Spatiotemporal Diversity of Tropical SST patterns in the Atlantic, Indian and Pacific Basins and Their Pantropical Interactions

Abstract:

As summarized in a recent review paper (Cai et al., 2019, Science), tropical atmosphere‐ocean processes in the Pacific, Indian and Atlantic Oceans are intimately coupled at various time scales. For instance, a rapid warming of the Indian Ocean and a warm phase of the Atlantic Multidecadal Oscillation during the past two decades have contributed to a strengthening of the Pacific trade winds, which caused an extended period of colder than normal sea surface temperatures (SSTs) in the Pacific Ocean. On interannual timescales, pantropical SST variability is dominated by the El Niño - Southern Oscillation (ENSO), which is tightly coupled to Indian Ocean variability through anomalous changes in the Walker Circulation. Extratropical atmospheric stationary waves forced by ENSO also significantly influence tropical North and South Atlantic SSTs from boreal fall in an ENSO developing year to spring in the following year. This ENSO-related rearrangement of pantropical SSTs is well captured by the first spatiotemporal empirical orthogonal function (EOF) mode of tropical Pacific SST anomalies, which describes a transition from El Niño to La Niña or La Niña to El Niño. The second EOF is characterized by two consecutive El Niño or La Niña events. The leading EOF modes of tropical Indian SST anomalies are characterized by the Indian Ocean Dipole mode in boreal fall and its transition to the Indian Ocean Basin mode in winter and spring. Both are tightly coupled and aligned with the leading modes of tropical Pacific SST anomalies. The leading modes of tropical Atlantic SST anomalies represent Atlantic Niño/Niña and its transition in boreal summer. To a large extent, these Atlantic modes are disconnected from the leading modes of Indo-Pacific SST anomalies. This suggests that pantropical SST anomalies are driven by two largely independent modes of variability: ENSO in the Indo-Pacific basin and Atlantic Niño/Niña in the Atlantic basin. Several fully coupled atmosphere-ocean model experiments are carried out using the Community Earth System Model version 2 to test this working hypothesis and to better understand the leading modes of SST variability in the Atlantic, Indian and Pacific basins and their pantropical interactions.