Beschreibung: The semienclosed western Mediterranean Sea has proven to be a useful location to evaluate surface heat flux estimates. In the past the directly measured average oceanic heat transport from the Atlantic into the Mediterranean Sea through the Strait of Gibraltar of similar to 5.2 +/- 1.3 W m(-2) has been compared to estimates of the average heat flux across the surface of the Mediterranean Sea. On long timescales both should closely balance each other. By using a monthly temperature climatology of the western Mediterranean Sea we offer the possibility to extend the comparison to the seasonal timescale. This gives additional information with which different surface heat flux data sets can be evaluated. The seasonal heat content changes of the western Mediterranean and the advective exchange of heat through the Straits of Gibraltar and Sicily are estimated on the basis of a new extensive hydrographic data set and of published values for the volume transports. To demonstrate the method, a limited number of surface heat flux data sets are compared with the oceanographically calculated counterpart. The comparison reveals that some heat fluxes do not only agree well for the long-term averages but also for the seasonal timescale, whereas others show larger deviations. The remaining rms discrepancies of +/-10.2 W m(-2) for the best heat flux data set are smaller than the uncertainty of the oceanographic estimate and of a reasonable magnitude compared to the uncertainty of the long-term average of similar to 5 W m(-2).

Beschreibung: A general circulation ocean model has been used to study the formation and propagation mechanisms of North Atlantic Oscillation (NAO)-generated temperature anomalies along the pathway of the North Atlantic Current (NAC). The NAO-like wind forcing generates temperature anomalies in the upper 440 m that propagate along the pathway of the NAC in general agreement with the observations. The analysis of individual components of the ocean heat budget reveals that the anomalies are primarily generated by the wind stress anomaly-induced oceanic heat transport divergence. After their generation they are advected with the mean current. Surface heat flux anomalies account for only one-third of the total temperature changes. Along the pathway of the NAC temperature anomalies of opposite signs are formed in the first and second halves of the pathway, a pattern called here the North Atlantic dipole (NAD). The response of the ocean depends fundamentally on Rt = (L/υ)/τ, the ratio between the time it takes for anomalies to propagate along the NAC [(L/υ) 10 years] compared to the forcing period τ. The authors find that for NAO periods shorter than 4 years (Rt 〉 1) the response in the subpolar region is mainly determined by the local forcing. For NAO periods longer than 32 years (Rt 〈 1); however, the SST anomalies in the northeastern part of the NAD become controlled by ocean advection. In the subpolar region maximal amplitudes of the temperature response are found for intermediate (decadal) periods (Rt 1) where the propagation of temperature anomalies constructively interferes with the local forcing. A comparison of the NAO-generated propagating temperature anomalies with those found in observations will be discussed.

Beschreibung: Historical winter sea ice concentration data are used to examine the relation between the Northern Annular Mode (NAM) and the sea ice concentration in the Nordic seas over the past 50 years. The well known basic response pattern of a seesaw between the Labrador Sea and the Greenland, Iceland and Barents seas is being reproduced. However, the response is not robust in the Greenland and Iceland seas. There the observed variability has a more complex relationship with surface temperatures and winds. We divide the sea ice response into three spectral bands: high (P 〈 5 year), band (5 〈 P 〈 15 year), and low pass (P 〉 15 year) filtered NAM indices. This division is motivated by the expected slow response of the ocean circulation which might play a significant role in the Greenland and Iceland seas. The response to the NAM is also examined separately for the periods before and after 1976 to identify variations due to the relocation of the northern centre of the North Atlantic Oscillation

Beschreibung: The response of the Arctic Ocean sea ice system to Northern Annular Mode-like wind forcing has been investigated using an ocean/sea ice general circulation model coupled to an atmospheric boundary layer model. A series of idealized experiments was performed to investigate the Arctic Ocean's response to idealized winter wind anomalies on interannual to multi-decadal time scales. The sea ice response of the model consists of a rapid change of ice movements leading to widespread variation in sea ice thickness and concentration. In most areas the response is largely independent of the forcing frequency with only a slight increase towards longer periods. Only the Greenland Sea exhibited a change in sign of sea ice concentration anomalies at about 20 years period which appears to be caused by slow adjustment of the oceanic circulation.

Beschreibung: Between 1996 and 1998, a concerted effort was made to study the deep open ocean convection in the Labrador Sea. Both in situ observations and numerical models were employed with close collaboration between the researchers in the fields of physical oceanography, boundary layer meteorology, and climate. A multitude of different methods were used to observe the state of ocean and atmosphere and determine the exchange between them over the experiment's period. The Labrador Sea Deep Convection Experiment data collection aims to assemble the observational data sets in order to facilitate the exchange and collaboration between the various projects and new projects for an overall synthesis. A common file format and a browsable inventory have been used so as to simplify the access to the data.

Beschreibung: Numerical experiments are performed to examine the causes of variability of Atlantic Ocean SST during the period covered by the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis (1958-98). Three ocean models are used. Two are mixed layer models: one with a 75-m-deep mixed layer and the other with a variable depth mixed layer. For both mixed layer models the ocean heat transports are assumed to remain at their diagnosed climatological values. The third model is a full dynamical ocean general circulation model (GCM). All models are coupled to a model of the subcloud atmospheric mixed layer (AML). The AML model computes the air temperature and humidity by balancing surface fluxes, radiative cooling, entrainment at cloud base, advection and eddy heat, and moisture transports. The models are forced with NCEP-NCAR monthly mean winds from 1958 to 1998. The ocean mixed layer models adequately reproduce the dominant pattern of Atlantic Ocean climate variability in both its spatial pattern and time dependence. This pattern is the familiar tripole of alternating zonal bands of SST anomalies stretching between the subpolar gyre and the subtropics. This SST pattern goes along with a wind pattern that corresponds to the North Atlantic Oscillation (NAO). Analysis of the results reveals that changes in wind speed create the subtropical SST anomalies while at higher latitudes changes in advection of temperature and humidity and changes in atmospheric eddy fluxes are important. An observational analysis of the boundary layer energy balance is also performed. Anomalous atmospheric eddy heat fluxes are very closely tied to the SST anomalies. Anomalous horizontal eddy fluxes damp the SST anomalies while anomalous vertical eddy fluxes tend to cool the entire midlatitude North Atlantic during the NAO's high-index phase with the maximum cooling exactly where the SST gradient is strengthened the most. The SSTs simulated by the ocean mixed layer model are compared with those simulated by the dynamic ocean GCM. In the far North Atlantic Ocean anomalous ocean heat transports are equally important as surface fluxes in generating SST anomalies and they act constructively. The anomalous heat transports are associated with anomalous Ekman drifts and are consequently in phase with the changing surface fluxes. Elsewhere changes in surface fluxes dominate over changes in ocean heat transport. These results suggest that almost all of the variability of the North Atlantic SST in the last four decades can be explained as a response to changes in surface fluxes caused by changes in the atmospheric circulation. Changes in the mean atmospheric circulation force the SST while atmospheric eddy fluxes dampen the SST. Both the interannual variability and the longer timescale changes can be explained in this way. While the authors were unable to find evidence for changes in ocean heat transport systematically leading or lagging development of SST anomalies, this leaves open the problem of explaining the causes of the low-frequency variability. Possible causes are discussed with reference to the modeling results.