Climate Fluctuations of Tropical Coupled Systems—The Role of Ocean Dynamics

Abstract

The tropical oceans have long been recognized as the most important region for large-scale oceanatmosphere interactions, giving rise to coupled climate variations on several time scales. During the Tropical Ocean Global Atmosphere (TOGA) decade, the focus of much tropical ocean research was on understanding El Niño-related processes and on development of tropical ocean models capable of simulating and predicting El Niño. These studies led to an appreciation of the vital role the ocean plays in providing the memory for predicting El Niño and thus making seasonal climate prediction feasible. With the end of TOGA and the beginning of Climate Variability and Prediction (CLIVAR), the scope of climate variability and predictability studies has expanded from the tropical Pacific and ENSO-centric basis to the global domain. In this paper the progress that has been made in tropical ocean climate studies during the early years of CLIVAR is discussed. The discussion is divided geographically into three tropical ocean basins with an emphasis on the dynamical processes that are most relevant to the coupling between the atmosphere and oceans. For the tropical Pacific, the continuing effort to improve understanding of large-and small-scale dynamics for the purpose of extending the skill of ENSO prediction is assessed. This paper then goes beyond the time and space scales of El Niño and discusses recent research activities on the fundamental issue of the processes maintaining the tropical thermocline. This includes the study of subtropical cells (STCs) and ventilated thermocline processes, which are potentially important to the understanding of the low-frequency modulation of El Niño. For the tropical Atlantic, the dominant oceanic processes that interact with regional atmospheric feedbacks are examined as well as the remote influence from both the Pacific El Niño and extratropical climate fluctuations giving rise to multiple patterns of variability distinguished by season and location. The potential impact of Atlantic thermohaline circulation on tropical Atlantic variability (TAV) is also discussed. For the tropical Indian Ocean, local and remote mechanisms governing low-frequency sea surface temperature variations are examined. After reviewing the recent rapid progress in the understanding of coupled dynamics in the region, this study focuses on the active role of ocean dynamics in a seasonally locked east-west internal mode of variability, known as the Indian Ocean dipole (IOD). Influences of the IOD on climatic conditions in Asia, Australia, East Africa, and Europe are discussed. While the attempt throughout is to give a comprehensive overview of what is known about the role of the tropical oceans in climate, the fact of the matter is that much remains to be understood and explained. The complex nature of the tropical coupled phenomena and the interaction among them argue strongly for coordinated and sustained observations, as well as additional careful modeling investigations in order to further advance the current...

Global Warming Pattern Formation: Sea Surface Temperature and Rainfall*

“…The IOD is a major mode of climate variability over the tropical Indian Ocean, strongly locked in phase to the July-October season when the southeasterly alongshore winds induce upwelling on the Indonesian coast and permit Bjerknes feedback (Saji et al 1999). For recent reviews, see Chang et al (2006) and Schott et al (2009) and references therein. Surprisingly, the shoaling thermocline and enhanced thermocline feedback in the eastern equatorial Indian Ocean do not lead to an increase in IOD interannual variability because atmospheric feedback weakens in response to strengthened tropospheric stability (Zheng et al 2010).…”

“…Small changes in tropical SST nevertheless can be highly influential on climate, as illustrated by observed climate variability. For example, over the tropical Atlantic and Indian Oceans, SST anomalies on the order of only 0.58C induce atmospheric anomalies not only locally but over remote areas (see recent reviews by Chang et al 2006;Schott et al 2009). Of larger magnitude and more expansive in space, SST anomalies over the large Pacific basin associated with El Niñ o-Southern Oscillation (ENSO) have long-lasting climatic effects over the entire globe (e.g., Alexander et al 2002).…”

Tropical Atlantic biases and their relation to surface wind stress and terrestrial precipitation

“…In the latter case, these coastal biases extend far offshore, covering almost the entire subtropical South Atlantic. Much of the tropical North Atlantic features cold SST biases of up to 2K associated with excessively strong northeast trades, reminiscent of the windevaporation-SST (WES) feedback (Xie and Carton 2004;Chang et al 2006). In association with the southward gradient of the SST bias, the Intertropical Convergence Zone (ITCZ) is shifted southward during MAM.…”

“…This is particularly evident along the equator (Breugem et al 2006) where the occurrence of interannual warm events (also termed Atlantic Niños) is anchored to the seasonal shoaling of the thermocline in boreal summer. Furthermore, errors in the mean state can affect ocean-atmosphere feedbacks and thus affect both the zonal and meridional modes of TAV (Xie and Carton 2004;Chang et al 2006;Keenlyside and Latif 2007). As a result, current prediction models perform poorly in the region (Nobre and Repelli 2004;Stockdale et al 2006;Huang et al 2007).…”

Growth and decay of the equatorial Atlantic SST mode by means of closed heat budget in a coupled general circulation model

“…Tropical Atlantic Variability (TAV) have important impacts over Africa and South America, where rain-fed agriculture is an important part of their economies (Xie and Carton, 2004;Chang et al, 2006a;Kushnir et al, 2006;Rodríguez-Fonseca et al, 2015). However, historically major efforts have been done in understanding the Tropical Pacific which holds the most important inter-annual variability mode in terms of the global impacts: El Niño and the Southern Oscillation (ENSO, Philander, 1990).…”