Abstract

A two-layer quasi-geostrophic channel model on a β-plane is used to investigate the properties of dipole eddies which may be relevant models for atmospheric blocking. It is shown that quasi-stationary equivalent barotropic dipole eddies, similar to the 1½-layer(reduced gravity) “modons” of Stern, can be resonantly excited in “realistic” westerly zonal wind conditions, including vertical shear, and that these eddies are persistent on lifetimes which are comparable with those of blocking.

These eddies cannot persist indefinitely due to interactions with stationary Rossby wave modes. The decay of these eddies into Rossby waves is studied under varying zonal flow conditions and it is found that the decay rate increases as the group velocity of the stationary Rossby waves increase. The decay rate is also found to be sensitive to the closeness of the channel walls which can restrict the development of the radiation field. Meanwhile the loss of fluid (and hence potential vorticity) from the vortices is achieved in narrow tongues which emerge from the stagnation point on the downstream side of the dipole. The decay due to Rossby-wave radiation does not appear to alter the propagation speed of the dipoles with respect to the zonal flow allowing them to remain as quasi-stationary anomalies despite large changes in circulation amplitude. This decay mechanism is briefly compared with spindown induced by Ekman friction and it is concluded that Rossby wave radiation is probably the more efficient process as it affects all vertical levels simultaneously.

Abstract

There is as yet no consensus an the best method for inserting surface data into ocean models so that appropriate information is transmitted to the deeper layers in a rapid and efficient manner. First we consider the correlation of sea surface pressure (which is related to sea surface height as measured by altimeters) and the surface and subsurface currents within the framework or a four-layer quasi-geostrophic model. We begin by suggesting that the homogenization of potential vorticity, q, in the unforced layers discussed by Rhines and Young, ensures that the potential vorticity anomalies in the deep ocean are weak. It follows that instantaneous q information from thew deep layers may not need to be accurately known in order to reconstruct the deep eddy currents, i.e., climatological q information may be sufficient. Taking the fields from the model we use the instantaneous surface streamfunction, ψ₁, data along with various approximate subsurface q climatologies and attempt to reconstruct the lower layer flow fields, including the eddies, to the best of our ability. We judge the success of the results both visually and by using the global measure of rms errors. This process is remarkably successful showing that much of the information on the deep eddy currants is contained in the surface ψ₁, field. We also try to use instantaneous surface q ₁ information to reconstruct the deep flow but with much less success. This is because the barotropic mode streamfunction ψ _B is not well constrained by q ₁ information alone whereas ψ₁ information does constrain ψ _B to some extent.

A new method of directly inserting data within the time integration scheme of the model is then suggested in which the q fields in the subsurface model layers are left unchanged by the assimilation procedure, and the method is tested with a twin experiment (i.e., a control ocean is defined by the same model at a different time in its evolution). In the assimilation run the top layer q ₁ field is changed so as to make the consistent ψ₁ field coincide with the control ocean values. The appropriate q ₁ is not the same as the control ocean q ₁ as it must compensate for the incorrect q fields below. The assimilation run is found to converge rapidly to the control ocean even with total surface data coverage every 40 days. The q fields in the deeper layers (particularly at the bottom) converge as the model evolves in between the data assimilation times. The method works very well because of the q homogenization in the unforced model layers; however, we argue that it would also be suitable under the less stringent constraint that the eddy q fields be uncorrelated in the vertical. Finally we discuss the two commonly used methods of nudging and direct insertion in the light of these results and consider whether this new method can be extended into a primitive equation framework.

Abstract

Weather regimes have been sought by examining the 500-mb streamfunction of the UGAMP GCM run for 10 yr at T42 resolution with perpetual January forcing. Five-day low-pass EOFs provide a low-order phase space in which to study dynamical aspects of the variability. The PNA pattern shows up as the first EOF over the Northern Hemisphere representing 12% of the variance, rising to 18.5% for Pacific-area-only EOFs.

Within the phase space of three to five EOFs, two local minima of the area-averaged ψ tendency (based on rotational velocity advection) are found. These two flow patterns both have a smaller implied tendency than the climatology and lie in the ±PNA regions of the phase space. It is suggested that these patterns may be acting as “fixed points” within the atmospheric attractor, encouraging persistent flows and the formation of weather regimes. These dynamical attracting points are compared with a more conventional means of identifying weather regimes using a statistical maximum likelihood analysis of all model states during the 10-yr GCM run. This analysis also indicates two preferred classes, separate from the climatology, in the ±PNA regions of phase space. These classes tend to be nearer the climatology than the dynamical states but have similar appearance otherwise.

Finally the role of low-frequency transients are examined to improve the dynamical interpretation of the regime centers. The method is first demonstrated for the extended Lorenz model of Molteni et al. The fixed points of the GCM attractor are assumed to be steady solutions to the 500-mb vorticity equation in the absence of contributions from transient eddies. The eddy contributions to the climatological vorticity budget are first determined, and then the deviations from the climatology that could provide similar contributions to the budget are found. Again two states in the ±PNA regions of phase space are found to satisfy the above conditions. The authors speculate that the attractors themselves are determined by the large-scale steady effects of topography and land-sea contrasts.

Abstract

A weakly nonlinear theory is presented that may explain the persistence of the two main types of low-frequency anomalies observed in the midlatitude jet stream by Dole and Gordon (1983). The theory describes how nonlinearity can balance dispersion effects for both split jet stream anomalies (which resemble blocking flows) and for jet intensification anomalies. It is shown that the variation of the potential or refractive-index function ≡ dq/dψ across the jet stream is crucial for determining which types of anomaly will tend to persist. Although the theory is only weakly nonlinear it is argued that the same dynamical mechanisms will remain important in the high-amplitude regime particularly for the intense-jet anomalies. In the split anomalies the potential vorticity contours can easily become closed at high amplitude hence trapping air parcels (this is the origin of the strongly nonlinear modon solutions). However, even for very strong intense-jet anomalies the potential vorticity contours may remain open and then no air trapping occurs, thus, the variations in the cross jet stream potential function remain important. Initial value numerical experiments are presented to demonstrate that both types of anomaly are close to persistent states of the full barotropic vorticity equation, even at amplitudes that are beyond the strict range of validity of the weakly nonlinear theory. Some discussion and investigation of the possible role of critical lines in preventing dispersion into equatorial latitudes is also presented. Finally, the possibility of testing this theory by making appropriate diagnostic measurements is considered.

Abstract

Theories which associate atmospheric blocking with isolated “free mode” solutions of the equations of motion are reviewed and the central role played by the potential function Λ ≡ dq/dψ (where q is the is the quasi-geostrophic potential vorticity and ψ is the streamfunction) is emphasized. This function provides the common dynamical link that draws together the weakly nonlinear (soliton) and fully nonlinear (modon) theories of isolated coherent structures.

A diagnostic study of the European blocking episode during October 1987 is presented and the relationship between q and ψ investigated by plotting scatter diagrams of quasi-geostrophic potential vorticity against the streamfunction on an isobaric surface. An approximate functional relationship is found allowing Λ to be defined. Over the blocking region, points on the scatter plot cluster around a straight line which is more steeply sloping than the straight line defined by points from nonblocking regions, demonstrating that the block exhibits a local minimum in Λ. Such a signature is characteristic of local fully nonlinear free mode structures, the prototype of which has been termed the “equivalent barottopic modon.” The data strongly suggest that blocking episodes can exhibit local free-mode dynamics and that their persistence may in part be attributed to the robustness and stationary nature of these local coherent structures.

Abstract

A diagnostic study of persistent intense jet stream events in the Pacific has been carried out with a 15 winter NMC dataset to assess the relevance of the weakly nonlinear model recently proposed by Haines and Malanotte-Rizzoli. Composited data from 14 episodes of persistent intensification anomalies in the central and eastern Pacific have been analyzed with scatter diagrams of potential vorticity q plotted against geopotential Φ on the 300-mb surface. The slope of the functional relationship gives a measure of the wavelength independent component of the refractive index (n ² = −Λ₀ = −dq ₀/dψ ₀). The theoretical model suggests that if dq ₀/dψ ₀ is more negative on the northern and southern flanks, a local intense region within the jet stream may be abnormally persistent. The composited dataset shows that this condition is satisfied during the postonset period as defined by Dole. In contrast, the climatology and the mean flow before onset does not show much variation in Λ₀ across the jet. Results are encouraging, but higher-resolution data is needed to draw firm conclusions.

Abstract

Idealized twin experiments with the HOPE ocean model have been used to study the ability of sea level data assimilation to correct for errors in a model simulation of the tropical Pacific, using the Cooper and Haines method to project the surface height increments below the surface. This work should be seen in the context of the development of the comprehensive real-time ocean analysis system used at ECMWF for seasonal forecasting, which currently assimilates only thermal data.

Errors in the model simulation from two sources are studied: those present in the initial state and those generated by errors in the surface forcing during the simulation. In the former, the assimilation of sea level data improves the convergence of the model toward its twin. Without assimilation convergence occurs more slowly on the equator, compared to an experiment using only correct surface forcing. With forcing errors present the sea level assimilation still significantly reduces the errors almost everywhere. An exception was in the central equatorial Pacific where assimilation of sea level did not correct the errors. This is mainly due to this region responding rapidly to errors in wind stress forcing and also to relatively large freshwater flux errors imposed here. These lead to errors in the mixed layer salinity, which the Cooper and Haines scheme is not designed to correct. It is argued that surface salinity analyses would strongly complement sea level assimilation here.

Abstract

The surface mixed layer (ML) governs atmosphere–ocean fluxes, and thereby affects Earth’s climate. Accurate representation of ML processes in ocean models remains a challenge, however. The O(100) m deep ML exhibits substantial horizontal thermohaline gradients, despite being near-homogenous vertically, making it an ideal location for processes that result from the nonlinearity of the equation of state, such as cabbeling and thermobaricity. Traditional approaches to investigate these processes focus on their roles in interior water-mass transformation and are ill suited to examine their influence on the ML. However, given the climatic significance of the ML, quantifying the extent to which cabbeling and thermobaricity influence the ML density field offers insight into improving ML representations in ocean models. A recent simplified equation of state of seawater allows the local effects of cabbeling and thermobaric processes in the ML to be expressed analytically as functions of the local temperature gradient and ML depth. These simplified expressions are used to estimate the extent to which cabbeling and thermobaricity contribute to local ML density differences. These estimates compare well with values calculated directly using the complete nonlinear equation of state. Cabbeling and thermobaricity predominantly influence the ML density field poleward of 30°. Mixed layer thermobaricity is basin-scale and winter intensified, while ML cabbeling is perennial and localized to intense, zonally coherent regions associated with strong temperature fronts, such as the Antarctic Circumpolar Current and the Kuroshio and Gulf Stream Extensions. For latitudes between 40° and 50° in both hemispheres, the zonally averaged effects of ML cabbeling and ML thermobaricity can contribute on the order of 10% of the local ML density difference.

Abstract

Assimilation of salinity into ocean and climate general circulation models is a very important problem. Argo data now provide far more salinity observations than ever before. In addition, a good analysis of salinity over time in ocean reanalyses can give important results for understanding climate change. Here it is shown from the historical ocean database that over large regions of the globe (mainly midlatitudes and lower latitudes) variance of salinity on an isotherm S(T) is often less than variance measured at a particular depth S(z). It is also shown that the dominant temporal variations in S(T) occur more slowly than variations in S(z), based on power spectra from the Bermuda time series. From ocean models it is shown that the horizontal spatial covariance of S(T) often has larger scales than S(z). These observations suggest an assimilation method based on analyzing S(T). An algorithm for assimilating salinity data on isotherms is then presented, and it is shown how this algorithm produces orthogonal salinity increments to those produced during the assimilation of temperature profiles. It is argued that the larger space and time scales can be used for the S(T) assimilation, leading to better use of scarce salinity observations. Results of applying the salinity assimilation algorithm to a single analysis time within the ECMWF seasonal forecasting ocean model are also shown. The separate salinity increments coming from temperature and salinity data are identified, and the independence of these increments is demonstrated. Results of an ocean reanalysis with this method will appear in a future paper.

Abstract

In this paper, the combined assimilation of satellite observed sea level anomalies and in situ temperature data into a global ocean model, which is used to initialize a coupled ocean–atmosphere forecast system, is described. The altimeter data are first used to create synthetic temperature observations, which are then combined with the directly observed temperature profiles in an optimum interpolation scheme. In addition to temperature, salinity is corrected based on a preservation of the model's local temperature–salinity relationship. Coupled forecasts with a lead time of up to 6 months are initialized from the ocean analyses and the impact of the data assimilation on both the ocean analysis and the coupled forecasts is investigated. It is shown that forecasts of sea surface temperature anomalies in the Niño-3 area can be improved by initializing the coupled forecast model with the ocean analysis in which temperature and altimeter data are assimilated in combination. The results further imply that a good simulation of the salinity field is required to make optimum use of the altimeter data.