Surface mixing and biological activity in the four Eastern Boundary Upwelling Systems

Eastern Boundary Upwelling Systems (EBUS) are characterized by a high productivity of plankton associated with large commercial fisheries, thus playing key biological and socio-economical roles. The aim of this work is to make a comparative study of these four upwelling systems focussing on their surface stirring, using the Finite Size Lyapunov Exponents (FSLEs), and their biological activity, based on satellite data. First, the spatial distribution of horizontal mixing is analysed from time averages and from probability density functions of FSLEs. Then we studied the temporal variability of surface stirring focussing on the annual and seasonal cycle. There is a global negative correlation between surface horizontal mixing and chlorophyll standing stocks over the four areas. To try to better understand this inverse relationship, we consider the vertical dimension by looking at the Ekman-transport and vertical velocities. We suggest the possibility of a changing response of the phytoplankton to sub/mesoscale turbulence, from a negative effect in the very productive coastal areas to a positive one in the open ocean.Comment: 12 pages. NPG Special Issue on "Nonlinear processes in oceanic and atmospheric flows". Open Access paper, available also at the publisher site: http://www.nonlin-processes-geophys.net/16/557/2009

A climate network perspective on the intertropical convergence zone

The intertropical convergence zone (ITCZ) is an important component of the tropical rain belt. Climate models continue to struggle to adequately represent the ITCZ and differ substantially in its simulated response to climate change. Here we employ complex network approaches, which extract spatiotemporal variability patterns from climate data, to better understand differences in the dynamics of the ITCZ in state-of-the-art global circulation models (GCMs). For this purpose, we study simulations with 14 GCMs in an idealized slab-ocean aquaplanet setup from TRACMIP – the Tropical Rain belts with an Annual cycle and a Continent Model Intercomparison Project. We construct network representations based on the spatial correlation patterns of monthly surface temperature anomalies and study the zonal-mean patterns of different topological and spatial network characteristics. Specifically, we cluster the GCMs by means of the distributions of their zonal network measures utilizing hierarchical clustering. We find that in the control simulation, the distributions of the zonal network measures are able to pick up model differences in the tropical sea surface temperature (SST) contrast, the ITCZ position, and the strength of the Southern Hemisphere Hadley cell. Although we do not find evidence for consistent modifications in the network structure tracing the response of the ITCZ to global warming in the considered model ensemble, our analysis demonstrates that coherent variations of the global SST field are linked to ITCZ dynamics. This suggests that climate networks can provide a new perspective on ITCZ dynamics and model differences therein

The role of cloud-radiative effects and diabatic processes for the dynamics of the North Atlantic Oscillation on synoptic time-scales

Clouds shape weather and climate by regulating the latent and radiative heating in the atmosphere. Recent work demonstrated the importance of cloud-radiative effects (CRE) for the mean circulation of the extratropical atmosphere and its response to global warming. In contrast, little research has been done regarding the impact of CRE on internal variability. During the northern hemisphere winter the dominant mode of atmospheric variability over the North Atlantic and the surrounding continental areas of North America and Europe is the North Atlantic Oscillation (NAO). Here, we study how clouds and the NAO couple on synoptic time-scales during northern hemisphere winter via CRE within the atmosphere (ACRE) in observations and model simulations. A regression analysis based on 5-day-mean data from CloudSat/CALIPSO reveals a robust dipole of cloud-incidence anomalies during a positive NAO, with increased high-level clouds along the storm track (near 45°N) and the subpolar Atlantic, and decreased high-level clouds poleward and equatorward of it. Opposite changes occur for low-level cloud incidence. Satellite retrievals from CloudSat/CALIPSO, CERES and GERB as well as ERA-Interim short-term forecast data show that these cloud anomalies lead to an anomalous column-mean heating due to ACRE over the region of the Iceland low, and to a cooling over the region of the Azores high. To quantify the impact of the ACRE anomalies on the NAO, and to thereby test the hypothesis of a cloud-radiative feedback on the NAO persistence, we apply the surface pressure tendency equation (PTE) to ERA-Interim short-term forecast data. The NAO-related surface pressure tendency anomalies due to ACRE amplify the NAO-related surface pressure anomalies over the Azores high but have no area-averaged impact on the Iceland low. In contrast, surface pressure tendency anomalies due to total diabatic heating, including latent heating and clear-sky radiation, strongly amplify the NAO-related surface pressure anomalies over both the Azores high and the Iceland low, and their impact is much more spatially coherent. This suggests that while ACRE lead to an increase in NAO persistence on synoptic time-scales, their impact is relatively minor and much smaller compared to other diabatic processes. To test the robustness of our PTE-based hypothesis, numerical simulations in ICON are carried out. The PTE analysis in ICON shows results that are qualitatively consistent with the observational analysis, in particular regarding the feedback mechanisms of ACRE and total diabatic heating, which is dominated by latent heating. These PTE-based results are further tested by means of sensitivity simulations in ICON, where a NAO-related diabatic heating pattern is imposed either due to ACRE or total diabatic heating. These heating patterns are based on 5-day-mean NAO regressions of either ACRE or total diabatic heating. The sensitivity simulations confirm the observational hypothesis and show that ACRE feed back positively by up to 1–2% of 1σ NAO, while the total diabatic heating feeds back positively by up to 10% of 1σ NAO. Overall, the observational and modeling work both illustrate the substantial impact of the total diabatic heating for the NAO, while ACRE play a minor role. This highlights that diabatic processes are essential for understanding and accurately modeling the NAO short-term dynamics

Seasonal and regional characterization of horizontal stirring in the global ocean

Recent work on Lagrangian descriptors has shown that Lyapunov Exponents can be applied to observed or simulated data to characterize the horizontal stirring and transport properties of the oceanic flow. However, a more detailed analysis of regional dependence and seasonal variability was still lacking. In this paper, we analyze the near-surface velocity field obtained from the Ocean general circulation model For the Earth Simulator (OFES) using Finite-Size Lyapunov Exponents (FSLE). We have characterized regional and seasonal variability. Our results show that horizontal stirring, as measured by FSLEs, is seasonally-varying, with maximum values in Summer time. FSLEs also strongly vary depending on the region: we have first characterized the stirring properties of Northern and Southern Hemispheres, then the main oceanic basins and currents. We have finally studied the relation between averages of FSLE and some Eulerian descriptors such as Eddy Kinetic Energy (EKE) and vorticity (w) over the different regions.Comment: 32 pages, 7 figure

Characterizing the Northern Hemisphere Circumpolar Vortex Through Space and Time

This hemispheric-scale, steering atmospheric circulation represented by the circumpolar vortices (CPVs) are the middle- and upper-tropospheric wind belts circumnavigating the poles. Variability in the CPV area, shape, and position are important topics in geoenvironmental sciences because of the many links to environmental features. However, a means of characterizing the CPV has remained elusive. The goal of this research is to (i) identify the Northern Hemisphere CPV (NHCPV) and its morphometric characteristics, (ii) understand the daily characteristics of NHCPV area and circularity over time, (iii) identify and analyze spatiotemporal variability in the NHCPV’s centroid, and (iv) analyze how CPV features relate to the air-sea teleconnections that are known to explain important variability in weather/climate. Daily data (1979─2017) were collected from the National Centers for Environmental Prediction at the 500-hPa geopotential height level, and processed and analyzed in Python, MATLAB, R, and ArcGIS Desktop platform. Results suggest that the innovative method improves the calculation of NHCPV area and circularity, proven with the significant correlations between the NHCPV and teleconnection indices. At a daily scale, both correlations and principal components analysis reveal that the NHCPV is closely related to some air-sea teleconnections. The NHCPV area has expanded linearly over the 1979─2017 period and within its four subperiods, likely because of the weakened gradient of atmospheric mass over time. On the other hand, the NHCPV has alternating periods of increasing and decreasing circularity, suggesting that it may have become more unstable in its delivery of west-to-east flow. Spectrum analysis shows distinct annual and semiannual cycles for the area and circularity over all periods. While the NHCPV centroid shifts annually and intra- annually throughout the time series, probably because of the seasonality and teleconnection linkage, the linear trend analysis shows that the day-to-day distance moved by the NHCPV centroid decreased significantly, suggesting stability in the centroid positions. Emerging hot spot analysis reveals that new and oscillating hot spots have been emerged over time. This research can be extended to understand the current and projected relationship between the full 4-D (x-y-z-t) feature-based CPV structure, ocean-air teleconnections, sea-ice forcing, and natural hazard impacts

Role of Ocean-Atmosphere Coupling in Regional Climatic Impacts of Anthropogenic Sulfate Aerosols

Using a suite of coupled and uncoupled climate model experiments, we explore the impacts of anthropogenic sulfate aerosols on tropical Pacific climate and its variability. The role of sea surface temperature (SST), ocean dynamics, and ocean-atmosphere interaction in climate response to aerosols is examined by appropriately choosing the ocean component namely, full ocean general circulation model (OGCM), slab ocean model (SOM) or prescribed climatological SST. The ensemble of shorter responses versus a long-term response highlights processes and coupled feedbacks that are active on seasonal-interannual timescales versus on multi-decadal timescales. We find that an abrupt increase in tropospheric sulfate aerosols gives rise to El Niño like warming of the eastern tropical Pacific on seasonal-interannual timescales. Dynamical interaction between the ocean and the atmosphere causes this equatorial warming, whereas the thermodynamic interaction is responsible for the off-equatorial warming. These two interactions are related to the presence of Bjerknes feedback over the equator versus the off-equatorial WES (Wind-Evaporation-SST) feedback in the tropical climate system. In long-term, ocean dynamics will remove the initial tropical warming leaving a weaker and negative SST in response to increased aerosols in the fully-coupled case. Absence of ocean dynamics in the partially-coupled case, on the other hand, will lead to amplification of the off-equatorial warming via positive WES feedback. These coupled feedbacks, therefore, control the intertropical convergence zone (ITCZ) shift in response to aerosols over the eastern tropical Pacific. In this study, we also identify a cloud microphysics based mechanism for the high cloud increase over the tropical Indian Ocean. This regional increase in high clouds results in local net positive radiative forcing in comparison to negative forcing elsewhere on the globe

A climate network perspective on the intertropical convergence zone

The intertropical convergence zone (ITCZ) is an important component of the tropical rain belt. Climate models continue to struggle to adequately represent the ITCZ and differ substantially in its simulated response to climate change. Here we employ complex network approaches, which extract spatiotemporal variability patterns from climate data, to better understand differences in the dynamics of the ITCZ in state-of-the-art global circulation models (GCMs). For this purpose, we study simulations with 14 GCMs in an idealized slab-ocean aquaplanet setup from TRACMIP – the Tropical Rain belts with an Annual cycle and a Continent Model Intercomparison Project. We construct network representations based on the spatial correlation patterns of monthly surface temperature anomalies and study the zonal-mean patterns of different topological and spatial network characteristics. Specifically, we cluster the GCMs by means of the distributions of their zonal network measures utilizing hierarchical clustering. We find that in the control simulation, the distributions of the zonal network measures are able to pick up model differences in the tropical sea surface temperature (SST) contrast, the ITCZ position, and the strength of the Southern Hemisphere Hadley cell. Although we do not find evidence for consistent modifications in the network structure tracing the response of the ITCZ to global warming in the considered model ensemble, our analysis demonstrates that coherent variations of the global SST field are linked to ITCZ dynamics. This suggests that climate networks can provide a new perspective on ITCZ dynamics and model differences therein