Anthony C. Hirst

Interannual variability in a tropical atmosphere−ocean model: influence of the basic state, ocean geometry and nonlinearity

Abstract The behavior of a tropical coupled atmosphere/ocean model is analyzed for a range of different background states and ocean geometries. The model is essentially that of Cane and Zebiak for the tropical Pacific, except only temporally constant background states are considered here. For realistic background states and ocean geometry, the model solutions feature oscillations of period of 3–5 yr. By comparing the full model solution with a linearized version of the model, it is shown that the basic mechanism of the oscillation is contained within linear theory. A simple linear analog model is derived that describes the nature of the interannual variability in the coupled tropical atmosphere–ocean system. The analog model highlights the properties that produce coupled atmosphere–ocean instability in the eastern ocean basin, and the equatorial wave dynamics in the western ocean basin that are responsible for a delayed, negative feedback into this instability growth. The growth rate of the local instabil...

Footprinting: A seasonal connection between the tropics and mid‐latitudes

A connection between the mid-latitude and tropical Pacific is identified in a coupled general circulation model (CGCM). The connection involves a seasonal coupling between winter mid-latitude atmospheric circulation anomalies, and summer equatorial wind stress anomalies. The seasonal coupling results from a “footprinting” mechanism, in which the summer tropical atmosphere responds to subtropical sea surface temperature (SST) anomalies that are generated by the mid-latitude atmospheric variability during the previous winter. Details of the connection, and of the footprinting mechanism are presented. Implications for interannual ENSO and decadal ENSO-like variability are discussed.

Developments in ocean climate modelling

This paper presents some research developments in primitive equation ocean models which could impact the ocean component of realistic global coupled climate models aimed at large-scale, low frequency climate simulations and predictions. It is written primarily to an audience of modellers concerned with the ocean component of climate models, although not necessarily experts in the design and implementation of ocean model algorithms.

Comparison of a coupled ocean-atmosphere model with and without oceanic eddy-induced advection. Part I: Ocean spinup and control integrations

Abstract The Gent and McWilliams (GM) parameterization for large-scale water transport caused by mesoscale oceanic eddies is introduced into the oceanic component of the Commonwealth Scientific and Industrial Research Organisation global coupled ocean–atmosphere model. Parallel simulations with and without the GM scheme are performed to examine the effect of this parameterization on the model behavior for integrations lasting several centuries under conditions of constant atmospheric CO2. The solution of the version with GM shows several significant improvements over that of the earlier version. First, the generally beneficial effects of the GM scheme found previously in studies of stand-alone ocean models, including more realistic deep water properties, increased stratification, reduced high-latitude convection, elimination of fictitious horizontal diffusive heat transport, and more realistic surface fluxes in some regions, are all maintained during the coupled integration. These improvements are especia...

The Seasonal Footprinting Mechanism in the CSIRO General Circulation Models

An influence of midlatitude atmospheric variability on interannual ENSO and decadal ENSO-like variability is established and investigated in the Commonwealth Scientific and Industrial Research Organisation (CSIRO) coupled general circulation models (CGCMs). The effect of midlatitude atmospheric variability is felt in the Tropics via the previously hypothesized ‘‘seasonal footprinting mechanism’’ (SFM), in which a tropical circulation is forced during spring and summer by tropical SST anomalies that are generated by midlatitude atmospheric variability during the previous winter. The tropical circulation includes equatorial zonal wind stress anomalies that act as a stochastic forcing for the CSIRO CGCM’s damped ENSO mode. Details of the SFM are investigated herein. A temporal analysis indicates that the SFM may explain 20%‐ 40% of the model’s interannual ENSO variability and nearly 70% of the model’s decadal to interdecadal tropical variability. An analysis of the physical mechanisms that govern the SFM highlights the role of relaxed trade winds in producing tropical SST anomalies during winter, and identifies a weak positive coupled feedback between off-equatorial tropical SST anomalies and the atmospheric response to those anomalies.

Global warming in a coupled climate model including oceanic eddy-induced advection

The Gent and McWilliams (GM) parameterization for large-scale water transport caused by mesoscale oceanic eddies is introduced into the oceanic component of a global coupled ocean-atmosphere model. Parallel simulations with and without the GM scheme are performed to examine the effect of this parameterization on model behavior under constant atmospheric CO2 and on the model response to increasing CO2. The control (constant CO2) runs show substantial differences in the oceanic stratification and extent of convection, similar to differences found previously using uncoupled ocean models. The transient (increasing CO2) runs show moderate differences in the rate of oceanic heat sequestration (less in the GM case), as expected based on passive tracer uptake studies. However, the surface warming is weaker in the GM case, especially over the Southern Ocean, which is contrary to some recent supposition. Reasons for the reduced warming in the GM case are discussed.

The simulated response of dimethylsulfide production in the Arctic Ocean to global warming

Sulfate aerosols (of both biogenic and anthropogenic origin) play a key role in the Earth’s radiation balance both directly through scattering and absorption of solar and terrestrial radiation, and indirectly by modifying cloud microphysical properties. However, the uncertainties associated with radiative forcing of climate due to aerosols substantially exceed those associated with the greenhouse gases. The major source of sulfate aerosols in the remote marine atmosphere is the biogenic compound dimethylsulfide (DMS), which is ubiquitous in the world’s oceans and is synthesized by plankton. Climate models point to significant future changes in sea-ice cover in the Arctic Ocean due to warming. This will have consequences for primary production and the sea-to-air flux of a number of biogenic compounds, including DMS. In this paper we discuss the impact of warming on the future production of DMS in the Arctic Ocean. A DMS production model has been calibrated to current climate conditions with satellite ocean colour data (SeaWiFS) using a genetic algorithm, an efficient non-derivative based optimization technique. We use the CSIRO Mk 2 climate model to force the DMS model under enhanced greenhouse climate conditions. We discuss the simulated change in DMS flux and its consequences for future aerosol production and the radiative budget of the Arctic. Significant decreases in sea-ice cover (by 18.5% annually and 61% in summer–autumn), increases in mean annual sea surface temperature of 1◦C, and a decrease of mixed layer depth by 13% annually are predicted to result in annual DMS flux increases of over 80% by the time of equivalent CO2 tripling (2080). Estimates of the impact of this increase in DMS emissions suggest significant changes to summer aerosol concentrations and the radiative balance in the Arctic region.

The Southern Ocean response to global warming in the CSIRO coupled ocean-atmosphere model

Abstract The Southern Ocean response to global warming is examined for a transient greenhouse gas integration using the Commonwealth Scientific and Industrial Research Organisation (CSIRO) coupled ocean-atmosphere model. The ocean component includes the Gent and McWilliams scheme for adiabatic eddy-induced transport. The atmospheric equivalent CO2 concentration (COe2) follows the IPCC/IS92a radiative forcing scenario from 1880 to 2082, and is then maintained at a constant value of three times the 1880 level for seven centuries. The simulated changes which occur in the Southern Ocean under global warming are very profound, and they begin to separate clearly from the background climate noise in the models 1990s. A major reduction in the depth and extent of convective mixing occurs by the time of COe2 doubling (year 2033), with near-cessation of convection by the time of COe2 tripling. Similarly major reduction occurs in the downwelling adjacent to Antarctica associated with Antarctic Bottom Water formation, which also nearly ceases by the time of COe2 tripling. These changes are associated with a marked reduction in surface density and salinity. By the time of COe2 tripling, both the pycnocline and halocline south from 60°S intensify over the control by about a factor of four. The changes in surface salinity and density continue to intensify for several centuries during the subsequent period of elevated stable COe2, and convection and Antarctic downwelling do not recover at all for the duration of the transient run. The water of the entire global ocean below about 1.5 km depth remains stagnant for the duration of the period of elevated stable COe2, retaining a density which is too great to allow renewal from any source. Possible caveats on the realism of these results are discussed, and potential consequences of the above changes are noted.

A Consistent Model for the Large-Scale Steady Surface Atmospheric Circulation in the Tropics*

The authors present a new model of the tropical surface circulation, forced by changes in sensible heat and evaporative flux anomalies that are associated with prescribed sea surface temperature anomalies. The model is similar to the Lindzen and Nigam (LN) boundary layer model, also driven by the above flux anomalies; but here, since the boundary layer is assumed well mixed and capped by an inversion, the model reduces to a twolayer, reduced-gravity system. Furthermore, the rate of exchange of mass across the boundary layer‐free atmosphere interface is dependent on the moisture budget in the boundary layer. When moist convection is diagnosed to occur, detrainment operates on the timescale associated with the life cycle of deep convection, approximately eight hours. Otherwise, the detrainment is assumed to be associated with the mixing out of the stable tropical boundary layer, which has a timescale of about one day. The model provides a diagnostic estimate of the anomalies in precipitation. However, it is assumed that the latent heat is released above the boundary layer, and it drives a circulation that does not impact the boundary layer. The authors discuss the derivations of the Gill‐Zebiak (GZ) and Lindzen‐Nigam models and highlight some apparent inconsistencies between their derivation and the values of several of the parameters that are required for these models to achieve realistic solutions for the circulations. Then, the new reduced-gravity boundary model equations are rewritten in the form of the GZ and LN models. Using realistic values for the parameters in the new model geometry, it is shown that the constants combine in the rewritten equations to produce the physically doubtful constants in the GZ and LN models, hence, the reason for the apparent success of these models.

Pacific Interannual and Interdecadal Equatorial Variability in a 1000-Yr Simulation of the CSIRO Coupled General Circulation Model*

Abstract The structure and evolution of, and the mechanisms responsible for, interannual and decadal equatorial variability in a 1000-yr simulation of the Commonwealth Scientific and Industrial Research Organisation (CSIRO) coupled general circulation model are examined. Principal component analysis is applied to the 0–270-m integrated heat content from the model to determine dominant modes of variability. The leading mode of unfiltered variability (annual) is best described by an AR2 null hypothesis with an implied periodic timescale of 6–10 yr. Spatial structures of the leading empirical modes of interannual (10-yr high-pass filtered) and decadal (9-yr low-pass filtered) variability closely resemble observations of interannual ENSO, and decadal ENSO-like variability. The amplitude of tropical SST anomalies is too small by a factor of 2–3 on interannual timescales, but is close to that observed for decadal timescales. For interannual timescales, an equatorial heat budget analysis shows a positive feedbac...