Jules B. Kajtar

SPH particle boundary forces for arbitrary boundaries

This paper is concerned with approximating arbitrarily shaped boundaries in SPH simulations. We model the boundaries by means of boundary particles which exert forces on a fluid. We show that, when these forces are chosen correctly, and the boundary particle spacing is a factor of 2 (or more) less than the fluid particle spacing, the total boundary force on a fluid SPH particle is perpendicular to boundaries with negligible error. Furthermore, the variation in the force as a fluid particle moves, while keeping a fixed distance from the boundary, is also negligible. The method works equally well for convex or concave boundaries. The new boundary forces simplify SPH algorithms and are superior to other methods for simulating complicated boundaries. We apply the new method to (a) the rise of a cylinder contained in a curved basin, (b) the spin down of a fluid in a cylinder, and (c) the oscillation of a cylinder inside a larger fixed cylinder. The results of the simulations are in good agreement with those obtained using other methods, but with the advantage that they are very simple to implement.

SPH simulations of swimming linked bodies

In this paper, we describe how the swimming of linked-rigid bodies can be simulated using smoothed particle hydrodynamics (SPH). The fluid is assumed to be viscous and weakly compressible though with a speed of sound which ensures the Mach number is ~0.1 and the density fluctuation (relative to the average density) is typically 0.01. The motion is assumed to be two-dimensional. The boundaries of the rigid bodies are replaced by boundary particles and the forces between these particles and the fluid particles determine the forces and torques on the rigid bodies. The links between the bodies are described by constraint equations which are taken into account by Lagrange multipliers. We integrate the equations by a second-order method which conserves linear momentum exactly and, in the absence of viscosity, is reversible. We test our method by simulating the motion of a cylinder moving in a viscous fluid both under forced oscillation, and when tethered to a spring. We then apply our method to three problems involving three linked bodies. The first of these consists of three linked diamonds in a periodic domain and shows the convergence of the algorithm. The second is the system considered by Kanso et al. [E. Kanso, J.E. Marsden, C.W. Rowley, J.B. Melli-Huber, Locomotion of articulated bodies in a perfect fluid, J. Nonlinear Sci. 15 (2005) 255-289 (referred to in the text as K05)] and our results show similar behaviour for the same gait. The third example clarifies the conservation of angular momentum by simulating the motion of three linked ellipses moving within and through the surface of an initially circular patch of fluid. Our method can be easily extended to linked bodies swimming near surfaces, or to swimming near fixed boundaries of arbitrary shape, or to swimming in fluids which are stratified. Some of these systems have biological significance and others are applicable to the study of undersea vessels which move because of shape changes.

Tropical climate variability: interactions across the Pacific, Indian, and Atlantic Oceans

Complex interactions manifest between modes of tropical climate variability across the Pacific, Indian, and Atlantic Oceans. For example, the El Niño–Southern Oscillation (ENSO) extends its influence on modes of variability in the tropical Indian and Atlantic Oceans, which in turn feed back onto ENSO. Interactions between pairs of modes can alter their strength, periodicity, seasonality, and ultimately their predictability, yet little is known about the role that a third mode plays. Here we examine the interactions and relative influences between pairs of climate modes using ensembles of 100-year partially coupled experiments in an otherwise fully coupled general circulation model. In these experiments, the air–sea interaction over each tropical ocean basin, as well as pairs of ocean basins, is suppressed in turn. We find that Indian Ocean variability has a net damping effect on ENSO and Atlantic Ocean variability, and conversely they each promote Indian Ocean variability. The connection between the Pacific and the Atlantic is most clearly revealed in the absence of Indian Ocean variability. Our model runs suggest a weak damping influence by Atlantic variability on ENSO, and an enhancing influence by ENSO on Atlantic variability.

Indo-Pacific Climate Interactions in the Absence of an Indonesian Throughflow

The Pacific and Indian Oceans are connected by an oceanic passage called the Indonesian Throughflow (ITF). In this setting, modes of climate variability over the two oceanic basins interact. El Nino‐Southern Oscillation (ENSO) events generate sea surface temperature anomalies (SSTAs) over the Indian Ocean that, in turn, influence ENSO evolution. This raises the question as to whether Indo-Pacific feedback interactions wouldstilloccurinaclimatesystemwithoutanIndonesianThroughflow.Thisissueisinvestigatedhereforthe firsttimeusingacoupledclimatemodelwithablockedIndonesiangatewayandaseriesofpartiallydecoupled experiments in which air‐sea interactions over each ocean basin are in turn suppressed. Closing the Indonesian Throughflow significantly alters the mean climate state over the Pacific and Indian Oceans. The Pacific OceanretainsanENSO-likevariability,butitisshiftedeastward.Incontrast,theIndianOceandipoleandthe Indian Ocean basinwide mode both collapse into a single dominant and drastically transformed mode. While the relationship betweenENSOandthealteredIndianOcean modeisweakerthanthatwhenthe ITF isopen, the decoupled experiments reveal a damping effect exerted between the two modes. Despite the weaker Indian Ocean SSTAs and the increased distance between these and the core of ENSO SSTAs, the interbasin interactions remain. This suggests that the atmospheric bridge is a robust element of the Indo-Pacific climate system, linking the Indian and Pacific Oceans even in the absence of an Indonesian Throughflow.

Model under-representation of decadal Pacific trade wind trends and its link to tropical Atlantic bias

The strengthening of the Pacific trade winds in recent decades has been unmatched in the observational record stretching back to the early twentieth century. This wind strengthening has been connected with numerous climate-related phenomena, including accelerated sea-level rise in the western Pacific, alterations to Indo-Pacific ocean currents, increased ocean heat uptake, and a slow-down in the rate of global-mean surface warming. Here we show that models in the Coupled Model Intercomparison Project phase 5 underestimate the observed range of decadal trends in the Pacific trade winds, despite capturing the range in decadal sea surface temperature (SST) variability. Analysis of observational data suggests that tropical Atlantic SST contributes considerably to the Pacific trade wind trends, whereas the Atlantic feedback in coupled models is muted. Atmosphere-only simulations forced by observed SST are capable of recovering the time-variation and the magnitude of the trade wind trends. Hence, we explore whether it is the biases in the mean or in the anomalous SST patterns that are responsible for the under-representation in fully coupled models. Over interannual time-scales, we find that model biases in the patterns of Atlantic SST anomalies are the strongest source of error in the precipitation and atmospheric circulation response. In contrast, on decadal time-scales, the magnitude of the model biases in Atlantic mean SST are directly linked with the trade wind variability response.

Model tropical Atlantic biases underpin diminished Pacific decadal variability

Pacific trade winds have displayed unprecedented strengthening in recent decades1. This strengthening has been associated with east Pacific sea surface cooling2 and the early twenty-first-century slowdown in global surface warming2,3, amongst a host of other substantial impacts4–9. Although some climate models produce the timing of these recently observed trends10, they all fail to produce the trend magnitude2,11,12. This may in part be related to the apparent model underrepresentation of low-frequency Pacific Ocean variability and decadal wind trends2,11–13 or be due to a misrepresentation of a forced response1,14–16 or a combination of both. An increasingly prominent connection between the Pacific and Atlantic basins has been identified as a key driver of this strengthening of the Pacific trade winds12,17–20. Here we use targeted climate model experiments to show that combining the recent Atlantic warming trend with the typical climate model bias leads to a substantially underestimated response for the Pacific Ocean wind and surface temperature. The underestimation largely stems from a reduction and eastward shift of the atmospheric heating response to the tropical Atlantic warming trend. This result suggests that the recent Pacific trends and model decadal variability may be better captured by models with improved mean-state climatologies.Simulation of observed Pacific wind trends is hampered by model limitations in representing variability or the forced response. Improved mean-state climatologies, including the recent Atlantic warming trend, should improve capture of Pacific trends.

On the Choice of Ensemble Mean for Estimating the Forced Signal in the Presence of Internal Variability

AbstractIn this paper we examine various options for the calculation of the forced signal in climate model simulations, and the impact these choices have on the estimates of internal variability. W...