After completing work along the Alaskan coast for the first week of this cruise RV Mirai turned north to challenge the ice edge. Up till now the polar pack has remained resolutely closer to shore than previous years, but in the last few days has retreated sufficiently for us to attempt to occupy the Northwind Abyssal Plain mooring NAP-12 at 75ºN 162ºW. Comparing the multiple sources of ice information received onboard, some collated by Martech Polar staff and forwarded to the ship, the Ice Navigator, Captain and Chief Scientist selected a northwesterly course toward station 033 at 74ºN 162ºW then north along the meridian to NAP-12. It should be noted that given her relatively light 1A ice class, the Mirai generally avoids ice, and preference is not to enter ice regimes of more than 1-3/10ths concentration.
RV Mirai relies primarily on open source ice information most of which is not real time. American NOAA and National Ice Centre charts are composite averages of past conditions issued three times and two times a week respectively. The scale permits reasonable strategic planning, but is not truly suitable for any tactical planning, however the dated nature of the data lessons the value for efficient route selection. Daily Japanese AMSR-2 sea ice and sea surface temperature and Norwegian TOPAZ sea ice charts have been proving to be more accurate compared to actual on location conditions. However the small scale – large areas and “macro” view with no clear ice regime partial definition makes them suitable for broad scale strategic planning, but still fall short of ideal for more accurate route planning. Taken all together however, reasonable decision making can be done as timing and routing are extremely flexible during a research cruise such as this.
As we neared the target station on 07 September it was clear that both the NOAA and NATICE charts were way off the mark. Where each had reported considerably more ice we encountered none. Both the TOPAZ and AMSR2 were far more accurate in showing virtual open water right to an ice edge, that due to the scale of both charts could have been north of or close to the NAP-12 mooring. We encountered our first ice mid day on the 7th, strips and patches of rotten old and thick first year ice along latitude 74º30’N. From that point on the Ice Navigator remained on the bridge as an active member of the bridge team. We were able to continue north, threading through the open patches between the very rotten strips and infrequent patches till we reached 74º50’N where 2.8nm ahead 6-8/10ths old ice blocked further passage to the light ice classed Mirai. We were ten miles short of our destination, but discretion is the better part of valour.
It is not a total loss. The mooring sensors and data collection equipment can comfortably remain another year until recovery during the 2014 voyage, or, if the pack continues to retreat northward and we have time later in this cruise we can recover NAP-12. But the decision of the Captain and Chief Scientist on the 7th was to move south and select an alternate location for the NAP-13 mooring at 74º34’N 161º58’W.
The new NAP-12 mooring was deployed 08 September. Given the depth of 1600m at the mooring site, the mooring string was just over 1000m long. At the top acting both as sensor and top float was a rigid foam encased ASL designed Ice Profiling Sonar, in place to measure ice thickness from below. In amongst numerous additional floats below were then rigged an acoustic doppler current profiler, several delicate gas and chemical sensors, the two sediment traps with conductivity and temperature sensors attached (perhaps the primary reason for the mooring) and a current meter. With the mooring successfully deployed, RV Mirai chalked up another successful mission objective completed.
Capt David (Duke) Snider
Ice Navigator
RV Mirai