Mary Jo Nath

The new GFDL global atmosphere and land model AM2-LM2: Evaluation with prescribed SST simulations

The configuration and performance of a new global atmosphere and land model for climate research developed at the Geophysical Fluid Dynamics Laboratory (GFDL) are presented. The atmosphere model, known as AM2, includes a new gridpoint dynamical core, a prognostic cloud scheme, and a multispecies aerosol climatology, as well as components from previous models used at GFDL. The land model, known as LM2, includes soil sensible and latent heat storage, groundwater storage, and stomatal resistance. The performance of the coupled model AM2‐LM2 is evaluated with a series of prescribed sea surface temperature (SST) simulations. Particular

The dynamical core, physical parameterizations, and basic simulation characteristics of the atmospheric component AM3 of the GFDL global coupled model CM3

AbstractThe Geophysical Fluid Dynamics Laboratory (GFDL) has developed a coupled general circulation model (CM3) for the atmosphere, oceans, land, and sea ice. The goal of CM3 is to address emerging issues in climate change, including aerosol–cloud interactions, chemistry–climate interactions, and coupling between the troposphere and stratosphere. The model is also designed to serve as the physical system component of earth system models and models for decadal prediction in the near-term future—for example, through improved simulations in tropical land precipitation relative to earlier-generation GFDL models. This paper describes the dynamical core, physical parameterizations, and basic simulation characteristics of the atmospheric component (AM3) of this model. Relative to GFDL AM2, AM3 includes new treatments of deep and shallow cumulus convection, cloud droplet activation by aerosols, subgrid variability of stratiform vertical velocities for droplet activation, and atmospheric chemistry driven by emiss...

Impact of ENSO on the Variability of the Asian–Australian Monsoons as Simulated in GCM Experiments

Abstract The influences of El Nino–Southern Oscillation (ENSO) on the summer- and wintertime precipitation and circulation over the principal monsoon regions of Asia and Australia have been studied using a suite of 46-yr experiments with a 30-wavenumber, 14-level general circulation model. Observed monthly varying sea surface temperature (SST) anomalies for the 1950–95 period have been prescribed in the tropical Pacific in these experiments. The lower boundary conditions at maritime sites outside the tropical Pacific are either set to climatological values [in the Tropical Ocean Global Atmosphere (TOGA) runs], predicted using a simple 50-m oceanic mixed layer (TOGA-ML runs), or prescribed using observed monthly SST variations. Four independent integrations have been conducted for each of these three forcing scenarios. The essential characteristics of the model climatology for the Asian–Australian sector compare well with the observations. Composites of the simulated precipitation data over the outstanding...

The Role of the “Atmospheric Bridge” in Linking Tropical Pacific ENSO Events to Extratropical SST Anomalies

Abstract The role of the atmospheric circulation as a “bridge” between sea surface temperature (SST) anomalies in the tropical Pacific and those in the midlatitude northern oceans is assessed. The key processes associated with this atmospheric bridge are described using output from four independent simulations with a general circulation model subjected to month to month SST variations observed in the tropical Pacific during the 1946–1988 period and to climatological SST conditions elsewhere (the “TOGA” runs). In episodes with prominent SST anomalies in the tropical Pacific, extratropical perturbations in the simulated atmospheric temperature, humidity, and wind fields induce changes in the latent and sensible heat fluxes across the air-sea interface of the midlatitude oceans. These anomalous fluxes in turn lead to extratropical SST changes. The relevance of the atmospheric bridge mechanism is evaluated by driving a motionless, 50-m deep oceanic negative mixed layer model at individual grid points with the...

A modeling study of the relative roles of tropical and extratropical SST anomalies in the variability of the global atmosphere-ocean system

Abstract In three parallel experiments, an atmospheric general circulation model has been subjected to observed, monthly varying sea surface temperature (SSI) conditions in each of the following domains: near-global ocean (GOGA run), tropical Pacific (TOGA run), and midlatitude North Pacific (MOGA run). Four independent realizations were obtained for the model response to the sequence of SST anomalies during the 1946–88 period in each of the above regions. The principal modes of coupling between the imposed SST forcing and the simulated Northern Hemisphere wintertime 5 1 5-mb height field in various experiments have been identified using a singular value decomposition (SVD) procedure. The leading SVD mode for the GOGA experiment is Qualitatively similar to that based on observational data, although the amplitudes of the simulated height anomalies are notably lower than the observed value. The SST pattern of this mode resembles that associated with El Nino events. The accompanying 5 1 5-mb height anomaly i...

Atmosphere–Ocean Variations in the Indo-Pacific Sector during ENSO Episodes

Abstract The influences of El Nino–Southern Oscillation (ENSO) events on air–sea interaction in the Indian–western Pacific (IWP) Oceans have been investigated using a general circulation model. Observed monthly sea surface temperature (SST) variations in the deep tropical eastern/central Pacific (DTEP) have been inserted in the lower boundary of this model through the 1950–99 period. At all maritime grid points outside of DTEP, the model atmosphere has been coupled with an oceanic mixed layer model with variable depth. Altogether 16 independent model runs have been conducted. Composite analysis of selected ENSO episodes illustrates that the prescribed SST anomalies in DTEP affect the surface atmospheric circulation and precipitation patterns in IWP through displacements of the near-equatorial Walker circulation and generation of Rossby wave modes in the subtropics. Such atmospheric responses modulate the surface fluxes as well as the oceanic mixed layer depth, and thereby establish a well-defined SST anom...

Variability of the Baroclinic and Barotropic Transient Eddy Forcing Associated with Monthly Changes in the Midlatitude Storm Tracks

Abstract The heat and vorticity transports by synoptic-scale eddies at various levels between 1000 and 100 mb have been compiled for each winter month of the 1966–84 period using time-filtered daily analyses produced by the U.S. National Meteorological Center. These circulation statistics were used to compute the three-dimensional distributions of the quasigeostrophic geopotential tendency and vertical motion induced by baroclinic and barotropic eddy processes in individual months. The latter fields serve as the basis for describing the synoptic-scale eddy forcing associated with the leading modes of month-to-month variability of the storm tracks over the North Pacific and North Atlantic. These modes are associated with the meridional displacements of the storm-track axes from their climatological positions. The placement of a storm track at a certain latitude ϕ in a certain month is accompanied by enhanced convergence of eddy heat fluxes poleward of ϕ. In the tropospheric column poleward of the storm tra...

Coupled GCM Simulation of Atmosphere–Ocean Variability Associated with Zonally Asymmetric SST Changes in the Tropical Indian Ocean

Abstract The nature of a recurrent pattern of variability in the tropical Indian Ocean (IO) during the boreal autumn has been investigated using a 900-yr experiment with a coupled atmosphere–ocean general circulation model. This Indian Ocean Pattern (IOP) is characterized by zonal surface wind perturbations along the equator, as well as east–west contrasts in the anomalous sea surface temperature (SST), surface pressure, and precipitation fields. The IOP is seen to be linked to the El Nino–Southern Oscillation (ENSO) phenomenon in the tropical Pacific. By constructing composite charts and analyzing the heat budget for the top ocean layer, it is illustrated that the ENSO-related changes in the surface wind modify the intensity of oceanic upwelling, horizontal temperature advection, and surface heat fluxes in various parts of the IO basin. These processes lead to SST perturbations with opposite signs in the eastern and western equatorial IO. Further diagnosis of the model output reveals that some strong IOP...

A General Circulation Model Study of the Atmospheric Response to Extratropical SST Anomalies Observed in 1950–79

Abstract A 30-year experiment with an atmospheric general simulation model has been performed. The lower boundary condition at all oceanic grid points between 38°S and 60°N has been prescribed to follow the observed month-to-month variation of the sea surface temperature (SST) field during the 1950–79 period. Much of the model diagnosis presented here pertains to the midlatitude atmospheric response to recurrent SST patterns in the North Pacific and North Atlantic in winter. The principal modes of variability of the seasonally averaged 515 mb height and SST fields have been identified using rotated principal component (RPC) analysis. The extrema of the first atmospheric mode reside over the North Atlantic and Eurasia, whereas the second mode is associated with height anomalies in the North Pacific/North American sector. Cross-correlation analysis reveals that these two atmospheric modes am linked to leading patterns of the SST field in the North Atlantic and North Pacific, respectively. It is also demonst...

Impact of ENSO on SST Variability in the North Pacific and North Atlantic: Seasonal Dependence and Role of Extratropical Sea–Air Coupling

Abstract The influences of El Nino–Southern Oscillation (ENSO) events in the tropical Pacific on interannual variability of the coupled ocean–atmosphere systems in the North Pacific and North Atlantic have been studied using a suite of experiments with a rhomboidal 30-wavenumber, 14-layer general circulation model (GCM). Observed month-to-month fluctuations of the sea surface temperature (SST) in the tropical Pacific during the 1950–95 period were prescribed as the lower boundary condition for the GCM. The SST conditions outside of the tropical Pacific were predicted by a simple ocean mixed layer model with a constant depth. Four independent integrations under this “Tropical Ocean–Global Atmosphere–Mixed Layer (TOGA-ML)” scenario were conducted. Both observational and model results indicate that the imposed ENSO forcing during midwinter is accompanied by prominent atmospheric circulation changes over the North Pacific and Atlantic. These teleconnection patterns in turn alter the heat exchange across the l...