Footprints of Southern Annular Mode on The Indian Ocean Waves

The annular modes are hemispheric scale patterns of climate variability as part of internal atmospheric dynamics in the middle latitudes. On riveting our attention to Indian Ocean, the potential imprints of tropical-extra tropical teleconnection of Southern Annular Mode (SAM) through sequential changes of Hadley cell on the North Indian Ocean (NIO) wave parameters are investigated in the present study. Marshall SAMI values are taken for the present study which was carried out from December 1979 to February 2018 . The positive (negative) phase of the SAM occurs when the pressure anomaly is low (high) on the pole, and the mid-latitude westerly jet is shifted poleward (equatorward). ERA5(ECMWF ReAnalysis) model data were used for analysing the variables under study, covered from january 1979 to within 5 days of real time. It is the fifth generation atmospheric reanalysis from ECMWF, ERA5-hourly data at 31km horizontal resolution with 137 levels up to 0.01 hPa. The composite analysis of Significant wave height( SWH), Swell wave height (SHTS), mean wave period (MWP), and wind sea height (SHWW ) anomalies for SAM positive and negative years reveal that JJA (June, July and August) season is most susceptible to other seasons. SWH, SHTS, MWP, and SHWW fields are stark opposites during the positive and negative phases of SAM and band-wise in the different IO regions. The southwest monsoon and the strengthening of SAM during the negative phase contribute to IO's highest mean wind fields during JJA. Wind speed is maximum (10m/s-14m/s) between 40°S-60°S. The dominance of wind seas during this period is visible from the lowest MWP fields of JJA. The swell fields are also indicative of this, with the highest swells during JJA. With the most dynamic season being JJA in the North Indian Ocean, a negative SAM event is seen to enhance the wind and in turn the SWH fields in JJA whereas, a negative SAM event weakens the wind causes low values of SWH as seen in the figure below. The signature of SAM is evident in the atmospheric signals linked to the change in wave parameters due to the altering wind as a result of modified Hadley cell over this region. A positive(negative) SAM event corresponds to a widespread (constricted) downdraft and intense (less severe) updraft of Hadley cell. These changes are a direct consequence of the pole-ward and equator-ward shift of wind during SAM phases. The study establishes the relevance of SAM as a persuasive factor towards wave climate and the co-varying atmospheric counterparts in the IO.