Cold Sea Surface Temperature Bias Research Articles

Evaluation of the South Pacific Convergence Zone in IPCC AR4 Climate Model Simulations of the Twentieth Century

Abstract Understanding how the South Pacific convergence zone (SPCZ) may change in the future requires the use of global coupled atmosphere–ocean models. It is therefore important to evaluate the ability of such models to realistically simulate the SPCZ. The simulation of the SPCZ in 24 coupled model simulations of the twentieth century is examined. The models and simulations are those used for the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC). The seasonal climatology and interannual variability of the SPCZ is evaluated using observed and model precipitation. Twenty models simulate a distinct SPCZ, while four models merge intertropical convergence zone and SPCZ precipitation. The majority of models simulate an SPCZ with an overly zonal orientation, rather than extending in a diagonal band into the southeast Pacific as observed. Two-thirds of models capture the observed meridional displacement of the SPCZ during El Niño and La Niña events. The four models that use ocean heat flux adjustments simulate a better tropical SPCZ pattern because of a better representation of the Pacific sea surface temperature pattern and absence of cold sea surface temperature biases on the equator. However, the flux-adjusted models do not show greater skill in simulating the interannual variability of the SPCZ. While a small subset of models does not adequately reproduce the climatology or variability of the SPCZ, the majority of models are able to capture the main features of SPCZ climatology and variability, and they can therefore be used with some confidence for future climate projections.

GIOTTO: A coupled atmosphere-ocean general-circulation model: The tropical Pacific

A new coupled general-circulation model (GIOTTO) has been developed. The individual components are composed of the atmosphere model, ECHAM-4, and the ocean model, MOM (Modular Ocean Model)-1.2. The model domain is global, and no flux correction is applied. The coupling is active between 60°N and 60° S. Poleward of 60° the atmosphere is forced by the climatological sea surface temperature (SST), and the ocean is relaxed towards the climatological SST and sea surface salinity. Further, the sea-ice coverage is prescribed. The coupling interval is set to two hours to resolve the diurnal cycle. In this paper we describe the design of the model, and discuss results of a coupled 20-year integration. The representation of the mean state is realistic, although there is an overall cold SST bias of about one degree centigrade in the tropics, and a tendency to simulate a double Inter Tropical Convergence Zone. The annual cycle, as simulated in the equatorial Pacific, is too weak in the east Pacific and too strong in the warm-pool region. The phase, however, is well captured. The SST variability in the equatorial Pacific is underestimated by about 30%, and the anomalies are too confined to the equator. The main features of El Niño-Southern Oscillation (ENSO) dynamics, like propagation of heat-content anomalies, reflection of equatorial Kelvin and Rossby waves, and westerly wind bursts, however, are correctly represented by the model. A variability analysis based on empirical orthogonal functions indicates that the ENSO mechanisms are simulated correctly. The model also appears to be well balanced with a remarkably low SST drift (0.5 degC decade−1), and a realistic equatorial thermal structure. We are, therefore, confident that the model can be used for experimental seasonal predictions.

The Double-ITCZ Problem in IPCC AR4 Coupled GCMs: Ocean–Atmosphere Feedback Analysis

Abstract This study examines the double–intertropical convergence zone (ITCZ) problem in the coupled general circulation models (CGCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). The twentieth-century climate simulations of 22 IPCC AR4 CGCMs are analyzed, together with the available Atmospheric Model Intercomparison Project (AMIP) runs from 12 of them. To understand the physical mechanisms for the double-ITCZ problem, the main ocean–atmosphere feedbacks, including the zonal sea surface temperature (SST) gradient–trade wind feedback (or Bjerknes feedback), the SST–surface latent heat flux (LHF) feedback, and the SST–surface shortwave flux (SWF) feedback, are studied in detail. The results show that most of the current state-of-the-art CGCMs have some degree of the double-ITCZ problem, which is characterized by excessive precipitation over much of the Tropics (e.g., Northern Hemisphere ITCZ, South Pacific convergence zone, Maritime Continent, and equatorial Indian Ocean), and are often associated with insufficient precipitation over the equatorial Pacific. The excessive precipitation over much of the Tropics usually causes overly strong trade winds, excessive LHF, and insufficient SWF, leading to significant cold SST bias in much of the tropical oceans. Most of the models also simulate insufficient latitudinal asymmetry in precipitation and SST over the eastern Pacific and Atlantic Oceans. The AMIP runs also produce excessive precipitation over much of the Tropics, including the equatorial Pacific, which also leads to overly strong trade winds, excessive LHF, and insufficient SWF. This suggests that the excessive tropical precipitation is an intrinsic error of the atmospheric models, and that the insufficient equatorial Pacific precipitation in the coupled runs of many models comes from ocean–atmosphere feedback. Feedback analysis demonstrates that the insufficient equatorial Pacific precipitation in different models is associated with one or more of the following three biases in ocean–atmosphere feedback over the equatorial Pacific: 1) excessive Bjerknes feedback, which is caused by excessive sensitivity of precipitation to SST and overly strong time-mean surface wind speed; 2) overly positive SST–LHF feedback, which is caused by excessive sensitivity of surface air humidity to SST; and 3) insufficient SST–SWF feedback, which is caused by insufficient sensitivity of cloud amount to precipitation. Off the equator over the eastern Pacific stratus region, most of the models produce insufficient stratus–SST feedback associated with insufficient sensitivity of stratus cloud amount to SST, which may contribute to the insufficient latitudinal asymmetry of SST in their coupled runs. These results suggest that the double-ITCZ problem in CGCMs may be alleviated by reducing the excessive tropical precipitation and the above feedback-relevant errors in the atmospheric models.

A Discussion on the Errors in the Surface Heat Fluxes Simulated by a Coupled GCM

Abstract This paper contrasts the sea surface temperature (SST) and surface heat flux errors in the Tropical Pacific simulated by the University of California, Los Angeles, coupled atmosphere–ocean general circulation model (CGCM) and by its atmospheric component (AGCM) using prescribed SSTs. The usefulness of such a comparison is discussed in view of the sensitivities of the coupled system. Off the equator, the CGCM simulates more realistic surface heat fluxes than the AGCM, except in the eastern Pacific south of the equator where the coupled model produces a spurious intertropical convergence zone. The AGCM errors are dominated by excessive latent heat flux, except in the stratus regions along the coasts of California and Peru where errors are dominated by excessive shortwave flux. The CGCM tends to balance the AGCM errors by either correctly decreasing the evaporation at the expense of cold SST biases or erroneously increasing the evaporation at the expense of warm SST biases. At the equator, errors in...

A 2XCO2 climate change scenario over Europe generated using a limited area model nested in a general circulation model: 1. Present‐day seasonal climate simulation

In this and the companion paper by Giorgi et al. (this issue) we present a regional climate change scenario for Europe and the western Mediterranean basin induced by doubling of carbon dioxide concentration as produced with a limited area model (LAM) nested in a general circulation model (GCM). In this paper we analyze the seasonal climatology of the present‐day climate simulation which is used as a control run in the generation of the scenario. The GCM and the LAM employed for this study are versions of the community climate model (CCM) of the National Center for Atmospheric Research (NCAR) and the NCAR/Pennsylvania State University mesoscale model (MM4). The CCM simulation generally reproduces the basic seasonal migration patterns of the storm tracks which affect the European region but also shows significant biases: (1) a general weakening and southward shift of the cold season North Atlantic jet; (2) cold sea surface temperature bias of up to 4°–6°C in the North Atlantic and the Mediterranean; (3) cold tropospheric temperature bias of several degrees; (4) low tropospheric relative humidity bias; and (5) underprediction of July precipitation, resulting in low soil moisture conditions and exceedingly high surface temperatures. Overall, the nested MM4 produces large‐scale circulations similar to those of the driving CCM and, because of its finer representation of topography and coastlines, substantially improves the spatial distribution of precipitation and surface air temperature not only at high resolution but also at resolutions close to the CCM's. Among the main biases in the nested MM4 simulation are (1) cold bias over land of 1°–5°C in January, April, and October and warm bias in July; (2) underprediction of precipitation over most of continental Europe, maximum in July (simulated precipitation is 60–80% lower than observed) and less pronounced in January, April, and October (simulated precipitation is 10–40% lower than observed). A strong contribution to these biases is given by errors in the driving CCM tropospheric fields. Overall, temperature and precipitation are better simulated in the colder seasons than in summertime and the seasonal cycle is captured better over the Mediterranean regions than over the continental interior.