Wind and Significant Wave Height at Indonesian Seas

based on ERA5 Reanalysis Data from 2008 to 2018

Ocha Bahtara and Muhammad Zikra

Department of Ocean Engineering, Institut Teknologi Sepuluh Nopember, Surabaya, Indonesia

Keywords: Significant Wave Height, Wind, ERA5 Reanalysis Data.

Abstract: Wind and waves are important elements in coastal engineering and oceanography. Research on variations of

in-situ winds and wave’s measurements is still very limited in Indonesia. Therefore, wind speed and

significant wave height data from ERA5 reanalysis data, provided by the European Centre for Medium-Range

Weather Forecast (ECMWF), and is used in this study. ERA5 data that has a spatial resolution of 0.25 ° x

0.25 ° is used to analyze correlation between wind speed and significant wave height at Indonesian seas in

2008 to 2018. The results show that wind speed and significant wave height in the closed water (inter-island

in Indonesian archipelago) has a higher correlation compared to the Indonesian open sea that facing the Indian

Ocean or Pacific Ocean.

1 INTRODUCTION

Wave height at sea level is very dependent on sea

wind conditions. Understanding of tides and waves of

sea water is a major requirement in the management

of coastal areas (Yulius et al, 2017, Khoiri, 2015).

Information about the sea wave itself is also very

much needed in various maritime interests and

activities. There are so many maritime studies that

require this information, for example in research on

breakwater structures (Torum et al, 2012). Significant

wave height data is used to determine the speed of

sediment transport on sandy beaches (Van Rijn,

2014). Significant wave height data is also needed in

planning offshore buildings, for example in research

on water breaking on ships (Dehghani et al, 2017) or

effect of global warming on wave climate (Zikra et al,

2015).

Significant wave height is one of the important

variables in disaster mitigation and coastal

vulnerability using the CVI (Coastal Vulnerability

Index) method (Serafim et al, 2019). This is used to

estimate the vulnerability of coastal areas to the

impacts of disasters that might occur. In the field of

shipping, the height of sea waves is one of the

important factors that are taken into account in

https://orcid.org/0000-0001-6400-8042

determining the recommendations of ship shipping

safety (Lutfiana and Tirono, 2013).

Analysis of significant wave height needs to be

done because of the importance of information about

significant wave heights needed for the benefit of

research and maritime activities. One of the data that

can be used in a significant wave height analysis is

satellite data. Satellite data from the European Center

for Medium Range Weather Forecast (ECMWF) will

be used in this final project. ECMWF is considered to

have reliable atmospheric data for research and

evaluation (Hwang et al, 2019). ERA5, the fifth

generation of ECMWF Reanalysis, is one of the

global atmospheric reanalysis data products provided

by ECMWF. ERA5 is an ECMWF product made to

replace its predecessor, ERA-Interim. ERA5 data has

a higher resolution than ERA-Interim (Hersbach and

Dee, 2016). ERA 5 has a grid size of 0.25° x 0.25°,

while the ERA-Interim has a grid resolution of 0.75°

x 0.75°. Olauson (2018) concluded that ERA5 has a

lower error value and has more advantages than

MERRA-2 (Olauson, 2018). Because of the various

advantages of ERA5, the ERA5 data will be used in

this research to analysis the variation of significant

wave height (Hs) and wind speed in Indonesian

waters.

Bahtara, O. and Zikra, M.

Wind and Signiﬁcant Wave Height at Indonesian Seas based on ERA5 Reanalysis Data from 2008 to 2018.

DOI: 10.5220/0010060001730180

In Proceedings of the 7th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management (ISOCEEN 2019), pages 173-180

ISBN: 978-989-758-516-6

173

2 DATA

The wind and wave data used in this study is obtained

from ECMWF ERA5 (https://www.ecmwf.int). Data

obtained is hourly data in Indonesia seas during 2008

to 2018. The location points reviewed are:

1. Western Sumatera, Indian Ocean (3,75° S and

99,75° E)

2. Sunda Strait (6° S and 105,75° E)

3. Southern Java (8° S and 110° E)

4. Malaka Strait (3,75° N and 99,75° E)

5. Java Sea (6,75° S and 112,5° E)

6. Flores Sea (6,75° S and 121,5° E)

7. Makassar Strait (1,5° S and 117,5° E)

8. Halmahera Sea (1,5° N and 129,25° E)

9. Northern Papua (0,25° Sand 135° E)

10. Jayapura (2,25° Sand 140,5° E)

Figure 1: Reviewed location points in Indonesian Seas.

These locations are divided into 3 zones, namely open

water zone facing the Indian Ocean (locations 1, 2

and 3), sheltered water zone (locations 4, 5, 6, and 7),

and open water zone facing the Pacific Ocean

(locations 8, 9 and 10).

3 RESULT AND DISCUSSION

3.1 Variation of Wind Speed

The trend of wind speed on monthly average during

2008-2018 can be seen in Table 1. In Southern

Sumatera, the statistically significant trend is only

appeared in September's with monthly maximum

wind speed of 0.193 m.s

-1

/year. In Sunda Strait, the

monthly maximum wind speed has a significant

increasing trend of 0.176 m.s

-1

/year. Monthly

maximum wind speed and mean wind speed in

Southern Java have a significant increasing trend in

November and December. The trend of monthly

maximum wind speed in Southern Java has a

significant increasing

The trend of monthly maximum wind speed in

November is 0.279 m.s

-1

/year and in December is

0.364 m.s

-1

/year. The monthly mean wind speed trend

in November is 0.104 m.s

-1

/year and in December is

0.126 m.s

-1

/year. Monthly maximum wind speed in

Malaka Strait has a value of increasing trend at 0.191

m.s

-1

/year in February, 0.201 m.s

-1

/year in April,

0.208 m.s

-1

/year in November, and 0.255 m.s

-1

/year in

December. Whereas the monthly mean wind speed in

Malaka Strait has a significant increasing trend in

January (0.147 m.s

-1

/year), February (0.066 m.s

/year), May (0.051 m.s

-1

/year), June (0.046 m.s

/year), September (0.174 m.s

-1

/year), October (0.147

m.s

-1

/year), and December (0.112 m.s

-1

/year). The

calculation of monthly mean wind speed trend in Java

Sea shows that there is no significant trend every

month throughout the period of 2008-2018.

Meanwhile the monthly maximum wind speed trend

value in Java Sea in June has a significant increasing

trend of 0.095 m.s

-1

/year. The calculation of

maximum wind speed and monthly mean trend in

Flores Sea shows that there is no significant trend

every month throughout 2008-2018. The monthly

average wind speed in Makassar Strait in December

had a significant increasing trend of 12.52 m.s

-1

/year.

The trend was also significantly increase by 0.125

m.s

-1

/year in March, in contrast there was no

significant monthly maximum wind speed trend value

in Makassar Strait throughout the years of 2008-2018.

Moreover, the monthly maximum wind speed in

Halmahera Sea has a significant decreasing trend in

May (-0.375 m.s

-1

/year) and a significant increasing

trend (0.172 m.s

-1

/year) in October. The monthly

mean wind speed in Halmahera Sea has a significant

decreasing trend in May (-0.07 m.s

-1

/year) and a

significant increasing trend in March and April of

0.117 m.s

-1

/year and 0.039 m.s

-1

/year.

Furthermore, monthly maximum wind speed in

Southern Papua does not have a significant trend

throughout 2008-2018. The monthly mean wind

speed in Utara Papua has a significant increasing

trend in February by 0.099 m.s

-1

/year, 0.081 m.s

/year in April, 0.091 m.s

-1

/year in November, and

0.113 m.s

-1

/year in December. Monthly maximum

wind speed in Jayapura in October has a decreasing

trend of -0.115 m.s

-1

/year. The mean monthly wind

speed in Jayapura has an increasing trend in February

(0.136 m.s

-1

/year), November (0.079 m.s

-1

/year), and

December (0.093 m.s

-1

/year). Wind rose at 10

reviewed location points during 2008-2018 can be

seen in Figure 3.

ISOCEEN 2019 - The 7th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management

174

Table 1: Monthly mean wind speed trend 2008-2018.

Month

Monthly Mean Wind Speed Trend (m.s

-1

/year)

1 2 3 4 5 6 7 8 9 10

January

0.095 0.029 -0.024 0.147 -0.074 -0.104 0.057 -0.003 -0.043 -0.068

February

-0.111 -0.060 -0.016 0.066 -0.081 -0.100 0.035 0.052 0.099 0.136

March

-0.063 -0.058 -0.018 0.004 0.042 0.121 0.074 0.117 0.071 0.081

April

0.018 -0.013 0.002 0.035 0.022 -0.063 0.009 0.039 0.081 0.056

May

-0.060 -0.015 0.046 0.051 0.059 0.032 -0.019 -0.070 -0.007 0.017

June

-0.041 0.004 0.019 0.046 0.007 -0.007 -0.029 -0.027 -0.013 0.029

July

-0.029 0.005 -0.015 0.073 0.050 -0.004 0.028 0.073 -0.015 -0.068

August

0.014 0.034 0.003 0.085 0.023 -0.006 0.097 0.120 -0.019 -0.033

September

0.084 -0.019 -0.025 0.174 0.061 0.052 -0.011 0.090 -0.026 -0.018

October

0.028 0.009 -0.001 0.147 0.021 -0.015 0.043 0.068 -0.018 -0.040

November

0.084 0.025 0.104 0.080 0.070 0.047 0.083 0.061 0.091 0.079

December

0.037 0.057 0.126 0.112 0.100 -0.007 0.125 0.060 0.113 0.093

Table 2: Monthly mean significant wave height trend 2008-2018.

Month

Monthly Mean Significant Wave Height Trend (m/year)

1 2 3 4 5 6 7 8 9 10

January

0.014 0.012 0.000 0.002 -0.013 -0.018 0.006 0.001 -0.012 -0.009

February

-0.008 -0.017 -0.005 0.007 -0.015 -0.024 0.007 0.013 0.014 0.015

March

0.000 -0.010 0.004 0.001 0.008 0.021 0.007 -0.004 -0.004 -0.001

April

-0.002 -0.001 0.006 0.003 0.003 -0.008 0.002 0.006 -0.005 0.006

May

0.004 -0.002 0.005 -0.004 0.012 0.010 0.000 -0.004 -0.010 -0.003

June

0.013 0.005 0.007 0.003 0.000 0.007 -0.003 -0.008 0.003 -0.004

July

0.019 0.006 0.024 -0.001 0.006 0.000 0.001 0.010 -0.001 0.001

August

0.019 0.004 0.011 -0.001 0.001 -0.003 0.012 0.013 0.001 -0.002

September

0.005 -0.001 -0.006 0.003 0.011 0.007 -0.001 0.014 0.011 0.002

October

0.006 0.002 -0.003 0.002 0.003 -0.001 0.007 0.018 0.053 0.010

November

0.009 -0.001 0.009 0.002 -0.002 -0.003 -0.001 0.007 0.003 0.010

December

0.014 0.014 0.016 0.010 0.016 0.008 0.008 0.001 0.013 0.005

3.2 Variation of Significant Wave

Height

Significant monthly mean wave height in Western

Sumatera has a significant increasing trend in August

at 0.019 m/year, while monthly maximum significant

wave height in Southern Sumatera has an increasing

significant trend in July by 0.081 m/year and 0.062

m/year in August. The calculation of the maximum

monthly significant wave height trend in Sunda Strait

shows that there is no significant trend every month

throughout the years 2008-2018, while the monthly

significant wave height trend value in Sunda Strait,

July, has a significant increasing trend of 0.006

m/year.

Wind and Signiﬁcant Wave Height at Indonesian Seas based on ERA5 Reanalysis Data from 2008 to 2018

175

Figure 3: Wind rose at 10 reviewed location points during 2008-2018.

Figure 4: Wave rose at 10 reviewed location points during 2008-2018.

Monthly maximum significant wave height in

Southern Java has a significant increasing trend of

0.05 m/year in May, 0.05 m/year in August, and 0.068

m/year in December.

Furthermore, the monthly mean significant wave

height in Southern Java has a significant increasing

trend in July of 0.024 m/year and December of 0.016

m/year. In February, there was a significant

ISOCEEN 2019 - The 7th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management

176

Figure 5: Regression graph of wind speed and significant wave height at Indonesian Seas.

Wind and Signiﬁcant Wave Height at Indonesian Seas based on ERA5 Reanalysis Data from 2008 to 2018

177

increasing trend in Malaka Strait both at monthly

maximum and mean significant wave height. For the

significant monthly mean wave height, the significant

trend is calculated by 0.007 m/year, while the

maximum monthly mean wave height has a significant

trend of 0.025 m/year. The calculation the monthly

maximum significant wave height trend in Java Sea

shows that there is no significant trend every month

throughout the years 2008-2018. The trend value of

monthly mean significant wave height in Java Sea has

a significant increasing trend of 0.012 m/year in May.

Monthly maximum significant wave height in

Makassar Strait has a significant increasing trend of

0.028 m/year in August. Monthly mean significant

wave height in Makassar Strait has a significant

increasing trend by 0.008 m/year in December. The

calculation of the monthly significant wave height

trend in Northern Papua shows that there is no

significant trend that occurs every month throughout

2008-2018. The trend of the maximum monthly

significant wave height in Northern Papua in

December has a significant decreasing trend of -0.055

m/year. There is no significant trend of monthly

significant wave height in Jayapura during 2008-2018.

Monthly maximum significant wave height in

Jayapura has a significant decreasing trend in June and

August, each at -0.054 m/year and at -0.03 m/year. The

calculation of the monthly maximum and the monthly

mean significant wave height trend in Flores Sea and

Halmahera Sea shows that the trend that exists every

month throughout the years 2008-2018 is not

statistically significant. The calculation of monthly

mean significant wave height trend in 11 years can be

seen in Table 2. Wave rose from 10 reviewed location

points during 2008-2018 can be seen in Figure 4.

3.3 Relationship of Wind Speed and

Wave Height

The relationship of wind speed and significant wave

height in Indonesia during 2008-2018 at each

reviewed location point has a positive correlation. It

can be seen from Table 3 that the sheltered water of

Indonesian archipelago, namely Malaka Strait, Java

Sea, Flores Sea, and Makassar Strait, have a

determination coefficient above 0.3. Even more, Java

Sea and Flores Sea each has a determination

coefficient 0.75. On the other hand, in waters facing

the ocean, Western Sumatera, Southern Java,

Halmahera Sea, Northern Papua, and Jayapura, the

determination coefficient is below 0.3. This means

that the relationship of significant wave height and

wind speed in those waters is not as high as the

relationship in the sheltered waters. Sunda Strait

which is categorized as an open water in this paper

turns out to have characteristics that tend to follow the

characteristics of sheltered waters which have a quite

high correlation between wind speed and significant

wave height with determination coefficient of 0.438.

Regression chart of the correlation between wind

speed and significant wave height in all reviewed

location points can be seen in Figure 5. The

regression method used is quadratic regression or

polynomial order 2.

Figure 6: Regression line of wind speed and significant wave height correlation at 10 reviewed location points in 11 years.

ISOCEEN 2019 - The 7th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management

178

Figure 7: Wind speed data distribution and significant wave height of ERA5 data compared to some studies regarding wave

forecasting in Java Sea.

It can be seen in Figure 6 that the regression lines

of sheltered waters (Malaka Strait, Java Sea, Flores

Sea, and Makassar Strait) are below the open water

regression line, which means that the significant wave

height in sheltered waters is lower than in open

waters. The regression lines of sheltered waters are

also more curved than open waters regression lines. It

means that the correlation between wind speed and

significant wave height in sheltered waters is higher

than the correlation of wind speed and significant

wave height in open waters. The graph only compares

the wind speed and significant wave height without

taking into account of the direction of the wind and

wave itself. Therefore, the graph cannot be used as a

reference in determining the significant wave height

based on wind speed or vice versa.

Figure 7 shows the distribution data of wind speed

and significant wave height from ERA5 compared to

direct measurement field data on some studies of

wave height forecasting in the Java Sea from

Thambas and Yuwono (2003), Denny et al (2017),

Basuki (2000). As seen in Figure 7, the wind speed

data range is limited between 5 knots to 30 knots.

Figure 7 indicated that ERA5 data has similar trend

as other studies in the Java Sea.

4 CONCLUSIONS

Conclusions that can be obtained in this paper

include:

1. Monthly mean wind speed at Indonesian Seas is

about 2 m/s to 9 m/s. Monthly maximum and

monthly mean wind speed at each reviewed

location points have statistically significant

trend, except at Flores Sea.

2. Monthly mean significant wave height at

Indonesian Seas is about 0,2 m to 2,8 m.

Monthly maximum and monthly mean

significant wave height at each reviewed

location points have statistically significant

trend, except at Flores Sea and Halmahera Sea.

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