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[1] We investigate the interpretation of tree-ring data using the Vaganov-Shashkin forward model of tree-ring formation. This model is derived from principles of conifer wood growth, and explicitly incorporates a nonlinear daily timescale model of the multivariate environmental controls on tree-ring growth. The model results are shown to be robust with respect to primary moisture and temperature parameter choices. When applied to the simulation of tree-ring widths from North America and Russia from the Mann et al. (1998) and Vaganov et al. (2006) data sets, the forward model produces skill on annual and decadal timescales which is about the same as that achieved using classical dendrochronological statistical modeling techniques. The forward model achieves this without site-by-site tuning as is performed in statistical modeling. The results support the interpretation of this broad-scale network of tree-ring width chronologies primarily as climate proxies for use in statistical paleoclimatic field reconstructions, and point to further applications in climate science.

[1] A dynamical mechanism is described that modulates the tilt of the sea-surface height and pycnocline depth between the central Arctic and the continental shelves. A simple analytical model is presented, forced with idealized zonal winds over an idealized, 2-layer, cylinder representing the Arctic Ocean. Ekman transports are linked to sea-surface and pycnocline tilt anomalies and basin-scale circulation in response to an annular wind anomaly. We compare the results to tide gauge data, as well as results from a more realistic numerical simulation and find that the model explains a major fraction of the interannual-to-decade scale sea-surface height anomalies at Arctic coastal tide gauges. The analytical model indicates, for example, that on the order of 10 cm of the observed rise of about 18 cm in coastal Arctic sea-surface height between about 1985 and 1993 was probably a response to increased Westerly winds associated with a strong positive phase of the Northern Annular Mode of atmospheric variability. The pycnocline depth anomaly time series from the model is used to calculate implied changes in the outflow of relatively fresh Polar Water to the North Atlantic. The comparisons indicate that the Ekman transport mechanism is important to changes in the export of buoyancy from the Arctic Ocean on seasonal, interannual, and decadal timescales.

[1] Monthly interannual anomalies of tropical Pacific sea level height from TOPEX/ Poseidon altimetry are compared with simulation and assimilation products from a variety of models, ranging from a simple linear long wave approximation to ocean general circulation models. Major spatial similarities in the error patterns are identified. These include zonally elongated maxima in the northwest and southwest tropical Pacific Ocean, a band of high values near 10°N, slightly inclined toward the equator from the Central American coast, and low values on the equator and in the southeastern tropical Pacific. These features are also present in the pattern of small-scale variability (SSV) of sea level height. Spatial and temporal components of this SSV are analyzed for predominant variability types. Monte Carlo experiments identify the areas where high SSV is wind-driven, caused by a similar pattern of variability in the wind stress. Model products systematically underestimate signal variance in such areas. Variability in other areas is due to the instability of ocean currents. The major component of uncertainty in the gridded satellite altimeter analyses is due to sampling error, for which estimates are developed and verified.

[1] Warren's [1983] “Why is no deep water formed in the North Pacific” is revisited. His box model of the northern North Pacific is used with updated estimates of oceanic volume transports and boundary freshwater fluxes derived from the most recent data sets, using diverse methods. Estimates of the reliability of the result and its sensitivity to error in the data are given, which show that the uncertainty is dominated by the large observational error in the freshwater fluxes, especially the precipitation rate. Consistent with Warren's conclusions, it is found that the subpolar Atlantic-Pacific salinity contrast is primarily explained by the small circulation exchange between the subpolar and subtropical gyres, and by the local excess of precipitation over evaporation in the northern North Pacific. However, unlike Warren, we attribute the latter excess to atmospheric water vapor transports, in particular the northern moisture flux associated with the Asian Monsoon. Thus the absence of such a large transport over the subpolar North Atlantic may partly explain why it is so salty, and why deep water can form there and not in the North Pacific.

Data assimilation via the extended Kalman filter can become problematic when the assimilating model is strongly nonlinear, primarily in connection with sharp, "switchlike" changes between different regimes of the system. The filter seems too inert to follow those switches quickly enough, a fact that can lead to a complete failure when the switches occur often enough. In this paper we replace the key feature of the filter, the use of local linearity for the error model update, with a principle that uses a more global approach through the utilization of a set of preselected regimes. The method uses all regime error models simultaneously. Being mutually incompatible, a compromise between the different error models is found through the use of a weighting function that reflects the 'closeness' of the error model to the correct model. To test the interactive Kalman filter a series of numerical experiments is performed using the double-well system and the well-known Lorenz system, and the results are compared to the extended Kalrnan filter. It turns out that, depending on the set of preselected regimes, the performance is worse than, comparable to, or better than that of the extended Kalman filter.

A linear box model of the Red Sea is presented. The model, which inverts hydrographic and 3He data for the main features of the width-integrated circulation, is able to describe a large part of the seasonal and annual mean circulation. By combining and reconciling major elements of current theories of the Red Sea circulation, available flow measurements, and tracer data the model provides a unified conceptual framework for the understanding of the circulation of the Red Sea. The model forms deep water in the extreme north in two modes, as first suggested by Cember (1988). The convective mode renews the bottom water at an annual mean rate of ∼0.04 Sv. In the winter the isopycnal mode water renews the uppermost part of the deep water mass. It flows to the south along the bottom of the pycnocline at an annual mean rate of 0.02–0.04 Sv. A new feature in the model's near-surface circulation scheme is the ventilation of the thermocline in the winter at a mean rate of ∼0.04 Sv, also from the extreme north. There is also a middepth return flow to the north at an annual mean rate of ∼0.07 Sv. This flow has been suggested but never rigorously quantified before.

The western tropical Pacific is thought to be an important zone for generating El Nino: reflections at the boundary make it a source region of equatorial Kelvin waves. Calculations of the effect of a gappy western boundary on the reflection process are carried out in the framework of the low-frequency limit of the shallow-water equations and thus are highly idealized. The method is also applied to a schematic version of the flow through the Indonesian seas from the western Pacific to the Indian Oceans. The results indicate some strong sensitivities to the location of the gap and to the structure of the incoming flows. In addition, the results can be quite different, depending on whether the zonal extend of the gap is assumed to be infinite or finite. (More precisely, the latter means that the extend of the gap is short compared with the zonal wavelength of the relevant free waves at that frequency.) In view of the complexity of the results for even such a simplified model, it will be very difficult to be confident of any modeling study of the Indonesian throughflow short of a highly resolved numerical calculation with a detailed representation of the geometry and bathymetry. Nonetheless, we offer tentative conclusions concerning the efficiency of the western Pacific boundary as a reflector. Our results suggest that the realistic boundary will not greatly alter expectations based on a simple solid boundary if the reflections important for El Nino are primarily in motion, represented by low-order Rossby modes. This is also consistent with observational evidence indicating no anomalous throughflow during El Nino events.

A series of observing system simulation experiments has been performed to assess the potential impact of marine surface wind data on numerical weather prediction. Care was taken to duplicate the spatial coverage and error characteristics of conventional surface, radiosonde, ship, and aircraft reports. These observations, suitably degraded to account for instrument and sampling errors, were used in a conventional analysis-forecast cycle. A series of five 72-hour forecasts were then made by using the analyzed fields as initial conditions. The forecast error growth was found to be similar to that in operational numerical forecasts. Further experiments simulated the time-continuous assimilation of remotely sensed marine surface wind or temperature sounding data in addition to the conventional data. The wind data were fabricated directly for model grid points intercepted by a Seasat-1 scatterometer (SASS) swath and were placed in the lowest active level (945 mbar) of the model. The temperature sounding experiment assimilated error-free data fabricated along actual Nimbus orbits. Forecasts were made from the resulting analysis fields, and the impact of the simulated satellite data was assessed by comparing these forecast errors with those of the control forecasts. When error-free winds were assimilated by using a localized successive correction method (SCM), the impacts in extratropical regions proved to be substantial, especially in lower tropospheric quantities such as surface pressure. In contrast, a less sophisticated assimilation method resulted in negligible impact. The assimilation of error-free sounder data (again by the SCM) gave impacts comparable to the wind data, suggesting that surface wind data alone may be as valuable as temperature soundings for numerical weather prediction. The effects of nominal SASS errors (±2 m/s in magnitude, ±20° in direction) on the impacts derived from wind data were found to be small.

A model for the climatological mean sea surface temperature (SST) of the tropical Pacific Ocean is developed. The upper ocean response is computed using a time dependent, linear, reduced gravity model, with the addition of a constant depth frictional surface layer. The full three-dimensional temperature equation and a surface heat flux parameterization that requires specification of only wind speed and total cloud cover are used to evaluate the SST. Specification of atmospheric parameters, such as air temperature and humidity, over which the ocean has direct influence, is avoided. The model simulates the major features of the observed tropical Pacific SST. The seasonal evolution of these features is generally captured by the model. Analysis of the results demonstrates the control the ocean has over the surface heat flux from ocean to atmosphere and the crucial role that dynamics play in determining the mean SST in the equatorial Pacific. The sensitivity of the model to perturbations in the surface heat flux, cloud cover specification, diffusivity, and mixed layer depth is discussed.