Graham J. Rickard

The New Hadley Centre Climate Model (HadGEM1): Evaluation of Coupled Simulations

Abstract A new coupled general circulation climate model developed at the Met Offices Hadley Centre is presented, and aspects of its performance in climate simulations run for the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) documented with reference to previous models. The Hadley Centre Global Environmental Model version 1 (HadGEM1) is built around a new atmospheric dynamical core; uses higher resolution than the previous Hadley Centre model, HadCM3; and contains several improvements in its formulation including interactive atmospheric aerosols (sulphate, black carbon, biomass burning, and sea salt) plus their direct and indirect effects. The ocean component also has higher resolution and incorporates a sea ice component more advanced than HadCM3 in terms of both dynamics and thermodynamics. HadGEM1 thus permits experiments including some interactive processes not feasible with HadCM3. The simulation of present-day mean climate in HadGEM1 is significantly better overall ...

Alfvén resonance excitation and fast wave propagation in magnetospheric waveguides

Magnetic pulsations are a robust feature of the Earths magnetosphere. It has been suggested recently that the magnetosphere is sometimes better modeled as a waveguide rather than a cavity. This paper presents numerical simulations of linear magnetohydrodynamic (MHD) waves in an inhomogeneous, low-β waveguide. Several features predicted by recent theoretical studies are confirmed in our simulations, notably that Alfven resonances are driven at frequencies corresponding to the natural frequency of the fast waveguide modes with Vg ≈ 0 (ky ≈ 0).

Impact of an Eddy-Permitting Ocean Resolution on Control and Climate Change Simulations with a Global Coupled GCM

Abstract Initial results are presented from a 150-yr control and an 80-yr transient simulation of a new global coupled climate model with an ocean model resolution of ⅓°, which is sufficient to permit ocean eddies to form. With no spinup procedure or flux correction, the coupled model remains close to radiative equilibrium, and the enhanced ocean resolution allows an improved ocean state to be simulated; this includes a general decrease in sea surface temperature errors compared to climatology and more realistic large-scale flows compared to previous lower-resolution models. However, the improvements in the atmospheric and coupled model climatology are less pronounced, with small improvements in atmospheric circulation counterbalanced by an El Nino–Southern Oscillation cycle that has peak power at too short a period and with too little power on longer time scales. With the model using exactly the same atmospheric component as a lower-resolution counterpart, the comparison gives some insight into the impac...

ULF pulsations in a magnetospheric waveguide: Comparison of real and simulated satellite data

The representation of the Earths magnetosphere as an MHD waveguide has prompted much recent speculation. In particular, the signatures associated with ULF pulsations sit well within the waveguide concept. We further explore the appositeness of the waveguide model by simulating data that a spacecraft would see when passing through a stimulated magnetospheric waveguide and by comparing the results with those obtained from a real spacecraft. We find that the waveguide results again compare favorably and can explain many features seen in the data. We also find that the magnetometer signature of the fast mode in a waveguide (unlike a cavity) does not have a regular oscillatory nature with constant period over a range of L shells.

ULF pulsations driven by magnetopause motions: Azimuthal phase characteristics

We present numerical simulations of wave coupling in a cold, ideal, inhomogeneous plasma waveguide which approximates the flanks of the magnetosphere. Two types of driving conditions are investigated. The first corresponds to a displacement of the magnetopause over a fixed azimuthal range. The pulsations driven by this boundary condition have an azimuthal wavenumber and phase speed determined by the equilibrium structure of the magnetosphere. The second condition corresponds to a displacement pulse which runs antisunward along the magnetopause and produces pulsations with a phase velocity strictly equal to that of the boundary pulse. Our results are compared briefly with data and suggest that on some occasions the magnetosphere is driven in a fashion that is well approximated by a “running” pulse or wave on the magnetopause.

Mean circulation and hydrography in the Ross Sea sector, Southern Ocean: representation in numerical models

Abstract Three models were used to look at the Southern Ocean Ross Sea sector circulation and hydrography. Two were climate models of low (1°) to intermediate resolution (1/3°), and one was an operational high resolution (1/10°) ocean model. Despite model differences (including physics and forcing), mean and monthly variability aspects of off-shelf circulation are consistently represented, and could imply bathymetric constraints. Western and eastern cyclonic gyral systems separated by shallow bathymetry around 180°E redistributing water between the wider Southern Ocean and the Ross Sea are found. Some model seasonal gyral transports increase as the Antarctic Circumpolar Current transport decreases. Model flows at 900 m at the gyral eastern end compare favourably with float data. On-shelf model depth-averaged west–east flow is relatively consistent with that reconstructed from longline fishing records. These flows have components associated with isopycnal gradients in both light and dense waters. The climate models reproduce characteristic isopycnal layer inflections (‘V’s) associated with the observed Antarctic Slope Front and on-shelf deep water formation, and these models transport some 4 Sv of this bottom water northwards across the outer 1000 m shelf isobath. Overall flow complexity suggests care is needed to force regional Ross Sea models.

A hydrodynamic model of Chatham Rise, New Zealand

Abstract A three‐dimensional hydrodynamic ocean model, the Regional Ocean Modelling System (ROMS), was applied to a region encompassing the Chatham Rise, New Zealand and forced by surface fluxes from an atmospheric reanalysis data set. The model outputs fields were validated against a number of observation‐based data sets, including the CSIRO Atlas of Regional Seas 2000 (CARS 2000) climatology of sub‐surface temperature and salinity, a sea surface temperature (SST) climatology from the NIWA SST Archive (NSA), and sea surface height (SSH) from the Archiving, Validation and Interpretation of Satellite data in Oceanography (AVISO) Mapped Sea Level Anomalies (MSLA) data set. The model reproduced the flow around Chatham Rise well and had a realistic seasonal cycle in the upper ocean. Its biggest deficiency was that the Wairarapa Eddy was too steady and pushed warm water too far south towards the head of Hikurangi Trough. This exercise confirms the need to validate a model against multiple data sets and shows the value of SST data for revealing underlying oceanographic features.

Development of a regional ocean model for New Zealand

Abstract A regional ocean model with a horizontal resolution of 1/6° encompassing the New Zealand Exclusive Economic Zone is described. The regional model successfully downscaled solutions from a high resolution, global, coupled model HadCEM. Transport estimates from the global and regional models were compared with observations, and both models supported largely consistent, climatological mean solutions. The regional model used monthly mean forcing at the surface. Nevertheless, the regional model eddy kinetic energy (EKE) spatial patterns compared favourably with long‐term mean satellite altimetric estimates, although the modelled background EKE amplitudes were much lower than observed. A series of permanent eddies associated with the western boundary current system around the top of the North Island of New Zealand were reproduced, and an eddy adjacent to Norfolk Ridge was identified in both the global and regional models. The western boundary current system around the North Island of New Zealand and the associated eddies were the most sensitive components of the model solutions, being influenced by initial conditions, wind forcing, and the model domain size.

CMIP5 Earth System Models with biogeochemistry: a Ross Sea assessment

Abstract An assessment is made of the ability of the Coupled Model Intercomparison Project 5 (CMIP5) models to represent the seasonal cycles of biogeochemistry of the Ross Sea over the late twentieth century. In particular, sea surface temperature, sea ice concentration, surface chlorophyll a, nitrate, phosphate and silicate, and the depth of the seasonal thermocline (measuring vertical mixing) are examined to quantify the physical-biogeochemical capabilities of each model, and to provide for ‘ranked’ model ensembles. This permits critical assessment of modelled Ross Sea biogeochemical cycling, including less well observed variables such as iron and vertically integrated primary production. The assessment enables determination of model output confidence limits; these confidence limits are used to examine future model scenario projections for consideration of potential ecosystem changes. The future scenarios examined are the representative concentration pathways rcp4.5 and rcp8.5. Our study suggests that by the end of the twenty-first century under rcp4.5 and/or rcp8.5 that there will be average increases in sea surface temperature, surface chlorophyll a, integrated primary production and iron, average decreases in surface nitrate, phosphate and silicate, and relatively large decreases in the depth of the seasonal thermocline and percentage coverage by sea ice in the Ross Sea.

ULF pulsations driven by a randomly varying magnetopause displacement

A magnetospheric cavity with a two-dimensional profile in Alfven speed has been driven with a nonmonochromatic source. Detailed numerical results show that the magnetospheric cavity filters the random driving signal and excites preferentially the fast modes whose eigenfrequencies lie within the driving spectrum. These fast modes may also couple to the Alfven mode, provided ky ≠ 0 and their eigenfrequencies lie within the Alfven continuum. The resulting Alfven modes and the position of the resonant field lines can be predicted to high accuracy by calculating the natural fast and Alfven frequencies of the undriven system. A preliminary investigation into the seismology of the magnetosphere has also been undertaken. The ratio of the energy density in the two resonant field lines depends only on the equilibrium of the cavity and not on the nature of the driving source (e.g., initial condition, impulsive excitation, or random forcing). Good agreement is found with the ratio predicted by an approximate analytical treatment based upon the eigenfunctions of the equilibrium magnetosphere. The new seismological technique may prove to be a useful diagnostic tool in future studies.