Coupled Climate System Model Research Articles

Simulation of Indian summer monsoon rainfall and its intraseasonal variability in the NCAR climate system model

The broad climatological features associated with the Asian monsoon circulation, including its mean state and intraseasonal and interannual variability over the Indian subcontinent as simulated in the National Center for Atmospheric Research (NCAR) global coupled climate system model (CSM) in its control reference experiment, are presented in this paper. The CSM reproduces the seasonal cycle as well as basic observed patterns of key climatic parameters reasonably well in spite of some limitations in simulation of the monsoon rainfall. However, while the seasonality in rainfall over the region is simulated well, the simulated area-averaged monsoon rainfall is underestimated to only about 60% of the observed rainfall. The centers of maxima in simulated monsoon rainfall are slightly displaced southward as compared to the climatological patterns. The cross-equatorial flow in simulated surface wind patterns during summer is also stronger than observed with an easterly bias. The transient experiment with a 1% per year compound increase in CO2 with CSM suggests an annual mean area-averaged surface warming of about 1.73 °C over the region at the time of CO2 doubling. This warming is more pronounced in winter than during the monsoon season. A net increase in area-averaged monsoon rainfall of about 1.4 mm day–1, largely due to increased moisture convergence and associated convective activity over the land, is obtained. The enhanced intraseasonal variability in the monsoon rainfall in a warmer atmosphere is confined to the early part of the monsoon season which suggests the possibility of the date of onset of summer monsoon over India becoming more variable in future. The enhanced interannual and intraseasonal variability in the summer monsoon activity over India could also contribute to more intense rainfall spells over the land regions of the Indian subcontinent, thus increasing the probability of extreme rainfall events in a warmer atmosphere.

Climate‐driven polar motion

The output of a coupled climate system model provides a synthetic climate record with temporal and spatial coverage not attainable with observational data, allowing evaluation of climatic excitation of polar motion on timescales of months to decades. Analysis of the geodetically inferred Chandler excitation power shows that it has fluctuated by up to 90% since 1900 and that it has characteristics representative of a stationary Gaussian process. Our model‐predicted climate excitation of the Chandler wobble also exhibits variable power comparable to the observed. Ocean currents and bottom pressure shifts acting together can alone drive the 14‐month wobble. The same is true of the excitation generated by the combined effects of barometric pressure and winds. The oceanic and atmospheric contributions are this large because of a relatively high degree of constructive interference between seafloor pressure and currents and between atmospheric pressure and winds. In contrast, excitation by the redistribution of water on land appears largely insignificant. Not surprisingly, the full climate effect is even more capable of driving the wobble than the effects of the oceans or atmosphere alone are. Our match to the observed annual excitation is also improved, by about 17%, over previous estimates made with historical climate data. Efforts to explain the 30‐year Markowitz wobble meet with less success. Even so, at periods ranging from months to decades, excitation generated by a model of a coupled climate system makes a close approximation to the amplitude of what is geodetically observed.

Five‐minute, 1/2°, and 1° data sets of continental watersheds and river networks for use in regional and global hydrologic and climate system modeling studies

A major shortcoming of the land surface component in climate models is the absence of a river transport algorithm. This issue becomes particularly important in fully coupled climate system models (CSMs), where river transport is required to close and realistically represent the global water cycle. The development of a river transport algorithm requires knowledge of watersheds and river networks at a scale that is appropriate for use in CSMs. These data must be derived largely from global digital topographic information. The purpose of this paper is to describe a new data set of watersheds and river networks, which is derived primarily from the TerrainBase 5′ Global DTM (digital terrain model) and the CIA World Data Bank II. These data serve as a base map for routing continental runoff to the appropriate coast and therefore into the appropriate ocean or inland sea. Using this data set, the runoff produced in any grid cell, when coupled with a routing algorithm, can easily be transported to the appropriate water body and distributed across that water body as desired. The data set includes watershed and flow direction information, as well as supporting hydrologic data at 5′, 1/2°, and 1° resolutions globally. It will be useful in fully coupled land‐ocean‐atmosphere models, in terrestrial ecosystem models, or in stand‐alone macroscale hydrologic‐modeling studies.

The Asian–Australian Monsoon and El Niño–Southern Oscillation in the NCAR Climate System Model*

Features associated with the Asian–Australian monsoon system and El Niño–Southern Oscillation (ENSO) are described in the National Center for Atmospheric Research (NCAR) global coupled Climate System Model (CSM). Simulation characteristics are compared with a version of the atmospheric component of the CSM, the NCAR CCM3, run with time-evolving SSTs from 1950 to 1994, and with observations. The CSM is shown to represent most major features of the monsoon system in terms of mean climatology, interannual variability, and connections to the tropical Pacific. This includes a representation of the Southern Oscillation links between strong Asian–Australian monsoons and associated negative SST anomalies in the eastern equatorial Pacific. The equatorial SST gradient across the Pacific in the CSM is shown to be similar to the observed with somewhat cooler mean SSTs across the entire Pacific by about 1°–2°C. The seasonal cycle of SSTs in the eastern equatorial Pacific has the characteristic signature seen in the observations of relatively warmer SSTs propagating westward in the first half of the year followed by the reestablishment of the cold tongue with relatively colder SSTs propagating westward in the second half of the year. Like other global coupled models, the propagation is similar to the observed but with the establishment of the relatively warmer water in the first half of the year occurring about 1–2 months later than observed. The seasonal cycle of precipitation in the tropical eastern Pacific is also similar to other global coupled models in that there is a tendency for a stronger-than-observed double ITCZ year round, particularly in northern spring, but with a well-reproduced annual maximum of ITCZ strength north of the equator in the second half of the year. Time series of area-averaged SSTs for the NINO3 region in the eastern equatorial Pacific show that the CSM is producing about 60% of the amplitude of the observed variability in that region, consistent with most other global coupled models. Global correlations between NINO3 time series, global surface temperatures, and sea level pressure (SLP) show that the CSM qualitatively reproduces the major spatial patterns associated with the Southern Oscillation (lower SLP in the central and eastern tropical Pacific when NINO3 SSTs are relatively warmer and higher SLP over the far western Pacific and Indian Oceans, with colder water in the northwest and southwest Pacific). Indices of Asian–Australian monsoon strength are negatively correlated with NINO3 SSTs as in the observations. Spectra of time series of Indian monsoon, Australian monsoon, and NINO3 SST indices from the CSM show amplitude peaks in the Southern Oscillation and tropospheric biennial oscillation frequencies (3–6 yr and about 2.3 yr, respectively) as observed. Lag correlations between the NINO3 SST index and upper-ocean heat content along the equator show eastward propagation of heat content anomalies with a phase speed of about 0.3 m s−1, compared to observed values of roughly 0.2 m s−1. Composites of El Niño (La Niña) events in the CSM show similar seasonal evolution to composites of observed events with warming (cooling) of greater than several tenths of a degree beginning early in northern spring of year 0 and diminishing around northern spring of year +1, but with a secondary resurgence in the CSM events later in northern spring of year +1. The CSM also shows the largest amplitude ENSO SST and low-level wind anomalies in the western tropical Pacific, with enhanced interannual variability of SSTs extending northeastward and southeastward toward the subtropics, compared to largest interannual SST variability in the central and eastern tropical Pacific in the observations.

Surface Ocean Fluxes and Water-Mass Transformation Rates in the Coupled NCAR Climate System Model*

Abstract The global distributions of the air–sea fluxes of heat and freshwater and water mass transformation rates from a control integration of the coupled National Center for Atmospheric Research (NCAR) Climate System Model (CSM) are compared with similar fields from an uncoupled ocean model equilibrium spinup and a new surface climatology. The climatology and uncoupled model use the same bulk-flux forcing scheme and are forced with National Centers for Environmental Predicition (formerly the National Meteorological Center) atmospheric reanalysis data and satellite-based cloud cover, solar flux, and precipitation estimates. The climatological fluxes for the open ocean are adjusted to give a global net balance and are in broad general agreement with standard ship-based estimates. An exception is the ice-free Southern Ocean, where the net heat and evaporative fluxes appear to be too weak but where the observational coverage underlying the reanalyis is quite poor. Major differences are observed between the...

A climate modelling primer

Preface. Acknowledgments. Chapter 1. Climate Chapter 2. History of and Introduction to Climate Models. Chapter 3. Energy Balance Models. Chapter 4. Intermediate Complexity Models. Chapter 5. Coupled Climate System Models. Chapter 6. Practical Climate Modelling. Appendix A: 20th Century Classics. Appendix B: Glossary. Appendix C: About the Primer CD. Index.