Decadal Variability of Global Ocean Significant Wave Height

ZHENG Chongwei, ZHOU Lin SHI Weilai, LI Xin and HUANG Chaofan

1) College of Meteorology and Oceanography, People’s Liberation Army University of Science＆Technology, Nanjing 211101, P. R. China

2) National Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics, the Chinese Academy of Sciences, Beijing 100029, P. R. China

3) Dalian Naval Academy, Dalian 116018, P. R. China

4) Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing 210044, P. R. China

Decadal Variability of Global Ocean Significant Wave Height

ZHENG Chongwei1),2),3),*, ZHOU Lin1), SHI Weilai1),4), LI Xin1), and HUANG Chaofan1)

1) College of Meteorology and Oceanography, People’s Liberation Army University of Science＆Technology, Nanjing 211101, P. R. China

2) National Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics, the Chinese Academy of Sciences, Beijing 100029, P. R. China

3) Dalian Naval Academy, Dalian 116018, P. R. China

4) Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing 210044, P. R. China

? Ocean University of China, Science Press and Springer-Verlag Berlin Heidelberg 2015

This paper presents the long-term climate changes of significant wave height (Hs) in 1958–2001 over the entire global ocean using the 45-year European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis (ERA-40) wave data. The linear trends in Hs and regional and seasonal differences of the linear trends for Hs were calculated. Results show that the Hs exhibits a significant increasing trend of about 4.6 cm decade-1in the global ocean as a whole over the last 44 years. The Hs changes slowly during the periods 1958–1974 and 1980–1991, while it increases consistently during the periods 1975–1980 and 1995–1998. The Hs reaches its lowest magnitude in 1975, with annual average wave height about 2 m. In 1992, the Hs has the maximum value of nearly 2.60 m. The Hs in most ocean waters has a significant increasing trend of 2–14 cm decade-1over the last 44 years. The linear trend exhibits great regional differences. Areas with strong increasing trend of Hs are mainly distributed in the westerlies of the southern Hemisphere and the northern Hemisphere. Only some small areas show obvious decreasing in Hs. The long-term trend of Hs in DJF (December, January, February) and MAM (March, April, May) is much more stronger than that in JJA (June, July, August) and SON (September, October, November). The linear trends of the Hs in different areas are different in different seasons; for instance, the increasing trend of Hs in the westerlies of the Pacific Ocean mainly appears in MAM and DJF.

global ocean; significant wave height; long-term trend; regional differences; seasonal differences; dominant season

1 Introduction

As the environment and resource crisis is becoming increasingly serious nowadays, people pay more attention to the global climate change. Ocean plays an important role in global climate change, oceanic and atmospheric circulations. The long-term changes of global surface temperature and sea surface temperature (SST) have been recognized by many experts, but there is little research about the long-term changes of Hs over the entire global ocean. By using the ICOADS (International Comprehensive Ocean-Atmosphere Data Set) data, Gulev and Grigorieva (2006) stated that in both North Atlantic and North Pacific mid-latitudes winter Hs shows a significant increasing trend from 10 to 40 cm decade-1during the last 45 years. The trend changes for wind sea and swell are quite different from each other, showing opposite signs of changes in the northeast Atlantic. Trend patterns of wind sea, swell, and Hs in the North Pacific are more consis-tent with each other. Dodet et al. (2010) have found that the North-East Atlantic Ocean Hs has obvious increasing trends (up to 0.02 m yr-1) during the last 57 years (1953–2009). Zheng et al. (Zheng and Li, 2011; Zheng et al., 2012a; Zheng and Li, 2015) found that the Hs and wave power density in most of the China seas have a significant increasing trend from 1988 to 2009. Semedo et al. (2011) reported that Hs in most of the North Pacific and the North Atlantic Oceans shows significantly increasing trend from year 1957 to 2002. Using a 109-year numerical wind-wave hindcast, Bertin et al. (2013) analyzed the long-term trend of Hs in the North Atlantic Ocean, and found a significant increasing trend of about 2.5–10 cm decade-1. Bertin et al. (2013) also pointed out that North Atlantic Oscillation (NAO) partly controls Hs inter- annual varibility.

As the observed Hs in global ocean is rare (the satellite data can cover the global ocean, but its time series is short, space resolution is low), the research about the long-term changes of Hs in the global ocean is still insufficient. By analyzing ERA-40 (40-year ECMWF Reanalysis) wave Reanalysis data 1957–2002 from ECMWF (EuropeanCentre for Medium-Range Weather Forecasts), the paper presents the overall linear trends of Hs in global ocean, their seasonal differences, and regional differences of the linear trend to study their global climate changes.

2 Data and Methods

The data used in this study is ERA-40 wave Reanalysis from ECMWF, with spatial extent 90?S–90?N, 180?W–180?E, spatial resolution 1.5?×1.5?, time extent from September 1957 to August 2002, and time resolution 6 hours; separating the wind waves and swell is the biggest advantage of the data. As the time series of ERA-40 wave Reanalysis is long enough and it covers the global ocean, this data is widely used to analyze the character of ocean wave in global ocean, especially in North Atlantic, North Pacific Ocean and South Ocean (Caires and Sterl, 2005; Zheng et al., 2012b, 2012c; Semedo et al., 2011; Hemer et al., 2007).

To analyze the long-term changes of Hs in the global ocean from year 1958 to 2001, linear regression is used to calculate the linear trends of Hs. The regional and seasonal differences of the linear trends for Hs at 1.5?×1.5? grid point are also presented.

3 Long-Term Trends of Hs in the Global Ocean

3.1 Overall Linear Trend

The Hs with global ocean average 1958–2001 is shown in Fig.1. By using the method of linear regression, correlation coefficient R=0.61＞r0.05=0.29, which passes the reliability test of 95%, the regression coefficient is 0.0046. It means that the Hs exhibits a significant increasing trends of 0.0046 m yr-1(4.6 cm decade-1) in the global ocean as a whole over the last 44 years. From 1958 to 1974, the changing trend of Hs is slow, the Hs is about 2.20 m, while in 1975 the Hs is about 2 m, which is the minimum value during the last 44 years. From year 1975 to 1980, the Hs increases remarkably. Then from year 1980 to 1991, the changing trend becomes slow again, the value being about 2.25 m. In 1992, the Hs has the maximum value of nearly 2.60 m during the last 44 years, and then from year 1992 to 1995, the decreasing trend is obvious. From 1995 to 1998, the changing trend becomes positive.

Fig.1 Linear trend of global ocean average significant wave height from year 1958 to 2001.

3.2 Regional Differences of the Long-Term Trend

The analysis method in Fig.1 is commonly used, which could demonstrate the linear trends of the global ocean Hs as a whole, but could not give the regional difference of the linear trend. In order to illustrate the regional difference, the linear trends in Hs at 1.5?×1.5? grid point year by year over the last 44 years is calculated as shown in Fig.2.

From Fig.2, some conclusions can be got. The Hs exhibits significant increasing trends of 2–14 cm decade-1in most global ocean waters over the last 44 years. Gulev and Grigorieva (2004) found positive trends for Hs over the North Pacific with a maximum of 8–10 cm decade-1in the northeast Pacific, even up to 14 cm decade-1in the North Atlantic and other ocean basins. The trends in the North Pacific and Atlantic in this study are close to the result reported by Gulev and Grigorieva (2004), but smaller than those reported by Gower (2002) form the northeast Pacific buoys data (12–27 cm decade-1). Dodet et al. (2010) found a significant increasing trend (up to 20 cm decade-1) of Hs for the North-East Atlantic Ocean over 1953–2009, using a 57-year hindcast with a spectral wave model forced by Reanalysis wind fields. The increasing trend in the North-East Atlantic Ocean in this study is smaller than the result of Dodet et al. (2010). Bertin et al. (2013) found a significant increasing trend of Hs in the North Atlantic Ocean, the rate being 2.5–10 cm·decade-1. Our result for the North Atlantic Ocean has a good consistency with the result of Bertin et al. (2013).

The increasing trend appears with obvious regional difference: in middle and high latitude waters the increasing trend is more obvious than that in low latitude waters; the increasing trend in the southern hemisphere is more obvious than that in the northern hemisphere. Areas with evident increasing trend of Hs are mainly distributed in the westerlies of the North Pacific (6–10 cm decade-1), the westerlies in the north of the Atlantic Ocean (6–8 cm decade-1), and most waters in the southern hemisphere (8–14 cm decade-1). In some areas the linear trends are not significant, such as the middle of the Pacific Ocean, middle of the South Atlantic Ocean, middle of the South Indian Ocean. Only a few waters show linear decreasing trend in Hs.

Gulev and Grigorieva (2004) pointed out that the longterm changes in wind wave height are closely associated with the NAO in the Atlantic and with North Pacific Oscillation (NPO) and El Ni?o-Southern Oscillation (ENSO) in the Pacific. Li (1990) and Chen et al. (2000) pointed out that there is a close relationship between the Eastsian Monsoon and the El Ni?o phenomenon. And the Eastsian Monsoon is stronger in the year of La Ni?a, weak in the year of El Ni?o. There may be a close relationship between the long-term trend of the global ocean Hs and NPO, ENSO and so on.

The changing trend of sea surface wind speed is considered as a possible reason for Hs trend. We calculated the long-term trend of wind speed for the period 1958–2001 (Figure omitted).

Conparing the long-term trends of wind speed and Hs, it is clearly that the distribution characteristics of the long-term trend for Hs has a good consistency with that for wind speeds in the waters of Southern Hemisphere westerlies, the North Pacific westerly, low latitudes of the Indian Ocean and low latitudes of the Atlantic Ocean. But this phenomenon does not exist in other waters, even the reverse appears in the central and eastern equatorial Pacific Ocean and west waters off the South America. This should be due to the fact that under the influence of the strong west wind, the wind sea plays an important role in the mixed waves. The long-term trend of wind sea has a good consistency with the wind speed, but no much consistency with the swell and mixed waves (Zheng et al., 2012b). Chen et al. (2002) pointed out that the swell index (the greater the swell index, the swell play a more important role in the mixed wave) in the westerlies waters is greater than that in low latitude waters, greater in east coastal waters than that in west coastal waters. Our result is consistent with Chen’s.

Fig.2 Long-term trend of significant wave height from year 1958 to 2001. Unit: cm·decade-1. Only trends significant at the 95% level are shown.

3.3 Seasonal Differences of the Long-Term Trend

In order to show the seasonal and regional differences, the linear trends of Hs at 1.5?×1.5? grid point are calculated in MAM (March, April, May), JJA (June, July, August), SON (September, October, November) and DJF (December, January, February) from 1958 to 2001 (Fig.3).

In MAM, the Hs in most waters of the Pacific Ocean and east of the Indian Ocean has obvious increasing trend and does not show any significant trends in most waters of the Atlantic Ocean and southwest of the Indian Ocean over the last 44 years. Areas with distinct increasing trends are mainly distributed in the middle of North Pacific Ocean (8–12 cm decade-1) and the large area off the south of Australia (12–20 cm decade-1).

In JJA, the Hs exhibits a significant increasing trend in most waters of the Indian Ocean (4–10 cm decade-1), the nearshore of northwest Pacific Ocean (2–4 cm decade-1), waters surrounding Australia (4–10 cm decade-1), most areas of north of 30?S in the Atlantic Ocean (4–8 cm decade-1), and waters surrounding Falkland Islands in the southeast of South America (about 8 cm decade-1). In the middle part of Pacific Ocean there are two small areas in which the linear trends of Hs are decreasing obviously. Hs does not show any significant trends in other waters.

In SON, areas with significant change trend are much smaller than that in MAM and JJA. The Hs exhibits significant increasing trend in most waters of the Atlantic Ocean, northwest of Pacific Ocean, the South China Sea, and the middle of East Indian Ocean. Areas with strong increasing trends are mainly located in the northwest of the Pacific Ocean (6–10 cm decade-1) and those surrounding Falkland Islands in southeast of the South America (10–14 cm decade-1). Only a small area in the middle of the Pacific Ocean shows significant decreasing trend in Hs. Hs does not show any significant trends in large scale waters.

In DJF, most oceans have obvious increasing trend of Hs. Only few scattered small waters exhibits significant decreasing trend over the last 44 years. Areas without significant change trend are mainly located in middle latitudes of Atlantic Ocean, middle latitudes of the South Indian Ocean, and some small areas in the middle of the South Pacific Ocean. Waters with strong increasing trend are mainly located in the westerlies of the North Pacific Ocean (of above 10 cm decade-1, even up to 18 cm decade-1), westerlies of the South Pacific Ocean (of about 8–16 cm decade-1) and westerlies of the North Atlantic Ocean (of about 10–14 cm decade-1). The results in this study are smaller than that reported by Gulev and Grigorieva (2006) for the North Atlantic and the North Pacific mid-latitudes waters (10–40 cm decade-1).

3.4 Dominant Seasons with Long-Term Trends in Different Waters

Fig.3 Long-term trends of global ocean significant wave height in MAM (a), JJA (b), SON (c) and DJF (d) from 1958 to 2001. Unit: cm decade-1. Only trends significant at the 95% level are shown.

From Fig.2 and Fig.3 it is noted that the significant increasing linear trends of Hs appears in MAM and in DJFin most oceans. The regional difference of linear trends of Hs is great. In different areas the trends are different by seasons. The increasing trend of the Hs in the westerlies of the North Pacific Ocean mainly appears in DJF, followed by MAM. Such a trend in the westerlies of the South Pacific Ocean mainly appears in MAM, followed by DJF. This trend in the westerlies of the North Atlantic Ocean mainly appears in DJF, followed by SON and JJA. Increasing trend of Hs in the areas surrounding Falkland Islands in southeast of the South America mainly appears in SON.

4 Conclusions

In this study, we mainly analyzed the linear trends in Hs in entire global ocean, regional and seasonal differences of the linear trends in Hs over 1958–2001 by using ERA-40 wave reanalysis data from ECMWF. Results show that:

1) The significant wave height exhibits a significant increasing trend of about 4.6 cm decade-1in the global ocean as a whole over the last 44 years. From 1958 to 1974, the changing trend of significant wave height is slow. From year 1975 to 1980, the significant wave height increases consistently. From year 1980 to 1991, the changing trend becomes slow again, and then from year 1992 to 1995, the decreasing trend is obvious. From 1995 to 1998, the changing trend becomes increasing. The Hs reaches its lowest magnitude in 1975, with annual average wave height about 2 m. In 1992, the Hs has its the maximum value of nearly 2.60 m.

2) The significant wave height in most oceans has a significant increasing trends of 2–14 cm decade-1over the last 44 years. The linear trend exhibits great regional difference: in the middle and high latitude waters the increasing trend of the significant wave height is more obvious than that in the low latitude area; the increasing trend in the southern hemisphere is more obvious than that in the northern hemisphere. Areas with strong increasing trend of the significant wave height are mainly distributed in the westerlies of the southern Hemisphere and the northern Hemisphere. Only some small areas show obvious decreasing in Hs.

3) The long-term trend of Hs in DJF and MAM is much more stronger than that in JJA and SON. The linear trends of the significant wave height in different areas are different in different seasons. The increasing trend of the Hs in the westerlies of the North Pacific Ocean mainly appears in DJF; in the westerlies of the South Pacific Ocean the trend appears in MAM; in the westerlies of the North Atlantic Ocean such a trend appears in DJF; in the areas surrounding Falkland Islands in southeast of South America this trend appears in SON.

Acknowledgements

This work was supported by the National Ky Basic Research Development Program (Grant Nos. 2015CB453200, 2013CB956200, 2012CB957803, 2010CB950400), and the National Natural Science Foundation of China (Grant Nos. 41430426, 41490642, 41275086, 41475070).

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(Edited by Xie Jun)

(Received September 12, 2013; revised December 31, 2013; accepted May 30, 2015)

J. Ocean Univ. China (Oceanic and Coastal Sea Research)

DOI 10.1007/s11802-015-2484-5

ISSN 1672-5182, 2015 14 (5): 778-782

http://www.ouc.edu.cn/xbywb/

E-mail:xbywb@ouc.edu.cn

* Corresponding author. E-mail: zhengzhang.xia@163.com