Introduction
The Arctic Ocean enters the climate system through its perennial ice cover acting as a global heat sink, and by exporting freshwater which has the potential to affect the Meridional Overturning Circulation (MOC). The fate of freshwater exported to lower latitudes through Fram Strait is a central component in this system. It is investigated under DAMOCLES, but with limited instrumentation and manpower allocated.

This component under iAOOS Norway aim to supplement DAMOCLES with instrumentation and manpower that will enable us to coherently observe the parameters governing the fate of freshwater exported through Fram Strait.

Oceanographic and sea ice observations will be made by permanent installations on bottom moorings, on drift stations, and by two intensive field campaigns using KV Svalbard as platform. The observations will cover the annual cycle of the observed parameters along the entire Transpolar Drift Stream length, from its origin, through Fram Strait, and well into the receiving basins. The logistics and details of the in situ observations are described under WP1.

The scientific questions and how to address them
This task supplements DAMOCLES task 3.4, “Output to impact”. It addresses the time varying processes by which the signals of Arctic change are transferred through subarctic seas to lower latitudes. The VEINS and ASOF cluster of EU projects left a legacy of three observation arrays across the East Greenland Current (EGC) at 79, 74 and 63 N. These observations will continue under DAMOCLES, but in addition to estimating the freshwater transport at each site, focus is now on calculating the freshwater transport divergence by comparing the transport at the different sites. The aim is to provide estimates of the leakage of freshwater from the EGC and Transpolar Drift into the Greenland and Iceland Seas. However, during the early phase of this work several scientific questions have come up. The most urgent questions for the DAMOCLES efforts may be listed as

Q1: A significant amount of liquid freshwater transport seems to take place on the shelf at the Fram Strait observation array at 79 N, also in winter. This is suggested by an ASOF-N wintertime section from May 2005 with helicopter, by the first mooring on the shelf, and by modeling results (NAOSIM, M. Karcher/AWI). What is the magnitude and variability of the liquid freshwater transport on the shelf through Fram Strait at 79 N?

Q2: ASOF-N results show that maximum liquid freshwater transport occurs in September. There is a sharp increase from the minimum in April. The annual maintenance cruises in September provide high resolution CTD sections necessary for the interpolation (horizontal and vertical) between the point measurements at the moorings in this period. What is the picture in May/June, a period where a substantial amount of freshwater transport takes place, but where a sudden freshwater input from melting and solar heating of the upper layers complicates our picture of the stratification and mixing processes taking place?

Q3: A significant melting of sea ice takes place between 79 and 74 N. This is evident from studies comparing upward looking sonar (ULS) observations from 74 and 75 N, and from an observed increased liquid freshwater content at 74 N compared to 79 N.  What are the melt rates across and along the ice stream, and what is the variability and chief controlling factors governing the transition of solid freshwater into liquid as sea ice in the Transpolar Drift Stream drifts into the Fram Strait region and further south?

Q4: During a cruise to Fram Strait in May 2005 an initial EM bird experiment revealed a pronounced gradient in the ice thickness distribution across the Transpolar Drift Stream where it exits the Arctic Ocean proper. This gradient is not readily captured with the existing 1-3 ULSes in the EGC at 79 N, with only 20 km spacing. The preliminary EM bird results indicate that there is significantly more ice in the thicker categories of the thickness distribution west of the ULSes in the EGC, than at the ULS site itself. If the present thickness estimates from the ULSes were assumed to represent the entire ice drift width, one would underestimate the ice volume leaving the Arctic. What is the magnitude of this gradient, how is it distributed across the thickness categories, and how is it varying with time?

These are the questions we seek to address under this task of iAOOS Norway. In the following we propose the instrumentation and field campaigns we deem necessary to pursuit the answers:

Addressing Q1: DAMOCLES will introduce a tube mooring on the shelf measuring T and S at several depths, along with an ADCP providing the corresponding current and transport. The shelf is very wide at this latitude, hence we propose to introduce an additional freshwater mooring under iAOOS Norway. It will consist of an Aanderaa CT string with pressure sensors, with data storage below a weak link. This setup increases the likelihood of data retrieval even after an iceberg collision. An accompanying IAOOS Norway funded ADCP will provide the current field at this third location on the shelf. We also propose to map the velocity field and do detailed CTD sections during two intensive field campaigns, using the KV Svalbard vessel as a platform. These campaigns will yield two snapshots of the freshwater content and transport over the shelf, during the IPY period where similar snapshots are taken upstream in the Arctic basin. If ice conditions allow, similar sections will be made across the shelf during the annual September cruises with RV Lance. Along with ongoing NAOSIM modeling efforts under DAMOCLES (R. Gerdes, M. Karcher), we are able to get a measure of the total liquid freshwater transport out of the Arctic through Fram Strait.

Addressing Q2: The two campaigns with KV Svalbard proposed above will take place in April/May, and will hence provide a picture of the stratification and mixing processes during spring. The winter/spring shelf velocity and TS field is mapped on sections at different latitudes using LADCP and portable CTD equipment from a helicopter. We propose to particularly focus on the 79 N section, where the high resolution TS and current fields are most urgently required for interpolation purposes and the general interpretation of the mooring based point measurements.

Addressing Q3: We propose to install an ASL Ice Profiling Sonar (IPS) on the 74 N array. For funding reasons such instrumentation has been missing on this location. This will provide timeseries of the changes in ice thickness from 79 to 74 N, and hence provide a measure of the transition from solid into liquid freshwater after the exit through Fram Strait. We also propose to do detailed thickness investigations drifting south along the ice drift, using the AWI EM bird, in situ drilling and EM31 observations.

Addressing Q4: We propose to do a detailed transect across the ice drift at 79 N during the KV Svalbard campaigns, using the EM bird, in situ drilling and EM31 measurements. Two detailed snapshots, combined with the data acquired during May 2005, would provide details that would improve our estimates of ice volume fluxes through Fram Strait. Such transects will also be performed at 74 N and midway between the two monitoring lines, further improving our ability to estimate the melt rates of sea ice going south and the corresponding transition into liquid freshwater.