Northern Arabian Sea Circulation-Autonomous Research (NASCar): A Research Initiative Based on Autonomous Sensors

Luca Centurioni,Bulusu Subrahmanyam,Rosalind Echols,Eric Terrill,Verena Hormann,Hyodae Seo,Amala Mahadevan,Janet Sprintall,Geno Pawlak,Patrick Conry,He Wang,Luc Rainville,Hans C Graber ,Eric D Skyllingstad ,Julie L Mcclean ,Ramsey R Harcourt ,Pierre L’hégaret ,Craig M Lee ,Tommy G Jensen ,Andrew J Lucas ,Robert E Todd ,Lynne D Talley ,Steven R Jayne ,Harindra J S Fernando ,Arnold L Gordon ,Hugo N Ulloa ,Sarah N Giddings ,Stephen C Riser ,Pierre F J Lermusiaux ,Michael J Caruso ,Corinne B Trott ,Isabella B Arzeno ,Lisa M Beal ,Andrey Y Shcherbina

doi:10.5670/oceanog.2017.224

Abstract

The Arabian Sea circulation is forced by strong monsoonal winds and is characterized by vigorous seasonally reversing currents, extreme differences in sea surface salinity, localized substantial upwelling, and widespread submesoscale thermohaline structures. Its complicated sea surface temperature patterns are important for the onset and evolution of the Asian monsoon. This article describes a program that aims to elucidate the role of upper-ocean processes and atmospheric feedbacks in setting the sea surface temperature properties of the region. The wide range of spatial and temporal scales and the difficulty of accessing much of the region with ships due to piracy motivated a novel approach based on state-of-the-art autonomous ocean sensors and platforms. The extensive data set that is being collected, combined with numerical models and remote sensing data, confirms the role of planetary waves in the reversal of the Somali Current system. These data also document the fast response of the upper equatorial ocean to monsoon winds through changes in temperature and salinity and the connectivity of the surface currents across the northern Indian Ocean. New observations of thermohaline interleaving structures and mixing in setting the surface temperature properties of the northern Arabian Sea are also discussed.

Highlights

The Arabian Sea, in the northwestern Indian Ocean, plays a critical role in the Asian monsoon and associated precipitation over the Indian subcontinent
The working hypothesis that connects the observational and modeling components of Northern Arabian Sea Circulation-autonomous research (NASCar) is that our limited knowledge of the local air-sea fluxes in the northwestern Indian Ocean limits the skills of models in correctly forecasting the onset and intraseasonal-to-interannual variability of the Asian monsoon, and that such fluxes are influenced by ocean mesoscale and submesoscale dynamics, planetary waves, and upper-ocean mixing, in addition to important ocean-atmosphere feedbacks (Vecchi et al, 2004)
In situ oceanographic observations in the northern Arabian Sea have been difficult to obtain for over two decades because of piracy, with the notable exception of measurements from Global Climate Observing System subarrays such as the Global Drifter Program (GDP), Argo, the High-Resolution Expendable Bathythermograph/Expendable ConductivityTemperature-Depth (HR XBT/XCTD) network, the Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA; McPhaden et al, 2009), and the developing Indian Ocean Observing System (IndOOS) effort in the northeastern

Summary

Introduction

The Arabian Sea, in the northwestern Indian Ocean, plays a critical role in the Asian monsoon and associated precipitation over the Indian subcontinent. New Global Climate Observing System observations (i.e., Argo and GDP arrays; RAMA moorings at 66°E) are complemented by (1) autonomous underwater gliders with repeated occupations of sections between the Seychelles, Oman, and the Maldives to sample latitudinal and longitudinal contrasts in upper-ocean structures associated with the monsoon forcing, and (2) a pair of rapidly profiling, drifting Wirewalker systems (Pinkel et al, 2011; Lucas et al, 2016) deployed at 1°S, 55.5°E to investigate diurnal cycling in the mixed layer and fine-scale temperature and salinity interleaving in the thermocline along the equator.

Results

Conclusion