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Thomas Jung and Jan Barkmeijer

Abstract

The sensitivity of the wintertime tropospheric circulation to changes in the strength of the Northern Hemisphere stratospheric polar vortex is studied using one of the latest versions of the ECMWF model. Three sets of experiments were carried out: one control integration and two integrations in which the strength of the stratospheric polar vortex has been gradually reduced and increased, respectively, during the course of the integration. The strength of the polar vortex is changed by applying a forcing to the model tendencies in the stratosphere only. The forcing has been obtained using the adjoint technique. It is shown that, in the ECMWF model, changes in the strength of the polar vortex in the middle and lower stratosphere have a significant and slightly delayed (on the order of days) impact on the tropospheric circulation. The tropospheric response shows some resemblance to the North Atlantic Oscillation (NAO), though the centers of action are slightly shifted toward the east compared to those of the NAO. Furthermore, a separate comparison of the response to a weak and strong vortex forcing suggests that to first order the tropospheric response is linear within a range of realistic stratospheric perturbations. From the results presented, it is argued that extended-range forecasts in the European area particularly benefit from the stratosphere–troposphere link.

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Carsten Eden and Thomas Jung

Abstract

In contrast to the atmosphere, knowledge about interdecadal variability of the North Atlantic circulation is relatively restricted. It is the objective of this study to contribute to understanding how the North Atlantic circulation responds to a forcing by the North Atlantic oscillation (NAO) on interdecadal timescales. For this purpose, the authors analyze observed atmospheric and sea surface temperature (SST) data along with the response of an ocean general circulation model to a realistic monthly surface flux forcing that is solely associated with the NAO for the period 1865–1997.

In agreement with previous studies, it is shown that the relationship between the local forcing by the NAO and observed SST anomalies on interdecadal timescales points toward the importance of oceanic dynamics in generating SST anomalies. A comparison between observed and modeled SST anomalies reveals that the model results can be used to assess interdecadal variability of the North Atlantic circulation.

The observed/modeled developments of interdecadal SST anomalies during the periods 1915–39 and 1960–84 against the local damping influence from the NAO can be traced back to the lagged response (10–20 yr) of the North Atlantic thermohaline circulation and the subpolar gyre strength to interdecadal variability of the NAO. Additional sensitivity experiments suggest that primarily interdecadal variability in the surface net heat flux forcing associated with the NAO governs interdecadal changes of the North Atlantic circulation.

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Thomas Jung and Michael Hilmer

Abstract

Recently, Hilmer and Jung have shown that the wintertime link between the North Atlantic oscillation (NAO) and the sea ice export through Fram Strait changed from zero correlation (1958–77) to about 0.7 (1978–97) during the last four decades. In the current study, the authors focus on the question of how the two phenomena are linked in a long-term context during wintertime (December–March). This is done on a statistical basis using data from a century-scale control integration of the coupled general circulation model ECHAM4–OPYC3 along with historical sea level pressure data for the period 1908–97.

From the results of this study there is less indication that a significant link on interannual and decadal timescales between the NAO and the sea ice export through Fram Strait is a characteristic property of the climate system—at least under present-day climate conditions. This missing link can be explained by a vanishing net impact of the NAO on sea ice thickness as well as sea ice drift near Fram Strait and thus the sea ice volume export through Fram Strait. It is argued that the spatial pattern of interannual NAO variability as observed during the last two decades of the twentieth century is unusual and so is the high correlation between the NAO and Arctic sea ice export for the period 1978–97.

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Thomas Jung and Frederic Vitart

Abstract

The ECMWF monthly forecasting system is used to investigate the impact that an interactive ocean has on short-range and medium-range weather predictions in the Northern Hemisphere extratropics during wintertime. On a hemispheric scale the predictive skill for mean sea level pressure (MSLP) with and without an interactive ocean is comparable. This can be explained by the relatively small impact that coupling has on MSLP forecasts. In fact, deterministic and ensemble integrations reveal that the magnitude of forecast error and the perturbation growth due to analysis uncertainties, respectively, by far outweigh MSLP differences between coupled and uncoupled integrations. Furthermore, no significant difference of the ensemble spread between the uncoupled and coupled system is found. The authors’ conclusions apply equally for a number of cases of rapidly intensifying extratropical cyclones in the North Atlantic region. Further experimentation with different atmospheric model versions, different horizontal atmospheric resolutions, and different ocean model formulation reveals the robustness of the findings. The results suggest that (for the cases, resolutions, and model complexities considered is this study) the benefit of using coupled atmosphere–ocean models to carry out 1–10-day MSLP forecasts is relatively small, at least in the Northern Hemisphere extratropics during wintertime.

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Thomas Jung and Peter B. Rhines

Abstract

Some effects of Greenland on the Northern Hemisphere wintertime circulation are discussed. Inviscid pressure drag on Greenland’s slopes, calculated from reanalysis data, is related to circulation patterns. Greenland lies north of the core of the tropospheric westerly winds. Yet strong standing waves, which extend well into the stratosphere, produce a trough/ridge system with jet stream lying close to Greenland, mean Icelandic low in its wake, and storm track that interacts strongly with its topography. In the lower troposphere, dynamic height anomalies associated with strongly easterly pressure drag on the atmosphere are quite localized in space and relatively short-lived compared to upper levels, yet they involve a hemispheric-scale dislocation of the stratospheric polar vortex. It is a two-scale problem, however; the high-pass time-filtered part of the height field, responsible for 73% of the pressure drag, is quite different, and expresses propagating cyclonic development in the Atlantic storm track. Eliassen–Palm flux (EP flux) analysis shows that the atmospheric response is (counterintuitively) an acceleration of the westerly winds. The hemispheric influence is consistent with the model results of Junge et al. suggesting that Greenland affects the stationary waves in winter.

This discussion shows that Greenland is not a simple “stirring rod” in the westerly circulation, yet involvement of Greenland’s topography with the shape, form, and intensity of the storm track is strong. Interaction of traveling storms, the jet stream, and the orographic wake frequently leads to increase of the lateral scale such that cyclonic system expands to the size of Greenland itself (∼2500 km). Using the global ECMWF general circulation model, the authors explore the effect of model resolution on these circulations. Statistically, in two case studies, and in higher-resolution global models at TL255 to TL799 resolution, intense tip jet, hydraulic downslope jet, and gravity wave radiation appear in strong flow events, in accord with the work of Doyle and Shapiro. Three-dimensional particle trajectories and vorticity maps show the nature and intensity of the summit-gap flow. Cyclonic systems in the lee of Greenland are strongly affected by the downslope jet. Penetration of the Arctic Basin by cyclonic systems arises from this source region, and the amplitude of the pressure drag is enhanced at high resolution. At the higher resolutions, storm-track analysis verifies the splitting of the storm track by Greenland with a substantial minority of storms moving northward through Baffin Bay. Finally, analysis of 20 winters of 40-yr ECMWF Re-Analysis (ERA-40) reforecasts shows little evidence that negative pressure-drag events are followed by anomalously large forecast errors over Europe, throughout the forecast. Forecast skill for the pressure drag is surprisingly good, with a correlation of 0.65 at 144 h.

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Richard J. Greatbatch and Thomas Jung

Abstract

In this paper, a version of the European Centre for Medium-Range Weather Forecasts (ECMWF) operational model is used to (i) diagnose the diabatic heating associated with the winter North Atlantic Oscillation (NAO) and (ii) assess the role of this heating in the dynamics of the NAO in the model. Over the North Atlantic sector, the NAO-related diabatic heating is dominated above the planetary boundary layer by the latent heat release associated with precipitation, and within the boundary layer by vertical diffusion associated with sensible heat flux from the ocean. An association between La Niña–El Niño–type conditions in the tropical Pacific and the positive/negative NAO is found in model runs using initial conditions and sea surface temperature (SST) lower boundary conditions from the period 1982–2001, but not in a companion set of model runs for the period 1962–81. Model experiments are then described in which the NAO-related diabatic heating diagnosed from the 1982–2001 control run is applied as a constant forcing in the model temperature equation using both 1982–2001 and 1962–81 model setups. To assess the local feedback from the diabatic heating, the specified forcing is first restricted to the North Atlantic sector alone. In this case, the model response (in an ensemble mean sense) is suggestive of a weak negative feedback, but exhibits more baroclinic structure and has its centers of action shifted compared to those of the NAO. On the other hand, forcing with only the tropical Pacific part of the diabatic heating leads to a robust model response in both the 1982–2001 and 1962–81 model setups. The model response projects on to the NAO with the same sign as that used to diagnose the forcing, arguing that the link between the tropical Pacific and the NAO is real in the 1982–2001 control run. The missing link in the corresponding run for 1962–81 is a result of a change in the tropical forcing between the two periods, and not the extratropical flow regime.

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Thomas Jung, Eberhard Ruprecht, and Friedrich Wagner

Abstract

A neural network (NN) has been developed in order to retrieve the cloud liquid water path (LWP) over the oceans from Special Sensor Microwave/Imager (SSM/I) data. The retrieval with NNs depends crucially on the SSM/I channels used as input and the number of hidden neurons—that is, the NN architecture. Three different combinations of the seven SSM/I channels have been tested. For all three methods an NN with five hidden neurons yields the best results. The NN-based LWP algorithms for SSM/I observations are intercompared with a standard regression algorithm. The calibration and validation of the retrieval algorithms are based on 2060 radiosonde observations over the global ocean. For each radiosonde profile the LWP is parameterized and the brightness temperatures (Tb’s) are simulated using a radiative transfer model.

The best LWP algorithm (all SSM/I channels except T85V) shows a theoretical error of 0.009 kg m−2 for LWPs up to 2.8 kg m−2 and theoretical “clear-sky noise” (0.002 kg m−2), which has been reduced relative to the regression algorithm (0.031 kg m−2). Additionally, this new algorithm avoids the estimate of negative LWPs.

An indirect validation and intercomparison is presented that is based upon SSM/I measurements (F-10) under clear-sky conditions, classified with independent IR-Meteosat data. The NN-based algorithms outperform the regression algorithm. The best LWP algorithm shows a clear-sky standard deviation of 0.006 kg m−2, a bias of 0.001 kg m−2, nonnegative LWPs, and no correlation with total precipitable water. The estimated accuracy for SSM/I observations and two of the proposed new LWP algorithms is 0.023 kg m−2 for LWP ⩽ 0.5 kg m−2.

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Sergey Gulev, Thomas Jung, and Eberhard Ruprecht

Abstract

Using the same approach as in Part I, here it is shown how sampling problems in voluntary observing ship (VOS) data affect conclusions about interannual variations and secular changes of surface heat fluxes. The largest uncertainties in linear trend estimates are found in relatively poorly sampled regions like the high-latitude North Atlantic and North Pacific as well as the Southern Ocean, where trends can locally show opposite signs when computed from the regularly sampled and undersampled data. Spatial patterns of shorter-period interannual variability, quantified through the EOF analysis, also show remarkable differences between the regularly sampled and undersampled flux datasets in the Labrador Sea and northwest Pacific. In particular, it is shown that in the Labrador Sea region, in contrast to regularly sampled NCEP–NCAR reanalysis fluxes, VOS-like sampled NCEP–NCAR reanalysis fluxes neither show significant interannual variability nor significant trends. These regions, although quite localized covering small parts of the globe, play a crucial role for the coupled atmosphere–ocean system. In the Labrador Sea, for instance, interannual and decadal-scale changes of the surface net heat fluxes are known to affect oceanic convection and, thus, the meridional overturning circulation of the Atlantic Ocean. From a discussion of current atmospheric data assimilation systems it is argued that in poorly sampled regions reanalysis products are superior to VOS-based products for studying interannual and interdecadal variations of atmosphere–ocean interaction. In well-sampled regions, on the other hand, conclusions about surface heat flux variations are relatively insensitive to the choice of the flux products used (VOS versus reanalysis data). The results are confirmed for two different datasets, that is, ECMWF 40-yr Re-Analysis (ERA-40) data and seasonal integrations with a recent version of the ECMWF model in which no actual data were assimilated.

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Thomas Jung, Laura Ferranti, and Adrian M. Tompkins

Abstract

The sensitivity of the atmospheric circulation to the warm Mediterranean sea surface temperature (SST) anomalies observed during the summer of 2003 (July and August) is studied using the European Centre for Medium-Range Weather Forecasts (ECMWF) model. A control integration imposes climatological Mediterranean SSTs as a lower boundary condition. The first sensitivity experiment uniformly increases these Mediterranean SSTs by 2 K, the approximate mean observed in the 2003 summer season. A second experiment then investigates the additional impact of the SST distribution by imposing the observed SST summer anomaly.

The response of the atmospheric circulation in the European area shows some resemblance to the observed anomaly. The weakness of this response suggests, however, that the warm Mediterranean played a minor role, if any, in maintaining the anomalous atmospheric circulation as observed in the summer of 2003. Increasing SST in the Mediterranean locally leads to an increase in precipitation, particularly in the western Mediterranean. Furthermore, significantly increased Sahelian rainfall is simulated, deriving from enhanced evaporation in the Mediterranean Sea. In the ECMWF model the anomalously high moisture is advected by the climatological Harmattan and Etesian winds, where enhanced moisture flux convergence leads to more precipitation. The associated diabatic heating leads to a reduction of the African easterly jet strength. A similar Sahelian response has been previously documented using a different atmospheric model, increasing confidence in the robustness of the result. Finally, the results are discussed in the context of the seasonal predictability of European and African climate.

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Sergey Gulev, Thomas Jung, and Eberhard Ruprecht

Abstract

Sampling uncertainties in the voluntary observing ship (VOS)-based global ocean–atmosphere flux fields were estimated using the NCEP–NCAR reanalysis and ECMWF 40-yr Re-Analysis (ERA-40) as well as seasonal forecasts without data assimilation. Air–sea fluxes were computed from 6-hourly reanalyzed individual variables using state-of-the-art bulk formulas. Individual variables and computed fluxes were subsampled to simulate VOS-like sampling density. Random simulation of the number of VOS observations and simulation of the number of observations with contemporaneous sampling allowed for estimation of random and total sampling uncertainties respectively. Although reanalyses are dependent on VOS, constituting an important part of data assimilation input, it is assumed that the reanalysis fields adequately reproduce synoptic variability at the sea surface. Sampling errors were quantified by comparison of the regularly sampled (i.e., 6 hourly) and subsampled monthly fields of surface variables and fluxes. In poorly sampled regions random sampling errors amount to 2.5°–3°C for air temperature, 3 m s−1 for the wind speed, 2–2.5 g kg−1 for specific humidity, and 15%–20% of the total cloud cover. The highest random sampling errors in surface fluxes were found for the sensible and latent heat flux and range from 30 to 80 W m−2. Total sampling errors in poorly sampled areas may be higher than random ones by 60%. In poorly sampled subpolar latitudes of the Northern Hemisphere and throughout much of the Southern Ocean the total sampling uncertainty in the net heat flux can amount to 80–100 W m−2. The highest values of the uncertainties associated with the interpolation/extrapolation into unsampled grid boxes are found in subpolar latitudes of both hemispheres for the turbulent fluxes, where they can be comparable with the sampling errors. Simple dependencies of the sampling errors on the number of samples and the magnitude of synoptic variability were derived. Sampling errors estimated from different reanalyses and from seasonal forecasts yield qualitatively comparable spatial patterns, in which the actual values of uncertainties are controlled by the magnitudes of synoptic variability. Finally, estimates of sampling uncertainties are compared with the other errors in air–sea fluxes and the reliability of the estimates obtained is discussed.

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