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Aaron Levine, Fei Fei Jin, and Michael J. McPhaden

Abstract

A major open question about El Niño–Southern Oscillation (ENSO) is what causes ENSO amplitude asymmetry, with strong El Niños generally larger than strong La Niñas. The authors examine a leading hypothesis—that the ENSO state modifies the fetch and/or wind speed of westerly wind bursts (WWBs) that create asymmetric forcing and an asymmetric ENSO response. Further, in El Niño forecasts, the number of WWBs expected increases in the month following a strong WWB when compared with the month preceding it. Using a conceptual model, a relationship is derived between the magnitude of the westerly wind burst state dependence on ENSO and ENSO asymmetry. It is found that this relationship between the magnitude of the state dependence and ENSO asymmetry holds in both the observations and 21 coupled climate models. Finally, it is found that because of state-dependent westerly wind burst forcing, extreme El Niño events tend to be of the eastern Pacific variety.

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Aaron F. Z. Levine, Dargan M. W. Frierson, and Michael J. McPhaden

Abstract

The Atlantic multidecadal oscillation (AMO) has been shown to play a major role in the multidecadal variability of the Northern Hemisphere, impacting temperature and precipitation, including intertropical convergence zone (ITCZ)-driven precipitation across Africa and South America. Studies into the location of the intertropical convergence zone have suggested that it resides in the warmer hemisphere, with the poleward branch of the Hadley cell acting to transport energy from the warmer hemisphere to the cooler one. Given the impact of the Atlantic multidecadal oscillation on Northern Hemisphere temperatures, we expect the Atlantic multidecadal oscillation to have an impact on the location of the intertropical convergence zone. We find that the positive phase of the Atlantic multidecadal oscillation warms the Northern Hemisphere, resulting in a northward shift of the intertropical convergence zone, which is evident in the Pacific climate proxy record. Using a coupled climate model, we further find that the shift in the intertropical convergence zone is consistent with the surface energy imbalance generated by the Atlantic multidecadal oscillation. In this model, the Pacific changes are driven in large part by the warming of the tropical Atlantic and not the extratropical Atlantic.

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Xin Geng, Wenjun Zhang, Fei-Fei Jin, Malte F. Stuecker, and Aaron F. Z. Levine

Abstract

Recent studies demonstrated the existence of a conspicuous atmospheric combination mode (C-mode) originating from nonlinear interactions between El Niño–Southern Oscillation (ENSO) and the Pacific warm pool annual cycle (AC). Here we find that the C-mode exhibits prominent decadal amplitude variations during the ENSO decaying boreal spring season. It is revealed that the Atlantic multidecadal oscillation (AMO) can largely explain this waxing and waning in amplitude. A robust positive correlation between ENSO and the C-mode is detected during a negative AMO phase but not during a positive phase. Similar results can also be found in the relationship of ENSO with 1) the western North Pacific (WNP) anticyclone and 2) spring precipitation over southern China, both of which are closely associated with the C-mode. We suggest that ENSO property changes due to an AMO modulation play a crucial role in determining these decadal shifts. During a positive AMO phase, ENSO events are distinctly weaker than those in an AMO negative phase. In addition, El Niño events concurrent with a positive AMO phase tend to exhibit a westward-shifted sea surface temperature (SST) anomaly pattern. These SST characteristics during the positive AMO phase are both not conducive to the development of the meridionally asymmetric C-mode atmospheric circulation pattern and thus reduce the ENSO/C-mode correlation on decadal time scales. These observations can be realistically reproduced by a coupled general circulation model (CGCM) experiment in which North Atlantic SSTs are nudged to reproduce a 50-yr sinusoidally varying AMO evolution. Our conclusion carries important implications for understanding seasonally modulated ENSO dynamics and multiscale climate impacts over East Asia.

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