Since the beginning of the Association of Southeast Asian Nations Climate Outlook Forum (ASEANCOF) in 2013, the most difficult challenge has been the rainfall forecast in boreal winter. This is the Maritime Continent monsoon season during which rainfall reaches maximum in the annual cycle. This forecast difficulty arises in spite of the general notion that seasonal predictability of the Maritime Continent rainfall may be higher than most places because of the strong and robust influences of ENSO. The lower predictability is consistent with the lower correlation between ENSO and western Maritime Continent rainfall that reaches minimum during the boreal winter monsoon. Various theories have been proposed to explain this low correlation. In this paper, we review the research on ENSO-Maritime Continent rainfall relationship and show that the main cause of the forecast difficulty is the wind-terrain interaction involving the Sumatran and Malay Peninsula mountains, rather than the effect of sea surface temperature (SST). The wind-terrain interaction due to the low-level regional scale anomalous horizontal circulation offsets the anomalous Walker circulation during both El Niño and La Niña. The net result of these two opposing responses to ENSO is a lower local predictability. We propose to call this low-predictability region the WIMP (Western Indonesia-Malay Peninsula) region both for its geographical location and its special characteristic of causing difficulties for forecasters to make a confident forecast for the boreal winter. Our result suggests that climate models lack skills in forecasting rainfall in this region because their predictability depends strongly on SST.

中文翻译：

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ENSO期间西部海域冬季北方冬季降水的季节预测

自2013年东南亚国家联盟气候展望论坛（ASEANCOF）成立以来，最困难的挑战是北方冬季的降雨预报。这是海洋大陆季风季节，在这一季节中，降雨在年度周期中达到最大。尽管由于ENSO的强而有力的影响，尽管人们普遍认为海洋大陆降雨的季节可预测性可能比大多数地方更高，但仍存在这种预测困难。较低的可预测性与ENSO和西部海洋大陆降雨之间的相关性较低有关，后者在冬季北方季风期间达到最小值。已经提出了各种理论来解释这种低相关性。在本文中，我们回顾了ENSO-海陆降水关系的研究，发现预报困难的主要原因是涉及苏门答腊和马来半岛山脉的风-地形相互作用，而不是海面温度（SST）的影响。低水平区域尺度异常水平环流引起的风地形相互作用抵消了厄尔尼诺和拉尼娜期间沃克环流的异常。这两个对ENSO的相反反应的最终结果是较低的局部可预测性。我们建议将此低可预测性地区称为WIMP（西印度尼西亚-马来半岛）地区，因为其地理位置和特殊特征使预报员难以对寒冬做出可靠预测。