 6Wm22, and they have reached one of the highest weak and enhanced convection centers swung across values on the global scale (more than 10 W m22) in the a wide area, the captured phases were generally found southeastern districts of Iran. to be different. On the other hand, for the episodes in Opposite to the positive phase, an enhanced convec- which the MJO amplitudes were strong and enhanced tion activity that leads an improved 850-hPa easterly and convection centers were consistently propagated east- southeasterly winds was centered over most parts of the ward, more conformity was found between the captured Indian Ocean tropics and the study areas in Iran and the phases. Arabian Peninsula as the negative MJO phase prevailed It is noteworthy to mention that the MH index com- (Fig. 2b). The positive anomaly of these easterlies is bines information from PC1 and PC2 of the 850-hPa about 1.5 and 0.5 m s21 over the central Indian Ocean zonal wind field. There are times when PC2 may have tropics and most of the study areas, respectively. As large amplitude whereas PC1 does not. On the other indicated, a band of negative OLR anomalies (with 22 hand, the RMM1 (the applied WH index) series is es- about 24Wm ) is extended from the central parts of sentially the time series of PC1, and it may need to be the African tropics to Afghanistan via Saudi Arabia and combined with RMM2 when comparing it with the MH southern parts of Iran. Maximum anomalies of positive index. Furthermore, as indicated earlier, when de- OLR (more than 10 W m22) are centered over the areas veloping a WH index a multivariate analysis with three between 908 and 1208E and between 58 and 108S along fields was used, and the 850-hPa wind fields, as a por- the Indonesian territory. Comparing Figs. 2a,b suggests tion of the leading empirical orthogonal functions that the intensity of convection activity over the Indo- (EOFs), may not have exactly the same structure as nesian region during the proposed positive phase is the MH index. Therefore, the applied MH and WH about 3 times more than the corresponding intensity indices could be shifted in phase a bit from each other over the study areas in Iran and the Arabian Peninsula to justify the observed differences in the captured during the negative phase. MJO phases. Figure 3 delineates the anomaly map of precipitable water during the considered MJO negative phase, which can be considered a diagnostic for the atmospheric b. Selection of the MJO phases moisture flux. As indicated, the augmented water vapor The analysis of daily series of the MH and WH indices anomaly over some parts of the Arabian Peninsula has has indicated that, out of 4887 available records (27 3 reached about 2.0 kg m22, which is the highest value in 181 5 4887 days), the indices were positive for 2511 and the global scale. It is noteworthy to mention that the 2485 days and negative for remaining days, respectively. maximum anomalies of precipitable water during the The given statistics suggest that, regardless of using ei- MJO positive phase were found to be around 50% more ther the WH or MH index, the frequency of positive and than this over the Indonesian and north Australian re- negative MJO events was almost identical (50% for each gions (not shown).

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FIG. 2. The anomalies of daily OLR and 850-hPa wind field for the (a) positive and (b) negative phases of the MJO for the period of November–April 1979–2005. The illustrated vector winds are significantly greater than 0.5 m s21. For the shaded areas OLR anomalies are either less than 22Wm22 or greater than 12Wm22.

The application of only the MH or WH time series as c. Precipitation the MJO index was also considered. However, after enormous computational analysis and a comparison Reliable time series of daily precipitation containing of the results, it was concluded that for some circum- the entire record length (1979–2005) was accessible for stances the outcomes of the MJO–precipitation com- only eight of the rain gauge stations in southern parts of posites are different if either the WH or MH is used as Iran (the stations denoted by a star in Table 1). In ad- the oscillation index. To resolve this discrepancy and to dition to analyzing these daily time series, the MJO– improve the results’ significance, as indicated earlier, precipitation relationships were also investigated using precipitation variability and atmospheric circulation monthly data for the November–April period. Monthly were analyzed for the episodes for which both indices precipitation data for 41 synoptic and climatology sta- were simultaneously positive and negative. tions in southern Iran and 11 stations in various parts of

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FIG. 3. The anomaly map of daily precipitable water during the negative MJO phase. Regions for which the anomalies are greater than 0.5 kg m22 or less than 20.5 kg m22 are shaded. the Arabian Peninsula (Fig. 1 and Table 2) were ob- considered stations in the Arabian Peninsula, these sta- tained from the Web sites of the Iranian Meteorological tistics vary from 36% in Salaha, Oman, to 0.99 in Bahrain Organization (IRIMO) and the Royal Netherlands (Table 2). Meteorological Institute, respectively. Only daily series d. Atmospheric variables from the Arsanjan site were gratefully supplied by the Ministry of Energy in Iran. Our attempt at accessing The girdded data of OLR, 850- and 200-hPa vector reliable and long-term data from the other station in the wind (u and y wind fields), 500-hPa omega, precipitable Arabian Peninsula failed. The analyses of daily pre- water, and precipitation rate were gratefully extracted cipitation data were used to confirm and validate the from the National Oceanic and Atmospheric Adminis- given results obtained by analyzing monthly series. tration (NOAA) Web site of the National Centers The missed precipitation data were estimated to be for Environmental Prediction–National Center for At- around 8% and 12%–20% for Iran and the Arabian mospheric Research (NECP–NCAR) reanalysis (http:// Peninsula, respectively. The results provided herein are, www.esrl.noaa.gov/psd/data/composites/day/). While the however, based only on the observed values because the spatial distribution of vector wind represents the di- missing data were ignored. As indicated in Fig. 4, about rection and magnitude of atmospheric airflow, the 77%–99% of the annual precipitation in southern Iran is omega field signifies the direction and intensity of ver- associated with the November–April period. For the tical flow. Because vertical velocity has shown to have

TABLE 1. The name of the gauge stations in the southern part of Iran is denoted by numbers in Fig. 1. Daily data were available for the stations that are flagged with an asterisk.

No. Station No. Station No. Station No. Station 1 Abadan 12 Borazjan 23 Iranshahr* 34 Shabankareh 2 Abadeh* 13 Chabahar* 24 Jask 35 Shahr-e-Kord 3 Adl(Dozak) 14 25 36 Shiraz* 4 15 Doroudzan 26 Kesht Sanat Karoon 37 Shoushtar 5 Arsanjan* 16 Emam-Ghaies 27 Khash* 38 Tasbit 6 Badjgah 17 Fasa 28 Lar 39 Zabol* 7 Bahookalat 18 Firouzabad 29 Makangan Manoojan 40 * 8 Bam 19 Ghalat 30 Minab 41 Ziaratgah 9 Bandar lengeh 20 Gorgin-Khobr 31 Molasany 10 Bandarabas 21 Haft Tappeh 32 Nayriz 11 Booshehr 22 33 Saman

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TABLE 2. Some statistical properties of the precipitation data as well as the results of the applied statistical tests for the considered 21 stations over the Arabian Peninsula. Mean and median are given (mm month ). RS/RA is the ratio of November–April precipitation to annual precipitation; Rn and Rp represent the mean precipitation during negative and positive phases of the MJO, respectively. The asterisk indicates that the applied test is significant at 95% level.

November–April period with 162 months Statistical test

No. Station Country Annual mm RS/RA Mean (mm) Median (mm) Rn/Rp Fisher exact Mann–Whitney 1 Bahrain Bahrain 69.3 0.96 12.22 2.25 1.20 — — 2 Dhahran Saudi Arabia 85.0 0.99 14.87 4.00 3.75 * * 3 Doha Qatar 84.6 0.98 14.00 2.40 5.50 * * 4 Kuwait Kuwait 87.4 0.93 14.02 9.00 2.10 * * 5 Masirah Oman 36.0 0.85 5.83 0.00 5.70 * * 6 Medina Saudi Arabia 66.0 0.79 9.69 2.00 1.10 — — 9 Muscat Oman 71.3 0.89 11.78 2.00 4.20 — * 7 Riyadh Saudi Arabia 104.6 0.92 21.22 7.00 3.00 — * 8 Salalah Oman 74.7 0.36 4.95 0.00 2.10 * * 10 Sur Oman 89.7 0.85 12.28 1.30 3.20 — * 11 Thumrait Oman 19.6 0.72 2.85 0.00 5.00 — * a more consistent relationship with tropical convection b. Monthly data analysis at 500 hPa (Lim and Wallace 1991; Robertson and Each month of the study period (1979–2005) was Mechoso 2000; Pattanaik 2007), the omega data of this characterized as a positive or negative MJO phase if, level were analyzed. The maps of precipitable water and for at least 60% of the days of that month the WH or precipitation rates were used as a signal for the magni- MK index was positive or negative, respectively. The tude of atmospheric water vapor and its transport during monthly spells for which both indices were simulta- various phases of the MJO. neously positive or negative were then selected and are summarized in Table 3. For each of the given months in 3. Methodology this table (even for February), both of the WH and MH values were either positive or negative for at least a. Daily data analysis 18 days (30 3 60% 5 18). As indicated, among the For the eight stations with daily data (in southern 162 months of the study period (27 3 6 5 162), each of Iran), the MJO–precipitation composites were con- the positive and negative MJO phases prevailed for only structed for both positive and negative phases of the 34 months, which is about 21% of the entire period. For MJO on a seasonal time scale. To quantify the measure these two sets of 34 months, the spatial distribution of of the effects of MJO extreme phases on precipita- the vector wind and OLR fields were generally found to tion amount, the ratio of mean precipitation during be similar to that in Figs. 2a,b (for the positive and the negative phase to its corresponding value during negative MJO phase, respectively). the positive MJO phase (Rneg/Rpos)wasthencomputed The MJO–precipitation composites associated with for each station. Furthermore, the ratios of Rpos/R and opposite phases of the oscillation were constructed on Rneg/R,whereR is the long-term average precipitation, monthly and seasonal time scales. For instance, for were also computed for each individual station. The the MJO positive phase, the February composite con- parametric Student’s t test and nonparametric Mann– sisted of seven precipitation values for 1985, 1992, 1994, Whitney test was then applied to investigate if mean 1997, 2000, 2001, and 2002. Likewise, the seasonal precipitation during the MJO positive phase is signifi- composites consisted of 34 records, including precipi- cantly different from its corresponding value during the tation data in January 1980, 1986, 1987, 1990, and 2005; negative phase. February 1985, 1992, 1994, 1997, 2000, 2001, and 2002; In addition to seasonal values, composite maps of ... and December 1984, 1987, 1993, 1996, and 2003 precipitation and other atmospheric variables were also (Table 3). A similar methodology was applied to con- produced for the monthly time scale. For instance, out of struct monthly and seasonal composites during the neg- the 810 days of November (27 3 30 5 810), the positive ative MJO phase. and negative phase prevailed for 345 and 268 days, re- Fourteen sets (one seasonal and six sets of monthly spectively. The considered composite maps of this month for each of the two phases) of the MJO–precipitation were, therefore, generated by obtaining the date of these composites were, therefore, constructed for every sta- days. tion. The nonparametric Mann–Whitney test was then

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FIG. 4. Spatial distribution of the ratio of November–April precipitation to annual precipitation for the period of 1979–2005 (for southern Iran). applied to investigate whether the precipitation mean for the four negative events were 70.0, 33.3 115.0, and during the MJO positive phase of each composite is sig- 144.0 mm. For this station, the mean values of pre- nificantly different from its corresponding value during cipitation for long-term (27 yr), positive, and negative the negative phase. Moreover, like the daily analysis, phases were found to be 56.5, 24.8, and 90.5 mm, re- the ratios of Rpos/R, Rneg/R,andRneg/Rpos were com- spectively. The applied Mann–Whitney test indicates puted for each station for monthly and seasonal time that the mean values of precipitation during the posi- scales. For instance, at Shiraz station, while February’s tive phase (24.8) are statistically less than the corre- precipitation during seven events of the positive MJO sponding values during the negative phase (90.5). The phase (Table 3) were recorded as 28.2, 31.0, 7.6, 5.7, ratios of Rpos/R, Rneg/R,andRneg/Rpos were computed 11.0, 29.8, and 60.2 mm, their corresponding records as 24.8/56.5 5 0.43, 90.5/56.5 5 1.6, and 90.5/24.8 5 3.6,

TABLE 3. The years during which monthly MJO was either in (a) positive or (b) negative phase. During each of the given months the time series of both WH and MH indices were simultaneously positive or negative for at least 18 days.

Month January February March April November December (a) Positive phase Year 1980, 1986, 1987, 1985, 1992, 1994, 1986, 1988, 1993, 1989, 1991, 1999, 1979, 1981, 1988, 1984, 1987, 1993, 1990, 2005 1997, 2000, 2001, 1999, 2004 2000 1995, 1998, 2001, 1996, 2003 2002 2002 (b) Negative phase Year 1979, 1985, 1992, 1982, 1986, 1990, 1981, 1983, 1984, 1988, 1995, 1997, 1982, 1991, 1992, 1984, 1986, 1989, 2004 1999 1985, 1989, 1991, 2003, 2004 1994, 2003 1995, 2001, 2002 1992, 1996, 1997, 2005 November–April period with 162 months Total events Positive phase Negative phase 34 months 34 months

Unauthenticated | Downloaded 10/07/21 04:00 AM UTC 15 FEBRUARY 2010 N A Z E M O S ADAT AND GHAEDAMINI 895 respectively. Because the ratio of Rneg/Rpos is greater TABLE 4. Seasonal values of the Rpos/R, Rneg/R, and Rneg/Rpos than one, precipitation is reduced (enhanced) in the ratios for the eight considered stations containing daily precipi- positive (negative) MJO phase. tation data. The ratios on the left and right sides of the table were computed using daily and monthly time series, respectively. Seasonal values of these ratios were obtained using a similar procedure, but for 34 records of both positive Daily Monthly and negative phases (Table 3). For the above-mentioned Station Rneg/Rpos Rpos/RRneg/RRneg/Rpos Rpos/RRneg/R analysis, the application of median instead of mean Abadeh 2.70 0.45 1.23 2.50 0.70 1.80 composites was also examined, but because of the small Arsanjan 2.60 0.55 1.40 3.00 0.45 1.50 sample size and the existence of some zero values in Chabahar 2.20 0.80 1.70 3.60 0.30 1.85 precipitation composites, the adopted methodology ob- Iranshahr 2.20 0.70 1.50 3.50 0.45 1.50 tained more reasonable results. Khash 1.15 0.90 1.10 2.50 0.60 1.50 Shiraz 2.20 0.60 1.30 2.60 0.55 1.40 Another examination was also performed to inves- Zabol 3.10 0.50 1.60 2.30 0.70 1.60 tigate whether the frequency of dry or wet events was Zahedan 1.70 0.75 1.30 3.0 0.45 1.45 significantly associated with the occurrence of positive or negative MJO phases, respectively. To conduct this examination, the events where the precipitation amount While daily composites for the positive and negative was below or above the long-term average were first phase, respectively, contained 1807 and 1714 records, counted and then considered as the frequency of dry or seasonal composites contained 34 records for each of wet incidents, respectively. For instance, out of the given these phases (Table 3). According to these statistics, the seven precipitation composites (for the Shiraz station in ratios are generally greater when the monthly time se- the above example) that occurred during the MJO ries is used instead of the daily series. The main reason positive phase, the frequency of the events in which for this discrepancy was found to be associated with the precipitation was either less or greater than 56.5 mm is 6 existence of many zero data in both daily and monthly or 1, respectively. Likewise, the frequency of dry or wet composites, in particular for southeastern districts. Be- events during the MJO negative phase (precipitation is cause daily rainfall was zero for more than 50% of the less than or above 56.5 mm) is 1 or 3. These frequencies days that the negative, and in particular positive, MJO were then put in a 2 3 2 contingency table to delineate phase prevailed, the values of Rneg or Rpos are generally the incidents of dry or wet events during each of the zero if they represent median precipitation. The ratios MJO phases. were, therefore, derived according to the mean values of The Fisher exact test (Fisher 1922; McKinney et al. either daily or seasonal composites. Comparison between 1989) was then applied to the constructed contingency the results of the daily and seasonal analyses suggests tables. The hypergeometric equation, as defined by that, for the stations without daily data, the applied Mason and Goddard (2001), was used to derive the methodology has obtained a good estimate of the mag- significance level ( p) of the relationships. If the com- nitude of the effects of MJO extreme phases on pre- puted p was less than 0.05, the frequency of wet or dry cipitation amount. events was significantly associated with the occurrence As indicated in Table 4, there is about a 10%–70% of the negative or positive phase of the MJO, re- increase from the long-term mean daily rainfall during spectively. The chi-square test was also considered in- the negative MJO phase. On the other hand, daily pre- stead of the Fisher exact test when the expected value in cipitation during the MJO positive phase was reduced each cell was found to be enough large (mostly for from about 10% to 55%. To consolidate the results, both seasonal analysis where the expected value was greater the Mann–Whitney and Student’s t tests were applied than 10). The results were, however, generally found to to investigate whether the mean value of seasonal pre- be similar for both of these tests. cipitation during the MJO positive phase was signif- icantly (at 95% significance level) different from the corresponding mean value during the negative phase. 4. Results and discussion The given results have indicated that, for each individual a. Precipitation composites station regardless of whether daily or monthly records were used, seasonal precipitation during the MJO pos- 1) COMPOSITES WITH DAILY DATA itive phase was consistently less than the corresponding

Table 4 delineates seasonal values of Rpos/R, Rneg/R, values during the negative phase. and Rneg/Rpos for the stations with daily data. Statistics Figure 5 depicts the spatial distribution of the daily in the left and right sides of the table were derived us- precipitation rate during the negative MJO phase minus ing daily and monthly precipitation data, respectively. their corresponding values during the positive phase for

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FIG. 5. Daily precipitation rates during the negative MJO phase minus their corresponding values during the positive phase for November–April 1979–2005. Contour intervals are 0.5 mm day21. The regions for which daily precipitation is significantly greater than 0.5 mm day21 or less than 21.5 mm day21 are shaded. the November–April period. As indicated, for south- tests) have indicated that, for each individual station, , some parts of Saudi Arabia, and a consid- the amounts of seasonal precipitation and the fre- erable part of Afghanistan, the average difference in quency of wet events during the MJO negative phase precipitation rate has reached to more than 0.5 mm were statistically greater than the corresponding val- day21, which is comparable to a similar difference over ues during positive phase. As indicated, the ratios the western part of the Indian Ocean and tropical Africa. vary from about 1.75 to 2.75 for the western half of the The maximum positive anomalies of precipitation were study area and from about 2.75 to 4.00 in the eastern observed over western parts of the South Pacific Ocean, districts. Alternation in the MJO extreme phases, approximately between 108–158S and 1608E–1808.In therefore, induces greater relative rainfall variability other words, the occurrence of dry or wet episodes over over the dry zones in eastern areas than over western this area is in phase with similar periods over most parts regions. However, because long-term precipitation in of the current study areas and Afghanistan. The in- western areas is generally a few times larger than the tensity of the precipitation difference over this part of corresponding values in the eastern parts, the effects the ocean is, however, about 3 times more than the of this alternation on national water resources are maximum observed intensity over the study regions. more considerable when the oscillation changes the Similar precipitation maps were also obtained for each western districts’ precipitation. As depicted in Figs. month of the study period (not shown) and the results 6b,c, the ratios of Rneg/R and Rpos/R are, respectively, confirmed the seasonal results given in Fig. 5. Because greater and smaller than one, indicating that seasonal climate characteristics of this part of the Pacific Ocean precipitation is significantly above and below the long- is the interest of the climatologists and oceanographer term mean for the negative and positive MJO phase. globally, a precipitation forecast for this region could In addition to seasonal values, the ratios were also potentially be used for the prediction of dry or wet computed for monthly precipitation and the results were conditions in southern Iran and the Arabian Peninsula. generally found to follow the seasonal pattern. As an example, Figs. 7a–c illustrates the spatial distribution of 2) COMPOSITES WITH MONTHLY DATA the ratio of Rneg/Rpos in southern Iran for November, Figure 6a depicts spatial distribution of the ratios of January, and March, respectively. According to these

Rneg/Rpos for southern Iran using precipitation data figures, for about 40% of the study area, the precipi- during 34 months of the positive and negative MJO tation amount and the frequency of wet events dur- phases (Table 3). Results of the applied significance ing the MJO negative phase were significantly greater tests (Mann–Whitney, Student’s t, and Fisher exact than corresponding values during the positive phase.

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FIG. 6. Spatial distribution of the ratios of (a) Rneg/Rpos, (b) Rneg/R, and (c) Rpos/R in southern Iran for the period of November–April 1979–2005. The results of the Student’s t, Mann– Whitney, and Fisher exact tests were significant for all the considered stations.

Because of the smaller sample size, less statistically precipitation was found to be inconsistent for some significant results were generally observed for monthly stations in southern Iran. For these stations, such as composites as compared with those of the seasonal Ahvaz and Chabahar, precipitation increased during the analysis. The effect of MJO opposite phases on March positive phase.

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FIG. 7. Same as Fig. 6a, but for (a) November, (b) January, and (c) March. The Fisher exact test is significant for the stations flagged with a red star. The shaded areas illustrate the regions for which mean precipitation during the negative MJO phase was significantly (according to Mann–Whitney test) greater than the corresponding values during the positive phase.

Unauthenticated | Downloaded 10/07/21 04:00 AM UTC 15 FEBRUARY 2010 N A Z E M O S ADAT AND GHAEDAMINI 899 b. The Arabian Peninsula of (warm) descending air over southwest Asia during the positive MJO phase. According to Gill (1980), in an Table 2, depicts the results of the applied Mann– idealized case for equatorially symmetric heating, the Whitney and Fisher exact tests for seasonal precipitation expected descent for this region should be around 1/6 of over the Arabian Peninsula. Because of the lack of sta- the maximum ascend in the enhanced convection re- tions with enough historical data and the uneven dis- gions. By solving the hydrostatic thermodynamic en- tribution of the available sites, the spatial variation of ergy equations, Barlow et al. (2005) have shown that the obtained ratios was not shown as a contour map. As both the advection of the MJO temperature anomalies indicated, the minimum and maximum values of the by the mean wind and the advection of the mean R /R ratio 5.70 and 1.10 are related to Masirah and neg pos thermal gradient by the MJO wind anomalies contrib- Medina, respectively. The ratio is generally greater than ute to air subsidence over southwest Asia, justifying 2.0 for the other stations, indicating the influential role less precipitation and hot events for this region during of the MJO extreme phases on the peninsula’s pre- the MJO positive phase. Our findings, however, in- cipitation variability. The presented significant results dicate that in addition to thermal advection, moisture suggest that during the negative MJO phase both the flux plays an influential role for describing the MJO precipitation amount and the frequency of the wet influence. events were consistently greater than corresponding Figures 8a–c illustrates the maps of the difference in values during the positive phase. Comparing the given daily precipitation rate and 850-hPa winds between the statistics in this table with the obtained information in opposite phases of the MJO for November, January, and Fig. 6a implies that precipitation variability during ex- March, respectively. For improving resolution, monthly treme phases of the MJO follows a similar pattern for precipitation maps were created for the Middle East. As both southern Iran and the Arabian Peninsula. indicated, the direction of the near-surface wind is c. Atmospheric circulation gradually changing from mostly southeasterly in No- vember to southwesterly in March, which affects the 1) SURFACE LEVELS rainfall pattern during these periods. The effect of wind As indicated in Fig. 2b, during the MJO negative direction on regional rainfall is also evident in Figs. 7a–c phase, anomalies of the low-level easterly winds across for which the areas with significant results generally the North Indian Ocean tropics tend to become southerly follow the pathway of moisture transport. Consistent near the coasts. These southerly winds transfer a sub- with the near-surface wind speed, the lowest and highest stantial amount of moisture to the study areas, precon- anomalies of precipitation rate over the study areas are, ditioning the atmosphere for deep convection (Fig. 3). A respectively, associated with November and January, southwest–northeast band of a negative OLR anomaly indicating that the alternation in the MJO extreme extends from western parts of Saudi Arabia (near the Red phases modulates wind characteristics and precipitation Sea) to northwestern regions of Afghanistan via southern rate in both seasonal and monthly scales. For instance, Iran, confirming such atmospheric perturbation during because the southerly or southwesterly wind prevailed the negative MJO phase (Fig. 2b). In addition to moisture in March, the precipitation difference has reached to transport from northwestern parts of the Indian Ocean more than 0.5 mm day21 in most parts of southern Iran, (via the Arabian Peninsula), the southerly winds also indicating a substantial enhancement in the rainfall of transfer a substantial amount of the Arabian Sea mois- this region during the negative MJO phase. An increase ture to southeastern parts of Iran in a direct way, inducing in the March precipitation rate (about 0.2–0.3 mm day21) heavy storms over these regions and possibly the coastal was also observed over the eastern coasts of the Arabian areas of Pakistan (Fig. 2b). Peninsula, though it is not delineated in Fig. 8c. Opposite to the negative phase, a dry near-surface 2) UPPER LEVELS northerly airflow that is concurrent with the positive anomalies of OLR was engulfed over the study area as The study areas mostly lie between two maxima in the the MJO positive phase prevailed (Fig. 2a). For south- subtropical westerlies: in the exit region of the North eastern parts of Iran and the areas alongside the Pakistan– Africa–Arabian jet and near the entrance region of Afghanistan borders, these anomalies reached to more the East Asian jet (Barlow et al. 2005). The entrance than 10 W m22, which is one of the highest values on the or exit regions of the aloft jets are associated with global scale. These positive anomalies are in balance moisture-bearing synoptic storms sometimes called with the negative OLR anomalies over the Indonesian ‘‘cyclone belts,’’ ‘‘storm tracks,’’ or baroclinic waveguides region, with magnitudes less than 212 W m22. Barlow (Blackmon et al. 1977; Wallace et al. 1988). The strong et al. (2005) have remarked about the large values horizontal gradients of wind speed at these regions are

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21 21 FIG. 8. Difference in daily precipitation rate (mm day ) and 850-hPa wind anomalies (m s ) between the positive and negative phases of the MJO for (a) November, (b) January, and (c) March. The denoted arrows signify that the areas with wind speed greater than 1 m s21. The precipitation differences are related to a particular month and could be greater or lower than the seasonal values that are shown in Fig. 5. generally balanced by vertical air motion, which affects a more diffuse jet exit region extending from southern the amount and strength of storm activity (Blackmon Iran through northern India. On the other hand, re- et al. 1977). Furthermore, the net effect of the upper-level ducing aloft wind speed during the negative MJO phase winds on the storm track appears to result from opposing enhances the exit region and its associated baroclinic in- contributions from baroclinic and barotropic energetics stability. They have reported that when the MJO rainfall (Cai and Mak 1990; Whitaker and Dole 1995). anomalies are in the eastern Indian Ocean tropics, the Barlow et al. (2005) have concluded that the occur- Northern Hemisphere Rossby gyre extends over south- rence of the MJO positive phase increases the aloft wind west Asia in terms of changes to the upper- and low-level speed over most parts of southwest Asia during boreal winds and their associated moisture transport. wintertime. However, the speed anomalies generally Figure 9a illustrates the maps of 200-hPa seasonal occur in the exit region of the jet maximum and result in mean wind field for the negative MJO phase and the a decrease in the gradient of wind speed, producing difference in 500-hPa omega for the opposite phases of

Unauthenticated | Downloaded 10/07/21 04:00 AM UTC 15 FEBRUARY 2010 N A Z E M O S ADAT AND GHAEDAMINI 901 the oscillation (omega values during negative phase Whereas the jet speed in January and March is mostly minus corresponding values during positive phase). The similar, the horizontal gradients of wind speed are exit region of the North Africa–Arabian jet, with its sharper and shifted farther to the east during latter remarkable horizontal wind gradient, has extended over months (Figs. 9c,d). As indicated in Fig. 9d, the wind northern parts of the Saudi Arabia and southern Iran gradient is about 10 m s21 for the areas between west- enhancing baroclinic instability, upward motion, and ern parts of Saudi Arabia and southeastern districts of precipitation. The negative values of the difference in Iran. In response to this horizontal wind gradient, the 500-hPa omega (around 20.01 Pa s21) are widespread difference in upward motion that is indicated by the over most parts of the study areas, with a significant en- negative anomalies of omega has also increased for all of hancement (around 20.03 Pa s21) over southern parts the study areas with maximum values (20.05 Pa s21) of Iran, where the positive anomalies of seasonal pre- above southeastern parts of Iran. As illustrated in Figs. 7c cipitation has reached 0.5 mm day21 (Fig. 5). Although and 8c, consistent with air circulation at the near- the occurrence of the MJO positive phase has increased surface, middle, and upper atmospheric levels, March the 200-hPa wind speed over the study areas by about precipitation has also significantly increased for this part 5ms21 from the mean speed (not shown), the hori- of Iran, western coasts of the Arabian Peninsula, and zontal wind gradient at the jet exit was found to be major parts of Afghanistan. around zero, suppressing upward motion and its asso- ciated precipitation. 5. Conclusions Figures 9b–d is like Fig. 9a, but for November, Janu- ary, and March, respectively. As indicated, the geo- The present study has made an effort to investigate graphical position and wind velocity of the jets are the effects of the Madden–Julian oscillation (MJO) different during various month of the study period, on November–April precipitation in both southern influencing both upward motion and storm tracks. In parts of Iran and the Arabian Peninsula for the period November (Fig. 9b), while the exit regions of the North of 1979–2005. The atmospheric circulation associated Africa–Arabian jet is above Egypt, the entrance of the with the MJO phenomenon was also analyzed to jus- East Asian jet is placed over southeastern parts of Iran, tify the obtained results. The positive and negative indicating a significant westward shift of the upper-level phases of the MJO were defined using daily records of jet compared to the seasonal mean. two indices comprising the time series of WH (the

Because of the lower wind speed at the lower and RMM1 series in Wheeler and Hendon 2004) and MH upper atmospheric levels, precipitation anomalies dur- (Maloney and Hartmann 1998; Maloney and Kiehl ing November are less than corresponding values in 2002). Of the 4887 days of the study periods, both January and March. In Fig. 9b, while the upward air indices were simultaneously positive (positive phase) motion in the southwestern parts of Saudi Arabia are or negative (negative phase) for 1807 or 1714 days, linked to the North Africa–Arabian jet exit, similar respectively. During the MJO positive phase, an en- motion over southern parts of Iran is mostly associated hanced convection center trailing a near-surface west- with the entrance region of the East Asian jet. erly wind anomaly was mostly extended over the south As indicated in Fig. 9c, compared to November, wind Indonesian and north Australian regions. Opposite to speed at January’s jet exit has increased by about 42% the positive phase, enhanced convection activity that and the exit region has displaced eastward, toward Iran’s leads a near-surface southerly wind anomaly centered territory (maximum jet speeds during November and over most parts of the study areas in Iran and the January are about 50 and 35 m s21, respectively). Al- Arabian Peninsula as the negative MJO phase pre- though, for most parts of the study areas, the difference vailed. This enhanced convection activity was, however, in upward motion (omega values at the 500-hPa level) weaker (about 1/3) than the atmospheric perturbation increased by about 2 times compared to November, the over the Indonesian region during the MJO positive increase is particularly remarkable for southwestern phase. In addition to being seasonal, the positive or parts of Iran and central-southern Saudi Arabia. Syn- negative phase of the MJO was also defined on a monthly chronized with this upward motion and the increased scale. horizontal wind speed gradients at the 200-hPa level, the The MJO–precipitation composites were then con- near-surface airflow augments a substantial amount of structed for the adapted positive and negative phases the nearby water bodies’ moisture (the Indian Ocean, using daily precipitation data for eight stations in Red Sea, and the Persian Gulf) to the study area pre- southern Iran. A similar compositing procedure was also conditioning the atmosphere for precipitation enhance- performed using the girdded values of OLR, omega, ment (Figs. 7b and 8b). precipitation rate, and wind fields to analyze the effects

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FIG. 9. Imposed map of zonal wind field at the 200-hPa level during the negative MJO phase and the difference in 500-hPa omega between the positive and negative phases of the MJO for (a) the seasonal, (b) November, (c) January, and (d) March period. Negative or positive omega values signify upward or downward air motion, respectively. Contour lines are not shown when wind speed is less than 20 m s21. of the MJO extreme phases on atmospheric circulation monthly data. According to the proposed definition, for and moisture transport on the regional scale. The anal- each month of the study period the positive or negative ysis of available daily precipitation data showed that the phase of the MJO prevailed if daily series of both MH MJO negative phase enhances seasonal (November– and WH were simultaneously positive or negative for at April period) precipitation over southern Iran from least 60% of the days of that month. Out of the 162 about 10% to 70%. On the other hand, the prevalence of months of the study period, each of the positive or the MJO positive phase reduced seasonal precipitation negative MJO phase prevailed for 34 months. from about 10% to 55%. For almost entire parts of the study area, seasonal A methodology was proposed to analyze the effects precipitation and the frequency of wet events were sta- of the MJO phases on precipitation variability using tistically increased during the MJO negative phase (and

Unauthenticated | Downloaded 10/07/21 04:00 AM UTC 15 FEBRUARY 2010 N A Z E M O S ADAT AND GHAEDAMINI 903 vice versa for the positive phase and dry events). The Cai, M., and M. Mak, 1990: On the basic dynamics of regional cy- ratio of seasonal precipitation during the negative phase clogenesis. J. Atmos. Sci., 47, 1417–1442. to the corresponding values during the positive phase was Carvalho, L. M. V., C. Jones, and B. Liebmann, 2004: The South Atlantic convergence zone: Intensity, form, persistence, and found to be about 1.75–2.75 and 2.75–4.00 for the western relationships with intraseasonal to interannual activity and and eastern parts of southern Iran, respectively. For the extreme rainfall. J. Climate, 17, 88–108. considered stations within the Arabian Peninsula, the Chakraborty, A., S. K. Behera, M. Mujumdar, R. Ohba, and highest and lowest values of this ratio (5.70 and 1.10) T. 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Ribbe, 2006: Near-global impact of the Gulf moisture to the study areas in Iran and the Arabian Madden-Julian Oscillation on rainfall. Geophys. Res. Lett., 33, Peninsula. At the same time, upward motion has im- L09704, doi:10.1029/2005GL025155. proved at the midatmosphere, causing precipitation en- Efron, B., and R. J. Tibshirani, 1993: An Introduction to the Bootstrap. Chapman and Hall, 436 pp. hancement. For the upper atmosphere levels, suppression Fisher, R. A., 1922: On the interpretation of x2fromcontin- of the total wind field and enhancement in the horizontal gency tables, and the calculation of P. J. Roy. Stat. Soc., 85, wind gradient are the other characteristics of the negative 87–94. MJO phase over the study regions. The geographical po- Gill, A., 1980: Some simple solutions for heat induced tropical sition of the North Africa–Arabian and East Asian jets circulation. Quart. J. Roy. Meteor. 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