David Marshall

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.

Abstract:

Purpose of ReviewAssessment of the impact of ocean resolution in Earth System models on the mean state, variability, andfuture projections and discussion of prospects for improved parameterisations to represent the ocean mesoscale.Recent FindingsThe majority of centres participating in CMIP6 employ ocean components with resolutions of about 1 degree intheir full Earth System models (eddy-parameterising models). In contrast, there are also models submitted to CMIP6 (both DECKand HighResMIP) that employ ocean components of approximately 1/4 degree and 1/10 degree (eddy-present and eddy-richmodels). Evidence to date suggests that whether the ocean mesoscale is explicitly represented or parameterised affects not onlythe mean state of the ocean but also the climate variability and the future climate response, particularly in terms of the Atlanticmeridional overturning circulation (AMOC) and the Southern Ocean. Recent developments in scale-aware parameterisations ofthe mesoscale are being developed and will be included in future Earth System models.SummaryAlthough the choice of ocean resolution in Earth System models will always be limited by computational consider-ations, for the foreseeable future, this choice is likely to affect projections of climate variability and change as well as otheraspects of the Earth System. Future Earth System models will be able to choose increased ocean resolution and/or improvedparameterisation of processes to capture physical processes with greater fidelity.

Abstract:

<jats:title>Abstract</jats:title> <jats:p>A substantial fraction of the deep ocean is ventilated in the high-latitude North Atlantic. Consequently, the region plays a crucial role in transient climate change through the uptake of carbon dioxide and heat. However, owing to the Lagrangian nature of the process, many aspects of deep Atlantic Ocean ventilation and its representation in climate simulations remain obscure. We investigate the nature of ventilation in the high latitude North Atlantic in an eddy-permitting numerical ocean circulation model using a comprehensive set of Lagrangian trajectory experiments. Backwards-in-time trajectories from a model-defined ‘North Atlantic DeepWater’ (NADW) reveal the locations of subduction from the surface mixed layer at high spatial resolution. The major fraction of NADW ventilation results from subduction in the Labrador Sea, predominantly within the boundary current (̴ 60% of ventilated NADW volume) and a smaller fraction arising from open ocean deep convection (̴ 25%). Subsurface transformations — due in part to the model’s parameterization of bottom-intensified mixing—facilitate NADWventilation, such that water subducted in the boundary current ventilates all of NADW, not just the lighter density classes. There is a notable absence of ventilation arising from subduction in the Greenland-Iceland-Norwegian Seas, due to the re-entrainment of those waters as they move southward. Taken together, our results emphasize an important distinction between ventilation and dense water formation in terms of the location where each takes place, and their concurrent sensitivities. These features of NADW ventilation are explored to understand how the representation of high-latitude processes impacts properties of the deep ocean in a state-of-the-science numerical simulation.</jats:p>