Papers by Annalisa Bracco
Geophysical flows are characterized by the presence of coherent vortices, localized concentration... more Geophysical flows are characterized by the presence of coherent vortices, localized concentrations of energy and vorticity that have a lifetime much longer than the local turbulence time (sometimes called the eddy turnover time).
Global Biogeochemical Cycles, 2002
1] Localized upwelling is a strong driver of primary production in many regions of the open ocean... more 1] Localized upwelling is a strong driver of primary production in many regions of the open ocean. This is achieved through providing an increased vertical flux of nutrients to otherwise starved surface waters. The impact of such upwelling hot spots on regional production is investigated with particular emphasis on three parameters: the fraction of the region experiencing upwelling, the increase in nutrient flux within upwelling regions with respect to ambient waters, and the rate of horizontal mixing between upwelled and ambient waters. It is demonstrated that although independently increasing one of these parameters has a limited effect on the production of the region, increasing them simultaneously can dramatically increase production. Furthermore, it is shown that the spatial distribution of upwelling regions can have a strong influence on productivity. Numerous small upwelling regions increase production significantly more than one large upwelling hot spot even if the total rate of upwelling is constant for the two cases. The magnitude of this discrepancy is shown to be influenced by the presence of coherent structures such as eddies in the surface ocean. The ocean components of global carbon cycle models are still far from being able to resolve mesoscale features fully. These results imply that such models may be greatly underestimating primary production in large parts of the ocean as a consequence.
Physical Review Letters, 2004
The dynamics of passive Lagrangian tracers in three-dimensional quasigeostrophic turbulence is st... more The dynamics of passive Lagrangian tracers in three-dimensional quasigeostrophic turbulence is studied numerically and compared with the behavior of two-dimensional barotropic turbulence. Despite the different Eulerian properties of the two flows, the Lagrangian dynamics of passively advected tracers in three-dimensional quasigeostrophic turbulence is very similar to that of barotropic turbulence. In both systems, coherent vortices play a major role in determining the mixing and dispersion properties. This work indicates that recent results on particle dynamics in barotropic, two-dimensional turbulence carry over to more realistic baroclinic flows, such as those encountered in the large-scale dynamics of the atmosphere and ocean.
Journal of Physical Oceanography, 2000
Probability density functions (PDFs) of daily velocities from subsurface floats deployed in the N... more Probability density functions (PDFs) of daily velocities from subsurface floats deployed in the North Atlantic and equatorial Atlantic Oceans are examined. In general, the PDFs are approximately Gaussian for small velocities, but with significant exponential tails for large velocities. Correspondingly, the kurtoses of the distributions are greater than three. Similar PDFs are found in both western and eastern regions, above and below 1000-m depth, with more significant non-Gaussianity in the North Atlantic than at the equator. Analogously, Lagrangian statistics in decaying two-dimensional turbulence also display non-Gaussian velocity PDFs with approximately exponential tails, in the limit of large Reynolds number.
Geophysical Research Letters, 2009
Geophysical Research Letters, 2009
Nature Geoscience, 2010
Decadal fluctuations of the ocean and atmosphere over the North Pacific Ocean significantly affec... more Decadal fluctuations of the ocean and atmosphere over the North Pacific Ocean significantly affect the weather and climate of North America and Eurasia. They also cause transitions between different states of marine ecosystems across the Pacific Ocean. An important fraction of North Pacific low-frequency variability is linked to the North Pacific Gyre Oscillation, a climate pattern associated with decadal fluctuations of the ocean circulation. Decadal variations in the North Pacific Gyre Oscillation are characterized by a pattern of sea surface temperature anomalies that resemble the central Pacific El Niño, a dominant mode of interannual variability with far-reaching effects on global climate patterns. Here we use an ensemble of simulations with a coupled ocean-atmosphere model to show that the sea surface temperature anomalies associated with central Pacific El Niño force changes in the extra-tropical atmospheric circulation. These changes in turn drive the decadal fluctuations of the North Pacific Gyre Oscillation. Given that central Pacific El Niño events could become more frequent with increasing levels of greenhouse gases in the atmosphere, we infer that the North Pacific Gyre Oscillation may play an increasingly important role in shaping Pacific climate and marine ecosystems in the twenty-first century.
Journal of Climate, 2009
Proxy-based paleoclimate reconstructions of tropical sea surface temperature (SST) fields may lea... more Proxy-based paleoclimate reconstructions of tropical sea surface temperature (SST) fields may lead to better constraints of tropical climate variability in climate model projections. In this study, the authors quantify uncertainties associated with two popular SST anomaly reconstruction methods that have been applied over the last millennium. The first reconstruction method exploits the high correlation between the leading modes of variability of global precipitation and SSTs; the second method uses a multiregression model that exploits the multiple modes of covariability between precipitation and SSTs. Regardless of the proxy network density, the first method has skill only in the tropical eastern Pacific and misses some ENSO events. By contrast, the multiregression approach demonstrates high skill throughout the tropical Indo-Pacific region and predicts all ENSO events correctly. The advantage of the multiregression method lies in the second mode of covariability between SSTs and precipitation, which explains nearly 15% of the covariability between the two variables. However, when the period 1950-2000 is considered, the authors find that the nonstationarity in the second mode of covariability between SST and precipitation leads to a significant reduction of skill in the Indian Ocean and the warm pool region. This change suggests that the underlying stationarity assumption common in most climate field reconstruction methods needs to be treated more carefully, particularly in the tropics.
Quarterly Journal of The Royal Meteorological Society, 2009
Recent studies using coupled atmosphere–ocean models have shown that the tropical Atlantic has a ... more Recent studies using coupled atmosphere–ocean models have shown that the tropical Atlantic has a significant impact on the Indian monsoon. In this article, the observational basis for this teleconnection is examined and the physical mechanism responsible for bridging sea-surface temperatures (SSTs) in the Atlantic and precipitation over India is investigated with idealized atmospheric general circulation model (AGCM) experiments in which constant SST anomalies are prescribed and ‘switched on’ in the tropical Atlantic region. A simple Gill–Matsuno-type quadrupole response is proposed to explain the teleconnection between the tropical Atlantic and the Indian basin, with an enforcement of the eastward response likely due to nonlinear interactions with the mean monsoon circulation. The simplicity of this mechanism suggests the reproducibility of this result with a broad range of AGCMs. Copyright © 2009 Royal Meteorological Society
Climate Dynamics, 2006
The relationship between interdecadal variations of tropical sea surface temperature (SST) in the... more The relationship between interdecadal variations of tropical sea surface temperature (SST) in the last 120 years and circulation anomalies related to the North Atlantic Oscillation (NAO) is investigated in this study. Using an atmospheric general circulation model (AGCM), we confirm observational evidence that variations in the SST gradient in the western tropical Pacific are related to the NAO anomalies on decadal timescale, and may be contributing to the shift towards the positive NAO phase observed in the late 20th century. The role played by the Indian Ocean-NAO teleconnection, advocated in recent studies focused on the last 50 years, is also assessed in the context of the 120-year long record. It is suggested that a positive feedback between the Pacific SST and the hemispheric circulation pattern embedding the decadal NAO signal may act to enhance the internal variability of the coupled ocean–atmosphere system, and justify the stronger teleconnection found in observational data than in SST-forced AGCM experiments.
Geophysical Research Letters, 2005
1] Ocean-atmosphere variability in the tropical Indian Ocean is investigated using observational ... more 1] Ocean-atmosphere variability in the tropical Indian Ocean is investigated using observational data and ensemble experiments with a coupled general circulation model. In one ensemble (IO runs) the ocean-model domain is limited to the Indian Ocean and observed sea surface temperatures force the atmospheric model elsewhere. In a second ensemble (TPIO) the coupled domain includes the Tropical Pacific. The IO runs display a coupled mode of variability with several characteristics of the Indian Ocean Dipole (IOD), but independent on ENSO (El-Niño Southern Oscillation). Changes in the Walker circulation induced by ENSO are insufficient to trigger IOD events. In the TPIO runs ENSO variability is correlated with the IOD mode as observed. Oceanic processes are responsible for an essential component of ENSO forcing in the Indian Ocean. The ENSO phase conditions the thermocline depth in the Indonesian Throughflow region and in the southeastern IO. TPIO results are in agreement with SODA reanalysis.
Geophysical Research Letters, 2008
1] The Indian monsoon interannual variability is modulated by the El Niño Southern Oscillation (E... more 1] The Indian monsoon interannual variability is modulated by the El Niño Southern Oscillation (ENSO), with a drier than normal monsoon season usually preceding peak El Niño conditions, and vice versa for La Niña phase. Pacific sea surface temperature (SST) anomalies, however, are not the only player. Building upon our recent discovery that atmospheric teleconnections between the tropical Atlantic and the Indian basin contributed to the weakening of the ENSO-monsoon anticorrelation during the '80s and '90s, we investigate the role of south equatorial Atlantic SSTs in forcing the Indian monsoon rainfall (IMR). Using two observational data sets and two ensembles of simulations we show that the residual in the IMR time series for observed and modeled data, obtained by subtracting the ENSO-forced component of the IMR that is linearly related to the NINO34 index, is significantly correlated with south equatorial Atlantic SSTs. Our results have important implications for seasonal forecast efforts.
Climate Dynamics, 2007
This study investigates how accurately the interannual variability over the Indian Ocean basin an... more This study investigates how accurately the interannual variability over the Indian Ocean basin and the relationship between the Indian summer monsoon and the El Niño Southern Oscillation (ENSO) can be simulated by different modelling strategies. With a hierarchy of models, from an atmospherical general circulation model (AGCM) forced by observed SST, to a coupled model with the ocean component limited to the tropical Pacific and Indian Oceans, the role of heat fluxes and of interactive coupling is analyzed. Whenever sea surface temperature anomalies in the Indian basin are created by the coupled model, the inverse relationship between the ENSO index and the Indian summer monsoon rainfall is recovered, and it is preserved if the atmospherical model is forced by the SSTs created by the coupled model. If the ocean model domain is limited to the Indian Ocean, changes in the Walker circulation over the Pacific during El-Niño years induce a decrease of rainfall over the Indian subcontinent. However, the observed correlation between ENSO and the Indian Ocean zonal mode (IOZM) is not properly modelled and the two indices are not significantly correlated, independently on season. Whenever the ocean domain extends to the Pacific, and ENSO can impact both the atmospheric circulation and the ocean subsurface in the equatorial Eastern Indian Ocean, modelled precipitation patterns associated both to ENSO and to the IOZM closely resemble the observations.
The Indian Monsoon-El Nino Southern Oscillation (ENSO) relationship, according to which a drier t... more The Indian Monsoon-El Nino Southern Oscillation (ENSO) relationship, according to which a drier than normal monsoon season precedes peak El Nino conditions, weakened significantly during the last two decades of the 20th century. In this work an ensemble of integrations of an Atmospherical General Circulation Model (AGCM) coupled to an ocean model in the Indian basin and forced with observed sea surface temperatures (SSTs) elsewhere is used to investigate the causes of such a weakening. The observed interdecadal variability of the ENSO-Monsoon relationship during the period 1950-1999 is realistically simulated by the model and a dominant portion of the variability is associated to changes in the tropical Atlantic SSTs in boreal summer. In correspondence to ENSO, the tropical Atlantic SSTs display negative anomalies south of the Equator in the last quarter of the 20th century and weakly positive anomalies in the previous period. Those anomalies in turn produce heating anomalies which excite a Rossby wave response in the Indian Ocean in both the model and in reanalysis data, impacting the time-mean monsoon circulation. The proposed mechanism of remote response of the Indian rainfall to tropical Atlantic sea surface temperatures is further tested forcing the AGCM coupled to the ocean model in the Indian basin with climatological SSTs in the Atlantic Ocean and observed anomalies elsewhere. In this second ensemble the ENSO-Monsoon relation is characterized by a stable and strong anticorrelation through the whole second half of the XX century. I mid 70's. The analysis of the interdecadal variability in the tropical Atlantic in a coupled model will be subject of future study.
Climate Dynamics, 2011
The focus of this paper is to assess the relative role of the north–south and east–west contrasts... more The focus of this paper is to assess the relative role of the north–south and east–west contrasts in atmospheric heating for the maintenance of the South Asian summer monsoon climatology. The juxtaposition of the Eurasian land mass and the Indian Ocean is responsible for the north–south contrast, while the greater diabatic heating above the western Pacific compared to the one over the African and the tropical South Atlantic Ocean region introduces the east–west gradient. With a series of idealized atmospheric general circulation model experiments, it is found that both contrasts contribute to the maintenance of the South Asian monsoon climatology, but their impact varies at regional scales. The surface atmospheric cyclone and precipitation over northern India are mainly due to the north–south contrast. On the other hand, when the Indian Ocean sea surface temperatures are close to their climatological mean values, the low-level cyclone and consequent rainfall activity in the Bay of Bengal and southern India result from the east–west gradient. The physical mechanism relays on the southern part of the upper-level South Asian monsoon high being forced by the east–west diabatic heating contrast via Sverdrup balance. The east–west heating difference controls also the strength of the Tropical Easterly Jet. Finally, the contribution of the El Niño Southern Oscillation to the interannual variability of the Indian monsoon is interpreted as the result of a longitudinal shift of one of the centers of diabatic heating contributing to the east–west contrast.
Climate Dynamics, 2004
An atmospheric general circulation model of intermediate complexity is used to investigate the or... more An atmospheric general circulation model of intermediate complexity is used to investigate the origin and structure of the climate change in the second half of the twentieth century. The variability of the atmospheric flow is considered as a superposition of an internal part, due to intrinsic dynamical variability, and an external part, due to the variations of the sea surface temperature (SST) forcing. The two components are identified by performing a 50-member ensemble of atmospheric simulations with prescribed, observed SSTs in the period 1949–2002. The large number of realizations allows the estimation of statistics of the atmospheric variability with a high confidence level. The analysis performed focuses on interdecadal and interannual variability of 500 hPa geopotential height in the Northern Hemisphere (NH) during winter. The model reproduces well the structure of the observed trend (defined as the difference in the two 25-year intervals 1977–2001 and 1952–1976), particularly in the Pacific region, and about half of the amplitude of the signal. The trend in 500 hPa height projects mainly onto the second empirical orthogonal function (EOF), both in the observations and in the model ensemble. However, differences between the modelled and the observed variability are found in the pattern of the second EOF in the Atlantic sector. SST changes associated with the El Niño southern oscillation (ENSO) are responsible for about 50% of the signal of the 500 hPa height trend in the Pacific. A second 50-member ensemble is used to evaluate the sensitivity of interdecadal variability to an increase in CO2 optical depth compatible with observed concentration changes. In this second experiment, the simulated trend includes a statistically significant contribution from the positive phase of the Arctic oscillation (AO). Such a contribution is also found in observations. Furthermore, the additional CO2 forcing accounts for part of the NH trend in near-surface temperature, and brings the zonal-mean temperature changes in the stratosphere and upper-troposphere closer to observations.
Meteorologische Zeitschrift, 2009
Recent studies found that the tropical Atlantic may exert a considerable teleconnection to both t... more Recent studies found that the tropical Atlantic may exert a considerable teleconnection to both the tropical Pacific and Indian Ocean basins, possibly modulating the Indian summer monsoon and Pacific ENSO events. A warm (cold) tropical Atlantic
Journal of Climate, 2007
The Indian Monsoon-El Nino Southern Oscillation (ENSO) relationship, according to which a drier t... more The Indian Monsoon-El Nino Southern Oscillation (ENSO) relationship, according to which a drier than normal monsoon season precedes peak El Nino conditions, weakened significantly during the last two decades of the 20th century. In this work an ensemble of integrations of an Atmospherical General Circulation Model (AGCM) coupled to an ocean model in the Indian basin and forced with observed sea surface temperatures (SSTs) elsewhere is used to investigate the causes of such a weakening. The observed interdecadal variability of the ENSO-Monsoon relationship during the period 1950-1999 is realistically simulated by the model and a dominant portion of the variability is associated to changes in the tropical Atlantic SSTs in boreal summer. In correspondence to ENSO, the tropical Atlantic SSTs display negative anomalies south of the Equator in the last quarter of the 20th century and weakly positive anomalies in the previous period. Those anomalies in turn produce heating anomalies which excite a Rossby wave response in the Indian Ocean in both the model and in reanalysis data, impacting the time-mean monsoon circulation. The proposed mechanism of remote response of the Indian rainfall to tropical Atlantic sea surface temperatures is further tested forcing the AGCM coupled to the ocean model in the Indian basin with climatological SSTs in the Atlantic Ocean and observed anomalies elsewhere. In this second ensemble the ENSO-Monsoon relation is characterized by a stable and strong anticorrelation through the whole second half of the XX century. I mid 70's. The analysis of the interdecadal variability in the tropical Atlantic in a coupled model will be subject of future study.
Journal of Geophysical Research, 2005
1] Oceanic carbon uptake depends on the productivity of the marine ecosystem. Here we study the d... more 1] Oceanic carbon uptake depends on the productivity of the marine ecosystem. Here we study the dependence of primary productivity on the spatial and temporal variability of the nutrient flux and the functional form used to parameterize it. We show that primary productivity is significantly affected by the form of the nutrient input. For a restoring flux, used to parameterize nutrient input by upwelling, primary productivity depends on the size and/or temporal duration of the upwelling events. For a fixed-flux nutrient input, we show that high-nutrient, low-chlorophyll (HNLC) regions can easily appear, without necessarily implying the lack of some micronutrient. These results have interesting implications on the interpretation of primary productivity estimates from observational data and ocean circulation models, and indicate a way to obtain upper and lower bounds to primary productivity in coarse-resolution models.
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Papers by Annalisa Bracco