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Aaron F. Z. Levine and Fei-Fei Jin

Abstract

The conceptual El Niño–Southern Oscillation (ENSO) recharge oscillator model is used to study the linear stability of ENSO under state-dependent noise forcing. The analytical framework developed by Jin et al. is extended to more fully study noise-induced instability of ENSO. It is shown that in addition to the noise-induced positive contribution to the growth rate of the ensemble mean (first moment) evolution of the ENSO cycle, there is also a noise-induced instability for the ensemble spread (second moment). These growth rates continue to increase as the strength of the multiplicative noise increases. In both the analytical solution and the numerical model, the criticality threshold for instability of the second moment occurs at a lower value of the parameter that measures multiplicative forcing than the threshold for the first moment. The noise-induced instability not only enhances ENSO activity but also results in a large ensemble spread and thus may reduce the effectiveness of ENSO prediction. As in the additive noise forcing case, the low-frequency variability in the forcing is the important part for forcing El Niño events and the high-frequency forcing alone cannot effectively excite ENSO.

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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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Jian Ma, Sara C. da Silva, Aaron Levine, Yang Yang, Paul Fuentes, Li Zhou, Chuan-Chi Tu, Jia Hu, I. M. Shiromani Jayawardena, Antti Pessi, and DaNa Carlis

A four-day educational cruise navigated around the leeward side of Oahu and Kauai to observe the thermodynamic and dynamic features of the trade-wind wakes of these small islands by using weather balloons and other onboard atmospheric and oceanographic sensors. This cruise was proposed, designed, and implemented completely by graduate students from the School of Ocean and Earth Science and Technology at the University of Hawaii. The data collected during the cruise show, for the first time, strong sea/land breezes during day/night and their thermal effects on the island wake. This cruise provided the students with a significant, valuable, and meaningful opportunity to experience the complete process of proposing and undertaking field observations, as well as analyzing data and writing a scientific article.

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Chidong Zhang, Aaron F. Levine, Muyin Wang, Chelle Gentemann, Calvin W. Mordy, Edward D. Cokelet, Philip A. Browne, Qiong Yang, Noah Lawrence-Slavas, Christian Meinig, Gregory Smith, Andy Chiodi, Dongxiao Zhang, Phyllis Stabeno, Wanqiu Wang, Hong-Li Ren, K. Andrew Peterson, Silvio N. Figueroa, Michael Steele, Neil P. Barton, Andrew Huang, and Hyun-Cheol Shin

Abstract

Observations from uncrewed surface vehicles (saildrones) in the Bering, Chukchi, and Beaufort Seas during June–September 2019 were used to evaluate initial conditions and forecasts with lead times up to 10 days produced by eight operational numerical weather prediction centers. Prediction error behaviors in pressure and wind are found to be different from those in temperature and humidity. For example, errors in surface pressure were small in short-range (<6 days) forecasts, but they grew rapidly with increasing lead time beyond 6 days. Non-weighted multimodel means outperformed all individual models approaching a 10-day forecast lead time. In contrast, errors in surface air temperature and relative humidity could be large in initial conditions and remained large through 10-day forecasts without much growth, and non-weighted multimodel means did not outperform all individual models. These results following the tracks of the mobile platforms are consistent with those at a fixed location. Large errors in initial condition of sea surface temperature (SST) resulted in part from the unusual Arctic surface warming in 2019 not captured by data assimilation systems used for model initialization. These errors in SST led to large initial and prediction errors in surface air temperature. Our results suggest that improving predictions of surface conditions over the Arctic Ocean requires enhanced in situ observations and better data assimilation capability for more accurate initial conditions as well as better model physics. Numerical predictions of Arctic atmospheric conditions may continue to suffer from large errors if they do not fully capture the large SST anomalies related to Arctic warming.

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