The origin and maintenance of these phenomena and their associations must be the subject of considerable future investigations. What are the mechanisms of interaction between the atmosphere and land surface processes on dec-cen timescales? Through what mechanisms does the planetary boundary layer mediate between dec-cen variability of the surface boundary layer and the free atmosphere?
One particular issue is how to average over short time- and space scales to study dec-cen processes, in particular boundary layer and interface processes. What are the mechanisms of region-to-region and basin-to-basin interactions on the dec-cen timescale? How do dec-cen changes in atmospheric trace gases and aerosols affect radiative balance and atmospheric circulation, and vice versa? The study of decadal to centennial variability of atmospheric circulation faces many challenges.
Much of our current understanding of the issue derives from the intense interest in anthropogenic climate change. These efforts should proceed side by side with establishing clear guidelines for future atmospheric observations and careful planning of the observational networks so that adequacy, continuity, and homogeneity of the records are assured as discussed earlier.
The observational efforts should focus on describing both state variables winds, pressure, temperature, humidity, and rainfall and forcing and other related variables solar radiation, clouds, aerosols, and chemical composition. Models of the climate system are powerful tools for the study of climate. Such models must be developed to allow the simulation of ocean, atmosphere, cryosphere, and changes in continental surface conditions.
Representation of the processes controlling the evolution of all of these important components must be. The ocean influences the climate system through surface exchange, storage, and redistribution of heat, fresh water, and carbon dioxide. Because of its large mass and heat capacity, the relatively slow moving ocean is responsible for approximately half of the global equator-to-pole meridional heat transport. Regarding climate variability, the ocean's influence becomes increasingly important as the timescales of variability increase. At seasonal to interannual timescales, the ocean influences climate primarily through its large heat capacity in the relatively thin surface layer, whereas on longer timescales the heat transport over basin and global scales predominates.
A key issue is the oceans' role in the longevity and long-term variability of climate patterns. Oceans are intimately tied to these patterns through SST and sea surface salinity fields. These fields typically covary with atmospheric surface layer pressure SLP fields, and an analysis of the nature of the covariation in the North Atlantic suggests a migration of SST anomalies along the primary circulation pathways of the North Atlantic.
This finding suggests that upper-ocean heat content likely plays a main role in the survival of surface anomalies and their migration from year to year. Unfortunately, climate patterns and their relationships with upper-ocean property fields are often difficult to evaluate because the empirical orthogonal function methodology most frequently applied for analysis tends to emphasize stationary patterns, precluding the identification of spatial migrations and covariability.
This situation highlights the methodological inadequacies and thus the need to apply more complex tools e. With the appropriate analysis tools, the first tasks are to identify clearly the oceanic and air-sea coupled signatures of these patterns, including their spatial and temporal linkages, and to explore data-poor regions to identify new patterns and better define existing ones. The relationships among ocean property fields and climate patterns must be more thoroughly documented and understood as well.
Specifically, we must improve our understanding of how the ocean regulates, maintains, and otherwise influences these patterns and their evolution, particularly through its couplings with the atmosphere, sea ice, and land as well as through internal ocean mechanisms. In addition to these general issues, a number of ocean-specific issues must be addressed.
They concern the internal ocean processes that influence SST, the principal property coupling the ocean to the atmosphere and climate. Such mechanisms also control the storage and redistribution of heat, fresh water, and atmo-. Specifically, we require improved understanding and parameterizations of diapycnal mixing mixing across density surfaces , surface layer processes, interbasin exchanges including marginal seas, throughflows, and overflows , subduction and ventilation processes, and mesoscale processes to answer fundamental questions.
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How does subducted water and anomalies in subducted water mix and evolve as it flows around the subtropical gyre, and define the vertical density and circulation structure of this gyre? Can we quantify transport pathways and mixing from the time the water is subducted into the subtropical gyre to when it is re-exposed to the atmosphere at the equator? Dec-cen ocean issues involve defining patterns and mechanisms of the participation of the ocean in climate change.
It is the formation and circulation of water masses that link surface forcings to the subsurface ocean. The variabilities of those water masses and forcings can alter subsurface ocean properties and circulation, and those subsurface changes can cause SST changes, locally or remotely, through advection, which feed back to alter the atmosphere. What are the dec-cen patterns of ocean variability and what dynamical mechanisms govern them at dec-cen timescales?
The rich literature on organized patterns of atmospheric variability is not paralleled in ocean research. Correlations of SST and associated forcing fields with these atmospheric patterns have been partly explored, but recent efforts have revealed a propagation of SST anomalies indicative of transseasonal memory of winter conditions, in particular heat content anomalies and movement of the stored anomalies by advection. Much remains to be done in documenting these anomalies and their relationships to the subsurface property and circulation changes.
Regional differences need to be explored.
For example, the atmospheric fields associated with NAO and PNA NPO seem rather similar, but the participation of the oceans beneath their action centers may differ because of the presence of deep overturning in the North Atlantic. What are the processes of formation and sequestering of water masses and of their subsequent modification and eventual return to the surface?
What are their dec-cen variabilities? How do anomalies of heat, fresh water, and chemical constituents translate into mixed-layer anomalies?
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How do the mixed-layer anomalies get into the ocean interior? How are they modified as they circulate through the interior and how are water masses re-entrained back to the mixed layer? How do freshwater fluxes evaporation minus precipitation, sea ice, and runoff anomalies modulate these processes through creation of salinity anomalies? Progress in. What are the dec-cen fluctuations of circulation structure and intensity and water mass pathways?
How are they affected by surface forcing? What are the mechanisms of the fluctuations?
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What are the relative roles, including the interaction of wind, thermal, and haline forcing? What are the surface expressions of these fluctuations? A number of processes are thought to modulate the intensity of meridional heat transport, as effected by gyre circulations and western boundary currents. These processes include eddy-driven subbasin- and basin-scale recirculations and the remote influence of wind stress and buoyancy anomalies via Rossby and coastal waves.
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What are the relative roles of these processes in dec-cen variability of heat transport and SST? What is the role of salinity advection feedback to surface freshwater anomalies when heat transport and SST anomalies exist? What feedback and coupling mechanisms maintain SST, heat, fresh water, sea ice, and chemical anomalies on dec-cen timescales? How do anomalies survive the seasonal cycle to reappear in subsequent winters, in particular, providing the observed long-lived recurrent winter SST anomalies?
How do the histories of water masses evolve at higher latitudes, where sequestering is only seasonal, so that in winter there are recurrent advected heat content anomalies manifested as SST anomalies propagating downstream through the warm-to-cold water transformation pathways? What mechanisms control the strength, heave, and wobble of gyres on dec-cen timescales?
What are the mechanisms of region-to-region and basin-to-basin interaction on dec-cen timescales? What mechanisms control the magnitude and other characteristics of waters exchanged among the oceanic circulation gyres and thus the amplitude and property fluxes of the full ocean overturning circulation?
Can the variability of this overturning circulation be measured? Decadal change evidence focuses mainly on isolated gyre or paired gyre-gyre phenomena—do more global patterns and interactions occur on interdecadal to century timescales? Are there unique patterns for Southern Ocean participation in dec-cen climate variability, reflecting the circumpolar flow and linkages provided by the Antarctic Circumpolar Current, including the effects on adjacent basins' subtropical gyres? What are the processes connecting tropical SST and the extratropical Pacific?
What oceanic processes modify ENSO events on deccen timescales? How is carbon partitioned in the ocean and what are the roles of physical processes in the carbon flux? What are the major processes controlling the partitioning of carbon among ocean reservoirs and between the ocean. Can these fluxes be quantified?