1SEPTEMBER 2001 MCHUGH AND ROGERS 3631 North Atlantic Oscillation In¯uence on Precipitation Variability around the Southeast African Convergence Zone MAURICE J. MCHUGH Department of Geography, University of WisconsinÐOshkosh, Oshkosh, Wisconsin JEFFREY C. ROGERS Department of Geography, The Ohio State University, Columbus, Ohio (Manuscript received 19 April 2000, in ®nal form 23 October 2000) ABSTRACT The relationship between the North Atlantic oscillation (NAO) and austral summer (December±February) rainfall variability over southeastern Africa is described. Thirty-one stations in 08±168S and 258±408E have statistically signi®cant correlations to the NAO index over varying periods of record starting since 1895 and form a regional normalized rainfall index of southeast African rainfall (SEAR) correlated to the NAO index (NAOI) at r 520.48 over 1894/95±1989/90, although the relationship is r 520.70 since 1958. The spectrum of the SEAR index has signi®cant amplitude at 7.6 yr, a periodicity commonly associated with the NAO, and the NAOI/SEAR cospectrum has its largest power at this periodicity. NCEP±NCAR reanalysis data, extending from 1958/59 to 1995/96 are used to evaluate moisture and circulation ®eld variations associated with both NAO and SEAR indices. Precipitable water over southeastern Africa varies signi®cantly such that anomalously high (low) convective rainfall occurs over southeast Africa when the NAO is weak (strong). Unusually wet summers are associated with anomalous equatorial westerly ¯ow originating in the subtropical Atlantic and traversing the continent. Relatively dry summers are associated with increased southeasterly monsoon ¯ow originating over the subtropical Indian Ocean. The NAO linkage to southeastern African rainfall is especially pronounced in 300-hPa zonal winds where ®ve highly signi®cant elongated bands of alternating zonal wind anomalies extend from the Atlantic Arctic to equatorial Africa. The latter 300-hPa equatorial band exhibits westerly (easterly) ¯ow during wet (dry) austral summers and undergoes regional divergence (convergence) over southeastern Africa. The westerly ¯ow, along with orographic uplift, has an element of instability due to the vertical component of the Coriolis parameter that assists rain production during wet summer. Potential interactions between the NAO and ENSO in producing regional latitudinal ITCZ shifts are discussed. 1. Introduction (northward) when the NAO westerlies are unusually strong (weak). This paper shows the characteristics, and causes, of African precipitation is characterized by an opposi- an austral summer (Dec±Feb) association between the tion in rainfall anomalies over eastern Africa south of North Atlantic oscillation (NAO) and precipitation var- the equator (Nicholson 1986; Ropelewski and Halpert iability along and north of the southeastern Africa con- 1987) in both annual data and especially the austral vergence zone. Meehl and van Loon (1979) ®rst dem- summer wet season (Janowiak 1988). Janowiak iden- onstrated that such a relationship may exist. They ti®ed an area (DJFMn) between the equator and 108S showed that the air temperature seesaw between Green- where rainfall departures tended to be opposite those of land and northern Europe was linked to latitudinal shifts DJFMs, lying between 158 and 308S, suggesting that the in January rainfall around the southeast Africa inter- out-of-phase relation implied a shift in the ITCZ. Jan- tropical convergence zone (ITCZ). Rainfall was higher owiak's DJFMn region partly corresponds to our study south (north) of 158S latitude during the ``Greenland area, an area where regionally coherent rainfall anom- Below'' (``Greenland Above'') Januaries than during alies can be linked to the NAO. Januaries dominated by the opposite seesaw phase. This Other interactions occur between African precipita- implies that the southeast African ITCZ shifts southward tion and large-scale teleconnections. Lamb and Peppler (1987) demonstrated a strong NAO link to interannual precipitation variability over Morocco, driven by the Corresponding author address: Dr. Maurice J. McHugh, Depart- ment of Geography, University of WisconsinÐOshkosh, 800 Algoma southward displacement of the Atlantic storm track and Blvd., Oshkosh, WI 54901-8642. precipitation-bearing storms when the Atlantic wester- E-mail: [email protected] lies are weak. El NinÄo±Southern Oscillation (ENSO) Unauthenticated | Downloaded 09/24/21 05:23 AM UTC 3632 JOURNAL OF CLIMATE VOLUME 14 Fig. 2; Janowiak 1988, his Fig. 2a). Many of the stations near the ITCZ (;108±158S) receive over 50% of their annual rainfall in summer while 25%±50% occurs in summer at stations extending farther north near 58S (McHugh 1999). 2. Data and methodology We use the National Centers for Environmental Pre- diction±National Center for Atmospheric Research (NCEP±NCAR) global reanalysis of atmospheric ®elds (Kalnay et al. 1996), which employ a global data as- similation system on a database that includes a wide variety of sources and instruments covering various spa- tial and temporal scales. Global climatological sets such as the Comprehensive Ocean±Atmosphere Data Set have been incorporated into the data assimilation system FIG. 1. The countries of equatorial and southern Africa. in addition to radiosonde, satellite, aircraft, merchant shipping, and meteorological station observations. Con- has been linked to unusually wet periods in equatorial sistent usage of the global data assimilation system in eastern Africa (Davies et al. 1985; Ogallo 1988) during the reanalysis results in a climate record that avoids any months June±August, and October±December. The area major analysis discontinuities due to incorporation of around Lake Victoria has an ENSO rainfall link in Oc- new data sources. The data have been quality controlled tober±December (Ropelewski and Halpert 1987; Nich- using a variety of spatial and temporal analyses. Re- olson 1996). Ropelewski and Halpert noted that the analysis of the combined datasets has been performed Lake Victoria rainfall anomaly is often opposite that of using an operational spectral forecast model with T62 southernmost Africa, such that an equatorward shift in spectral resolution, 28 vertical levels, and a grid size of the convergence zone occurs during ENSO (warm) ep- approximately 210 km (Kalnay et al. 1996). isodes. Variables used here include u- and y-component In South Africa, aspects of the tendency for summers winds and sea level pressures. These are among the most to be dry (wet) during equatorial Paci®c warm (cold) reliable (class A) variables, little in¯uenced by the mod- events are explored by van Heerden et al. (1988), Jury el although statistical interpolation of observations may et al. (1994), Jury (1996), and Kruger (1999). Rope- be employed in creating the gridded ®elds. Data were lewski and Halpert (1996) similarly ®nd a statistically evaluated in this study at all mandatory pressure levels signi®cant ENSO/rainfall signal in southeastern Africa, (McHugh 1999), but only 1000-, 850-, and 300-hPa south of Lake Malawi in Mozambique, Africa (see Fig. results are presented here, being representative of tro- 1) southward of 148S and extending across eastern South pospheric geopotential height and u- and y-wind com- Africa. This area corresponds closely to Janowiak's ponent ®elds. Streamlines are calculated at each level, using the u- and -component geostrophic wind ®elds. (1988) DJFMs region. y The analysis area used here is concentrated on 08± Atmospheric moisture variables used here (speci®c hu- 168S and 258±408E, covering several African countries, midity and precipitable water) are determined partially identi®able in Fig. 1, including the Democratic Republic by the model and partly by observations of that variable of Congo (D.R.C.; formerly Zaire), Rwanda, Burundi, (class-B ®elds). Precipitable water is calculated as ver- Uganda, Kenya, Tanzania, Zambia, Malawi, and Moz- tically integrated speci®c humidity from 1000 to 300 ambique. Per the preceding discussion, the region to hPa. 168S is potentially in¯uenced by ENSO along the equa- The North Atlantic oscillation index (NAOI) is cal- tor around Lake Victoria and eastern Kenya during De- culated since 1895 from normalized values of seasonal cember, and possibly over latitudes 148±168S, where mean sea level pressure values at Ponta Delgada, Ropelewski and Halpert (1987, 1996) suggest an ENSO Azores, and Akureyri, Iceland (Rogers 1984). Positive impact during austral summer months. For the most part (negative) NAOI values indicate that pressure is si- however, there is little evidence of an ENSO impact in multaneously higher (lower) than normal over the our study area; it is a part of Africa where rainfall link- Azores and lower (higher) than normal over Iceland, ages to large-scale circulation disturbances are not well representing an increase (decrease) in the North Atlantic known (Nicholson 1996). The region between 108 and Ocean pressure gradient and zonal wind speed. The 168S is especially uni®ed by having a broad rainfall Azores and Iceland pressure difference will be used here maximum during the December±January±February to measure the NAO's phase and strength, rather than (DJF) summer period (Meehl and van Loon 1979, their the air temperature seesaw (Meehl and van Loon 1979), Unauthenticated | Downloaded 09/24/21 05:23 AM UTC 1SEPTEMBER 2001 MCHUGH AND ROGERS 3633 which is merely a response to the NAO pressure gradient This often diminishes the signi®cance of zonally and extremes.