Email: j.f.talib@pgr.reading.ac.uk

Every year PhD students from the Department of Meteorology at the University of Reading welcome a distinguished scientist in the field of environmental sciences. Previous scientists include Richard Rotunno (UCAR), Isaac Held (GFDL) and Susan Solomon (NOAA). This year’s honoured visitor was Professor Tapio Schneider from the climate dynamics research group from California Institute of Technology (Caltech), the academic home of NASA’s Jet Propulsion Laboratory. Tapio is a well-known contributor to our understanding of global climate dynamics and it was a pleasure to welcome him to our department.

Our visiting scientist programme in the department is an opportunity for PhD students to share and explain their research to an external visitor. It allows for PhD research to be looked at from a completely new perspective which will hopefully improve the PhD studies. In a typical PhD visiting scientist week, the visiting scientist meets students one to one, attends departmental research groups and presents work in departmental seminars.

Tapio Schneider presented two departmental seminars during his time with us titled How low clouds respond to warming: Observational, numerical and physical constraints and Model hierachies: From advancing climate dynamics to improving predictions. The latter of these seminars encouraged a discussion to rethink how we approach advancing our modelling capabilities. Tapio argued that the atmospheric modelling community had not fully engaged in the benefits that observations offer. He suggested that our goal should be a heirarchical system that integrates both observational data and models. We should look into creating “machine-learning” models, those which use observational data to improve our modelling capabilities through altering parameterisation schemes and radiative balance calculations at the top of the atmosphere (as two examples).

As already mentioned, the visiting scientist also meets with students one-to-one and it was highly beneficial for my own project to have a meeting with Tapio Schneider. We discussed papers released by himself alongside his former PhD student Tobias Bischoff (for example, The Equatorial Energy Balance, ITCZ position and Double-ITCZ bifurications) which concentrate on creating a diagnostic framework with which we can estimate the location and structure of the Inter-Tropical Convergence Zone (ITCZ). We discussed conclusions reached from my own aquaplanet simulations and how they relate to the proposed diagnostic framework. Keep an eye on the blog for a post coming soon on the developments in my own PhD project, (titled, what determines the location and intensity of the ITCZ?).

To bring this blog post to a close I would like to thank Professor Tapio Schneider for his time, knowledge and wisdom that he shared with the PhD cohort whilst at Reading. Thank you also to those from the University of Reading who supported Tapio’s visit. Feedback from the PhD cohort is extremely positive and I would highly recommend a similar scheme for other scientific departments.

Email: n.e.smith@pgr.reading.ac.uk

The Arctic’s climate is one of those most rapidly changing globally, and as such the region has become a poster-child of climate change. Sea ice area is frequently used as an indicator of the rate of change of the system, providing striking visualisations of the rapidity of the change in recent years. The sea ice is also a driver of climate change, with areal cover greatly affecting the planet’s albedo and ice melt cooling and desalinating the Arctic ocean, altering circulation globally.

Photo: Haakon Hop, Norsk Polarinstitutt

During the summer months, incoming solar radiation melts the surface layers of the sea ice. This melt water collects in hollows on the surface of the sea ice forming pools called melt ponds. Since the sea ice is porous, water from these ponds can percolate down or flow down through macroscopic flaws in the ice and out of the base of the ice. The melt water is relatively warm and fresh compared to the ocean below, so it floats between the ice and the ocean, gathering in pools beneath the sea ice called under-ice melt ponds. [1]

A couple of types of ice growth have been observed associated with these ponds. Most importantly, a sheet of ice can form at the interface between the pond and the ocean, completely isolating the fresh water from the ocean. As they create the illusion that they are the base of the sea ice, these sheets of ice are commonly referred to as ‘false bottoms’. [2]

We have developed a one-dimensional thermodynamic model of under-ice melt ponds to investigate how they affect their surroundings. We have carried out a number of sensitivity studies using this model, which have lead to some interesting conclusions about how these ponds evolve and affect the ice above them.

For example, the thicker the sea ice above an under-ice melt pond, the longer it takes to freeze due to a shallower temperature gradient above. As a result, more ice is gained due to under-ice melt ponds beneath thicker ice. This could be a positive feedback cycle, since we expect to see thinner ice on average as the Arctic warms, leading to less ice gained due to the ponds beneath it.

We also see that, as well as the outcome observed in the field, in which the false bottom migrates upwards and thickens as it freezes through the pond, it can also ablate under certain conditions. For example, ponds that are relatively salty at the start of the simulation freeze more slowly, and the false bottom ablates before it is able to reach the base of the sea ice.

Our sensitivity studies show that under-ice melt ponds could be responsible for up to 7.9% additional ice thickness at the end of a 50 day simulation. This would equate to up to 3.2% more ice volume across the Arctic dependent on the area of the ice underlain by these pools.

Recently, we have coupled our under-ice melt pond model with a simple, zero-dimensional model of the oceanic mixed layer. Using this coupled model, we see that the false bottom ablates more rapidly than a slab of sea ice, releasing more fresh water into the mixed layer. This strong reduction of salinity causes a shallowing of the mixed layer. We are currently further investigating the effects that the ponds have on the ocean below them.

Under-ice melt ponds and false bottom insulate the sea ice from below and affect the basal fluxes of salt and fresh water into the mixed layer, and thicken the ice above them allowing less radiation to penetrate through from the surface. They are clearly significant to the mass balance of the ice and the ocean below them, yet are not currently accounted for in the sea ice components of climate models. A parameterisation of their effects would be useful to include.

[1] Notz, Dirk, et al. “Impact of underwater‐ice evolution on Arctic summer sea ice.” Journal of Geophysical Research: Oceans 108.C7 (2003).

[2] Martin, Seelye, and Peter Kauffman. “The evolution of under-ice melt ponds, or double diffusion at the freezing point.” Journal of Fluid Mechanics 64.3 (1974): 507-528.