Chapter 5: Wave Climatology of Indian Ocean Annual...

Chapter -5 Wave Climatology of Indian Ocean

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Chapter 5: Wave Climatology of Indian Ocean – Annual Cycle

5.1 Introduction

The routine collection of information on wave characteristics, in Indian Ocean,

was predominantly based on visual observations carried out using ships of

opportunity. Further using data from Indian Daily Weather Report (IDWR)

(published by Indian Meteorology Department, New Delhi) Vijayarajan et al.

(1978) prepared a wave atlas for Arabian Sea. Subsequently another wave atlas

was prepared by National Institute of Oceanography, India for North Indian Ocean

(NIO, 1982) which was further updated by Chandramohan et al. (1991). Apart

from these, there were also measurements of wave characteristics onboard

research vessels during oceanographic surveys. The availability of in-situ wave

data was very sparse and inhomogeneous over Indian Ocean region. This adds to

the limitation of establishing a reliable wave climatology using in-situ data.

The global coverage and periodic measurements of ocean waves by the altimeters

onboard Geosat, ERS and TOPEX/ POSEIDON made an important contribution

in building up reliable wave data at larger spatio-temporal scale. The wave data

acquired from various satellite platforms has been effectively utilized to

understand the variability and also processes. Rajkumar et al. (2009) utilized the

ERS-1 Synthetic Aperture Radar (SAR) wave mode data along with TOPEX/

POSEIDON altimeter – derived SWH to study the monthly variation of waves off

the west coast of India. A comparative study between Geosat, in-situ and model

data conducted by Vethamony et al. (2000) reveals that the satellite derived


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significant wave heights were extremely valuable to evolve long term distribution

for generating wave climatology.

In this chapter the detailed discussion given on daily and monthly Climatology of

SWH computed from model data covering a period from 1998 to 2010 with 6hr

time interval. Modulo re-gridding transformation available with Ferret

(Ferret_user_guide_v602) was used for computation of wave climatology. Prior to

analysis of SWH, the wind climatology has been discussed considering as primary

forcing mechanism for waves.

5.2 Spatial Distribution of SWH Mean Annual Cycle

The climate in the Indian Ocean is characterized by an alteration of seasons

known as monsoons. The effect of monsoons is felt even in the subtropical

regions of the south Indian Ocean. During northern winter, dry surface air blow

from land to sea in the north-east direction, resulting in the north-east monsoon

season and in summer there is a complete reversal of these conditions with the

moist winds blowing from sea to land in the south-west direction, resulting in the

south-west monsoon season.

The hovmollar diagram (Figure 5.1) of longitudinally averaged monthly fields of

SST and winds derived from COADS monthly climatology over the Indian Ocean

representing mean latitudinal distribution. The differential heating of land and

ocean causes the formation of pressure gradients, which drives the winds from one

place to another place. During north-east monsoon, a high pressure area was

present at about 35o S in the south Indian Ocean. Towards north of this

subtropical high, the pressure decreases till the equatorial low. At this time, there

was also another subtropical belt of high pressure in the northern hemisphere over


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Central Asia. This belt was more irregular due to the larger proportion of land in

the northern hemisphere and its seasonal changes were more prominent. During

this period, winds blow from the continent towards the ocean in a north-east

direction as these winds cross the equator, they change in direction and become

north-westerly and westerly and meet the south-east trade winds at about 5-15oS

in the south Indian Ocean. The Inter Tropical Convergence Zone (ITCZ) was

found between 5 and 15oS during this period of the year. Southward of this zone,

south-east trade winds blow due to the presence of the high pressure area. The

north-east winds blowing in winter monsoon were relatively weaker than that of

south-west monsoon.

Figure 5.1: Monthly variation of longitudinally averaged (a) SST and (b)

wind speed from COADS climatology.

During south-west monsoon, the southern high pressure area moves up towards

30o S and becomes stronger. In the northern part of this high pressure area, the

south-east trade winds blow from 30oS. The trade winds in the south Indian

Ocean have a high mean rate of travel. These winds blow almost constantly


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throughout the year from 30oS towards the equator. The highest mean wind speed

(33 km/hr) was found in the south-east trades of the south Indian Ocean. In this

season, the Asiatic high disappears and is replaced by a deep and extensive low.

During this period low pressure was found over northern India. The incoming

solar radiation heat very large land mass of Asia, resulting in steady falling of

pressure from subtropical high (at about 30oS). Hence, the south-east trade winds

cross the equator and become south-west monsoon wind in the north Indian

Ocean. These winds blow parallel to the coast of Africa and Arabia. The Inter

Tropical Convergence Zone (ICTZ) is not easily identified in the Indian Ocean

and the only winds present in this season originate in the south Indian Ocean. In

the Indian Ocean, north of the equator, winds from south-west blow very strong

from June till September. The wind speeds reach values of about 25-30 km/hr

between India, Arabia and Africa. During transition periods (Sep-Oct-Nov and

Mar-Apr-May) the wind speeds reach minimum magnitudes over entire north

Indian Ocean. Because of this high seasonality of winds over north Indian Ocean

the annual mean of the wave heights were comparatively lower than that of

Tropical South Indian Ocean (TSIO). The mean annual spatial distribution of

Significant Wave Height (SWH) by averaging 12 month climatology (Figure 5.2)

of Indian Ocean shows a monopole structure with high wave condition of 3 m to

3.5 m exists around 80oE and 20

oS. The wave heights of Tropical South Indian

Ocean (TSIO) are high compared to Tropical North Indian Ocean (TNIO). It is

clear from the figure 5.1-b that the existence of high and moderate winds over

TSIO all along the year, resulting in large magnitude of the mean annual wind

fields. Waves with more than 2 m exist between 10oS to 30

oS, except the region

west of Madagascar. The mean annual pattern shows decreasing of SWH from


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south to north. Low wave conditions exist over northwest of Madagascar and

northern most Indian Ocean.

Figure 5.2: Spatial distribution of SWH mean annual cycle.

5.3 Daily Climatology of SWH:

The results presented in this section intend to illustrate the optimum ranges in

SWH within the study area. Initially, spatially averaged daily climatology of

SWH was analyzed (Fig. 5.3). The results show a clear annual cycle ranging from

0.5 m to 3.5 m. Further it was clear that the annual cycle is dominated by high

wave conditions during summer monsoon, low wave conditions during winter

monsoon and moderate wave conditions during transition periods.

Figure 5.3 : Daily Climatology of SWH annual cycle.


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In order to find the dominant wave heights in different sectors of Indian Ocean

i.e., AS (40˚-77˚E; 5˚-25˚N), BoB (77˚-100˚E; 5˚-25˚N) and Tropical South Indian

Ocean (TSIO) (5˚N-30˚S; 40˚-100˚E), the percentage histograms were also

computed. Figures 5.4, 5.5 and 5.6 shows the histograms of spatially averaged

SWH daily climatology for AS, BoB and TSIO respectively. The histograms were

plotted by separating the total range of SWH by 0.5 m and percentage of

occurrence of each range was given on y-axis. The histogram of SWH over AS

covers a wider range from 0.5 m to 3.5 m which was higher as compared to the

other sectors. SWH with 0.5-1 m and 1-1.5 m were predominant over AS with

29.9% and 34.8% of occurrence in an annual cycle respectively. However, SWH

with 1.5-3.5 m had less significance of occurrence. This range of wave heights

were encountered only during peak of summer monsoon (July) when winds are

the strongest with speed more than 10 m/s in the western Arabian Sea (Rao and

Behera, 2005). The wide spectrum of wave heights over AS was due to high

contrasting seasonality with comparatively very high magnitudes of wave heights

during summer monsoon and very low wave heights during transition periods.

Chandramoham et al. (1991) also observed the occurrence of high wave during

summer monsoon due to persistent winds.


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Figure 5.4: Histogram of spatially averaged SWH daily climatology for Arabian Sea.

For Bay of Bengal (BoB) SWH daily climatology ranged from with a minimum of

0.5 m to a maximum of 2.5 m. This was comparatively smaller than that in AS.

The predominant wave heights between 1 to 1.5 m occur 38.9% of the total waves

in a year (Figure 5.5). The occurrence of remaining each range of wave heights

0.5 to 1 m and 1.5 to 2.5 m is less than 22%. Even though BoB is generally

characterized by seasonality of winds, the mean monthly wave height with high

magnitudes such as 3.5 m in AS not exist because of weakening of monsoon

winds when reaching BoB. A survey conducted using IDWR wave data also

revealed that the predominant wave height was comparatively lower in BoB than

that of AS (Chandramohan, Sanil Kumar and Nayak, 1991).


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Figure 5.5: Histogram of spatially averaged SWH daily climatology for

Bay of Bengal.

The southern part of Indian Ocean covering from equator to 30˚S was

characterised by high wave heights compared to both AS and BoB (Figure 5.6).

Winds at the equator change direction, but are generally weak (speed < 2 m/s

during March-April and November-December). The southeast trades were

stronger, compared to other oceans and extend from about 5° to 20° S with speed

more than 6 m/s during July-August. The sustained high trade winds all along the

year were responsible for the generation of high wave in this region. It was clear

that by comparing the three histograms that the presence of wave with magnitude

less than 1.5 m was totally absent in the daily climatology of TSIO. The SWH

between 3.0-3.5m were with low percentage (10.1%) of occurrence. The

availability of wave information over TSIO was sparse. However, few studies

based on numerical models and satellite altimetry shown that the existence of

comparatively high waves in all seasons due to persistent strong trade winds and

large fetch (Vethamony et al., 2000; Young, 1994).


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Figure 5.6: Histogram of spatially averaged SWH daily climatology for

South Indian Ocean.

5.4 Monthly Climatology of SWH:

The annual cycle of spatially averaged SWH monthly climatology for TNIO and

TSIO along with the sun’s declination and position of ITCZ was presented in

Figure 5.7. Monthly climatology of SWH show a strong seasonal signal with

primary maximum during Summer monsoon and secondary maximum during

Winter monsoon over Tropical North Indian Ocean (TNIO). The seasonal signal

over Tropical South Indian Ocean (TSIO) consists of single maximum during

summer monsoon. It is important to note that the monthly mean SWH over TSIO

was greater than that of TNIO for entire annual cycle due to strong sustained SE

trade winds. As the magnitude of the wave depends on the availability of fetch

and duration, these wind fields over Indian Ocean can be well explained by the

position of ITCZ. The location of ITCZ and its utility in synoptic practice is an

important aspect. At the height of the monsoon, a trough runs regularly along the


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Gangetic valley, called the monsoon trough which is normally identified with the

ITCZ over the Indian area at surface level. While the air-mas to the south of the

trough line can be traced back to the southern hemisphere, whereas that to the

north is a continuation of the same air mass. The monsoon trough is not the cause

of much weather, though its position affects the development of other systems

which have a profound influence on wave generation. ITCZ reaches its northern

end during June and southern end during February.

The detailed discussion on major features of SWH spatial fields for each month is

given in the following section (figures from 5.8 to 5.19). The Sea Surface

Temperature (SST) and Sea Surface Pressure (SLP) drives the wind field which

intern responsible for the generation and growth of waves (Pond and Pickard,

1983; Ippen, 1966). Therefore the SWH pattern was discussed based on the SST,

SLP and wind field. The spatial fields of SWH and Wind computed from model

data whereas, SLP and SST were extracted from COADS climatology.

Figure 5.7: Monthly variation of spatially averaged SWH climatology of TNIO

(line with empty circles) and TSIO (line with filled circles). Blue line

represents the sun’s declination and red line represents mean monthly

position of the ITCZ.


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January:

The spatial average of SWH, wind vector, SLP and SST for the month of January

was given in Figure 5.8. The result shows that more than 50% of the region was

covered with heights less than 2m. Also low wave heights of less than 1 m were

observed as a narrow belt along northern boundary of Indian Ocean. Further low

wind conditions were seen around 10˚S latitude. The SW sectors of AS and BoB

were characterized by strong NE component of wind, which generates the

secondary maximum of wave field over north Indian Ocean. However, the

southern tropical Indian Ocean with strong sustained trade winds generates high

waves with magnitude between 2 to 2.5m. January is the peak period for northern

winter characterised by low SST over north Indian Ocean and relatively high SST

over south of the equator (Figure 5.8). A strong spatial gradient in SST and SLP

on either side of the equator drives the trade winds towards the equator.

According to the study conducted by Waliser & Gautier (1993), the ITCZ reaches

its southern most end at around 5˚S and stays upto February. As the ITCZ

accelerates the trade winds and a high pressure gradient over TSIO further

strengthen the wind field. This is the reason why the maximum wave heights (>

2.0 m) exists over TSIO (Figure 5.8-a). More than 50% of the region is covered

with heights less than 2m. Low wave heights of less than 1 m exist as a narrow

belt along the northern boundary of Indian Ocean.


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Figure 5.8: Spatial distribution of (a) SWH, (b) wind, (c) sea level pressure and (d)

sea surface temperature climatology for January month.

February:

The reduction of winds over the TNIO results in very calm sea state condition

with wave heights <1.5m for entire AS and BoB (Figure 5.9). The wind

direction is almost similar to that of January over the entire region with slight

higher winds off Somali coast. The increased pressure gradient over TSIO

causes enhancement of wind speed with SE component on east and easterly

component on west. This causes the occurrence of waves with >2.5 m over

western region of TSIO. By this month, as the sun starts retreating from the

tropic of Capricorn, the ITCZ shifts towards north. This causes the reduction

of surface pressure gradient over TNIO and increase of pressure gradient over

SIO. The mean monthly SWH over TNIO (TSIO) is comparatively low


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(high) than that of January. The monthly variation of SWH over TNIO and

TSIO are in opposite phase with each other from November to March (Figure

5.7). This occurs due to the weak winter monsoon signal over TNIO cause

generation of waves with moderate to high amplitudes.


sea surface temperature climatology for February month.

March:

March is the month of least mean monthly SWH (0.95 m) of TNIO (Figure

5.7). Wave heights over BoB and AS were in similar pattern as February

month, whereas the high wave heights of 2.5 to 3 m extends to southeast region

(Figure 5.10). Development of a low pressure due to increased heating over

India starts in March itself, with slightly higher pressures over the AS and the

BoB. During this period northeast monsoon starts decaying by the spreading of

warm SST towards north which results comparatively uniform low pressure


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over entire TNIO. This causes the further reduction of wind speed over TNIO.

The enhancement of pressure gradient over TSIO causes further enhancement

of wind speed and with slight increase in SWH.


sea surface temperature climatology for March month.

April:

Because of the weak pressure gradient over TNIO the wind fields were more or

less same as in March with a small increase in SWH. However, comparatively

high winds exist over TSIO due to sustained gradients in SST and SLP. Wave

heights over BoB were high (1 to 1.5 m) compared to AS (0.5 to 1 m) during

this month (Figure 5.11). The region of wave heights between 2.5 to 3 m

extends further north upto 15˚S. The global land lows have begun establishing

themselves along about 10˚ N in North Africa and about the Tropic of Cancer

in the Indian region and Burma. By this month the sun’s lies over the northern


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hemisphere causes the ITCZ migrates further north and the mean position was

more or less over the equator. This makes the heating over land more

prominent to the north of 20˚N and hence the axis of the low was at more

northerly latitude over India. A ridge runs from Arabia into the west Arabian

Sea where a clock wise wind circulation was found around 14˚N, 60˚E. A

similar circulation was also present over the BoB around 14˚N, 90˚E. The sub-

tropical high in the TSIO is along 30˚S.


sea surface temperature climatology for April month.

May:

A large shift in mean monthly SWH from 1 m (April) to 1.6 m (May) show the

increased amplitudes of wave due to enhanced wind forcing over TNIO.

However, there was gradual increase in SWH over TSIO with significant cross

equatorial flow over western region. The enhanced gradients cause further


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strengthening of the winds over TSIO. Large fetch over TSIO generates waves

with SWH between 3 to 3.5 m (Figure 5.12). During this period the wave

heights over BoB were high compared to AS. By May, the summer continental

low pressure areas completely dominate North Africa and Asia. Its main centre

over India was near 30˚N, 75˚E with an extension as a trough upto Orissa.

Close to the equator, in the BoB and east of 70˚E in the AS, there was a

substantial percentage of occasions when sea surface temperature warmer than

the surface air temperature by more than 1˚C. In such cases, the warm sea may

cause convection in the overlying air and lead to the moist air mass building up

in depth. Whether spell of monsoon activity is related to the sea surface being

warmer than the air mass requires to be studied. If there was any such

relationship, bursts of air at high speed from across the equator with rather low

air temperatures, perhaps from high latitudes in the southern hemisphere, may

be most suited for convective modification in the east AS. The warm SST over

TNIO and the high surface air temperatures over India cause increase in

pressure gradient over TIO (Rao and Raghavendra, 1967). Thus there can be

substantial contribution of air flow from across the equator with SE wind

component over TSIO and SW wind component over TNIO.


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sea surface temperature climatology for May month.

June:

During this month a low wave region of ocean state with magnitudes of about 2

m was observed over the equatorial region (5˚N to 5˚S) and very rough sea

state was seen on either side of this region. The sea state condition over AS

was rough with high waves compared to BoB. However, the mean monthly

SWH of TSIO was also higher than that of TNIO. At this month, the sun

reaches its northern most end called Topic of Cancer and causes raise in

surface air temperature. Subsequently the low heat region was significant with

the main centre over Pakistan. These low pressure systems were responsible for

well establishment of ITCZ at its northern most end. As this is the month of

summer solstice a fully developed warm SST region with temperatures

between 29 to 30oC, over northern most Indian Ocean, causes the formation of

low pressure region with closed isobars. These strong gradients accelerate the


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wind with large amplitudes up to 14 m/s (Figure 5.13). This high wind forcing

was responsible for the generation and growth of waves with large amplitude

of mean monthly SWH of 2.4 m and 2.9 m over TNIO and TSIO respectively.

Compared to the month of May, a large variation in ocean state condition exists

over entire TIO in terms of wind speed and wave heights. High gradient of

SST and SLP over TSIO drives the SE trade winds with a magnitude of 8 to 10

m/s and undergoes strong cross equatorial flow and become SW over TNIO.

The work of Findlater (1969a; 1969b) identified the manner of feed from the

south of the equator into the Indian Southwest monsoon in the Arabian Sea.

Apart from the flow at the surface into the northern hemisphere, strong flow

from south with a mean speed of about 15.5 m/s at the equator prevails over

eastern Africa. This mean flow across equator from 35˚E to 75˚E, from surface

to 600 hPa amounts to 77x1012

metric ton per day. In the earlier studies also

reported total mean flow of 16.2 x 1012

metric ton per day in lower troposphere

(Rao, 1964). As speeds between 20.6 m/s and 51.4 m/s were frequently reached

at one point or the other in this cross-equatorial flow below three km, Findlater

calls it ‘low-level jet stream near the equator’. Low level southeasterly jet

streams flow intermittently from the vicinity of Mauritius, over the northern tip

of Madagascar, to reach Kenya coast as southerlies. Sometimes, this

southeasterly jet from Mauritius, joined by or even replaced temporarily by low

level jet streams moving northward through the Mozambique Channel after

bursts of cooler air around the tip of southern Africa. The jet stream was not

always a single core but made up of a series of segments. This cross equatorial

winds continues towards north and accelerates further off Somali region with

magnitudes of about 12 to 14 m/s called as ‘Somali Jet’ (Rao, 1976; Rao, 2002;


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Gadgil, 2003). Waves with magnitude of 3.5 to 4 m exists at the core of the Jet

over AS (Figure 5.13).


sea surface temperature climatology for June month.

July:

As per the entire TIO is concern July was characterized by very rough sea

conditions with high wind (Fig. 5.14(b)) and wave magnitudes among all the

months. Except around Madagascar region Wave heights with 3.5 to 4 m

exists over SIO. Hence, this month was known to be roughest month over the

entire annual cycle. The monsoon activity was maximum in July when the low

pressure area extending from North Africa to North-East Siberia. Its main

centre was over north Baluchistan and neighborhood. A trough lies over north

India with axis from Sriganganagar to the Head Bay, which was referred to as

the ‘monsoon trough’. Pressure gradient was strong south of this trough. The


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Indian Ocean ‘High’ strengthened and was centred at about 30˚S, 60˚E. The

SLP continuously decreases over the Indian Ocean northwards of this high

pressure belt. Weak ridges were present in the Arabian Sea off the west coast

of India and in the Bay of Tennaserim coast and over Burma. The weak trough

of the pre-monsoon months, in the eastern Peninsula, now lies just off the east

coast of the south Peninsular India whichpersists through the monsoon months.

A fully developed Somali jet can be observed (Fig 5.14(b)) with highest wind

speed of 16 m/s at the core. The observations made during International Indian

Ocean Expedition (IIOE,1958) shows that the sea surface temperatures were

least (23 -24 ˚C) along and off the coasts of Africa (north of the equator) and

Arabia (except in the Gulf of Aden), owing to strong upwelling due to strong

winds parallel to the coast. The SST increases to 31˚C in the Gulf of Aden and

towards east in the Arabian Sea (28 – 29 ˚C) between 65˚ and 75˚ E, after

which there is slight decrease of SST (1˚C) upto the west coast of the Indian

peninsula (south of 20˚N) (Rao, 1976). In the AS there was more or less an

east to west gradient of SST, with maximum gradient to the west of 60˚E. In

the BoB, very little west to east gradient was observed towards north of 9˚N.

These gradients drive strong winds to blow over the surface and responsible for

generation of large spectrum of waves (from 0.5m to 3.5 m, as shown in figure

5.14). A fully developed Somali Jet over western AS was responsible for

comparatively very rough conditions over AS (Rao, 1976; Rao, 2002; Gadgil,

2003). Bunker (1965) traced a low level jet off Somalia and then across the

central parts of the AS to the coast of India, decreasing in speed progressively

to east. A maximum speed of about 25.7 m/s was attained at the top of a 1000

m thick layer of air, cooled by contact with cold upwelling water, which may

have as low a temperature as 13˚C. The jet was a result of the strong pressure


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gradient at right angle to the Somalia coast, between the air over the heated

land and cold off-shore waters. The upwelling and very low sea surface

temperatures (Fig. 5.14(d)) were due to the strong wind transporting large

quantities of surface water eastwards. The jet crossing equator pointed out by

Findlater (1969a), Bunker’s jet off Somalia and the low level jet over the

peninsula brought out by Joseph & Raman (1966) may all the different

segments of the same feature. As the colder air warmed during its further

passage over the Arabian Sea, the horizontal temperature gradient decreases

and the jet’s maximum value decreases progressively to east, dropping to less

than 15.4 m/s near India. These strong wind fields were responsible for the

existence of high wave with amplitudes 4 to 4.5 m over western AS and 3 to

3.5 m over eastern AS.

Figure 5.14: Spatial distribution of (a) SWH, (b) wind, (c) sea level pressure and

(d) sea surface temperature climatology for July month.


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August:

The weakening of cross equatorial flow and Somali Jet causes major changes

in wave field over western parts of TIO. Due to these reduced wind fields the

mean monthly SWH over TNIO reduced to 2.3 m and over TSIO, it was 3.0m

(Figure 5.15). A large region of high waves with mean monthly SWH of 3.5 to

4 m exists over central region of TSIO. In August, the intensity of the Afro-

Asian pressure low decreases. The waters off the coasts of North Africa and

Arabia are colder by 1˚C and the maximum temperature in the east Arabian

Sea, north of 15˚N and along 70˚E, is less marked. South of 10˚N and east of

60˚E, rather waters develop. The spatial pattern of maritime pressure fields

over TIO is more or less similar to that of July.


(d) sea surface temperature climatology for August month.


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September:

The sustained pressure gradient over TSIO drives the SE trade winds with

magnitudes almost similar to that of July. In figure 5.16, it can be observed

that the decrease of SWH over TSIO during this month was not so significant

when compared to TNIO. Because of the retrieval of southwest monsoon, the

wind speed over north Indian Ocean start decreases and a minimum wind speed

of 4 to 6 m/s exist over central and east equatorial Indian Ocean.

Comparatively high wave heights exist over BoB because of the intrusion of

swell generated over south-east Indian Ocean. By September, the sun retreats

completely from northern hemisphere and reaches over the equator, hence

ITCZ also starts migrating towards south. The high pressure over Asia (north

of 40˚N) and the Afro-Asian low was oriented east-west. The pressure gradient

south of monsoon trough which is maximum in July and slightly less in June

and August, and September. The ridge off the west coast was displaced in

September to about 65˚E. The SST pattern during this month is slightly similar

to August, but due to the increase in SLP over Indian continent, the north-south

gradient decreases. The well marked decrease of pressure gradients over TNIO

causes the SW winds to decrease (Rao, 1976). This results in decrease of mean

monthly SWH over TNIO to 1.8 m.


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(d) sea surface temperature climatology for September month.

October:

The wind speed over TNIO weaken and ranges from 4 to 6 m/s. The mean

monthly SWH over TNIO was reduced to 1.3 m, which was half of the SWH

(2.6 m) during July (Figure 5.17). Due to sustained wind speeds of magnitudes

6 to 10 m/s over TSIO causes for the occurrence of mean monthly SWH of 2.5

m. Even though the strong wind existed, the sea state over TSIO was calm

compared to July. This is due to the sustained trade winds over TSIO for entire

length of the year with very low seasonality compared to TNIO. Since October

was the period of transition for seasonally reversing winds from southwest to

northeast direction over north Indian Ocean. Due to decrease in wind forcing

the wave heights over entire Indian Ocean start decaying and reaches a

magnitude of less than three meters over TNIO.


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(d) sea surface temperature climatology for October month.

By October, the trough over northern India shifts to the Bay of Bengal, with the

trough line along 13˚N and the pressure field is flat over the country. The low

pressure belt runs from Africa to the west Pacific between the equator and

20˚N with centres over Africa, the BoB and the west Pacific. The Asian ‘High’

was establishing along 50˚N and was centered at about 90˚E (Rao, 1976). This

spatial variability of pressure and wind fields were responsible for variability in

SWH spatial distribution

November:

The mean monthly SWH (1.2 m) for November over TNIO during this month

was slightly lower than that of October. Very low wind speeds 2 to 6 m/s exist

over equatorial Indian Ocean with moderate wind conditions on either side.


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The region of low wave heights (i.e., > 1.5 m) extend further south, which was

south of equator for all along Indian longitudes except over BoB region. High

sea state condition exist over central and eastern region of TSIO associated

with magnitudes of 2.5 to 3 m (Figure 5.8(a)). Since the sun reaches very near

to Tropic of Capricorn, ITCZ also migrates further south. The development of

Siberian High at subtropics creates the weak pressure gradient decreasing

towards low latitude. The pressure gradient drive cold dry air towards low

latitude and trigger the northeast monsoon over India. Hence a weak

component of NE wind blow over the TNIO.


(d) sea surface temperature climatology for November month.


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December:

The wind speed ranges between 6 to 8 m/s distributes over most of the TNIO,

particularly off Somali coast the fetch spreads over large area and causes the

waves with 1.5 to 2 m monthly mean. However, as per the entire TNIO was

concern, the mean spatial wave height (1.2 m) was more or less similar to

November (Figure 5.19). The wind field over TSIO weaken by this month due

to decreased pressure gradients. The mean monthly SWH over TSIO decreases

(2.0 m) compared to the month of November. The Siberian High over

subtropics further strengthen and results in increase of surface pressure

gradients over TNIO. The sun reaches its northern most end and ITCZ lies

over equator. The pattern of SST and SLP is similar to that of January, which

were characterized by the region of warmest SST and low SLP over its

southern most extremes.


(d) sea surface temperature climatology for December month.


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5.5 Seasonality of wave climate:

Apart from the daily and monthly climatology discussed in earlier sections, an

attempt has been also made to understand the seasonality of the wave climate. The

results were presented along two meridional transects (65˚ E and 90˚ E), three

zonal transects (15˚ N, equator and 15˚ S). Further Principal Component Analysis

(PCA) of the annual cycle was also carried out. The latitudinal variation of SWH

monthly climatology at 65˚E longitude and 90˚E longitude is shown in figure

5.20. There were three major features of the wave climate were observed. First

one was a clear decreasing gradient of SWH towards north for all months except

during July at 65˚E. This was due to the presence of Somali Jet. A large fetch and

sustained winds for the entire annual cycle over TSIO was responsible for the

generation of high waves and propagates towards north with decreasing

magnitude. However, the sea state over TNIO was dominant during summer

monsoon with strong wind forcing. A study conducted by Chandramohan et al.

(1991) also reveals that the wave heights during summer monsoon were

comparatively high than other months. The second feature was, a clear

classification of annual cycle into three seasons i.e., summer monsoon (JJA), one

month each of pre and post monsoon (May and Sep) and rest of the year

(ONDJF)(figure 5.20). This classification was particularly significant over TNIO

(i.e., over AS and BoB) due to seasonal reversal of winds. Kesavadas et al. (1979)

were analyzed the wave data collected from ship borne wave recorder over eastern

AS (Lakshadweep) in 1978. They classified the wave data in to two seasons (June

to September and October to May) based on magnitude of wave height and period.

The third feature was the contrasting sea state condition over TNIO. The sea state

over AS was rough compared to BoB, from May to September, which was due to


108

strong summer monsoon signal over AS also due to the presence of Somali Jet

over western AS.

Figure 5.20: Latitudinal variation of monthly climatology of SWH at 65ºE and 90ºE.

The longitudinal variation of SWH monthly climatology at 15˚N, Equator and

15˚S are shown in figure 5.21. The wave heights were maximum over central

parts of the AS and BoB. Further, waves with more than 2.5m height were also

encountered over central Arabian Sea during summer monsoon and were high

compared to BoB. This was due to the existence of low SST, high salinity and

strong winds over Arabian Sea compared to BoB (Shenoi, Shankar and Shetye,

2002; Sprintall and Tomezak, 1992; McCreary, Kundu and Molinari, 1993; Schott

and McCreary, 2001). Almost similar wave heights were observed along Equator

for all months. The wave heights were slightly lower on western region of TSIO

(at 15˚S). Moderate wave conditions were also seen over entire Indian Ocean

during two months of pre and post monsoon (May and Sep) as compared to other

seasons, whereas low wave conditions were seen during the rest of the year

(Figure 5.21). Hence, the seasonality of SWH was clearly reflected in the latitude

and longitudinal transects. As earlier studies (Kesavadas, Varkey and Ramaraju,


109

1979; Sathe, Somayajulu and Gopalakrishna, 1979; Chandramohan, Sanil Kumar

and Nayak, 1991; Vethamony et al., 2006) suggested, the SWH during summer

monsoon was high compared to other seasons. Particularly in the present

transactional plots there were three different conditions of ocean state over TNIO

was observed. Whereas, over TSIO the seasonal variation was comparatively less

significant.

Figure 5.21: Longitudinal variation of monthly climatology of SWH at 15ºN,

Equator and 15ºS.

5.5.1 PCA of SWH Monthly Climatology

A Principal Component Analysis (PCA) was used to deduce the SWH annual

cycle and the principal characteristic of the wave regimes. This statistical

technique (Von Storch and Zwiers, 1999) allow to discriminate the meaningful


110

component and the “noise”, which can be considered irrelevant in the process

description and understanding. The results show that most of the variability was

described by the first two PCs, accounting for 91% and 5% of the total variance,

respectively. The first two PCs present a clear annual cycle (Fig 5.22). The first

component splits the year into a period (May to September) characterized by

waves produced by the summer monsoon winds and another signal (October to

April) corresponding to those produced by winter monsoon winds. The second PC

splits the year in four parts: the positive phase characterizes the periods Nov-Feb

and Jun-Aug, while the negative one characterizes the periods Mar-May and Sep-

Oct. Hence, the average behavior of the two components suggest to consider four

seasons: summer monsoon (Jun-Aug) with waves generated by summer monsoon

winds, winter monsoon (Dec-Feb) with wave generated by Winter monsoon winds

and transition periods (Mar-May and Sep-Nov).

Figure 5.22: First and second PCs of SWH annual cycle.


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5.6 Summary

The chapter presents detailed discussion on daily and monthly climatology of

SWH computed from model data. The spatially averaged daily climatology of

SWH for different sectors of Indian Ocean i.e., AS (40˚-77˚E; 5˚-25˚N), BoB

(77˚-100˚E; 5˚-25˚N) and Tropical South Indian Ocean (TSIO) (5˚N-30˚S; 40˚-

100˚E) was analyzed. The annual cycle of TIO was dominated by high wave

conditions during southwest monsoon, low wave conditions during winter

monsoon and moderate wave conditions during transition periods. Due to high

contrasting seasonality of winds over AS, the wider range (0.5 m to 3.5 m) of

wave heights were observed. For BoB, SWH daily climatology ranged from 0.5 m

to 2.5 m. The southern part of Indian Ocean covering from equator to 30˚S was

characterized by high waves compared to both AS and BoB. Monthly climatology

of SWH over TNIO showed strong seasonality with primary maximum during

summer monsoon and secondary maximum during winter monsoon. The seasonal

signal over TSIO consists of single maximum during summer monsoon. The

highest sea state condition over entire TIO was observed during July and lowest

during transition months (i.e., Mar and Nov). Another important observation was

that the monthly mean SWH over TSIO was greater than that of TNIO for entire

annual cycle due to strong sustained SE trade winds. There were three major

features of the wave climate observed in latitudinal transects. First one was a

clear decreasing gradient of SWH towards north for all months except during July

at 65˚E. The second feature was, a clear classification of annual cycle into three

seasons i.e., summer monsoon (JJA), one month each of pre and post monsoon

(May and Sep) and rest of the year (ONDJF). This classification was particularly

significant over TNIO (i.e., over AS and BoB) due to seasonal reversal of winds.


112

The third feature was the contrasting sea state condition over TNIO. The sea state

over AS was rough compared to BoB, from May to September, which was due to

strong summer monsoon signal over AS also due to the presence of Somali Jet

over western AS. The wave heights were maximum over central parts of the AS

and BoB. The average behavior of the two components of PCA suggest to

consider four seasons: summer monsoon (Jun-Aug) with waves generated by

Summer monsoon winds, winter monsoon (Dec-Feb) with wave generated by

Winter monsoon winds and transition periods (Mar-May and Sep-Nov).

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