Abstract
The wind–evaporation–SST (WES) feedback describes a coupled mechanism by which an anomalous meridional sea surface temperature (SST) gradient in the tropics evolves over time. As commonly ...posed, the (positive) WES feedback depends critically on the atmospheric response to SST anomalies being governed by a process akin to that argued by Lindzen and Nigam in 1987, and omits an alternative process by which SST anomalies modulate surface wind speed through vertical momentum mixing as proposed by Wallace et al. and Hayes et al. in 1989. A simple model is developed that captures the essential coupled dynamics of the WES feedback as commonly posed, while also allowing for momentum entrainment in response to evolving SST anomalies. The evolution of the coupled system depends strongly on which effects are enabled in the model. When both effects are accounted for in idealized cases near the equator, the initial anomalous meridional SST gradient grows over a time scale of a few months but is damped within one year. The sign and magnitude of the WES feedback depend on latitude within the tropics and exhibit hemispheric asymmetry. When constrained by realistic profiles of prevailing zonal wind, the model predicts that the WES feedback near the equator is stronger during boreal winter, while the domain over which it is positive is broader during boreal summer, and that low-frequency climate variability can also modulate the strength and structure of the WES feedback. These insights may aid in the interpretation of coupled climate behavior in observations and more complex models.
Significance Statement
Regional climate variability on time scales from months to decades, including El Niño, relies heavily on feedbacks between the atmosphere and the ocean in which some initial change in the environment is either amplified or damped over time. Several conceptual models for such feedbacks have been devised over the years to explain the coupled climate behavior seen in observations and computer simulations. A rather ubiquitous one is called the wind–evaporation–SST (WES) feedback, but the typical phrasing of it does not incorporate a potentially important influence of ocean temperature changes on the stability of the atmosphere above it. This study adds that effect to the WES feedback framework and examines climate variability through the lens of the augmented conceptual model.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Widespread coral bleaching across the Great Barrier Reef (GBR) in 2016 is often reportedly caused by El Niño and/or global warming. However, the GBR is not in a region where it is straightforward to ...anticipate sea surface temperature (SST) warming during El Niño, and the role of climate change is unclear. This study uses a diverse range of observations to investigate the physical causes of SST anomalies that developed on the GBR in 2016. Warm SST anomalies developed in two stages. Initial warming was caused by El Niño shifting the global‐scale pattern of convection, increasing solar radiation in the Coral Sea. The warm anomaly was extended and amplified near the coast by a terrestrial heat wave propagating across eastern Australia, further warming the GBR through turbulent heat flux. It is concluded that El Niño caused the SST anomaly, and global warming increased its amplitude and extended it by several months.
Key Points
A quantitative heat budget analysis of the 2016 marine heat wave in the Great Barrier Reef lights a path to large‐scale climate processes
The waning El Niño of 2015/2016 warmed regional seas with solar radiation; a terrestrial heat wave in Australia intensified coastal warming
Long‐term trends in marine and terrestrial records clarify role of global warming, widespread coral mortality in 2016 unlikely without it
Following the recent discovery of the “Modoki” El Niño, a proliferation of studies and debates has ensued concerning whether Modoki is dynamically distinct from “Canonical” El Niño, how Modoki ...impacts and teleconnections differ, and whether Modoki events have been increasing in frequency or amplitude. Three decades of reliable, high temporal‐resolution observations of coupled ocean‐atmosphere variability in the equatorial Pacific reveal a rich diversity of El Niños. Although central and eastern Pacific sea surface temperature (SST) anomalies appear mechanistically separable in terms of local and remote forcing, their frequent overlap precludes robust classifications. All observed El Niños appear to be a mixture of locally (central Pacific) and remotely forced (eastern Pacific) SST anomalies. Submonthly resolution appears essential for this insight and for the proper dynamical diagnosis of El Niño evolution; thus, the use of long‐term monthly reconstructions for classification and trend analysis is strongly cautioned against.
Key Points
Reliable weekly measurements of SST and other variables are analyzed
Central and eastern Pacific SST anomalies are mechanistically distinguishable
All El Niños are mixtures of central and eastern Pacific SST anomalies
The effects of externally forced tropical sea surface temperature (SST) anomalies on long-term Walker circulation changes are investigated through numerical atmospheric general circulation model ...(AGCM) experiments. In response to the observed tropics-wide SST trend, which exhibits a prominent interbasin warming contrast (IBWC) with smaller warming in the Pacific than the Indian and Atlantic Oceans that includes a weak La Niña–like pattern in the equatorial Pacific, pronounced low-level easterly anomalies emerge over the equatorial Pacific. Through sensitivity experiments, the intensification of the Pacific trade winds (PTWs) is attributable to the IBWC, whereas the slightly enhanced zonal SST gradient within the equatorial Pacific plays a small role relative to the observed IBWC. It is further demonstrated that the greater Indian Ocean warming forces low-level easterly anomalies over the entire equatorial Pacific, while the greater tropical Atlantic warming-driven enhancement of PTWs is located over the central equatorial Pacific. In contrast to observations, a negligible IBWC emerges in the tropical SST trends of CMIP5 historical simulations due to a strong El Niño–like warming in the tropical Pacific. Lacking the observed IBWC (and the observed enhancement of the zonal SST gradient within the equatorial Pacific), the PTWs in the CMIP5 ensemble can only weaken.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The double Intertropical Convergence Zone bias remains a persistent problem in coupled general circulation model simulations. Due to the strong sea surface temperature (SST)‐convection relationship ...in the tropics, precipitation biases are sensitive to background SST. Using historical simulations of 24 coupled general circulation models and an atmospheric general circulation model, we show that cold equatorial SST biases at least exacerbate double Intertropical Convergence Zone biases in the Pacific. A linear regression model is used to demonstrate that improved predictability of precipitation trends is possible with such model‐dependent information as mean‐state SST biases accompanying projected SST trends. These results provide a better understanding of the root of the double Intertropical Convergence Zone bias and a possible path to reduced uncertainty in future tropical precipitation trends.
Plain Language Summary
Dozens of complex computer models of the climate system (accounting for both atmosphere and ocean) are used for climate change predictions over the coming decades as anthropogenic emissions of greenhouse gases persist. These computer models are essential yet imperfect. Two well‐known mismatches (i.e., biases) in models that have persisted for many years include (1) too cold sea surface temperature (SST) along the equatorial Pacific Ocean and (2) excessive precipitation south of the equator, which appears as a double‐peaked precipitation pattern known as the double Intertropical Convergence Zone (ITCZ). These are commonly referred to as the “cold tongue bias” and “double ITCZ bias,” respectively. Our analysis confirms that they are closely related in models; the worse the cold tongue bias is, the more the ITCZ is split into two. We also found that predicted trends in tropical rainfall depend on these biases; models with the least severe SST bias will improve relative to today's climate as SST warms, whereas the double ITCZ in models with the most severe biases will remain the same even as equatorial SSTs warm. Finally, we demonstrate that rainfall predictions can be improved if information about biases is accounted for.
Key Points
Equatorial Pacific sea surface temperature bias contributes to double ITCZ bias in climate models
Interpretation of future tropical rainfall projections requires care as double ITCZ bias affects precipitation trends
Improvement of bias in equatorial Pacific sea surface temperature may reduce uncertainty in rainfall projections
Equatorial islands have distinct oceanographic signatures, including cool sea surface temperature and high productivity immediately to their west. It has long been hypothesized that topographic ...upwelling is responsible for such characteristics—upward deflection by the islands of the eastward‐flowing equatorial undercurrent (EUC). Using 22 years of in situ measurements by Argo, we provide the first direct observations of this process occurring with consistency at two prominent archipelagos in the equatorial Pacific. Argo measurements resolve a clear subsurface thermal fingerprint of vertical divergence at the depth of the EUC, confined to within 100 km of both the Gilbert (∼175°E) and Galápagos Islands (∼90°W). This signal at the Galápagos is well‐reproduced by a high‐resolution ocean reanalysis, enabling the estimation of vertical velocities balancing the zonal convergence of the EUC upon the islands. This sharpened view of the physics underpinning such important tropical ecosystems has implications for strategies to model and predict them.
Plain Language Summary
Upwelling fuels marine life by bringing nutrient‐rich water into the sunlit surface layer. Upwelling can also impact the atmosphere, because the water brought to the surface is colder, which affects the wind and clouds. The surprising abundance and diversity of life—from corals to fish to seabirds—around equatorial islands are thought to be caused by a unique form of upwelling. A global array of thousands of floats called Argo is used to reveal the structure of this unique form of upwelling. Unlike most upwelling zones (e.g., California), which are driven directly by the wind combined with Earth's rotation, upwelling at equatorial islands is shown here to be due to a massive underwater current that flows along the equator colliding with the islands. The results are corroborated with a high‐resolution model and recent data from underwater gliders near the Galápagos. These insights have immediate implications for predictions of how such important tropical ecosystems will change in the future.
Key Points
Twenty‐two years of Argo profiles reveal the localized topographic upwelling signal west of the two equatorial archipelagos
The observed thickening of the thermocline west of the Galápagos is associated with the localized mean upwelling velocity of 12 m per day
The blockage of the equatorial undercurrent by islands contributes to the productivity of waters west of the equatorial archipelagos
Tropical cyclogenesis in the Atlantic is influenced by environmental parameters including vertical wind shear, which is sensitive to forcing from the tropical Pacific. Reliable projections of the ...response of such parameters to radiative forcing are key to understanding the future of hurricanes and coastal risk. One of the least certain aspects of future climate is the warming of the eastern tropical Pacific Ocean. Using climate model experiments isolating the warming of the eastern Pacific and controlling for other factors including El Niño‐Southern Oscillation (ENSO), changes in Atlantic tropical cyclogenesis potential by the end of this century are ∼20% lower with enhanced eastern Pacific warming. The ENSO signal in Atlantic tropical cyclogenesis potential amplifies with global warming, and that amplification is larger with enhanced eastern Pacific warming. The largest changes and dependencies on eastern Pacific warming are found in the south‐central main development region, attributable to changes in zonal overturning.
Plain Language Summary
Today, there are about 15 tropical storms per year in the Atlantic. That number varies considerably from year to year, with El Niño being one major factor. When the eastern Pacific Ocean warms temporarily, Atlantic hurricanes tend to be suppressed (and vice versa for La Niña). As the climate warms due to greenhouse gas emissions, hurricanes are expected to change. Such changes could include the average number of tropical storms per year, where they tend to form, how strong they become, how far and fast they travel, how much rain they produce, and how El Niño affects them. This study investigates how the formation regions of Atlantic hurricanes may change in the future, particularly as a function of how much the eastern Pacific Ocean warms in the future, which is one of the most uncertain aspects of climate change. We find that the warming of the eastern Pacific strongly influences predictions of future changes in Atlantic hurricanes, including how El Niño affects them. Specifically, a strong eastern Pacific warming causes a change in the winds over the tropical Atlantic, which shifts where hurricanes will tend to form in the future, and increases the effect of El Niño.
Key Points
Enhanced surface warming in the eastern equatorial Pacific Ocean impacts the response of Atlantic hurricanes to global warming
Genesis potential decreases in the south‐central part of the main development region, but only with enhanced eastern Pacific warming
The El Niño/La Niña signal in Atlantic genesis potential amplifies with global warming—more so with enhanced eastern Pacific warming
A greater warming trend of sea surface temperature in the tropical Indian Ocean than in the tropical Pacific is a robust feature found in various observational data sets. Yet this interbasin warming ...contrast is not present in climate models. Here we investigate the impact of tropical Indian Ocean warming on the tropical Pacific response to anthropogenic greenhouse gas warming by analyzing results from coupled model pacemaker experiments. We find that warming in the Indian Ocean induces local negative sea level pressure anomalies, which extend to the western tropical Pacific, strengthening the zonal sea level pressure gradient and easterly trades in the tropical Pacific. The enhanced trade winds reduce sea surface temperature in the eastern tropical Pacific by increasing equatorial upwelling and evaporative cooling, which offset the greenhouse gas warming. This result suggests an interbasin thermostat mechanism, through which the Indian Ocean exerts its influence on the Pacific response to anthropogenic greenhouse gas warming.
Key Points
The interbasin warming contrast is a robust observational feature, which is not present in climate models
The Indian Ocean warming effectively reduces the Pacific warming response to anthropogenic greenhouse gases
The reason that climate models fail to capture the interbasin warming contrast is likely due to model bias
Recent Tropical Expansion Grise, Kevin M.; Davis, Sean M.; Simpson, Isla R. ...
Journal of climate,
03/2019, Letnik:
32, Številka:
5
Journal Article
Recenzirano
Odprti dostop
Previous studies have documented a poleward shift in the subsiding branches of Earth’s Hadley circulation since 1979 but have disagreed on the causes of these observed changes and the ability of ...global climate models to capture them. This synthesis paper reexamines a number of contradictory claims in the past literature and finds that the tropical expansion indicated by modern reanalyses is within the bounds of models’ historical simulations for the period 1979–2005. Earlier conclusions that models were underestimating the observed trends relied on defining the Hadley circulation using the mass streamfunction from older reanalyses. The recent observed tropical expansion has similar magnitudes in the annual mean in the Northern Hemisphere (NH) and Southern Hemisphere (SH), but models suggest that the factors driving the expansion differ between the hemispheres. In the SH, increasing greenhouse gases (GHGs) and stratospheric ozone depletion contributed to tropical expansion over the late twentieth century, and if GHGs continue increasing, the SH tropical edge is projected to shift further poleward over the twenty-first century, even as stratospheric ozone concentrations recover. In the NH, the contribution of GHGs to tropical expansion is much smaller and will remain difficult to detect in a background of large natural variability, even by the end of the twenty-first century. To explain similar recent tropical expansion rates in the two hemispheres, natural variability must be taken into account. Recent coupled atmosphere–ocean variability, including the Pacific decadal oscillation, has contributed to tropical expansion. However, in models forced with observed sea surface temperatures, tropical expansion rates still vary widely because of internal atmospheric variability.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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