M4PE Seminar: August 27th, Bishakdhatta Gayen (ANU)

ARC Future Fellow Dr Bishak Gayen (ANU) will discuss his research in the M4PE seminar at UNSW Sydney on Monday 27 August 2018.

Title: Spanning 10 billion scales from millimetre turbulence to global circulation

Speaker: Bishakhdatta Gayen (Australian National University)

Date & Time: 4pm, Monday 27 August 2018. (Seminar will be followed by refreshments.)

Location: Red Centre room RC-3085, School of Mathematics and Statistics, UNSW Sydney

Abstract: The general ocean circulation, of crucial importance to the global climate, involves fluid motion on scales ranging from turbulence, internal waves, eddies and fronts, planetary Rossby waves and basin-scale gyre recirculation. Equilibrium is maintained between continuous large-scale forcing and energy dissipation. Understanding the physics of various dissipation mechanisms is important for improving the dynamical description of large-scale circulation. Large-scale ocean models do not accurately model turbulent convection, breaking waves, and turbulence, providing motivation to develop a better understanding of these mechanisms. In this presentation, my primary focus will be on understanding the role of turbulence and convection in ocean circulation.

In order to examine the effect of convection in ocean circulation, we have developed a model of circulation with flow driven by surface buoyancy in a closed basin using Direct Numerical Simulations. The circulation cell involves a horizontal boundary flow, turbulent plume motion and week interior return flow. We show that under planetary rotation, even in the absence of wind stress, the flow becomes three-dimensional with small-scale deep convection and broad basin-scale gyres. For the first time, DNS is used to model this circulation and quantify the heat transfer and flow energetics, demonstrating several dynamical regimes. I will also discuss the role of turbulent convection in melting of basal ice shelves and circulation around the Antarctic basin.

About the speaker: Dr Bishakhdatta Gayen is a Research Fellow at the Research School of Earth Sciences at Australian National University. His current research interests are nonlinear internal waves in the ocean, turbulent convection, modeling of Antarctic ice melting and Southern ocean dynamics. Bishak is a 2018 ARC Future Fellow, and has previously been awarded a 2013 ARC DECRA Fellowship. He has also received the RJL Hawke post-doctoral fellowship from the Australian Antarctic Science Program to study subsurface melting of ice shelves around Antarctica with implications for future sea-level rise.

About the main image: A snapshot from simulation of circulation in a closed ocean basin forced by imposed constant temperature having a variation with latitude, showing the kinetic energy on a horizontal plane near the upper boundary, temperature contours on a vertical section near the western boundary and vertical velocity on a vertical section near the northern boundary. Time averaged near-surface transport streamfunction is shown above.


Study uncovers role of ocean warming in water cycle and salinity changes

A recent study by researchers at UNSW Sydney and the UK National Oceanography Centre has revealed how ocean warming has changed the pattern of salinity observed at the surface of the ocean. Their results, published last month in Environmental Research Letters, have significant implications for our ability to measure changes to Earth’s water cycle in a changing climate.

Changes in ocean salinity – the concentration of salt in the ocean – is an important signature of past changes in the water cycle. Over a given area of the ocean, net evaporation increases salt concentration, while net precipitation lowers salt concentration. Observed changes in surface salinity reveal global warming trends: in particular, regions of the globe with high salinity are become more salty, while regions that have low salinity are become less salty.



Figure (a) Mean fresh water flux out of the ocean from reanalysis data. (b) Mean sea surface salinity from observational analysis. From Zika et al. Environmental Research Letters (2018).


This has led climate scientists to think of the ocean itself as a “rain gauge” that can reveal historical changes in the water cycle, which is tightly linked to climate change. Global climate models project overall increases in evaporation and precipitation and rainfall extremes, with wet regions of the globe getting wetter and dry regions getting ever drier. Changes in the global water cycle will critically impact environmental, agricultural, and energy systems relied upon by humanity.

However, the relationship between surface salinity patterns in the ocean and changes in the water cycle is puzzling. While the surface salinity pattern has increased by 5-8% since 1950, the water cycle has changed by a smaller amount, 2-3%, over the same period. So what is driving the discrepancy?

Dr Jan Zika (UNSW Sydney) and colleagues at the National Oceanography Centre in the United Kingdom addressed this puzzle using realistic numerical global ocean models. Their results indicate that surface ocean warming is a key and previously overlooked process driving sea surface salinity changes. Warming increases near-surface stratification, which amplifies surface salinity patterns. As a result, approximately half of the observed surface salinity pattern changes can be accounted for by ocean warming, with the remaining changes due to melting ice and changes in the water cycle.

Dr Zika’s research indicates that changes in Earth’s water cycle can be monitored using sea-surface salinity observations, once ocean warming effects are appropriately accounted for.

Citation: Zika, J.D., Skliris, N., Blaker, A.T., Marsh, R., Nurser, A.J.G. and Josey, S.A. [2018]. Improved estimates of water cycle change from ocean salinity: the key role of ocean warming. Environmental Research Letters, 13 (7). https://doi.org/10.1088/1748-9326/aace42


Trevor McDougall elected a Fellow of the American Geophysical Union

The world’s leading society of Earth and space science, the American Geophysical Union, has elected UNSW Professor Trevor McDougall to its class of 2018 Fellows. Professor McDougall adds the award to an already impressive array of honours, including the Jaeger medal, awarded by the Australian Academy of Sciences, and the Prince Albert I medal, awarded by International Association for the Physical Sciences of the Oceans. Earlier this year, Trevor was appointed a Companion of the Order of Australia (AC).

Trevor’s research focuses on the mathematical and physical foundations of the mixing in the ocean. His work has advanced our understanding of the thermodynamics of seawater, resulting in dramatic improvements in the accuracy of climate models. He is a Scientia Professor and ARC Laureate Fellow at the School of Mathematics and Statistics at UNSW Sydney.

The American Geophysical Union (AGU) is the premier scientific body dedicated to the geophysical sciences, including atmospheric and ocean sciences, geology, hydrology, and space science.Each year, the AGU elects as Fellows scientists whose “visionary leadership and scientific excellence have fundamentally advanced research in their respective fields”. The honor is bestowed on only 0.1% of AGU’s 62,000 members from around the world.

Congratulations Trevor!


Public lecture on ozone and climate launches M4PE initiative

Renowned atmospheric scientist Professor Darryn Waugh (UNSW and Johns Hopkins University) delivered a public lecture, The enduring impact of the ozone hole on climate, to a packed audience at UNSW Sydney on Monday 30 July.

The event marked the official launch of the Mathematics for Planet Earth (M4PE) Initiative, which promotes mathematical research that benefits society and the environment. Dr Shane Keating, co-director of M4PE, opened the lecture in the Scientia’s Tyree Room, welcoming the 100 attendees and introducing Professor Waugh.


During his lecture, Prof Waugh spoke about the history of the ozone debate, and the good news story of the Montreal Protocol. The Protocol, ratified three decades ago, was the first concerted international effort to impose short-term economic cost to protect against a “speculative” global environmental problem.


However, he warned that vigilance was still needed due to the slow recovery of the ozone, and potential non-compliance of countries to the Montreal regulations, as recently seen in China where widespread use of CFC-11 in the construction industry has been detected.

The lecture was held on the same day that an article on the ozone hole by Dr Keating and Prof Waugh appeared in The Conversation.

Main image credit: Catherine Pogonowski