The climate over the north Indian Ocean (NIO) is one of the most dynamic in the world because of seasonally reversing monsoon winds. In this study, the climate of the NIO and the variability of its surface waves using the European Centre for Medium-Range Weather Forecasts (ECMWF) global atmospheric reanalysis product (ERA-Interim) for the period 1979–2012 are analyzed. Annual average significant wave height (SWH) of the NIO ranges from 1.5 to 2.5 m and the seasonal average is highest (3–3.5 m) during the monsoon period [June–September (JJAS)]. Swells propagating from the Southern Hemisphere are present in the NIO during the premonsoon [February–May (FMAM)] and postmonsoon [October–January (ONDJ)] periods. The waves are separated into wind seas and swells based on the wave energy statistical method. The results show that the NIO is swell dominated and that wind sea heights are lower compared to the swell heights. Higher wind sea and swell heights are observed during the monsoon in the western NIO because of strong cross-equatorial winds of the Somali (Findlater) jet. In the postmonsoon period, the eastern NIO shows a higher swell height than the western NIO shows. SWH shows an annual increasing trend in the western NIO. On a seasonal scale, the trends are increasing significantly in the monsoon compared to the postmonsoon period in a major part of the NIO, whereas the premonsoon period shows a decline in SWH. In the NIO, the monsoon is the dominant mode of variability and it covers 92% of the total variability. Wave climate is also influenced by the annual and interannual variability in monsoon wind and rainfall.

In this study, the wind and the surface waves in the Indian Ocean (IO) during 1979–2017 are studied based on the ECMWF ERA5 Reanalysis data. Long‐term statistical analysis of extreme waves is carried out based on Generalized Extreme Value distribution using annual maxima and the spatial distribution of return levels for 50 and 100 years are studied. In general, the ERA5 significant wave height (H_s) and maximum wave height (H_max) show a good agreement with measured buoy data in the coastal (bias ~0.29 m) and deep waters (bias ~0.18 m). During the tropical cyclone, underestimation of H_s and H_max in the ERA5 data compared to buoy data is 2.7 and 1.4%, but in general the bias is large (~0.69 m). Swell domination is observed in larger regions of the IO, whereas wind‐seas are comparable to swells in the Southern Ocean. The meridional wind speed largely influences the spatial pattern of H_s in the North IO. The stronger winds over the Southern Ocean play a major role in generating higher waves at higher latitudes. Maximum value of the 100‐year return level for H_s in IO is 17.8 m, whereas highest value of the H_s is 16.7 m and H_max is 32.0 m. Severe wave events are common at 50°–60°S and only during 25% of the time in a year, H_max is less than 5 m in this region. Ratio of the H_max to H_s varies from 1.46 to 2.3 with a mean value of 1.87. The 100‐year return value of H_s changed by −4 to 5 m, when the length of the dataset is decreased from 39 years (1979–2017) to recent 20 years. On an average, from 1979 to 2017, the annual average H_max increased by 0.73 cm/year. In major areas of the IO, a clear decrease of the H_s is observed during 1991–2017, whereas during 1979–2017, an increase in H_s is found.

Wave data collected off Ratnagiri, west coast of India, during 1 May 2010 to 30 April 2012 are used in this study. Seasonal and annual variations in wave data controlled by the local wind system such as sea breeze and land breeze, and remote wind generated long period waves are also studied. The role of sea breeze on the sea state during pre- and postmonsoon seasons is studied and it is found that the maximum wave height is observed at 15:00 UTC during the premonsoon season, with an estimated difference in time lag of 1–2 h in maximum wave height between premonsoon and postmonsoon seasons. Observed waves are classified in to (i) short waves (T_p < 8 s), (ii) intermediate waves (8 < T_p < 13 s), and (iii) long waves (T_p> 13 s) based on peak period (T_p) and the percentages of occurrence of each category are estimated. Long period waves are observed mainly during the pre- and the postmonsoon seasons. During the southwest monsoon period, the waves with period > 13 s are a minimum. An event during 2011 is identified as swells propagated from the Southern Ocean with an estimated travelling time of 5–6 days. The swells reaching the Arabian Sea from the south Indian Ocean and Southern Ocean, due to storms during the pre- and postmonsoon periods, modify the near surface winds due to higher phase wave celerity than the wind speed. Estimation of inverse wave age using large-scale winds such as NCEP (National Centers for Environmental Prediction) reflects the presence of cyclonic activity during pre- and postmonsoon seasons but not the effect of the local sea breeze/land breeze wind system.

The Arabian Sea and Bay of Bengal (BoB) regions are special interested sea areas in the Northern Hemisphere with large seasonal variability. This study focused on the long-term wind and wave in the central BoB from 1979 to 2012 based on the ECMWF ERA-Interim reanalysis data sets. Data were validated with the nearest available buoy data for the years 2003, 2004 and 2005, and good correlation was observed (root mean square error ≈ 0.17–0.43 m). A clear seasonality was noted with intensified wind and waves during the southwest monsoon season. We observed statistically declining trends in the mean and extreme wind speed (90th percentile) with increasing trend in extreme significant wave height (SWH). Seasonal analysis also investigated and found that the stronger events/winds during the southwest (SW) monsoon season are weakening. Conflicting trends in the wind and wave height were mainly due to the swell dominance at the region which was identified by the separate trend analysis of wind-sea and swell height. The area average analysis is carried out to investigate the sensitivity of the identified trend results for the point location and found similar trends for extreme wind speed and SWH. The high (>5 m) annual maximum SWH in the study area was attributed to the influence of tropical cyclones in the BoB, and all of these high waves occurred before 1996 indicating that the influence of tropical cyclones in the study area decreased after 1996.