Paola Malanotte-Rizzoli

Data Assimilation in Meteorology and Oceanography

Publisher Summary This chapter provides a review of current operational practice and of advanced data assimilation techniques in meteorology. Numerical models can be used to assimilate meteorological and oceanographic data, creating a dynamically consistent, complete and accurate “movie” of the two geofluids, atmosphere, and ocean in motion. The oceans strong stratification helps determine the most energetic scales and processes for the global ocean circulation. Active research on data assimilation is burgeoning rapidly in both meteorology and oceanography. Operational NWP requirements have produced a mature data-assimilation technology in meteorology, from which climatic research has benefitted as well. Ocean is characterized by transient, energetic motions with a broad spectrum in frequency and wave number. A steady component of the circulation may not even exist, and be only a model resulting from the analysis of data sets sparse in space and time, like hydrographic data sets, for which steadiness is assumed a priori. Thus, in oceanic data-assimilation problems, the choice of a model and related data assimilation scheme and the definition of success of the assimilation process itself depend crucially on the scientific issue of interest as the starting point.

Model evaluation experiments in the North Atlantic Basin: simulations in nonlinear terrain-following coordinates

Abstract A primitive equation ocean circulation model in nonlinear terrain-following coordinates is applied to a decadal-length simulation of the circulation in the North Atlantic Ocean. In addition to the stretched sigma coordinate, novel features of the model include the utilization of a weakly dissipative, third-order scheme for tracer advection, and a conservative and constancy-preserving time-stepping algorithm. The objectives of the study are to assess the quality of the new terrain-following model in the limit of realistic basin-scale simulations, and to compare the results obtained with it against those of other North Atlantic models used in recent multi-model comparison studies. The new model is able to reproduce many features of both the wind-driven and thermohaline circulation, and to do so within error bounds comparable with prior model simulations (e.g., CME and DYNAMO). Quantitative comparison with comparable results obtained with the Miami Isopycnic Coordinate Model (MICOM) show our terrain-following solutions are of similar overall quality when viewed against known measures of merit including meridional overturning and heat flux, Florida Straits and Gulf Stream transport, seasonal cycling of temperature and salinity, and upper ocean currents and tracer fields in the eastern North Atlantic Basin. Sensitivity studies confirm that the nonlinear vertical coordinate contributes significantly to model fidelity, and that the global inventories and spatial structure of the tracer fields are affected in important ways by the choice of lateral advection scheme.

General Circulation of the Eastern Mediterranean

Abstract A novel description of the phenomenology of the Eastern Mediterranean is presented based upon a comprehensive pooled hydrographic data base collected during 1985–1987 and analyzed by cooperating scientists from several institutions and nations (the POEM project). Related dynamical process and modeling studies are also overviewed. The circulation and its variabilities consist of three predominant and interacting scales: basin scale, subbasin scale, and mesoscale. Highly resolved and unbiased maps of the basin wide circulation in the thermocline layer are presented which provide a new depiction of the main thermocline general circulation, composed of subbasin scale gyres interconnected by intense jets and meandering currents. Semipermanent features exist but important subbasin scale variabilities also occur on many time scales. Mesoscale variabilities modulate the subbasin scale and small mesoscale eddies populate the open sea, especially the south-eastern Levantine basin. Clear evidence indicates Levantine Intermediate Water (LIW) to be present over most of the Levantine Basin, implying that formation of LIW is not localized but rather is ubiquitous. The Ionian and Levantine basins are confirmed to form one deep thermohaline cell with deep water of Adriatic origin and to have a turnover time of one and a quarter centuries. Prognostic, inverse, box and data assimilative modeling results are presented based on both climatological and POEM data. The subbasin scale elements of the general circulation are stable and robust to the dynamical adjustment process. These findings bear importantly on a broad range of problems in ocean science and marine technology that depend upon knowledge of the general circulation and water mass structure, including biogeochemical fluxes, regional climate, coastal interactions, pollution and environmental management. Of global ocean scientific significance are the fundamental processes of water mass formations, transformations and dispersion which occur in the basin.

A synthesis of the Ionian Sea hydrography, circulation and water mass pathways during POEM-Phase I

Abstract In this paper we revisit, with a thorough in-depth analysis, the dataset collected in the hydrographic surveys of the international collaborative programme POEM (Physical Oceanography of the Eastern Mediterranean) in the period 1986–1987. The work has two major objectives. The first is to refine the dynamic picture of the Ionian upper thermocline sub-basin scale circulation, rather less definitive than the dynamic picture of the Levantine Sea circulation. The second is to identify the pathways of the major water masses of the basin not only in the near-surface, but also in the intermediate and deep layers. To our knowledge, this is the first work defining the pathways of the Levantine Intermediate Water (LIW) and of the Adriatic Deep Water (ADW)/Eastern Mediterranean Deep Water (EMDW) that characterize the intermediate and deep circulations. The major novel results can be summarized as follows. In the upper thermocline: (1) The Atlantic Ionian Stream (AIS) jet entering the Sicily Straits bifurcates into two main branches at 37°N, ∼ 17°E. It advects the Modified Atlantic Water (MAW) into the Ionian Sea interior. The first branch turning directly southward encloses an overall anticyclonic area comprising multiple centers around which the MAW is advected. (2) The second AIS branch extends further into the northeastern Ionian, where it too turns southward before crossing the entire Ionian Sea meridionally, advecting MAW on its left side and Ionian Surface Water (ISW) on its right. This branch of the jet is confined to the Ionian margin and does not pass around the Pelops gyre. (3) A new water mass, the LSW, is formed in the Levantine basin and enters the Cretan passage, then is first veered cyclonically south of Crete by the Cretan gyre and successively is entrained anticyclonically around the Pelops gyre, and then enters the Aegean Sea. (4) A permanent cyclone located in the northeastern Ionian determines the pathway of mixed Adriatic Surface Water/Ionian Surface Water (ASW/ISW). (5) A permanent cyclone is found in all the surveys near the tip of the Italian boot. This novel analysis of the LIW pathways shows that: (1) The major source of intermediate LIW during the period 1986–1987 was actually in the Levantine Sea. LIW formed there entered the Cretan passage, was veered cyclonically by the Cretan gyre south of Crete and then entered the southern Ionian Sea. The major LIW pathway was westward directly to the Sicilian Straits. (2) Secondary important LIW pathways were determined by the interior structures. The strong Pelops anticyclone was entraining LIW around its periphery and was determining the LIW northward pathway that closely followed the eastern Greek coastline. It was along this pathway that LIW entered the Otranto Strait. A further branch of LIW was entrained and recirculated around the multiple Ionian Anticyclones (IA) of the western Ionian Sea. (3) The Cretan cyclone is a feature confined to the upper thermocline-intermediate layer. It disappears at ∼ 400 dbar while the Pelops anticyclone is strongly barotropic below the upper 100 dbar and penetrates quite intense down to 800 dbar. (4) A further completely novel result concerns the new water mass found in the deep layer that spreads on the 29.15 kg/m3 isopycnal surface. This water mass, characterized by high salinity and high oxygen content, is formed inside the Aegean Sea and is observed to spread out all around the Cretan Arc Straits. The final fully novel result is the demonstration of a second pathway for the ADW exiting from the Otranto Strait that is transformed into EMDW and occupies the abyssal layers of the Ionian Sea interior. The traditional pathway for EMDW is along the isobaths along the western side of Italy but ADW was observed to be exiting from the Otranto Strait in the eastern Hellenic trench at 39.5°N, both during POEM-ON86 and POEM-AS87. This second pathway for EMDW follows isobath contours along the western side of Greece. The two EMDW routes converge and merge between 36°N and 35°N, so producing a deep layer of EMDW that occupies uniformly the abyssal plain of the interior of the Ionian Sea.

An approximate Kaiman filter for ocean data assimilation: An example with an idealized Gulf Stream model

A practical method of data assimilation for use with large, nonlinear, ocean general circulation models is explored. A Kaiman filter based on approximations of the state error covariance matrix is presented, employing a reduction of the effective model dimension, the errors asymptotic steady state limit, and a time-invariant linearization of the dynamic model for the error integration. The approximations lead to dramatic computational savings in applying estimation theory to large complex systems. We examine the utility of the approximate filter in assimilating different measurement types using a twin experiment of an idealized Gulf Stream. A nonlinear primitive equation model of an unstable east-west jet is studied with a state dimension exceeding 170,000 elements. Assimilation of various pseudomeasurements are examined, including velocity, density, and volume transport at localized arrays and realistic distributions of satellite altimetry and acoustic tomography observations. Results are compared in terms of their effects on the accuracies of the estimation. The approximate filter is shown to outperform an empirical nudging scheme used in a previous study. The examples demonstrate that useful approximate estimation errors can be computed in a practical manner for general circulation models.

Simulation of annual plankton productivity cycle in the Black Sea by a one‐dimensional physical‐biological model

The annual cycle of the plankton dynamics in the central Black Sea is studied by a one-dimensional vertically resolved physical-biological upper ocean model, coupled with the Mellor-Yamada level 2.5 turbulence closure scheme. The biological model involves interactions between the inorganic nitrogen (nitrate, ammonium), phytoplankton and herbivorous zooplankton biomasses, and detritus. Given a knowledge of physical forcing, the model simulates main observed seasonal and vertical characteristic features, in particular, formation of the cold intermediate water mass and yearly evolution of the upper layer stratification, the annual cycle of production with the fall and the spring blooms, and the subsurface phytoplankton maximum layer in summer, as well as realistic patterns of particulate organic carbon and nitrogen. The computed seasonal cycles of the chlorophyll and primary production distributions over the euphotic layer compare reasonably well with the data. Initiation of the spring bloom is shown to be critically dependent on the water column stability. It commences as soon as the convective mixing process weakens and before the seasonal stratification of surface waters begins to develop. It is followed by a weaker phytoplankton production at the time of establishment of the seasonal thermocline in April. While summer nutrient concentrations in the mixed layer are low enough to limit production, the layer between the thermocline and the base of the euphotic zone provides sufficient light and nutrient to support subsurface phytoplankton development. The autumn bloom takes place sometime between October and December depending on environmental conditions. In the case of weaker grazing pressure to control the growth rate, the autumn bloom shifts to December–January and emerges as the winter bloom, or, in some cases, is connected with the spring bloom to form one unified continuous bloom structure during the January–March period. These bloom structures are similar to the year-to-year variabilities present in the data.

Water mass pathways between the subtropical and tropical ocean in a climatological simulation of the North Atlantic ocean circulation

Abstract A primitive equation, hydrostatic, terrain-following coordinate ocean general circulation model (OGCM) is used to investigate the mean water mass pathways from the subtropics to the tropics in the Atlantic Ocean. The OGCM is used in a fully realistic configuration of the Atlantic, from 30°S to 65°N, with realistic bathymetry. Surface forcings are provided by the COADS climatology. A non-eddy-resolving numerical simulation is analyzed with 3/4° horizontal resolution and 20 terrain-following vertical levels. The primary objective of this study is to assess the theoretical framework extending the ventilated thermocline theory to the equator in the context of the numerical calculation, and to establish whether the predictions of a steady-state theory can be verified in a time-dependent simulation, in which rectified seasonal effects on the time mean yearly circulation may be important. The Bernoulli function is evaluated on isopycnal surfaces outcropping in the subtropics in both hemispheres and floats are injected at different northern and southern latitudes. In both hemispheres, the interior flow velocities are parallel to the Bernoulli streamlines that are significantly modified by inertia only very near the equator and on the Equatorial UnderCurrent (EUC). In the Northern Atlantic, pathways from the subtropics to the tropics exist for the isopycnal surfaces outcropping at 20–22°N. The injected floats reach the EUC following a zigzag pattern determined by the tropical current system. It is impossible to distinguish between the western boundary and the interior exchange windows as they are merged together forming a broad exchange pathway east of the northwestward flowing North Brazil Current (NBC). This exchange window disappears for the floats injected north of ∼30°N, and corresponding outcropping isopycnals σ θ >25.5 kg/m 3 , where only the recirculating window of the subtropical gyre remains. In the Southern Atlantic, all the floats injected between 6° and 15°S migrate to the western boundary where they are entrained in the NBC. There is no interior exchange window. At the equator, some are directly entrained into the EUC, some overshoot and retroflect at ∼8°N, then join the EUC. As the numerical simulation is carried out under surface forcings that include the seasonal cycle, we can assess the impact of the seasonal cycle on the steady-state analysis. The most important effect is due to the Atlantic Intertropical Convergence Zone (ITCZ), which in summer is strong, and produces an “island” of Ekman upwelling between 10° and 20°N, which is reflected in the yearly mean properties. The ICTZ-induced upwelling and interior stratification support a corresponding “island” of high potential vorticity that penetrates in depth to all the isopycnals outcropping between 20° and 25°N. This high potential vorticity island creates a barrier that constrains the floats injected at and north of 20°N to flow around it to reach the Equator and the EUC.

Modeling the response of top‐down control exerted by gelatinous carnivores on the Black Sea pelagic food web

Recent changes in structure and functioning of the interior Black Sea ecosystem are studied by a series of simulations using a one-dimensional, vertically resolved, coupled physical-biochemical model. The simulations are intended to provide a better understanding of how the pelagic food web structure responds to increasing grazing pressure by gelatinous carnivores (medusae Aurelia aurita and ctenophore Mnemiopsis leidyi) during the past 2 decades. The model is first shown to represent typical eutrophic ecosystem conditions of the late 1970s and early 1980s. This simulation reproduces reasonably well the observed planktonic food web structure at a particular location of the Black Sea for which a year-long data set is available from 1978. Additional simulations are performed to explore the role of the Mnemiopsis-dominated ecosystem in the late 1980s. They are also validated by extended observations from specific years. The results indicate that the population outbreaks of the gelatinous species, either Aurelia or Mnemiopsis, reduce mesozooplankton grazing and lead to increased phytoplankton blooms as observed throughout the 1980s and 1990s in the Black Sea. The peaks of phytoplankton, mesozooplankton, Noctiluca, and gelatinous predator biomass distributions march sequentially as a result of prey-predator interactions. The late winter diatom bloom and a subsequent increase in mesozooplankton stocks are robust features common to all simulations. The autotrophs and heterotrophs, however, have different responses during the rest of the year, depending on the nature of grazing pressure exerted by the gelatinous predators. In the presence of Mnemiopsis, phytoplankton have additional distinct and pronounced bloom episodes during the spring and summer seasons. These events appear with a 2 month time shift in the ecosystem prior to introduction of Mnemiopsis.

Tropical instability waves in the Atlantic Ocean

Abstract An idealized numerical model of the tropical Atlantic Ocean is used to study the structure, energetics and heat flux of the Atlantic tropical instability waves (TIWs). The model results compare well with the observations, and they both show that, unlike commonly assumed, the TIWs in the Atlantic exist on both sides of the equator and are generated not only in the summer but from May to January. Furthermore it is demonstrated that the Atlantic TIWs are generated by barotropic instability of the shear between the equatorial undercurrent and the northern south equatorial current and make a surprisingly small contribution to the heat budget of the equatorial mixed layer. The model results reveal that the often published strong meridional heat flux divergence of the TIWs is largely compensated for by their vertical heat flux divergence.

Modeling distinct vertical biogeochemical structure of the Black Sea: Dynamical coupling of the oxic, suboxic, and anoxic layers

A one-dimensional, vertically resolved, physical-biogeochemical model is used to provide a unified representation of the dynamically coupled oxic-suboxic-anoxic system for the interior Black Sea. The model relates the annual cycle of plankton production in the form of a series of successive phytoplankton, mesozooplankton, and higher consumer blooms to organic matter generation and to the remineralization-ammonification-nitrification-denitrification chain of the nitrogen cycle as well as to anaerobic sulfide oxidation in the suboxic-anoxic interface zone. The simulations indicate that oxygen consumption during remineralization and nitrification, together with a lack of ventilation of subsurface waters due to the presence of strong stratification, are the two main factors limiting aerobic biogeochemical activity to the upper ∼75 m of the water column, which approximately corresponds to the level of nitrate maximum. The position of the upper boundary and thus the thickness of the suboxic layer are controlled by upper layer biological processes. The quasi-permanent character of this layer and the stability of the suboxic-anoxic interface within the last several decades are maintained by a constant rate of nitrate supply from the nitrate maximum zone. Nitrate is consumed to oxidize sinking particulate organic matter as well as hydrogen sulfide and ammonium transported upward from deeper levels.