Inter-annual hydroclimatic variability in coastal Tanzania

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 Nino-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 Nino amplify this hydroclimatic signal. Because previous work suggests the likelihood of increased tendency for positive IOD and increased moisture variability associated with El Nino 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.