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--> 8 Empirical and Diagnostic Studies Empirical studies and diagnostic analyses are essential elements of GOALS. Empirical studies describe phenomenological relationships within and among fields of different variables, both temporally and spatially. Diagnostic studies include quantitative analyses of processes and budgets and extend beyond empirical relationships. Together they lead towards an ability to describe and understand what is occurring in nature and how such knowledge can be used to improve models. Empirical studies proposed under GOALS should encompass a broad spectrum of activities ranging from investigations that search for empirical relationships with predictive value to investigations intended to produce a better understanding of processes leading to improved parameterizations for prediction models. In keeping with the overall objectives of GOALS, priority is accorded to diagnostics and systems studies pertaining to relationships and processes that influence the seasonal-to-interannual predictability of climate variations, first over North America and then globally. The role of diagnostic and empirical studies in the composite structure of GOALS is shown in Figure 4-1. The basic strategy for GOALS calls for an extension of the TOGA emphasis on the Pacific to the global tropics in order to encompass the major heat sources and sinks associated with the tropical oceans and major land masses. This expanded scope should include the interaction between, for example, the monsoons and ENSO, and between the tropics and the mid-latitudes. The probable interac-
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--> tion of phenomena on a wide range of space and time scales will require very careful empirical and diagnostic analyses. Structures and Interrelationships It is possible that hitherto unknown relationships between the major components of the climate system will add to current estimates of its predictability. Such relationships could suggest physical mechanisms that influence the system and indicate possible sources of predictability that can be exploited. Possible candidates are the interaction between Intraseasonal Oscillations and ENSO, and between ENSO and the Tropical Biennial Oscillation, and the Quasi-Biennial Oscillation. Diagnostics Diagnostic studies are excellent tools for posing and testing hypotheses as well as providing basic information about budgets, fluxes, and other parameters not measured directly. The climate system is so complex and interactive that a wide range of diagnostic studies will be needed to investigate and test hypotheses on the underpinning physical processes governing various interrelationships. Examples of other specific products obtainable from diagnostic studies include estimates of budgets of atmospheric mass and moisture and of atmospheric and oceanic heat; fresh water and momentum fluxes and budgets; and the movement and evolution of ocean heat content, and the surface heat balance. The new ocean data collected under GOALS, will enable the determination of the role of planetary-scale ocean waves (including the ''delayed oscillator'' mechanism) in seasonal-to-interannual climate variability. Forcings—Response And Feedbacks Central to the understanding of seasonal-to-interannual climate variations is the response of the atmosphere and oceans to external forcings. Also important are linkages between local and remote locations and the impact of various feedback mechanisms and processes between components of the ocean—atmosphere—land system. Several aspects of forcings and feedbacks are proposed below for further study. The spatial and temporal distribution of atmospheric forcing should be determined, with the aim of finding which aspects are most important for seasonal-to-interannual variability and why such forcings change. Among the most easily observable symptoms of a forcing change are changes in the convection zones in the tropics. These changes provide two of the most important feedbacks in the atmospheric system—latent heating and cloud—radiative interaction—and
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--> respond to changes in boundary conditions both locally and remotely. Perhaps the most important hypothesis to be tested is that changes in convection zones provide the major pathway connecting seasonal-to-interannual atmospheric variability with forcing by the more slowly evolving parts of the climate system. Convection anomalies are of interest both because of the importance of precipitation to human activities and because of teleconnections to other parts of the atmosphere. Thus, identification of mechanisms by which boundary condition forcing produces such changes in convection zones is a priority. The response of the atmosphere and oceans to local and remote atmospheric forcings, and remote responses in the oceans arising from atmospheric teleconnections, should be assessed empirically. Many questions concerning wave propagation, interaction of quasi-stationary waves with transients, and so forth, can be addressed empirically as well as with modeling studies. These questions are important because they lead to a physical understanding of the way predictable features of the tropics are projected to other regions. The impacts of local and remote forcing on the hydrological cycle (especially over land) should be determined, and feedbacks to the atmosphere through changes in ground hydrology and other land surface processes should be estimated. The role of snow cover and soil moisture over land in inducing anomalies in large-scale circulations should likewise be determined. It is important to assess the extent to which land surface anomalies are stochastic or part of long-term fluctuations. The importance of other feedbacks on the coupled system, such as changes in storm tracks, interactions with sea ice, and effects of extratropical SST anomalies, should be assessed. Predictability A primary focus of GOALS is the investigation of predictability with empirical and diagnostic studies. To this end it is recommended that analogues be used to determine the rate of separation of initially similar states of the coupled system. But because of the limited time span of historical data record, innovative approaches are needed to address the long time scales of interest in GOALS. Empirical searches should also be made for regular behavior; very-low-frequency phenomena; and spatial, temporal, or mechanistic links between elements of the coupled system, in order to identify elements or features that may lead to improvements in prediction skill.
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