5.6.1 The sources
The deep and abyssal waters of the global ocean
are produced at near-polar latitudes by the intense
cooling of winter storms. The waters convect
downwards and spread slowly equatorward,
renewing the deepest and most distant layers of
the global ocean on a time scale of several hundred
years. In the south convection carries such water
from the fringes of the Antarctic continent directly
to great depths, particularly in the Weddell and
Ross Seas. In the north, because of the topography
of the seabed, the cold dense waters are dammed
up behind the Greenland–Iceland–Scotland ridge
(Fig. 5.6.1) and enter the world ocean at shallow
levels. They are known as the northern overflows
of the Atlantic Ocean and form the deepest component of North Atlantic Deep Water (NADW),
whose course can be traced into the Southern,
Indian and Pacific Oceans. In the Atlantic this
water mass forms the deep component of the overturning circulation, southward at depth, northward at shallower levels, which give rise to the
transports discussed in Section 6 of this volume.
The goals of this chapter are to review what is
currently known about the northern overflows on
the basis of observations, models and theory providing, as project WOCE (World Ocean Circulation
Experiment) has done, a baseline of knowledge for
future research. Special emphasis will be given to
the assembly of a coherent body of observations in
one location in order to simplify the evaluation of
the increasingly sophisticated numerical models
expected in the future. In the following pages we
review the sources of the overflows and their paths
(north of 60°N), the measurements of their transport, and of the estimates of errors, uncertainties
and variability. The successes and failures of modelling the overflows to date will also be considered,
along with the topics of hydraulic control (at the
sills) and the nature and magnitude of mixing.
5.6.1.1 Sources of the overflow
The nature of the barrier separating the North
Atlantic from the Nordic seas is revealed in
Figure 5.6.1. The principal channels for the overflows are the Denmark Strait in the west and the
Faroe Bank Channel in the east. Their sill depths
are at 620 m and 840 m, respectively, whereas
between them the maximum barrier depth is about
480 m. Minor episodic overflows occur principally
just west of the Faroe Islands and on the Wyville–
Thomson ridge where the maximum depth is
625 m. Here we will concentrate on the two major
overflows that principally determine both the
properties and flux of NADW. A detailed discussion of the minor overflow components can
be found in the recent review by Hansen and
Østerhus (2000).
The outflow of cold water from the region
north of the ridge is approximately balanced by
the inflow of warm Atlantic water, the imbalance
arising from the net inflow to the Arctic through
the Bering Strait and the excess of fresh water
from runoff and precipitation at polar latitudes.
The inflowing waters are saline and because of the
surplus of fresh water the outflowing waters
are cold and fresh. Mauritzen (1996a) describes
the conversion between these two types as follows.
5.6
The Dense Northern Overflows
Peter M. Saunders
401
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER
The deep and abyssal waters of the global ocean
are produced at near-polar latitudes by the intense
cooling of winter storms. The waters convect
downwards and spread slowly equatorward,
renewing the deepest and most distant layers of
the global ocean on a time scale of several hundred
years. In the south convection carries such water
from the fringes of the Antarctic continent directly
to great depths, particularly in the Weddell and
Ross Seas. In the north, because of the topography
of the seabed, the cold dense waters are dammed
up behind the Greenland–Iceland–Scotland ridge
(Fig. 5.6.1) and enter the world ocean at shallow
levels. They are known as the northern overflows
of the Atlantic Ocean and form the deepest component of North Atlantic Deep Water (NADW),
whose course can be traced into the Southern,
Indian and Pacific Oceans. In the Atlantic this
water mass forms the deep component of the overturning circulation, southward at depth, northward at shallower levels, which give rise to the
transports discussed in Section 6 of this volume.
The goals of this chapter are to review what is
currently known about the northern overflows on
the basis of observations, models and theory providing, as project WOCE (World Ocean Circulation
Experiment) has done, a baseline of knowledge for
future research. Special emphasis will be given to
the assembly of a coherent body of observations in
one location in order to simplify the evaluation of
the increasingly sophisticated numerical models
expected in the future. In the following pages we
review the sources of the overflows and their paths
(north of 60°N), the measurements of their transport, and of the estimates of errors, uncertainties
and variability. The successes and failures of modelling the overflows to date will also be considered,
along with the topics of hydraulic control (at the
sills) and the nature and magnitude of mixing.
5.6.1.1 Sources of the overflow
The nature of the barrier separating the North
Atlantic from the Nordic seas is revealed in
Figure 5.6.1. The principal channels for the overflows are the Denmark Strait in the west and the
Faroe Bank Channel in the east. Their sill depths
are at 620 m and 840 m, respectively, whereas
between them the maximum barrier depth is about
480 m. Minor episodic overflows occur principally
just west of the Faroe Islands and on the Wyville–
Thomson ridge where the maximum depth is
625 m. Here we will concentrate on the two major
overflows that principally determine both the
properties and flux of NADW. A detailed discussion of the minor overflow components can
be found in the recent review by Hansen and
Østerhus (2000).
The outflow of cold water from the region
north of the ridge is approximately balanced by
the inflow of warm Atlantic water, the imbalance
arising from the net inflow to the Arctic through
the Bering Strait and the excess of fresh water
from runoff and precipitation at polar latitudes.
The inflowing waters are saline and because of the
surplus of fresh water the outflowing waters
are cold and fresh. Mauritzen (1996a) describes
the conversion between these two types as follows.
5.6
The Dense Northern Overflows
Peter M. Saunders
401
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER