• Aboobacker, V. M., , Vethamony P. , , and Rashmi R. , 2011: “Shamal” swells in the Arabian Sea and their influence along the west coast of India. Geophys. Res. Lett., 38, L03608, doi:10.1029/2010GL045736.

    • Search Google Scholar
    • Export Citation
  • Barstow, S. F., , and Kollstad T. , 1991: Field trials of the directional waverider. The Proceedings of the First (1991) International Offshore and Polar Engineering Conference, J. S. Chung, M. Isaacson, and H. Maeda, Eds., Vol. III, International Society of Offshore and Polar Engineers, 55–63.

  • Barth, H. J., 2001: Characteristics of the wind regime north of Jubail, Saudi Arabia, based on high resolution wind data. J. Arid Environ., 47, 387402.

    • Search Google Scholar
    • Export Citation
  • Haver, S., 1980: Analysis of uncertainties related to the stochastic modelling of ocean waves. Division of Marine Structures, Norwegian Institute of Technology, Rep. UR-80-09, 187 pp.

  • Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77, 437471.

  • Kuik, A. J., , Vledder G. , , and Holthuijsen L. H. , 1988: A method for the routine analysis of pitch and roll buoy wave data. J. Phys. Oceanogr., 18, 10201034.

    • Search Google Scholar
    • Export Citation
  • Kumar, V. S., , Pathak K. C. , , Pednekar P. , , and Gowthaman R. , 2006: Coastal processes along the Indian coastline. Curr. Sci., 91, 530536.

    • Search Google Scholar
    • Export Citation
  • Kumar, V. S., , Philip S. , , and Nair T. N. B. , 2010: Waves in shallow water off west coast of India during the onset of summer monsoon. Ann. Geophys., 28, 817824.

    • Search Google Scholar
    • Export Citation
  • Membery, D. A., 1983: Low level wind profiles during the Gulf shamal. Weather, 38, 1824.

  • Neetu, S., , Shetye S. R. , , and Chandramohan P. , 2006: Impact of sea breeze on wind-seas off Goa, west coast of India. J. Earth Syst. Sci., 115, 20312038.

    • Search Google Scholar
    • Export Citation
  • Perrone, T. J., 1979: Winter shamal in the Persian Gulf. Naval Environmental Prediction Research Facility Tech. Rep. TR 79-06, 180 pp.

  • Pierson, W. J., Jr., , and Moskowitz L. , 1964: A proposed spectral form for fully developed wind seas based on the similarity theory of A. A. Kitaigoirodskii. J. Geophys. Res., 69 (24), 51815190.

    • Search Google Scholar
    • Export Citation
  • Portilla, J., , Ocampo-Torres F. J. , , and Monbaliu J. , 2009: Spectral partitioning and identification of wind sea and swell. J. Atmos. Oceanic Technol., 26, 117122.

    • Search Google Scholar
    • Export Citation
  • Rao, P. G., , Hatwar H. R. , , Al-Sulaiti M. H. , , and Al-Mulla A. H. , 2003: Summer shamals over the Arabian Gulf. Weather, 58, 471477.

  • Reynolds, R. M., 1993: Physical oceanography of the Persian Gulf, Strait of Hormuz, and the Gulf of Oman—Results from the Mt. Mitchell expedition. Mar. Pollut. Bull., 27, 3559.

    • Search Google Scholar
    • Export Citation
  • Verhoef, A., , Portabella M. , , and Stoffelen A. , 2012: High-resolution ASCAT scatterometer winds near the coast. IEEE Trans. Geosci. Remote Sens., 50, 24812487.

    • Search Google Scholar
    • Export Citation
  • View in gallery

    Study area showing the wave measurement buoy location. The depth contours are in meters.

  • View in gallery

    Composite average map of wind speed and wind direction during weak southwest winds and strong summer shamal (northwest) winds in (a) 2010 and (b) 2011, and during strong southwest winds and weak summer shamal (northwest) winds in (c) 2010 and (d) 2011.

  • View in gallery

    ASCAT wind direction and wind speed at the Persian Gulf during (a) 2010 and (b) 2011.

  • View in gallery

    Measured AWS wind direction and wind speed at Ratnagiri during (a) May and (b) June 2010. The meteorological convention is used for presenting the data (0° and 360° for wind from the north, 90° from the east, 180° from the south, and 270° from the west).

  • View in gallery

    ASCAT wind direction at the Persian Gulf and Ratnagiri along the west coast of India from 20 May to 19 Jul 2011.

  • View in gallery

    (a) Wave direction, (b) significant wave height, and (c) wave period of sea, swell, and resultant of northwesterly summer shamal swells in 2010. Peak is the dominant direction value without separating into sea and swell. Sea and swell indicate the wave characteristics of the sea and swell components. Resultant is the estimated value without separating into sea and swell.

  • View in gallery

    As in Fig. 6, but for 2011. Peak is the dominant direction value without separating into sea and swell. Sea and swell indicate the wave characteristics of sea and swell component. Resultant is the estimated value without separating into sea and swell.

  • View in gallery

    Wave direction contours on a frequency–time domain for May (top) 2010 and (bottom) 2011.

All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 6 6 3
PDF Downloads 2 2 1

Observational Evidence of Summer Shamal Swells along the West Coast of India

View More View Less
  • 1 Ocean Engineering, CSIR-National Institute of Oceanography, CSIR, Dona Paula, Goa, India
  • 2 Ocean Science and Information Services Group, Indian National Centre for Ocean Information Services, Ministry of Earth Sciences, Hyderabad, India
  • 3 Ocean Engineering, CSIR-National Institute of Oceanography, CSIR, Dona Paula, Goa, India
© Get Permissions
Full access

Abstract

Wave data collected off Ratnagiri, which is on the west coast of India, in 2010 and 2011 are used to examine the presence of the summer shamal swells. This study also aims to understand variations in wave characteristics and associated modifications in wind sea propagation at Ratnagiri. Wind data collected using an autonomous weather station (AWS), along with Advanced Scatterometer (ASCAT) and NCEP data, are used to identify the presence of summer shamal winds along the west coast of the Indian subcontinent and on the Arabian Peninsula. NCEP and ASCAT data indicate the presence of summer shamal winds over the Arabian Peninsula and northwesterly winds at Ratnagiri. This study identifies the presence of swells from the northwest that originate from the summer shamal winds in the Persian Gulf and that reach Ratnagiri during 30% of the summer shamal period. AWS data show the presence of northwest winds during May and southwest winds during the strong southwest monsoon period (June–August). Another important factor identified at Ratnagiri that is associated with the summer shamal events is the direction of wind sea waves. During the onset of the southwest monsoon (May), the sea direction is in the direction of swell propagation (northwest); however, during the southwest monsoon (June–August), a major part of the wind sea direction is from the southwest. The average occurrence of summer shamal swells is approximately 22% during the southwest monsoon period. An increase in wave height is observed during June and July at Ratnagiri due to the strong summer shamal event.

CSIR-National Institute of Oceanography Contribution Number 5256.

Corresponding author address: V. Sanil Kumar, Principal Scientist, Ocean Engineering, CSIR-National Institute of Oceanography, CSIR, Dona Paula, Goa 403 004 India. E-mail: sanil@nio.org

Abstract

Wave data collected off Ratnagiri, which is on the west coast of India, in 2010 and 2011 are used to examine the presence of the summer shamal swells. This study also aims to understand variations in wave characteristics and associated modifications in wind sea propagation at Ratnagiri. Wind data collected using an autonomous weather station (AWS), along with Advanced Scatterometer (ASCAT) and NCEP data, are used to identify the presence of summer shamal winds along the west coast of the Indian subcontinent and on the Arabian Peninsula. NCEP and ASCAT data indicate the presence of summer shamal winds over the Arabian Peninsula and northwesterly winds at Ratnagiri. This study identifies the presence of swells from the northwest that originate from the summer shamal winds in the Persian Gulf and that reach Ratnagiri during 30% of the summer shamal period. AWS data show the presence of northwest winds during May and southwest winds during the strong southwest monsoon period (June–August). Another important factor identified at Ratnagiri that is associated with the summer shamal events is the direction of wind sea waves. During the onset of the southwest monsoon (May), the sea direction is in the direction of swell propagation (northwest); however, during the southwest monsoon (June–August), a major part of the wind sea direction is from the southwest. The average occurrence of summer shamal swells is approximately 22% during the southwest monsoon period. An increase in wave height is observed during June and July at Ratnagiri due to the strong summer shamal event.

CSIR-National Institute of Oceanography Contribution Number 5256.

Corresponding author address: V. Sanil Kumar, Principal Scientist, Ocean Engineering, CSIR-National Institute of Oceanography, CSIR, Dona Paula, Goa 403 004 India. E-mail: sanil@nio.org

1. Introduction

Waves are the dominant factor influencing the near-shore processes. The waves along the west coast of the Indian subcontinent primarily depend on the wind conditions prevailing over the three different seasons: southwest monsoon (June–September), northeast monsoon (October–January), and premonsoon (February–May). The general wave conditions in the Arabian Sea during the premonsoon period also depend on the swells coming from the far northwest Arabian Sea because of the northwesterly blowing shamal winds (Aboobacker et al. 2011). According to Aboobacker et al. (2011), northwest waves are observed along the west coast of India with mean periods ranging between 6 and 8 s. These waves are due to the strong northwesterly winds blowing over the Arabian Peninsula and the northwestern Arabian Sea, and result in an increase in wave height, a decrease in swell period, and common northwest direction during the northeast monsoon and early premonsoon seasons. During the northeast monsoon and early premonsoon seasons, the maximum significant wave height observed is 3.5 m near the Arabian Peninsula and 2 m along the west coast of India. Kumar et al. (2010) studied the characteristics of swells and wave growth during the onset of the summer monsoon. Coastal processes along the Indian subcontinent are a function of wave parameters, such as wave height, wave period, and wave direction. During the Indian summer monsoon (southwest monsoon), the dominate swells are from the southwest because of the strong southwest wind. During the Indian winter monsoon (northeast monsoon) and the calm premonsoon season, the swells observed along the west coast of the Indian subcontinent are from the west and west-northwest and the southwest, respectively, because of the weak northeast winds. The shift in wave direction from southwest to northwest will change the direction of the longshore drift from south to north along the west coast of India. The present study examines the presence of summer shamal waves and winds in the near-shore region of the Indian west coast. The variation in the existence of summer shamal swells and wind sea propagation along the west coast of India is studied during the late premonsoon (May) and strong Indian summer monsoon (June–August) seasons.

The most well-known weather phenomenon in the Persian Gulf is the shamal, a northwest wind that occurs year-round (Perrone 1979). The shamal is the only persistently strong wind in the region that can last for several days and has winds that can reach strong to gale force over the open sea and routinely produce wind waves of 3–4 m in height. According to Barth (2001), a high-energy summer shamal regime occurs from June to August, with a complex transition phase in May, in the Arabian region. A moderate eastern spring phase occurs in April. Membery (1983) found that the summer shamal is persistent over Iraq and the Gulf during the summer from May to July. Rao et al. (2003) reported that 51% of shamal days occurred from May to July compared to the winter months of November–March. Aboobacker et al. (2011) used the February data collected from the west coast of India to identify the winter shamal events occurring on the Arabian Peninsula from November to March and the corresponding swells reaching the Indian west coast. The present study focuses on identifying the presence of summer shamal winds and swells along the west coast of India. This study will help in understanding the effect of summer shamal waves and winds on the near-shore regions of the Indian west coast. These winds are often strong during the day, but they decrease at night. The summer shamal events are longer in duration compared to the winter shamal events. Because of their longer duration, summer shamal events are also known as “40-day shamal.” The summer shamal is practically continuous from early June through July, and it is associated with the relative strengths of the Indian and Arabian thermal lows (Reynolds 1993).

2. Study area

The wind and wave data measured at Ratnagiri (Fig. 1) were used in the study. The measurement location lies along the west coast of the Indian subcontinent. The Arabian Peninsula and the Persian Gulf are located northwest of Ratnagiri. The wave climate of the Arabian Sea and the climate along the west coast of India are influenced by the monsoonal winds during the southwest monsoon with high wave activity. A relatively calm condition prevails during the rest of the year. The direction of approaching waves is west and west-southwest during the southwest monsoon, west and northwest during the northeast monsoon, and southwest during the fair-weather period (Kumar et al. 2006). The sea breeze has an impact on the diurnal cycle of the sea state along the west coast of India during the premonsoon season because of the weak winds (Neetu et al. 2006).

Fig. 1.
Fig. 1.

Study area showing the wave measurement buoy location. The depth contours are in meters.

Citation: Journal of Atmospheric and Oceanic Technology 30, 2; 10.1175/JTECH-D-12-00059.1

3. Data and methodology

Waves off Ratnagiri (16°58′48.324″N, 73°15′30.312″E) at a water depth of 13 m were measured using a moored Datawell Directional Waverider buoy DWR-MkIII (Barstow and Kollstad 1991). The Waverider buoy measures heaves in the range of −20 to +20 m and periods between 1.6 and 30 s, with a resolution of 1 cm in heave. The cross sensitivity of the heave is less than 3%. The measurement of the wave direction using DWR-MkIII is in the range of 0°–360° and a resolution of 1.5°, with an accuracy of 0.5° reference to the magnetic north. Data were recorded for 30 min at a frequency of 1.28 Hz every half hour from May 1 to August 31 in 2010 and 2011. The collected time series was subjected to standard error checks for spikes, steepness, and constant signals (Haver 1980). The wave data analysis was similar to that reported in Kumar et al. (2010). Wave spectra were obtained through fast Fourier transform (FFT). An FFT of eight series, each consisting of 256 measured vertical elevations of the buoy data, was added to obtain spectra with a high-frequency cutoff at 0.58 Hz and a resolution of 0.005 Hz. The significant wave height (Hs), or 4, and the mean wave period (Tm), or , were obtained from the wave spectrum, whereas mn is the nth-order spectral moment and is given by , n = 0, and 2, S(f) is the spectral energy density at frequency f. The period corresponding to the maximum spectral energy [i.e., spectral peak period (Tp)] was estimated from the wave spectrum. Peak wave direction (Dp) corresponding to the spectral peak was estimated based on circular moments (Kuik et al. 1988). The meteorological convention is used for presenting the wind and wave direction data (0° and 360° for wind/wave from the north, 90° from the east, 180° from the south, and 270° from the west).

Wind seas and swells from the measured data were separated using the method described by Portilla et al. (2009). Portilla et al. (2009) proposed a 1D separation algorithm based on the assumption that the energy at peak frequency of a swell system cannot be higher than the value of a Pierson–Moskowitz (PM) spectrum (Pierson and Moskowitz 1964) with the same peak frequency. The algorithm calculates the ratio γ* between the peak energy of a wave system and the energy of a PM spectrum at the same frequency. If γ* is above a threshold value of 1, then the system is considered to represent the wind sea; otherwise, it is taken to be swell. The wind sea and swell components of significant wave height, mean wave period, and mean wave direction were computed by integrating the respective spectral parts. Resultant or total is the estimated values, without separation into wind sea and swell.

Simultaneous wind measurements were carried out using an autonomous weather station (AWS) at 10-min intervals. The AWS measures the wind speed in the range of 0–60 m s−1 with an accuracy of 0.2 m s−1 and a direction from 0° to 360° with an accuracy of 3°. AWS data were used to analyze the wind pattern of Ratnagiri in 2010 during strong and weak summer shamal winds. Because of the unavailability of AWS wind data during 2011, these data were not used in the study. AWS wind data are useful for understanding the variation in wind speed and direction associated with the summer shamal wind, local wind system (sea breeze and land breeze), and southwest monsoon winds during strong and weak summer shamal events.

Reanalysis data of zonal and meridional components of wind speed at a height of 10 m, real-time observations at 6-h intervals from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) (Kalnay et al. 1996), and daily data from Advanced Scatterometer (ASCAT) wind (Verhoef et al. 2012) obtained over the Arabian Sea and the Persian Gulf were also used to analyze the wind pattern. The NCEP–NCAR data were provided by the National Oceanic and Atmospheric Administration (NOAA) Clouds and the Earth’s Radiant Energy System (CIRES) Climate Diagnostics Center in Boulder, Colorado. These data can be found online (at http://www.cdc.noaa.gov/). The ASCAT data are derived from the ASCAT on board the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) Meteorological Operation-A (MetOp-A) and are downloaded from the Physical Oceanography Distributed Active Archive Center (online at http://podaac.jpl.nasa.gov/dataset/ASCAT-L2-25km?ids=Sensor&values=ASCAT). The data are used to calculate the relative wind strength between the summer shamal wind at the Persian Gulf and the southwest winds off Somalia because of the daily average data with high spatial resolutions of 50 km. NCEP data are used to analyze and study the combined effect of northwest summer shamal winds and southwest Indian summer monsoon winds over the Arabian Sea. This analysis will give a good indication of the temporal variation of summer shamal events at the Persian Gulf and its propagation over the Arabian Sea.

4. Results and discussion

a. Wind pattern over the Arabian Sea

ASCAT wind data are used to study the presence of weak and strong shamal and southwest monsoon winds. For this purpose, we used the average wind speed and direction obtained from three boxes: 1) near the Arabian Peninsula (B1), 2) near the northeast region of the African continent (B2), and 3) near Ratnagiri (B3). The ratio between the daily average wind speeds near the Arabian Peninsula at B1 to the southwest monsoonal winds at B2 is used to estimate the relative strength of winds between the summer shamal and southwest monsoon winds. Occurrences of summer shamal swells at Ratnagiri depend on the comparative strengths of these wind systems. The combined effect of northwest shamal winds and southwest monsoon winds along the study region was examined based on the B1/B2 value and was characterized as summer shamal winds and weak southwest winds when the B1/B2 value was greater than 1.0 and as summer shamal winds and strong southwest winds when the B1/B2 was less than 1.0 for northwest winds blowing over the Persian Gulf region. The correlation coefficient between the shamal winds and winds at Ratnagiri during the shamal events is 0.16. This coefficient indicates the near absence of summer shamal winds at Ratnagiri due to the strong SW monsoon season in the Arabian Sea. NCEP data are used to study and analyze the change in wind pattern over the Arabian Sea near the west coast of India and over the Arabian Peninsula during the summer shamal event.

1) Summer shamal winds and weak southwest winds

Figure 2 depicts the composite average of the wind pattern over the Arabian Sea, under the combined effect of the strong northwest summer shamal and the weak southwest Indian summer monsoon winds, in 2010 and 2011. Six-hourly averages of NCEP data over the Arabian Sea during summer shamal winds and weak southwest monsoon winds (Figs. 2a and 2b) indicate the presence of northwest summer shamal winds over the Persian Gulf and the Arabian Peninsula. When the summer shamal dominates over the weak southwest monsoon, southwest winds from the southwestern Arabian Sea are deviated by the northwest winds from the Arabian Peninsula and propagate toward Ratnagiri as west-northwest winds. The change in wind direction depends on the strength of northwest winds, and it has enough strength to reach the west coast of India as northwest winds. This condition mainly prevails over the Arabian Sea during May.

Fig. 2.
Fig. 2.

Composite average map of wind speed and wind direction during weak southwest winds and strong summer shamal (northwest) winds in (a) 2010 and (b) 2011, and during strong southwest winds and weak summer shamal (northwest) winds in (c) 2010 and (d) 2011.

Citation: Journal of Atmospheric and Oceanic Technology 30, 2; 10.1175/JTECH-D-12-00059.1

2) Summer shamal winds and strong southwest winds

Figures 2c and 2d depict the composite average map of NCEP wind over the Arabian Sea during different periods of the strong southwest Indian summer monsoon. During this period with a strong southwest monsoon, shamal winds dominate the Arabian Peninsula and the Persian Gulf region (see Fig. 2). The wind pattern over the Arabian Sea is driven by the southwest monsoonal winds, with the small variation in the direction of northwest winds toward the north that is observed at the Arabian Peninsula. This phenomenon is due to the interaction between northwesterly and southwesterly winds and mainly prevails during June through August, when the B1/B2 value is less than 1.0.

b. Wind pattern over the Arabian Gulf

Figure 3 depicts the wind pattern over the Persian Gulf using the data derived from the wind scatterometer ASCAT. The wind direction during May and June indicates the presence of summer shamal (northwest winds) events. The summer shamal events persisted over the Persian Gulf for about 10 days (Fig. 3a) during 2010, whereas in 2011, the summer shamal events had a longer duration, starting in late May and persisting for more than 20 days (Fig. 3b). The wind direction also showed a more consistent pattern from the northwest side, indicating the summer shamal events at the Persian Gulf where the summer shamal swell originated. The correlation between the B1/B2 ratio and wind direction time series showed a positive correlation of 0.40 and 0.38, respectively, during the summer shamal period of 2010 and 2011.

Fig. 3.
Fig. 3.

ASCAT wind direction and wind speed at the Persian Gulf during (a) 2010 and (b) 2011.

Citation: Journal of Atmospheric and Oceanic Technology 30, 2; 10.1175/JTECH-D-12-00059.1

c. Winds along the west coast of India

The AWS wind data at Ratnagiri in May (Fig. 4a) show that during the onset of the southwest monsoon period (May), the wind is predominantly from the northwest. During this period, the northwest wind from the Arabian Peninsula shows the continuous arrival of wind, without the presence of a land breeze, during the night in the premonsoon (May). Aboobacker et al. (2011) observed the presence of northwest winds during the winter due to weak northeast winds. In May (premonsoon period), southwest winds are weak, and northwest winds are strong enough to affect the near-shore regions of Ratnagiri and cause wind sea from the northwest. AWS wind data during June (Fig. 4b) show that during this period, the wind is predominantly from the southwest, indicating the presence of southwest winds at Ratnagiri. This wind will cause the generation and propagation of wind sea either from the northwest or southwest, depending on the direction of the winds blowing over the near-shore area of Ratnagiri. Figure 5 indicates that a summer shamal wind over the Persian Gulf shows a wind propagation period of 2–3 days to reach the coast of Ratnagiri during peak time. The low correlation coefficient of 0.21 obtained between the wind direction at the Persian Gulf and the wave direction at Ratnagiri during the study period indicates that the wind direction at Ratnagiri is primarily influenced by the strong southwest monsoon seasonal winds in the Arabian Sea.

Fig. 4.
Fig. 4.

Measured AWS wind direction and wind speed at Ratnagiri during (a) May and (b) June 2010. The meteorological convention is used for presenting the data (0° and 360° for wind from the north, 90° from the east, 180° from the south, and 270° from the west).

Citation: Journal of Atmospheric and Oceanic Technology 30, 2; 10.1175/JTECH-D-12-00059.1

Fig. 5.
Fig. 5.

ASCAT wind direction at the Persian Gulf and Ratnagiri along the west coast of India from 20 May to 19 Jul 2011.

Citation: Journal of Atmospheric and Oceanic Technology 30, 2; 10.1175/JTECH-D-12-00059.1

d. Existence of summer shamal swells

Figures 6 and 7 depict the presence of northwest swells reaching the Indian coastline at Ratnagiri during the summer shamal period between May and August in 2010 and 2011. The northwest swells show stronger shamal events during the summer of 2011 than those in 2010 at the Arabian Peninsula. The measured waves during May 2010 (Fig. 6a) and 2011 (Fig. 7a) show a steady decrease in swell direction toward the Indian summer monsoon. This change in swell direction from northwest to west-northwest occurred during May and continued in the west-northwest direction in June and July. This continuation occurred because of the interaction of waves from the northwest swells produced by the summer shamal events and southwest swells, owing to the strong SW monsoon over the Arabian Sea. The presence of strong summer shamal swells in May is due to the weak southwest monsoon winds during the onset of the summer monsoon. After the summer monsoon, there is a decrease in the number of swells coming from the northwest sector because of the dominance of swells from the southwest Indian summer monsoon. The southwest swells have higher amplitudes compared to the northwest swells. During 2010 and 2011, an absence of northwest swells was observed during the weak northwest winds at the Arabian Peninsula due to the strong southwest winds of the Indian summer monsoon and the rough sea state of the Arabian Sea. The swells propagating from the Persian Gulf travel approximately 1200–1500 km to reach the Ratnagiri coast (Aboobacker et al. 2011). This will trigger a time difference between the arrival of summer shamal swells and winds at Ratnagiri. The correlation between Ratnagiri wind and waves is 0.61.

Fig. 6.
Fig. 6.

(a) Wave direction, (b) significant wave height, and (c) wave period of sea, swell, and resultant of northwesterly summer shamal swells in 2010. Peak is the dominant direction value without separating into sea and swell. Sea and swell indicate the wave characteristics of the sea and swell components. Resultant is the estimated value without separating into sea and swell.

Citation: Journal of Atmospheric and Oceanic Technology 30, 2; 10.1175/JTECH-D-12-00059.1

Fig. 7.
Fig. 7.

As in Fig. 6, but for 2011. Peak is the dominant direction value without separating into sea and swell. Sea and swell indicate the wave characteristics of sea and swell component. Resultant is the estimated value without separating into sea and swell.

Citation: Journal of Atmospheric and Oceanic Technology 30, 2; 10.1175/JTECH-D-12-00059.1

Contours of wave direction in a frequency–time domain (Fig. 8) during May of 2010 and 2011 show the northwest swells reaching the near-shore area of Ratnagiri. Periods of northwest swells ranged between 6.5 and 11 s, and the swell direction was up to 315°. The increase in swell period is associated with the decrease in swell direction due to the onset of the strong Indian southwest monsoon season. Figure 8 also shows the presence of long-period swells dominate in the range of less than 240° from the southwest. At the end of May, the swells from the southwest are dominant throughout the Arabian Sea because of the break in the summer shamal winds and the strong Indian summer monsoon.

Fig. 8.
Fig. 8.

Wave direction contours on a frequency–time domain for May (top) 2010 and (bottom) 2011.

Citation: Journal of Atmospheric and Oceanic Technology 30, 2; 10.1175/JTECH-D-12-00059.1

e. Wave characteristics

The measured waves at Ratnagiri in 2010 and 2011 from May to August show the presence of northwest swells during a particular interval, in contrast to the normal condition of southwest waves. The swells during May show an increase in wave height and a decrease in wave period similar to the winter shamal swells along the west coast of India (Aboobacker et al. 2011). The direction of the swell and resultant waves shows a shift in the wave direction toward the west from the northwest during June and July compared to May. The direction of the wind sea depends on the wind condition along the west coast of India. Therefore, the direction of the wind sea in the first half of May is from the northwest, and during the remaining period, the direction shifts from the northwest to the west and then to the southwest, depending on the strength of the winds reaching the west coast of the Indian subcontinent. During the strong southwest monsoon period (June–August), the direction of the wind sea is mainly from the southwest.

During the period when the summer shamal swells reach the west coast of India at Ratnagiri, the mean wave period is in the range of 3.7–7.7 s (Tables 1 and 2), with the mean swell period in the range of 6.6–11.2 s. Significant wave height varied between 0.7 and 1.7 m during May, and during the southwest monsoon period, significant wave height increased and varied between 1.4 and 3.4 m. This increase in wave height during June is also due to the strong northwesterly winds from late May to early July (Membery 1983). The swell direction is more than 270° and reaches up to 308° 30% of the time. However, the wind sea shows both a southwest and northwest direction, depending on the prevailing wind condition along the west coast of the Indian subcontinent.

Table 1.

Monthly variation of peak wave period (Tp), wave direction (Dp), mean wave period (Tm), significant wave height (Hs), swell direction (Dswell), sea direction (Dsea), significant swell height (Hswell), significant sea height (Hsea), swell period (Tswell), and sea period (Tsea) of northwest swells during the southwest monsoon (May–August) in 2010 measured off Ratnagiri at a depth of 13 m.

Table 1.
Table 2.

As in Table 1, but during the southwest monsoon (May–August) in 2011.

Table 2.

f. Interaction between southwest swells and northwest swells in the Arabian Sea

The propagation of swells from the NW shamal event is smooth, and continuous detection of these swells is possible along the west coast of India. The swell direction is well above 270°. However, during the southwest monsoon, the directions of observed northwest swells show a decrease in wave direction and are just above 270°. The decrease in wave direction from northwest to west-northwest is due to the interaction of swells between the northwest and southwest (Aboobacker et al. 2011). The observed direction of northwest swells in May clearly indicates the decrease in wave direction with the increase in Indian summer monsoon conditions.

g. Effect of summer shamal swells along the Indian coast

Because of the northwest–southeast inclination of the coastline, the swells propagating from the northwest will change the alongshore current and sediment transport regime toward the south from the prevailing northerly direction. This change in near-shore currents and the sediment transport from north to south depends on the strength of the shamal swells and shamal events at the Arabian Peninsula together with the prevailing summer monsoon winds over the Arabian Sea and propagating swells from the southwest. During the southwest monsoon, the wave heights are typically higher compared to the rest of the season. The observed wave data for shamal swells also show an increase in the wave height during the Indian southwest monsoon with corresponding high energy. Because of the increase in wave height, the amount of energy carried by these swells is large during June and July compared to winter shamal swells at the Indian coast. In May, the presence of a large number of waves coming from the northwest due to summer shamal winds will play an important role in the sea state of the northern Arabian Sea and near-shore circulation of the Indian coastline.

Table 3 shows the variation in the number of days of summer shamal swells observed at Ratnagiri along the west coast of India. Observed summer shamal swells show an increase during 2011 (43 days) compared to 2010 (30 days) due to the stronger summer shamal winds in 2011 than in 2010. The maximum observed summer shamal swell was in May and showed a decrease in number as the southwest monsoon reached the Indian subcontinent, and the observed summer shamal swell was minimized in July due to the strong southwest monsoon.

Table 3.

Monthly percentage variation of summer shamal swells observed at Ratnagiri during the southwest monsoon of 2010 and 2011.

Table 3.

5. Conclusions

Wave data measured off Ratnagiri during the time of summer shamal events (between May and August) in the Arabian Peninsula are analyzed, along with the ASCAT wind, NCEP data, and AWS wind collected at Ratnagiri. The presence of a northwest swell is identified at Ratnagiri along the west coast of India from the measured wave data. ASCAT data indicate the northwest winds in the Persian Gulf during the study period. An analysis of swell and wind sea direction indicates the presence of swells from the northwest direction during May and from the west-northwest direction during the remaining months. The prevailing wind condition in the Arabian Sea influences the generation of wind sea in the region during the arrival of northwest swells and shows the wind sea from the southwest direction. The mean period of shamal swells is in the range of 6.5–11.2 s during summer, whereas during winter, the mean wave period is in the range of 6–8 s. The significant wave height associated with the shamal swells is high, and it reaches a maximum of 3.4 m and is twice the wave height observed during the winter season. The passage of summer shamal swells through the Arabian Sea is dependent on the strength of the southwest Indian monsoon winds blowing over the Arabian Sea. The wind pattern over the west coast of India at Ratnagiri is driven by northwest winds in May. However, during the summer monsoon, the wind pattern depends on the strength of the monsoon winds from the southwest and summer shamal winds from the northwest.

Acknowledgments

The authors thank the director of CSIR-NIO and the director of INCOIS for providing facilities and encouragement. The authors also thank Mr. K. Ashok Kumar for his help during the wave data collection and Mr. G. Udhaba Dora for his help in preparing Fig. 2. We thank Prof. Ramola Antao, consultant, Senior Cambridge English examinations in India, for editing the manuscript. This work forms part of the Ph.D. thesis of the first author.

REFERENCES

  • Aboobacker, V. M., , Vethamony P. , , and Rashmi R. , 2011: “Shamal” swells in the Arabian Sea and their influence along the west coast of India. Geophys. Res. Lett., 38, L03608, doi:10.1029/2010GL045736.

    • Search Google Scholar
    • Export Citation
  • Barstow, S. F., , and Kollstad T. , 1991: Field trials of the directional waverider. The Proceedings of the First (1991) International Offshore and Polar Engineering Conference, J. S. Chung, M. Isaacson, and H. Maeda, Eds., Vol. III, International Society of Offshore and Polar Engineers, 55–63.

  • Barth, H. J., 2001: Characteristics of the wind regime north of Jubail, Saudi Arabia, based on high resolution wind data. J. Arid Environ., 47, 387402.

    • Search Google Scholar
    • Export Citation
  • Haver, S., 1980: Analysis of uncertainties related to the stochastic modelling of ocean waves. Division of Marine Structures, Norwegian Institute of Technology, Rep. UR-80-09, 187 pp.

  • Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77, 437471.

  • Kuik, A. J., , Vledder G. , , and Holthuijsen L. H. , 1988: A method for the routine analysis of pitch and roll buoy wave data. J. Phys. Oceanogr., 18, 10201034.

    • Search Google Scholar
    • Export Citation
  • Kumar, V. S., , Pathak K. C. , , Pednekar P. , , and Gowthaman R. , 2006: Coastal processes along the Indian coastline. Curr. Sci., 91, 530536.

    • Search Google Scholar
    • Export Citation
  • Kumar, V. S., , Philip S. , , and Nair T. N. B. , 2010: Waves in shallow water off west coast of India during the onset of summer monsoon. Ann. Geophys., 28, 817824.

    • Search Google Scholar
    • Export Citation
  • Membery, D. A., 1983: Low level wind profiles during the Gulf shamal. Weather, 38, 1824.

  • Neetu, S., , Shetye S. R. , , and Chandramohan P. , 2006: Impact of sea breeze on wind-seas off Goa, west coast of India. J. Earth Syst. Sci., 115, 20312038.

    • Search Google Scholar
    • Export Citation
  • Perrone, T. J., 1979: Winter shamal in the Persian Gulf. Naval Environmental Prediction Research Facility Tech. Rep. TR 79-06, 180 pp.

  • Pierson, W. J., Jr., , and Moskowitz L. , 1964: A proposed spectral form for fully developed wind seas based on the similarity theory of A. A. Kitaigoirodskii. J. Geophys. Res., 69 (24), 51815190.

    • Search Google Scholar
    • Export Citation
  • Portilla, J., , Ocampo-Torres F. J. , , and Monbaliu J. , 2009: Spectral partitioning and identification of wind sea and swell. J. Atmos. Oceanic Technol., 26, 117122.

    • Search Google Scholar
    • Export Citation
  • Rao, P. G., , Hatwar H. R. , , Al-Sulaiti M. H. , , and Al-Mulla A. H. , 2003: Summer shamals over the Arabian Gulf. Weather, 58, 471477.

  • Reynolds, R. M., 1993: Physical oceanography of the Persian Gulf, Strait of Hormuz, and the Gulf of Oman—Results from the Mt. Mitchell expedition. Mar. Pollut. Bull., 27, 3559.

    • Search Google Scholar
    • Export Citation
  • Verhoef, A., , Portabella M. , , and Stoffelen A. , 2012: High-resolution ASCAT scatterometer winds near the coast. IEEE Trans. Geosci. Remote Sens., 50, 24812487.

    • Search Google Scholar
    • Export Citation
Save