Introduction

R/V Pelagia Cruise LOCO/IW 2009

INTRODUCTION

The ocean is vertically stratified in density, with relatively warm and fresh water overlying relatively dense, cold and salty water. This stratification supports waves in the ocean interior ('internal waves') that propagate into the deep ocean. Such waves can attain large amplitudes, up to 100 m vertically. When they encounter large topography such as underwater sea-mounts or the continental shelf, they may break and induce large mixing. Internal wave mixing is thought to be the key in maintaining the general ocean circulation, induced about in half by tidal motions and in half by atmospheric (wind) induced (inertial) motions. As sinusoidal waves do not mix, non-linear interaction between internal waves is assumed to transfer energy to smaller scales, eventually leading to wave breaking, and mixing. In this respect, the low-frequency near-inertial internal waves are considered to be important, because of their strong vertical shear of horizontal currents. This shear is one of the causes for linear waves to become non-linear and eventually break. Internal tidal motions are important because of their persistent generation and focusing in basins. During this cruise of the NWO-funded LOCO-program (Long-term Ocean Current Observations), in part also funded by BSIK Climate changes Spatial Planning programme, the aim is on studying the variability with time of deep-ocean near-inertial and internal tidal motions and their influence on other internal waves and internal-wave induced breaking. This study is carried out at various sites in the North-Atlantic Ocean, ranging from the convectively active Irminger Sea including its Miid-Atlatic Ridge extension, the Canary Basin to the near-equatorial Brazil Basin. During the present cruise in the Canary Basin we will study in particular the effects of deterministic (diurnal tidal) forcing and sub-harmonic instabilities that may reinforce atmospherically induced or other geostrophically adjusted near-inertial motions. The associated reduction in vertical scales may imply larger mixing than elsewhere in the ocean. In the Brazil Basin we will study the effects of the lack of the near-inertial motions, as the vertical component of the Coriolis force becomes negligible. This area is particularly important for the meridional transport of water masses. The measurements will be mainly done using current and temperature sensors on long (3.7 km) underwater moorings. Additionally, we will make specific, fast sampled measurements using special NIOZ-designed instrumentation. During the next few weeks you may learn more on details of this cruise through our subsequent reporting on this site.