Physical oceanography

Abstract:

<p>The upper limb of the Atlantic Meridional Overturning Circulation draws waters with negative potential vorticity from the southern hemisphere into the northern hemisphere. The North Brazil Current is one of the cross-equatorial pathways in which this occurs. It is known that upon crossing the equator fluid parcels within this current must modify their potential vorticity, to render them stable to symmetric (inertial) instability and to merge smoothly with the ocean interior.</p><p>A hierarchy of models predict the excitement of inertial instability in cross-equatorial flows dynamically similar to the North Brazil Current. A linear stability analysis of a barotropic flow is able to predict the structure and growth rate of the instability. A two-dimensional numerical model verifies these predictions and shows how the instability is able to stabilise unstable potential vorticity configurations. A simplified three-dimensional model demonstrates how large anti-cyclonic rings spun up at the equator entrain waters with negative PV, before the rings themselves become inertially unstable. The high-resolution, observationally constrained, MITgcm LLC4320 model is probed for signs of this instability process.</p>

Abstract:

<jats:title>Abstract</jats:title><jats:p>Recent studies show that the western boundary acts as a ‘graveyard’ for westward-propagating ocean eddies. However, how the eddy energy incident on the western boundary is dissipated remains unclear. Here we investigate the energetics of eddy-western boundary interaction using an idealised MIT ocean circulation model with a spatially variable grid resolution. Four types of model experiments are conducted: (1) single eddy cases, (2) a sea of random eddies, (3) with a smooth topography and (4) with a rough topography. We find significant dissipation of incident eddy energy at the western boundary, regardless of whether the model topography at the western boundary is smooth or rough. However, in the presence of rough topography, not only the eddy energy dissipation rate is enhanced, but more importantly, the leading process for removing eddy energy in the model switches from bottom frictional drag as in the case of smooth topography to viscous dissipation in the ocean interior above the rough topography. Further analysis shows that the enhanced eddy energy dissipation in the experiment with rough topography is associated with greater anticyclonic-ageostrophic instability (AAI), possibly as a result of lee wave generation and non-propagating form drag effect.</jats:p>

Abstract:

The evolution of upper-ocean potential vorticity (PV) over a full year in a typical midocean area of the northeast Atlantic is examined using submesoscale- and mesoscale-resolving hydrographic and velocity measurements from a mooring array. A PV budget framework is applied to quantitatively document the competing physical processes responsible for deepening and shoaling the mixed layer. The observations reveal a distinct seasonal cycle in upper-ocean PV, characterized by frequent occurrences of negative PV within deep (up to about 350 m) mixed layers from winter to mid-spring, and positive PV beneath shallow (mostly less than 50 m) mixed layers during the remainder of the year. The cumulative positive and negative subinertial changes in the mixed layer depth, which are largely unaccounted for by advective contributions, exceed the deepest mixed layer by one order of magnitude, suggesting that mixed layer depth is shaped by the competing effects of destratifying and restratifying processes. Deep mixed layers are attributed to persistent atmospheric cooling from winter to mid-spring, which triggers gravitational instability leading to mixed layer deepening. However, on shorter time scales of days, conditions favorable to symmetric instability often occur as winds intermittently align with transient frontal flows. The ensuing submesoscale frontal instabilities are found to fundamentally alter upper-ocean turbulent convection, and limit the deepening of the mixed layer in the winter-to-mid-spring period. These results emphasize the key role of submesoscale frontal instabilities in determining the seasonal evolution of the mixed layer in the open ocean.

Abstract:

Many different physical processes contribute to mixing in the ocean. Mixing plays a significant role in shaping the mean state of the ocean and its response to a changing climate. This chapter provides a review of some recent work on the processes driving mixing in the ocean, on techniques for parameterizing the various mixing processes in climate models, and on the role of ocean mixing in the climate system. For the latter, this chapter illustrates how ocean mixing shapes the contemporary mean climate state by focusing on key ocean features influencing the climate (such as the meridional overturning circulation and heat transport, ocean heat and carbon uptake, ocean ventilation, and overflows from marginal seas), how ocean mixing participates in shaping the transient climate change (including anthropogenic ocean heat and carbon uptake, sea level rise and changes in nutrient fluxes that impact marine ecosystems), how ocean mixing is projected to change under future climate change, and how tides and related mixing differed for paleoclimates. Improving our collective understanding of the dynamics of mixing processes and their interactions with the large-scale state of the ocean will lead to greater confidence in projections of how the climate system will evolve under climate change and to a better understanding of the feedbacks that will act to regulate this evolution.