POSSIBLE TELECONNECTION BETWEEN THE INDIAN OCEAN DIPOLE (IOD) AND RAINFALL DISTRIBUTION OVER NIGERIA

ABSTRACT

The El Nino Southern Oscillation (ENSO) has some level of control over the weather of Nigeria and Africa seasonally but there has been enough debate as to the extent of the control. ENSO magnitudes do not necessarily translate into impacts in the same direction hence, attempt to investigate possible teleconnections with the Indian Ocean Dipole Mode Index (IODMI) phases over equatorial Indian Ocean that could explain the subsisting gaps in the knowledge of large scale controls on Nigerian Monsoon Seasonal Rainfall (NMSR) pattern is the basis of this paper. Daily rainfall data was obtained from Nigerian Meteorological Agency (NiMet) and Sea Surface Temperatures (SSTs) over the Indian Ocean and the Pacific Ocean were obtained from National Oceanic and Atmospheric Administration (NOAA) site for a period of 1983-2015. Pearson correlation coefficient method was used to assess the spatial and temporal relationship between the IODMI/NMSR and ENSO/NMSR while the test-of-significance of the correlation results were carried out using two methods- Critical r value and p-value. The results revealed that there is a significant correlation at alpha value 0.1 between the IODMI and NMSR over the selected stations with contrasting tendencies to increase rainfall at a certain period of the year or decrease it at another period. However, the comparison of the influence of IODMI and ENSO indices negates their effects on NMSR which is dependent on the strength of correlation, thus with both having a decreasing north to south spatial pattern. Hence, the study concluded that there is a teleconnection between the NMSR and IODMI over the stations considered in the study and it is recommended that the incorporation of IODMI for the climate prediction of the monsoon rainfall over Nigeria would improve the resilience of different specific sectors of the economy especially agriculture.

CHAPTER ONE

1.0   INTRODUCTION

1.1 Background of Study

Despite the availability of oil revenues that form the backbone of the economy and foreign exchange earnings in Nigeria, agriculture still remains major source of livelihood in terms of food and employment for local communities. Therefore, no sustainable development can be Gachieved today and in the future without effectively addressing challenges associated with the threats of present and future rainfall variability and change. These include increased frequency and intensity of drought, floods, delayed onset, early cessation, and irregular distribution, leading to widespread impacts on infrastructure, agriculture, land degradation and soil erosion, livestock (including fisheries), loss of property, human health and life (Ukhurebor and Abiodun, 2018).

Rainfall variability and change impacts are already being felt in many parts of the world especially Africa, and are likely to worsen in the future, with clear evidences of changes in climate and weather patterns in many parts of the world based on Assessment Reports produced by the Working Groups of the Inter-Governmental Panel on Climate Change (IPCC, 2001b; 2012; 2013a).

However, the rainfall over the western region of Africa is directly affected by circulation over the Atlantic and indirectly by that over the Indian Ocean. (Preethi, 2015). Among the tropical indo-pacific climate drivers, the canonical El Niño/Southern Oscillation (ENSO), (Ramussen and Carpenter, 1983), is thought to play a dominant role in rainfall distribution over various parts of Africa during all the seasons (Janowiak, 1988; Janicot et al., 1998).

ENSO has been recognized as an important manifestation of the tropical ocean-atmosphere- land coupled system. The more recently discovered Indian Ocean Dipole (IOD). (Saji et al., 1999; Behera et al., 1999; Webster et al., 1999; Guanghan et al., 2016) is another important manifestation of the tropical air-sea interaction.

IOD also known as the Indian Nino, is an irregular oscillation of Sea Surface Temperature (SST) in which the western Indian Ocean becomes alternately warmer and cooler than the eastern part of the ocean. This leads to a coupled ocean-atmosphere phenomenon in which convection, winds, SST and thermocline take part actively (Vinayachandran, et al., 2007).

The IOD measured using a Dipole Mode Index (DMI) is calculated from the difference in SST anomaly between the tropical western Indian Ocean and the tropical south eastern Indian Ocean, (Saji et al., 1999). It usually starts around May or June, peaks between August and October and then rapidly decays when the monsoon arrives in the southern hemisphere around the end of the spring. It involves an aperiodic oscillation of SST between “positive”, “neutral” and “negative” phases. A positive phase has been shown to be associated with greater than average SST and greater precipitation in the western Indian Ocean region with a corresponding cooling of waters in the eastern Indian Ocean, which tends to cause droughts in adjacent land areas of Indonesia and Australia. The positive phase peaks is in September- October (Murtugadde et al., 2000). The negative phase of the IOD brings about the opposite conditions with warmer water and greater precipitation in the eastern Indian Ocean and cooler and drier conditions in the west.

Hence, the negative phase of IOD can be considered as an intensification of the normal state whereas as a positive phase of the IOD represents conditions nearly opposite to the normal.

Thus, Positive IOD = Western Equatorial SST > Eastern Equatorial SST and Negative IOD = Eastern Equatorial SST > Western Equatorial SST.  (Vinayachandran, et al., 2007, Saji et al., 1999).

The positive phase SST anomaly (SSTA) can be accompanied by above average rainfall in eastern Africa, Kavango-Zambezi Transfrontier Conservation Area (KAZA) in Southern Africa and the tropical western Indian Ocean but diminished rainfall over Indonesia and the tropical southeastern Indian Ocean (Saji et al., 1999; Webster et al., 1999; Black et al., 2003; Gaughan et al., 2016). It is understood that during the positive phase of IOD, unusually strong winds from the east push warm surface water towards Africa, allowing cold water to upwell along the Sumatran coast. Strong zonal wind anomalies trapped to the equatorial Indian Ocean are a characteristic atmospheric feature during such SSTA events (Reverdin et al., 1986; Murtugudde et al., 2000).

However, the perception that the Indian Ocean is passive and merely responds to the atmospheric forcing has been shown to be untrue. The discovery of the IOD and the studies that followed have demonstrated that the Indian Ocean can sustain its own intrinsic coupled ocean-atmosphere processes and is not merely a slave to the events happening over the Pacific Ocean in connection with ENSO. (Schott et al., 2009; Cherchi and Navara 2013).

In addition, in March and April 2019 there were unprecedented atmospheric circulations over the Indian Ocean such as cyclones Idai and Kenneth that had devastating effects on the southern east African coast in Mozambique, Tanzania and Kenya (“Cyclones Idai and Kenneth”, 2019 March 15th & April 25th), strongly indicating that independent active IOD- related SST influences exist east and south  on the African continent.

El Niño and IOD events account for 30% and 12% of the tropical Indian Ocean SST variability respectively (Saji et al., 1999). It means that both of the aforementioned phenomena explain a significant part of the tropical Indian Ocean variability. As alluded to earlier, the IOD events have a strong influence on the climate of the immediate neighboring regions such as East Africa, Indonesia and southern Africa (Saji et al., 1999; Black et al., 2003; Gaughan et al., 2016), and also on the Indian summer monsoon region (Ashok et al., 2001; Gouda et al., 2017), East Asia (Saji and Yamagata, 2002b, Guan et al., 2002), the Mediterranean, Australia, and Brazil (Saji and Yamagata, 2002b).

Significant increase in rainfall is seen in eastern Africa in association with canonical El Nino and positive IOD events. This significant positive correlation observed between the East African rainfall and IOD events during October-December season are in general agreement with the earlier studies (Ummenhofer et al., 2009) which show that the East African short rains are predominantly driven by the warm SST anomalies in the western equatorial Indian Ocean. However, the IOD event has been observed to affect east African rainfall independent of ENSO especially during September and November period (Behera et al., 2005) but have a weak influence on South Sudan rainfall pattern (Omay, 2015).

Over Nigeria, the seasonal cycle of rainfall is controlled by large scale monsoon circulations, the migration/oscillation of an Inter Tropical Discontinuity (ITD) and regional orography (Nicholson, 1996) as well as the African Easterly and Tropical Jet streams, and the weather- producing synoptic-scale wave disturbances (Sultan and Janicot, 2003), the westward- propagating mesoscale convective systems and African easterly waves (Kiladis et al., 2006).

The West African Monsoon circulation is also known to combine with monsoonal circulation over the Indian sub-continent to define summer circulation features over Nigeria. Thus, good amount of warm waters over western Indian Ocean encourages good monsoon condition. As Oguntoyinbo (1986) puts it, there exists interaction between the atmosphere, land and ocean over large geographical scales, so that climatic anomalies tend to be extensive in space. Thus, it is common to find that an intensification in a feature in one area signifies an abatement in another feature in another region and vice versa. Such linkages are called teleconnections.

Many studies have established a correlation between SST and rainfall in Africa (Berte and Ward, 1998, Colma et al., 2000). Rowell et al., (1993), William and Hanan (2011), Okonkwo, (2014),  Gaughan et al., (2016) among others, established a significant empirical relationship between SSTAs and African precipitation variability. In addition, there are suggestions that there is a relationship between the strength and/or position of the African Easterly Jet and precipitation (Fontaine and Janicott 1992; Rowell et al., 1993). Therefore, it is of interest to consider the dependence of the jet on structure in the SST field. Understanding this connection may help us to better understand the mechanisms that connect SST distributions and African rainfall. It is noteworthy that over Nigeria, the correlation between the tropical Indo-Pacific SSTA and East/South African rainfall has been established (Black et al., 2003; Omogbai, 2010; Gaughan et al., 2016).

Having established the facts that ENSO plays a huge role in the determination of the rainfall regime in Africa (Preethi et al., 2015) this study is focused on investigating the unexplored influence of IOD SST anomalies on rainfall distribution over Nigeria.

1.2 Statement of the Research Problem

Rainfall patterns in Nigeria can be affected by a lot of factors which include; large scale monsoon circulation, the migration/oscillation of ITD, orography and other forcing functions, but the effects of IOD is yet to be investigated (specifically for Nigeria) even though established to have direct influence and impact on East and South African, and Sahelian rainfall patterns. (Black et al., 2003; Behera et al., 2005; Williams and Hannan, 2011; Preethi et al., 2015; Gaughan et al., 2016).

This gap is significant on the backdrop of the propagation of active mesoscale convective systems (line squalls) from the tropical western Indian ocean affecting both the Zonal (east- west) circulation and the meridional (north-south) circulation in the troposphere, as shown in the RGB satellite imagery in Figure 1.1.

1.3 Justification for the Study

While it is a well-known fact that ENSO events exercise some control over the climate of Nigeria and West Africa at large and this knowledge has since been incorporated in the seasonal rainfall prediction for Nigeria. It is very imperative to investigate any possible interfaces with IODMI phases over equatorial Indian Ocean that could explain subsisting gaps in the Knowledge of large scale controls on rainfall over Nigeria.

This study seeks to answer the question as to whether there is a significant correlation between the SST anomalies over the tropical Indian ocean and the rainfall patterns over Nigeria. If this is proven, measures of Indian ocean SSTAs could be adopted, in addition to the ENSO already in use, to better improve the seasonal forecast of rainfall and for the better understanding of multi-decadal modulations of global interannual teleconnections.

For Nigeria to ensure food security through sustainable rainfed agriculture, there is need to understand the spatial and temporal characteristics of rainfall, and any teleconnection between these meteorological parameters and systems causing potential climate variability, such as Indian Ocean Dipole Mode Index (IODMI) and El Niño Southern Oscillation (ENSO). It will thus be the focus of this study to assess and fill the gaps and recommend possible means to better understand and forecast intra-seasonal and interannual variability in rainfall over Nigeria.

1.4 Aim and Objectives:

The aim of this research investigated the possible teleconnections between the Indian Ocean Dipole (IOD) phases and rainfall amount and distribution over Nigeria.

The specific objectives are to;

i.          Examine the nature of the temporal relationship between IOD phases and intra- seasonal rainfall variability in parts of Nigeria.

ii.        Assess the spatial distribution of rainfall across Nigeria in relation to IODMI phases.

iii.       Compare the teleconnection between rainfall and IODMI and ENSO indices.

1.5 Research Questions

The following questions will be raised in order to achieve the above objectives;

i.   How do IODMI phases affect the rainfall amount over selected stations in Nigeria?

ii.  How do IODMI phases affect the spatial distribution of rainfall over Nigeria?

iii. How do the IODMI and ENSO indices compare in their influences on rainfall amount and distribution over Nigeria?

1.6 Research Hypothesis– Some hypotheses are put forth to wholly or partly address some of the objectives in this study. Hence,the following hypothesis;

i.          Null hypothesis (Ho)– There is no significant relationship between Indian Ocean Dipole Mode Index (IODMI) and the Nigerian Monsoon Seasonal Rainfall (NMSR).

ii.         Alternative hypothesis (Hi)- There is a significant relationship between Indian Ocean Dipole Mode Index (IODMI) and the Nigerian Monsoon Seasonal Rainfall (NMSR).

1.7       Scope and Limitations of the Study

The scope of this research focused on examining the temporal relationship between IOD phases and intra-seasonal rainfall variability in parts of Nigeria, was restricted to a domain delimited by Latitudes 4  –14  N and Longitudes 3  –15  E. Data utilised were daily rainfall data from NiMet and monthly IODMI/SOI indices from NOAA website covering a 33-year period (1983-2015). Synoptic stations included in the study, for which data were readily available, were; Sokoto, Maiduguri, Katsina, Kano, Yelwa, Yola, Ibi, Abuja, Ilorin, Jos, Ikeja, Benin, Enugu, Owerri, Portharcourt and Calabar, spread across the eco-climatic zones of the country.

1.8  Study Area

Nigeria is located between Latitudes 4  –14  N and Longitudes 3  –15  E and covers a total area of 923,768 km². It shares border with the Republics of Niger, Cameroon and Benin and the Gulf of Guinea, to the north, east, west and south respectively.

Due to its location just north of the equator, Nigeria enjoys a tropical climate characterized by the hot and wet conditions associated with the movement of the Inter-Tropical Convergence Zone (ITCZ) north and south of the equator. The climate of Nigeria is dominated by the influence of two major wind currents, namely: the south-westerly tropical maritime (mT) air mass which is prevalent during the wet season because it is moisture laden as a result of it has traversed through the Atlantic ocean where it acquired moisture and the north-easterly tropical continental (cT) air mass that prevails during the dry season because of its dryness since there is no ocean along its continental path (Adeniyi, 2014; Odekunle, 2004). Two main seasons; wet (April-October) and dry (November- March), result from the interplay of the aforementioned two major wind currents.

Rainfall in Nigeria falls within a distinct period (Adeniyi et al., 2009). These periods vary from the northern part to the southern part of the country because of their relative distances from the Atlantic Ocean. Rainfall starts earlier and ceases late in the southern parts while it starts later and ceases earlier in the north. The onset month in the south varies between March and April while the cessation month is October. In the North, however, rainfall starts in May and ends in September (Adeniyi, 2014). There is rainfall occurrence all over the country during June to September. However, in August, there is a period of little dry season in the southern part of the country (Odekunle, 2004). Overall two rainfall peaks occur in the south whereas only one rainfall peak occurs in the north. This makes the climate to be humid in the south with mean annual rainfall of over 2000 mm and semi-arid in the north with mean annual rainfall less than 600 mm (Ojo, 1977). However, topographic relief plays a significant role in the local climate only around the Jos Plateau and along the eastern border highland.

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