Strengthening of the Walker Circulation in recent decades and the role of natural sea surface temperature variability

The recent borderline El Niño events of 2014/2015 and 2018/2019 provided operational centers with unique challenges because of the apparent absence of typical coupling between the tropical atmosphere and ocean before onset. The mismatch between atmosphere and ocean raises questions about its causes and predictability. Here we analyze observational data since 1979 to show that a sea surface temperature pattern characterized by an anomalous gradient in the western and central equatorial Pacific played a critical role in inhibiting the expected onset of central tropical Pacific deep convection during these events. This sea surface temperature pattern, which produces an atmospheric response that opposes the response to elevated eastern Pacific sea surface temperatures, has become more prevalent over the past 40 years. Plain Language SummaryThe El Niño-Southern Oscillation, a naturally occurring climate pattern that impacts most of the planet, develops through interactions between the tropical atmosphere and ocean. During the development of recent borderline El Niño episodes in 2014 and 2018, however, the tropical atmosphere failed to behave in a typical manner despite conducive oceanic conditions, providing forecasters with challenges in classifying the events and in forecasting their future development and global impacts. A key question is whether forecasters could have anticipated the atypical tropical atmospheric pattern or if it was the result of chaotic weather variability that could not have been forecast with much advance warning. Here we analyze 40 years of observational data and find that a sea surface temperature pattern in the western and central equatorial Pacific offsets the typical El Niño impacts during these events. This sea surface temperature pattern has become more prevalent over the past 40 years, which indicates a recent tendency for El Niño tropical atmospheric conditions to be weaker relative to past episodes. The identification of this characteristic sea surface temperature pattern provides hope that forecasters may be able to anticipate when it may reinforce or offset the atmospheric conditions associated with the El Niño-Southern Oscillation.

Section: Discussionsupporting

confidence: 91%

On the Delayed Coupling Between Ocean and Atmosphere in Recent Weak El Niño Episodes

Johnson

L’Heureux

Chang

et al. 2019

“…We finally examine trends in the ELI (Figure 5f), as prior studies have shown that ENSO is a significant driver of global TC activity (e.g., Camargo et al, 2007). The 1990-2021 period exhibited a statistically significant westward shift in the annually averaged ELI, indicating a stronger and westward-shifted Walker Circulation, as recently noted by Seager et al (2019) and Zhao and Allen (2019). This trend toward a more La Niña-like base state is consistent with observed atmospheric/oceanic pattern changes as well as the observed decreasing trend in western North Pacific ACE and increasing trend in North Atlantic ACE since 1990.…”

Section: Large-scale Drivers Of Observed Trendsmentioning

confidence: 85%

Trends in Global Tropical Cyclone Activity: 1990–2021

Klotzbach

Wood

Schreck

et al. 2022

This study investigates global tropical cyclone (TC) activity trends from 1990 to 2021, a period marked by largely consistent observational platforms. Several global TC metrics have decreased during this period, with significant decreases in hurricane numbers and Accumulated Cyclone Energy (ACE). Most of this decrease has been driven by significant downward trends in the western North Pacific. Globally, short‐lived named storms, 24‐hr intensification events of ≥50 kt day−1, and TC‐related damage have increased significantly. The increase in short‐lived named storms is likely due to technological improvements, while rapidly intensifying TC increases may be fueled by higher potential intensity. Damage increases are largely due to increased coastal assets. The significant decrease in hurricane numbers and global ACE are likely due to the trend toward a more La Niña‐like base state from 1990 to 2021, favoring North Atlantic TC activity and suppressing North and South Pacific TC activity.

“…An in‐depth analysis of these causes is beyond the scope of this paper, but we discuss some possible explanations found in the current literature. One of the reasons for the diminished DIC ′ variability may be related to the fact that models simulate a weaker Walker circulation in response to global warming (Vecchi et al., 2006; Zhao & Allen, 2019). A weaker Walker circulation would weaken the upwelling of DIC‐rich waters during La Niña conditions.…”

Section: Resultsmentioning

confidence: 99%

Anthropogenic Intensification of Surface Ocean Interannual pCO₂ Variability

Gallego

Timmermann

Friedrich

et al. 2020

We use several global coupled atmosphere‐ocean‐biogeochemistry models from the Coupled Model Intercomparison Project (CMIP5) to show that the global interannual variability of the sea surface pCO2 (calculated as 1σ) will increase by ∼64 ± 20% by 2040–2090 relative to the beginning of the industrial revolution under the RCP8.5 scenario. All models agree that the increase in variability is a consequence of a larger background pCO2 and a lower buffering capacity that enhance the response of pCO2 to the fluctuations of surface temperature (T) and dissolved inorganic carbon (DIC). The most skillful group of models under present‐day conditions shows a future global decrease in DIC fluctuations that will weaken the pCO2 interannual variability (IAV). The remaining uncertainties in the projected evolution of pCO2 variability regionally highlight the need for continuous carbon monitoring programs which will contribute to a better understanding of the oceanic carbon sink's response to increased green house emissions.