Halifax: 19th - 21st October, 2015

VITALS
Ventilation, Interactions and Transports Across the Labrador Sea

Mobile Sampling Team

Objectives

  1. To characterize the spatial scales of convection and re-stratification in the Labrador Sea.
  2. To examine the horizontal exchange influencing properties at the central mooring.
  3. To explore the impact of frontal dynamics on phytoplankton biomass and export productivity.

Background

Mobile sampling with periodic cruises and autonomous profiling and drifting platforms will provide crucial information on the temporal evolution and lateral structures in the Labrador Sea. Many challenges persist in understanding spring/summer re-stratification, which allows the spring bloom to occur, thereby driving the biological carbon pump in this region, and setting up the conditions for the deep convection the following winter. This re-stratification is not a one-dimensional warming of the upper water by the sun; solar insolation can only account for half of the observed stratification increase (Lilly et al. 2003), and the increasing salinity at depth can only be explained by non-local processes. Eddies from the warm, saline boundary currents that ring the Labrador Sea intrude laterally and carry significant fluxes of heat, salt, and distinct nutrients signatures. In order to observe these lateral re-stratification process, we will primarily focus our sampling effort on the central Labrador Sea, in the 50-100 km around the mooring array, using a combination of mobile platforms including ocean gliders and deep ocean profiling drifters. A primary goal of the central sampling will be to determine how the mesoscale coherent features that are observed in the Labrador Sea decay in the interior.

The structures that drive submesoscale mixing must be understood to improve the fidelity of models to produce accurate representations of not just physical exchange rates and processes (Capet et al 2008, Spall 2003), but also of biological productivity (Kortzinger et al. 2008; Fraijka-Williams and Rhines 2010). These processes are poorly sampled, with the basic statistics only starting to be collected (Cole et al. 2010). We will also use the same tools is to carefully measure the Labrador Current front in order to quantify its transports, and to better understand the interplay between physical frontal processes and biological processes. Fronts influence biological productivity, as they are regions of lateral nutrient exchange (Palter et al. 2011) and create strong vertical velocities over narrow spatial scales (D'Asaro et al. 2011). Thus, fronts are intimately linked to the vertical transport of nutrients, carbon, and oxygen. Until recently, observations of the physics and biology of fronts at relevant spatial scales in the open ocean have been extremely limited; as a result, our understanding of the biology at and near these frontal zones remains in its infancy. The Labrador Current is a pathway of Arctic water that is freshening due to changing climate and will be a second focus area for the Mobile Sampling team. The productive Labrador Current influences water properties to the south and is critical to Canadian fisheries. It also provides an accessible analogue for fronts elsewhere in the Labrador Sea and the global ocean.

Methods

Our main tools are:

  1. Gliders: autonomous underwater vehicles capable of sampling the top kilometer of the ocean at high spatial and temporal resolution. They will measure T, S, oxygen, chlorophyll, and depth-averaged currents. One glider will also measure current shear. Gliders can operate for many months, making them ideally suited for monitoring changes in the inclement weather of the Labrador Sea.
  2. Profiling Floats: Free-floating, and profiling between 0 and 1000 m, Argo floats currently collect 100 to 150 temperature and salinity profiles in the Labrador Sea each month. However, the Argo program does not typically sample oxygen concentrations or directly measure velocity, and they leave key features of the Labrador Sea under-sampled. We will add oxygen sensors to 4 Argo floats released by DFO each year, augmenting the oxygen measurements made at our fixed mooring site) and deploy three additional (EM/APEX) floats to measure oxygen, absolute velocity, and vertical shear. These floats will have rapid communication with shore to retrieve data and adapt their sampling plan. They also operate during the extreme weather events that are so important in this region.
  3. Undulating CTD Profiler: Deployed from a ship, these profilers can measure T,S profiles very quickly and match the results to current profiles collected by the ship’s acoustic Doppler current profiler.

Labrador Current

This study will be conducted first and give us valuable experience piloting gliders and analyzing the results. We will use the motion of the Labrador Current to carry two gliders downstream, while glider propulsion will be used to cross the current. Each glider will complete about 5 cross-current transects along its flight while it travels downstream more than 750 km, permitting detailed characterization of properties in the cross-stream direction as well as the downstream evolution of the current. By launching the gliders in pairs, we will be able to sample at fine scales in the along-shelf direction, calculate gradients, and quantify spatio-temporal changes to the current. The glider resolution (4-6 km of horizontal distance between each vertical profile) will provide data at a much finer resolution than the hydrographic sections typically used to characterize the volume, heat, and freshwater transports along the slope (Palter and Lozier 2008). The current is the primary export pathway from the Arctic, and this sampling will elucidate the dynamics of the exchange from the shelf to the Labrador Sea, which is poorly understood (Loder et al., 1998).

Lateral Scales at the Central Mooring

We will use the gliders and Argo float arrays to quantify lateral scales and advection around the mooring array. The spatial separation scale of the deployments will be chosen with the help of the numerical modeling teams through Observing System Simulation Experiments (OSSEs). This mobile array will assess the heat, salinity, and gas budgets at the fixed mooring. Early in the program, before the full mooring array is deployed, we will fly two gliders in the open Labrador Sea to develop sampling protocols and determine the seasonal dynamics of the spatial scales of frontal systems. In years 2-4, we will use the gliders and float array to map the spatial structures in which the moorings are embedded. Beyond helping the central mooring calculations, these observations will provide valuable information about evolving lateral structures and mixing in a very dynamic environment. These observations are sorely lacking globally, and are very important for improving numerical parameterizations of upper ocean processes.

Re-stratification Due to Coherent Eddies

In order to better capture the annual eddy-mediated re-stratification in the Labrador Sea we also propose a Lagrangian experiment that follows 6 months of a single eddy’s interaction with its surroundings. For this experiment, satellite information, and the undulating profiler on the ship will be used in early spring to find an Irminger Ring north east of the mooring location. The eddy will be mapped out, sampled biogeochemcially with CTD rosettes, and seeded with floats and a pair of gliders. The float and glider array will monitor the eddy dynamics through the summer, and then (ship time permitting) the eddy will be resampled by the ship in the fall, and the mobile assets retrieved.

Deliverables

These mobile sensors will provide a number of observations that are vitally important to understanding the ocean's role in the climate system, very challenging to model, and of high-impact. Specifically, our observations will yield:

  1. Better understanding of frontal dynamics in a moderately strong current. Both the physical and biological effects of these fronts are not well understood.
  2. An increased ability to quantify the transport of heat and freshwater out of the Arctic along the Labrador current and improved resolution of the mechanisms of exchange of this water with the Labrador Sea.
  3. Enhanced understanding of lateral processes that drive mixing in the central Labrador Sea, which will lead directly to model improvements, as convection is very sensitive to the parameterization of unresolved lateral processes.
  4. Better understanding of eddy dynamics, which are crucial to re-stratification, and presently inadequately represented numerically.