Forgotten accounts of tropical cyclones making landfall in Tanzania

Landfalling tropical cyclones are essentially unheard of in Tanzania. It has been suggested that Cyclone Kenneth, which occurred in 2019, was the first tropical cyclone to make landfall in Tanzania. This paper brings to light tropical cyclones that made landfall in Tanzania in 1872 and 1952. We draw on eyewitness accounts and meteorological observations to raise the profile of these seemingly forgotten tropical cyclones, which devastated lives and economic activities. The severe societal impacts, and potential link to climate change, of landfalling tropical cyclones in Tanzania encourages further investigation into the drivers of tropical cyclones in the southwest Indian Ocean.


Introduction
Tropical cyclones, also known as hurricanes in the Atlantic and typhoons in the Pacific, typically form over the ocean where there is a sufficiently strong Coriolis force and when the sea surface temperature is at least 26.5°C (Dare and Mcbride, 2011). Several regions of the global oceans meet these criteria at certain times of year, allowing cyclone formation to take place. The timing of the cyclone season varies among the different oceanic regions, with November to April being the tropical cyclone season for the southwest Indian Ocean.
Close to the equator, the Coriolis force is weak, meaning the rotation of winds needed for tropical cyclone formation does not occur. Far from the equator, cooler sea surface temperatures inhibit the formation and maintenance of tropical cyclones. As a result, tropical cyclones tend to form between 5° and 15° from the equator (Anthes, 2016). During the tropical cyclone season in the southwest Indian Ocean, tropical cyclones rarely make landfall north of the Mozambique border (Mavume et al., 2013). The Kenyan coast does not experience tropical cyclones directly and even further south, along the Tanzanian coast, landfalling tropical cyclones are unheard of in recent times. A record from 1872, however, provides the first known instance of a tropical cyclone making landfall along the Tanzanian coast, which crossed over Zanzibar and passed over Bagamoyo city in April 1872, devastating the two towns ( Figure 1).
It was recently suggested in an article in The Conversation that Cyclone Kenneth in 2019 was the first cyclone to make landfall in Tanzania (Fitchett, 2019). Whilst this is true of recent history (notably, since the start of the meteorological satellite era in the 1970s), Tanzania has felt the devastating impacts of tropical cyclones before. In 1872 and 1952 the country was hit by devastating cyclones that made landfall in the far north and south respectively (Figure 1). Meteorological records of these events are limited, but there are eyewitness accounts which provide interesting and important information on tropical cyclones along the coast of Tanzania.  April 1872 and1952, respectively. The approximate landfall position of cyclone Kenneth is also noted as ~12°S to highlight that this occurred much further south than the two historical cyclones presented here.

Behaviour of tropical cyclones over the southwestern Indian Ocean
Tropical cyclones in the southwestern Indian Ocean are associated with violent weather that cause death and injuries, and damage socioeconomic activities and community livelihoods. In December 2019, tropical cyclones in the Indian Ocean were associated with enhanced rainfall over the East Africa region, even though these cyclones did not make landfall there (Wainwright et al., 2020). The October-December 2019 rainy season saw floods and landslides across the region, with initial estimates suggesting over 2.8 million people were adversely affected (Wainwright et al., 2020). Along with the tropical cyclones, the 2019 rainy season saw a strongly positive Indian Ocean Dipole (IOD), with such events having been shown to play a key role in the high rainfall over East Africa (Black, 2005). In addition, the Madden-Julian Oscillation (MJO) was active over Africa, which can also enhance rainfall. A link between the MJO, Indian Ocean tropical cyclones and rainfall over East Africa has been presented by Finney et al., (2019).
The severity of tropical cyclone impacts varies by location and depends on the intensity and position relative to the coast, as well as community preparedness. A direct hit occurs when the eyewall comes onshore while the centre of the storm may stay offshore (NHC, 2020), as was the case during Cyclone Idai in 2019. The eyewall is the most dangerous and destructive part of a tropical cyclone characterised by the strongest winds, heavy rainfall, and deep convective clouds that rise from close to Earth's surface to a height of around 15 000m (49 000ft) (Zehnder, 2019). Tropical cyclone landfall is associated with heavy rain, strong sustained winds and storm surges, which can lead to flooding of coastal areas. Floods exacerbate stress to the community by causing secondary impacts such as breeding sites for parasites and outbreaks of diseases such as malaria, dengue and cholera (WHO, 2020). In addition, through circulation changes, cyclones can indirectly be associated with enhanced or suppressed rainfall in more equatorial locations, as was the case with the very wet long rains in Kenya during 2018 (Kilavi et al., 2018;Finney et al., 2019).

Evidence of the Zanzibar tropical cyclone
The April 1872 tropical cyclone was reported as the 'terrible Zanzibar Cyclone' by the Times newspaper of India in its special report, which provides eyewitness evidence of the tropical cyclone eye at ~6.5°S (Trove, 1872). The impact of the April 1872 tropical cyclone triggered the documen-tation of subsequent disaster data by the colonial government ( Figure 2 This account provides clear evidence of tropical cyclone features. Interestingly, the article describes how an older resident had warned others that calming of the southerly winds was just the eye: 'An old friend, who was better acquainted than I was with the laws of storms, advised me to go home and secure the windows and doors having a northerly exposure. At this time the calm was almost as oppressive as the hurricane. Not a ray of sunshine pierced the lead-coloured sky, which deepened to an inky black towards the horizon on the north, with here and there a lurid glare' (Trove, 1872). This shows that they were aware of the nature of tropical cyclones and the circular wind field, which would cause the winds to swap direction after the passage of the eye. Without advice from his elderly friend, the eyewitness would not have taken precautions during the calm of the eye, thereby highlighting the importance of providing information to residents if another tropical cyclone were to be forecast in future.

Evidence of the Lindi tropical cyclone
The Lindi tropical cyclone made landfall on 15 April 1952, almost exactly 80 years after the Zanzibar tropical cyclone, and was reported in the Survey of Meteorological History and Behaviour of the East African Meteorological Department (Samson, 1953). The 0600 gmt surface pressure analysis from 15 April 1952 ( Figure 3a) shows a deep low pressure system of less than 1002mb over the Lindi coastline during the event (Samson, 1953).  The Oversee Food Corporation health inspector, who was in Lindi for the Easter weekend (12-13 April 1952), wrote a brief note on the development of the heavy storm over the sea. This situation encouraged him return to Nachingwea town early for the fear of the road being impassable (Samson, 1953). A pilot balloon at Lindi Meteorological station on the morning of 13 April showed unusually strong southsoutheasterly upper winds. At 1500ft above sea level, the wind speed was 37kn (19ms −1 ).
Rainfall observations reports on 14 and 15 April 1952 indicate heavy rainfall at various observation stations. During the Lindi tropical cyclone the 24-hour rainfall total was 142mm at Mikindani, 160mm at Mtwara port and 178mm at Kitunda weather stations (Table 1). These individual amounts are close to the monthly average rainfall or more in some areas based on the 1981-2010 climatology. The average rainfall over the southern parts of the Tanzania coastline for the month of April is 163mm at the southernmost point, and increases northwards.

Societal and human impacts of tropical cyclones in Tanzania
The Lindi tropical cyclone resulted in extensive damage to buildings and property and a considerable loss of life (UNDRR, 2019). The devastated area was confined to a narrow strip of the coast near Lindi, though damage also occurred at Mikindani and Mtwara further south. Fifty percent of buildings in Mtwara, and tugs and cranes at port were badly damaged. Half the buildings in Lindi lost their roofs; electricity, telephone and telegraph services were damaged; and boats were driven from the shore and onto reefs by the cyclone. Houses were flattened, and roads and bridges damaged (UNDRR, 2019). The damage was estimated at 2.5 million GBP, equivalent to 72 million GBP in 2020 (CPI, 2020). There is no evidence of appreciable damage being caused by the cyclone after it had moved inland.
The Zanzibar tropical cyclone caused damage to houses and the harbour (Figure 2). The Sultan lost all his vessels, with the excep-  14-15 April 1952respectively. (Source: Samson, 1953 Samson (1953). Amounts recorded at 0830h the following morning. Samson (1953)  tion of one small steamer, which happened to be at Mombasa at the time. The Roman Catholic Mission Hospital at Bagamoyo was also destroyed, along with two thirds of the clove and coconut plantations on Zanzibar (UNDRR, 2019). About 150 Arab and Indian boats, many full of cargo, either sank or were wrecked along the shore (Lindström, 2019). It is further reported that in Bagamoyo the tropical cyclone destroyed 50 houses and agricultural scheme of the Catholic Father (Lindström, 2019).

Climate change and tropical cyclone activity
Most studies in the scientific literature suggest that tropical cyclone activity is expected to be modified by climate change (Malan et al., 2013;Noy, 2016;Knutson et al., 2019). The intensity and frequency of the most intense cyclones are expected to increase over tropical regions in the twentyfirst century (Peduzzi et al., 2012;Schaeffer et al., 2014;Knutson et al., 2019), though it is not clear if there will be an increase or decrease in frequency of weaker tropical cyclones. Due to the vulnerability of coastal regions to storm surges, flooding is expected to increase with future sea-level rise, and further coastal development will elevate the risk to coastal livelihoods. Rising global temperatures could lead to tropical cyclones forming in locations where they are currently unheard of.
A study conducted in the Northern Hemisphere over the Bay of Bengal had iden-tified severe cyclones as being among the possible triggers for IOD occurrences (Francis et al., 2007). As previously mentioned, one of the important climate drivers that prevailed during the 2019 southwest Indian Ocean tropical cyclone season was an extremely positive IOD (Wainwright et al., 2020). The positive IOD is characterised by warmer than average ocean waters in the western Indian Ocean and cooler than average conditions in the eastern Indian Ocean. The 2019 IOD event lead to prolonged heavy rains during the preceding seasons in East Africa, and as such its predictability has become a focus area of study (Doi et al., 2020;Wainwright et al., 2020). Predictability of rare tropical cyclones forming further north in the East Africa region and their potential in triggering strongly positive IOD events requires further research.

Conclusion
The Zanzibar and Lindi tropical cyclones both occurred during the month of April, separated by 80 years, in 1872 and 1952 respectively. These accounts provide clear historical evidence that tropical cyclones do make landfall in Tanzania, even in the more northerly locations towards the equator where there is a lack of evidence of such events in more recent satellite records. The two cyclones discussed here are almost a lifetime apart and even the 1952 cyclone may not be in the living memory of most people. However, the impacts can be devastating, possibly even more so because people have no first-hand experience of tropical cyclones along the Tanzania coastline. The Zanzibar and Lindi tropical cyclones demonstrate that those with experience take the cyclone eye as an opportunity to prepare for the onslaught to come when exiting the eye of the cyclone. But for those unfamiliar with tropical cyclones, the calm of the cyclone eye can encourage complacency.
Nowadays, meteorologists are able to track and model the path of cyclones and can provide impact-based forecasts of cyclones. These are delivered through early warning systems to inform and protect people on the Tanzania coast from these rare and extreme weather events. Although awareness of such events is crucial, it may not always be seen as practical to design infrastructure to protect against such rare events, nor account fully for the effects of climate change on the frequency and severity of these event. Events such as the Lindi and Zanzibar cyclones may not well be captured in the observational records normally used to understand extreme weather. However, we have shown with eyewitness accounts that the recent Cyclone Kenneth is not the most northward cyclone to affect the East African coast, as has been suggested. An additional observation is that all three of the northerly-occurring cyclones (Lindi, Zanzibar and Kenneth) formed in April, late in the tropical cyclone season for the southwest Indian Ocean.
The reported impacts of these historical cyclones highlights the phenomenon as an important research topic. In particular, exploring teleconnections in the climate system to these rare landfalling tropical cyclone events in Tanzania will be useful for informing and developing preparedness within coastal communities.