Large-scale climate response to regionally confined extratropical cooling: effect of ocean dynamics Climate Dynamics

This study investigates the effect of ocean dynamics on the tropical climate response to localized radiative cooling over three northern extratropical land regions using hierarchical model simulations that vary in the degree of ocean coupling. Without ocean dynamics, the tropical climate response is independent of the extratropical forcing location, characterized by a southward tropical precipitation shift with a high degree of zonal symmetry, a reduced zonal sea surface temperature gradient along the equatorial Pacific, and the eastward-shifted Walker circulation. When ocean dynamical adjustments are allowed, the zonal-mean tropical precipitation shift is damped primarily via Eulerian-mean ocean heat transport. The oceanic damping effect is strongest (weakest) for North Asian (American) cooling, associated with the largest (smallest) Eulerian-mean ocean heat transport across the equatorial Pacific. The cross-equatorial ocean heat transport in the Pacific is anchored to the North Pacific subtropical high, the response of which can be inferred from the corresponding slab ocean simulations. Hence, the slab ocean simulations provide useful a priori prediction for oceanic damping efficiency. Ocean dynamics also modulates the spatial pattern of climate response in a distinct manner depending on the zonal distribution of imposed forcing. North Asian forcing induces a pronounced eastern equatorial Pacific cooling extending to the western basin, accompanying the westward shifted Walker circulation. European forcing causes cooling confined to the eastern equatorial Pacific and strengthens the Walker circulation. The tropical precipitation response in these two cases exhibits large zonal variations with a high degree of equatorial symmetry, being essentially uncorrelated with the corresponding slab ocean simulations. By contrast, North American forcing induces a sufficiently strong inter-hemispheric contrast in the tropical Pacific SST response, due to the relatively weak oceanic damping effect, producing a weaker but spatially similar tropical response to that in the slab ocean simulation. This study demonstrates that the effect of ocean dynamics in modulating the tropical climate response depends on the extratropical forcing location. The results are relevant for understanding the distinct climate response induced by aerosols from different continental sites.

本研究采用海洋耦合程度各异的层级化模型模拟（hierarchical model simulations），探究海洋动力过程对北温带三个陆地区域局地辐射冷却响应的热带气候影响。若无海洋动力过程参与，热带气候响应与温带强迫位置无关，其特征表现为热带降水向南偏移且纬向对称性极高、赤道太平洋纬向海表温度梯度降低，以及沃克环流（Walker circulation）东移。当允许海洋动力调整时，纬向平均热带降水偏移主要通过欧拉平均海洋热输送（Eulerian-mean ocean heat transport）得到抑制。海洋的抑制效应以北亚（北美）冷却下最强（最弱），对应赤道太平洋跨区欧拉平均海洋热输送最大（最小）。太平洋跨赤道海洋热输送锚定北太平洋副热带高压，其响应可通过对应平板海洋模拟（slab ocean simulations）推断得到。因此，平板海洋模拟可为海洋抑制效率提供有效的先验预测。海洋动力过程还会根据强迫场的纬向分布，以独特方式调控气候响应的空间格局。北亚强迫会引发显著的赤道东太平洋冷却并延伸至西太平洋海盆，同时伴随沃克环流西移。欧洲强迫仅导致赤道东太平洋冷却，并增强沃克环流。上述两种情形下的热带降水响应呈现显著纬向变化且赤道对称性极高，与对应平板海洋模拟结果基本不相关。相较而言，由于海洋抑制效应相对较弱，北美强迫会引发热带太平洋海表温度响应出现显著的跨半球差异，进而产生与平板海洋模拟结果空间分布相似但强度更弱的热带气候响应。本研究表明，海洋动力过程对热带气候响应的调控效应取决于温带强迫位置。该研究结果有助于理解不同大陆区域气溶胶引发的差异化气候响应。