The South Pacific Meridional Mode as a Thermally Driven Source of ENSO Amplitude Modulation and Uncertainty
The South Pacific Meridional Mode as a Thermally Driven Source of ENSO Amplitude Modulation and Uncertainty
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DOI：
10.1175/jcli-d-17-0722.1
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发表时间：
2018-06
影响因子：
4.9
通讯作者：
S. Larson;K. Pegion;B. Kirtman
中科院分区：
文献类型：
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作者：
S. Larson;K. Pegion;B. Kirtman
这项研究旨在确定厄尔尼诺/南方涛动幅度和不确定性的热驱动源，因为与风驱动源相比，它们相对未被探索。太平洋纬向模态被认为是ENSO事件的风触发器。这项研究提供了一个替代的作用，南太平洋纬向模态（SPMM）在ENSO动力学，而不是作为一个ENSO触发器，但作为一个符合源的潜热通量（LHF）强迫ENSO SSTA，如果正确（错误）预测，可以减少（增加）ENSO预测误差。我们利用耦合模式模拟，其中ENSO变率是完美的周期性和每个厄尔尼诺经历相同的风应力强迫。厄尔尼诺振幅的差异主要是通过SPMM热驱动的。当厄尔尼诺与正位相SPMM同时发生时，正SPMM LHF异常抵消了厄尔尼诺LHF阻尼的一小部分，允许更强烈的厄尔尼诺。如果SPMM相位相反是负的，SPMM LHF放大了厄尔尼诺的LHF阻尼，降低了事件的振幅。因此，SPMM LHF异常可能对ENSO事件产生相长或相消的干扰，从而调制ENSO的振幅。海洋也发挥了作用，因为热强迫的海表温度异常随后被平均纬向速度向西平流，除了风强迫的分量外，还在ENSO海表温度异常趋势中产生变暖或变冷。结果表明，除了风驱动的来源，存在一个热驱动的一块ENSO振幅和不确定性，通常被忽视。SPMM和太平洋年代际变率之间的联系进行了讨论。
This study seeks to identify thermally driven sources of ENSO amplitude and uncertainty, as they are relatively unexplored compared to wind-driven sources. Pacific meridional modes are argued to be wind triggers for ENSO events. This study offers an alternative role for the South Pacific meridional mode (SPMM) in ENSO dynamics, not as an ENSO trigger, but as a coincident source of latent heat flux (LHF) forcing of ENSO SSTA that, if correctly (incorrectly) predicted, could reduce (increase) ENSO prediction errors. We utilize a coupled model simulation in which ENSO variability is perfectly periodic and each El Niño experiences identical wind stress forcing. Differences in El Niño amplitude are primarily thermally driven via the SPMM. When El Niño occurs coincidentally with positive phase SPMM, the positive SPMM LHF anomaly counteracts a fraction of the LHF damping of El Niño, allowing for a more intense El Niño. If the SPMM phase is instead negative, the SPMM LHF amplifies the LHF damping of El Niño, reducing the event’s amplitude. Therefore, SPMM LHF anomalies may either constructively or destructively interfere with coincident ENSO events, thus modulating the amplitude of ENSO. The ocean also plays a role, as the thermally forced SSTA is then advected westward by the mean zonal velocity, generating a warming or cooling in the ENSO SSTA tendency in addition to the wind-forced component. Results suggest that in addition to wind-driven sources, there exists a thermally driven piece to ENSO amplitude and uncertainty that is generally overlooked. Links between the SPMM and Pacific decadal variability are discussed.