Erratum to: Initialisation and predictability of the AMOC over the last 50 years in a climate model

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Initialisation and predictability of the AMOC over the last 50 years in a climate model

The mechanisms involved in Atlantic meridional overturning circulation (AMOC) decadal variability and predictability over the last 50 years are analysed in the IPSL–CM5A–LR model using historical and initialised simulations. The initialisation procedure only uses nudging towards sea surface temperature anomalies with a physically based restoring coefficient. When compared to two independent AMOC reconstructions, both the historical and nudged ensemble simulations exhibit skill at reproducing AMOC variations from 1977 onwards, and in particular two maxima occurring respectively around 1978 and 1997. We argue that one source of skill is related to the large Mount Agung volcanic eruption starting in 1963, which reset an internal 20-year variability cycle in the North Atlantic in the model. This cycle involves the East Greenland Current intensity, and advection of active tracers along the subpolar gyre, which leads to an AMOC maximum around 15 years after the Mount Agung eruption. The 1997 maximum occurs approximately 20 years after the former one. The nudged simulations better reproduce this second maximum than the historical simulations. This is due to the initialisation of a cooling of the convection sites in the 1980s under the effect of a persistent North Atlantic oscillation (NAO) positive phase, a feature not captured in the historical simulations. Hence we argue that the 20-year cycle excited by the 1963 Mount Agung eruption together with the NAO forcing both contributed to the 1990s AMOC maximum. These results support the existence of a 20-year cycle in the North Atlantic in the observations. Hindcasts following the CMIP5 protocol are launched from a nudged simulation every 5 years for the 1960–2005 period. They exhibit significant correlation skill score as compared to an independent reconstruction of the AMOC from 4-year lead-time average. This encouraging result is accompanied by increased correlation skills in reproducing the observed 2-m air temperature in the bordering regions of the North Atlantic as compared to non-initialized simulations. To a lesser extent, predicted precipitation tends to correlate with the nudged simulation in the tropical Atlantic. We argue that this skill is due to the initialisation and predictability of the AMOC in the present prediction system. The mechanisms evidenced here support the idea of volcanic eruptions as a pacemaker for internal variability of the AMOC. Together with the existence of a 20-year cycle in the North Atlantic they propose a novel and complementary explanation for the AMOC variations over the last 50 years

Decadal predictability of the Atlantic meridional overturning circulation and climate in the IPSL-CM5A-LR model

International audienceThis study explores the decadal potential predictability of the Atlantic Meridional Overturning Circulation (AMOC) as represented in the IPSL-CM5A-LR model, along with the predictability of associated oceanic and atmospheric fields. Using a 1000-year control run, we analyze the prognostic potential predictability (PPP) of the AMOC through ensembles of simulations with perturbed initial conditions. Based on a measure of the ensemble spread, the modelled AMOC has an average predictive skill of 8 years, with some degree of dependence on the AMOC initial state. Diagnostic potential predictability of surface temperature and precipitation is also identified in the control run and compared to the PPP. Both approaches clearly bring out the same regions exhibiting the highest predictive skill. Generally, surface temperature has the highest skill up to 2 decades in the far North Atlantic ocean. There are also weak signals over a few oceanic areas in the tropics and subtropics. Predictability over land is restricted to the coastal areas bordering oceanic predictable regions. Potential predictability at interannual and longer timescales is largely absent for precipitation in spite of weak signals identified mainly in the Nordic Seas. Regions of weak signals show some dependence on AMOC initial state. All the identified regions are closely linked to decadal AMOC fluctuations suggesting that the potential predictability of climate arises from the mechanisms controlling these fluctuations. Evidence for dependence on AMOC initial state also suggests that studying skills from case studies may prove more useful to understand predictability mechanisms than computing average skill from numerous start dates

Reconstructing the subsurface ocean decadal variability using surface nudging in a perfect model framework

Initialising the ocean internal variability for decadal predictability studies is a new area of research and a variety of ad hoc methods are currently proposed. In this study, we explore how nudging with sea surface temperature (SST) and salinity (SSS) can reconstruct the three-dimensional variability of the ocean in a perfect model framework. This approach builds on the hypothesis that oceanic processes themselves will transport the surface information into the ocean interior as seen in ocean-only simulations. Five nudged simulations are designed to reconstruct a 150 years “target” simulation, defined as a portion of a long control simulation. The nudged simulations differ by the variables restored to, SST or SST + SSS, and by the area where the nudging is applied. The strength of the heat flux feedback is diagnosed from observations and the restoring coefficients for SSS use the same time-scale. We observed that this choice prevents spurious convection at high latitudes and near sea-ice border when nudging both SST and SSS. In the tropics, nudging the SST is enough to reconstruct the tropical atmosphere circulation and the associated dynamical and thermodynamical impacts on the underlying ocean. In the tropical Pacific Ocean, the profiles for temperature show a significant correlation from the surface down to 2,000 m, due to dynamical adjustment of the isopycnals. At mid-to-high latitudes, SSS nudging is required to reconstruct both the temperature and the salinity below the seasonal thermocline. This is particularly true in the North Atlantic where adding SSS nudging enables to reconstruct the deep convection regions of the target. By initiating a previously documented 20-year cycle of the model, the SST + SSS nudging is also able to reproduce most of the AMOC variations, a key source of decadal predictability. Reconstruction at depth does not significantly improve with amount of time spent nudging and the efficiency of the surface nudging rather depends on the period/events considered. The joint SST + SSS nudging applied everywhere is the most efficient approach. It ensures that the right water masses are formed at the right surface density, the subsequent circulation, subduction and deep convection further transporting them at depth. The results of this study underline the potential key role of SSS for decadal predictability and further make the case for sustained large-scale observations of this field

Acyclovir-resistant HSV-1 keratitis: a concerning and emerging clinical challenge

Purpose To describe the clinical and virological profiles of patients with herpes simplex keratitis (HSK) caused by acyclovir-resistant (ACV(R)) strains of herpes simplex virus 1 (HSV-1). Design Multicenter retrospective case series. Methods HSV-1 resistance to ACV was confirmed using sequencing of genes encoding HSV-1 thymidine kinase (TK) and DNA polymerase (DNA pol). Data were collected on the number of HSK episodes before and after the diagnosis of resistance, ocular findings including the type of HSK, immune status of patients, antiviral treatments and HSV-1 genotypic resistance profiles. Results This study evaluated 18 HSK patients (13 males, 5 females, 66.8±4.7 years) with ACV(R) HSV-1 positive ocular samples. Genotypic resistance testing was performed due to frequent recurrences despite adequate antiviral prophylaxis (AVP) (N=13, 72%), or poor response to suppressive antiviral therapy (N=5, 28%). Resistance mutations were found in the TK (N=15, 83%) or in the DNA pol gene (N=3, 17%). Prior to the diagnosis of resistance, duration of disease was 29.8±20.4 years with more than 10 HSK recurrences in 15 patients (83%). The number of recurrences between the first episode and the diagnosis of resistance was significantly lower in immunocompromised patients (N=6, 33%), than in immunocompetent patients (N=12; 67%) (11.5±4.9 versus 16.4±1.9, P=0.05). Conclusion HSV-1 resistance to ACV must be suspected in HSK patients with recurrences despite AVP and/or in cases that respond poorly to a suppressive antiviral regimen. Immunocompromised patients and/or those with a long-standing disease, may be particularly at risk for developing resistance

Antiviral Effects of Cacicol\textregistered, a Heparan Sulfate Biomimetic for Corneal Regeneration Therapy, for Herpes Simplex Virus Type-1 and Varicella Zoster Virus Infection

Background Cacicol\textregistered, a topical eye biopolymer containing a poly-carboxymethylglucose sulfate solution that is a regenerating matrix therapy agent, intended for wound healing of persistent corneal epithelial defects. Based on the chemical composition, we hypothesized that Cacicol\textregistered may compete with natural heparan sulfate (HS) which initiates cell surface attachment of herpes simplex virus type-1 (HSV-1), varicella zoster virus (VZV) and human adenovirus (HAdV), three viruses associated with corneal infections. Methods Cacicol\textregistered was compared to vehicle in the following viral strains: HSV-1 SC16 strain and HSV-1 PSLR, a clinical isolate highly resistant to acyclovir and foscarnet; VZV ATH and VZV FLO, two VZV clinical isolates; and HAdV-D37 strain. Viruses in Cacicol\textregistered or vehicle were added to cells for 1 h during adsorption then viral replication was assessed by plaque reduction assays on Vero cells for HSV-1 and MeWo cells for VZV and by immunostaining assay on Hep-2 cells for HAdV-D37. Results The vehicle had no effect, dose-dependent effects were demonstrated when HSV-1 SC16, HSV-1 PSLR, VZV ATH and VZV FLO were inoculated in the presence of Cacicol\textregistered, inhibiting viral replication by 98.4%, 98.9%, 90.1% and 89.0%, respectively. Cacicol\textregistered had no antiviral effect against HAdV-D37. Conclusions Cacicol\textregistered has a significant antiviral activity on HSV-1 and VZV, but not on HAdV-D37. The lack of effect on HAdV is probably because it is less dependent on HS interactions for cell entry. Clinical studies are necessary to determine Cacicol\textregistered for an adjunct or alternative therapy of corneal HSV-1 or VZV infection, particularly for the management of antiviral resistant HSV-1

Recurrent herpetic keratitis despite antiviral prophylaxis: A virological and pharmacological study

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