Revisiting the Linkage Between the Pacific-Japan Pattern and Indian Summer Monsoon Rainfall: The Crucial Role of the Maritime Continent

The Pacific-Japan (PJ) pattern traditionally refers to the meridional dipole mode of rainfall and the low-level circulation over the tropical western North Pacific and mid-latitude East Asia. However, recent studies have reported that the PJ pattern can also affect the Indian summer monsoon (ISM) via the anomalous circulation over the North Indian Ocean. We summarize the currently available PJ indices and re-examine the linkage between the PJ pattern and the ISM. We found that the only PJ indices that are significantly correlated with rainfall in southern India are the two indices containing signals of the Maritime Continent. The Maritime Continent rainfall can also stimulate circulation anomalies in the North Indian Ocean, thereby strengthening the PJ-ISM linkage. When the signals associated with the Maritime Continent are removed, the PJ-ISM linkage becomes weak and insignificant. The PJ indices should be chosen carefully when studying the climatic impacts of the PJ pattern. The Pacific-Japan (PJ) pattern is the dominant climate mode over East Asia and the western North Pacific and reflects the out-of-phase variation of rainfall and the low-level circulation between the tropics and mid-latitudes. However, recent studies have reported that the PJ impacts are not limited to these regions, but also include the Indian summer monsoon (ISM). We summarize here the currently available PJ indices and re-examine the PJ-ISM linkage. We found that the PJ-ISM linkage depends on the definition of the PJ indices. Most of the PJ indices are not significantly correlated with rainfall in southern India and only those indices that include signals from the equatorial regions (especially the Maritime Continent) are closely associated with the ISM. The circulation anomalies associated with the northern lobe of the PJ are confined to the extratropical region, whereas those associated with the southern lobe can extend westward into the North Indian Ocean. The circulation anomalies associated with the PJ are too weak to affect the ISM. Rainfall in the Maritime Continent can also stimulate circulation anomalies in the North Indian Ocean, thereby strengthening the PJ-ISM linkage. When the signals associated with the Maritime Continent are removed, the PJ-ISM linkage becomes insignificant. The linkage between the Pacific-Japan (PJ) pattern and Indian summer monsoon (ISM) rainfall is sensitive to the definition of the PJ index Only the PJ indices containing signals of the Maritime Continent are linked significantly to the ISM Equatorial Rossby waves excited by Maritime Continent rainfall are crucial in the PJ-ISM linkage