[4.Jul.'17] Arctic Amplification weakens as sea ice becomes increasingly seasonal
The Arctic Ocean has less and less sea ice cover from year to year. We demonstrate using a new metric, the seasonality number, that the northern hemisphere sea ice cover is now as seasonal as the one in the southern hemisphere, which is considered to be seasonal as long as modern observation exist. The presence of a relatively thin summer sea ice cover vulnerable to complete melt is a key factor in positive ice feedbacks leading to the sso-called Arctic Amplification, the faster warming of northern high latitudes compared to the global mean. As sea ice coverage in the summer shrinks Arctic warming stagnates and the amplification reduces.



[10.Mar.'14] Increasing Arctic Ocean surface stress as sea ice retreats—except in summer
The Arctic sea ice cover has thinned dramatically over the last two decades. Consequently, the ice cover is weaker, less resistive against the wind forcing, and moves faster. Using a regional sea-ice ocean simulation forced by realistic winds we show that these sea ice changes cause an enhanced momentum influx into the Arctic Ocean on annual average. The ocean surface stress increased by 23% from 1979-2012.
However, we also discovered a reduction in Arctic-wide ocean surface stress for the summer months. This is due to the extensive retreat of the sea ice cover in this season. The ice surface is generally rougher than the sea surface and thus less momentum is transferred in open water than in areas of loosely drifting ice floes.
A follow up study based on a model with variable sea ice roughness supports these findings but also highlights the competing effects of decreasing roughness and ice strength--both due to sea ice thinning--on the momentum transfer.



[28.Nov.'12] Southern Ocean deep convection oscillations driving internal climate variability
Many climate models tend to form bottom water in open ocean deep convection (instead of on the continental shelves), which bares similarities to the the Weddell Polynya observed in the 1970s. We identified a mechanism driving the recurrence of such events and involving slow heat accumulation at intermediate ocean depths and its rapid release to the atmosphere in polynya years. This variability is present in many CMIP5 simulations and it turns out that sea ice volume and stratification of the Southern Ocean are its key controls common to different models. It links to low frequent variability of the Atlantic meridional overturning circulation and warming over Antarctica. The variability patterns associated with the shutdown of the dee convection have the potential to mask global warming trends.



[26.May'10] Introducing fully coupled interactive icebergs to complex climate models
The impact of icebergs on the ocean has long not been appreciated by the climate modeling community. We showed by introducing Lagrangian particle icebergs to the GFDL coupled model system that the distinct pattern of meltwater from drifting icebergs impacts both sea ice thickness and bottom water formation around Antarctica.
The iceberg model code has recently been picked up by the NEMO ocean model community.