Impact of Sea-Ice Bottom Topography on the Ekman Pumping - Towards an Interdisciplinary Approach in Earth System Science - page 128

1 Introduction

The sea ice in the Arctic Ocean has a surface and bottom topography that is
characterized by many different scales from small hummocks and piles of ice to
large ridges. A pressure ridge consists of a part that extends into the atmosphere
(sail) and a part that extends into the ocean (keel).
The sails are usually above one meter, sometimes they can be as high as 2 m. In
order to satisfy the hydrostatic equilibrium, the keels usually extend much deeper
into the ocean and may reach depths of 30 m (Pite et al. 1995 ). The formation of
these topographic features depends on the ice motion. In particular, large pressure
ridges are formed when the ice is exposed to strong convergence.
The main forces that govern the ice motion are the internal forces (Steele et al.
1989 ), the local winds and the ocean currents (Thorndike and Colony 1982 ). In the
momentum balance equation that describes the ice motion, the interactions between
air, ice and water are parameterized by drag coef

cients
must account for sea-ice surface characteristics on the near-surface transport of
momentum. The sea-ice surface is spatially and temporally inhomogeneous and
thus we can expect spatial and temporal variations of the drag coef

cients. These drag coef

cients as well.

cients on the surface
topography of the ice. In particular, for the atmospheric drag coef

Many studies addressed the dependence of the drag coef

cients, param-
eterizations for numerical models have been developed (see, e.g., Birnbaum and
L ü pkes 2002 ; Garbrecht et al. 2002 ;L ü pkes and Birnbaum 2005 ;L ü pkes et al.

2012 , 2013 ). In these parameterizations the atmospheric drag coef cients are a

function of surface characteristics of the ice (i.e., melt ponds, pressure ridges, fl oe

edges). Only very few studies focused on the oceanic drag coef cients. Among

these few, the studies by Steiner et al. ( 1999 ) and Steiner ( 2000 ) relate the drag
coef cients to the roughness of the ice, whereas in Lu et al. ( 2011 ) the oceanic drag

coef cients are expressed as a function of observable geometric parameters of the

sea ice such as the depth of keels, the mean separation between ridges, and the fl oe

edges.
The momentum transferred by wind or ice to the ocean is redistributed by
vertical turbulent mixing from the surface to a certain depth. The layer with tur-
bulence, that is where the vertical variations of the surface stress are not negligible,
is called the Ekman layer. The

fl fluxes of momentum lead to the formation of a

velocity field in the surface layer of the ocean. Associated with the induced velocity

is the vertical Ekman pumping (when directed downwards) or Ekman suction (when
directed upwards). The Ekman pumping (suction) depends on the wind stress
applied at the upper surface and represents the amount of volume pumped from
below into (or from above out of) the Ekman layer. It was also shown (Rabe et al.
2011 ) that variations in Ekman pumping affect the depth of the 34-isohaline with
consequences for the entire ocean circulation. In most state-of-the-art global cir-
culation models, the stress at the ice-ocean interface depends on the variability of
the wind

cients are usually not taken into
account. In the present study we calculate oceanic drag coef

field, while variations in the drag coef

cients as function of