The dominant modes of the long rains interannual variability over Tanzania and their oceanic drivers

Abstract

Mitigating the impacts of droughts and floods over East Africa (EA) may largely be possible through accurate predictions of the long rains (March–May). Though the close interannual variability between the long rains, Indian and Pacific Oceans has been widely studied, Tanzania's long rain variability remains unclear. This paper examines the Tanzanian long rains dominant modes through an empirical orthogonal function (EOF) analysis, and their relationship with winter sea surface temperature (SST) for the period 1951–2018. Correlation and composite analyses were applied to explore the associated mechanisms. The first EOF (EOF1) mode is characterized by negative anomalies over Tanzania. The second EOF (EOF2) mode depicts positive (negative) anomalies over northern (southern) Tanzania. The third EOF (EOF3) mode reveals positive (negative) anomalies over eastern (western) Tanzania. EOF1 is closely related to the SST anomalies (SSTAs) in the southeastern Indian Ocean (SEIO). When the SSTAs are positive, they subsequently linked to upper (lower) level convergence (southeasterly winds) over Tanzania. Associated with this convergence entails a sinking motion, leading to a negative rainfall anomaly. The southeasterly winds indicate an early onset of the southeast monsoon, leading to normal rainfall been recorded. EOF2 is linked to the SSTAs in the central Pacific (CP). Positive SSTAs are related to reversed Walker circulation (WC) over northern Tanzania, while the upper‐level convergence was observed around southern Tanzania. This meridional dipole vertical motion pattern enhances (suppresses) rainfall anomalies. EOF3 is associated with mega El Niño–Southern Oscillation (ENSO)‐like SSTAs. The SSTAs are related to eastward shift of WC over the Indian Ocean, and zonal cell over the western Indian Ocean with rising limb is located east of Tanzania. The ascending motion enhances rainfall over eastern Tanzania. Predictions based on the SEIO, CP, and mega ENSO index may significantly support long rains prediction efforts and provide early warning.

Modulating role of the Interdecadal Pacific Oscillation on the relationship between interannual variation of the long rains over Tanzania and south‐central tropical Indian Ocean sea surface temperature

“…The delineation in quantifying the sliding correlation vertically integrated moisture flux and moisture divergence (VIMFC) are consistent with the findings of Kebacho and Chen (2022). is used to explore how correlation between two variables varies in time or space.…”

“…The CRU and GPCC data cover a longer period and were assessed based on their performance in Tanzania (Koutsouris et al, 2016). The datasets were utilized by Kebacho and Chen (2022) to examine the TLR variability. These long‐term gridded rainfall datasets from 1950 to 2010 are more suitable for examining decadal variability in this study.…”

“…Camberlin et al (2010) posited that changes in the intensity of Southeast Asian monsoon (SAM) have a significant influence on both the cessation date and rainfall during the long rainy season in northern Tanzania. Kebacho and Chen (2022) claimed that SSTAs across the central Pacific Ocean and southeastern Indian Ocean are crucial to the TLR's variation. Moreover, TLR variability was strongly affected by the mega ENSO‐like SSTA.…”

Coupled Model Intercomparison Project Phase 6 simulations of the spatial structure of rainfall variability over East Africa: Evaluation and projection

“…The monopole structure suggests that during the positive (negative) phase, the whole region experiences above (below) normal rainfall associated with floods (droughts) conditions. Such orientation has been dominantly linked to the Pacific and Indian Ocean sea surface temperature (SST) anomalies (Saji et al ., 1999; Bowden and Semazzi, 2007; Smith and Semazzi, 2014; Kebacho and Chen, 2022). The EOF2 exhibits a dipole‐like pattern characterized by positive loading to the north and negative loading to the south of the study region (Figure 5b,g).…”

“…This orientation indicates increasing rainfall over northern parts and declining amounts over the southern parts of the region, also known as trend mode (Schreck and Semazzi, 2004; Bowden and Semazzi, 2007). The un‐detrended EOF2 mode is attributed to the general global warming, especially over the tropical South Atlantic (Schreck and Semazzi, 2004; Bowden et al ., 2005), whereas the corresponding interannual variability is linked to the SST anomalies over the central Pacific (Kebacho and Chen, 2022). In contrast to the meridional opposite rainfall pattern observed in EOF2, EOF3 is oriented as a zonal dipole pattern between the eastern and western parts of the region, with a positive loading to the east and a negative loading to the west (Figure 5c,h).…”