Klaus Peter Koltermann

North Atlantic Ocean control on surface heat flux on multidecadal timescales

Nearly 50 years ago Bjerknes suggested that the character of large-scale air–sea interaction over the mid-latitude North Atlantic Ocean differs with timescales: the atmosphere was thought to drive directly most short-term—interannual—sea surface temperature (SST) variability, and the ocean to contribute significantly to long-term—multidecadal—SST and potentially atmospheric variability. Although the conjecture for short timescales is well accepted, understanding Atlantic multidecadal variability (AMV) of SST remains a challenge as a result of limited ocean observations. AMV is nonetheless of major socio-economic importance because it is linked to important climate phenomena such as Atlantic hurricane activity and Sahel rainfall, and it hinders the detection of anthropogenic signals in the North Atlantic sector. Direct evidence of the oceanic influence of AMV can only be provided by surface heat fluxes, the language of ocean–atmosphere communication. Here we provide observational evidence that in the mid-latitude North Atlantic and on timescales longer than 10u2009years, surface turbulent heat fluxes are indeed driven by the ocean and may force the atmosphere, whereas on shorter timescales the converse is true, thereby confirming the Bjerknes conjecture. This result, although strongest in boreal winter, is found in all seasons. Our findings suggest that the predictability of mid-latitude North Atlantic air–sea interaction could extend beyond the ocean to the climate of surrounding continents.

Topographic effects of the Maud Rise on the stratification and circulation of the Weddell Gyre

Abstract Hydrographic data indicate that due to the Maud Rise (65°S, 3°E) the upwelling of relatively warm and more saline Lower Circumpolar Deep Water (LCDW) in the Weddell Gyre and the entrainment of LCDW into the upper layer are enhanced, which increase the ocean heat loss and enrich the salinity of the upper layer. By these processes the water column is preconditioned for deep convection. An anomalous water column with a diameter of about 150–200 km, notable for its relatively cold deep water and its relatively saline upper layer, is situated above the crest of the Maud Rise. Water from its main pycnocline spreads laterally within the upper 500 m and compensates for the entrainment of LCDW into the upper layer. Current meter data from the Winter Weddell Sea Project 1986 show a marked temporal and spatial variability of the southward transport of LCDW in the lee of the Maud Rise. An anticyclone of LCDW, which had cut off from the main body of LCDW east of the Maud Rise, migrated southward along the western slope with a velocity of about 3 cm s−1. It was followed by a cyclone, which had probably formed at the front of the anomaly due to a strengthened flow, undergoing an enhanced meandering. The wavelength of the entire disturbance is estimated to be about 380 km, one-third of the circumference of the Maud Rise anomaly. If the disturbance repeats continuously, about two to three anticyclones detach per year.

On the currents over the shelf off cap blanc in the northwest African upwelling area

An interpretation of current measurements yields, as a typical feature of the cross-component over the upper slope, two circulation cells with opposite rotation. Here coastal upwelling has been assumed to be mainly a two-dimensional phenomenon. Over the upper continental slope the core of the subsurface onshore return flow (∼+10 cm s−1) occurs at intermediate depths of 60 m to 70 m and above the bottom over the shelf. This flow causes a convergence just beneath the actual relatively shallow upwelling layer. A great part of the water masses of the return flow feeds the Ekman flux in the surface layer. Another minor part, however, sinks down the slope contributing to a weak offshore component above the bottom. It is suggested that the strength, the thickness and the depth of the onshore compensation flow increase with the intensity of upwelling.

On the relation between residual currents and the wind field in the German Bight

The long-period part (T>25 h) of currents measured at close spacing in the German Bight in June 1968 is investigated. The north component of the bottom current is shown to be highly correlated with the east component of the wind in this area, the wind-generated changes of the mass-field giving rise to a quasi-geostrophic current near the bottom.

Klimaschwankungen im Atlantik: globale Zusammenhänge und regionale Effekte

Die Rolle des Ozeans im Klimasystem der Erde ist zunehmend in den Vordergrund der Klimadiskussion geraten. Besonders der globale ozeanische Transport von Warme und Suswasser wurde dabei herausgearbeitet. Dabei ist der asymetrische Warmetransport des Atlantiks und seine Schwankungen von besonderer Bedeutung fur Europa. Gekoppelte Klimamodelle erlauben heute globale und regionale Szenarien. Wahrend in den Beobachtungen naturliche und anthropogene Schwankungen nicht sauber getrennt werden konnen, lassen Szenarienrechnungen Trendaussagen zu. Dennoch bestehen zahlreiche offene Fragen zur Physik und Chemie des Ozeans, die daher bislang nicht in die Modelle eingearbeitet werden konnen. Um Schwankungen und Trends sauber trennen zu konnen- und die notwendigen wissenschaftlichen Erkenntnisse fur die Modellformulierungen bereitstellen zu konnen—brauchen wir besonders fur der Ozean ein langfristig angelegtes, zweckdienlich konzipiertes und okonomisch vertretbares globales Beobachtungssystem. An seinem Entwurf wird verstarkt gearbeitet. Dabei wird neben wissenschaftlichem und technologischem Neuland auch verwaltungsmasig Neuland betreten. Wie kann eine globales Beobachtungssystem funktionieren, wenn es nicht mehr wird als die Summe seiner Einzelteile? Da die Industrienationen auf der Nordhalbkugel siedeln, richtet sich ihr Augenmerk naturlich auf den Ozean vor der Haustur. Sie mussen sich aber vorbereiten, auch auf der Sudhalbkugel dieses Beobachtungssystem zu unterstutzen. Sind sie dazu in der Lage?