Modulations of climate variability on global and Southern Ocean changes
Date
2021
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
The historical global surface temperature (GST) exhibits staircase-like evolutions with both accelerated warming periods and warming slowdown periods. The rate of GST change slowed down during 2003-2012, relative to the warming acceleration in the late 20th century and is termed as the global warming slowdown period. Since 1850, other global warming slowdown periods have also been identified. The proposed explanations for this warming slowdown can be understood as whether it is caused by the external forcing (e.g. the increased volcano activities, the increased aerosols and decreased water vapor concentration) or it is due to the internal variability from the Pacific Ocean, the Indian Ocean, or the Atlantic Ocean. This study reviews the observed historical records, previous analysis, and offers interpretations of coupled climate model experiments, reconciles the proposed mechanisms and quantifies their relevant contributions to the GST evolution. ☐ The first section of this study focuses on the surface signal, the evolution of GST, and the main drivers for the GST change in different time scales. In this section, the Ensemble Empirical Mode Decomposition (EEMD) method is applied to the observed GST time series for the period of 1850-2020, which is decomposed into a group of signals respectively on inter-annual (< 8 years), inter-decadal (9-20 years) and multi-decadal (60-80 years) timescales as well as a non-linear secular trend. Both the inter-annual and inter-decadal signals in GST can be linked to the El Niño-Southern Oscillation (ENSO)-like variability in the Pacific Ocean. In contrast, the multi-decadal signal in GST is in phase with the Atlantic Multi-decadal Oscillation (AMO) and the associated sea surface temperature (SST) patterns resemble the AMO in the North Atlantic and its trans-basin footprints in the other oceans. In order to assess the respective roles of the Pacific, the Atlantic, and the external forcing in driving the GST change in historical records, a suit of Pacemaker experiments has been examined. It has been revealed that the Pacific is the main driver of the GST change on inter-annual and inter-decadal time scales. The Atlantic also contributes to the GST change on inter-decadal time scales through the atmosphere bridge teleconnection. The external forcing is capable of reproducing the timing and phase of the GST change and can successfully reproduce the observed multidecadal variability (MDV) spatial pattern. The results further suggest that the prolonged hiatus periods of ~30 years (1945-1975) are mainly attributed to the multi-decadal signal, whereas the relatively short-term hiatus events, such as the recent one over 2003-2012, are mainly caused by the inter-decadal signal. The findings in this section reconcile the debate on whether the Atlantic Ocean or the Pacific plays a critical role in modulating the GST changing rates. ☐ The second section of this study emphasizes the energy redistribution in the Southern Ocean which also modulates the GST changes addressed in the first section. The recent global surface warming slowdown is associated with an increased heat uptake in the deep ocean, particularly, the Southern Hemisphere oceans experienced rapid warming during the decadal long global surface warming slowdown (2003–2012) and the earlier Argo period over 2006–2013. However, in this section, updated observations are examined to show that this rapid warming has slowed down, leading to less contribution of the Southern Hemisphere oceans to the global ocean heat storage (~65% over the available Argo period 2006–2019). Two warming hotspot regions, the Southeast Indian Ocean and the South Pacific Ocean have experienced cooling over 2013–2019. This decadal shift is related to variations in the Southern Annular Mode (SAM) and Interdecadal Pacific Oscillation (IPO). It has been further suggested that the isopycnal deepening (shoaling) forced by changing winds dominated the regional ocean temperature changes over the earlier warming (later cooling) period. The finding in this section demonstrates how decadal variability modulates long-term climate change and provides important observational information for the ongoing calibration of decadal prediction systems. ☐ In the third section of this study, the role of Pacific forcing in enhancing the early 21st-century rapid warming in the Southern Ocean is further investigated. Using CESM large ensemble (CESM-LE) experiment and Pacific Pacemaker (CESM-PAC) experiments help to better understand the contributions of internal forcing associated with the eastern tropical Pacific SST change and impacts of external forcing on the Southern Ocean heat content change. Consistent with ocean reanalysis, the tropical Pacific forced signals can reproduce a pronounced La Niña-like warming pattern with enhanced warming in the Southeast Indian Ocean (SEIO) and South Pacific Ocean (SPAC), and cooling in the eastern Pacific Ocean. Around 1/3 of the peaks of warming in the mid-latitude of Southern Hemisphere oceans warming is estimated to be controlled by the tropical Pacific forcing through Pacific teleconnection. On the contrary, the external forcing performed by the CESM-LE experiment can only generate uniform warming, indicating the tropical Pacific is the main contributor to the rapid Southern Ocean warming during 2003-2012. Furthermore, the Empirical Orthogonal Function (EOF) decomposition method is utilized to identify the dominant spatial mode of ocean heat variability in the Southern Hemisphere oceans. The extracted first mode explaining 24% of the variance is the most influential mode related to the tropical interdecadal Pacific Oscillation (IPO) and is consistent with the CESM-PAC simulation. The second mode is related to the Southern Annular Mode (SAM). These two modes show that both SAM and IPO are the drivers of this Southern Ocean warming slowdown since 2013. ☐ This study reconciles the explanations of whether the external forcing, the Atlantic Ocean or the Pacific Ocean plays a vital role in modulating the GST changing rates. And it further demonstrates how the Pacific together with the SAM drives the decadal variability in the Southern Ocean. Overall, this study provides important observational information for the ongoing calibration of climate prediction systems.
Description
Keywords
Climate model, Global surface temperature, Ocean heat content, SAM, Southern Ocean warming