Expert Q&A Done Your research involves studying O2 concentrations in the mixed l

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Expert Q&A Done Your research involves studying O2 concentrations in the mixed layer. Upon arriving at your hydrographic station, you observed that the mixed layer is 100 m deep and supersaturated by 5 mmol m-3. The water temperature is 20°C, the salinity is 35 psu, and the saturation water vapor pressure (i.e., humidity ) is 2.3 %. The gas transfer velocity (K+" DAz) is cqual to 20 cm hr. (hint: Refer to chapter 6 by Libes) a. What was the normal atmospheric equilibrium concentration (NAEC) of O,? b. What is the concentration of Oz in the mixed layer? c. Calculate the air-sca flux of O2. (Hint: make sure all quantities have the same units) d. Is Oz moving into or out of the water? Explain. e. What is one possible reason that could explain why the O; was supersaturated? 3.

Explanation / Answer

The mixed layer of the ocean and the processes therein affect the ocean’s biological production, the exchanges with the

atmosphere, and the water modification processes important in a climate change perspective. To provide a better under-

standing of the variability in this system, this paper presents time series of the mixed layer properties depth, temperature,

salinity, and oxygen from Ocean Weather Station M (OWSM; 66-

N,2-

E) as well as spatial climatologies for the Norwe-

gian Sea. The importance of underlying mechanisms such as atmospheric fluxes, advective signals, and dynamic control of

isopycnal surfaces are addressed. In the region around OWSM in the Norwegian Atlantic Current (NwAC) the mixed layer

depth varies between -

20 m in summer and -

300 m in winter. The depth of the wintertime mixing here is ultimately

restrained by the interface between the Atlantic Water (AW) and the underlying water mass, and in general, the whole

column of AW is found to be mixed during winter. In the Lofoten Basin the mean wintertime mixed layer reaches a depth

of -

600 m, while the AW fills the basin to a mean depth of -

800 m. The temperature of the mixed layer at OWSM in

general varies between 12 -

C in summer and 6 -

C in winter. Atmospheric heating controls the summer temperatures while

the winter temperatures are governed by the advection of heat in the NwAC. Episodic lateral Ekman transports of coastal

water facilitated by the shallow summer mixed layer is found important for the seasonal salinity cycle and freshening of the

northward flowing AW. Atmospheric freshwater fluxes have no significant influence on the salinity of the AW in the area.

Oxygen shows a clear annual cycle with highest values in May–June and lowest in August–September. Interannual vari-

ability of mixed layer oxygen does not appear to be linked to variations in any of the physical properties of the mixed layer.

PACS: Surface mixed layer; Hydrography; Air–sea interaction; Coastal waters

Keywords: Atlantic water; Interannual variability; North Atlantic; Norwegian Sea; Multidecadal time series; Ocean Weather Station Mike

doi:10.1016/j.pocean.2006.03.014

* Corresponding author. Tel.: +47 5520 5881; fax: +47 5520 5801.

E-mail address: even@nersc.no (J. Even Ø. Nilsen). 1 Present address: Geophysical Institute, University of Bergen, Alle´gt. 70, N-5007 Bergen, Norway.

1. Introduction

Understanding and monitoring the seasonal cycle of the mixed layer in the Norwegian Sea and its interan-

nual variability is of key importance to enhance our knowledge about the Atlantic flow towards the Arctic. An

understanding of the processes by which the mixed layer properties vary is essential for quantitative diagnos-

tics for the coupled ocean and atmosphere system and its effect on biogeochemical cycles and ecosystems. Sea-

sonal and interannual fluctuations in the upper ocean climate are also of great importance in other aspects like

biological production (Polovina et al., 1995). Knowledge of the variability in the mixed layer and its properties

in the Norwegian Sea is for instance highly important in the context of heat release from the Norwegian Atlan-

tic Current (NwAC), which is the most important contribution of oceanic heat transport toward high latitudes

in the Northern Hemisphere. The Ocean Weather Station M (OWSM; 66-

N,2-

E) has been operating for

more than 50 years in the Norwegian Sea. Being strategically placed in the western branch of the northward

flowing NwAC (Fig. 1) OWSM is a unique place to monitor possible climatic fluctuations (Gammelsrød et al.,

1992). The long time series that now exists from OWSM provides an excellent means to analyse variations in

mixed layer properties for this area over different time scales.

The mixed layer (ML) is defined as a surface layer where there is nearly no variation in density

with depth, i.e. a quasi-homogeneous region (Kantha and Clayson, 2000). This homogeneity is caused

20o

10o

0o 10o

20o

60o

63o

66o

69o

72o

NORTH

ATLANTIC

VØRING

PLATEAU

Shetland

NORWEGIAN

BASIN

NORWAY

MOHN RIDGE

LOFOTEN

BASIN

Jan

Mayen

ICELAND

SEA

ICELAND

HALTEN

BANK

Faroe

BARENTS

SEA

GREENLAND BASIN GREENLAND

ICELANDFAROE RISE FAROESHETLAND CHANNEL

3000m

400m

3000m

1400m

400m

3000m

NORTH

SEA

ONA

SULA

Fig. 1. Bathymetry of the Norwegian Sea. Isobaths every 100 m from 200 to 3000 m, and at 3500 m. OWSM (circled M) is situated above

the slope southwest of the Vøring Plateau in or near the topographically steered baroclinic western branch of the Norwegian Atlantic

Current (NwAC; arrows). Open circles indicate positions of NCEP/NCAR atmospheric reanalysis data. Points indicate centers of the bins by turbulence through the action of surface forcing, like mechanical mixing by wind stress and convective

mixing by surface buoyancy fluxes. The mixed layer can often show a strong diurnal cycle (Brainerd and

Gregg, 1995), with night-time convection due to cooling driving active mixing from the surface to the sea-

sonal pycnocline, while during daytime a shallower restratification may occur due to heating. Salinity may

also be of importance to mixed layers, especially after heavy rainfall when pools of freshwater at the sur-

face can generate strong haloclines at their base (Lukas and Lindstro¨m, 1991), which can suppress mixing.

We find Niiler and Kraus (1977) to provide most extensive and practical inputs on these issues. More

recent is the book by Kantha and Clayson (2000), which gives a good introduction to the mechanisms

that govern the depth and properties of the mixed layer. Spatial and monthly variability of the mixed

layer depth (MLD) for the global ocean is provided and examined by Kara et al. (2003). However, their

paper does not cover any part of the Arctic Mediterranean. Apart from the work done by Johannessen

and Gade (1984), who used a one-dimensional model to simulate variations in MLD at OWSM for a 17

day period of March 1967, little has been done in the context of studying the evolution and variability of

the mixed layer in the Norwegian Sea.

Since the start of hydrographical observation at OWSM in 1948, several papers have been written to under-

stand the seasonal and interannual variability in the region and its underlying causes. Seasonal variations of

temperature and salinity in the upper layer from 1948 to 1958 have been studied by Helland (1963) and annual

variations of energy exchange across the air–sea interface for the same period by Bøyum (1966). Leinebø

(1991) has looked at the oxygen variations in the intermediate water from 1953 to 1990. The long time series

has been used by Østerhus and Gammelsrød (1999) to show that the temperature of the deep water at OWSM

has increased considerable during the 1990s, while Blindheim et al. (2000) showed a long term decrease in both

temperature and salinity in the upper 500 m of the water column. Furevik et al. (2002) have used the OWSM

data in their study of the temporal and spatial variability of the sea surface salinity in the Norwegian Sea.

Time series of temperature and salinity anomalies for selected depths are presented in Gammelsrød and Holm

(1984) for the years 1948–1981 and in Gammelsrød et al. (1992) for 1948–1991. Series from all depths for

1948–1999 are presented in Nilsen (2003).

In contrast to these earlier time series studies of the OWSM data, which were applied to fixed depths,

we will in this paper present time series of mixed layer averages of temperature and salinity, together with

time series of MLD and mixed layer averages of oxygen for OWSM as well as winter and summer clima-

tologies for the Norwegian Sea. When considering the hydrographic conditions in the area of OWSM,

with a frequent vertical movement of the transition layer between the Atlantic Water (AW) and the inter-

mediate waters below (Mosby, 1962), an anomaly at a fixed depth may be the result of waters having

different origins and consequently of different properties and therefore not represent a variability in time

of the water mass under consideration. We believe that time series of integrated properties over the mixed

layer give a more ideal picture of the variability of the AW than time series of properties from fixed

depths. In this respect, ML-averages provide more reliable basis for creating spatial averages from oceanic

stations, since the bulk of the mixed layer is taken into account and not only the highly variable surface

measurements.

The climatologies of ML-properties are compared to relevant fields of atmospheric fluxes, in order to eval-

uate the importance of horizontal gradients in the area. For the annual cycle and interannual variations, the

role of atmospheric forcing is examined, and effects of advection and entrainment are briefly discussed. Fur-

thermore, the study describes some important mechanisms involving mixed layer processes through which the

northward flowing Atlantic Water can be modified. The main objective of the present work is to show how

different ML-properties in the Norwegian Sea vary on different time scales, and to reveal some of the under-

lying processes that cause such variations. The results (Section 6) have been divided into four sections treating

the Mixed layer depth, ML-temperature, ML-salinity, and ML-oxygen separately. In each of these sections,

the spatial and seasonal differences in the Norwegian Sea, the mean annual cycle at OWSM, and the multi-

year time series from OWSM will be discussed.

Before the results are treated, we will present an overview of the current knowledge of the hydrography in

the Norwegian Sea (Section 2), discuss important mechanisms governing the oxygen content of this part of the

world oceans (Section 3), and account for the data (Section 4) and methods of analysis (Section 5) used in this

paper

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