In this study, we report significant biennial variability or oscillation (BO) in the boreal spring (March–May) Surface Air Temperature (SAT) over India and unravelled the causative mechanisms. The positive phase of the BO exhibit significant seasonal warming over India, whereas seasonal cooling is observed during the negative phase of BO. Heat wave days are more during the positive phase of BO compared to negative or neutral phase. The positive (negative) phase of BO is generally coherent with the central (eastern) Pacific warming (cooling) years. The anomalous low-level divergence associated with low-level anticyclonic circulation induces less cloudiness and intense surface solar radiation ovr India during the positive phase, favouring surface warming. The evolution of some positive and/or negative phases of BO without any large scale forcing from the equatorial Pacific suggested the possibility of alternate pathways. The strong anomalous upper-level (at 200 hPa) anticyclonic circulation provoked by mid-latitude Rossby waves is found contributing to the positive phase, thereby highlighting the role of dominant mid-latitude pathways in the biennial SAT variability in addition to El Niño forcing. The sinking motion associated with persistent high, and the associated adiabatic compression also supported surface heating during the positive phase of BO. On the other hand, the mid-latitude Rossby wave induced upper-level cyclonic circulation is found contributing to the negative phase. The sinking motion associated with persistent high, and the associated adiabatic compression also supported surface heating during the positive phase of BO. In contrast, negative soil temperature anomalies and high latent heat flux release to the atmosphere supported surface cooling during the negative phase.

中文翻译：

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印度北方春季表面气温的两年变率


在这项研究中，我们报告了印度北方春季（3 月至 5 月）地表气温 （SAT） 的显着两年变率或振荡 （BO），并揭示了致病机制。BO 的正相在印度表现出显着的季节性变暖，而在 BO 的负相期间观察到季节性降温。与负或中性阶段相比，BO 正阶段的热浪天数更多。BO 的正（负）相通常与太平洋中部（东部）变暖（冷却）年一致。与低层反气旋环流相关的异常低层散度在正阶段导致印度云量减少和强烈的表面太阳辐射，有利于地表变暖。BO 的一些正和/或负相的演变没有来自赤道太平洋的任何大规模强迫，这表明可能存在替代途径。中纬度罗斯比波引发的强烈异常高层（200 hPa）反气旋环流被发现有助于正相，从而突出了除厄尔尼诺强迫外，中纬度路径在两年一次的 SAT 变率中的作用。与持续高温相关的下沉运动和相关的绝热压缩也支持 BO 正相期间的表面加热。另一方面，发现中纬度罗斯比波引起的高层气旋环流导致了负相。与持续高温相关的下沉运动和相关的绝热压缩也支持 BO 正相期间的表面加热。 相比之下，负土壤温度异常和高潜热通量释放到大气中支持负相期间的地表冷却。