The tropospheric biennial oscillation (TBO) is conventionally considered to involve transitions between the Indian and Australian summer monsoons and the interactions between these two monsoons and the underlying Indo-Pacific Oceans. Here we show that, since the early 1990s, the TBO has evolved to mainly involve the transitions between the western North Pacific (WNP) and Australian monsoons. In this framework, the WNP monsoon replaces the Indian monsoon as the active northern hemisphere TBO monsoon center during recent decades. This change is found to be caused by stronger Pacific-Atlantic coupling and an increased influence of the tropical Atlantic Ocean on the Indian and WNP monsoons. The increased Atlantic Ocean influence damps the Pacific Ocean influence on the Indian summer monsoon (leading to a decrease in its variability) but amplifies the Pacific Ocean influence on the WNP summer monsoon (leading to an increase in its variability). The shift from the strong Pacific-Indian Ocean coupling to the strong Pacific-Atlantic coupling may support the shift of biennial monsoon transitions from the Indian-Australian monsoon transitions to the WNP-Australian monsoon transitions. The stronger Pacific-Atlantic coupling during recent decades is considered to be the primary reason why we observe a shift in the monsoon centers in the biennial monsoon transitions associated with the TBO. These results suggest that the Pacific-Atlantic interactions have become more important to the TBO dynamics during recent decades.