Yun Yang, Lixin Wu, Ying Guo, Bolan Gan, Wenju Cai, Gang Huang, Xichen Li, Tao Geng, Zhao Jing, Shujun Li, Xi Liang and Shang-Ping Xie.
August, 2021.
Abstract
Variability of North Tropical Atlantic (NTA) sea surface temperature (SST), characterized by a near-uniform warming at its positive phase, is a consequential mode of climate variability. Modulated by El Niño–Southern Oscillation (ENSO) and the North Atlantic Oscillation, NTA warm anomalies tend to induce La Niña events, droughts in Northeast Brazil, increased frequency of extreme hurricanes, and phytoplankton blooms in the Guinea Dome. Future changes of NTA variability could have profound socioeconomic impacts yet remain unknown. Here, we reveal a robust intensification of NTA variability under greenhouse warming. This intensification mainly arises from strengthening of ENSO-forced Pacific-North American pattern and tropospheric temperature anomalies, as a consequence of an eastward shift of ENSO-induced equatorial Pacific convection and of increased ENSO variability, which enhances ENSO influence by reinforcing the associated wind and moist convection anomalies. The intensification of NTA SST variability suggests increased occurrences of extreme NTA events, with far-reaching ramifications.
INTRODUCTION
Variability of North Tropical Atlantic (NTA) sea surface temperature (SST) (hereafter the NTA) typically peaks in boreal spring (March, April, and May, MAM1, where “1” refers to the current year) and is characterized by basin-wide SST warming at its positive phase (1, 2). Coupled with a latitudinal movement of the intertropical convergence zone, the NTA profoundly influences precipitation in Northeast Brazil and Sahel (3–6). An anomalous NTA warming during 1979 to 1981 induced severe drought in Northeast Brazil, leading to a more than 70% reduction in production of rice, beans, and cotton (7). An intense NTA warming contributed to the 2012 to 2016 drought in Northeast Brazil, affecting 33.4 million people and resulting in losses of U.S. $30 billion (8). By modulating the intensity, number, and track pattern of Atlantic tropical cyclones, an NTA warm event increases the number of major hurricanes and their landfall frequency along the U.S. East Coast (9–12). Moreover, the NTA has a prominent influence on the Guinea Dome (13), chlorophyll-ɑ concentration, and ecosystems (14, 15). In addition to regional influences, the NTA exerts its climatic impacts over the globe, including El Niño–Southern Oscillation (ENSO) in the Pacific (16), sea ice distribution in the Antarctic (17), and anomalous global mean temperature (18). Because of these severe effects, determining the response of NTA to greenhouse warming is an issue of great importance.
The NTA mainly arises from latent heat flux anomalies associated with anomalous northeasterly trades (19–21). Specifically, the wind-evaporation-SST feedback (22), mainly confined to the deep tropics, contributes to the development of anomalous SST (23, 24), but forcings outside of the tropical Atlantic are required to reinforce the NTA temperature anomaly (2, 25, 26). The North Atlantic Oscillation (NAO) is one such forcing (27, 28). During a negative NAO event, the northeasterly trades weaken in response to a slackened Subtropical High, reducing surface latent heat flux and leading to an anomalous NTA warming. Another important forcing comes from ENSO (29, 30), which exerts its influence in several pathways. During El Niño, tropospheric temperature over the central and eastern equatorial Pacific increases and propagates eastward in the form of equatorial Kelvin waves, which reduces moist convection over the northern tropical Atlantic and gives rise to warm SST anomalies, a process referred to as “tropospheric temperature” mechanism (20, 31). Furthermore, convective anomalies associated with increased precipitation in the western and central equatorial Pacific excite the Pacific–North American (PNA) pattern (32), a Rossby wave train with a ridge over the western North America and troughs over the Aleutians and the southeastern United States. In response, the northeasterly trades weaken over the northern tropical Atlantic, reducing evaporation and generating warm SST anomalies. In addition, El Niño induces a negative diabatic heating over the Amazon basin, which, in turn, generates an anomalous Atlantic Hadley circulation (33) and a Gill-type response (34), contributing to the weakened northeasterly trades.
Despite the advances described above, how the NTA may respond to greenhouse warming remains unknown. Below, we show that most of the Coupled Model Intercomparison Project Phase 6 (CMIP6) models simulate an increase in NTA variability.
