NOAA/WDS Paleoclimatology - Griffin - Sunspot - PSME - ITRDB NM609

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）便饱受持续性严重干旱的侵扰。墨西哥境内的干旱活动受到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，而此类预报可助力缓解未来干旱事件带来的经济与社会影响。1920年之前，墨西哥气候的仪器观测记录极为有限，但依托墨西哥老龄林构建的树木年轮年表，能够出色地替代反映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研究提供有效支撑。研究团队基于墨西哥境内及周边覆盖252条对气候敏感的树木年轮年表的广泛网络，构建了墨西哥干旱图集（Mexican Drought Atlas, MXDA）。该图集的重建序列时间跨度为公元1400年至2012年，校准阶段采用1944-1984年的仪器观测夏季（6-8月，JJA）自校准帕尔默干旱严重指数（self-calibrating Palmer Drought Severity Index, scPDSI），以逐点集合回归（Ensemble Point-by-Point Regression, EPPR）方法，在覆盖14°N至34°N、75°W至120°W陆地区域的0.5°经纬度网格上开展校准。针对1920-1943年时段，研究团队利用该时段内有限气象站点观测得到的网格化scPDSI仪器数据，对网格点重建结果进行了验证。MXDA为过去600年间墨西哥境内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年的一兔年阿兹特克大干旱、1785-1786年的饥荒年干旱，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检测到的影响干湿变率的最重要海气强迫因子。研究重建结果显示，北美中部赤道太平洋海表温度（sea surface temperature, SST）与北美土壤水分平衡之间最强的遥相关关系，或许就存在于墨西哥北部地区。相较于大西洋多年代际振荡与太平洋年代际振荡，该ENSO信号的相关性更强且时间稳定性更高。扩展多变量ENSO指数与墨西哥北部地区重建的scPDSI相关性最高，暖事件会使得该区域冬季、春季及初夏的水汽条件更为湿润。这一与墨西哥北部相关的ENSO遥相关在过去150年间始终显著，但在墨西哥中部与南部地区，该遥相关则相对较弱且存在非平稳性，该区域的热带东太平洋与加勒比海/热带大西洋海表温度似乎发挥了更为关键的作用。现有仪器观测PDSI数据中，与墨西哥北部相关的ENSO遥相关强度较弱，但基于社区地球系统模型（Community Earth System Model）开展的千年气候模拟分析表明，墨西哥北部地区冬季、春季及初夏的土壤水分平衡，或许确实对ENSO强迫尤为敏感。随着人为气候变化的推进，全国性干旱预计将愈发频发，但MXDA的重建结果显示，过去600年间罕见出现大范围的“全墨西哥”型强干旱，且近数十年来这类干旱的发生频率并未出现显著提升。