Natalie J. Burls

Energetics of the Tropical Atlantic Zonal Mode

Sea surface temperature in the central-eastern equatorial Atlantic has a seasonal cycle far bigger than that of the Pacific, but interannual anomalies smaller than those of the Pacific. Given the amplitude of seasonal SST variability, one wonders whether the seasonal cycle in the Atlantic is so dominant that it is able to strongly influencetheevolutionofitsinterannualvariability.Inthisstudy,interannualupper-oceanvariabilitywithinthe tropical Atlanticis viewed from an energetics perspective, and the role ofocean dynamics, in particularthe role ofoceanmemory,withinzonalmodeeventsisinvestigated.UnlikeinthePacificwhereseasonalandinterannual variability involve distinctly different processes, the results suggest that the latter is a modulation of the former

Variability in the South Atlantic Anticyclone and the Atlantic Niño Mode

Sea surface temperature (SST) anomalies in the eastern equatorial Atlantic are connected to modulations in the strength of the South Atlantic subtropical high-pressure system, referred to as the South Atlantic Anticyclone (SAA). Using ocean and atmosphere reanalysis products we show here that the strength of the SAA from February to May impacts the timing of the cold tongue onset and the intensity of its development in the eastern equatorial Atlantic (EEA) via anomalous tropical wind power. This modulation of the timing and amplitude of the seasonal cold tongue development manifests as anomalous SST events peaking between June and August. The timing and impact of this connection is not completely symmetric for warm and cold events. For cold events, an anomalously strong SAA in February and March leads to positive wind power anomalies from February to June resulting in an early cold tongue onset and subsequent cold SST anomalies in June and July. For warm events the anomalously weak SAA persists until May, generating negative wind power anomalies that lead to a late cold tongue onset as well as a suppression of the cold tongue development and associated warm SST anomalies. Mechanisms by which SAA induced wind power variations south of the equator influence EEA SST are discussed, including ocean adjustment via Rossby and Kelvin wave propagation, meridional advection, and local intraseasonal wind variations

What Controls the Mean East–West Sea Surface Temperature Gradient in the Equatorial Pacific: The Role of Cloud Albedo

AbstractThe mean east–west sea surface temperature gradient along the equator is a key feature of tropical climate. Tightly coupled to the atmospheric Walker circulation and the oceanic east–west thermocline tilt, it effectively defines tropical climate conditions. In the Pacific, its presence permits the El Nino–Southern Oscillation phenomenon. What determines this temperature gradient within the fully coupled ocean–atmosphere system is therefore a central question in climate dynamics, critical for understanding past and future climates. Using a comprehensive coupled model [Community Earth System Model (CESM)], the authors demonstrate how the meridional gradient in cloud albedo between the tropics and midlatitudes (Δα) sets the mean east–west sea surface temperature gradient in the equatorial Pacific. To change Δα in the numerical experiments, the authors change the optical properties of clouds by modifying the atmospheric water path, but only in the shortwave radiation scheme of the model. When Δα is va...

Simulating Pliocene warmth and a permanent El Niño‐like state: The role of cloud albedo

Available evidence suggests that during the early Pliocene (4–5 Ma) the mean east-west sea surface temperature (SST) gradient in the equatorial Pacific Ocean was significantly smaller than today, possibly reaching only 1–2°C. The meridional SST gradients were also substantially weaker, implying an expanded ocean warm pool in low latitudes. Subsequent global cooling led to the establishment of the stronger, modern temperature gradients. Given our understanding of the physical processes that maintain the present-day cold tongue in the east, warm pool in the west and hence sharp temperature contrasts, determining the key factors that maintained early Pliocene climate still presents a challenge for climate theories and models. This study demonstrates how different cloud properties could provide a solution. We show that a reduction in the meridional gradient in cloud albedo can sustain reduced meridional and zonal SST gradients, an expanded warm pool and warmer thermal stratification in the ocean, and weaker Hadley and Walker circulations in the atmosphere. Having conducted a range of hypothetical modified cloud albedo experiments, we arrive at our Pliocene simulation, which shows good agreement with proxy SST data from major equatorial and coastal upwelling regions, the tropical warm pool, middle and high latitudes, and available subsurface temperature data. As suggested by the observations, the simulated Pliocene-like climate sustains a robust El Nino-Southern Oscillation despite the reduced mean east-west SST gradient. Our results demonstrate that cloud albedo changes may be a critical element of Pliocene climate and that simulating the meridional SST gradient correctly is central to replicating the geographical patterns of Pliocene warmth.

Wetter subtropics in a warmer world: Contrasting past and future hydrological cycles

Significance The subsiding flow of the atmospheric Hadley circulation controls dry conditions over vast subtropical bands where the main arid regions of the globe reside. In the context of future changes in the atmospheric hydrological cycle, understanding precipitation changes in the subtropics is of particular importance given the scarcity of water resources in these locations. A major puzzle arises when contrasting the drier conditions in the subtropics predicted by climate models under global warming scenarios against the wetter conditions seen in reconstructions of past warm climates, including the warm, ∼400 ppm CO2, Pliocene. Our modeling results address this puzzle and highlight the importance of correctly predicting how the meridional temperature gradient between the tropics and the subtropics will change with global warming. During the warm Miocene and Pliocene Epochs, vast subtropical regions had enough precipitation to support rich vegetation and fauna. Only with global cooling and the onset of glacial cycles some 3 Mya, toward the end of the Pliocene, did the broad patterns of arid and semiarid subtropical regions become fully developed. However, current projections of future global warming caused by CO2 rise generally suggest the intensification of dry conditions over these subtropical regions, rather than the return to a wetter state. What makes future projections different from these past warm climates? Here, we investigate this question by comparing a typical quadrupling-of-CO2 experiment with a simulation driven by sea-surface temperatures closely resembling available reconstructions for the early Pliocene. Based on these two experiments and a suite of other perturbed climate simulations, we argue that this puzzle is explained by weaker atmospheric circulation in response to the different ocean surface temperature patterns of the Pliocene, specifically reduced meridional and zonal temperature gradients. Thus, our results highlight that accurately predicting the response of the hydrological cycle to global warming requires predicting not only how global mean temperature responds to elevated CO2 forcing (climate sensitivity) but also accurately quantifying how meridional sea-surface temperature patterns will change (structural climate sensitivity).

Extra-tropical origin of equatorial Pacific cold bias in climate models with links to cloud albedo

General circulation models frequently suffer from a substantial cold bias in equatorial Pacific sea surface temperatures (SSTs). For instance, the majority of the climate models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) have this particular problem (17 out of the 26 models evaluated in the present study). Here, we investigate the extent to which these equatorial cold biases are related to mean climate biases generated in the extra-tropics and then communicated to the equator via the oceanic subtropical cells (STCs). With an evident relationship across the CMIP5 models between equatorial SSTs and upper ocean temperatures in the extra-tropical subduction regions, our analysis suggests that cold SST biases within the extra-tropical Pacific indeed translate into a cold equatorial bias via the STCs. An assessment of the relationship between these extra-tropical SST biases and local surface heat flux components indicates a link to biases in the simulated shortwave fluxes. Further sensitivity studies with a climate model (CESM) in which extra-tropical cloud albedo is systematically varied illustrate the influence of cloud albedo perturbations, not only directly above the oceanic subduction regions but across the extra-tropics, on the equatorial bias. The CESM experiments reveal a quadratic relationship between extra-tropical Pacific albedo and the root-mean-square-error in equatorial SSTs—a relationship with which the CMIP5 models generally agree. Thus, our study suggests that one way to improve the equatorial cold bias in the models is to improve the representation of subtropical and mid-latitude cloud albedo.

Active Pacific meridional overturning circulation (PMOC) during the warm Pliocene

The warm, ~400-ppmv CO2 world of the Pliocene supported a northern-sourced meridional overturning cell in the Pacific. An essential element of modern ocean circulation and climate is the Atlantic meridional overturning circulation (AMOC), which includes deep-water formation in the subarctic North Atlantic. However, a comparable overturning circulation is absent in the Pacific, the world’s largest ocean, where relatively fresh surface waters inhibit North Pacific deep convection. We present complementary measurement and modeling evidence that the warm, ~400–ppmv (parts per million by volume) CO2 world of the Pliocene supported subarctic North Pacific deep-water formation and a Pacific meridional overturning circulation (PMOC) cell. In Pliocene subarctic North Pacific sediments, we report orbitally paced maxima in calcium carbonate accumulation rate, with accompanying pigment and total organic carbon measurements supporting deep-ocean ventilation-driven preservation as their cause. Together with high accumulation rates of biogenic opal, these findings require vigorous bidirectional communication between surface waters and interior waters down to ~3 km in the western subarctic North Pacific, implying deep convection. Redox-sensitive trace metal data provide further evidence of higher Pliocene deep-ocean ventilation before the 2.73-Ma (million years) transition. This observational analysis is supported by climate modeling results, demonstrating that atmospheric moisture transport changes, in response to the reduced meridional sea surface temperature gradients of the Pliocene, were capable of eroding the halocline, leading to deep-water formation in the western subarctic Pacific and a strong PMOC. This second Northern Hemisphere overturning cell has important implications for heat transport, the ocean/atmosphere cycle of carbon, and potentially the equilibrium response of the Pacific to global warming.