Changes in Arctic Stratification and Mixed Layer Depth Cycle: A Modeling Analysis

Abstract. The sensitivity of the Indian monsoon to the full spectrum of climatic conditions experienced during the Pleistocene is estimated using the climate model HadCM3. The methodology follows a global sensitivity analysis based on the emulator approach of Oakley and O'Hagan (2004) implemented following a three-step strategy: (1) development of an experiment plan, designed to efficiently sample a five-dimensional input space spanning Pleistocene astronomical configurations (three parameters), CO2 concentration and a Northern Hemisphere glaciation index; (2) development, calibration and validation of an emulator of HadCM3 in order to estimate the response of the Indian monsoon over the full input space spanned by the experiment design; and (3) estimation and interpreting of sensitivity diagnostics, including sensitivity measures, in order to synthesise the relative importance of input factors on monsoon dynamics, estimate the phase of the monsoon intensity response with respect to that of insolation, and detect potential non-linear phenomena. By focusing on surface temperature, precipitation, mixed-layer depth and sea-surface temperature over the monsoon region during the summer season (June-July-August-September), we show that precession controls the response of four variables: continental temperature in phase with June to July insolation, high glaciation favouring a late-phase response, sea-surface temperature in phase with May insolation, continental precipitation in phase with July insolation, and mixed-layer depth in antiphase with the latter. CO2 variations control temperature variance with an amplitude similar to that of precession. The effect of glaciation is dominated by the albedo forcing, and its effect on precipitation competes with that of precession. Obliquity is a secondary effect, negligible on most variables except sea-surface temperature. It is also shown that orography forcing reduces the glacial cooling, and even has a positive effect on precipitation. As regards the general methodology, it is shown that the emulator provides a powerful approach, not only to express model sensitivity but also to estimate internal variability and detect anomalous simulations.

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What controls the mixed-layer depth in deep-sea sediments? The importance of particulate organic carbon flux

Limnology and Oceanography ◽

10.4319/lo.2004.49.2.0620 ◽

2004 ◽

Vol 49 (2) ◽

pp. 620-622 ◽

Cited By ~ 2

Author(s):

Bernard P. Boudreau

Keyword(s):

Organic Carbon ◽

Particulate Organic Carbon ◽

Mixed Layer ◽

Deep Sea ◽

Carbon Flux ◽

Mixed Layer Depth ◽

Layer Depth ◽

Organic Carbon Flux ◽

Particulate Organic Carbon Flux ◽

Deep Sea Sediments

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An alternative method for correcting fluorescence quenching

Ocean Science ◽

10.5194/os-11-83-2015 ◽

2015 ◽

Vol 11 (1) ◽

pp. 83-91 ◽

Cited By ~ 12

Author(s):

L. Biermann ◽

C. Guinet ◽

M. Bester ◽

A. Brierley ◽

L. Boehme

Keyword(s):

Mixed Layer ◽

Mixed Layer Depth ◽

Fluorescence Emission ◽

Euphotic Zone ◽

Light Level ◽

High Light Intensity ◽

Photochemical Quenching ◽

Layer Depth ◽

Southern Elephant Seals ◽

Non Photochemical Quenching

Abstract. Under high light intensity, phytoplankton protect their photosystems from bleaching through non-photochemical quenching processes. The consequence of this is suppression of fluorescence emission, which must be corrected when measuring in situ yield with fluorometers. We present data from the Southern Ocean, collected over five austral summers by 19 southern elephant seals tagged with fluorometers. Conventionally, fluorescence data collected during the day (quenched) were corrected using the limit of the mixed layer, assuming that phytoplankton are uniformly mixed from the surface to this depth. However, distinct deep fluorescence maxima were measured in approximately 30% of the night (unquenched) data. To account for the evidence that chlorophyll is not uniformly mixed in the upper layer, we propose correcting from the limit of the euphotic zone, defined as the depth at which photosynthetically available radiation is ~ 1% of the surface value. Mixed layer depth exceeded euphotic depth over 80% of the time. Under these conditions, quenching was corrected from the depth of the remotely derived euphotic zone Zeu, and compared with fluorescence corrected from the depth of the density-derived mixed layer. Deep fluorescence maxima were evident in only 10% of the day data when correcting from mixed layer depth. This was doubled to 21% when correcting from Zeu, more closely matching the unquenched (night) data. Furthermore, correcting from Zeu served to conserve non-uniform chlorophyll features found between the 1% light level and mixed layer depth.

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Numerical investigation of the Arctic ice-ocean boundary layer; implications for air-sea gas fluxes

10.5194/os-2016-4 ◽

2016 ◽

Cited By ~ 1

Author(s):

A. Bigdeli ◽

B. Loose ◽

S. T. Cole

Keyword(s):

Sea Ice ◽

Water Column ◽

Mixed Layer ◽

Mixed Layer Depth ◽

The Arctic ◽

Layer Depth ◽

Near Surface ◽

Resolution Model ◽

Simplifying Assumptions ◽

Vertical Spacing

Abstract. In ice-covered regions it can be challenging to determine air-sea exchange – for heat and momentum, but also for gases like carbon dioxide and methane. The harsh environment and relative data scarcity make it difficult to characterize even the physical properties of the ocean surface. Here, we seek a mechanistic interpretation for the rate of air-sea gas exchange (k) derived from radon-deficits. These require an estimate of the water column history extending 30 days prior to sampling. We used coarse resolution (36 km) regional configuration of the MITgcm with fine near surface vertical spacing (2 m) to evaluate the capability of the model to reproduce conditions prior to sampling. The model is used to estimate sea-ice velocity, concentration and mixed-layer depth experienced by the water column. We then compared the model results to existing field data including satellite, moorings and Ice-tethered profilers. We found that model-derived sea-ice coverage is 88 to 98 % accurate averaged over Beaufort Gyre, sea-ice velocities have 78 % correlation which resulted in 2 km/day error in 30 day trajectory of sea-ice. The model demonstrated the capacity to capture the broad trends in the mixed layer although with a bias and model water velocities showed only 29 % correlation with actual data. Overall, we find the course resolution model to be an inadequate surrogate for sparse data, however the simulation results are a slight improvement over several of the simplifying assumptions that are often made when surface ocean geochemistry, including the use of a constant mixed layer depth and a velocity profile that is purely wind-driven.

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