Ocean transport processes

Surface drifters and subsurface floats are commonly used to study transport properties in the mesoscale range.

Analysis on Lagrangian data were performed and new techniques were developped in order to classify lagrangian trajectories in different classes characteristics of different dynamical regimes and charcatreized by different dispersion properties.

Figure. Example of different classes of trajectories. Classes are obtained using the ratio between the acceleration and velocity time scale

Such analysis on global data set leads to a dynamical chracterization of the main current system and may be useful to develop new diffusivity parameterization to be inserted in OGCM

Figure. Map of the ratio between the acceleration and the velocity lagrangian time scales obtained from surface drifters (left) and subsurface floats (right). A ratio near to 1 is indicative of high turbulent activity.

References

• Rupolo, V., B. L. Hua, A. Provenzale, and V. Artale, 1996: Lagrangian spectra at 700 m in the western North Atlantic J. of Phys. Oceanogr ,26,1591-1
• Rupolo, V., A. Babiano, V. Artale, and D. Iudicone, 2003: Sensitivity of the Mediterranean circulation to horizontal space-time-dependent tracer diffusivity field in a OGCM. Nuovo Cimento,26 C 4 ,387-415
• Rupolo, V.,,2007: 'Observing Turbulence Regimes and Lagrangian Dispersal Properties in the Oceans'. in Lagrangian Analysis and Prediction of Coastal and Ocean Dynamics. Ed. by A. Griffa, A.D. Kirwan, A.J. Mariano, T. Ozgokmen and T. Rossby, Cambridge University Press.
• Rupolo, V. 2007: A Lagrangian based approach for determining trajectories Taxonomy and Turbulence regimes. J. of Phys. Oceanogr. Vol. 37 1584-1609
• Rupolo V., T. Dubos ,A. Babiano, 2009: ' On the sensitivity of OGCM to the parameterization of diffusive processes'. Environmental Fluid Dynamics ED CIMNE Barcelona 2003.A. Babiano. P Fraunie, J.M. Redondo andC. Vasilicos (eds).

Tropospheric high and low frequency variability

Our research wishes to propose a description of the statistical properties of mid- latitude atmospheric waves for observational data-sets and state-of-the-arts climate models simulations in order to construct process-oriented metrics pointing at the diagnostics of specific physical processes in climate models .

Therefore, a non-linear theory, accounting for a relevant part of the mid-latitude variability, has been revised in terms of its signature in observational and model data. Theoretical and observational arguments suggest that two main features of mid-latitude northern hemispheric winter variability can be somewhat unambiguously separated, both in terms of signal and physical processes.

The synoptic phenomena, which are travelling waves characterized by time scales of the order of 2-7 days and by spatial scales of the order of few thousands Km., can be associated with release of available energy driven by conventional baroclinic conversion. At lower frequencies (10-40 days), the planetary scale variability is related to the non linear orographic resonance processes .

Collaboration with V. Lucarini

Figure. Climatological average over 45 winters of Hayashi spectra for 500 hPa geopotential height
(relative to the latitudinal belt 33°N-68°N) from NCEP data

Global and regional water cycle

Long-term changes in Mediterranean water cycle

An observational analysis of Mediterranean Sea water cycle variability based on recently available datasets provides new insights on the long-term changes which affected the region since the 1960s.

Results indicate an overall increase in evaporation during 1958-2006, with a decrease up until the mid-1970s and an increase from thereon. Precipitation variability is characterized by substantial interdecadal variations and a negative long-term trend. Evaporation increase, primarily driven by SST variability, together with precipitation decrease resulted in a substantial increase in the loss of fresh water from the Mediterranean Sea toward the overlying atmosphere. An increase in fresh water deficit is consistent with observed Mediterranean Sea salinity tendencies and has broad implications for the Mediterranean water cycle and connected systems.

These observational results are in qualitative agreement with simulated Mediterranean Sea water cycle behavior from a large ensemble of CMIP3 models. However, simulated anomalies are about one order of magnitude smaller than observed. This inconsistency and the large uncertainties associated with the observational rates of change highlight the need for more research to better characterize and understand Mediterranean water cycle variations in recent decades, and better simulate crucial underlying processes in global models.

Figure. Decadal variations in Mediterranean Sea mean evaporation (E) minus precipitation (P) over the period 1958-2007. Shown are 6-yrs running means of Mediterranean Sea area-averaged evaporation anomalies relative to the period 1988-2000 (lines, mm/d). Various observational sources are used (see legend; left hand scale). Annual mean values based on OAFlux and GPCP data are also represented (symbols). CMIP3 models ensemble running mean averages are also displayed (note different scale at right). Adapted from Mariotti A, 2010: Recent changes in Mediterranean water cycle: a pathway toward long-term regional hydroclimatic change? J of Climate, Vol. 23, No. 6.: 1513-1525.