Marcelo Barreiro

The Pliocene Paradox (Mechanisms for a Permanent El Niño)

During the early Pliocene, 5 to 3 million years ago, globally averaged temperatures were substantially higher than they are today, even though the external factors that determine climate were essentially the same. In the tropics, El Niño was continual (or “permanent”) rather than intermittent. The appearance of northern continental glaciers, and of cold surface waters in oceanic upwelling zones in low latitudes (both coastal and equatorial), signaled the termination of those warm climate conditions and the end of permanent El Niño. This led to the amplification of obliquity (but not precession) cycles in equatorial sea surface temperatures and in global ice volume, with the former leading the latter by several thousand years. A possible explanation is that the gradual shoaling of the oceanic thermocline reached a threshold around 3 million years ago, when the winds started bringing cold waters to the surface in low latitudes. This introduced feedbacks involving ocean-atmosphere interactions that, along with ice-albedo feedbacks, amplified obliquity cycles. A future melting of glaciers, changes in the hydrological cycle, and a deepening of the thermocline could restore the warm conditions of the early Pliocene.

GFDL's CM2 Global Coupled Climate Models. Part II: The Baseline Ocean Simulation

The current generation of coupled climate models run at the Geophysical Fluid Dynamics Laboratory (GFDL) as part of the Climate Change Science Program contains ocean components that differ in almost every respect from those contained in previous generations of GFDL climate models. This paper summarizes the new physical features of the models and examines the simulations that they produce. Of the two new coupled climate model versions 2.1 (CM2.1) and 2.0 (CM2.0), the CM2.1 model represents a major improvement over CM2.0 in most of the major oceanic features examined, with strikingly lower drifts in hydrographic fields such as temperature and salinity, more realistic ventilation of the deep ocean, and currents that are closer to their observed values. Regional analysis of the differences between the models highlights the importance of wind stress in determining the circulation, particularly in the Southern Ocean. At present, major errors in both models are associated with Northern Hemisphere Mode Waters and outflows from overflows, particularly the Mediterranean Sea and Red Sea.

Inferring long memory processes in the climate network via ordinal pattern analysis

We use ordinal patterns and symbolic analysis to construct global climate networks and uncover long- and short-term memory processes. Data analyzed are the monthly averaged surface air temperature (SAT field), and the results suggest that the time variability of the SAT field is determined by patterns of oscillatory behavior that repeat from time to time, with a periodicity related to intraseasonal oscillations and to El Niño on seasonal-to-interannual time scales.

The Freshening of Surface Waters in High Latitudes: Effects on the Thermohaline and Wind-Driven Circulations

Abstract The impacts of a freshening of surface waters in high latitudes on the deep, slow, thermohaline circulation have received enormous attention, especially the possibility of a shutdown in the meridional overturning that involves sinking of surface waters in the northern Atlantic Ocean. A recent study by Fedorov et al. has drawn attention to the effects of a freshening on the other main component of the oceanic circulation—the swift, shallow, wind-driven circulation that varies on decadal time scales and is closely associated with the ventilated thermocline. That circulation too involves meridional overturning, but its variations and critical transitions affect mainly the Tropics. A surface freshening in mid- to high latitudes can deepen the equatorial thermocline to such a degree that temperatures along the equator become as warm in the eastern part of the basin as they are in the west, the tropical zonal sea surface temperature gradient virtually disappears, and permanently warm conditions prevail...

Atlantic modulation of El Niño influence on summertime rainfall over southeastern South America

[1] This study addresses the effect of Atlantic sea surface temperature (SST) anomalies on rainfall over southeastern South America during January-February, particularly during El Nino years, using observations as well as model simulations. It is found that the state of the equatorial Atlantic during El Nino years can modulate its influence on rainfall over southeastern South America, such that when the equatorial Atlantic is warm, the El Nino influence is weaker. This Atlantic influence is shown to occur through the response of the low level winds to equatorial SST anomalies: the convergence of westerly anomalies onto the warm anomaly decreases the equatorial trades and moisture flow into the Amazon and, moreover, reduces the northerly flow that brings moisture to southeastern South America. The total rainfall response in this region can thus be thought as the combination of rainfall anomalies from the equatorial Pacific and Atlantic oceans.

On the Role of the South Atlantic Atmospheric Circulation in Tropical Atlantic Variability

One dominant manifestation of tropical Atlantic variability (TAV) takes place in March-April-May in the form of a strong inter-hemispheric sea surface temperature gradient coupled to a cross-equatorial near surface atmospheric flow. The variability of this circulation pattern affects the position of the intertropical convergence zone and the regional climate in the surrounding areas. In this study, we investigated the effect of the South Atlantic atmospheric variability on this phenomenon. We found that southern summer atmospheric variability (and to a lesser extent winter variability) can play a preconditioning role in the onset of inter-hemispheric anomalies in the deep tropics during the following austral fall. It does so by inducing a sea surface temperature anomaly in the southern tropics that initiates local thermodynamic air-sea feedbacks. This remote influence of the Southern Hemisphere on TAV is contrasted with the remote influence of El Nino-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) during austral summer. The results suggest that to fully understand TAV and its predictability it is necessary to consider not only the remote influences from ENSO and NAO, but also the influence from the South Atlantic atmospheric circulation.

Assessing the direction of climate interactions by means of complex networks and information theoretic tools

An estimate of the net direction of climate interactions in different geographical regions is made by constructing a directed climate network from a regular latitude-longitude grid of nodes, using a directionality index (DI) based on conditional mutual information (CMI). Two datasets of surface air temperature anomalies-one monthly averaged and another daily averaged-are analyzed and compared. The network links are interpreted in terms of known atmospheric tropical and extra-tropical variability patterns. Specific and relevant geographical regions are selected, the net direction of propagation of the atmospheric patterns is analyzed, and the direction of the inferred links is validated by recovering some well-known climate variability structures. These patterns are found to be acting at various time-scales, such as atmospheric waves in the extratropics or longer range events in the tropics. This analysis demonstrates the capability of the DI measure to infer the net direction of climate interactions and may contribute to improve the present understanding of climate phenomena and climate predictability. The work presented here also stands out as an application of advanced tools to the analysis of empirical, real-world data.

Role of the global oceans and land–atmosphere interaction on summertime interdecadal variability over northern Argentina

This study uses experiments with an atmospheric general circulation model (AGCM) to address the role of the oceans and the effect of land–atmosphere coupling on the predictability of summertime rainfall over northern Argentina focusing on interdecadal time scales during 1901–2006. Ensembles of experiments where the AGCM is forced with historical sea surface temperature (SST) in the global, Pacific and tropical-North Atlantic domains are used. The role of land–atmosphere interaction is assessed comparing the output of simulations with active and climatological soil moisture. A maximum covariance analysis between precipitation and SST reveals the impact of the Pacific Decadal Oscillation, the Atlantic Multidecadal Oscillation and the equatorial–tropical South Atlantic on rainfall over northern Argentina. Model simulations further show that while the dominant influence comes from the Pacific basin, the Atlantic influence can explain a large transition from dry to wet decades over northern Argentina during the beginning of the 1970s. Analysis of anomalies before and after the transition reveals an upper level anticyclonic circulation off the Patagonian coast with barotropic structure. This circulation enhances the moisture transport and convergence in northern Argentina and, together with enhanced evaporation, increased the rainfall after 1970. The SST pattern is dominated by cold conditions in the equatorial Atlantic and warm eastern Pacific and South Atlantic. We also found that land–atmosphere interaction leads to a representation of the long term rainfall evolution over northern Argentina that is closer to the observed one. Moreover, it leads to a smaller dispersion among ensemble members, thus resulting in a larger signal-to-noise ratio.

A linear tendency correction technique for improving seasonal prediction of SST

tropical Atlantic sector using an AGCM coupled to a slab ocean. By construction, the model has systematic biases in its deterministic dynamics because the slab ocean has no ocean dynamics. We show that the linear correction procedure successfully corrects the model biases in the regions where ocean dynamics are expected to be important. [4] In a recent study, Chang et al. [2003] (hereafter CSJ) show that SST anomalies in the tropical north Atlantic (TNA) can be predicted two seasons in advance using an AGCM coupled to a slab ocean, owing to the combined effect of the remote ENSO influence and local thermodynamic air-sea feedbacks. But in the tropical south and equatorial Atlantic, the simple coupled model revealed poor skills and under-performed the persistence forecast. Here we use CSJ’s results as a reference with the exception that the prediction and verification period is expanded from 1959 to 2000. We focus on February–May, the peak months of the gradient mode of variability in the tropical Atlantic [Chiang et al., 2002], allowing us to investigate the importance of ocean dynamics in its evolution.

Climate Sensitivity to Changes in Ocean Heat Transport

AbstractUsing an atmospheric general circulation model coupled to a slab ocean, the effects of ocean heat transport (OHT) on climate are studied by prescribing OHT from 0 to 2 times the present-day values. In agreement with previous studies, an increase in OHT from zero to present-day conditions warms the climate by decreasing the albedo due to reduced sea ice extent and marine stratus cloud cover and by increasing the greenhouse effect through a moistening of the atmosphere. However, when the OHT is further increased, the solution becomes highly dependent on a positive radiative feedback between tropical low clouds and sea surface temperature. The strength of the low cloud–SST feedback combined with the model design may produce solutions that are globally colder than in the control run, mainly due to an unrealistically strong equatorial cooling. Excluding those cases, results indicate that the climate warms only if the OHT increase does not exceed more than 10% of the present-day value in the case of a s...