Details

Autor(en) / Beteiligte

• Delorme, Bertrand Louis Rene

Titel

Enhanced Abyssal Mixing in the Equatorial Ocean Associated with Non-Traditional Effects

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

2021

Link zum Volltext

• ProQuest

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• The ocean plays a key role in regulating the Earth’s climate by storing vast quantities of heat and carbon and distributing them all over the world. Schematically, the ocean circulation can be represented by two meridional overturning cells stacked in the vertical, which form together the so-called Meridional Overturning Circulation (MOC). The upper cell is associated with sinking of polar water to mid-depth in the North Atlantic, which then upwells back to the surface along constant density surfaces under the pull exerted by strong winds blowing over the Southern Ocean. This upwelling process therefore requires very little mixing. However, the lower cell, associated with sources of abyssal water around Antarctica, inherently requires mixing processes to lift bottom waters towards mid-depth and eventually close the MOC. Although the polar source regions for these overturning cells have been identified, the energy sources driving mixing and their spatial distribution are still debated. Inverse models of the ocean’s circulation suggested that much of the mixing needed to close the abyssal meridional overturning cell occurs in the tropical oceans. However, the abyssal near-equatorial oceans are poorly sampled; the inferences from the inverse models have thus not been fully tested in the field.To address this problem and study mixing in the deep equatorial regions, instruments were recently deployed from the surface all the way to the seafloor to gather information on flow velocity, stratification of the water column, and turbulence in the Eastern Equatorial Pacific. Researchers reported evidence of strong turbulence in this region, finding rates of kinetic energy dissipation typical of flows over rough topography. However, these observations were collected over smooth topography. Hence, the common interpretation that bottom-intensified mixing is driven by internal waves v breaking locally through interactions with rough topography is not applicable in this region. In addition, the observations also had evidence of a downward-propagating Equatorial Wave (EW) in the data and researchers suggested that the mixing may have been energized by the EW whose physics had been modified by the horizontal component of the Coriolis parameter, ƒ, which is traditionally ignored in most theories and models of the ocean circulation.This hypothesis was motivated by previous studies that investigated the nontraditional (NT) effects (i.e. the effects induced by a non-zero ƒ ) on mid-latitude waves. These studies showed that NT effects can lead to critical reflection at the inertial latitude (i.e., the latitude where the wave frequency is equal to f, the vertical component of the Coriolis parameter). Critical refection consists of a focusing of the wave’s energy, leading to wave amplification and possibly breaking. However, it remains unknown how this mechanism applies to EWs, whether it can explain the enhanced mixing observed during the campaign, and how much it could contribute to closing the abyssal cell of the MOC.In this dissertation, it is shown analytically and numerically that the reflection of EWs of the bottom leads to seafloor-intensified mixing and substantial diapycnal upwelling near the equator when the NT effects are taken into account.