Robert M. Chervin

Ocean general circulation from a global eddy‐resolving model

A concerted effort has been made to simulate the global ocean circulation with resolved eddies, using a highly optimized model on the best available supercomputer. An earlier 20-year spin-up has been extended for 12.5 additional years: the first 2.5 with continued annual mean forcing and the final 10.0 with climatological monthly forcing. Model output archived at 3-day intervals has been analyzed into mean fields, standard deviations, products, and covariances on monthly, annual, and multiyear time scales. The multiyear results are examined here in order to give insight into the general circulation of the world ocean. The three-dimensional flow fields of the model are quite realistic, even though resolution of eddies in high latitudes is marginal with a 0.5°, 20-level grid. The use of seasonal forcing improves the simulation, especially in the tropics and high northern latitudes. Mid-latitude gyre circulations, western boundary currents, zonal equatorial flows, and the Antarctic Circumpolar Current (ACC) all show mean and eddy characteristics similar to those observed. There is also some indication of eddy intensification of the mean flow of the ACC and of separated boundary jets. A global thermohaline circulation of North Atlantic Deep Water is identified in deep western boundary currents connected by the ACC. This deep circulation rises mainly in the equatorial Pacific. Several zonal jets are an integral part of this circulation near the equator. The deep flow rises toward the surface in a series of switchbacks. Much of the thermohaline return flow then follows an eddy-rich warm-water route through the Indonesian archipelago and around the southern tip of Africa. However, some intermediate level portions of the thermohaline circulation return south into the ACC and follow a cold water route through the Drake Passage. The representation of a global “conveyor belt” circulation with narrow and relatively high-speed currents along most of its path may be the most important result of this modeling study. Statistics of scalar fields such as transport stream function and surface height are exhibited, as are time series and frequency spectra of certain variables at selected points. These provide a baseline for comparison both with observations and with other model studies at higher resolution. Mean and eddy characteristics of the near-surface temperature and salinity fields are discussed, and surface forcing fields are examined. In particular, combined thermal and hydrological forcing effects are found to drive a conveyor belt circulation between the tropical Pacific and the high-latitude North Atlantic. The effect of weak restoring terms to observed temperature and salinity at great depth and in polar latitudes is found mainly to augment the models convective processes, which are poorly resolved with 0.5° grid spacing. However, the deep restoring terms are insignificant in both the tropics and the mid-latitudes. The geographical distributions of eddy heat and salt transport are discussed. The eddies transport heat and salt down the gradients and along the mean flow in many regions of strong currents. Net meridional transports of heat and salt by both the total currents and the eddies are computed for the Atlantic, the Indo-Pacific, and the global ocean. The total currents provide for poleward heat transport (except near 40°S, where the contribution from ACC instabilities is rather weak) and, in particular, for that needed to sustain the conveyor belt transport. Meridional eddy transports are especially important for warming the Pacific upwelling branch of the thermohaline circulation and for transporting salt across the equator into the North Pacific. Planned improvements to the model include a free-surface treatment of the barotropic mode and additions of the Arctic Basin and sea ice. A fully prognostic extension of the existing integration is intended, with subsequent transitioning of the model onto a 0.25° grid having very realistic geometry. The 0.25° version of the model will run effectively on newly available supercomputers.

On Determining the Statistical Significance of Climate Experiments with General Circulation Models

Abstract One of the key problems in analyzing the results of climate experiments with general circulation models (GCMs,) is the matter of estimating the statistical significance of a prescribed change response. This question involves separating the signal (that part of the response attributable to the prescribed change) and the noise (some measure of the inherent variability of model statistics). In this paper we present maps showing the geographical distribution of estimates of the standard deviations at each grid point for the January climatological statistics of the NCAR GCM based on a sample of five independent realizations. Also, a formalism for estimating the statistical significance of a prescribed change response is given based on the classical Students t-test, and the implications of varying the sample size are discussed. The most telling implication of the results is that thus statistical significance questions could mean that a large percentage of total computational effort in a particular pr...

Interannual Variability and Seasonal Climate Predictability

Abstract A twenty-year integration of an atmospheric general circulation model with identically evolving prescribed surface boundary conditions each year is employed to provide a measure of the interannual variability obtainable from internal atmospheric dynamics alone. In particular, the variability of seasonal mean sea level pressure and 700-mb geopotential height is considered by means of a sampled climate ensemble approach. This model-generated internal dynamics variability is assumed to be identical to that resulting from the real atmosphere if it operated without anomalous boundary conditions and is considered unpredictable since the time scales involved are beyond the traditional limits of deterministic predictability. By means of objective statistical tests, sampled model variances for these fields are compared to sampled variances of observed seasonal means (which have contributions from anomalous boundary conditions as well) for all four seasons in order to ascertain if, in an infinite populatio...

Cloudiness as a Climatic Feedback Mechanism: Effects on Cloud Amounts of Prescribed Global and Regional Surface Temperature Changes in the NCAR GCM

Abstract In a series of January experiments, prescribed changes of 2 K to the ocean surface temperatures of the NCAR GCM are imposed either globally or in one of two zonal strips centered at 5°S and 15°N. The global perturbations cause a global response in 3 km cloudiness of opposite sign to the surface temperature change, whereas a warm surface temperature perturbation at 15°N, under the subsiding branch of the Hadley cell, causes a cloudiness increase over the strip and smaller cloudiness decreases in zones adjacent to the strip. On the other hand, a 2 K strip placed at 5°S under the ascending branch of the Hadley cell causes little cloudiness response over the strip, but a statistically significant cloudiness decrease in adjacent zones. It is concluded that, in addition to a term which accounts for the effects on global cloudiness of global temperature changes, a comparably important additional term is needed to compute the global cloudiness feedback problem. This additional term integrates the local c...

Effects of a Variety of Indian Ocean Surface Temperature Anomaly Patterns on the Summer Monsoon Circulation: Experiments with the NCAR General Circulation Model

The time mean response of the summer monsoon circulation, as simulated by the 2.5° latitude-longitude resolution, July version of the National Center for Atmospheric Research (NCAR) General Circulation Model (GCM), to a variety of Indian Ocean surface temperature anomaly patterns is examined. In separate experiments, prescribed changes in surface temperature are imposed in the Western Arabian Sea, the Eastern Arbian Sea or the Central Indian Ocean. The influence of these anomaly patterns on the simulated summer monsoon circulation is evaluated in terms of the geographical distribution of the prescribed change response for any field of interest. This response is defined as the grid point difference between a 30-day mean from a prescribed change experiment and the ensemble average of the 30-day means from the control population for which the same set of climatological ocean surface temperatures are used in each simulation. The statistical significance of such a prescribed change response is estimated by relating the normalized response (defined as the ratio of the prescribed change response to the standard deviation of 30-day means as estimated from the finite sample of control cases) to the classical Studentst-statistic. Using this methodology, the most prominent and statistically significant features of the models response are increased vertical velocity and precipitation over warm anomalies and typically decreased vertical velocity and precipitation in some preferred region adjacent to the prescribed change region. In the case of cold anomalies, these changes are of opposite sign. However, none of the imposed anomaly patterns produces substantial or statistically significant precipitation changes over large areas of the Indian sub-continent. The only evidence of a major nonlocal effect is found in the experiment with a large positive anomaly (+3°C) in the Central Indian Ocean. In this instance, vertical velocity and precipitation are reduced over Malaysia and a large area of the Equatorial Western Pacific Ocean. Thus, while these anomaly experiments produce only a local response (for the most part), it is hoped, as one of the purposes of the planned Monsoon Experiment (MONEX), that the necessary data will be provided to produce detailed empirical evidence on the extent to which Indian Ocean surface temperature anomalies correlate with precipitation anomalies over the Indian subcontinent—a correlation which generally does not appear in these GCM results.

On the Comparison of Observed and GCM Simulated Climate Ensembles

Abstract Within the sampled climate ensemble framework for describing the climate, objective univariate statistical tests are presented which permit a straightforward assessment of the extent to which observed and GCM simulated climates agree or differ with respect to various first- and second-moment measures (i.e., ensemble averages and standard deviations) of the climate. As an example of this approach, the vertically averaged transient beat flux, (??)¯V′T′ is considered as a basic climate element and ensemble averages and standard deviations of this climate element are objectively compared fox. the same number of samples from a global data set assembled by A. Oort of GFDL and from sets of 2.5 and 5° resolution realizations with a GCM developed several years ago at NCAR. It is found that the degree of agreement between observed and simulated climate is highly dependent on the geographical location, the statistical moment used as the climate measure and whether zonally averaged or grid-point values const...

Response of the NCAR General Circulation Model to Prescribed Changes in Ocean Surface Temperature Part I: Mid-Latitude Changes

Abstract Four numerical experiments are analyzed to determine the three-dimensional response of the NCAR general circulation model to large prescribed changes in mid-latitude North Pacific Ocean surface temperature. The ocean surface temperature (OST) boundary conditions are subjected to changes of opposite sign in the eastern and west-central portions of the North Pacific Ocean. The maximum amplitude of the temperature changes is either 12°C or 4°C. The model atmosphere response in the North Pacific sector includes changes in amplitude and vertical tilt of the long waves, an increased direct thermal circulation (i.e., warm air rises over the positive OST change and cold air sinks over the negative OST change), and locally enhanced westerlies to the north of the positive OST change. Cyclones form and/or intensify over the positive OST change and tend to be absent or weak over the negative OST change. The mid-tropospheric response extends downstream from the prescribed change region, and the response both ...

Testing the statistical significance of the response of the NCAR general circulation model to North Pacific Ocean surface temperature anomalies

Abstract The response of the six-layer NCAR atmospheric general circulation model (GCM) to mid-latitude North Pacific Ocean surface temperature anomalies is examined. In particular, January and winter season averages of the 1.5 km temperature fields generated by the GCM with prescribed ocean surface temperatures (i.e., the control experiment) are subtracted from January and winter season averages of three separate prescribed change experiments in which ocean surface temperature anomalies are superimposed on the ocean surface temperature patterns of the control case. These differences between prescribed change and control experiments are compared at each grid point to the standard deviations of the 1.5 km temperature field constructed from five independent GCM generated January simulations. The statistical significance of the results is evaluated using a methodology developed for the GCM by Chervin and Schneider based on the classical Students t-test. The results suggest that a statistically significant s...

The Observed and Simulated Atmospheric Seasonal Cycle. Part I: Global Wind Field Modes

Abstract The seasonal cycle of the global wind field is documented for both a decadal set of analyses from the National Meteorological Center (NMC) and an extended term integration of a research version of the Community Climate Model developed at the National Center for Atmospheric Research. Composite eigenvector analysis is used to establish the dominant three dimensional coherent structures characteristic of the datasets while gridpoint harmonic analysis provides evidence of the extent to which these structures describe conventional seasonal modes. These quantitative indicators of spatial and temporal variance form a stringent measure of model performance with respect to seasonal variation. The model appears to be far more successful at capturing the annual harmonic contained in the NMC analyses than the semiannual harmonic. This discrepancy may be related to the absence of the requisite tropical forcings due to either inadequate parameterizations of certain physical processes or the lack of interannual...

A Study of the Response of NCAR GCM Climatological Statistics to Random Perturbations: Estimating Noise Levels

Abstract General Circulation Models (GCMs) of the earths atmosphere have been frequently used to test different mechanisms for climate change. Typically, these tests involve comparing the statistics of the model with a prescribed change of a variable, system parameter or boundary condition, to the statistics from an unperturbed control case. However, it is not uncommon for the prescribed change experiment statistics to be comparable to statistics from a random perturbation experiment. Consequently, it is essential to determine the inherent “noise climatology” of GCMs in order to distinguish between signal and noise in climate experiments. We have examined the time-averaged response of the NCAR GCM to random perturbations in the initial conditions, while leaving all boundary conditions fixed. The dependence on time averaging interval of the noise level of a number of globally- and zonally-averaged GCM variables has been computed and gives an indication of how long to time average in order to reduce nois...