Extreme rainfall in the Namib Desert during late summer 2006 and influences of regional ocean variability

INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 28: 1061 1070 (2008) Published online 7 August 2007 in Wiley InterScience (www.interscience.wiley.com).1603 Extreme rainfall in the Namib Desert during late summer 2006 and influences of regional ocean variability A. Muller, C. J. C. Reason* and N. Fauchereau Department of Oceanography, University of Cape Town, Private Bag X3, Rondebosch, 7701, South Africa ABSTRACT: The desert country of Namibia (southwestern Africa) and adjoining areas received anomalously heavy rains in austral summer 2006. In particular, the hyper-arid southern Namib Desert experienced an extreme rainfall event in mid- April 2006 which led to the coastal station of Luderitz receiving 102 mm, or about six times its annual average rainfall. Based on extreme value theory, this event appears to have a return period of 145 years the rainfall received at Luderitz during 16 23 April 2006 was about twice that of the annual rainfall recorded at this station in the previous wettest year (1956). It is argued that this event occurred as a result of the unusual interaction of an anomalously westward-located tropicalextratropical cloudband, and an anomalously northward-located cut-off low. In turn, the location and behaviour of these synoptic features may have been aided by regional ocean conditions, a strong warm event in the tropical southeast Atlantic and a positive dipole-like sea surface temperature event in the south Indian Ocean, together with La Niña conditions. It is argued that the combination of these regional ocean patterns with the La Niña event led to the very wet conditions over Namibia and neighbouring countries during austral summer 2005 2006. Copyright 2007 Royal Meteorological Society KEY WORDS climate variability; Namibia; oceans Received 29 January 2007; Revised 20 June 2007; Accepted 23 June 2007 1. Introduction The western coast of southern Africa is characterized by a cool upwelling current system (the Benguela), and one of the driest deserts in the world, the Namib, that extends north from about 30 S to near 15 S. A combination of cool coastal sea surface temperatures (SSTs) and sub-tropical subsidence leads to a strongly stable lower atmosphere that acts to suppress convection. Annual precipitation along this coast ranges from 50 to 100 mm in the far south (e.g. Port Nolloth at 29.3 S on the South African west coast has an annual average of 64 mm) to 5 18 mm along the central coast of Namibia, and less than 50 mm along the southern Angolan coast. Rainfall events are relatively few and tend to be of low intensity. South of about 25 S, the meagre rainfall occurs mainly in winter due to cold fronts penetrating anomalously far north or the edge of a cut-off low, whereas north of this latitude, the even sparser rainfall tends to be in late summer when it occurs, and as a result of an easterly trough displaced anomalously far west. This very sparsely inhabited region is known as the Skeleton Coast by virtue of its aridity and often treacherous sea conditions that historically resulted in many shipwrecks with little hope of rescue. * Correspondence to: C. J. C. Reason, Department of Oceanography, University of Cape Town, Private Bag X3, Rondebosch, 7701, South Africa. E-mail: cjr@egs.uct.ac.za Austral summer 2005 2006 was anomalously wet over large areas of southern Africa but particularly in parts of the Namib Desert and also the Kalahari Desert located over eastern Namibia, Botswana and northwestern South Africa. For example, Luderitz located on the Namibian coast near 26.6 S has an annual average rainfall of 16.7 mm but received over 100 mm in 1 week in late summer 2005 2006. Although the coefficient of variation of Namibian rainfall is the greatest in southern Africa, which, in itself is a highly variable and mainly semiarid region, the question arises as to how anomalous in terms of rainfall the April 2006 event actually was. For example, although it is known that the long-term rainfall average of desert stations like Luderitz is made up of relatively few rain events, if an event of 100 mm is not highly anomalous, then that would imply very long periods (years to decades) of virtually no rainfall in order that the long-term annual average would still only be 16.7 mm. Several circulation and sea surface temperature features, on both seasonal and weather scales, suggest that the April 2006 event was unusual in many respects. In addition to the moderate La Niña, two regional SST patterns existed in summer 2005 2006 that are known to be often associated with above-average summer rainfall over southern Africa. First, a large-scale warming occurred in November 2005 in the tropical southeast Atlantic and persisted past June 2006. The strongest SST anomalies existed off the Angolan and Namibian coasts. Warm events of this type have been observed to be often Copyright 2007 Royal Meteorological Society

1062 A. MULLER ET AL associated with anomalously wet conditions over western Angola and the northern Namibian coast (Hirst and Hastenrath, 1983; Rouault et al., 2003) although not to date over southern coastal Namibia (e.g. Luderitz). Second, the South Indian Ocean displayed warm SST anomalies in the southwest, and cool anomalies west of Australia during summer 2005 2006, a dipole-like pattern often linked with above-average rainfall over southern Africa (Behera and Yamagata, 2001; Reason, 2001, 2002; Washington and Preston, 2006). Thus, regional SST patterns may have combined with the moderate La Niña to produce favourable conditions for above-average rainfall over the sub-continent during this summer (note that El Niño southern oscillation (ENSO) rainfall impacts over the region are typically positive (negative) during La Niña (El Niño) Lindesay, 1988; Allan et al., 1996; Reason et al., 2000). Large-scale circulation anomalies during this summer were also favourable for wet conditions with an enhanced ascending branch of the regional convective cell located over southern Africa, low-level anti-cyclonic anomalies over the southwest Indian Ocean and low-level cyclonic anomalies over the western interior of southern Africa and neighbouring sub-tropical south Atlantic Ocean as well as south of Africa. The combination of cyclonic anomalies over Namibia/Angola, and enhanced troughing south of Africa, is conducive to the development of tropical-extratropical cloudbands, the synoptic system that brings most of southern Africa s summer rainfall (Tyson and Preston-Whyte, 2000). However, in the April 2006 event, it appears that there were also anomalous cyclonic conditions off the coast of Namibia and extending well west into the central south Atlantic Ocean. These conditions then led to the development of a cut-off lowlike feature, which combined with a pre-existing cloudband to produce the extreme rainfall over the Luderitz region. The purpose of this study is, therefore, to provide evidence that this event was rather unusual, and to examine the associated large-scale circulation and SST patterns that may have contributed to it. 2. Data Daily rainfall data for Luderitz (Diaz Point station) were obtained from the Namibian Meteorological Service for the period after 1940, as were monthly rainfall data extending over the last century. Although some daily data exists for the decades prior to 1940, there are significant gaps in the early records, and so for the purposes of comparing the 2005 2006 extreme rainfall event with previous heavy rain events, and for computing the statistical return period, only the post-1940 record is used since it does not have any missing data. The 2.5 horizontal resolution NCEP NCAR re-analyses (Kalnay et al., 1996), available for 1948 2006, were used to assess hemispheric and regional circulation anomalies, while the 1 resolution Reynolds Optimally Interpolated SST data (Reynolds and Smith, 1994) were used to assess regional SST patterns during the summer. Moisture fluxes and convergence were calculated by integrating the product of the daily averaged re-analysis specific humidity and the wind from the surface to the 850 hpa level for 16 22 April 2006. 3. The April 2006 event 3.1. Rainfall Figure 1 shows the rainfall recorded at Luderitz during 16 22 April 2006 together with the long-term average annual rainfall at this station (the April monthly average is 2 mm), as well as annual rainfall for the previous six wettest years over the last century. It is clear that not only did just over 6 times as much rain fall in this week in April 2006 (101.8 mm) as the long-term annual average (16.7 mm), but that this weekly amount corresponds to almost twice as much rain as has ever fallen in the previous wettest year of the last century. These rainfall statistics, therefore, suggest that the April 2006 event was highly unusual. As discussed below, three days out of 16 22 April 2006 each received well above the annual average rainfall in a single day with one day recording 39.4 mm. Examination of daily rainfall data for Luderitz back to 1940 Rainfall (mm) 100 90 80 70 60 50 40 30 20 10 0 1917 1953 1956 1963 1965 1987 2006Apr Ave Figure 1. The far right hand column shows the annual average precipitation for Luderitz located in the southern Namib coastal desert. Next to this column is the rainfall recorded during the extreme event of 16 22 April 2006. To the left of this April 2006 column are the annual rainfall amounts recorded during the six wettest years between 1900 and 2005.

EXTREME RAINFALL IN THE NAMIB DESERT REGIONAL OCEAN VARIABILITY 1063 indicates that the previous daily maximum rainfall was 31.0 mm which occurred in March 1956, and that this event was also the only other occurrence besides April 2006, of two days of consecutive rainfall each receiving more than the annual average (16.7 mm). Although two consecutive days of rainfall at Luderitz has happened on several occasions, there is only one other occurrence, besides April 2006, of three days of consecutive rainfall in the post-1940 record (in August 1963). During March 1917, there was a period of four consecutive rain days, but one must treat the pre-1940 data with caution since there are significant gaps in the data. Thus, the 5 days of consecutive rainfall in April 2006 (and in fact seven rain days out of eight during 16 23 April 2006 totalling 102.0 mm) has no precedent in the daily record. Since 1940, 10 years have recorded zero or trace rainfall (i.e. 0.1 1 mm) during a given January December period, with 20 months being the longest period without any rainfall at all. Given these results, and the interest in rainfall amounts, we apply the extreme value theory to annual maxima of cumulative five-day rainfall computed from the daily Luderitz rainfall data post-1940. The Generalized Extreme Value distribution (Coles, 2001) is fitted to the post-1940 time series of rainfall maxima to determine the return period of a 5-days cumulative rainfall of 100 mm as was recorded in 2006. The return period is defined as the average time interval (expressed in years) between occurrences of another rainfall event at Luderitz of five days of cumulative rainfall of 100 mm, and is a statistical measure of how often such an event is likely to happen. Thus, a 10-year return period for this event is one that is expected to only occur every 10 years. Application of the extreme value theory determined the return period of this rainfall event at Luderitz as 145 years, confirming the unusualness of the April 2006 event. It may also be noted that, in terms of the six next wettest years shown in Figure 1, 1917 was the onset year ofalaniña, 1956 was a La Niña mature phase year, 1963 corresponds to a Benguela Niño as well as the onset year of an El Niño, 1965 was an El Niño onset year, 1987 was an El Niño mature phase year, and that 1953 was neutral with respect to ENSO. Thus, it appears that extremely wet years in the southern Namib Desert can occur during all phases of ENSO. Daily rainfall data from the Namibian Meteorological Service indicates that the 16th, 19th, 20th and 21st of April 2006 were the main rainy days at Luderitz with the 16th alone receiving 39.4 mm or more than double the long-term annual average. Almost 20 mm fell on the 19th, 27.4 mm on the 20th, (second wettest day) with the 21st receiving 11.2 mm. April 22 experienced 2.6 mm and the 23rd was the last rainy day (0.2 mm). Peak rainfall intensities occurred on the mid-afternoon of the 16th and around 11 pm on the 19th (about 15 mm/hr in each case). Examination of daily Global Precipitation Climatology Project (GPCP) rainfall data during the pentad indicates that most of Namibia received some rainfall on April 16. On the next day, the rain was confined to the northwest and north central areas of the country, whereas a band of rain stretched from far northwestern Namibia southeastwards across the country and into central South Africa on April 18. Southern and western Namibia received rainfall on the 19th while during the last two days of the period only the southwestern part of the country seems to have experienced precipitation. Thus, it appears that the southern Namibian Desert (which includes Luderitz) received some rainfall on all days during the week except the 18th. Heavy falls occurred on April 16, 19, 20 and 21. 3.2. Circulation In order for such an arid area to receive so much rainfall in one event, the atmospheric circulation patterns are likely to be rather anomalous and this was indeed the case during April 16 22. It appears that the heavy rainfall resulted from the interaction between a truncated tropicalextratropical cloudband or a tropical-temperate trough (TTT), itself located anomalously far west, and a cutoff low, itself located anomalously far north. TTTs are the dominant summer rainfall producing synoptic system over sub-tropical southern Africa (Harrison, 1984; Todd and Washington, 1999) and, during years with average or above average rainfall, tend to extend southeast from a heat low located over southeastern Angola (the Angola low) to a mid-latitude disturbance located southeast of South Africa. Occasionally they are located farther west, but it is rare for them to be located over the Benguela upwelling system as was the case during the event (Figure 2). On average about 1 2 cut-off lows occur over subtropical southern Africa and neighbouring ocean areas (20 40 S, 10 40 E) each April. However, they are less frequently found in the northwestern quadrant of this region (southern Namibia and northwestern South Africa) than in the other three quadrants (Singleton and Reason, 2007). In addition, as is shown below, the particular system that occurred during April 16 22 seemed to Figure 2. EUMETSAT satellite image for 16 April 2006 with Namibia outlined in black.

1064 A. MULLER ET AL originate anomalously far north over the sub-tropical south Atlantic Ocean further pointing to the unusual nature of the synoptic situation during the period. Figure 3 shows the daily sequence of 500 hpa geopotential height anomalies for the 13 21 April 2006. On the 13th, a deep trough extended from the southern tropics of the southeast Atlantic Ocean to as far south as the Antarctic coast, with an anti-cyclonic anomaly to its west. This anomaly ridged farther east to almost the South African west coast on the 14th leading to a strong cyclonic anomaly being isolated to the west of Namibia. At upper levels (200 and 50 hpa not shown), a deep trough extended from this region all the way to the Antarctic. This upper-level trough together with the input of high-potential vorticity air into the upper troposphere is typical of cut-off low formation. Examination of 200 hpa height vorticity fields indicated a region of strong cyclonic vorticity over the southeast Atlantic. Over the next few days, the mid- and lower-level ridge extended farther east over South Africa with the cut-off low moving close to the Namibian coast on the very wet day (16th). Convection over the heat-low region in Angola linked up with the mid-latitude cyclone located southwest of South Africa on that day to form a truncated TTT just to the east of the cut-off low (Figure 2). The latter was present over and west of the Namibian coast for the next two days before retreating on the 19th and then approaching again on the 20th. On the 21st, the system linked up with a strong cyclonic anomaly in the Southern Ocean to form a deep trough again extending to Antarctica. Examination of the anomalies in the 500 hpa omega field (not shown) indicated very strong uplift along the Namibian coast on all days during the period except the 17th. However, Figure 4, the daily 850 hpa moisture flux and convergence sequence during the period provides some insight into why the southern Namib Desert received so much rain on the 16th, 19th, 20th and 21st, but little on the other two days. The interaction between the offshore cut-off low and the tropical easterly flow into the low-latitude part of the truncated TTT to its east is apparent on the 16th. Low-level moisture emanating from tropical southern Africa reached anomalously far to the southwest (i.e. the southern Namib) and encountered the unstable conditions associated with the cut-off low. Low-level moisture convergence is evident along Figure 3. Daily anomalies in 500 hpa geopotential height for 13 21 April 2006. Contour interval is 50 m.

EXTREME RAINFALL IN THE NAMIB DESERT REGIONAL OCEAN VARIABILITY 1065 Figure 4. Daily anomalies in 850 hpa moisture flux for 16 21 April 2006. Grey areas indicate areas of convergence contoured at 1 10 8 kg kg 1 s 1 with a scale moisture flux vector shown as part of the magnitude bar at the bottom of each panel. the Namibian coast and just offshore, as well as over the central interior of the landmass. On the 17th, the moisture flux anomalies were strong southeasterly, and therefore, offshore of the west coast with the main convergence area well away from Luderitz so that only light rain occurred in this region. Strong northerly flux anomalies and associated moisture convergence over the southern Namib and much of the west coast were again in evidence for 19 21 April consistent with Luderitz receiving heavy rain again on those three days. 4. Seasonal anomalies during austral summer 2005 2006 Figure 5 indicates that large areas of southern Africa, particularly Namibia, received above average rainfall during each month of January to April 2006. In April, the wet conditions were mainly confined to western Namibia, interior South Africa and northern Mozambique. The period was characterized by the mature phase of a La Niña event, often associated with above average summer

1066 A. MULLER ET AL rainfall over southern Africa (Lindesay, 1988; Reason et al., 2000) via an enhanced local Walker-type circulation over the southern African landmass/south Indian Ocean and anomalous uplift over the sub-continent. In addition, two regional SST patterns present in summer 2005 2006 also likely contributed to the above average rainfall (Figure 6). The first of these is a strong warm anomaly in the Angola Benguela Frontal Zone (ABFZ) area off the southern Angolan coast. Warm SST events in this area known as Benguela Niños, if they are connected to sub-surface anomalies in the equatorial Atlantic via the thermocline (Shannon et al., 1986; Florenchie et al., 2003, 2004), have been shown to lead to above average rainfall over western Angola Figure 5. Regional monthly rainfall anomalies for January April 2006 from the International Research Institute for Climatology and Society (IRI). Contour interval is 50 mm. This figure is available in colour online at www.interscience.wiley.com/ijoc Figure 6. Regional SST anomalies during JFM 2006. Contour interval is 0.3 C.

EXTREME RAINFALL IN THE NAMIB DESERT REGIONAL OCEAN VARIABILITY 1067 and Namibia (Hirst and Hastenrath, 1983) and sometimes inland (Rouault et al., 2003). Warm SST anomalies in this region lead to increased local instability and enhanced low-level moisture inflow from the tropical southeast Atlantic towards the Angola heat-low, the source region for the cloudbands. This SST anomaly was in existence from December 2005 but was particularly strong in April 2006, the month of the extreme rainfall event at Luderitz. Thus, an extended period of warm SST off the northern Namibian/southern Angolan coast and associated lowlevel moisture flux into the Angola low existed before the April 16 22 event. Second, a positive phase sub-tropical south Indian Ocean SST pattern existed throughout this summer which was also particularly strong in April 2006. Both observations (Behera and Yamagata, 2001) and experiments with atmospheric general circulation models (Reason, 2001, 2002; Washington and Preston, 2006) and a regional climate model (Hansingo and Reason, 2006) have shown that when this dipole is oriented with warm anomalies in the southwest of the basin and cool anomalies in the southeast, above average summer rainfall tends to occur over much of sub-tropical southern Africa. The atmospheric model experiments suggest that the increased rainfall results from enhanced transport of moist marine air from the sub-tropical southwestern Indian Ocean over southern Africa, and increased low-level convergence and uplift over the adjacent landmass. Thus, at least three SST patterns were in existence during the period that likely contributed to the wet summer conditions. Although model experiments would be required to determine the relative contributions of the La Niña, south Indian Ocean SST dipole and the Benguela Niño-like event in the tropical southeast Atlantic, it is likely that their coincidence during this summer and the strengthening of the regional SST patterns in April played an important role in the development of the April 2006 event. Examination of the regional monthly 850 hpa height (Figure 7) and 500 hpa omega (Figure 8) anomalies provides some indications as to the distribution and intensity of the monthly rainfall anomalies (Figure 5). The height anomalies show a stronger south Indian Ocean anti-cyclone, advecting more moist, warm marine air over southern Africa for each month, and particularly, January and February. The omega anomalies show strongly ascending conditions over the preferred TTT location from the Angola low source southeast to frontal disturbances tracking south of Africa. This pattern was particularly prevalent in January and February consistent with the large and widespread positive rainfall anomalies. January was also characterized by a strong cyclonic anomaly over the western part of the domain, and hence, a much stronger Angola low. In March, these favourable conditions weakened slightly with evidence of some anticyclonic ridging from the southeast Atlantic Ocean (and hence, advection of cooler, drier air over western South Africa) although there was still widespread uplift. The fields for April appear to be dominated by the cut-off low and TTT interaction that occurred over western Namibia in the middle weeks of this month. The upper level velocity potential anomalies (Figure 9) reflect the strongly ascending conditions in the troposphere over southern Africa during summer 2005 2006. Consistent with the rainfall, geopotential and omega fields, these anomalies are strong in January and February, weaken slightly in March and then become prominent over the Namibian region in April. In summary, the regional circulation patterns were favourable for above-average rainfall over southern Africa during summer. The shift of these favourable conditions towards Namibia in April helps explain why an environment conducive for the April 16 22 extreme Figure 7. Monthly 850 hpa geopotential height anomalies during JFMA 2006. Contour interval is 5 m.

1068 A. MULLER ET AL Figure 8. Monthly 500 hpa omega anomalies during JFMA 2006. Negative (positive) values imply relative ascent (subsidence). Contour interval is 0.01 Pa s 1. Figure 9. Monthly anomalies in upper level velocity potential during JFMA 2006. Contour interval is 5 10 5 m 2 s 1. rainfall event existed over western Namibia. Some suggestions as to the contribution of the 2005 2006 La Niña to these patterns relative to the regional SST patterns may be obtained by subtracting the strong La Niña composite fields formed from the ten strong events since 1948. This calculation shows that stronger uplift occurred over southern Africa, and particularly the western region, in 2006 than for the La Niña average event, suggesting that the unusually wet 2006 summer was unlikely to have been caused by La Niña acting alone, and instead it was

EXTREME RAINFALL IN THE NAMIB DESERT REGIONAL OCEAN VARIABILITY 1069 also strongly contributed to by the regional SST anomalies. Further evidence for the suggestion that all three SST patterns contributed to the anomalously wet 2006 season follows from the fact that wetter conditions over Namibia during subtropical south Indian Ocean SST dipole events only seem to happen when there are also positive SST anomalies in the tropical southeast Atlantic (e.g. 1974), whereas, drier conditions occur there during those dipole events when there are also cool events in the tropical southeast Atlantic (e.g. 1981). If one then looks at the seasons with strong warm events in the tropical southeast Atlantic, one finds that the rainfall anomalies extend much farther inland over Namibia and Angola during those events which also have a positive SST dipole in the south Indian Ocean (e.g. 1986, 2001, 2006). As a result, it seems reasonable to suggest that the 2006 anomalously wet conditions over Namibia were most likely due to a combination of La Niña, the positive SST dipole in the south Indian Ocean and the strong warming in the tropical southeast Atlantic. 5. Summary Austral summer 2005 2006 was unusually wet over large parts of southern Africa, and particularly over Namibia. The most extreme rainfall occurred in the southern Namib Desert, one of the driest regions in the world, with the town of Luderitz recording 101.8 mm during the week of April 16 22. By comparison, the long-term annual average rainfall for this station is only 16.7 mm, and the previous wettest year recorded 59.4 mm. It is suggested that the wet conditions over southern Africa resulted from the combination of a La Niña event, strong warming in the tropical southeast Atlantic, and a positive phase sub-tropical SST dipole in the south Indian Ocean. Each of these climate events has been associated with above average summer rainfall over parts of southern Africa in the past and although it is not uncommon for two of them to occur simultaneously, it is extremely rare for all three to co-exist at the same time. In fact, since 1940, the only other case seems to be 1999 2000. In this case, the eastern half of Namibia as well as much of sub-tropical southern Africa received well above average rainfall but not the southern Namib Desert as in 2005 2006. In 2005 2006, the warm SST anomalies in the tropical southeast Atlantic were larger and more extensive than in 1999 2000 and the warm anomaly in the southwest Indian Ocean was larger and shifted further into the tropics suggesting a larger rainfall response downstream over southern Africa (Reason, 2002; Washington and Preston, 2006). Thus, the regional SST and circulation patterns present during summer 2005 2006 were favourable for widespread good rains, and particularly over Namibia. The southern Namib Desert is not a region that is typically strongly impacted on by any of these events and it appears that it was the very unusual interaction of an anomalously equator-ward located cut-off low with an anomalously westward-located TTT over the central Benguela upwelling system that led to the extreme rainfall over this region in April 2006. Although we have not specifically considered the role that these climate events may have had in promoting the specific synoptic conditions during April 2006 over the southern Namibian region, the following remarks may be made. During La Niña,TTTsdotendtooccurpreferentially across southern Africa from southern Angola towards southeastern South Africa, rather than farther to the east as in neutral or El Niño years, but not as far west as in April 2006. Thus, although the large-scale circulation and SST conditions were favourable for a TTT formation, the location of the April 2006 case was anomalous. Likewise, although there is some suggestion of increased cut-off low occurrences in the South African region during La Niña, they typically occur much farther south than the April 2006 case (Singleton and Reason, 2007). It therefore seems that the April 2006 event over the southern Namib Desert resulted from both unusual synopticscale interactions as well as the co-existence of the three climate events previously mentioned. This unusual combination of synoptic and inter-annual regional-tohemispheric scale factors led to the extreme rainfall and strongly suggests that this event is a highly anomalous case. Consistent with that suggestion, the return period of another rainfall event of 100 mm at Luderitz was determined using the extreme value theory as being 145 years. Acknowledgements This study emanates from the BSc (Hons) thesis of AM. 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