The Role of Ocean Dynamics in the Cross-equatorial Energy Transport under a Thermal Forcing in the Southern Ocean

Abstract

Under external heating forcing in the Southern Ocean, climate models project anomalous northward atmosphere heat transport (AHT) across the equator, accompanied by a southward shift of the intertropical convergence zone (ITCZ). Comparison between a fully coupled and a slab ocean model shows that the inclusion of active ocean dynamics tends to partition the cross-equatorial energy transport and significantly reduce the ITCZ shift response by a factor of 10, a finding which supports previous studies. To understand how ocean dynamics damps the ITCZ’s response to an imposed thermal heating in the Southern Ocean, we examine the ocean heat transport (OHT) and ocean circulation responses in a set of fully coupled experiments. Results show that both the Indo-Pacific and the Atlantic contribute to transport energy across the equator mainly through its Eulerian-mean component. However, different from previous studies that linked the changes in OHT to the changes in the wind-driven subtropical cells or the Atlantic meridional overturning circulation (AMOC), our results show that the cross-equatorial OHT anomaly is due to a broad clockwise overturning circulation anomaly below the subtropical cells (approximately bounded by the 5°C to 20°C isotherms and 50°S to 10°N). Further elimination of the wind-driven component, conducted by prescribing the climatological wind stress in the Southern Ocean heat perturbation experiments, leads to little change in OHT, suggesting that the OHT response is predominantly thermohaline-driven by air-sea thermal interactions.

摘要

当南大洋受到外界热强迫的扰动时, 气候模型通常能够模拟出跨赤道的大气热输运异常, 并伴随着热带辐合带 (ITCZ) 位置的偏移. 对比全耦合模型和混合层海洋模型的实验结果发现, 海洋动力过程的引入能够产生极强的跨赤道海洋热输运 (OHT) 异常, 从而大大减弱大气跨赤道能量输运的负担, 同时抑制了 ITCZ 的偏移幅度. 前人的研究认为该 OHT 异常的产生与副热带海洋环流圈 (STC) 或者大西洋经向翻转环流的变化有关. 为了探究海洋动力学如何抑制 ITCZ 对南大洋热强迫的响应, 本研究在全耦合模型中对南大洋海域施加理想的热强迫探究 OHT 和海洋环流的变化, 发现在印度-太平洋海盆和大西洋海盆都存在跨赤道 OHT 异常, 该 OHT 异常源于 STC 下方广泛分布的顺时针环流异常 (等温线 5° C −20° C, 50° S−10° N). 本研究进一步地通过风应力部分耦合实验, 揭示了该环流异常主要受海气热交换过程的控制而并非通常认为的风应力过程驱动, 成功地阐明了南大洋热吸收通过海洋动力过程影响低纬度气候的机制.