ABSTRACT

The surface warming of northern continents during the twentieth century is not uniform across seasons. Surface warming is particularly pronounced over northwestern Canada, where winter trends are much larger than summer ones. The upper-air temperature trends over the region are analyzed in three radiosonde datasets from 1958 to 2012 to assess their seasonal structure. The seasonal variation of upper-air trends can provide insights into the dynamical and thermodynamical processes generating these trends, including warming at the surface. The focus is not on the canonical structure of secular (i.e., long-term) trends—tropospheric warming and stratospheric cooling—but its seasonal variation. We find the boreal winter-minus-summer difference in trends over northwestern Canada to be positive and large in the lower troposphere (p ≳ 500 hPa) and lower stratosphere (50 hPa ≲ p ≲ 150 hPa); it is largest at the surface and smallest at the tropopause. The decreasing seasonality of the tropospheric trend with height supports the attribution of the notable seasonality of surface warming in this region to both land–surface–hydroclimate interactions and changes in winter circulation. In the lower stratosphere, a cooling trend is evident in all seasons, not unexpectedly, but a pronounced seasonality is again apparent, with the strongest cooling in summer. The near-zero trend tropopause region is a rare point of confluence for seasonal trends.

摘要

二十世纪北部大陆的地表变暖在各个季节都不均匀。在加拿大西北部，地表变暖尤其明显，那里的冬季趋势比夏季的趋势要大得多。在1958年至2012年的三个无线电探空仪数据集中分析了该地区的高层气温趋势，以评估其季节性结构。高空趋势的季节性变化可以洞悉产生这些趋势的动力学和热力学过程，包括地表变暖。重点不是长期（即长期）趋势的标准结构（对流层变暖和平流层变冷），而是季节性变化。我们发现加拿大西北部地区冬季的北-北-北-北-北-北-北-北-北-北-北-北-南-北-北-北-北-北-北-北-北p  ≳500百帕）和较低的平流层（50百帕≲  p  ≲150百帕）; 它在表面最大，在对流层顶最小。随着高度的增加，对流层趋势的季节性下降支持该地区表面变暖的明显季节性归因于土地-地-水气候相互作用和冬季环流的变化。在平流层下部，所有季节都有明显的降温趋势，这并非出乎意料，但明显的季节性又明显，夏季降温最强。接近零趋势的对流层顶区域是季节性趋势汇合的罕见点。