The Eastern Pacific Double ITCZ during Boreal Spring in Observations: Is It Modulated by the Central Pacific La Nin ̃a?

Abstract:

During boreal spring, the eastern Pacific is characterized by double ITCZs (Intertropical Convergence Zones) seen from monthly mean fields, yet the daily ITCZ state can vary among different configurations, including the northern, the double and the southern ITCZ. Based on the daily states of the eastern Pacific ITCZ during March-April 1980–2012, composites of the global observed thermodynamical and dynamical fields associated with the daily double and the southern ITCZs are analyzed, with the northern ITCZ as the reference state. It is found that the double ITCZs are related to the central Pacific La Nin ̃a-like states: anomalous positive (negative) SSTs and atmospheric heating (cooling) over the western (central) tropical Pacific. The daily southern ITCZs show similar patterns but with a much greater magnitude and therefore can be seen as the extreme case of the double ITCZs. Air-sea interaction plays an important role in developing the double/southern ITCZ related anomalies as revealed by surface heat fluxes. Further analysis reveals that these patterns can be largely reproduced by the regression of the corresponding monthly mean fields onto the leading mode of the tropical Pacific SSTs during the same season, suggesting that the composites associated with the eastern Pacific ITCZ daily variability during boreal spring can be attributed to the tropical Pacific interannual variability.