More of our team flew in to join the voyage. Their journey south was a remarkable chain of weather delays, from Christchurch, to Scott Base, then a Basler flight north to Mario Zuchelli Station. Photo: Lana Young.
Ross Sea Voyage Update #4: Katabatic winds
We’ve just emerged from a multi-day katabatic wind event. It’s one thing to write about them in a research paper, but quite another to experience one!
Sixty knot freezing winds cascaded off the ice sheet and blasted out over the coastal ocean – in an event that persisted for several days. It gave us an inside look into the major driver of a wind-forced coastal polynya.
Despite the relatively small area given over to polynya in global terms, they provide a localised connection between the atmosphere, cryosphere and ocean that has implications at local, regional and global scales. Katabatic winds blow off the land, or off fringing ice shelves, cooling the surface ocean until it is sufficiently cold to change phase and form nascent sea ice, which is then swept away to form consolidated sea ice. As well as influencing sea ice production, this coverage feeds-back on regional thermal dynamics and is a major influence on the regional ecosystem.
The ice formation process also results in salt being excluded from the seawater into the surface ocean, which is destabilizing and causes convective plumes to sink through the water column. This denser, oxygenated, salt-enriched water (high salinity shelf water, or HSSW) aggregates and flows according to buoyancy, bathymetry and the Coriolis force. Southward pathways transport water beneath ice shelves where, despite their cold origins, they are warmer than local melting temperatures. That's what drives ice shelf basal melting (melt on the underside of the shelf). At the same time, northward flows drain off the Antarctic continental shelf and enter the global thermohaline systems, and there are indications that this flux is changing. We hope to capture these HSSW fluxes in our data.
Sunshine and katabatic winds in Terra Nova Bay. Photo: Craig Stevens.
An iceberg near Campbell Glacier. Photo: Craig Stevens.
The RV Laura Bassi approaching the Italian base. Photo: Christina Riesselman.
The ship and team spent several days tidying up sampling in Terra Nova Bay, including the final Mario Zuchelli Station re-supply.
The base is a summer-only base, serviced by the RV Laura Bassi, and is around 8km from Korea's Jang Bogo Station, and 25km north of the new Chinese base. We toured the Mario Zuchelli Station and environs; several of us had visited or worked there previously. The base has a contingent of around 45 people and is built from interlocking containers with several large storage buildings and a wharf and some vessels. It operates using helicopters and also supports the Dome C Concordia ice sheet station that is 1200km inland.
The team made a brief visit to Mario Zuchelli Station and explored its surrounds. Photo: Craig Stevens.
The voyage has now shifted from Leg 1 to Leg 2, and with it we have had some personnel changes.
Christina Riesselman (University Otago) returned to New Zealand via Mario Zuchelli, Scott Base and an eventual flight north from McMurdo Station. Her team, PhD students Meghan Duffy and Luisa Fontanot, are well-prepared for their upcoming sampling using the Sub-Bottom Profiler, as well as corer deployments.
The ship was joined by NIWA’s Svenja Halfter, Alina Madita Wieczorec, Craig Stewart and Lana Young, as well as University of Waikato PhD student Georgia Pollard. Their journey south was a remarkable chain of weather delays, followed by just sneaking through for several flights. They flew from Christchurch to McMurdo Sound on a US LC-130 then, after an overnight at Scott Base, waited for weather forecasts to indicate a suitable weather window. At the last minute, they were out on the road to catch a Basler flight north.
We had a “see you in 10 minutes” WhatsApp call while they sat at Mario Zuchelli Station. However, the winds increased to the point where the ship was unable to hold position close enough for the usual barge transfer. Worse still, the winds were set to persist for several more days.
Our planning looked like having a major multi-day delay to pick up our team and the new Italian Party Chief (coordinator). However, a thirty-minute lull in wind enabled the skipper to hold station in a bay a little to the south of the wharf and the base’s helicopter was able to drop the team and their voluminous pile of gear off on the RV Laura Bassi’s helo deck.
Our Italian friends from Mario Zuchelli Station helicoptered our team to RV Laura Bassi when there was a break in the weather. Photo: Lana Young.
A downside of the katabatic event is we’ve had to leave our remaining DITN mooring in place. This sits very close to the north side of the giant Drygalski Ice Tongue, monitoring how it affects the Terra Nova Bay polynya. Its instruments will continue to sample for most of the coming year anyway and so we plan to pick it up, possibly the Korean RV Araon icebreaker can pick it up for us next year. (They also did the original deployment for us.) Waiting for the wind to ease sufficiently would have cost us several days, and we really need to get away.
The full team is now swinging into action as we get going on a major work package for the front of the Ross Ice Shelf, the home of one of Antarctica’s major polynyas. We are busy setting up instruments to be dropped off into the polynya area, as well as getting underway gear ready sampling from the CTD bottle samples. This requires very substantial fine tuning of the schedule as well as a degree of flexibility as the expedition evolves.
I’m writing this update as we pass Franklin Island north of Ross Island. James Clark Ross discovered the islands on this same day of the year, 27 January, 183 years ago.
ASP Project Two: Ocean Mechanics has been maintaining hydrographic moorings here for close on a decade to look at the interaction of the Drygalski Ice Tongue and the polynya.
Porthole view. Photo: Lana Young.
This update was sent from the ship by New Zealand voyage leader Prof Craig Stevens and revised by the NZ-based communications team.