CLIMATE, MONSOONS AND LAND-SEA BREEZES

Transcription

1 Chapter 5 CLIMATE, MONSOONS AND LAND-SEA BREEZES Beaches and coastal zone studies between Talapadi and Kapu, Dakshina Kannada, India.

2 THE CLIMATE 5.0. General The climate is considered to be an important factor in many of the coastal processes. The necessity to understand the present as well as the past climates has been stressed by many coastal researchers for the evident reason that the Pleistocene/Holocene climate was responsible for the development of the present-day coastal landforms. The role of climate as a factor in the ongoing coastal process can be of three types (Davies, 1977, p.14): a) Direct involvement in the shore processes like, platform weathering, dune building and coastal circulation. b) Effect on subaerial geomorphic processes in the coastal zone behind. the shore. c) Influence on the nature and rate of supply of sediment from the coastal hinterland. Broadly speaking, the climate of India is the product of the Asiatic monsoon circulation process. Because, it is the monsoons that bring rain over India and determine the moisture content of the land, and air above. Situated in between the tropical latitudes and influenced by the monsoon circulation system, India has varied climatic regime. In a first order, non-rationale classification scheme, the climate of India can be described as tropical wet-dry' to 'tropical semi-arid' type The Monsoons A brief description on monsoons is made here for the reason that they are responsible for the dynamicity of the climate of the region. The factors that drive the 127

3 coastal processes, like winds, waves and rainfall, are directly connected to these monsoons. The Asiatic monsoon circulation system has two distinct monsoons, which are of roughly equal duration and flow from opposite directions. The summer monsoon, which begins by about the end of May, is from SW direction. Hence it is called the southwest monsoon. This monsoon is responsible for most of the rainfall over India and is characterised by mt (tropical monsoon) and me (maritime equatorial) air masses. The winter monsoon begins when the southwest monsoon starts withdrawing and it flows from NE direction, hence the name northeast monsoon. The onset of this monsoon begins in the north by about mid September and reaches the southern tip of India by about first week of December. It is characterised by dry, ct (continental tropical) type of air masses. The aspects of genesis and dynamics of these monsoons have been discussed in detail by Das (1986). The pattern of southwest monsoon is an interesting aspect. An important regional characteristic is the surprising regularity of onset of this monsoon. The standard deviation of the date of arrival of monsoon rains over the southern tip of India is of the order of a week (Das, 1986, p.x). The normal date of onset is May 25 (Subbaramayya et al., 1988) and it will reach the study area by about first day of June. The onset of this monsoon is like a burst and it spreads quickly over the entire India in about three weeks time. 128

4 The southwest monsoon is also known for the consistency of its winds' direction. During the onset time the winds will be from south and later they become southwesterly. Along the west coast they become more of westerly. The speed of winds will be great at the beginning like a gust, later they will be of higher order breezes, occasionally reaching storm intensity. There are two patterns in the southwest monsoon, the active monsoon and the break in monsoon (Das, 1986, p.29; Riley and Spolton, 1981, p.42). During the active monsoon, depressions move from the Bay of Bengal into northern or central India. Along the west coast, winds will be at their maximum velocity and rainfall will be continuous. During the break in monsoon (or weak monsoon), there will be a weak pressure gradient over the Peninsula. This result in decreased wind speeds, and rain over the Western Ghats and coastal plain gets reduced. The breaks in monsoon can last from three to ten days. The general pattern is that there will be a few and short breaks in June and July, but longer and more frequent ones in August and September. The ratio of the total duration of active monsoon to the total duration of break monsoon, in a given year, is of vital importance, because it represents the success of the monsoon of that year. Observation and analysis have indicated great inter-annual variability of this ratio (Iyengar, 1991; Das, 1986, p.42). Unlike the southwest monsoon, which onsets as burst and spreads quickly over the entire India, the northeast monsoon advances very gradually. It reaches the study area by about November end. The effect of northeast monsoon (either of winds or rainfall) is hardly felt over the west coast. Therefore, in many of the coastal studies, 129

5 the duration of southwest monsoon is addressed as monsoon period, and rest of the year as fair weather period or non-monsoon period' Climatic features of the west coast The west coast of India receives a very high rainfall during the summer monsoon period and its annual temperature average is well above 20 C. Hence, it falls under the 'Amg' (Tropical monsoon) category of Koeppen's (Critchfield, 1979, p.147) and Aw' (Tropical wet-dry) category of Trewartha's (Riley and Spolton, 1981, p.98) classification schemes. listed thus: The main characteristic climatic features of the west coast of India can be 1. The annual rainfall average is very high (>250 cm) and most of the rain (more than 80%) falls during summer monsoon (i.e., June through September). During this period a strong onshore flow of moist air crosses the coast. The coast is backed by a mountain barrier (the Western Ghats), hence an exceptionally heavy rainfall is encountered. 2. The average annual potential evaporation is of the order of 1500 to 2000 mm and the average annual runoff exceeds 1000 mm. Being a tropical region, the average annual solar radiation will be about 150 to 200 k cal cm'2 and is free from occurrence of frost. 3. The annual temperature average is about 27 C and the variation in monthly mean temperature is very small. The high temperature is experienced just before the rainy season (hottest months, April and May). The summer monsoon months are relatively cooler. The diurnal variation of temperature is greatest in the drier months (December, January and February) and least in the rainy days. 4. The humidity is high throughout the year (> 65%) and more so in the rainy season. The diurnal variation in humidity is more pronounced during the drier months. 130

6 Climate of the study area In order to present a better picture of the climate of the study area, an analysis of the main climatic elements is brought out in the following sections. For this purpose, the data recorded at the Panamburu meteorological observatory of the India Meteorological Department has been utilised as this observatory is located in the middle of the study area Temperature The annual temperature average being 27 C and having peaks before the summer rainy season, the area experiences the Ganges type of temperature regime. The variation in mean monthly temperature is very small and it is between 25.5 C (August) and C (April). The highest maximum temperature ever recorded at Mangalore was 37.8 C on February , and the lowest minimum was 16.7 C recorded on January 13, 1911, February 8, 1911 and December 10, 1950 (Abhishankar, 1973). The mean monthly maximum temperature varies between 28.1 C (August) and 33.0 C (April), and the highest temperatures occur during the months of March, April and May (fig.5.1). The mean monthly minimum temperature varies between 21.0 C (January) and 25.3 C (April & May) and the lowest temperature is encountered in the months of December, January and February. The diurnal variation in temperature is maximum during November, December, January and February (9 to 11 C), and it is minimum during June, July, August and September (5.2 to 6.1 C) (fig.5.1 A, inset). 131

7 o CO o CO LO (N snisieo aajbap ui ajn}djaduiei 0O CM CO o uo < < Figure 5.1: The Mean monthly maximum and minimum temperatures: A - Average of 18 years from 1973 to 1990, B - during the study period. Inset in A - monthly mean diurnal variation in temperature. 132

8 The diurnal variation in temperature has an important bearing on the land-sea breeze system. More the diurnal variation in temperature in the coastal terrain, higher will be the intensity and dimension of land-sea breeze system. This has been reflected in the more pronounced land-sea breezes in the study area during the months of December through April. The characteristic feature of the area is the prevalence of relatively cool summer. It is because of the flow of moist southwest monsoon winds, which bring in heavy rainfall also. In most of the days of this rainy period, there will be a cover of cloud, which drastically reduces the amount of solar radiation reaching the surface. Also, the cloud cover may prevent the outgoing long-wave radiation, resulting in warming up to a little extent. The small diurnal variation in temperature encountered during rainy months may be attributed to these phenomena. Figure 5.IB is the plot of mean monthly maximum and minimum temperatures for the study period, which indicates no major departure from the 18 years average values (fig.5.1 A) Rainfall The monsoons have been described as harbingers of rain in the literary works of India. This is because most of the rainfall over India is associated with the southwest monsoon. Along the west coast of India, there is a one-to-one correlation between the intensity of monsoon and the amount of rainfall received. About 87% of the annual rainfall in the study area occurs in just four months of southwest monsoon (fig.5.2a). Before the onset of southwest monsoon, in the months of April and May. there will be some occasional rains, which are called pre-monsoon showers. 133

9 X = mm r = CD L_ O o in CD CO o\ 7 I I i o o _L o in (UiUl) 1104UIDJ 1D40J. o o in < Figure 5.2: A - Mean monthly rainfall for 18 years between 1973 and B - Total rainfall received during different months of the study period. C - Interannual variation in rainfall (in standard deviation units) and five year running mean of rainfall. 134

10 Characteristically, these showers are attendant with thunders and lightning. The month of May accounts for about 5% of the total annual rain, which may consist of pre-monsoon showers, rain due to early onset of southwest monsoon or both (fig.5.3). The pattern of rainfall during the southwest monsoon is not a continuous one (fig.5.3). There are dry spells between the periods of heavy rain. This is because of the active and the break characters of the monsoon. Spatially, the amount of rainfall gradually increases from the coast towards the Western Ghats. The annual total exceeds 4000 mm on the top of the Ghats. This phenomenon is attributed to the orographic effect of the Ghats (Das, 1986, p.l 13). The Ghats, being a barrier (with an average height more than 1000 m) across the path of the monsoon, disturb the low-level jet (air) streams resulting in a heavy rainfall. If the intensity of the monsoon is weak, the jet-streams get deflected/reflected back creating offshore-vortices which brings in rain over the coastal plain. This kind of a situation prevails more in the second half of the southwest monsoon period. The rainfall occurring during the months of October and November is attributed to the retreating southwest monsoon or the advancing northeast monsoon. Most of the winter season is devoid of rain except for some rare showers, which are due to the effect of cyclones passing along the east coast. Even though winter is a rain-less period, the moisture reserve in the soil supports a good amount of vegetation in the area. There is a large variation in the amount of total rainfall from year to year, and there is no pattern in this interannual variability. The Analysis made by Iyengar (1991), for the entire coastal Karnataka, has indicated the persistence of a sign 135

11 E o c oi_ 5 Q Figure 5.3: The pattern of rainfall during the study period indicating active and break phases in the southwest monsoon. The months December, January and February have been excluded in the figure as there was no rain during those months

12 (positive or negative from the zero mean) for a predominant period of three years before it changes. However, a five years running mean, for the 18 years data of the study area, indicates a probable cycle (fig.5.2c). Rainfall is a significant geomorphie agent. Its activity/influence is perceptible in two processes of the study area. Even though the rivers are having small drainage area, their drainage density and stream order are very high (as indicated in table 4.2). This is because, the heavy rain, concentrated in just four months of the year, results in overloading of the drainage which leads to a linear/ headward erosion. The other process is, the sudden flux of water on peak rainy days causing flooding in the coastal plain (lowlands) Humidity The relative humidity is very high through out the year. The winter months (November through March) are relatively drier with an average humidity around 70% (fig.5.4). During the summer (southwest) monsoon months, it exceeds 85% owing to the influx of moist air and accompanying rainfall. The diurnal variation in humidity will be more during January and February, and very less during June, July and August (fig.5.4a, inset). Factors that contribute to the high humidity even during the rainless period are; evaporation from the coastal water bodies (like, estuaries, marshes and swamps), evaporation from the sea (transported by the sea breeze) and evapotranspiration. The aeolian transport of the sediment in the coastal zone is partially controlled by the humidity, because a higher shear stress is required to move the wet sand. This aspect is reflected in the absence of dune building activity in the study area. The 137

13 \ : 1 :l / \. / ( \\t \ 31 H \\v 0 8 :3 0 h r s v: V. \; f. i: i: I- r - / / - - :/.1 :l I I i!.-/rv. '/ W I \ \ \ / / 'N t i \ j. 'I > j "V, A1 - * / / /. / ; / -** /, \ \ \ ' \ 17:30 h r s Q >> 70 "O i 70 D X QJ > <u QC 09 < ft) LT C B » a> ct I I I I I l l I I I I J F M A M J J A S O N O Figure 5.4; Monthly variation in relative humidity: A - averages for 18 year between 1973 and 1990, and B - averages during the study period. Inset in A - difference (of average values) between morning (08:30 hrs) and evening (17:30 hrs) observations

14 humidity being more than 85% during the strong wind days (southwest monsoon) prevent the removal of sand from the beaches. However, a very minor scale transportation due to strong sea breeze has been observed on the beaches on a few days during December and January. 139

15 THE WINDS General Wind is a factor responsible for many of the beach and other coastal processes. In the micro- to meso-tidal environments, its importance is next only to waves. Apart from generating waves, it plays an important part in the movement of beach material (King, 1971, p.165; Davis, 1985). The process of dune building and creation of wind-tide (storm surge) are its direct effects while the influence on surfzone and other nearshore circulations are its indirect effects Winds in the study area The wind system of the study area comprises mainly two elements; the seasonal trade winds of southwest monsoon and the land-sea breezes. Among them, the southwest monsoon winds are of great importance as they are responsible for the generation of seas (waves) in the Arabian Sea (Mukherjee and Sivaramakrishnan, 1982; Thiruvengadathan, 1984; Pandey et al., 1986; Raj Kumar et al., 1988) Southwest monsoon winds On a regional scale, the winds of the southwest monsoon (here-after referred to as monsoon ) are known for their consistency in direction. But locally, variations do occur. This may be due to local topography and inherent dynamics of the winds. During the onset-days of the monsoon, the direction of winds is southerly and their intensity varies between a gale and a storm. The duration of each gale/storm 140

16 will be about 30 minutes to an hour, which reflects the intermittent nature in the advancement of the monsoon winds. Soon after the monsoon crosses the area, the winds attain a steady state and their velocity will be of higher order breezes with occasional gales. By this time, the winds take the direction of southwesterly and then westerly. This condition prevails through out the monsoon period, but for the speed reaching storm intensity occasionally. The properties of active and break monsoons are also reflected in the wind pattern, but only in its velocity. An active phase of monsoon is characterised by the presence of pulses of higher order wind velocities. Repeated occurrence of many such pulses in a short period (of about 2 to 3 days) results in storm surge at the coast. This storm surge coupled with heavy rain causes flooding in the low-lying coastal areas. Two to four such floods are common every year. There are some instances when flooding occurred four to six times in a year. Also, floods have been persistent for longer periods (5 to 7 days) at times. Figure 5.5A is the plot of the percentage observations of wind direction and figure 5.6 is the wind-rose for the year It is inferred from these figures that the dominant direction is westerly with subordinate peaks at northwesterly and southwesterly directions. Among them, the southwesterly and westerly are the monsoon components where as the northwesterly is of sea breezes (discussed in the next section). An important feature that has to be noticed in wind pattern is the subsistence of calm weather (fig.5.5b). Even though the monsoon is the period of persistent winds, there are occasions of calm conditions (about 7 to 10 % of times) within it. 141

17 Direction in degrees from true north Percentage observations of calm weather Figure 5.5: A - Percentage observation of winds from different directions during the years 1988 and B - Percentage observation of calm conditions during different months. 142

18 Figure 5.6: Annual wind-rose for the year Index: 1. light air, 2. light breeze, 3. gentle breeze, 4. moderate breeze and 5. fresh breeze. Note the missing strong breeze, gales and storms because the wind observations have been made only twice a day. 143

19 5.4.3 Land and sea breezes The land and sea breezes are mesoscale, thermally driven circulations and they are attributed to the difference in heat response between land and water under an equal heat supply (Defant, 1951). Little cloudiness and strong sunshine are decisive factors in prompting the occurrence of these breezes. Basically, these winds are circulatory in nature but to a ground observer only the horizontal part of circulation is noticeable, in the form of relatively cool breeze from the sea in the afternoons. The nocturnal land breeze is of lower intensity when compared to its counterpart, the sea breeze (Hsu, 1970a). During the course of a day, these winds rotate in a clockwise direction in the northern hemisphere due to the influence of Coriolis force (Defant, 1951). The sea breeze plays an important part in the dynamic processes operating at the coast. It is effective in generating waves and currents, which in turn transport sediments in the littoral environment. It can produce a high frequency peak in the nearshore wave spectrum that dominate the background swell in the afternoon and evening (Sonu et al ). Also, the aeolian sand transport and air pollution dispersal in the coastal zone are closely controlled by this localised wind (Hsu, 1970b). In areas where the sea breeze prevails for many months of the year, the cumulative effect of its winds, albeit modest, on coastal processes should reach a significant extent (Sonu et al., 1973). The factors that control the distributions of intensity, direction and onset time of the land and sea breezes are mainly: the coastal lithology and topography, the diurnal variation in temperature, regional (gradient) winds (Defant, 1951; Hsu, 1979a & b; Sonu et al, 1973). 144

20 The study area, being a tropical sea-coast, experiences these breezes through out the non-monsoon period (October through May). The diurnal variation in temperature, being 7 C to 11 C. is favourable for the development of these breezes to full scale. The non-existing/low intensity regional wind (northeast monsoon) does not surpass/hinder the formation of these breezes. But the coastal lithology and topography have some control. Usually, the sea breeze elsewhere starts between 10:00 hrs and 11:00 hrs, reaches its maximum velocity between 13:00 hrs and 14:00 hrs and subsides between 14:00 hrs and 20:00 hrs, after which it is replaced by the nocturnal land breeze (Defant, 1951). But the field observations in the study area reveals that it sets in between 11:30 hrs and 13:00 hrs, reaches its maximum velocity between 15:00 hrs and 17:00 hrs and subsides between 18:00 and 21:00 hrs. This delay in the sea breeze cycle can be attributed to the lithology and topography of the area. The shore adjacent coastal zone, having estuaries, marshes and swamps, do not heat up and create the pressure difference quickly. The topography, having ridges, and broad valleys with good amount of vegetation, create the same effect. Further, the feeble intensity regional wind (northeast monsoon) may delay the progress of the sea breeze front on some occasions. The onset direction of the sea breeze in the area is from west. Later, it rotates clockwise, to west-northwest and then to northwest at its peak velocity time. Regarding the land breeze, proper observations have not been made as its velocity is of lower order. However, it is not incorrect to generalise that there will be a delay in its onset time. Usually, the land breeze will be at its peak velocity around 06:00 hrs in the early morning elsewhere. But, the 08:30 hrs meteorological 145

21 observations indicate winds of 1.5 to 2 m sec'1 velocity, coming from the east and southeast directions. Hence, it is reasonable to assume that the land breeze will be at its maximum velocity by about 08:00 hrs in the morning. The land and sea breezes can stretch up to 50 to 60 km inland, even up to 100 km some time, in a tropical terrain (Defant, 1951). The geographical set-up of the area has some restriction/influence on aerial extent of these breezes. The coast is backed by the Western Ghats, at about 50 to 70 km away from the shore. The valley breeze and mountain breeze can be expected along the Ghats. The valley breeze blows upward with maximum velocity in the late afternoon and the mountain breeze flows downward with maximum velocity by about early morning. Thus, there will always be a possibility of these two local wind systems merging with each other, or at least on- peak sunny days (during December and January). Alongshore variation in the land and sea breeze can be expected as there are many water bodies, in the form of estuaries, along the coast, but their influence on the direction of the breezes may not be considerable. Temporally, the land and sea breeze circulations begin to appear by the end of monsoon period (September). But they develop to full scale between December and April. Through out this period, the sea breeze will be having maximum velocity of around 4 m sec'1, occasionally reaching 6 m sec'1, whereas, the land breeze appears to be having maximum velocity during December and January. Possibility of the regional wind (northeast monsoon) coupling with the land breeze cannot be ruled out during that period. In this case, the resultant direction will be from East (NE of regional wind and SW of land breeze at its maximum velocity time). The presence of second maximum peak at 90 (East) direction in the figure 5.5A, and the higher 146

22 velocities of about 2 m sec'1 in the 08:30 hrs observations during December and January may be because of this reason. It is generally assumed that the land and sea breezes develop/merge with the regional circulation during the monsoon period (Jayappa, 1987). But it is unlikely for the reasons that the monsoon winds are strong and consistent, the diurnal variation in temperature is minimum and there will be a cloud cover in most of the days during that period. However, the possibility of development of these local winds in the 'break monsoon' phase cannot be ruled out. In such cases, their aerial extent and intensity may not be of significant scale, and there may not be a complete cycle of these winds Constraints of wind data It has to be pointed out here that wind data of the study area has some constraints. The observations are made twice a day, at 08:30 hrs and 17:30 hrs (local times), and the data has been collected manually. Therefore, the full spectrum of winds is not available, and the storms are out of records. Because of these reasons, the data is not useful for the calculation of percentage persistence of different category of winds. However, in order to elucidate the role of wind in the coastal processes, the wind data of the area have been separated into four components - northerly, southerly, onshore and offshore - with respect to the orientation of shoreline, and the mean monthly wind speeds have been calculated (figs.5.7 & 5.8). 147

23 M onthly m ean v e lo c it y (m s e c 1) TO NORTH > <-----OFFSHORE ONSHORE >. U l -F- U1 J l ir \, \ / -DAY (mean) MORNING _l I I I 1 I I 1 Jan 1988 I i i i i i i r 1 l_ EVENING -i.._.l L_..A _ i...j I I I I 1 J Dec Jan 1989 Dec 1 I l i i i i i i i i i i I i Figure 5.7: Mean monthly velocities of different components of wmd: A - onshore and offshore components, and B - northerly and southerly components. The morning and the evening observations are shown separately to highlight the nature of land and sea breezes. 148

24 s Figure 5.8: Percentage share of four components of wind during different months from the annual total. Percentage share of a component depends on the contribution of total wind speed during a particular month to the annual total. 149

25 Alongshore component The northerly and the southerly winds constitute the shore-parallel or alongshore component. This is an important component, because it can generate/strengthen the alongshore currents, especially in the surfzone. At times, it has a capacity to modify the breaking pattern of waves. The mean monthly velocities of the alongshore component are in phase with the monsoon and the non-monsoon periods (fig.5.7b). During the non-monsoon period, this component is strongly southward (fig.5.8). This is because the predominant winds during that period are sea-breezes which, at their peak time of the day, are northwesterly. The only departure from this are the morning winds during October through January. During this period there prevails some strong southeasterly winds, but they occur only on a few days. Occurrence of these southeasterly winds can be attributed to strong land-breeze. A prominent feature in figure 5.7B is of the winds during June, which have a strong and uniform southerly component (also in fig.5.8). June is the month of onset of monsoon and the winds are southerly at that time. Rest of the monsoon period is characterised by weak southerly component or it oscillates between southerly and northerly directions. From figure 5.8, it is inferred that the northerly component predominates over the southerly component during the non-monsoon period. There is also a progressive increase of this component from September to November and again from February to May. The break in this progressive trend around December and January may be due to the influence of regional (NE monsoon) winds as they enhance the land breeze 150

26 velocity during that time. In general, the northerly component of wind predominates through out the year resulting in net southward transport of water mass at the shore Shore-normal component The onshore and the offshore winds constitute the shore-normal component. The westerly winds of the monsoon, and the land and sea breezes contribute to this t component. The morning and evening observations of the winds indicate a clear-cut monsoon and non-monsoon patterns of this component (fig.5.7a). The evening winds are onshore throughout the year and their mean monthly velocities are about 3 to 5 m sec'1. The morning winds indicate an offshore component during non-monsoon period, and an onshore component during the monsoon. This is because, the winds of the southwest monsoon, which are strong and consistent, do not allow the land breeze to develop/exist (or their velocity simply exceeds that of the land breeze). A striking feature in the offshore component is the linear increase in its velocity from September to December/January, and a linear decrease from January to May (fig.5.8). There are two aspects attributable to this trend. The most possible one is the range of diurnal variation in temperature, which has a similar trend (fig.5.1a, inset). The other possibility is the influence of regional (northeast monsoon) wind on land breeze. * 151