Introducing katabatic winds in global ERA40 fields to simulate their impacts on the Southern Ocean and sea-ice

doi:10.1016/j.ocemod.2010.07.001

Ocean Modelling

Volume 35, Issue 3, 2010, Pages 146-160

https://doi.org/10.1016/j.ocemod.2010.07.001 Get rights and content

Abstract

A medium resolution (10–20 km around Antarctica) global ocean/sea-ice model is used to evaluate the impact of katabatic winds on sea-ice and hydrography. A correction is developed to compensate for the drastic underestimation of these katabatic winds in the ERA40 reanalysis. This correction derives from a comparison over 1980–1989 between wind stress in ERA40 and those downscaled from ERA40 by the MAR regional atmospheric model. The representation in MAR of the continental orography surrounding the ocean, like the Transantarctic Mountains, and a specific parameterisation of roughness length in the planetary boundary layer yield a major improvement in the representation of katabatic winds along the coast of Antarctica. Wind stress directions at the first ocean point are remarkably similar in ERA40 and MAR, but MAR wind stress amplitudes are much greater. From this comparison, a scale factor constant in time (i.e. no seasonal variation) but spatially varying (decreasing off-shore over a distance of about 150 km) is created for the meridional and zonal wind stress components and adapted to the wind vector. The correction thus consists of a local amplification of the amplitude of the 6-hourly ERA40 wind vector components at ocean points near the coast. The impact of katabatic wind correction is investigated in 40-year long twin simulations of a global ocean/sea-ice model. The wind stress over polynyas is increased by a factor of 2, and amplitudes of sensible and latent air–sea heat exchanges are increased by 28% and 18%, respectively. Sea-ice thickness and ice-fraction near the coast of Antarctica show a marked decrease. The amplified katabatic winds also increase the extent of coastal polynyas by 24% (i.e. the total polynya area is augmented by 60,000 km³ around Antarctica), and the winter sea-ice production in polynyas is greater by 42%. Outside polynyas, the impact is a reduction of sea-ice production in the Southern Ocean sea-ice pack. Impacts on the ocean circulation are also marked. Katabatic wind amplification strengthens the local overturning in coastal polynyas with a more intense transformation of Antarctic Surface Waters into colder and denser shelf waters (in total over all polynyas around Antarctica, the overturning reaches 4.7 Sv in annual mean, an increase of 1.8 Sv, and peaks to 6 Sv in winter). The modification of shelf water properties and of the zonal surface winds yields an increase of the amplitude of the seasonal cycle of the Antarctic Coastal Current.

Introduction

Antarctic katabatic winds are gravity winds generated by intense radiation cooling of air adjacent to ice sheet surfaces, especially in winter. Their strength is largely determined by local orography, which explains that they are rather persistent in strength and direction, and why they are particularly strong in the presence of a topographic confluence.

As they get in contact with the ocean surface, these cold winds promote high rates of sea-ice formation as they continually push sea-ice away from the coast line, creating ice free areas (i.e. polynyas) in winter at selected locations along the Antarctica coast. The effect of katabatic winds can extend over 100 km off-shore (Adolphs and Wendler, 1995), although they typically lose most of their momentum within a few tens of kilometres from the coast (Bromwich and Kurtz, 1984).

According to Massom et al. (1998), katabatic winds are responsible for the formation of 16 polynyas of the 28 that have been observed along East Antarctica. Oceanic effects of these polynyas are an intense cooling of surface waters, and a brine rejection associated with the formation of new sea-ice. Ocean vertical mixing and convection are consequently increased, and dense waters formed on the shelf will later influence the properties of intermediate and deep waters around Antarctica. A description of katabatic winds and of the dynamics of polynyas may be found in Parish, 1988, Maqueda et al., 2004, respectively.

The small scale orography (e.g. glacier valleys crossing the Transantarctic Mountains) that largely influences katabatic winds is generally not well represented in global atmospheric general circulation models (AGCMs) that are used for global weather analyses or reanalyses (Van Den Broeke et al., 1997, Petrelli et al., 2008). The regional atmospheric models MAR (Modèle Atmosphérique Régional, Gallée and Schayes, 1994) has parameterisations dedicated to ice–air interactions, and a locally tuned orography roughness. These improved configurations help simulate more realistic katabatic winds, as shown by Petrelli et al. (2008). However, no reanalysis was produced with such models over the last 50 years, and their use in constructing an atmospheric forcing of a global ocean general circulation model (OGCM) relevant to the period 1960 to present requires dedicated downscaling and blending work. A way to do this, attempted by few authors in various manners (see Section 3), is to work out a correction of the reanalysed winds that improves the coastal effects of katabatic winds on the simulated sea-ice and ocean properties.

The ultimate objective of the present work is to study the impact of katabatic winds on sea-ice and ocean properties around Antarctica in a global ocean/sea-ice model. For that purpose, we first develop a correction to ERA40 winds in order to better account for the katabatic winds effects. The MAR model, forced laterally by the ERA40 reanalysis, was run over 10-years (1980–1989), and thus provided a regional downscaling of ERA40 over Antarctica with improved boundary layer dynamics. Comparing the outputs of the regional model with the reanalysis yields a local correction of the global ERA40 winds. The effects of katabatic winds on the ocean properties are then assessed by simulations carried out with a global model configuration of the NEMO OGCM (Madec, 2008), at a resolution of 1/2°.

Section 2 provides a short description of the characteristics of ERA40 and MAR models, along with a comparison between their surface fields. The description of the katabatic wind correction follows in Section 3. Section 4 presents the various NEMO based model configurations and the simulation strategy. The ocean model results are analysed in Section 5, sorting out the effects of katabatic winds on sea-ice and water masses.

Section snippets

Near-coast atmospheric surface conditions in ERA40 and MAR

To provide guidelines for the construction of the correction of ERA40 winds, we compare the wind stresses of the global reanalysis with those produced by the MAR atmospheric regional model. The comparison is focused on the wind stress component at the coast around Antarctica. This regional model, which has slightly finer horizontal and vertical resolutions, uses a bottom boundary layer adapted to ice-covered regions, and an orography calibrated for katabatic winds.

Correction of katabatic winds

Other studies have proposed corrections for katabatic winds in the ocean modelling community. Kim and Stossel (1998) proposed a basic correction in their coarse resolution ocean circulation model, and fixed the wind to 20 m s⁻¹ (northward) along the coast of the Bellingshausen Sea, the Amundsen Sea and the East coast, and to 10 m s⁻¹ (northward) elsewhere. Roeske (2005) proposed to correct the meridional wind by the addition of an offset which is proportional to the difference between the wind over

Ocean circulation model

The effect of katabatic winds on ocean/sea-ice dynamics is assessed through twin experiments performed with the global ocean sea-ice DRAKKAR configuration ORCA05 (resolution of 0.5°). This section briefly describes the ORCA05 configuration and the atmospheric forcing (including the application of the katabatic correction) used over the period 1958–2001.

Impact of katabatic winds

Katabatic winds are known to be one of the main factors driving polynya dynamics around Antarctica. Massom et al. (1998) showed that 60% of the polynyas found along the East coast are forced, at least partly, by katabatic winds (the Mertz Glacier Polynya or Terra Nova Bay Polynyas for example). Dense Antarctic waters are primarily formed on the Antarctic shelf by formation of sea-ice during winter months. This process is assisted by coastal polynyas. The strong cooling and salting (due to brine

Impact on sea-ice in Polynyas

Coastal polynyas exhibit a large rate of sea-ice production on shelf areas. Therefore, we define here as polynyas areas which satisfy the following twofold criterion: ice production greater than 0.7 m per month; and ocean depth shallower than 1200 m.

With this criterion, coastal polynyas are strictly limited to small areas of a few tens of kilometres spread along the continent (note that ice fraction can be lower than 40% in polynyas as shown by the map displayed in Fig. 10).

The global

Conclusion

A correction of ERA40 wind velocity around the coast of Antarctica that accounts for the underestimation of the katabatic winds in ERA40 has been constructed. The correction is based on a comparison over the period 1980–1989 of ERA40 reanalysis wind stress and the wind stress obtained by a downscaling of ERA40 by the MAR regional atmospheric model. The correction takes the form of a constant in time but spatially varying multiplying factor (ranging, for the stress, from 6 to slightly less than

Acknowledgements

The authors acknowledge support from Ministère de l’Education Nationale et de la Recherche and from Centre National de la Recherche Scientifique (CNRS). This work is a contribution of the DRAKKAR project. Support to DRAKKAR comes from various grants and programs listed hereafter: French national programs GMMC, LEFE, and PICS2475. The contribution of Institut National des Sciences de l’Univers (INSU) to these programmes is particularly acknowledged. DRAKKAR acknowledge the support from the

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Introducing katabatic winds in global ERA40 fields to simulate their impacts on the Southern Ocean and sea-ice

Abstract

Introduction

Section snippets

Near-coast atmospheric surface conditions in ERA40 and MAR

Correction of katabatic winds

Ocean circulation model

Impact of katabatic winds

Impact on sea-ice in Polynyas

Conclusion

Acknowledgements

Deep Sea Res. II

Ocean Modell.

Deep Sea Res.

Ocean Modell.

J. Mar. Syst.

A pilot study on the interactions between katabatic winds and polynyas at the Adelie Coast, Eastern Antarctica

Antarctic Sci.

Observations and modelling of Antarctic downslope flows: a review

Antarctic Res. Ser.

Impact of partial steps and momentum advection schemes in a global ocean circulation model at eddy-permitting resolution

Ocean Dynam.

A method for improved representation of dense water spreading over topography in geopotential-coordinate models

J. Phys. Oceanogr.

Causes of interannual – decadal variability in the meridional overturning circulation of the mid-latitude North Atlantic Ocean

J. Clim.

Agulhas leakage dynamics affects decadal variability in Atlantic overturning circulation

Nature

Katabatic wind forcing of the terra Nova Bay Polynya

J. Geophys. Res.

Transport and variability of the Antarctic circumpolar current in Drake Passage

J. Geophys. Res.

The development of Antarctic katabatic winds and implications for the Coastal Ocean

J. Atmos. Sci.

Application of the e-turbulence closure model to neutral and stable atmospheric boundary layer

J. Atmos. Sci.

Sea-ice index monitors polar ice extent

Eos Trans. AGU

Sensitivity of a global sea-ice model to the treatment of ice thermodynamics and dynamics

J. Geophys. Res.

Dynamical aspects of katabatic winds evolution in the antarctic coastal zone

Boundary Layer Meteorol.

Development of a three dimensional meso-scale primitive equations model, katabatic winds simulation in the area of Terra Nova Bay

Antartic Mon. Weather Rev.