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Ute Luksch

Abstract

The role of atmospheric circulation anomalies in generating midlatitude sea surface temperature (SST) variability is investigated by means of ocean general circulation model (OGCM) experiments, in which observed winds are prescribed during the period 1950–1979. The heat flux parameterization involves atmospheric advection equivalent to specifying an interactive atmosphere, albeit a simple one.

Forced by the observed wind field, the model is successful in reproducing the large-scale patterns of the observed wintertime SST in the North Atlantic. The local correlations between observed and simulated winter-time SSTs are high enough to be significant around 40°N but are not as high in summer (when the scales of the anomalies are smaller) and north of 50°N. The model results are used to understand more about the mechanisms of creating interannual SST variability in winter. During wintertime, strong northwesterly winds from the American continent cool the northwestern pan of the ocean, and advective (Ekman) transports of cold water from the north intensify the SST anomaly. The influence of the sensible and latent heat fluxes is slightly higher than that of the advective transport.

During this 30-yr period the atmospheric circulation and the (observed and simulated) SST also show anomalies that persist for several years. In the 1970s, a strong cyclone over Greenland was repeated in several consecutive winters, and the simulated SST was anomalously cold from 1973 to 1978. Weaker northwesterly winds around 40°–50°N in the 1950s led to an anomalously warm model SST from 1951 to 1955. The simulation is particularly close to the observation during the warm 1950s and delayed and reduced in amplitude in the cold 1970s when a great salinity anomaly was observed in the northern North Atlantic.

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Ute Luksch and Hans von Storch

Abstract

The question of whether the large-scale low-frequency sea surface temperature (SST) variability in the North Pacific can be interpreted as a response to large-scale wind anomalies is studied by an ocean general circulation model coupled to an advective model for the air temperature. Forced with observed monthly mean winds, the model is successful in reproducing the main space and time characteristics of the large-scale low-frequency SST variability. In winter also the simulated and observed SSTs are highly correlated.

The dominant process in producing wintertime SST tendencies is the anomalous turbulent beat exchange with the atmosphere that is parameterized by the bulk aerodynamic formula and takes into account the simulated air temperature, the simulated SST, and the observed winds. The oceanic response to turbulent momentum fluxes is much smaller. The horizontal scale of the simulated air temperature is induced by advective transports with the observed winds and transferred to the ocean by anomalous turbulent latent and sensible heat fluxes. The ocean response is lagging the atmospheric forcing by about one month and persists over much longer time than the atmospheric anomalies, particularly in winter.

Part of the observed low-frequency SST variance can be explained by teleconnection. A wind field that is directly related to the tropical El Niño–Southern Oscillation (ENSO) phenomenon produces SST anomalies with an ENSO-related variance of more than 50% instead of 10% to 30% as observed.

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Ute Luksch and Hans von Storch

Abstract

A stochastic specification for monthly mean wintertime eddy heat transport conditional upon the monthly mean circulation is proposed. The approach is based on an analog technique. The nearest neighbor for the monthly mean streamfunction (at 850 and 300 hPa) is searched for in a library composed of monthly data of a 1268-yr control simulation with a coupled ocean–atmosphere model. To reduce the degrees of freedom a limited area (the North Atlantic sector) is used for the analog specification. The monthly means of northward transient eddy flux of temperature (at 750 hPa) are simulated as a function of these analogues.

The stochastic model is applied to 300 years of a paleosimulation (last interglacial maximum around 125 kyr BP). The level of variability of the eddy heat flux is reproduced by the analog estimator, as well as the link between monthly mean circulation and synoptic-scale variability. The changed boundary conditions (solar radiation and CO2 level) cause the Eemian variability to be significantly reduced compared to the control simulation. Although analogues are not a very good predictor of heat fluxes for individual months, they turn out to be excellent predictors of the distribution (or at least the variance) of heat fluxes in an anomalous climate.

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Katrin Walter, Ute Luksch, and Klaus Fraedrich

Abstract

Several GCM studies indicate that eddy activity may have a considerable influence on the atmospheric response to midlatitude sea surface temperature anomalies. The effect of eddy activity on the atmospheric equilibrium response to idealized midlatitude thermal forcing is analyzed for an atmosphere with or without an idealized storm track. Experiments using a simplified global circulation model forced by thermal anomalies of different sign and location are discussed.

Consistent with the linear theory the geopotential height field displays a baroclinic response with a shallow low (high) somewhat downstream of the warm (cold) anomaly; farther downstream an equivalent barotropic response occurs with positive (negative) amplitude increasing with height. Eddy feedbacks have weak impact on the baroclinic part, but the equivalent barotropic response is strongly enhanced if the bandpass-filtered streamfunction tendency is in-phase with the linear geopotential height response. This is the case in an experiment with a warm anomaly near 40°N, located southwesterly of the idealized storm track. In the corresponding experiment with a cold anomaly the two patterns are out-of-phase and the equivalent barotropic response is slightly reduced. It is weakened (strengthened) if a warm (cold) anomaly is shifted about 10° poleward or equatorward relative to the idealized storm track. Midlatitude heat sources generate wave trains that extend equatorward and poleward developing large-scale correlations between the flow at remote locations (teleconnections). The space–time variability can be changed considerably by eddy feedbacks developing stronger variance for large-scale retrogressive traveling and standing waves. Partially, blocking-like events develop.

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