NOAA/WDS Paleoclimatology - Villanueva - Fule Cuevecillas - PSME - ITRDB MEXI080

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.

墨西哥自有史乃至史前时期以来，便长期遭受持续性严重干旱的困扰。墨西哥地区的干旱事件受大西洋与太平洋的海气变率调控，这为开展长期季节气候预测提供了可行路径，而此类预测可有效缓解未来干旱期带来的经济与社会负面影响。1920年之前，墨西哥气候的器测观测记录极为有限，但依托该国老龄林开发的树轮年表，可高精度重建过去干湿状况的空间格局与强度，为气候动力学及气候影响评估研究提供优质代用资料。
研究团队基于墨西哥境内及周边地区共计252条气候敏感型树轮年表，构建了墨西哥干旱图集（Mexican Drought Atlas, MXDA）。该数据集的重建序列时间跨度为公元1400年至2012年，校准过程以1944—1984年的器测夏季（6—8月，JJA）自校准帕默尔干旱强度指数（self-calibrating Palmer Drought Severity Index, scPDSI）为基准，基于覆盖14°N—34°N、75°W—120°W陆地区域的0.5°经纬网格，采用集合逐点回归（Ensemble Point-by-Point Regression, EPPR）方法完成。针对1920—1943年时段，研究人员利用该时段有限气象站点观测得到的网格化器测scPDSI数据，对网格点重建结果进行了独立验证。
MXDA为探究过去600年间墨西哥干湿极端事件的历史影响提供了全新的空间视角，其中包括1454年阿兹特克历法“一兔年”大干旱（Aztec Drought of One Rabbit）、1785—1786年的“饥饿之年（El Ano de Hambre）”干旱，以及1909—1910年墨西哥革命爆发前的干旱事件。
厄尔尼诺/南方涛动（El Niño/Southern Oscillation, ENSO）是MXDA检测到的对干湿变率影响最为显著的海气强迫因子。研究重建结果显示，北美地区土壤水分平衡与赤道中太平洋海表温度（sea surface temperature, SST）的最强遥相关，可能位于墨西哥北部。该ENSO信号的相关性较大西洋多年代际振荡及太平洋年代际振荡更强，且时间稳定性更优。扩展多变量ENSO指数与墨西哥北部重建的scPDSI相关性最高，暖事件会使得该区域冬季、春季及初夏降水偏多。这种ENSO与墨西哥北部的遥相关在过去150年间始终强劲，但在墨西哥中部与南部，该遥相关则相对较弱且非稳态，此时热带东太平洋及加勒比/热带大西洋SST的影响更为显著。现有器测PDSI数据中，ENSO与墨西哥北部的遥相关强度较弱，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、春季及初夏的土壤水分平衡，确实对ENSO强迫尤为敏感。
研究预测，随着人为气候变化加剧，全国性干旱事件将愈发频发，但MXDA的重建结果表明，过去600年间高强度的“全墨西哥”干旱事件较为罕见，且近几十年来其发生频率并未出现显著提升。