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.

Abstract. 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.

Abstract. Intrinsic modes of variability have a significant role in driving the climatic oscillations in the oceanic processes. In this paper, we investigate the influence of an interannual mode of variability, the Indian Ocean Dipole (IOD), on the wave climate of the eastern Arabian Sea (AS). Using measured, modeled and reanalysis wave data and reanalysis wind data, we show that the IOD plays a major role in the variability of wave climate of the study region. Due to the IOD-induced changes in equatorial sea surface temperature and sea level pressure, the winds from the northern AS gets modified and cause inter-annual variability in the wave climate over the eastern AS. The changes in wind field over the AS due to the IOD influence the generation or dissipation of the wave field and hence cause a decrease in northwest short-period waves during positive IOD and an increase during negative IOD.