Inter‐annual hydroclimatic variability in coastal Tanzania

Abstract

Climatic controls regulate the coupled natural and human systems in coastal Tanzania, where mangrove wetlands provide a wealth of ecosystem services to coastal communities. Previous research has explained the precipitation seasonality of eastern Africa in terms of the local monsoons. This research examines a wider range of hydroclimatic variables, including water vapour flux, evapotranspiration, runoff, and ocean salinity, and the sources of low‐frequency atmosphere–ocean variability that support mangrove productivity and associated ecosystem services. Results confirm previous work suggesting that the northeast monsoon (kaskazi) largely corresponds to the “short rains” of October–December and extends through February, while the southeast monsoon (kusi) corresponds to the “long rains” of March–May and the drier June–September. The Indian Ocean Dipole (IOD) and, to a lesser extent, El Niño–Southern Oscillation (ENSO) are important modulators not only of precipitation (as has been shown previously) but also of water vapour flux, evapotranspiration, runoff, and salinity variability. During kaskazi, positive (negative) hydroclimatic anomalies occur during positive (negative) IOD, with a stronger IOD influence occurring during its positive phase, when seasonal anomalies of precipitation, evapotranspiration, and runoff exceed +50, 25, and 100%, and nearby salinity decreases by 0.5 practical salinity units. During kusi, the contrast between the positive and negative IOD modes is subtler, and the pattern is dictated more by variability in “long rains” months than in the dry months. The coincidence of the positive IOD and El Niño amplify this hydroclimatic signal. Because previous work suggests the likelihood of increased tendency for positive IOD and increased moisture variability associated with El Niño events in the future, wetter conditions may accompany the kaskazi, with less change expected during the kusi. These results advance understanding of the key environmental drivers controlling mangrove productivity and wetland spatial distribution that provide ecosystem services essential to the well‐being of the human population.

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

“…Figure 5 shows that the ITCZ span a longitudinal between 15 to 60 E and a latitudinal extent of roughly 12 S-5 N. The interaction between southeasterly and northeasterly winds reveals remarkable features of the ITCZ consistent with previous studies (e.g., Borhara et al, 2020). The southeasterly winds are part of the southeast monsoon prevailing from May to October, consistent with the findings of Rohli et al (2019) and Kalisa et al (2019).…”

“…According to Williams and Funk (2011), a reduction in the rainfall amount is related to an extension of the tropical WC and the western edge of the tropical Indian Ocean warm pool. Rohli et al (2019) described a link between higher-than-normal rainfall during March-September (kusi) season and a positive Indian Ocean Dipole (IOD). They found a positive and a negative IOD to have limited influence on rainfall during kusi season compared to October-February (kaskazi) season.…”

Identifying global and local drivers of change in mangrove cover and the implications for management

“…While this implies that the climate is getting drier, future projections suggest wet seasons will become wetter and dry seasons drier (Maina et al., 2013). Changing rainfall patterns are likely to influence the distribution, extent, and growth rates of mangrove forests, particularly in mangroves at the edge of their tolerances (Rohli et al., 2019). For example, a decrease in rainfall and an increase in evaporation may lead to an increase in soil salinity, resulting in a decrease in seedling survival, productivity and growth rate (Duke et al., 1998).…”

Fine-Tuning Climate Resilience in Marine Socio-Ecological Systems: The Need for Accurate Space-Time Representativeness to Identify Relevant Consequences and Responses

“…Sea surface temperature ranges from 25 to 30.5 • C along the year, presenting significant decreases during intense rainfall (Nyandwi and Dubi, 2001). According to Rohli et al (2019), the rainfall regime corresponds closely with the monsoonal circulation.…”

“…Unguja Ukuu is located at 6 • S of latitude and 39 • E of longitude, in the southwest of Unguja Island, the biggest one of the Zanzibar archipelago, on the Tanzanian coasts in East Africa. Average annual air temperature in the region is about 27 • C, with minor seasonal variation (Rohli et al, 2019). In contrast, the wind shows clear seasonal shifts: the southeast monsoon (locally known as "kusi") usually occurs from April to October, while the northerly monsoon ("kaskazi") blows from November to March (Mahongo et al, 2011).…”