The spring phytoplankton bloom and vertical velocities in the stratified and deep convecting Labrador Sea, as observed by Seagliders

Abstract

The Labrador Sea is a critical region in the world’s ocean: a region where the effects of climate change are seen quickly and strikingly, and where dynamic processes that affect climate change are observed. A recent effect of climate change is the increase of freshwater to high latitudes, as Arctic pack ice and Greenland glaciers melt at accelerating rates. In times of rapid climate change, it is more important than ever to understand the influence these changes have on the status quo of ecosystems and physical processes. The focus of this thesis is to examine the current state of the Labrador Sea biological and physical system through in situ Seaglider and surface satellite observations. The influence of physical processes, including increased freshwater runoff, on the biological system is the subject of chapters 2 and 3. The deep convective process, one of the downwelling branches of the global oceanic overturning circulation is the subject of chapters 4 and 5. Seaglider is capable of making novel measurements of vertical water velocity to better than 1 cm s^{−1} accuracy. Using these measurements along with hydrographic observations, we describe deep convection during the 2004-05 winter. Besides the scientific merits of the results shown here, this thesis also demonstrates the ability of Seaglider to observe bio-optical properties and vertical velocities, two relatively newer observations in the oceanography literature.

Figure 5.18: WKB-scaling of w_rms in the stratified ocean, sg015. Vertical velocity measurements were binned by N and wind speed (either > 10 m s⁻¹ or < 10 m s⁻¹). Winds in these two bands averaged 5 and 16 m s⁻¹. For N < 0.0013 rad s⁻¹, measurements matched theoretical expectations for a GM-internal wave field, scaling as < w² >≈ 0.25N₀/N. The WKB-scaling breaks for N > 0.0013 rad s⁻¹. A weak dependence of the coefficient cGM on winds was found, though 95% confidence intervals overlap. c_GM ≈ 0.18 for weaker winds and 0.25 for higher winds.

Figure 5.20: Scales and magnitude of vertical velocities in the surface mixed layer from sg015, January through March, 2005. a) Decorrelation length scale of vertical velocities in the mixed layer and the mixed layer depth. Notably, the length scales appear to correlate with mixed layer depths, with a ratio of about L = 0.27 MLD. b) Magnitude of vertical velocities in the surface mixed layer estimated as w_rms in 1-daily periods corresponding to the daily period of the heat fluxes and winds from Yu and Weller 2007. Wind speeds and heat fluxes are also shown.

Eleanor Frajka-Williams

Professor of Ocean Dynamics in a Changing Climate

I am a physical oceanographer who uses ocean observations to investigate ocean dynamics and circulation in a changing climate. I have a particular interest in problems spanning scales (from micro- to large-scale) or spheres (biogeosphere, cryosphere, atmosphere), and in methods that leverage traditional observations with new platforms and satellite data.