Theoretical and Practical Perspectives in Geophysical Fluid Dynamics

Atmospheric Cold Pools (ACP) or regions of large-scale masses of cold air are often observed beneath precipitating deep convective clouds as a result of rain evaporation. An ACP is typically identified as a drop in air temperature that is greater than 10C within a period of 30 minutes to an hour at a given location. The dense air pockets relative to warmer surroundings sink and lead to low-level outflows that may propagate as gravity currents. It has been postulated that propagating ACPs could trigger secondary convection when ensuing gravity currents undercut and mechanically lifts warm air to the level of free convection. Detailed understanding of ACP dynamics has been stymied by the lack of high-resolution field data or numerical simulations, in particular in cases where a cold-pool induced gravity current is propagating in an ambience with a mean flow. As such, and motivated by observations of ACPs in the Bay of Bengal during recent MISO-BOB field studies, laboratory experiments we...

The Indian Ocean has been warming rapidly over the last few decades. However, this warming is not uniform, with the South Indian Ocean (SIO, south of 5S) exhibiting the strongest warming after 2000, an abrupt reversal from the cooling trend observed until the late 20th Century. Increased Indonesian throughflow (ITF) into the Indian Ocean during the recent climate hiatus was considered to be the primary reason for this SIO warming. Here, we show that the role of ITF on the IO decadal variability has reduced considerably after 2010. We find that the warming of the SIO during the climate hiatus (1998-2010) resulted in a weaker Mascarene High and decoupled it from the Southern Ocean atmospheric variabilities. Subsequently, while the Pacific Ocean subtropical gyre continued to migrate poleward in response to the anthropogenic warming in the Southern Ocean, it stalled in the Indian Ocean. This caused a three-fold increase in the Tasman inflows into the Indian Ocean, compensating for the weak...

 It is well known that heatwaves are influenced by both atmospheric and land-surface forcings. As the climate warms, both these forcings are likely to change. To clarify the role of each these forcings on the intensity-duration-frequency (IDF) characteristics of heatwaves, we use an idealised to study the dry static energy (DSE) budget of heatwaves, and how the sources and sinks of DSE are affected when atmospheric opacity and Bowen ratio are separately changed. Furthermore, we will look at how the changing energetics impacts the IDF characteristics and return times of heatwaves. Since the heatwaves in this model are primarily driven by the circulation, this configuration also provides insight into the character oF atmospheric macroturbulence near the tail of the distribution.
 

Consider a system described by a multi-dimensional state vector X. The evolution of x is governed by a set of equations in the form of dx/dt=F(X(t)). x is a component of X. F(X(t)), the differential forcing of x, is a deterministic function of X. The solution of such a system often exhibits randomness, where the solution at one time is independent of the solution at another time. This study investigates the mechanism responsible for such randomness. We do so by exploring the integral forcing of x, G_T (t), a definite integral of F over the time span extending from t to t+T, which links the solution at two times, t and t+T.

We show that, for a system in equilibrium, G_T (t) can be expressed as G_T (t)=c_T+d_T x(t)+f_T (t), which consists of (apart from constant c_T) a dissipating component with strength d_T and a fluctuating component f_T (t), in line with the fluctuation-dissipation theorem that for a system in equilibrium, anything that generates fluctuations must also damp the flu...

Consider a system described by a multi-dimensional state vector X. The evolution of x is governed by a set of equations in the form of dx/dt=F(X(t)). x is a component of X. F(X(t)), the differential forcing of x, is a deterministic function of X. The solution of such a system often exhibits randomness, where the solution at one time is independent of the solution at another time. This study investigates the mechanism responsible for such randomness. We do so by exploring the integral forcing of x, G_T (t), a definite integral of F over the time span extending from t to t+T, which links the solution at two times, t and t+T.

We show that, for a system in equilibrium, G_T (t) can be expressed as G_T (t)=c_T+d_T x(t)+f_T (t), which consists of (apart from constant c_T) a dissipating component with strength d_T and a fluctuating component f_T (t), in line with the fluctuation-dissipation theorem that for a system in equilibrium, anything that generates fluctuations must also damp the flu...

We present a data-driven framework for dimensionality reduction and causal inference in climate fields. Given a high-dimensional climate field, the methodology first reduces its dimensionality into a set of regionally constrained patterns. Causal relations among such patterns are then inferred in the interventional sense through the fluctuation-response formalism. To distinguish between true and spurious responses, we propose an analytical null model for the fluctuation-dissipation relation, therefore allowing us for uncertainty estimation at a given confidence level. The framework is then applied to understand the relation between sea surface temperature warming patterns and changes in the net radiative flux at the top of the atmosphere, the so-called "pattern effect". We present a set of new results on the pattern effect and discuss the role of different processes, active at different spatiotemporal scales, in establishing the causal linkages between warming at the surface and radiat...

Horizontal and vertical distributions of mesoscale eddy kinetic energy (EKE), the dominant reservoir of ocean kinetic energy, are influenced by both environmental and dynamical factors. Compared to partitioning across horizontal scales, distributions of EKE in the vertical have been relatively under-observed and under-studied. Using newly collected full-depth observations of horizontal velocity from four unique mooring sites and output from the NOAA GFDL CM2.6 suite, this work presents a characterization of eddy vertical structure and investigates the factorings controlling its spatio-temporal variability. Time series analysis and application of clustering tools reveal geographic patterns in vertical structure. These patterns indicate the role of latitude and bathymetry in moderating the vertical partitioning of EKE. These relationships are interpreted through the lens of theoretical expectation and considered in the context of techniques used to infer or impose vertical structure.

The Bay of Bengal (bay) is a semi-enclosed tropical basin driven by seasonally reversing monsoon winds and a huge quantity of freshwater from rainfall and river runoff. The bay plays a fundamental role in controlling weather systems that make up the Asian summer monsoon system, including monsoon depressions and tropical cyclones. We have used a high resolution (~1 km) regional ocean model of the Bay of Bengal to explore the sub-mesoscale variability in the bay. Model simulations show that the East India Coastal Current (EICC) is extremely rich in submesoscale features compared to the open ocean and exhibit significant seasonal variations. Submesoscale activity over the EICC region is weakest during spring (March-May), slightly stronger during summer monsoon (June-September) and strongest during winter monsoon (November-January). Weak winds during spring and a huge fresh-water gain during summer monsoon tend to weaken submesoscale activity. Investigation of conversion rates of APE to KE...