1. Introduction
Compared to the extensive exploration of the winter and summer monsoons over the Indian Ocean Basin (Ramage 1971; Hastenrath 1985; Pant and Kumar 1997), much less is known about the monsoon transitions, which are the rainy seasons in equatorial East Africa. Of these the boreal autumn “short rains” are of interest here. Attention has been given to the circulation mechanisms of climate anomalies (Hastenrath et al. 1993; Goddard and Graham 1999; Hastenrath and Polzin 2003, 2004, 2005), seasonal forecasting (Farmer 1988; Hutchinson 1992; Mutai et al. 1998; Mutai and Ward 2000; Philippon et al. 2002; Black et al. 2003; Clark et al. 2003; Hastenrath et al. 2004), processes in the upper ocean (Hastenrath et al. 1993; Anderson 1999; Webster et al. 1999; Saji et al. 1999; Baquero-Bernal et al. 2002; Lau and Nath 2004), and particularly three extreme flood events during the second half of the twentieth century. Thus, the disastrous floods of 1961 sparked interest, which was sustained over decades (Thompson and Mörth 1965; Lamb 1966; Morth 1967; Hastenrath 1984, 42–49; Reverdin et al. 1986; Flohn 1987; Kapala et al. 1994). Then came the torrential floods of 1994 (Behera et al. 1999) and 1997 (Birkett et al. 1999; Webster et al. 1999; Latif et al. 1999; Murtugudde et al. 2000). By comparison, little attention has been given to drought events in East Africa.
During January–February 2006 repeated radio reports were received of severe drought conditions in East Africa. This prompted the present endeavor to diagnose this extreme climatic anomaly in the context of the causative circulation mechanisms. Indeed, numerous Web sites pay attention to this extreme climatic event and its human impact (more information available online at http://iri.columbia.edu/climate/cid, http://www.em-dat.net, http://www.reliefweb.int/rw/dbc.nsf/doc100?OpenForm, http://news.bbc.co.uk/1/hi/in_depth/africa/2006/africa_food_crisis/default.stm, http://www.fews.net/centers/?f=ke, http://www.fews.net/special/index.aspx?f=al&pageID=specialDoc&g=1000929). Section 2 describes the data, section 3 summarizes the circulation background gained from a long-term reference period, section 4 examines the evidence for the boreal autumn of 2005 in this context, and a synthesis is offered in section 5.
2. Data
As in earlier related work (Hastenrath et al. 1993, 2004; Hastenrath and Polzin 2003, 2004, 2005), the data sources for the present study consist of rain gauge measurements, surface ship observations in the Indian Ocean from the Comprehesive Ocean–Atmosphere Data Set (COADS; Woodruff et al. 1987, 1993), and global surface and upper-air fields from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis (Kalnay et al. 1996; Kistler et al. 2001). The spatial resolution in COADS is 2.0 and in NCEP–NCAR is 2.5° latitude–longitude square. We are primarily interested in the surface wind, sea level pressure (SLP), sea surface temperature (SST), and 500-mb omega vertical motion.
In our earlier work (Hastenrath et al. 1993, 2004; Hastenrath and Polzin 2004, 2005) various index series for October–November were compiled from these data, as detailed in Fig. 1. UEQ (4°N–4°S, 60°–90°E) is an index of the zonal component of surface wind over the central-equatorial Indian Ocean. The pressure index PW and the SST index TW are for a domain in the west (8°N–8°S, 40°–50°E), the pressure index PE and the SST index TE are for a block in the east (8°N–8°S, 90°–100°E), and PWE = PW − PE and TWE = TW − TE represent the zonal pressure and SST gradients along the equator. Further SST indices are STW (10°N–5°S, 45°–55°E) and STE (5°–15°S, 90°–110°E). SIW (4°–12°S, 60°–90°E) is an index of the total wind speed in the downstream portion of the South Indian Ocean trade winds. Both W5ω (2.5°N–2.5°S, 30°–50°E) and E5ω (2.5°N–2.5°S, 100°–120°E) are indices of the 500-mb omega vertical motion. The October–November rainfall is captured by the index RON (Rain, October–November) for East Africa and SJB (Sumatra–Java–Borneo) for Indonesia. These index series were compiled as “all-station-average normalized departures” using the procedure first introduced by Hastenrath (1976). The index RON is based on seven stations at the coast of East Africa and the index SJB is based on eight stations in Indonesia.
Our earlier investigations into circulation mechanisms of climate anomalies could draw on the base period 1958–97 available for both NCEP–NCAR and COADS. By contrast, for 2005 the COADS compilations were not yet adequate, so that the NCEP–NCAR set was used for both the upper-air and surface fields; accordingly, the latitude domains of the indices PW, TW, PE, TE, UEQ, and SIW, are slightly different, as detailed in the caption to Fig. 1. However, the October–November 2005 analyses are compared to the 1958–97 reference period, both consistently based on the NCEP–NCAR dataset. The Southern Oscillation index (SOI), showing the phases of the difference of pressure anomalies Tahiti minus Darwin, was used as in earlier studies (Hastenrath et al. 1993; Hastenrath 2000; Hastenrath and Polzin 2003).
Similarly, consistency was ensured for rainfall, although information was insufficient to compile RON and SJB values for 2005. Thus, the 2005 precipitation was compared to the 1948–87 reference mean for two groups of four and of five stations in Kenya, as illustrated in Fig. 2 and listed in Table 1.
For an illustration of hydrological conditions of Lake Victoria, data on water levels and discharge were drawn from printed tabulations (United Nations Development Programme–World Meteorological Organization 1974), and a USDA Web site (see online at http://www.pecad.fas.usda.gov/cropexplorer/global_reservoir); further pertinent is an earlier report (Morth 1967).
3. Background
For comprehensive documentation of the annual cycle of circulation and climate in the equatorial Indian Ocean, reference is made to a series of atlases, a book, and journal articles (Hastenrath and Lamb 1979a, b; Hastenrath and Greischar 1989, 1991; Hastenrath 1995, 57–66, 186–197; Hastenrath 2000; Hastenrath et al. 2002; Hastenrath and Lamb 2004). A brief synopsis must suffice here, with focus on the October–November season of a zonal circulation cell along the Indian Ocean equator and the peak of the short rains at the coast of East Africa.
In the transition between the retreating boreal summer monsoon flow from the Southern Hemisphere and the evolution of the winter monsoon emanating from south Asia, intense surface westerlies sweep the equatorial zone of the Indian Ocean. The westerly surface winds drive the eastward equatorial jet, or Wyrtki jet, in the upper ocean (Wyrtki 1973; Hastenrath and Greischar 1991). These westerlies are the surface manifestation of an equatorial zonal circulation cell in the atmosphere, with ascending motion over Indonesia, divergent westward flow in the upper troposphere, and subsidence over the western Indian Ocean and East Africa. This feeds into the surface westerlies, which are forced by the strong zonal pressure gradient, with higher pressure in the west and lower pressure in the east. The equatorial zonal circulation cell is confined to the October–November core of the short rains at the coast of East Africa. Figures 1a and 3 schematically illustrate the equatorial zonal circulation cell, and Figs. 4a–d map the long-term mean surface and upper-air fields. Thus, Fig. 4a shows the high pressure in the west and low pressure in the east over Indonesia. Consistent with Fig. 4a, Fig. 4b exhibits the strong westerlies sweeping the equatorial zone, sandwiched between the downstream portion of the South Indian Ocean trade winds in the south and the incipient winter northeast monsoon winds to the north. Most remarkable in Fig. 4c is the ascending motion over Indonesia and the subsidence centered over the coast of East Africa, characteristics of the equatorial zonal circulation cell. Figure 4d portrays the warmer surface waters in the equatorial zone and particular to the east.
Turning to interannual variability, reference is made in particular to two publications on recent work (Hastenrath and Polzin 2004, 2005) also containing reviews of other papers on the subject. Hastenrath et al. (1993) documented the atmospheric forcing on the upper hydrosphere in the equatorial Indian Ocean, and later papers also dealt with the atmosphere–ocean coupling (Anderson 1999; Webster et al. 1999; Saji et al. 1999; Baquero-Bernal et al. 2002; Lau and Nath 2004), as indicated in the introduction. The atmosphere can force the ocean through wind stress. The ocean can affect the atmosphere through SST by hydrostatic forcing (boundary layer temperature and latent heat release in the free atmosphere) of the pressure field. In good development these couplings may cooperate, as borne out by the observed typical seasonal evolution of the complex of components in the coupled atmosphere–ocean system. Thus, observations show that pressure and wind anomalies precede those in SST (Hastenrath and Polzin 2005). The SST departures, once in place, may then feed back on the atmosphere as simulated by numerical modeling experiments (Latif et al. 1999; Goddard and Graham 1999), and then contribute constructively to other atmospheric circulation processes to produce the anomalies in rainfall (Hastenrath and Polzin 2005).
As requested by a reviewer, attention is called to some challenges in the literature. There is no dipole/seesaw between the west and east (Anderson 1999; Saji et al. 1999); the lag relations between the anomaly fields of pressure, wind, and SST merit appreciation (Anderson 1999; Saji et al. 1999; Latif et al. 1999; Goddard and Graham 1999); while we are dealing with a coupled system, explanation is invited on feedback sequences (Anderson 1999; Webster et al. 1999; Saji et al. 1999; Latif et al. 1999; Goddard and Graham 1999); and atmospheric general circulation models driven by prescribed SST (Latif et al. 1999; Goddard and Graham 1999) are intrinsically unsuited to diagnose the essential processes.
The findings on circulation mechanisms and forcings obtained in earlier work (Hastenrath et al. 1993, 2004; Hastenrath and Polzin 2004, 2005) are in compact form presented in Fig. 3 and Table 2. The pertinent circulation indices are detailed in the caption to Fig. 1. Thus, Table 2 lists (in its part A) correlations between indicative indices in October–November over the long-term reference period 1958–97. The equatorial westerlies UEQ are correlated at +0.79 with precipitation in Indonesia SJB and at −0.85 with rainfall at the coast of East Africa, arguably the strongest such correlation on the planet. Plausibly, the westerlies UEQ are correlated positively with pressure in the west PW, negatively with pressure in the east PE, and have an even stronger correlation with the zonal pressure gradient PWE (viz. +0.86), while the correlation is negative with the wind speed in the downstream portion of the South Indian Ocean trade winds SIW. Symptomatic for the equatorial zonal circulation cell, the surface westerlies UEQ are strong with enhanced subsidence in the west (W5ω) and ascending motion in the east (E5ω). Furthermore, with strong westerlies UEQ surface waters are anomalously cold in the west (TW, STW) and warm in the east (TE, STE). Correlations of these indices with rainfall in Indonesia and East Africa are consistent with the strong correlations of UEQ with SJB and RON. Complementing Table 2, Fig. 3 illustrates in compact form the very tight concurrent correlations between the various components of the system. Likewise very strong are the correlations with the upper-tropospheric divergent zonal wind component and the Wyrtki jet in the upper ocean, but these were not included in Fig. 3 because such information was not available in 2005 for comparison.
Given the very finely tuned machine illustrated by Fig. 3 for the October–November core of the East African short rains, the question arises about the seasonal evolution of relationships and chain of causality. This has been explored in two recent studies (Hastenrath and Polzin 2004, 2005). A brief synopsis must suffice here. The surface equatorial westerlies UEQ form the backbone of the system. Plausibly, UEQ is strong with a steep eastward pressure gradient along the equator; it is further favored by weak wind in the downstream portion of the South Indian Ocean trade winds SIW, because this allows a recurvature from southwesterly to southeasterly relatively far to the south of the equator, thus, producing a broad equatorial band in which westerlies can develop. The surface westerlies UEQ are a necessary condition for the development of the equatorial zonal circulation cell, which entails subsidence in the west and ascending motion in the east, conducive to contrasting rainfall anomalies. These are further favored by the kinematic and thermodynamic conditions, in that, for example, with lower-tropospheric divergence in the west subsidence and cold surface waters combine to reduce precipitable water, further conducive to deficient rainfall (Latif et al. 1999; Black et al. 2003; Hastenrath and Polzin 2005). There is no seesaw between the west and east in either pressure or temperature, and no indication of local forcing of temperature on pressure. Fast UEQ steepens the zonal temperature gradient, thus tightening the inverse relationships between the zonal gradients of pressure and temperature. A new frontier is defined in the question: what controls the pressure pattern over the Indian Ocean Basin?
The evidence conveyed in part A of Table 2 and in Fig. 3 draws on earlier evaluations of COADS, which is however not yet available in adequate form for 2005. Therefore, the 1958–97 mean maps in Figs. 4a–d were produced from the NCEP–NCAR reanalysis, so as to provide a consistent reference for appraising the circulation conditions during October–November 2005, which is the objective of section 4.
4. The 2005 drought
Radio reports of severe drought in East Africa are confirmed by rain gauge measurements in Kenya, as shown in Table 1 for two groups of stations identified in Fig. 2. At the Kenya coast as well as in the interior highlands the 2005 totals for both the October–November peak and the September–December rainy season as a whole stayed well below the long-term reference mean.
The circulation anomalies that led to these drought conditions in equatorial East Africa shall be examined here with reference to Fig. 4 and Tables 2 and 3. In particular, the maps of departures in Figs. 4e,f shall be considered in comparison to the corresponding long-term mean maps in Figs. 4a–d; and the listing of differences in part B of Table 2 shall be appreciated in the context of the long-term mean assessment in part A.
To begin with the large-scale pressure pattern, the map in Fig. 4e is dominated by strong positive departures in the west, entailing an enhanced eastward pressure gradient. Complementing the map in Fig. 4e, part B of Table 2 and Table 3 list the values corresponding to the domains of PW and PE, also marked in Figs. 4a,e, as well as PWE. Thus, the strong eastward pressure gradient PWE in 2005 results mainly from the enhanced pressure in the west PW; the positive departure in SOI is consistent with the weak negative pressure departure in the east PE, but has no functional bearing on the steepened PWE, and hence the anomalous UEQ and deficient rainfall in East Africa. It may be recalled (Hastenrath and Polzin 2003) that in the long-term mean UEQ has correlations of similar magnitude with PW and PE; just in 2005 PW contributed particularly strongly to the zonal pressure gradient PWE.
The map of wind departures in Fig. 4f most prominently features drastically accelerated westerlies over the central-equatorial Indian Ocean, consistent with the enhanced zonal pressure gradient evident from Fig. 4e and Table 2. The greatly increased zonal wind in the domain of UEQ is also listed in part B of Table 2. As apparent from Fig. 4f and part B of Table 2, the total speed in the downstream portion of the South Indian Ocean trade winds SIW was near the long-term mean, thus not unconducive to faster UEQ.
The map of midtropospheric vertical motion in Fig. 4g and part B of Table 2 show enhanced subsidence in the west and ascending motion in the east. Along with the evidence on UEQ in Fig. 4f and Table 2, this is further symptomatic of an enhanced equatorial zonal circulation cell.
The map of SST in Fig. 4h features warm departures in most of the equatorial and northern Indian Ocean. Noting from Fig. 4f and Table 2 that 2005 featured fast UEQ, it is interesting to compare in Table 2 the temperature anomalies of 2005 with the correlations over the period 1958–97. Although fast UEQ tends to come with cold anomalies in the west, such patterns did not materialize in 2005.
Complementing Table 2 (part B), Table 3 lists 2005 departures of circulation indices as dimensionless and absolute values, for both October–November (ON) and August–September (AS). Already known as typical from previous empirical investigations (Hastenrath and Polzin 2004, 2005), the departures of the pressure and wind fields evolved in AS as in ON. Thus, the pressure gradient PWE, subsidence in the west W5ω and ascending motion in the east E5ω, and the equatorial westerlies UEQ were already enhanced. Also typical for the development of fast UEQ, the speed in the downstream portion of the South Indian Ocean trade winds SIW was weak, and along with that the surface waters in the east, TE and STE, were anomalously warm from AS into ON. By contrast, not typical for fast UEQ and East African drought (Hastenrath and Polzin 2005), the surface waters in the west, TW and STW, were warm and further warmed anomalously from AS into ON. It may be recalled that warm waters in the west are deemed favorable for East African rainfall (Hastenrath et al. 1993; Latif et al. 1999; Goddard and Graham 1999; Hastenrath and Polzin 2003, 2005; Black et al. 2003; Clark et al. 2003). As apparent from Table 2 (part A), such association of warm departures in the west with fast UEQ is not common; the seasonal evolution of the anomalous pressure and wind fields (PWE, UEQ, W5ω) appears as the cause of the 2005 drought in equatorial East Africa, consistent with previous empirical experiences (Hastenrath and Polzin 2003, 2004, 2005).
In synthesis from this evidence, the anomalously high pressure in the west was crucial to the dynamics of the 2005 East African drought event. This caused fast westerlies over the central-equatorial Indian Ocean. The enhanced surface westerlies, in turn, were essential for the development of a vigorous equatorial zonal circulation cell, which featured enhanced subsidence over East Africa and the western Indian Ocean, unconducive to precipitation activity.
Complementing Fig. 4e, Fig. 5 presents a circumglobal map of the pressure differences ON 2005 minus the 1958–97 mean. It illustrates positive departures extending from the western Indian Ocean westward across the Atlantic and to the date line in the Pacific (except for negative values in the western part of the low-latitude North Atlantic). The largest positive departures are found over the eastern Pacific. Negative departures extend from the date line westward into the eastern Indian Ocean, with largest values somewhat to the west of the date line. Within this circumglobal context, the enhanced pressure at the western extremity of the equatorial Indian Ocean Basin was pivotal for the circulation and climate anomalies of the 2005 short rains.
The human impact of the drought may have been aggravated by unfavorable conditions during the preceding boreal spring “long rains” of 2005. Although an equatorial zonal circulation cell during this season is not found in the long-term mean (Hastenrath 2000; Hastenrath et al. 2002; Hastenrath and Lamb 2004), such development may not be precluded in individual extreme years. Indeed, during April–May 2005 as compared to the 1958–97 mean, departures were for the equatorial westerlies UEQ +0.3 m s−1, and for the omega vertical motion in the west W5ω and east E5ω +1.8 and −0.4 × 10−4 mb s−1, respectively. Consistent with the enhanced westerlies and subsidence in the west, precipitation activity in East Africa was reduced, as shown in Table 1.
The longer-term challenges in water resources may be further appreciated by an inspection of the annual cycles in the level of Lake Victoria (Figs. 6 and 2): in the long-term mean, water discharge and lake level are lowest around November–February and highest in May (Figs. 6a,b); by comparison to the seasonal variation proper, a dominant and sustained drop of lake level has taken place over the years 2004, 2005, and into 2006 (Figs. 6c,d,e). Human activity (i.e., water use) may have combined with the drought to lead to these extreme hydrological conditions.
The drought also poses a threat to the glaciers of equatorial East Africa. For Mount Kenya in particular we ascertained the progressive ice shrinkage since the end of the nineteenth century with a last airborne photogrammetric mapping in September 2004 (Hastenrath 2005, 2006; Rostom and Hastenrath 2007), and severe decay is also documented for Kilimanjaro (Hastenrath and Greischar 1997; Hastenrath 2006). The drought can harm the glaciers primarily through increased net shortwave radiation resulting from reduced cloudiness and surface albedo. A severe glacier decay can be expected.
While this paper continued in the editorial process, the observations of the rainy seasons of 2006 became available. Departures for April–May 2006 were for the equatorial westerlies UEQ −0.3 m s−1, and for the omega vertical motion in the west W5ω and east E5ω +0.9 and −0.3 × 10−4 mb s−1, respectively. Departures for ON 2006 were for the equatorial westerlies UEQ −3.3 m s−1, and for the omega vertical motion in the west W5ω and east E5ω −2.6 and +1.7 × 10 4 mb s−1, respectively. Along with this, precipitation was abundant, particularly in the boreal autumn short rains, as shown in Table 1. Consistent with Table 1, Fig. 6 also shows some recovery of Lake Victoria’s water level following the core of the short rains. Thus, the symptoms of a weak equatorial zonal circulation cell were consistent with the abundant rainfall and, in part, disastrous floods in East Africa.
The boreal autumn rainy season may bring severe drought and also disastrous floods to East Africa, and such extreme events have a strong human impact. Anomalies in the rainfall activity are remarkably closely related to the intensity of the boreal autumn zonal circulation cell over the equatorial Indian Ocean. Indeed the concurrent correlation of −0.85 between rainfall at the coast of East Africa and the equatorial westerlies (i.e., section 3) may be the tightest such correlation on the planet. Considering the severe human impact along with the close relation to circulation mechanisms, the “short rains” of East Africa may seem an ideal target for seasonal forecasting. Indeed, there has been no lack of endeavors; however, appealing results over limited periods or using cross validation were found to be painful illusions (Hastenrath et al. 2004). While there are some weak precursors, the causes for the scant predictability may be due to the circumstance that the equatorial zonal circulation cell over the Indian Ocean in boreal autumn is both strong and short lived, as is the Wyrtki jet in the upper ocean, which it drives.
5. Conclusions
Equatorial East Africa has two rainy seasons, in the transition between the basinwide winter and summer monsoons, when the near-equatorial low pressure trough is closest to the region. During these short time intervals, surface westerlies sweep the central-equatorial Indian Ocean. In boreal autumn these westerlies become part of a powerful zonal circulation cell along the Indian Ocean equator (Hastenrath 2000; Hastenrath et al. 2002; Hastenrath and Lamb 2004). As shown in earlier work (Hastenrath et al. 1993; Hastenrath and Polzin 2003, 2004, 2005; Hastenrath et al. 2004), the intensity of this circulation cell dominates the interannual variability of the short rains at the coast of East Africa. The extreme flood events during boreal autumn of 1961, 1994, and 1997 aroused recurrent interest, and all three resulted from a weakened zonal circulation cell during these years. Remarkably, no dry event of comparable intensity has been described.
In this context, recent alarming reports of severe drought in equatorial East Africa deserve appraisal. The deficient precipitation during the short rains of 2005 could be confirmed from rain gauge measurements in coastal as well as central Kenya. Thanks to the timely availability of the NCEP–NCAR reanalysis, it here became possible to promptly diagnose the circulation setting in which the drought occurred. The circulation evidence of October–November 2005 is generally consistent with earlier empirical findings. The controls are understood from the pressure and wind anomalies. Thus, during the 2005 short rains of East Africa, the precipitation activity was hampered by enhanced subsidence at the western extremity of the intensified equatorial zonal circulation cell, favored by the anomalously fast surface equatorial westerlies. The acceleration of the westerlies was due to a steepened eastward pressure gradient along the equator, resulting from anomalously high pressure in the west. The causes for enhanced pressure in the west merit further attention.
Acknowledgments
This study was supported by the Variability of Tropical Climate Fund of the University of Wisconsin Foundation. We appreciate the timely availability of the NCEP–NCAR reanalysis, which made this prompt appraisal of the recent circulation and climate anomaly possible. We thank the anonymous reviewers for constructive comments.
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(a) Zonal–vertical cross section and (b) orientation map detailing ON circulation indices. In the map, solid lines delineate the domains of the indices of surface zonal wind UEQ (4°N–4°S, 60°–90°E), pressure PW and SST TW (8°N–8°S, 40°–50°E), pressure PE and SST TE (8°N–8°S, 90°–100°E), dashed lines the domains of the indices of 500-mb omega vertical motion W5ω (2.5°N–2.5°S, 30°–50°E) and E5ω (2.5°N–2.5°S, 100°–120°E), and the total wind speed in the downstream portion of the South Indian Ocean trade winds SIW (4°–12°S, 60°–90°E); and dash–dotted lines the domains of the SST indices STW (10°N–5°S, 45°–55°E) and STE (6°–15°S, 90°–100°E). Dotted lines indicate the domains of the rainfall indices RON for the coast of East Africa and SJB for Indonesia (5°N–10°S, 100°–120°E), and the dotted line quadrangle in East Africa encloses the domains of the rain gauge stations shown in Fig. 2. In the NCEP–NCAR dataset used for 2005, the meridional domains for PW, TW, PE, TE, are 7.5°N–7.5°S. Of further interest are the indices for the zonal gradients of pressure PWE = PW − PE and temperature TWE = TW − TE, as well as the SOI (pressure difference Tahiti minus Darwin). Shading in the zonal—vertical section in (a) schematically illustrates the equatorial zonal circulation cell in ON.
Citation: Journal of Climate 20, 18; 10.1175/JCLI4238.1
Orientation map of rain gauge stations in Kenya, with the domain indicated by the dotted lines in Fig. 1. Stations available for 1948–87 and 2005 are, at the coast Lamu (LM), Malindi (MD), Mombasa (MB), and Voi (VO); and in the interior highlands Nanyuki (NK), Nyeri (NY), Meru (ME), Embu (EM), and Machakos (MK).
Citation: Journal of Climate 20, 18; 10.1175/JCLI4238.1
Diagram of correlations between the circulation indices in Fig. 1, for ON 1958–97. Correlation coefficients are in hundredths and significant at the 1% level. Shading schematically illustrates the equatorial zonal circulation cell in ON.
Citation: Journal of Climate 20, 18; 10.1175/JCLI4238.1
Maps of ON circulation (from NCEP–NCAR). (a), (b), (c), (d) The 1958–97 mean patterns and (e), (f), (g), (h) the departures of 2005 from the 1958–97 mean. Maps in (a) and (e) depict SLP with isobar spacing of 2 and 0.2 mb, respectively; maps in (b) and (f) depict the wind field with isotach spacing of 2 and 1 m s−1, respectively; maps in (c) and (g) depict the 500-mb omega vertical motion with isoline spacing of 2 × 10−4 mb s−1; and maps in (d) and (h) depict SST with isotherm spacing of 2° and 0.5°C, respectively.
Citation: Journal of Climate 20, 18; 10.1175/JCLI4238.1
Circumglobal map of the difference in SLP in ON 2005 minus the 1958–97 mean with isobar spacing of 0.5 mb.
Citation: Journal of Climate 20, 18; 10.1175/JCLI4238.1
Lake Victoria’s annual cycle: (a) water discharge at Jinja, 1948–70 mean in 108 m3; (b) water level from satellite, 1993–2002 mean, in m; (c) same as in (b), but for 2004; (d) same as in (b), but for 2005; (e) same as in (b), but for 2006.
Citation: Journal of Climate 20, 18; 10.1175/JCLI4238.1
Rainfall totals (mm) at station groups shown in Fig. 2, in March–May (MAM), ON, and September–December (SOND), during 2005 and 2006 as compared to the long-term reference mean 1948–87.
Circulation characteristics of ON captured by the indices identified in Fig. 1. Part A (from COADS) presents coefficients of correlation over the period 1958–97, in hundredths. Part B (from NCEP–NCAR) lists the differences of the 2005 values from the 1958–97 mean (in units of mb for PW, PE, PWE, SOI; m s−1 for UEQ and SIW; 10−4 mb s−1 for W5ω and E5ω; and °C for TW, TE, TWE, STW, STE).
Circulation indices identified in Fig. 1 for AS and ON of 2005. To the left the dimensionless values are listed in terms of standard deviation (1958–97 reference period). To the right they are given in units of mb for PW, PE, PWE, SOI; m s−1 for UEQ and SIW; 10−4 mb s−1 for W5ω and E5ω; and °C for TW, TE, TWE, STW, STE.
