U.S. Dept. of Commerce / NOAA / OAR / GFDL *Disclaimer

 

6. CLIMATE DIAGNOSTICS

GOALS
  6.1 COMPILATION OF A TEMPORALLY HOMOGENEOUS RADIOSONDE
        TEMPERATURE DATASET

ACTIVITIES FY00

     6.1.1 Sensitivity of Radiosonde Temperature Trends to Data Quality           Progress has been made in applying adjustments to a global network of 87 radiosonde stations so as to render their temperature time series more temporally homogeneous. Adjustment is needed due to artificial discontinuities introduced when instruments or recording practices were changed. These adjusted data are intended for use in the study of long-term climate variability, particularly with regard to global temperature trends. This effort builds on progress made recently (1699) in quantifying the sensitivity of various factors involved in the process of making adjustments and estimating trends.

          The laborious task of identifying artificial change-points (abrupt changes in time series) has been completed. This procedure entailed visual examination and consideration of all data and a variety of ancillary information on a station by station, level by level basis. The tools employed were a unique compilation of station history meta-data (created at ARL), diurnal differences in the temperatures, vertical temperature structure, and non-temperature measures of climate variability, among others.

          A complex algorithm for adjusting the data has been developed and coded, along with related graphical tools. The scheme employed attempts to make adjustments in such a way as to preserve the natural vertical coherence of the temperature data. This is accomplished by making adjustments using "reference levels" from the same station (i.e., vertical levels which do not require adjustment and, excluding the effects of any artificial discontinuities, correlate highly with the level to be adjusted). After a level has been adjusted it can potentially serve later as a reference level. This scheme is an attempt to remove the artificial component of change while retaining the natural component. In a minority of cases when suitable reference levels are not available, adjustment is made by a method which assumes that the difference in mean temperature across the change-point is entirely artificial.

          After completing the identification of artificial change-points, the newly developed adjustment method was applied to the radiosonde station data. This process was repeated more than once in order to create different "scenarios" of adjusted data based on either various levels of confidence in the identification of artificial change-points or other procedural differences. Such an approach allows for testing the sensitivity of low frequency variability (especially trends) to some of the details of the identification and adjustment processes. Having completed these initial steps, various analyses involving internal sensitivity and consistency checks, comparisons with other observed datasets, and comparison with model (GCM) data were started.

ACTIVITIES FY01

          The effort to create and examine a more temporally homogeneous radiosonde temperature data base will continue. The adjusted data will be used to assess long-term trends and compared to a number of observational and GCM datasets for the assessment of global change. It is anticipated that the observed data used for comparison will include Microwave Sounding Unit (MSU), surface station, and NCEP/NCAR Reanalysis temperature data. An additional project involving wider external collaboration will be initiated and will focus on development of operational radiosonde products for use in near real-time diagnosis of temperatures in the free atmosphere.

  6.2 ANALYSIS OF DATASETS BASED ON SATELLITE OBSERVATIONS

ACTIVITIES FY00

     6.2.1 Decadal Variations in Tropical Water Vapor: An Evaluation of Satellite Observations and a Model Simulation           It is widely believed that water vapor amounts will increase in response to global warming. Climate models predict that the column-integrated water vapor, or total precipitable water (W), will increase by ~7% per 1°C increase in surface temperature. Consequently, if the global warming in response to a doubling of CO2 approaches the upper range of current model predictions (~4.5°C), water vapor amounts could increase in excess of 20% during the next half century. This amplified moistening of the atmosphere in response to a surface warming not only highlights the importance of water vapor feedback in determining the climate sensitivity of GCMs, but also underscores the importance of long-term monitoring of water vapor to the detection and attribution of climate change.

          Despite its obvious importance, there have been relatively few observational studies of the long-term variations or trends in water vapor. Moreover, there have been few, if any, attempts to compare observed trends against model-predicted trends, or to inter-compare observed trends from different observing systems. To address this issue, multiple satellite records of tropical-mean water vapor were compared with a general circulation model (GCM) simulation to assess our ability to monitor and predict low-frequency changes in W (1741). Particular attention was focused on the drying between 1979-1995 recorded by a TOVS-statistical retrieval which is calibrated to radiosondes. A version of the GFDL GCM integrated with observed time-varying SST forcing, as well as observational estimates based on microwave and TOVS-physical retrievals, show no sustained drying in the last two decades (Fig. 6.1). This discrepancy is consistent with a previous assertion that the TOVS-statistical

algorithm is vulnerable to radiosonde instrumentation changes over this period, which introduce an artificial drying trend into the retrieval. The comparison also demonstrated excellent agreement between the observed variations in tropical water vapor over the 1979-1999 period and those predicted by the GCM integration.

     6.2.2 Reconciling Surface and Satellite Temperature Records

          The nature of temperature trends in the lower troposphere is a subject of considerable debate. Satellite observations from the Microwave Sounding Unit (MSU) channel 2LT exhibit little warming over the period of record (1979-present), in contrast to the distinct warming trend evident in surface records. A key point of debate in interpreting the MSU temperature records centers on the ability to correct for known degradations in the satellite orbit, which have been shown to introduce a spurious cooling trend in the record. While attempts have been made to correct for these spurious drifts, comparison with radiosonde temperature records suggests that a cooling drift, consistent with that expected from changes in the satellite orbit, still remains in the MSU record.

          Figure 6.2 compares the globally averaged difference in the lower tropospheric temperature between MSU and an objectively-analyzed radiosonde dataset. The MSU record exhibits a distinct cooling trend over the period 1979-1994 relative to the radiosonde data. Also shown in this figure is the expected spurious trend in the MSU lower tropospheric temperature record based upon known changes in satellite orbit. A notable similarity between the two time series is discernible, suggesting that there still remains a spurious downward (cooling) drift in the MSU temperature record.


 

PLANS FY01

          Future intercomparisons will be performed between the radiosonde and satellite observations of upper tropospheric water vapor on a station-by-station and satellite-by-satellite basis, so as to assess their utility for climate monitoring. In order to construct a homogeneous record of water vapor for trend detection, particular effort will focus on identifying and quantifying discontinuities associated with radiosonde instrumentation or satellite calibration changes.

  6.3 AIR-SEA INTERACTION

ACTIVITIES FY00

     6.3.1 Experimentation with an Atmospheric General Circulation Model Coupled to an Ocean Mixed-Layer with Variable Depth           A coupled model has been developed utilizing a one-dimensional ocean mixed-layer model (MLM) developed at the Climate Diagnostics Center (CDC) and a GFDL atmospheric GCM. The mixed-layer in the oceanic component has a variable depth, with computations of temperature and salinity changes at 31 levels in the vertical. The ocean model considers fluxes through the air-sea interface and vertical re-distribution in the column, but excludes any horizontal communication in the ocean. The atmospheric component is a full GCM with horizontal resolution of R30 and 14 levels in the vertical.

          Having completed the initial testing of this new coupled model, several ensembles of experiments were conducted using different spatial configurations of the ocean model. Since the main goal of this project is to assess the impact of El Niño-Southern Oscillation (ENSO) on global sea surface temperature (SST) variations, all of the experiments entail prescription of SST anomalies in the deep tropical eastern Pacific (DTEP, defined as the region between ~173°E and the South American coast, and within ~15°N-15°S) as observed in the 1950-1999 period. These experiments will henceforth be referred to as the Deep Tropical Eastern Pacific Ocean-Global Atmosphere (DTEPOGA) runs.

          The "standard" experiment uses the SST forcing in DTEP, with the MLM employed elsewhere in the World Oceans. A sixteen-member ensemble of this experiment has been completed, with each run being initiated from an independent set of atmospheric conditions. To compliment this model scenario, a set of eight "control" integrations has been completed in which the MLM was not used; instead, seasonally varying climatological SSTs (computed from the "standard" DTEPOGA runs) were specified outside of DTEP. Finally, an eight-member ensemble of the "North Pacific" (NP) experiment has been completed. In this scenario, the MLM was employed only in the North Pacific Ocean, and observed climatological SSTs were specified elsewhere outside of DTEP. The purpose of the NP experiment is to isolate the influences of ENSO on SST variability in the North Pacific through changes in the overlying atmospheric circulation. The data generated by the coupled model has been distributed to external collaborators. In particular, a large subset of the model output was sent to CDC.

          The DTEPOGA experiments have been analyzed in conjunction with a previous series of integrations based on coupling of the same R30L14 atmospheric GCM to an oceanic mixed layer model with a constant depth of 50 m. The domain of SST prescription in the model runs using fixed mixed layer depth extends across the entire width of the tropical Pacific between ~25°N and ~25°S. Integrations with the 50-m mixed layer incorporated at all ocean grid points outside of the tropical Pacific are referred to as the TOGA-ML runs; whereas the corresponding control experiment (i.e., with climatological SST conditions prescribed outside of the tropical Pacific) is labeled as the TOGA experiment. Four-member ensembles of the TOGA-ML and TOGA experiments have been completed for the 1950-1999 period.

     6.3.2 Atmospheric Bridge Linking ENSO to SST Variability in the North Pacific and North Atlantic

          Both observational data and output from the TOGA-ML experiment (6.3.1) indicate that the imposed ENSO forcing during mid-winter is accompanied by prominent atmospheric circulation changes over the North Pacific and Atlantic. These teleconnection patterns in turn alter the heat exchange across the local sea-air interface. The extratropical SST anomalies generated by this "atmospheric bridge" mechanism (1393) typically attain maximum amplitudes in late winter or early spring.

          Detailed diagnoses of the monthly evolution of the surface heat budget during ENSO episodes at selected sites have been performed (nk). The simulated SST response exhibits a 1-2 month delay relative to temperature changes in the overlying atmosphere. This lag relationship results in a polarity change of sea-to-air gradients in temperature and water vapor mixing ratio in late winter. From late autumn through mid-winter, the action of the climatological wind on these gradients results in enhancement of the developing SST anomalies. In the months thereafter, the reversed gradients are accompanied by attenuation of the SST signal. Shortwave radiative fluxes associated with variations in cloud cover play an important role in SST variability at some of the subtropical sites.

          The nature of sea-air feedbacks in the extratropics has been studied by contrasting the output from the TOGA-ML and TOGA experiments (6.3.1). Incorporation of sea-air coupling in TOGA-ML is seen to enhance the persistence of the ENSO-related atmospheric anomalies in the extratropics through late winter and early spring. Comparison with results from previous studies on midlatitude sea-air interactions suggests that part of the atmospheric signal in TOGA-ML may be attributed to forcing from extratropical SST anomalies produced by the atmospheric bridge mechanism.

          Preliminary diagnosis of the output from the DTEPOGA experiments (6.3.1) confirms the principal findings based on the TOGA-ML and TOGA runs. Comparison between the results from the "standard" and "NP" scenarios (6.3.1) reveals that the atmospheric signals in the latter scenario are stronger, thus suggesting the presence of negative interferences from atmospheric responses to SST anomalies generated in the Indian Ocean or tropical western Pacific through tropical atmospheric bridges.

     6.3.3 Impact of ENSO on Monsoon Systems in East Asia and Australia

          Previous observational studies have indicated that warm ENSO events are accompanied by the development of a wintertime near-surface anticyclone over the Philippine Sea region. The southwesterly wind anomaly on the northern flank of this circulation feature is opposite to the climatological northeasterly (dry) winter monsoon over East Asia. The resultant weakening of the monsoon flow brings about above-normal precipitation and temperature in that region. The air-sea interaction associated with the Philippine anticyclone is also known to influence the SST conditions in the South China Sea and tropical western Pacific. Such altered conditions could affect the subsequent development of the Meiyu/Baiu rainbands over eastern China and Japan in the following spring and early summer.

          Diagnosis of both the TOGA and "control" DTEPOGA experiments (6.3.1) illustrates that the GCM simulates the above-mentioned ENSO influences on the East Asian winter monsoon with a high degree of fidelity. In accord with the observations, the composite model anomaly near the surface in the control DTEPOGA integration is characterized by an anticyclone over the Philippines during warm events (Fig. 6.3). The southerly flow to the west of the anomalous high pressure center leads to warmer and wetter conditions than normal. The reduced wind speeds and cloud cover over the South China Sea result in considerable warming of the oceanic mixed layer, whereas the enhanced northeasterly circulation to the east of the anomalous anticyclone is conducive to oceanic cooling. Both the atmospheric and oceanic anomalies over the subtropical western Pacific exhibit a tendency to migrate eastward during the spring season.

          Also noteworthy in the patterns of Fig. 6.3 is the appearance of the anticyclone off the eastern Australian coast, which exerts a considerable influence on the summertime circulation in that sector. The salient symmetry about the equator of the anomalous features over the western Pacific is suggestive of a Rossby-wave response to reduced near-equatorial convective heating in that sector, when the ascending branch of the Walker Cell is displaced eastward during warm ENSO events.

     6.3.4 Modulation of Tropical Transient Activity by ENSO

          The impact of ENSO events on the geographical distribution and intensity of tropical atmospheric variability on synoptic (~several days) and intraseasonal (30-60 days) time scales has been examined using both NCEP/NCAR reanalyses and output from the TOGA experiment (6.3.1). The preferred sites of eddy activity, propagation behavior and

growth/decay characteristics of these disturbances have been identified using lagged regression statistics, variance analysis, and complex empirical orthogonal functions. It has been demonstrated that the GCM is capable of reproducing the essential regional and seasonal characteristics of the observed transient activity in the tropics. The results also indicate that the occurrence of ENSO has noticeable effects on the propagation speed, as well as locations of growth and attenuation, of the these disturbances. Further analysis suggests that such influences are attributable to spatial shifts in convective zones accompanying the development of ENSO events, and to changes in the atmospheric steering flow when such events occur.
 

PLANS FY01

          With the integration of the coupled oceanic mixed-layer model/atmospheric GCM having been completed, future efforts will be devoted to more detailed examination of the model output, with particular emphasis on remote atmospheric and oceanic responses to ENSO forcing, and on local air-sea interactions in the extratropics.

          The relative contributions of enhanced persistence and positive dynamical air-sea feedbacks to the springtime atmospheric signals associated with ENSO will be further investigated using the suite of DTEPOGA integrations. The nature of the atmospheric responses in the North Pacific to SST forcing in the Indian Ocean and western tropical Pacific will be studied. The role of tropical Atlantic SST anomalies in atmospheric variability in the North Atlantic sector will also be analyzed.

          The relationships between ENSO-related processes and the Asian-Australian monsoon systems will be further delineated using the available observational and model datasets. Particular emphasis will be placed on the origin of the variability of the air-sea coupled system on biennial time scales, and on extratropical influences of monsoon fluctuations during the summer season.

          The characteristics of air-sea interactions accompanying the passage of atmospheric disturbances on synoptic and intraseasonal time scales will be documented using output from the TOGA-ML and DTEPOGA experiments. The teleconnections between extratropical flow features and tropical Madden-Julian Oscillations will be examined.

  6.4 DEVELOPMENT OF WEB-BASED TOOLS FOR VISUALIZING AND EVALUATING MODEL OUTPUT

ACTIVITIES FY00

          Continued efforts have been made to design and improve a user-friendly web-based software package for visualizing and validating the output of the future FMS. The graphical and model evaluation tools assembled in a preliminary version of this package have been introduced to the GFDL community.

          This project has been substantially reorganized to include applications to oceanic datasets, and to incorporate the advanced features in the data display and management toolkit maintained by the Thermal Modeling and Analysis Project at the Pacific Marine Environmental Laboratory (PMEL). A long-term partnership between GFDL and PMEL has been established to facilitate this collaboration. The latest version of the PMEL Live Access Server (LAS), which is a web-based interface to data sharing, visualization and analysis, has been successfully installed and tested at GFDL. Plans to link existing model evaluation tools to this framework have been formulated. Enhancements and modifications to LAS aimed at improving its functionality in displaying both meteorological and oceanographic fields have been made. It is envisioned that the LAS facility, in conjunction with the generalized Internet-wide approach to data sharing through the Distributed Oceanographic Data Systems (DODS) network, will constitute a powerful and convenient tool for accessing and visualizing datasets from research centers throughout the world, and for evaluating the performance of various FMS experiments conducted at GFDL.

PLANS FY01

          With the assistance of the PMEL staff, a fully operational version of LAS, with appropriate connections to various standard model evaluation routines, will be adapted for laboratory-wide use within GFDL. New output from various FMS experiments will be assembled and linked to this new software package. The multitude of data resources available through the DODS network will be exploited to facilitate intercomparisons between model-simulated and observational datasets.

  6.5 GFDL/UNIVERSITIES COLLABORATIVE PROJECT FOR
MODEL DIAGNOSIS

ACTIVITIES FY00

          The primary goals of this collaborative effort are to involve the university community in the analysis and design of numerical model experiments at GFDL for identifying the effects of anomalies in surface boundary conditions on interannual and interdecadal variability of the atmosphere, and to develop procedures for insightful analysis and comparison of GCM predictions of regional climate anomalies. The efforts of this collaboration are focussed on the mutual interactions between stationary eddies, low-frequency variability and storm tracks, the maintenance of regional climates and their sensitivities to ocean surface temperature, and the applications of these results to atmospheric predictability. Since 1990, this collaboration has received financial support from the Climate and Global Change Program of the NOAA/Office of Global Programs. The present group of extramural investigators include scientists from CDC, University of Washington, University of Illinois, Florida State University, University of Wisconsin at Milwaukee, and Pennsylvania State University. The final workshop for this decade-long effort was held in September 2000 at GFDL, at which the progress of our understanding of various scientific issues was reviewed, and the future continuation of cooperative arrangements among interested scientists was considered.

          Significant achievements by GFDL investigators associated with this collaborative project during the past year include the completion of a comprehensive suite of 50-year (1950-1999) long experiments in which a variable-depth ocean mixed layer model is coupled to the atmosphere under different scenarios (6.3.1), improved understanding of the role of air-sea coupling in enhancing the persistence of extratropical atmospheric anomalies associated with ENSO (6.3.2), and identification of the impact of ENSO on tropical monsoon systems and transient activity (6.3.3, 6.3.4). Progress has also been made in summarizing the principal findings of this collaborative effort in a set of review articles to be published in a special issue of a journal.

PLANS FY01

          The drafting process of the set of review papers on various research foci of this collaborative project will be completed. The manuscripts will be edited for publication in a scientific journal.

          Efforts will be made to sustain this collaboration through identification of appropriate scientific problems and interested scientists, and consultation with funding agencies.


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