Interannual monsoon wind variability as a key driver of East African small pelagic fisheries

Small pelagic fisheries provide food security, livelihood support and economic stability for East African coastal communities—a region of least developed countries. Using remotely- sensed and field observations together with modelling, we address the biophysical drivers of this important resource. We show that annual variations of fisheries yield parallel those of chlorophyll-a (an index of phytoplankton biomass). While enhanced phytoplankton biomass during the Northeast monsoon is triggered by wind-driven upwelling, during the Southeast monsoon, it is driven by two current induced mechanisms: coastal “dynamic uplift” upwelling; and westward advection of nutrients. This biological response to the Southeast monsoon is greater than that to the Northeast monsoon. For years unaffected by strong El-Niño/La-Niña events, the Southeast monsoon wind strength over the south tropical Indian Ocean is the main driver of year-to-year variability. This has important implications for the predictability of fisheries yield, its response to climate change, policy and resource management.

The region chosen for the spatiotemporal analysis of satellite Chl-a presented in Fig. 2 is as follow: -Setting the eastern boundary of the black box at 40.15˚E in order include the entire islands East-coast production areas.
-Including the shallow areas between the islands and the mainland coastline as suspended materials are low due to the abundance of coral reefs 59,60,61 .
-Excluding an area around the Rufiji River outflow due to the significant amount of suspended material 62 . The Rufiji outflow area is excluded using a mask limited by the lines of latitude between the coast and Mafia Island, north and south of the river mouth (See white mask on Fig. 2a). Testing the sensitivity by varying the mask size, had little effect on the averaged Chla timeseries (small change in the magnitude of Chl-a timeseries but no difference in the overall behaviour).    T2   T3   T1   T2   T3   T1   T2   T3   T1   T2   T3   T1   T2   T3   T3   T2   T1  T1   T2   T3   T1   T2   T3   T1   T2  Aug04 T2   T3   T1   T2   T3   T1   T2   T3   T1   T2   T3   T1   T2   T3   T3   T2   T1  T1   T2   T3   T1   T2   T3   T1   T2

Strong dynamic uplift upwelling along Tanzanian and Kenyan coasts in Aug04
A similar to Sep02, but less pronounced situation is observed in Aug04 relative to the 19-year climatological mean (1997-2015) (Fig. S4, a, d, h and k). The satellite data reveals a coastal band of elevated Chl-a (up to 0.75 mg/m 3 ) from Mafia Island (8.5°S) to Kenya (1°S), contrasting with the waters further East with low Chl-a (down to 0.3 mg/m 3 ) (Fig. S4 d and k). These Chl-a concentrations intensified in Aug04 by about 1.5 times their climatological mean (Fig. S4 d and k). The observed SST have cooled down to 25.1°C as compared to ~ 25.5°C in a normal August ( Fig.  S4a and h). In the model, Aug04 Chl-a is less well resolved than in Sep02 (cf. Fig. 4e) with maximum values not exceeding 0.45 mg/m 3 (Fig. S4e). But in the model Aug04 shows high Chl-a concentrations along the coasts of Tanzania and Kenya, in agreement with remote sensing. The modelled SST also shows anomalously low values, decreasing from 25.2°C in normal conditions to below 24.7°C for Aug04, over a similar spatial extent as in the satellite observations ( Fig. S4b and i).
There is a clear pattern of high Chl-a concentrations associated with cool waters along the coastline during Aug04, as the case of Sep02 indicating an intensified upwelling or enhanced vertical mixing signal. The fact that the MLD in Aug04 across three vertical crossections along the elevated Chl-a coastal band between Mafia and Kenya (T1 to T3, see Fig. S4e for exact positions) is shallower than the climatology (Fig. S4f), suggests that the interannual Chl-a intensifications are not caused by enhanced mixing but likely by upwelling.
The upwelling subsurface signature can be assessed in the modelled temperature, Chl-a, Dissolved Inorganic Nitrogen (DIN) of sections T1 to T3. In a climatological August, the sections T1 to T3 indicate cool SST, high Chl-a and elevated nutrients near the coast (Fig. S4j, m and n), typical of an upwelling regime. The 25°C isotherm rises around 39.5°E from ~125m to 40-50 m on sections T1-T3. This uplifts the 25°C isotherms on sections T1 to T3 in the climatological August. These cold temperatures are accompanied by high Chl-a concentrations of 0.4-0.5 mg/m 3 in the upper 80 m. The elevated Chl-a is consistent with the doming of DIN isopleths on sections T1 to T3 near the coasts. During Aug04, the same situation occurs with more intensity (Fig. S4c, f and g). The cold waters with elevated nutrients are closer to the surface than in the climatological year, leading to higher Chl-a concentrations (Fig. S4f and m). The elevated Chl-a over 80 m depth is around 0.45 mg/m 3 in Aug04. The 25°C isotherm in Aug04 is uplifted to 20-30 m on sections T1 and T3 and outcrops completely on section T2 (Fig. S4c). Elevated Chl-a concentrations of around 0.5 mg/m3 are visible over the upper 80 m in Aug04 (Fig. S4f). The prominent Chl-a signal near the coasts coincides with a more accentuated doming of the 2 mmol/m 3 DIN isopleth for Aug04 than the climatology (Fig. S4g).  Fig. S4 (d). Note that these locations are as close as possible (but not exactly) to the model sections T1 and T2. The 2 mmol/m 3 isopleth is highlighted with a thick black line on panel (c).

Quality of simulated upwelling signature (temperature, Chl-a, DIN) at Tanzanian waters
The modelled 2004 subsurface pattern is coherent with in situ temperature, fluorescence and Nitrate & Nitrite sections sampled from South of Zanzibar to Pemba. The presented in-situ cross-sections (C1 and C2) locations are on the same latitude as model sections (T1 and T2) but with a shorter longitudinal extent (see Fig. S4d). The observed temperatures on Fig. S5a show cooler waters of ~25.2°C reaching the surface on sections C1-C2 along the coasts. The thermocline is pushed upwards towards the surface. High levels of fluorescence (a proxy for Chl-a) ranging between 1 and 1.5 mg/m 3 are seen on Fig. S5b in the upper 40 m, which signify high phytoplankton biomass along the coastal band from South of Zanzibar to Pemba. This is coherent with the downward sloping of the 2 mg/l Nitrate & Nitrite isopleth (Fig. S5c), which is located at the ~80 m layer during an inter-monsoon period 80 , but is now observed at a depth of 50 m, which is within the photic zone.  Figure S4. Maps on panels (a), (b) T1   T2   T3   T1   T2   T3   T1   T2   T3   T1   T2   T1   T2   T3   T3   T2   T1  T1   T2   T3   T1   T2   T3   T1   T2 T1   T2   T3   T1   T2   T3   T1   T2   T3   T1   T2   T1   T2   T3   T3   T2   T1  T1   T2   T3   T1   T2   T3   T1   T2 Figure  S9. Maps on all panels were created by the authors using MATLAB software vR2013a (see https://uk.mathworks.com/videos/r2013a-release-highlights-75269.html and https://uk.mathworks.com/products/matlab.html).

Differences of the ocean conditions between the high and low catch years
To further highlight how different are the biophysical ocean conditions during the Southeast monsoon of the high and low catch years, we assess two composites of modelled (Fig. S12) and remotely sensed (Fig. S13) SST, Chl-a and surface current speeds constructed as high minus low catch years following Jury et al., 30