Introduction

Lake Victoria is the biggest freshwater lake in the tropical regions of the world. It straddles the Equator and lies within the countries of Kenya, Tanzania and Uganda, having a surface area of approximately 70 000 km2. Compared to the other freshwater lakes of the world, it is the second largest, being next to Lake Superior. The River Nile originates from this lake at Jinja in Uganda and, passing through Lakes Kyoga and Mobutu Sese-Seko, is joined by the Blue Nile from Ethiopia and then flows through the Sudan into Egypt. The catchment area of the Upper Nile up to Nimule at the Uganda–Sudan border is 41 000 km2 of which 87%; lies in Kenya, Uganda and Tanzania and the remaining 13%; in Rwanda, Burundi and Zaire.

Since the River Nile has a direct bearing on the economic development of all these countries, study of the water balance of Lake Victoria, which regulates the flow of the Nile, is of immense importance.

The degree of predictability of monsoons is a matter of considerable social and economic importance. Large agrarian populations exist in monsoon areas, and monsoon rains have a critical influence on food production and human welfare. The long-range prediction of average rainfall could be of immense value for water management and agricultural planning. In this chapter, evidence that mean flow conditions and precipitation patterns at low latitudes are in principle more predictable than those at high latitudes is presented. Among the low-latitude circulations are the African and Asian monsoons and perhaps also the southeasterly monsoon flow east of the North American Cordillera. In particular we wish to show that the natural flow instabilities on synoptic scales account for most of the interannual variability of monthly mean quantities at midlatitudes, but cannot explain the observed variability at low latitudes.

Introduction

The Indian Ocean is of especial significance to oceanographers because of its unique feature of currents that switch direction on an annual cycle. This feature makes the Indian Ocean a natural ‘laboratory’ where oceanographers can seek to test their understanding of the dynamics of how ocean currents respond to changing wind patterns. In addition, Parts I and II of this book have suggested how the oceanography of the Indian Ocean may be of crucial importance to meteorologists. Charney and Shukla, in Chapter 6 of this book, indicated the potential ‘predictability’ of monsoon meteorology given a prediction of boundary values, including, especially, sea-surface temperatures. In this context, they emphasized that timescales of oceanic responses are all relatively slower than their atmospheric counterparts, so that longer-term forecasting of ocean boundary values (for use as the input to atmospheric models) might ultimately be attainable.

They are certainly right about timescales of response. Indeed, it has long been known for a midlatitude ocean that the timescale associated with the accumulation of the baroclinic part of its response to wind stress (including its western boundary current) may be as much as a decade (Veronis and Stommel, 1956). On the other hand, for a low-latitude system such as the Indian Ocean, the response is by no means as slow as that, and almost a decade ago Lighthill (1969) suggested that the corresponding timescale would be about a month rather than a decade.

Introduction

A large number of observational and numerical studies have been carried out in recent years to investigate the general circulation of the tropical atmosphere. The observational studies were greatly affected by the recent development of a wide variety of global observational systems (satellite, aircraft, etc.). Most of these studies have dealt with regional or global monthly/seasonal mean analyses of the tropical flow fields (e.g., Aspliden et al., 1965; Flohn, 1971 and Newell et al., 1972) and provided characteristic mean features of the tropospheric motion fields. However, not many long sequences of daily analyses required for a quantitative investigation have ever been performed. Only two such studies are available (Krishnamurti and Rogers, 1970 and Krishnamurti et al., 1975). These studies provided daily analyses of the upper-tropospheric motion fields during northern summer seasons. More recently, it has been recognized that the analyses of the tropical motion fields prepared at the US National Meteorological Center (NMC) have been improved as a result of extensive data-gathering efforts and have become useful data sources.

Introduction

An elongated trough, running parallel to the southern periphery of the Himalayas, is an important feature of the monsoon. It is known as the monsoon trough, and its normal position in June is shown in Fig. 23.1. The line of symmetry, shown by a dashed line on the figure, is the axis of the trough. The trough is most marked at sea level, and rarely extends above 700 mb.

Short-period variations in monsoon rain are closely associated with the position of the trough.

Introduction

The net energy accumulated over the tropics is transported to higher latitudes by the atmospheric and oceanic circulations in maintaining the thermal equilibrium of the earth–atmosphere system. Earlier studies have indicated that the atmosphere transports more energy than the oceans. However, the studies of Von der Haar and Oort (1973) based on satellite radiation data clearly indicate that the oceans play the larger role in transporting the surplus energy from the tropics. The relative contributions of the Indian, Atlantic and Pacific oceans must clearly differ.

Introduction

Because of the lack of data over vast areas of the Indian Ocean, the details of the monsoonal airflow at low levels, where the development of the southwest monsoon takes place, still remains largely unknown.

All previous studies over the ocean have been related to monthly mean fields or local measurements, while transient phenomena of interest occurring with shorter periods over the Indian monsoon region as a whole are still largely uninvestigated. For example, one of the most important phenomena, which is the alternating of break and strong activity of the monsoon over the Indian subcontinent, takes place with a period of about two weeks. The study of the relationship between the variability of monsoon rainfall and that of the fields of other meteorological variables requires a knowledge of the variability of these fields averaged over a few days.

Brief description of the models

The formulation of the 5-layer model is given in Corby et al. (1977). It has an irregular grid-mesh on the sphere designed to achieve a quasi-constant mesh-length of approximately 330 km.

Introduction

When the governing equations of a system are nonlinear, one of the analytical methods for obtaining the solution is the perturbation technique. In such a situation the system is reduced to a non-dimensional form by referring different physical variables to characteristic scales. The dependent variables are expanded asymptotically in terms of small parameters which occur in a natural way depending on the physics of the problem. It is assumed that each term of the perturbation series is smaller in magnitude than its preceding term throughout the region of interest. If this condition breaks down in any part of the region, the asymptotic expansion is no longer valid. An important reason why perturbation solutions are often not uniformly valid is concerned with the ‘large integrated effect’.

The final part of this book is devoted to the last part of the cycle of important fluid-dynamical processes which constitutes monsoon dynamics. The analysis of these concluding processes gains interest primarily from the hope that it can lead to practical methods for producing adequate and reliable advance warning of flooding dangers.

First, we have two chapters by leading experts on the prediction of marine flooding associated with storm surges. The numerical modelling involved allows for the combined input from atmospheric processes and astronomical tide-raising forces, together with a comprehensive treatment of depth-distribution effects and boundary conditions. These chapters describe both the successful present use of these techniques in connection with flood prediction for the British Isles, and their potential for application in the Bay of Bengal to forecast flooding surges resulting from monsoon wind stresses acting upon the shallow water at the mouth of the Ganges delta.

Although in these chapters the storm-surge experts concentrate on flooding dangers associated with monsoons, they were, of course, consulted during their stay in Delhi on the implications of the tragic events of the month (November, 1977) immediately preceding the Symposium upon which this book is based. Flooding of great severity was produced by a tropical cyclone in the state of Tamil Nadu. This was followed, within a few days, by a still more devastating, and indeed unprecedented, degree of flooding in Andhra Pradesh caused by another cyclone.

Introduction

Since the pioneering work of Richardson in the 1920s, numerical methods have been used increasingly to solve the equations governing the motion of the atmosphere and the sea. During the 1960s atmospheric models were put to work in operational weather prediction, and their use and development has continued up to the present. Similar developments have taken place in oceanography, where models have been used to investigate motion in oceans and shelf seas.

Introduction

A special feature of the low-level airflow over eastern Africa and the western Indian Ocean during the northern summer is that it is organized into a relatively narrow high-speed transequatorial current in the western periphery of the monsoon regime. The flow is strongest where the current is blocked or guided by high ground, and is weakest in the vicinity of the oceanic Equator. The current is characterized by a system of daily low-level jet streams, sufficiently persistent to show up markedly in monthly-averaged wind data.

The characteristics of the major current and the daily low-level jet streams have been described in detail by Findlater (1966, 1967, 1969a, b, 1970, 1971a, b, 1972, 1974, 1977b), but the general form of the current at the 1 km level in July can be seen in Fig. 20.1.

Introduction

The monsoonal rainfall over south Asia and west Africa undergoes interannual variations. Although some of the anomalous periods have more of a regional character, i.e., a period of drought over parts of a continent may not occur at the same period in other regions, there do exist periods of widespread drought that extend from west Africa to India. Such droughts occurred during the summers of 1877, 1899, 1918 and 1972.

Introduction

General circulation models (GCMs) have recently been applied to the study of the summer and winter monsoon patterns over eastern Africa, southern Asia, and the nearby oceans. For convenience, we refer to the entire region as the Indian monsoon region. Although GCMs have led to a better understanding of the large-scale features of the monsoon, they have not properly simulated all the small-scale features. This review discusses the ‘state of the art’ of Indian monsoon simulations and points out problems in, and prospects for, improving our understanding of this interesting and important meteorological phenomenon.

Washington (1970) experimented with a 5-degree, latitude–longitude grid version of the National Center for Atmospheric Research (NCAR) GCM showing the basic features of the monsoon. Even in this early experiment, the strong cross-equatorial jet near Somalia, the formation of a tropical easterly jet, and the low-level westerly flow in the vicinity of India were apparent.

Introduction

The Somali Current is the most dramatic feature of the seasonally reversing circulation in the northern part of the western Indian Ocean. At its peak, at the height of the southwest monsoon, surface currents of 3 to 4 ms-1 have been reported, and its total transport has been estimated at 70 million tonnes per second. The present state of knowledge of this current has been thoroughly reviewed in two recent papers, by Düing (1978) on observations and by Anderson (1978) on theoretical studies. From the point of view of monsoon dynamics, there are two particularly significant aspects. For monsoon meteorologists, the upwelling related to the Somali Current and its effect on sea-surface temperatures in the western Indian Ocean may be most important. For dynamical oceanographers, the onset of the Somali Current has been a particular concern: how do the monsoon winds generate such a strong current so quickly? The purpose of this chapter is to briefly review observations that bear on these two aspects of the Somali Current. First, though, for the benefit of those to whom it may be unfamiliar, a short description of the fullydeveloped current will be given.

The fully-developed Somali Current

It is convenient to think of the Somali Current as the seasonally reversing part of the boundary current along the east coast of Africa; that is, the part extending northwards from approximately 3° S latitude.

Introduction

Many studies have been carried out of the rainfall characteristics of most of the regions affected by the monsoons, and extensive records exist in many countries. In more recent years, however, the emphasis in regional observational studies has tended to be placed more on the analysis of weather patterns at various levels in the atmosphere in order to identify the structures of particular weather systems responsible for rainfall. Research has also been carried out which attempts to evaluate the various terms in the moisture budget in particular regions, for instance the net water vapour transport into the region and the accompanying evaporation and rainfall. Chapters 12 to 28 describe such regional studies.

Chapter 12 concentrates on the role of the upper-tropospheric anticyclone during the summer monsoon onset, while in Chapters 13 and 14 aspects of the structures of weather systems responsible for particular monsoon rain events are described. Such events are shown, in Chapter 15, to be closely related to the pattern of sea-surface temperature in the Arabian Sea and Bay of Bengal. The rainfall distribution in the Himalayas is discussed in Chapter 16, and the rainfall over India as a whole is discussed in the full water vapour budget context in Chapter 17.

One of the basic problems of tropical meteorology is to determine the precise way in which the atmosphere organizes the upward transports of heat, moisture and momentum on a variety of different scales.

Introduction

It is recognized that boundary-layer studies are likely to play an important role in improving our understanding of the monsoon. In recent years, experimental data have been collected by the NCAR 1972 Puerto Rico experiment (Pennell and LeMone, 1974) for the mixed layer over a tropical ocean. A three-dimensional model by Sommeria (1976 and 1978) has shown good agreement with the observed data. In this chapter, a similar one-dimensional model is presented which may be used for similar experiments for the monsoon region.

Basic equations

Atmospheric turbulence is assumed to be statistically homogeneous. The mean values of the dependent variables, namely, the components of the wind vector, potential temperature and the pressure gradient are assumed to be independent of horizontal space coordinates. This implies that the horizontal variation of the mean flow is assumed to be small compared with its vertical variation.