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.
The Madden–Julian oscillation (MJO) is known as the primary mode of large-scale intraseasonal variability in tropical regions (Madden and Julian 1994). Donald et al. (2004) characterized the MJO as a 40-day wave that develops over the tropical Indian Ocean and then travels east across the tropics at 5–10 m s−1. The phenomenon has a frequency of 6–12 events per year, with an associated period ranging from 30 to 60 days. In its active stage, the MJO is associated with increased convective activity over the equatorial eastern Indian and western Pacific Oceans. Trailing the active center is a region of suppressed convective activity and near-surface-level westerly wind.
Since the discovery of the oscillation, many studies have shown that MJO also affects extratropical weather and climate systems (Liebmann and Hartmann 1984; Krishnamurti et al. 1997). Moreover, it has been shown that the MJO strongly influences the monsoon-related precipitation patterns in Asia and Australia, and moderately influences precipitation in North and South America (Lau and Chan 1986; Mo 2000; Paegle et al. 2000; Higgins and Shi 2001; Carvalho et al. 2004; Donald et al. 2006). The MJO has also been linked with enhanced precipitation and increased incidence of floods in northwestern parts of the United States, although its influence was found to be substantially different during early versus late winter (Bond and Vecchi 2003). Hendon et al. (2000) found that forecasts in the tropics and midlatitudes of the Northern Hemisphere during boreal winter have less skill when they are initialized either during or prior to periods of active MJO as opposed to quiescent episodes of the oscillation. Donald et al. (2006) have provided a mechanistic basis for an MJO-based forecasting capacity that bridges the weather–climate divide for tropical and extratropical weather states. Maloney and Hartmann (1998) and Maloney and Kiehl (2002) introduced an MJO index for characterizing the intensity and state of the MJO. After this, Wheeler and Hendon (2004) developed another index for monitoring and predicting the MJO.
Barlow et al. (2005) analyzed the impact of the MJO on daily precipitation in southwest Asia for the period from 1979 to 2002. They showed that the MJO signals can modulate regional precipitation with strength comparable to interannual variability. Furthermore, the MJO-associated winds aloft, which are largest poleward and to the west of the primary tropical rainfall anomalies, are shown to have a clear influence on the local jet structure in southwest Asia. They have suggested further research by including several key areas with more observational data of precipitation and consistent reporting, for the verification of their results based on outgoing longwave radiation (OLR) data. Also, Mariotti (2007) has indicated that the enhanced precipitation in southwest central Asia during El Niño–Southern Oscillation (ENSO) events results from an anomalous southwesterly moisture flux coming from the Arabian Sea and tropical Africa, which is generated along the northwestern flank of the high pressure anomaly over the Indian and western Pacific Oceans, which is part of the canonical ENSO sea-saw pressure anomalies.
An assessment of the effects of the MJO with respect 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 circulation over Iran and the Arabian Peninsula that is the main concern of the present study was not the theme of their investigation. In addition to a seasonal analysis, the present study also examines the effects of the MJO extreme phases on precipitation and atmospheric circulation on the monthly time scale, which was not the focus of earlier studies.
Almost all of the Arabian Peninsula and the southern regions of Iran (Fig. 1) are situated in a region that is characterized by low amounts and high variability of precipitation as well as a high potential for evapotranspiration (Sadeghi et al. 2002; Sormana and Abdulrazzak 1995). The average annual precipitation in southern Iran varies from about 600 mm in western and central areas to less than 100 mm in the east, with precipitation generally occurring between November and April. The intensity and frequency of hydrometeorological disasters are generally greater for this part of the country than other areas. For the accessible stations in the Arabian Peninsula, annual precipitation varied from about 105 mm in Riyadh to 20 mm in Thumrait (Fig. 1). Reliable prediction of seasonal and intraseasonal precipitation in these regions is considered a challenge for climate researchers.
Recent studies have shown that major teleconnection patterns, including ENSO and sea surface temperatures over both the Persian Gulf and Caspian Sea, have a significant impact on rainfall in Iran (Nazemosadat and Cordery 2000; Nazemosadat 2001; Nazemosadat and Mousavi 2001; Nazemosadat and Ghasemi 2004; Nazemosadat et al. 2006). Chakraborty et al. (2006) have analyzed the influences of the Indian Ocean dipole (IOD) and ENSO on tropospheric moisture over Saudi Arabia. They reported an increased atmospheric moisture flux during El Niño and the positive IOD phase.
Given the predictability of the MJO out to at least 4 weeks in combination with its strong influence on regional precipitation (Wheeler and Hendon 2004; Donald et al. 2006), the outcome of the study could potentially improve subseasonal forecasting of dry and wet periods over the study regions. The applied analysis, hopefully, bridges the gap between synoptic and subseasonal forecasting, which is an essential component for risk management in west Asian areas. The goals of the study are, therefore, to examine the effects of the MJO extreme phases on precipitation and the frequency of dry or wet events in southern Iran and the Arabian Peninsula. Moreover, the impact of these phases on atmospheric circulations over the study region is analyzed.
a. The MJO indices and their relationships
Based on the first two principal components (PC1 and PC2) of the bandpass-filtered 850-hPa zonal wind data over the equatorial (5°N–5°S) ocean waters (globally), Maloney and Hartmann (1998) and Maloney and Kiehl (2002) introduced an MJO index by adding PC1 to the value of PC2 12 days later (hereafter the MH index). Their PC1 time series is most closely associated with zonal wind over the Indian Ocean and eastern Pacific, while PC2 emphasizes variance over the western Pacific. Daily time series of the MH index was gratefully supplied by E. Maloney (2006, personal communication) for the of period 1979–2005.
In addition to the MH index, Wheeler and Hendon (2004) developed a pair of principal component time series, called the real-time ultivariate MJO series 1 (RMM1) and series 2 (RMM2), as the oscillation indices. These indices, which use satellite-based outgoing longwave radiation and near-equatorial 850- and 200-hPa zonal winds as input, have subsequently been widely used for monitoring and predicting the state of the MJO. The two multivariate series were most highly correlated when RMM2 lags RMM1 by 10–15 days. The time series of RMM1 (hereafter the WH index) were also used as another MJO index to assess the oscillation impact on precipitation variability in southern Iran and the Arabian Peninsula. The RMM2 series was also examined but did not provide meaningful results compared to those of RMM1. The MH and WH indices are mostly positive (or negative) when the enhanced (or suppressed) precipitation anomalies are propagating from equatorial parts of the eastern Indian Ocean region to the western parts of the Pacific Ocean. Negative values of these indices suggest a weaker positive precipitation anomaly over the westernmost Indian Ocean and central Pacific. More details about the MH or WH indices are given on the referred references.
The correlation coefficients between the daily series of these two indices were examined for all 27 yr (with 365 records in each analysis) and were found to be around 0.70 for each analysis (significant at the 99% level). As an example, the coefficients were found to be 0.69, 0.66, 0.75, and 0.75 for 1981, 1990, 1997, and 2002, respectively. In addition to the concurrent correlations, the lag-1–5-day correlations were also examined, but the best results were found for the contemporaneous analysis. The resampling bootstrap analysis (with replacement) with 1000 synthetic pairs of samples was then performed to examine the approximation of the confidence intervals around the computed correlation coefficients (r) using observed data (Efron and Tibshirani 1993; Wilks 1995; Wood 2004). Each computed (r) was significant (at the 95% level) if, among the 1000 sorted series of the simulated coefficients, it does not fall within the 25 greatest or 25 lowest of the series. In addition to yearly analysis, the correlation coefficients between these two indices were also examined for each month of all 27 yr. Again, the results were generally found to be around 0.6–0.7.
In spite of these strong relationships that indicate that the variance in each of the considered time series accounts for about 50% of the variance in the other index, some discrepancies exist in the captured phases by these two indices. For finding the reasons of the observed discrepancies, the concurrent variations of daily and monthly series of the MH and WH indices were compared (not shown). According to the given results, for the episodes in which the MJO amplitudes were weak and enhanced convection centers swung across a wide area, the captured phases were generally found to be different. On the other hand, for the episodes in which the MJO amplitudes were strong and enhanced convection centers were consistently propagated eastward, more conformity was found between the captured phases.
It is noteworthy to mention that the MH index combines information from PC1 and PC2 of the 850-hPa zonal wind field. There are times when PC2 may have large amplitude whereas PC1 does not. On the other hand, the RMM1 (the applied WH index) series is essentially the time series of PC1, and it may need to be combined with RMM2 when comparing it with the MH index. Furthermore, as indicated earlier, when developing a WH index a multivariate analysis with three fields was used, and the 850-hPa wind fields, as a portion of the leading empirical orthogonal functions (EOFs), may not have exactly the same structure as the MH index. Therefore, the applied MH and WH indices could be shifted in phase a bit from each other to justify the observed differences in the captured MJO phases.
b. Selection of the MJO phases
The analysis of daily series of the MH and WH indices has indicated that, out of 4887 available records (27 × 181 = 4887 days), the indices were positive for 2511 and 2485 days and negative for remaining days, respectively. The given statistics suggest that, regardless of using either the WH or MH index, the frequency of positive and negative MJO events was almost identical (50% for each phase). Furthermore, it was found that both indices were simultaneously positive or negative for 1807 and 1714 days (37% and 35% of the study periods), respectively. This means that the sign of WH and MH indices was different for about 28% [100% − (37% + 35%)] of the considered period.
Figures 2a,b shows the anomaly maps of OLR and the 850-hPa vector wind fields for the days on which both indices were concurrently positive and negative (1807 and 1714 days, respectively). For the purpose of this study, the given conditions in Figs. 2a,b were correspondingly referred to as the positive and negative MJO phase. As indicated, during the MJO positive phase an enhanced convection center (with OLR anomalies around −12 W m−2) trailing a near-surface westerly wind (with maximum anomalies more than 1.5 m s−1) was extended over the south Indonesian–north Australian regions. For such conditions, the OLR anomalies over most parts of the study regions were more than 6 W m−2, and they have reached one of the highest values on the global scale (more than 10 W m−2) in the southeastern districts of Iran.
Opposite to the positive phase, an enhanced convection activity that leads an improved 850-hPa easterly and southeasterly winds was centered over most parts of the Indian Ocean tropics and the study areas in Iran and the Arabian Peninsula as the negative MJO phase prevailed (Fig. 2b). The positive anomaly of these easterlies is about 1.5 and 0.5 m s−1 over the central Indian Ocean tropics and most of the study areas, respectively. As indicated, a band of negative OLR anomalies (with about −4 W m−2) is extended from the central parts of the African tropics to Afghanistan via Saudi Arabia and southern parts of Iran. Maximum anomalies of positive OLR (more than 10 W m−2) are centered over the areas between 90° and 120°E and between 5° and 10°S along the Indonesian territory. Comparing Figs. 2a,b suggests that the intensity of convection activity over the Indonesian region during the proposed positive phase is about 3 times more than the corresponding intensity over the study areas in Iran and the Arabian Peninsula during the negative phase.
Figure 3 delineates the anomaly map of precipitable water during the considered MJO negative phase, which can be considered a diagnostic for the atmospheric moisture flux. As indicated, the augmented water vapor anomaly over some parts of the Arabian Peninsula has reached about 2.0 kg m−2, which is the highest value in the global scale. It is noteworthy to mention that the maximum anomalies of precipitable water during the MJO positive phase were found to be around 50% more than this over the Indonesian and north Australian regions (not shown).
The application of only the MH or WH time series as the MJO index was also considered. However, after enormous computational analysis and a comparison of the results, it was concluded that for some circumstances the outcomes of the MJO–precipitation composites are different if either the WH or MH is used as the oscillation index. To resolve this discrepancy and to improve the results’ significance, as indicated earlier, precipitation variability and atmospheric circulation were analyzed for the episodes for which both indices were simultaneously positive and negative.
Reliable time series of daily precipitation containing the entire record length (1979–2005) was accessible for only eight of the rain gauge stations in southern parts of Iran (the stations denoted by a star in Table 1). In addition to analyzing these daily time series, the MJO–precipitation relationships were also investigated using monthly data for the November–April period. Monthly precipitation data for 41 synoptic and climatology stations in southern Iran and 11 stations in various parts of the Arabian Peninsula (Fig. 1 and Table 2) were obtained from the Web sites of the Iranian Meteorological Organization (IRIMO) and the Royal Netherlands Meteorological Institute, respectively. Only daily series from the Arsanjan site were gratefully supplied by the Ministry of Energy in Iran. Our attempt at accessing reliable and long-term data from the other station in the Arabian Peninsula failed. The analyses of daily precipitation data were used to confirm and validate the given results obtained by analyzing monthly series.
The missed precipitation data were estimated to be around 8% and 12%–20% for Iran and the Arabian Peninsula, respectively. The results provided herein are, however, based only on the observed values because the missing data were ignored. As indicated in Fig. 4, about 77%–99% of the annual precipitation in southern Iran is associated with the November–April period. For the considered stations in the Arabian Peninsula, these statistics vary from 36% in Salaha, Oman, to 0.99 in Bahrain (Table 2).
d. Atmospheric variables
The girdded data of OLR, 850- and 200-hPa vector wind (u and υ wind fields), 500-hPa omega, precipitable water, and precipitation rate were gratefully extracted from the National Oceanic and Atmospheric Administration (NOAA) Web site of the National Centers for Environmental Prediction–National Center for Atmospheric Research (NECP–NCAR) reanalysis (http://www.esrl.noaa.gov/psd/data/composites/day/). While the spatial distribution of vector wind represents the direction and magnitude of atmospheric airflow, the omega field signifies the direction and intensity of vertical flow. Because vertical velocity has shown to have a more consistent relationship with tropical convection at 500 hPa (Lim and Wallace 1991; Robertson and Mechoso 2000; Pattanaik 2007), the omega data of this level were analyzed. The maps of precipitable water and precipitation rates were used as a signal for the magnitude of atmospheric water vapor and its transport during various phases of the MJO.
a. Daily data analysis
For the eight stations with daily data (in southern Iran), the MJO–precipitation composites were constructed for both positive and negative phases of the MJO on a seasonal time scale. To quantify the measure of the effects of MJO extreme phases on precipitation amount, the ratio of mean precipitation during the negative phase to its corresponding value during the positive MJO phase (Rneg/Rpos) was then computed for each station. Furthermore, the ratios of Rpos/R and Rneg/R, where R is the long-term average precipitation, were also computed for each individual station. The parametric Student’s t test and nonparametric Mann–Whitney test was then applied to investigate if mean precipitation during the MJO positive phase is significantly different from its corresponding value during the negative phase.
In addition to seasonal values, composite maps of precipitation and other atmospheric variables were also produced for the monthly time scale. For instance, out of the 810 days of November (27 × 30 = 810), the positive and negative phase prevailed for 345 and 268 days, respectively. The considered composite maps of this month were, therefore, generated by obtaining the date of these days.
b. Monthly data analysis
Each month of the study period (1979–2005) was characterized as a positive or negative MJO phase if, for at least 60% of the days of that month the WH or MK index was positive or negative, respectively. The monthly spells for which both indices were simultaneously positive or negative were then selected and are summarized in Table 3. For each of the given months in this table (even for February), both of the WH and MH values were either positive or negative for at least 18 days (30 × 60% = 18). As indicated, among the 162 months of the study period (27 × 6 = 162), each of the positive and negative MJO phases prevailed for only 34 months, which is about 21% of the entire period. For these two sets of 34 months, the spatial distribution of the vector wind and OLR fields were generally found to be similar to that in Figs. 2a,b (for the positive and negative MJO phase, respectively).
The MJO–precipitation composites associated with opposite phases of the oscillation were constructed on monthly and seasonal time scales. For instance, for the MJO positive phase, the February composite consisted of seven precipitation values for 1985, 1992, 1994, 1997, 2000, 2001, and 2002. Likewise, the seasonal composites consisted of 34 records, including precipitation data in January 1980, 1986, 1987, 1990, and 2005; February 1985, 1992, 1994, 1997, 2000, 2001, and 2002; … and December 1984, 1987, 1993, 1996, and 2003 (Table 3). A similar methodology was applied to construct monthly and seasonal composites during the negative MJO phase.
Fourteen sets (one seasonal and six sets of monthly for each of the two phases) of the MJO–precipitation composites were, therefore, constructed for every station. The nonparametric Mann–Whitney test was then applied to investigate whether the precipitation mean during the MJO positive phase of each composite is significantly different from its corresponding value during the negative phase. Moreover, like the daily analysis, the ratios of Rpos/R, Rneg/R, and Rneg/Rpos were computed for each station for monthly and seasonal time scales. For instance, at Shiraz station, while February’s precipitation during seven events of the positive MJO phase (Table 3) were recorded as 28.2, 31.0, 7.6, 5.7, 11.0, 29.8, and 60.2 mm, their corresponding records for the four negative events were 70.0, 33.3 115.0, and 144.0 mm. For this station, the mean values of precipitation for long-term (27 yr), positive, and negative phases were found to be 56.5, 24.8, and 90.5 mm, respectively. The applied Mann–Whitney test indicates that the mean values of precipitation during the positive phase (24.8) are statistically less than the corresponding values during the negative phase (90.5). The ratios of Rpos/R, Rneg/R, and Rneg/Rpos were computed as 24.8/56.5 = 0.43, 90.5/56.5 = 1.6, and 90.5/24.8 = 3.6, respectively. Because the ratio of Rneg/Rpos is greater than one, precipitation is reduced (enhanced) in the positive (negative) MJO phase.
Seasonal values of these ratios were obtained using a similar procedure, but for 34 records of both positive and negative phases (Table 3). For the above-mentioned analysis, the application of median instead of mean composites was also examined, but because of the small sample size and the existence of some zero values in precipitation composites, the adopted methodology obtained more reasonable results.
Another examination was also performed to investigate whether the frequency of dry or wet events was significantly associated with the occurrence of positive or negative MJO phases, respectively. To conduct this examination, the events where the precipitation amount was below or above the long-term average were first counted and then considered as the frequency of dry or wet incidents, respectively. For instance, out of the given seven precipitation composites (for the Shiraz station in the above example) that occurred during the MJO positive phase, the frequency of the events in which precipitation was either less or greater than 56.5 mm is 6 or 1, respectively. Likewise, the frequency of dry or wet events during the MJO negative phase (precipitation is less than or above 56.5 mm) is 1 or 3. These frequencies were then put in a 2 × 2 contingency table to delineate the incidents of dry or wet events during each of the MJO phases.
The Fisher exact test (Fisher 1922; McKinney et al. 1989) was then applied to the constructed contingency tables. The hypergeometric equation, as defined by Mason and Goddard (2001), was used to derive the significance level (p) of the relationships. If the computed p was less than 0.05, the frequency of wet or dry events was significantly associated with the occurrence of the negative or positive phase of the MJO, respectively. The chi-square test was also considered instead of the Fisher exact test when the expected value in each cell was found to be enough large (mostly for seasonal analysis where the expected value was greater than 10). The results were, however, generally found to be similar for both of these tests.
4. Results and discussion
a. Precipitation composites
1) Composites with daily data
Table 4 delineates seasonal values of Rpos/R, Rneg/R, and Rneg/Rpos for the stations with daily data. Statistics in the left and right sides of the table were derived using daily and monthly precipitation data, respectively. While daily composites for the positive and negative phase, respectively, contained 1807 and 1714 records, seasonal composites contained 34 records for each of these phases (Table 3). According to these statistics, the ratios are generally greater when the monthly time series is used instead of the daily series. The main reason for this discrepancy was found to be associated with the existence of many zero data in both daily and monthly composites, in particular for southeastern districts. Because daily rainfall was zero for more than 50% of the days that the negative, and in particular positive, MJO phase prevailed, the values of Rneg or Rpos are generally zero if they represent median precipitation. The ratios were, therefore, derived according to the mean values of either daily or seasonal composites. Comparison between the results of the daily and seasonal analyses suggests that, for the stations without daily data, the applied methodology has obtained a good estimate of the magnitude of the effects of MJO extreme phases on precipitation amount.
As indicated in Table 4, there is about a 10%–70% increase from the long-term mean daily rainfall during the negative MJO phase. On the other hand, daily precipitation during the MJO positive phase was reduced from about 10% to 55%. To consolidate the results, both the Mann–Whitney and Student’s t tests were applied to investigate whether the mean value of seasonal precipitation during the MJO positive phase was significantly (at 95% significance level) different from the corresponding mean value during the negative phase. The given results have indicated that, for each individual station regardless of whether daily or monthly records were used, seasonal precipitation during the MJO positive phase was consistently less than the corresponding values during the negative phase.
Figure 5 depicts the spatial distribution of the daily precipitation rate during the negative MJO phase minus their corresponding values during the positive phase for the November–April period. As indicated, for southwestern Iran, some parts of Saudi Arabia, and a considerable part of Afghanistan, the average difference in precipitation rate has reached to more than 0.5 mm day−1, which is comparable to a similar difference over the western part of the Indian Ocean and tropical Africa. The maximum positive anomalies of precipitation were observed over western parts of the South Pacific Ocean, approximately between 10°–15°S and 160°E–180°. In other words, the occurrence of dry or wet episodes over this area is in phase with similar periods over most parts of the current study areas and Afghanistan. The intensity of the precipitation difference over this part of the ocean is, however, about 3 times more than the maximum observed intensity over the study regions. Similar precipitation maps were also obtained for each month of the study period (not shown) and the results confirmed the seasonal results given in Fig. 5. Because climate characteristics of this part of the Pacific Ocean is the interest of the climatologists and oceanographer globally, a precipitation forecast for this region could potentially be used for the prediction of dry or wet conditions in southern Iran and the Arabian Peninsula.
2) Composites with monthly data
Figure 6a depicts spatial distribution of the ratios of Rneg/Rpos for southern Iran using precipitation data during 34 months of the positive and negative MJO phases (Table 3). Results of the applied significance tests (Mann–Whitney, Student’s t, and Fisher exact tests) have indicated that, for each individual station, the amounts of seasonal precipitation and the frequency of wet events during the MJO negative phase were statistically greater than the corresponding values during positive phase. As indicated, the ratios vary from about 1.75 to 2.75 for the western half of the study area and from about 2.75 to 4.00 in the eastern districts. Alternation in the MJO extreme phases, therefore, induces greater relative rainfall variability over the dry zones in eastern areas than over western regions. However, because long-term precipitation in western areas is generally a few times larger than the corresponding values in the eastern parts, the effects of this alternation on national water resources are more considerable when the oscillation changes the western districts’ precipitation. As depicted in Figs. 6b,c, the ratios of Rneg/R and Rpos/R are, respectively, greater and smaller than one, indicating that seasonal precipitation is significantly above and below the long-term mean for the negative and positive MJO phase.
In addition to seasonal values, the ratios were also computed for monthly precipitation and the results were generally found to follow the seasonal pattern. As an example, Figs. 7a–c illustrates the spatial distribution of the ratio of Rneg/Rpos in southern Iran for November, January, and March, respectively. According to these figures, for about 40% of the study area, the precipitation amount and the frequency of wet events during the MJO negative phase were significantly greater than corresponding values during the positive phase. Because of the smaller sample size, less statistically significant results were generally observed for monthly composites as compared with those of the seasonal analysis. The effect of MJO opposite phases on March precipitation was found to be inconsistent for some stations in southern Iran. For these stations, such as Ahvaz and Chabahar, precipitation increased during the positive phase.
b. The Arabian Peninsula
Table 2, depicts the results of the applied Mann–Whitney and Fisher exact tests for seasonal precipitation over the Arabian Peninsula. Because of the lack of stations with enough historical data and the uneven distribution of the available sites, the spatial variation of the obtained ratios was not shown as a contour map. As indicated, the minimum and maximum values of the Rneg/Rpos ratio 5.70 and 1.10 are related to Masirah and Medina, respectively. The ratio is generally greater than 2.0 for the other stations, indicating the influential role of the MJO extreme phases on the peninsula’s precipitation variability. The presented significant results suggest that during the negative MJO phase both the precipitation amount and the frequency of the wet events were consistently greater than corresponding values during the positive phase. Comparing the given statistics in this table with the obtained information in Fig. 6a implies that precipitation variability during extreme phases of the MJO follows a similar pattern for both southern Iran and the Arabian Peninsula.
c. Atmospheric circulation
1) Surface levels
As indicated in Fig. 2b, during the MJO negative phase, anomalies of the low-level easterly winds across the North Indian Ocean tropics tend to become southerly near the coasts. These southerly winds transfer a substantial amount of moisture to the study areas, preconditioning the atmosphere for deep convection (Fig. 3). A southwest–northeast band of a negative OLR anomaly extends from western parts of Saudi Arabia (near the Red Sea) to northwestern regions of Afghanistan via southern Iran, confirming such atmospheric perturbation during the negative MJO phase (Fig. 2b). In addition to moisture transport from northwestern parts of the Indian Ocean (via the Arabian Peninsula), the southerly winds also transfer a substantial amount of the Arabian Sea moisture to southeastern parts of Iran in a direct way, inducing heavy storms over these regions and possibly the coastal areas of Pakistan (Fig. 2b).
Opposite to the negative phase, a dry near-surface northerly airflow that is concurrent with the positive anomalies of OLR was engulfed over the study area as the MJO positive phase prevailed (Fig. 2a). For southeastern parts of Iran and the areas alongside the Pakistan–Afghanistan borders, these anomalies reached to more than 10 W m−2, which is one of the highest values on the global scale. These positive anomalies are in balance with the negative OLR anomalies over the Indonesian region, with magnitudes less than −12 W m−2. Barlow et al. (2005) have remarked about the large values of (warm) descending air over southwest Asia during the positive MJO phase. According to Gill (1980), in an idealized case for equatorially symmetric heating, the expected descent for this region should be around ⅙ of the maximum ascend in the enhanced convection regions. By solving the hydrostatic thermodynamic energy equations, Barlow et al. (2005) have shown that both the advection of the MJO temperature anomalies by the mean wind and the advection of the mean thermal gradient by the MJO wind anomalies contribute to air subsidence over southwest Asia, justifying less precipitation and hot events for this region during the MJO positive phase. Our findings, however, indicate that in addition to thermal advection, moisture flux plays an influential role for describing the MJO influence.
Figures 8a–c illustrates the maps of the difference in daily precipitation rate and 850-hPa winds between the opposite phases of the MJO for November, January, and March, respectively. For improving resolution, monthly precipitation maps were created for the Middle East. As indicated, the direction of the near-surface wind is gradually changing from mostly southeasterly in November to southwesterly in March, which affects the rainfall pattern during these periods. The effect of wind direction on regional rainfall is also evident in Figs. 7a–c for which the areas with significant results generally follow the pathway of moisture transport. Consistent with the near-surface wind speed, the lowest and highest anomalies of precipitation rate over the study areas are, respectively, associated with November and January, indicating that the alternation in the MJO extreme phases modulates wind characteristics and precipitation rate in both seasonal and monthly scales. For instance, because the southerly or southwesterly wind prevailed in March, the precipitation difference has reached to more than 0.5 mm day−1 in most parts of southern Iran, indicating a substantial enhancement in the rainfall of this region during the negative MJO phase. An increase in the March precipitation rate (about 0.2–0.3 mm day−1) was also observed over the eastern coasts of the Arabian Peninsula, though it is not delineated in Fig. 8c.
2) Upper levels
The study areas mostly lie between two maxima in the subtropical westerlies: in the exit region of the North Africa–Arabian jet and near the entrance region of the East Asian jet (Barlow et al. 2005). The entrance or exit regions of the aloft jets are associated with moisture-bearing synoptic storms sometimes called “cyclone belts,” “storm tracks,” or baroclinic waveguides (Blackmon et al. 1977; Wallace et al. 1988). The strong horizontal gradients of wind speed at these regions are generally balanced by vertical air motion, which affects the amount and strength of storm activity (Blackmon et al. 1977). Furthermore, the net effect of the upper-level winds on the storm track appears to result from opposing contributions from baroclinic and barotropic energetics (Cai and Mak 1990; Whitaker and Dole 1995).
Barlow et al. (2005) have concluded that the occurrence of the MJO positive phase increases the aloft wind speed over most parts of southwest Asia during boreal wintertime. However, the speed anomalies generally occur in the exit region of the jet maximum and result in a decrease in the gradient of wind speed, producing a more diffuse jet exit region extending from southern Iran through northern India. On the other hand, reducing aloft wind speed during the negative MJO phase enhances the exit region and its associated baroclinic instability. They have reported that when the MJO rainfall anomalies are in the eastern Indian Ocean tropics, the Northern Hemisphere Rossby gyre extends over southwest Asia in terms of changes to the upper- and low-level winds and their associated moisture transport.
Figure 9a illustrates the maps of 200-hPa seasonal mean wind field for the negative MJO phase and the difference in 500-hPa omega for the opposite phases of the oscillation (omega values during negative phase minus corresponding values during positive phase). The exit region of the North Africa–Arabian jet, with its remarkable horizontal wind gradient, has extended over northern parts of the Saudi Arabia and southern Iran enhancing baroclinic instability, upward motion, and precipitation. The negative values of the difference in 500-hPa omega (around −0.01 Pa s−1) are widespread over most parts of the study areas, with a significant enhancement (around −0.03 Pa s−1) over southern parts of Iran, where the positive anomalies of seasonal precipitation has reached 0.5 mm day−1 (Fig. 5). Although the occurrence of the MJO positive phase has increased the 200-hPa wind speed over the study areas by about 5 m s−1 from the mean speed (not shown), the horizontal wind gradient at the jet exit was found to be around zero, suppressing upward motion and its associated precipitation.
Figures 9b–d is like Fig. 9a, but for November, January, and March, respectively. As indicated, the geographical position and wind velocity of the jets are different during various month of the study period, influencing both upward motion and storm tracks. In November (Fig. 9b), while the exit regions of the North Africa–Arabian jet is above Egypt, the entrance of the East Asian jet is placed over southeastern parts of Iran, indicating a significant westward shift of the upper-level jet compared to the seasonal mean.
Because of the lower wind speed at the lower and upper atmospheric levels, precipitation anomalies during November are less than corresponding values in January and March. In Fig. 9b, while the upward air motion in the southwestern parts of Saudi Arabia are linked to the North Africa–Arabian jet exit, similar motion over southern parts of Iran is mostly associated with the entrance region of the East Asian jet.
As indicated in Fig. 9c, compared to November, wind speed at January’s jet exit has increased by about 42% and the exit region has displaced eastward, toward Iran’s territory (maximum jet speeds during November and January are about 50 and 35 m s−1, respectively). Although, for most parts of the study areas, the difference in upward motion (omega values at the 500-hPa level) increased by about 2 times compared to November, the increase is particularly remarkable for southwestern parts of Iran and central-southern Saudi Arabia. Synchronized with this upward motion and the increased horizontal wind speed gradients at the 200-hPa level, the near-surface airflow augments a substantial amount of the nearby water bodies’ moisture (the Indian Ocean, Red Sea, and the Persian Gulf) to the study area preconditioning the atmosphere for precipitation enhancement (Figs. 7b and 8b).
Whereas the jet speed in January and March is mostly similar, the horizontal gradients of wind speed are sharper and shifted farther to the east during latter months (Figs. 9c,d). As indicated in Fig. 9d, the wind gradient is about 10 m s−1 for the areas between western parts of Saudi Arabia and southeastern districts of Iran. In response to this horizontal wind gradient, the difference in upward motion that is indicated by the negative anomalies of omega has also increased for all of the study areas with maximum values (−0.05 Pa s−1) above southeastern parts of Iran. As illustrated in Figs. 7c and 8c, consistent with air circulation at the near-surface, middle, and upper atmospheric levels, March precipitation has also significantly increased for this part of Iran, western coasts of the Arabian Peninsula, and major parts of Afghanistan.
The present study has made an effort to investigate the effects of the Madden–Julian oscillation (MJO) on November–April precipitation in both southern parts of Iran and the Arabian Peninsula for the period of 1979–2005. The atmospheric circulation associated with the MJO phenomenon was also analyzed to justify the obtained results. The positive and negative phases of the MJO were defined using daily records of two indices comprising the time series of WH (the RMM1 series in Wheeler and Hendon 2004) and MH (Maloney and Hartmann 1998; Maloney and Kiehl 2002). Of the 4887 days of the study periods, both indices were simultaneously positive (positive phase) or negative (negative phase) for 1807 or 1714 days, respectively. During the MJO positive phase, an enhanced convection center trailing a near-surface westerly wind anomaly was mostly extended over the south Indonesian and north Australian regions. Opposite to the positive phase, enhanced convection activity that leads a near-surface southerly wind anomaly centered over most parts of the study areas in Iran and the Arabian Peninsula as the negative MJO phase prevailed. This enhanced convection activity was, however, weaker (about ⅓) than the atmospheric perturbation over the Indonesian region during the MJO positive phase. In addition to being seasonal, the positive or negative phase of the MJO was also defined on a monthly scale.
The MJO–precipitation composites were then constructed for the adapted positive and negative phases using daily precipitation data for eight stations in southern Iran. A similar compositing procedure was also performed using the girdded values of OLR, omega, precipitation rate, and wind fields to analyze the effects of the MJO extreme phases on atmospheric circulation and moisture transport on the regional scale. The analysis of available daily precipitation data showed that the MJO negative phase enhances seasonal (November–April period) precipitation over southern Iran from about 10% to 70%. On the other hand, the prevalence of the MJO positive phase reduced seasonal precipitation from about 10% to 55%.
A methodology was proposed to analyze the effects of the MJO phases on precipitation variability using monthly data. According to the proposed definition, for each month of the study period the positive or negative phase of the MJO prevailed if daily series of both MH and WH were simultaneously positive or negative for at least 60% of the days of that month. Out of the 162 months of the study period, each of the positive or negative MJO phase prevailed for 34 months.
For almost entire parts of the study area, seasonal precipitation and the frequency of wet events were statistically increased during the MJO negative phase (and vice versa for the positive phase and dry events). The ratio of seasonal precipitation during the negative phase to the corresponding values during the positive phase was found to be about 1.75–2.75 and 2.75–4.00 for the western and eastern parts of southern Iran, respectively. For the considered stations within the Arabian Peninsula, the highest and lowest values of this ratio (5.70 and 1.10) were observed in Masirah and Medina located in Oman and Saudi Arabia, respectively. The precipitation response to the prevalence of the opposite MJO phases was found to be almost similar using either daily or monthly records.
During the negative MJO phase, enhanced near-surface southeasterly or southwesterly winds transfer substantial amounts of the Arabian Sea, Red Sea, and Persian Gulf moisture to the study areas in Iran and the Arabian Peninsula. At the same time, upward motion has improved at the midatmosphere, causing precipitation enhancement. For the upper atmosphere levels, suppression of the total wind field and enhancement in the horizontal wind gradient are the other characteristics of the negative MJO phase over the study regions. The geographical position of the North Africa–Arabian and East Asian jets shifts to the east from November to March, affecting the storm track and rainfall variability over the study area.
Southern Iran and the Arabian Peninsula are mostly engulfed by a dry northerly airflow during the positive MJO phase. Because of the suppression in moisture transport, the study area encounters an intense dry event during this phase. In addition to the suppressed moisture transport, the horizontal wind speed gradient is very low at the top levels of atmosphere that prevent upward air motion and baroclinic instability during the positive MJO phase.
This work was supported by Iran’s Water Resources Management Co. (Ministry of Energy) through Grant Code WRE1-84087. The authors are grateful to the two anonymous reviewers who reviewed the manuscript thoroughly and to Eric D. Maloney for providing MJO index data and informative remarks. A great thank you to Anthony D. Del Genio from the journal editorial board for his useful comments and advice.
Corresponding author address: Mohamad J. Nazemosadat, Water Engineering Department, College of Agriculture, Climate Research Center, Shiraz University, Shiraz 71441-65186, Iran. Email: firstname.lastname@example.org