The composites of OND standardized rainfall anomalies corresponding to positive IOD and negative IOD for the two chosen periods are depicted in Figure 5. The positive IOD composites during 1976-2010 (Figure 5(a)) show above-normal all over the region, with largest significant values in the eastern part. Meanwhile, the northern part of the country around Lake Victoria basin was observed to have above normal nonsignificant amounts. The results were opposite during 2011-2022, whereby the northern part was observed to have largest above-normal significant

values compared to the eastern part, which shows below-normal nonsignificant values (Figure 5(b)).

On the other hand, negative IOD composites during 1976-2010 show below-normal significant values in most parts of the country, with the largest values being on the northern part, while the central and southwestern parts show nonsignificant values (Figure 5(c)). During 2011-2022, northeastern and southwestern parts show above-normal values, while the rest of the country shows below-normal values, with significant values over the central and southern (largest values) parts (Figure 5(d)).

The most important point to note here is that for positive IOD events, some areas such as the northern (western) part, which received less(much) amount of rainfall during 1976-2010, received much(less) rainfall during 2011-2022 (Figure 5(a) & Figure 5(b)). For negative IOD events, the northeastern and southwestern parts turned from below average during 1976-2010 (Figure 5(c)) to above average during 2011-2022 (Figure 5(d)). This implies that there is a changing relationship between OND IOD and OND rainfall across different places over the study domain.

In order to confirm the validity of rainfall distribution over the study domain, as shown on Figure 5, we determined percentage departure for composites of OND rainfall during the two periods (Figure 6). The two figures (Figure 5 & Figure 6) show the same rainfall patterns. Positive IOD composites during 1976-2010 show above-normal all over the region, with largest significant values in the eastern part. The northern part of the country around Lake Victoria basin was observed to have above normal nonsignificant percentage departure values. The results were a bit different during 2011-2022, whereby the northern part was observed to have above-normal significant percentage departure values compared to the eastern part, which shows below-normal percentage departure nonsignificant values (Figure 6(b)).

Furthermore, negative IOD composites during 1976-2010 show below-normal significant percentage departure values in most parts of the country (Figure 6(c)). During 2011-2022, northeastern and southwestern parts show above-normal percentage departure values, while the rest of the country shows nonsignificant below-normal values, with an exception of the central part which show significant values (Figure 6(d) ).

More interesting, for positive IOD composites, some of the areas such as the northern part, which had nonsignificant positive percentage departure rainfall during 1976-2010 (Figure 6(a)), observed to have significant positive percentage departure values during 2011-2022 (Figure 6(b)). For negative IOD events, the northeastern and southwestern parts which was observed to be below-normal percentage departure during 1976-2010 (Figure 6(c)) turned to above-normal percentage departure during 2011-2022 (Figure 6(d)), the same as features observed in rainfall anomalies (Figure 5(a) & Figure 5(b)).

Figure 7 displays the composite of detrended OND sea surface temperature anomalies (SSTA) corresponding to positive IOD and negative IOD years for the two chosen periods. During positive IOD, the SSTA composite shows a wider warm pool over the western Indian Ocean during 2011-2022 (Figure 7(b)) compared to that of 1976-2010 which was cooler on the southwestern part (Figure 7(a)). For

negative IOD composites, the central Indian Ocean was observed to be cooler during 2011-2022 (Figure 7(d)) compared to 1976-2010 (Figure 7(c)). Shifting of warmer cooler patterns was also noticed, for example, the northwestern part which was warmer during 1976-2010 (Figure 7(c)) was observed to be cooler during 2011-2022 (Figure 7(d)). The same phenomenon was observed over the northeastern part. Furthermore, the southwestern part of Indian Ocean which was cooler during 1976-2010 turned to warmer during 2011-2022 (Figure 7(c) & Figure 7(d)). Due to temperature differences, positive IOD is accompanied by easterly winds while negative IOD is accompanied by westerly wind at the surface.

The findings shows that both negative and positive IOD events strengthened, with the cautionary note that some areas turned from cooler to warmer while others from warmer to cooler during the two periods.

1) Velocity Potential and Divergent Wind at 850 hPa

In order to understand the influencing factor mechanisms behind the observed rainfall patterns, positive and negative IOD composite anomalies of velocity potential along with divergent wind vectors at the 850 hPa pressure level were analyzed during the two periods (Figure 8). During 1976-2010, positive IOD composite anomalies show convergence over the central Pacific, America and western part of Indian Ocean including Tanzania while divergence was observed to cover the western Pacific (Southern Asia and Australia) and western Africa (Figure 8(a)). Both convergence and divergence were found to be shifting eastward, indicating the eastward shift of the Walker circulation but with a weaker strength during 2011-2022, with an exception over the Atlantic Ocean, where divergence was enhanced (Figure 8(b)).

Moreover, negative IOD events shows divergence over central and eastern Pacific as well as western Indian Ocean including Africa and Tanzania, while convergence was observed over the western Pacific (Figure 8(c)). Contrary to what was observed during positive IOD events, during 2011-2022, both convergence and divergence were enhanced and noticed to shift westwards, indicating the westward shift of the Walker circulation (Figure 8(d)). A northward shift of systems was also evident, indicating the northward shift of the Hadley circulation (Figure 8).

2) Velocity Potential and Divergent Wind at 200 hPa

In order to understand the mechanisms behind the observed rainfall patterns,

positive and negative IOD composite anomalies of velocity potential along with divergent wind vectors at the 200 hPa pressure level were also analyzed during the two periods (Figure 9).

During 1976-2010, positive IOD composite anomalies shows divergence over the central Pacific, America, as well as over western Indian Ocean, including Tanzania while convergence was observed to cover the western Pacific (including Southern Asia and Australia) and western Africa (Figure 9(a)). Both convergence and divergence were found to be shifting westwards, indicating the westward shift of the Walker circulation but with a weaker strength during 2011-2022, with an exception over the Indian Ocean and Atlantic Ocean, where the convergence were enhanced (Figure 9(b)). The convergence and divergence systems also exhibited a bit northward shift, especially over Indian Ocean, indicating a northward propagation of the Hadley circulation (Figure 9(b)).

On the other hand, during 1976-2010, negative IOD events shows convergence over central Pacific and western Indian Ocean, including Africa and Tanzania, while divergence was observed over the western Pacific (including Southern Asia and Australia) (Figure 9(c)). Contrary to what was experienced during positive IOD events, during 2011-2022, both convergence and divergence were enhanced and noticed to shift eastward, indicating the eastward shift of the Walker circulation (Figure 9(d)). A southward shift of systems was also noticeable,

indicating the southward shift of the Hadley circulation (Figure 9).

Generally, the study proposes that the changing relationship between IOD and OND rainfall might be attributed by the east-west (north-south) shifting of Walker (Hadley) circulation, which, on the other hand, due to teleconnection, changed the upper-level velocity potential and divergent wind systems, thus impacting the rainfall distribution over the region (Figure 5). The study observed that the lower and upper-level velocity potential & divergent wind systems were weakened (enhanced) during positive (negative) IOD on 2011-2022 as compared to 1976-2010 (Figure 8 & Figure 9). The same atmospheric circulations were evident on Figure 8 whereby the IOD structure favors westerly winds during positive IOD while negative IOD is accompanied by easterly winds. Past researchers have associated the position of Walker circulation with HREs over the northern part of Tanzania, in such a way that when its ascending limb is around 60˚E, it suppresses HREs (Japheth et al., 2021) .

Additionally, the study analyzed the Walker and Hadley circulations. To account for the contribution of the Pacific Ocean wind patterns to the OND rainfall variability over Tanzania, we compare the east-west Walker circulation patterns over the Indo-Pacific region (Figure 10).

During positive IOD composites, the western Pacific was generally dominated by a strong (weak) ascending limb during 1976-2010 (2011-2022) (Figure 10(a) & Figure 10(b)), which appears to enhance the amount of rainfall over Tanzania (Figure 5(a) & Figure 5(b)). However, during 1976-2010, Tanzania was characterized by descending limb (Figure 10(a)) compared to 2011-2022, whereby ascending motion dominated the area (Figure 10(b)). Contrary to what was observed during positive IOD events, the western Pacific was dominated by a descending limb during negative IOD events (Figure 10(c) & Figure 10(d)). During the time, Tanzania was ruled by a descending (ascending) limb during 1976-2010 (2011-2022) (Figure 10(c) & Figure 10(d)), suppressing (enhancing) the amount of rainfall specifically in the southwestern and northeastern parts of our study domain (Figure 5(c) & Figure 5(d)).

It has been documented that the ascending limb of the Walker circulation over the study domain enhances convection, with the strongest uplift accompanied by convective activity of the moisture flux from the Congo airmass and that from the tropical Indian Ocean leading to anomalous rainfall over Tanzania (Limbu & Tan, 2019) .

As depicted in Figure 11(a) & Figure 11(b), the study found that during positive IOD events, the Hadley circulation generally show a strong (weak) ascending motion over the study domain during 1976-2010 (2011-2022). This was also evident in the velocity potential and divergent wind, which explained about Hadley circulation shift towards the north and its role as a moisture supplier

over the study domain (Figure 9(a) & Figure 9(b)). This impact resulted in an enhanced amount of rainfall during 1976-2010 compared to 2011-2022 (Figure 5(a) & Figure 5(b)).

During negative IOD events, the Hadley circulation showed that the study domain was generally dominated by descending motion during 1976-2010 (Figure 11(c)), while the findings were opposite during 2011-2022, whereby ascending motion was evident (Figure 11(d)). This caused the study domain to experience anomalous rainfall, especially in the southwestern and northeastern parts, where above-normal rainfall was observed, leaving the rest of the country below normal (Figure 5(d)).

Past studies have documented that Hadley circulation over the Indian landmass is recognized as a meridional overturning circulation in which hot air rises at the equatorial region and moves poleward, where it cools, sinks and moves back to the equator (Sharma et al., 2023) . This coupled air-sea feedback over

the Indian Ocean is said to modify the meridional overturing during ENSO and IOD phases hence affect the global rainfall distribution, including Indian monsoon summer rainfall (Wu & Kirtman, 2004) .