Connection of the interannual seesaw of the Somali-Australian cross-equatorial flows with China summer rainfall

Abstract

The low-level Cross-Equatorial Flows (CEFs) over the Indian Ocean and western Pacific regions have been recognized as essential components of the Asian-Australian monsoon systems. The variations of the CEFs play a key role in the year-to-year monsoon rainfall anomalies. Recently, several studies have illustrated that the changes of the two CEFs are not independent with each other. A negative (seesaw) correlation at interannual timescale between the Somali CEF and the Australian CEF during boreal summer (Jun-July-August) has been noticed. In contrast to the Indian summer monsoon (ISM), for which the Somali CEF plays a dominant role in the moisture transport, both the Somali and Australian CEFs show significant contributions to the East Asian summer monsoon (EASM). Thus, it may be more useful to consider the effect of the combined CEFs on the EASM. To describe the opposite interannual variability between the Somali and Australian CEFs, our previous study has proposed a CEF-seesaw index which is defined by the difference between the standardized Somali and Australian CEFs. A higher (lower) CEF-seesaw index represents an intensified (weakened) Somali CEF but a weakened (intensified) Australian CEF. The CEF-seesaw index shows a closer correlation with EASM than the individual CEF does. In addition, the seesaw-EASM connection is even closer than the ENSO-EASM connection. Thus, the CEF-seesaw and its close connection to Asian summer monsoon may represent an intrinsic variability within the Asian-Australian monsoon system. This contributes to better understanding of the interannual variability of summer rainfall in the Asian-Australian monsoonal region.