His movements during his childhood were influenced by the turmoil of World War II, until his family was finally reunited in 1946 in Plön, Schleswig-Holstein, West Germany.

He obtained his habilitation in oceanography and geophysics from the University of Kiel in 1966 for his work on the circulation and stratification at Bab-el-Mandeb, Red Sea.

During his tenure at the Institut für Meereskunde and as professor of the University of Kiel, he held the following positions:[2] His work contributed in advancing knowledge on ocean circulation and climate, boundary currents, oceanic fronts, flow through straits, mixing processes, and internal waves.

[9][10][11] Over the years, Siedler held several positions as visiting scientist in the USA, France, Spain and South Africa.

[2] German Research Foundation: German federal ministry for science and technology (now Bundesministerium für Bildung und Forschung): GKSS Research Centre Geesthacht [de] Scientific Committee on Oceanic Research (SCOR) of the International Council for Science (ICSU): International Association for the Physical Sciences of the Oceans (IAPSO) of the International Union of Geodesy and Geophysics (IUGG): Programme National d'Etude de la Dynamique du Climat (PNEDC), France: Scientific Steering Group of the World Ocean Circulation Experiment (WOCE): Woods Hole Oceanographic Institution, USA: European Commission, Brussels Mid-Term Evaluation Panel, Marine Science and Technology Program: