PT5 work

In recent years, the oceanographic community has gained access to information that constitutes a formidable leap for our science. This information includes data from remote sensing instruments such as satellite altimetry and ice thickness from upward looking sonar, in situ observations like hydrography from SeaSoar-CTD and current measurements from VM-ADCP, and model simulations of the ocean circulation that produce results with an increasing level of details. The expression "high resolution information" is here used as a reference to the joint sets of information just listed. In Task 5, we seek to take advantage of these advances in order to arrive at an enhanced understanding and improved description of processes relevant to the ocean climate of the Nordic Seas and the Arctic Sea. The work planned for Task 5 includes studies of (1) variability in the transports of the northern branches of the Gulf Stream system due to oceanic teleconnections by signal propagation, (2) variability in the fluxes and paths of Atlantic Water that enters the Nordic Seas, (3) variability related to the front between Atlantic Water and Arctic Water in the Nordic Seas, and (4) variability and trends related to the ice cover and ice transport in the Arctic Sea and Nordic Seas.

In Norway, there is a tradition for regional model studies of the ocean circulation. Little attention has been paid to examine and understand how the variability of the regional circulation is affected by the variability of the circulation of the North Atlantic Ocean. Hence, one goal here is to investigate the relations between anomalous events in the North Atlantic Ocean and variability of the flux of Atlantic Water into the Nordic Seas. The length of the available time series from the joint sets of high resolution observations and simulations fall short of the period required for studying decadal variability. Hence, we will use these sets in a study of seasonal and interannual variability and episodic events during the last 10-20 years.

One of the model simulations involved has a global perspective on the mesoscale, and it is therefore considered pertinent for examination of variability of transports of mass and heat in the mixed layer of the Atlantic Ocean along the pathways in the Gulf Stream system towards the Nordic Seas. However, this model is unfit to describe variability within the Nordic Seas, and uses open boundaries at the Greenland-Iceland-Scotland ridge with specified lateral fluxes. In subtask 5.1, we will use supplementary results from another model that is truly Atlantic/Arctic and has a focus with high resolution in the region of AW inflow to the Nordic Seas. The models in question are the NLOM [Wallcraft, 1991] and the MICOM [Bleck and Boudra, 1986], respectively.

In order to gain further understanding of AW inflow variability, in situ measurements and observations from remote sensing instruments will also be studied. These data are valuable for examinations of variability and events in their own right. In the present effort, we shall seek to combine the observational data and model output in order to obtain an improved description and understanding by a synthesis of these sets of information. The observations should also prove helpful in sorting out any discrepancies between the two model simulations.

The inflow of AW to the Nordic Seas may be considered as being divided between two pathways, an eastern branch and a western branch. Flow along the two pathways may differ both in the characteristics of the water masses as they enter the Nordic Seas, and in the modifications of AW by mixing with adjacent water masses while in the Nordic Seas. In the first phase, we will describe and analyze to what degree the variability is forced by oceanic teleconnections through signal propagation, and local atmospheric forcing (subtasks 5.1 and 5.2). Particular attention will be paid to the period 1992-present, for which simulation results high quality altimeter data, data from hydrography crossections, and current measurements are all available. The work to be carried out in subtasks 5.1 and 5.2 will be coordinated and performed as a collaborative effort. Here, we will address the question of whether the widely different AW transport values that has been reported are related to oceanic variability or observational inaccuracies, or a combination of the two.

While the warm and salty water masses of the northeastern Atlantic Ocean are generally recognized as a key feature of the climate of Northwestern Europe, trends in the ice coverage of the Arctic Ocean is recognized as being an important indicator of climate change in these and other regions. Also, sea ice is by itself important for the climate, since there is a strong feedback in the heat fluxes between the atmosphere and ocean related to melting and freezing of ice. In subtask 5.3 we will work towards an improved description and understanding of sea ice variability using the relevant high quality information that has been made available in the recent years from, e.g., upward-looking sonar, passive microwave data (SMMR and SSM/I), and model simulations using MICOM coupled to NERSC's Hibler ice model [Drange and Simonsen, 1996; Drange, 1999; Bentsen et al., 1999]. The study will also include ocean variables in the Arctic Sea. Results from subtask 5.2 may provide clues about relations between variability of AW and variability related to Arctic sea ice and hydrography due to signal propagation. The interannual variability of the ice thickness will be given particular attention, and results should prove valuable in order to sort out the widely different trends that have been reported in the literature.

Variability in the AW inflow to the Nordic Seas and variability in the East Greenland Current related to events in the Arctic Ocean have the potential for strongly influencing processes in the frontal zone between these water masses in the Nordic Seas. Obviously, the local winds are also important in this context. A process study of the mixing of water masses across this front will be undertaken in a separate task (Principle Task 4). In subtask 5.4 we aim at describing the seasonal and interannual variability of the position and strength of this front.. Again, the work will be performed using a combination of observational data and model output from MICOM. Results from MICOM have previously been used in conjuncture with observations in a study of fluxes through the boundaries of the Greenland Sea basin [Chierici et al., 1999]. Results from subtasks 5.2 and 5.3 will certainly be relevant for subtask 5.4, and a collaborative effort will be undertaken on this basis.

Subtask 5.1:
Seasonal and interannual variability of the origins of the Norwegian Atlantic Current

Identification and examination of anomalies that either propagate towards the Nordic seas, or cause variability by obstructing the normal pathways of the Atlantic Water, will be at the heart of the study. An important goal is to obtain an understanding and description of how and to what degree such anomalies cause the flow of warm and salty Atlantic Water towards the Nordic Seas to intensify or retard.

Subtask 5.2:
Seasonal and interannual variability of the inflow of Atlantic Water into the Nordic Seas

A key question that will be addressed here is what causes the variability of the Atlantic Water fluxes in the eastern and western branches of the inflow, and the variability of their relative distribution. Also, how is the seasonal variability of the inflow forced, by thermohaline processes or winds? In order to quantify this variability, sea level altimeter data, observations and model results will be studied.

Subtask 5.3:
Seasonal and interannual variability, and trends, of the sea ice and ocean parameters in the Arctic Ocean

In order to describe and understand the variability and trends of the Arctic and sub-Arctic ocean-ice system during the last 20 years, anomalies of selected climate sensitive variables will be studied. These variables include ice extent, area and thickness, first and multiyear ice fraction, ice drift, and ocean parameters, all from observations and available models with a high spatial resolution.

Subtask 5.4:
Seasonal and interannual variability in the mixing of Atlantic and Arctic Water in the Nordic Seas

Processes related to the front between Atlantic and Arctic Water in the Nordic Seas will be studied in detail in Task 4 of the present project. Here, we will use available high resolution information in order to examine the ocean variability which is related to the frontal zone. Key elements in this study of ocean variability will include the position of the front, and its strength in terms of the density gradient.

Arne Melsom
Last modified: Fri Mar 30 13:15:20 GMT 2001