CGD Research Catalog

	Dr. Kevin Trenberth

Dr. Kevin Trenberth

Su, F., J. C. Adam, K. E. Trenberth, D. P. Lettenmaier, 2006: Evaluation of surface water fluxes of the pan-Arctic land region with a land surface model and ERA-40 reanalysis. Journal of Geophysical Research, 111, D05 110, doi:10.1029/2005JD006387.

Abstract: The seasonal, spatial, and latitudinal variability of precipitation (P), evapotranspiration (E), and runoff (R) are examined for large Arctic river basins and for the entire pan-Arctic domain using a 21-year off-line simulation of the Variable Infiltration Capacity (VIC) macroscale hydrology model and the ERA-40 reanalysis. Observed P used in the VIC model (corrected for gauge catch deficiency) is compared with that from the ERA-40 reanalysis. Gridded values of evapotranspiration minus precipitation (E-P) are calculated from the ERA-40 atmospheric water budget, and estimates of implied E are obtained as the residual of observed P and ERA-40 E-P. The ERA-40 P is surprisingly close to observations on an annual basis over the large river basins (especially accounting for known errors in the observations). Furthermore, ERA-40 P is quite consistent with observations in terms of interannual, spatial, and latitudinal variations. ERA-40 E is generally higher than both VIC E and implied E in spring and autumn. However, VIC estimates more E in June and July than either ERA-40 or the atmospheric budget for the Yenisei, Ob, and Mackenzie River basins. The ERA-40 bias toward early snowmelt and a double runoff peak (not present in VIC or observations) indicates the need for improvements in the ECMWF land surface scheme. The long-term means of ERA-40 vapor convergence P-E for the Lena, Yenisei, Ob, and Mackenzie are not in balance with observed runoff, mainly due to the uncertainties in computed P-E and observed streamflow.

Supported by the National Science Foundation.

Figure: (High resolution figure.) Fields of (a) VIC simulated annual mean runoff (mm) (1979–1999), (b) annual mean moisture convergence P-E (mm), and (c) annual mean ERA-40 runoff (mm).

Trenberth, K. E., B. Moore, T. R. Karl, and C. Nobre, 2006: Monitoring and Prediction of the Earth's Climate: A Future Perspective. Journal of Climate, 19, (CLIVAR special issue), 5001-5008.

Abstract: The climate is changing and will continue to do so regardless of any mitigation actions. Observations and information available are also changing as technological advances take place. Accordingly, an overview is given of a much-needed potential climate information system that embraces a comprehensive observing system to observe and track changes and the forcings of the system as they occur, and which develops the ability to relate one to the other and understand changes and their origins. Observations need to be taken in ways that satisfy the climate monitoring principles and ensure long-term continuity, and which have the ability to discern small but persistent signals. Some benchmark observations are proposed to anchor space-based observations and trends, including a much-needed step forward in the quality of water vapor observations. Satellite observations must be calibrated and validated, with orbital decay and drift effects fully dealt with, and adequate overlap to ensure continuity. The health of the monitoring system must be tracked and resources identified to fix problems. Fields must be analyzed into global products and delivered to users while stakeholder needs are fully considered. Data should be appropriately archived with full and open access, along with metadata that fully describe the observing system status and environment in which it operates. Reanalysis of the records must be institutionalized along with continual assessment of impacts of new observing and analysis systems. Some products will be used to validate and improve models, as well as initialize models and predict future evolution on multiple time scales using ensembles. Modeling and assimilation aspects are dealt with in accompanying papers. Attribution of changes to causes is essential and it is vital to fully assess past changes and model performance and results in making predictions to help appraise reliability and assess impacts regionally on the environment, human activities, and sectors of the economy. In particular, we expect to see a revolution in the way developing countries use and apply climate information. Such a system will be invaluable and further provides a framework for priority setting new observations and related activities. Without the end-to-end process our investments will not deliver adequate return and our understanding will be much less than it would be otherwise.

NSF Partly sponsored by NOAA under grant NA56GP0247 and by a joint NOAA/NASA grant NA87GP0105.

Figure: (High resolution figure.) Trends in SSTs 1950-1999 and time series of monthly SST anomalies over the equatorial Indian and western Pacific Oceans (lower left) and eastern equatorial Pacific Ocean (lower right).

Trenberth, K. E., and D. J. Shea, 2006: Atlantic hurricanes and natural variability in 2005. Geophysical Research Letters, 33, L12704, doi:10.1029/2006GL026894.

Abstract: The 2005 North Atlantic hurricane season (1 June to 30 November) was the most active on record by several measures, surpassing the very active season of 2004 and causing an unprecedented level of damage. Sea surface temperatures (SSTs) in the tropical North Atlantic (TNA) region critical for hurricanes (10° to 20°N) were at record high levels in the extended summer (June to October) of 2005 at 0.9°C above the 1901–70 normal and were a major reason for the record hurricane season. Changes in TNA SSTs are associated with a pattern of natural variation known as the Atlantic Multi-decadal Oscillation (AMO). However, previous AMO indices are conflated with linear trends and a revised AMO index accounts for between 0 and 0.1°C of the 2005 SST anomaly. About 0.45°C of the SST anomaly is common to global SST and is thus linked to global warming and, based on regression, about 0.2°C stemmed from after-effects of the 2004–05 El Niño.

Supported by NSF.

Figure. (High resolution figure.) Figure. SST anomalies (?C) relative to 1951-80 for 10–20ºN 0–80ºW in the tropical North Atlantic for June to October seasonal averages plus the low-pass filtered values (shaded).

Trenberth, K. E., and L. Smith, 2006: The vertical structure of temperature in the tropics: Different flavors of El Nino. Journal of Climate, 19, 4956-4973.

Abstract: To explore the vertical coherence of the vertical temperature structure in the atmosphere, an analysis is performed of the full three-dimensional spatial structure of the temperature field monthly mean anomalies from ERA-40 for a core region of the tropics from 30°N to 30°S, with results projected globally. The focus is on the first three empirical orthogonal functions (EOFs), two of which have primary relationships to El Niño Southern Oscillation (ENSO) and feature rather different vertical structures. The third (EOF-2) also has a weak ENSO signature but a very complex vertical structure and reflects mainly nonlinear trends, some real but also some in large part spurious and associated with problems in assimilating satellite data. The dominant pattern (EOF-1) in its positive sign, features highly coherent zonal mean warming throughout the tropical troposphere from 30°N to 30°S that increases in magnitude with height to 300 hPa, drops to zero about 100 hPa at the tropopause, and has reverse sign to 30 hPa with peak negative values at 70 hPa. Spatially at low levels it shows warmth throughout most of the tropics although with weak or slightly opposite sign in the western tropical Pacific and a strong reversed sign in the Pacific subtropics. Coherent wave structures below 700 hPa at higher latitudes cancel out in the zonal mean. However, the structure becomes more zonal above about 700 hPa and features off-equatorial maxima straddling the equator in the eastern Pacific in the upper troposphere with opposite sign at 100 hPa, as a signature of a forced Rossby wave. The corresponding sea level pressure pattern is similar to but more focused in equatorial regions than the Southern Oscillation pattern. The time series highlights the 1997-98 El Niño along with those in 1982-83, 1986-87, and the 1988-89 La Niña, and correlates strongly with global mean surface temperatures. Missing, however, is the prolonged sequence of 3 successive El Niño events in the early 1990s, which are highlighted in EOF-3 as part of a mainly lower frequency decadal variation that features modest zonal mean warming below 700 hPa, cooling from 700 to 300 hPa, and warming above there, peaking at 100 hPa and extending from 40°N to 50°S. Spatially at the surface this pattern is dominated by Southern Oscillation wave-1 structures throughout the tropics and especially subtropics. The regional temperature structures are coherent throughout the troposphere, with strongest values in the Pacific and extending well into the extratropics, with a sign reversal at and above 100 hPa. Strong Rossby wave signatures are featured in the troposphere with a distinctive quadrupole pattern that reverses at 100 hPa. The vertical coherence of all patterns suggests that they should be apparent in broad layer satellite temperature records but that stratospheric anomalies are not independent. The quite different three-dimensional structure of these different patterns highlights the need to consider the full structure outside of the Pacific and at all vertical levels in accounting for impacts of ENSO, and how they relate to the global mean.

Partially sponsored by the NOAA CLIVAR and CCDD programs under grants NA17GP1376 and NA04OAR4310073 and NSF.

Figure. (High resolution figure.) Shown is the first EOF pattern zonal mean resulting from the analysis of the three dimensional temperature field (top) principle component monthly anomaly time series for 1979-2001, and the regression (middle) and correlation (bottom) patterns for zonal mean temperature anomalies as latitude-pressure cross sections. The contour interval is 0.1 K per standard deviation (middle) and 0.1 for correlation.

Trenberth, K.E., L. Smith, T. Qian, A. Dai, and J. Fasullo, 2006: Estimates of the global water budget and its annual cycle using observational and model data. Journal of Hydrometeorology, accepted.

Abstract: A brief review is given of research in the Climate Analysis Section at NCAR on the water cycle. Results are used to provide a new estimate of the global hydrological cycle for long-term annual means that includes estimates of the main reservoirs of water as well as the flows of water among them. For precipitation P over land a comparison among three datasets enables uncertainties to be estimated. In addition, results are presented for the mean annual cycle of the atmospheric hydrological cycle based on 1979 to 2000 data. These include monthly estimates of P, evapotranspiration E, atmospheric moisture convergence over land, and changes in atmospheric storage, for the major continental land masses, zonal means over land, hemispheric land means and global land means. The evapotranspiration is computed from the Community Land Model run with realistic atmospheric forcings, including precipitation that is constrained by observations for monthly means but with high frequency information taken from atmospheric reanalyses. Results for P-E are contrasted with those from atmospheric moisture budgets based on ERA-40 reanalyses. The latter show physically unrealistic results, because evaporation often exceeds precipitation over land, especially in the tropics and subtropics.

Partially Supported by NSF and NOAA.

Figure. (High resolution figure.) The hydrological cycle. Estimates of the main water reservoirs, given in plain font in 103 km3, and the flow of moisture through the system, given in slant font in103 km3/yr, equivalent to Exagrams (1018 g) per year.

Uppala, S. M., P. W. Kållberg, A. J. Simmons, U. Andrae, V. da Costa Bechtold, M. Fiorino, J. K. Gibson, J. Haseler, A. Hernandez, G. A. Kelly, X. Li, K. Onogi, S. Saarinen, N. Sokka, R. P. Allan, E. Andersson, K. Arpe, M. A. Balmaseda, A. C. M. Beljaars, L. van de Berg, J. Bidlot, N. Bormann, S. Caires, A. Dethof, M. Dragosavac, M. Fisher, M. Fuentes, S. Hagemann, E. Hólm, B. J. Hoskins, L. Isaksen, P. A. E. M. Janssen, A. P. McNally, J.-F. Mahfouf, R. Jenne, J.-J. Morcrette, N. A. Rayner, R. W. Saunders, P. Simon, A. Sterl, K. E. Trenberth, A. Untch, D. Vasiljevic, P. Viterbo, and J. Woollen, 2005: The ERA-40 reanalysis. Quart. Journal of Royal Meteorological Society, 131, 2961-3012.

Summary: ERA-40 is a re-analysis of meteorological observations from September 1957 to August 2002 produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) in collaboration with many institutions. The observing system changed considerably over this re-analysis period, with assimilable data provided by a succession of satellite-borne instruments from the 1970s onwards, supplemented by increasing numbers of observations from aircraft, ocean-buoys and other surface platforms, but with a declining number of radiosonde ascents since the late 1980s. The observations used in ERA-40 were accumulated from many sources. The first part of this paper describes the data acquisition and the principal changes in data type and coverage over the period. It also describes the data assimilation system used for ERA-40. This benefited from many of the changes introduced into operational forecasting since the mid-1990s, when the systems used for the 15-year ECMWF re-analysis (ERA-15) and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) re-analysis were implemented. Several of the improvements are discussed. General aspects of the production of the analyses are also summarized. A number of results indicative of the overall performance of the data assimilation system, and implicitly of the observing system, are presented and discussed. The comparison of background (short-range) forecasts and analyses with observations, the consistency of the global mass budget, the magnitude of differences between analysis and background fields and the accuracy of medium-range forecasts run from the ERA-40 analyses are illustrated. Several results demonstrate the marked improvement that was made to the observing system for the southern hemisphere in the 1970s, particularly towards the end of the decade. In contrast, the synoptic quality of the analysis for the northern hemisphere is sufficient to provide forecasts that remain skilful well into the medium range for all years. Two particular problems are also examined: excessive precipitation over tropical oceans and a too strong Brewer–Dobson circulation, both of which are pronounced in later years. Several other aspects of the quality of the re-analyses revealed by monitoring and validation studies are summarized. Expectations that the ‘second-generation’ ERA-40 re-analysis would provide products that are better than those from the first generation ERA-15 and NCEP/NCAR re-analyses are found to have been met in most cases.

Supported by NOAA and NSF.

Figure. (High resolution figure.) Daily values of the ERA-40 RMS background (grey) and analysis (black) fits to 12 UTC 850 hPa radiosonde measurements of specific humidity (g kg-1) over the Tropics.