NOAA/WDS Paleoclimatology - Villanueva - Bayas - PSME - ITRDB MEXI071

Mexico has suffered a long history and prehistory of severe sustained drought. Drought over Mexico is modulated by ocean-atmospheric variability in the Atlantic and Pacific, raising the possibility for long-range seasonal climate forecasting, which could help mediate the economic and social impacts of future dry spells. The instrumental record of Mexican climate is very limited before 1920, but tree-ring chronologies developed from old-growth forests in Mexico can provide an excellent proxy representation of the spatial pattern and intensity of past moisture regimes useful for the analysis of climate dynamics and climate impacts. The Mexican Drought Atlas (MXDA) has been developed from an extensive network of 252 climate sensitive tree-ring chronologies in and near Mexico. The MXDA reconstructions extend from 1400 CE-2012 and were calibrated with the instrumental summer (JJA) self-calibrating Palmer Drought Severity Index (scPDSI) on a 0.5 latitude/longitude grid extending over land areas from 14 to 34N and 75-120W using Ensemble Point-by-Point Regression (EPPR) for the 1944-1984 period. The grid point reconstructions were validated for the period 1920-1943 against instrumental gridded scPDSI values based on the fewer weather station observations available during that interval. The MXDA provides a new spatial perspective on the historical impacts of moisture extremes over Mexico during the past 600-years, including the Aztec Drought of One Rabbit in 1454, the drought of El Ano de Hambre in 1785-1786, and the drought that preceded the Mexican Revolution of 1909-1910.
The El Nino/Southern Oscillation (ENSO) is the most important ocean-atmospheric forcing of moisture variability detected with the MXDA. In fact, the reconstructions suggest that the strongest central equatorial Pacific sea surface temperature (SST) teleconnection to the soil moisture balance over North America may reside in northern Mexico. This ENSO signal has stronger and more time-stable correlations than computed for either the Atlantic Multidecadal Oscillation or Pacific Decadal Oscillation. The extended Multivariate ENSO Index is most highly correlated with reconstructed scPDSI over northern Mexico, where warm events favor moist conditions during the winter, spring, and early summer. This ENSO teleconnection to northern Mexico has been strong over the past 150 years, but it has been comparatively weak and non-stationary in the MXDA over central and southern Mexico where eastern tropical Pacific and Caribbean/tropical Atlantic SSTs seem to be more important. The ENSO teleconnection to northern Mexico is weaker in the available instrumental PDSI, but analyses based on the millennium climate simulations with the Community Earth System Model suggest that the moisture balance during the winter, spring, and early summer over northern Mexico may indeed be particularly sensitive to ENSO forcing. Nationwide drought is predicted to become more common with anthropogenic climate change, but the MXDA reconstructions indicate that intense "All Mexico" droughts have been rare over the past 600 years and their frequency does not appear to have increased substantially in recent decades.

墨西哥长期以来（包括史前时期）便饱受持续严重干旱的困扰。墨西哥干旱活动受大西洋与太平洋的海气变率调控，这为开展长期季节性气候预报提供了可能，而这类预报可帮助缓解未来干旱事件带来的经济与社会冲击。
墨西哥气候的器测记录在1920年之前极为有限，但基于墨西哥境内原始老龄林构建的树轮年表，可提供优质的代用指标序列，用以表征过去水分状况的空间格局与强度，为气候动力学及气候影响分析提供支撑。
墨西哥干旱图集（Mexican Drought Atlas, MXDA）基于墨西哥境内及周边广泛分布的252条气候敏感树轮年表构建而成。该图集的重建序列时间跨度为公元1400年至2012年，校准时段为1944年至1984年，采用逐点集合回归（Ensemble Point-by-Point Regression, EPPR）方法，以14°N-34°N、75°W-120°W陆地区域内0.5°经纬度网格上的器测夏季（6-8月，JJA）自校准帕默尔干旱严重指数（self-calibrating Palmer Drought Severity Index, scPDSI）作为校准依据。
网格点重建结果针对1920年至1943年时段进行了验证，验证数据为该时段内基于少量气象站观测得到的网格化器测scPDSI值。MXDA为研究过去600年间墨西哥境内极端水分状况的历史影响提供了全新的空间视角，其中包括1454年的“一只兔子”阿兹特克大干旱、1785年至1786年的“饥荒之年”干旱，以及1909年至1910年墨西哥革命前夕的干旱事件。

厄尔尼诺-南方涛动（El Niño/Southern Oscillation, ENSO）是MXDA检测到的影响水分变率的最主要海气强迫因子。研究结果显示，北美地区与赤道中太平洋海表温度（sea surface temperature, SST）关联性最强的土壤水分平衡区或许位于墨西哥北部。该ENSO信号的相关性更强且时间稳定性更高，优于大西洋多年代际振荡（Atlantic Multidecadal Oscillation）与太平洋年代际振荡（Pacific Decadal Oscillation）的相关结果。
扩展多变量ENSO指数与墨西哥北部重建的scPDSI相关性最高，暖事件会使该区域冬季、春季及初夏的湿度条件偏高。这一针对墨西哥北部的ENSO遥相关在过去150年间始终显著，但在墨西哥中部与南部，该遥相关则相对较弱且具有非稳态特征，该区域的热带东太平洋与加勒比/热带大西洋SST的影响似乎更为显著。现有器测PDSI数据中，针对墨西哥北部的ENSO遥相关强度较弱，但基于社区地球系统模型（Community Earth System Model, CESM）的千年气候模拟分析显示，墨西哥北部冬季、春季及初夏的水分平衡或许确实对ENSO强迫尤为敏感。
研究表明，随着人为气候变化加剧，全国性干旱的发生概率或将提升，但MXDA重建结果显示，过去600年间“全墨西哥”级别的极端干旱十分罕见，且近数十年来其发生频率并未出现显著增长。