Boreal Summer Season Research Articles

Investigation of the 2016 Eurasia heat wave as an event of the recent warming

This study investigates the physical mechanisms that contributed to the 2016 Eurasian heat wave during boreal summer season (July–August, JA), characterized by much higher than normal temperatures over eastern Europe, East Asia, and the Kamchatka Peninsula. It is found that the 2016 JA mean surface air temperature, upper-tropospheric height, and soil moisture anomalies are characterized by a tri-pole pattern over the Eurasia continent and a wave train-like structure not dissimilar to recent (1980–2016) trends in those quantities. A series of forecast experiments designed to isolate the impacts of the land, ocean, and sea ice conditions on the development of the heat wave is carried out with the Global Seasonal Forecast System version 5. The results suggest that the tri-pole blocking pattern over Eurasia, which appears to be instrumental in the development of the 2016 summer heat wave, can be viewed as an expression of the recent trends, amplified by record-breaking oceanic warming and internal land-atmosphere interactions.

Recent weakening in MJO-related convective activity over the equatorial Indian Ocean and Maritime Continent

We document the observed changes in the intraseasonal variability of precipitation and outgoing longwave radiation (OLR) associated with the Madden-Julian Oscillation (MJO) from 1998 to 2015 when reliable and simultaneous satellite-derived daily precipitation and OLR are available. Tropical Rainfall Measuring Mission (TRMM) observations show a statistically significant weakening trend in variance of convective activity associated with the MJO across the equatorial Indian Ocean (EQIO) and Maritime Continent (MC) in boreal summer and winter seasons. During northern summer, the MJO variability in the POST period (2007–2015) displays a remarkable reduction in convection at all wavenumbers compared with that in the PRE period (1998–2006). During northern winter, along with a reduction in variance, the MJO-related activity is shifted from lower to higher wavenumbers by breaking down the associated convection into smaller scales. EQIO and MC experience a shift from a higher probability of stronger events to weaker events during both seasons in the multivariate Wheeler-Hendon Index. Moreover, there is a southward shift in the location of maximum variance, increasing (reducing) meridional symmetry in MJO variance during boreal summer (winter). This is manifested by a significant increase in boreal winter MJO variance over northern Australia. We also observed that over western EQIO, OLR anomalies associated with dry (wet) phase of MJO are higher (lower) in POST as compared with the PRE period indicating warmer surface temperatures and taller clouds during dry and wet phases of MJO respectively, which is consistent with observation. The exact opposite behavior is seen over the MC and Western Pacific Oceans. These variations in MJO can have important implications for the local and remote climate systems across the globe.

Skilful interannual climate prediction from two large initialised model ensembles

Climate prediction skill on the interannual timescale, which sits between that of seasonal and decadal, is investigated using large ensembles from the Met Office and CESM initialised coupled prediction systems. A key goal is to determine what can be skillfully predicted about the coming year when combining these two ensembles together. Annual surface temperature predictions show good skill at both global and regional scales, but skill diminishes when the trend associated with global warming is removed. Skill for the extended boreal summer (months 7–11) and winter (months 12–16) seasons are examined, focusing on circulation and rainfall predictions. Skill in predicting rainfall in tropical monsoon regions is found to be significant for the majority of regions examined. Skill increases for all regions when active ENSO seasons are forecast. There is some regional skill for predicting extratropical circulation, but predictive signals appear to be spuriously weak.

Predictability of Seasonal Streamflow and Soil Moisture in National Water Model and a Humid Alabama–Coosa–Tallapoosa River Basin

AbstractThis study investigates the potential predictability of streamflow and soil moisture in the Alabama–Coosa–Tallapoosa (ACT) river basin in the southeastern United States. The study employs the state-of-the-art National Water Model (NWM) and compares the effects of initial soil moisture condition with those of seasonal climate anomalies on streamflow and soil moisture forecast skills. We have designed and implemented seasonal streamflow forecast ensemble experiments following the methodology suggested by Dirmeyer et al. The study also compares the soil moisture variability in the NWM with in situ measurements and remote sensing data from the Soil Moisture Active and Passive (SMAP) satellite. The NWM skillfully simulates the observed streamflow in the ACT basin. The soil moisture variability is 46% smaller in the NWM compared with the SMAP data, mainly due to a weaker amplitude of the seasonal cycle. This study finds that initial soil moisture condition is a major source of predictability for the seasonal streamflow forecast. The contribution of the initial soil moisture condition is comparable or even higher than that of seasonal climate anomaly effects in dry seasons. In the boreal summer season, the initial soil moisture condition contributes to 65% and 48% improvements in the seasonal streamflow and soil moisture forecast skills, respectively. This study attributes a greater improvement in the streamflow forecast skill to the lag effects between the soil moisture and streamflow anomalies. The results of this study can inform the development and improvement of the operational streamflow forecasting system.

The role of extreme rain events in Peninsular Florida’s seasonal hydroclimate variations

The role of extreme rains over Peninsular Florida (PF) in modulating the seasonal rainfall characteristics is investigated in this study. The paper is motivated on its potential implication on the seasonal predictability of the hydroclimate of PF by relatively coarse global seasonal climate models. A majority of these climate models are unable to resolve the weather events like tropical cyclones that produce such extreme rain events. Therefore, a legitimate question to ask is if this limits the model’s seasonal predictability of the hydroclimate of PF.In this paper, extreme rain events over PF within a season are defined as days with daily rain amount at or above the 95th percentile over 39 years from 1979 to 2017 at the grid resolution of the observed rainfall dataset (0.5° × 0.5°). The thresholds for extreme rain days range from 16 mmday−1 to 36 mmday−1 depending on the season and the location over PF, while the heaviest rainfall range from 58 mmday−1 to 278 mmday−1. These extreme rain events occur most often across PF in the boreal summer season followed by the fall season with the least in the boreal winter season. Our study reveals that removing the days of extreme rain events has the largest impact on the corresponding seasonal anomalies and daily rainfall distribution in the dry winter season and least in the wet summer season.The impact of El Niño and the Southern Oscillation (ENSO) on the extreme rain events was evaluated by contrasting the differences in the shape and the scale parameters of the fitted Gamma distribution on daily rainfall in winter/spring seasons during warm and cold phases. Results revealed that the warm ENSO phases make the tails of the daily rainfall distribution over PF heavier and longer relative to the cold ENSO phases in the winter and spring seasons. In essence, our study reveals that the extreme rain events that are critical for the overall seasonal distribution of rainfall over PF in the first half of the year is modulated by large-scale phenomenon (e.g., ENSO). In the latter half of the year (summer and fall), the extreme rain events are not as critical to the seasonal rainfall anomaly or the overall seasonal distribution of rainfall over PF. Therefore, resolving the extreme rain events need not be as critical for the seasonal predictability of the hydroclimate of PF.

Variation of thermodynamic layers over the South Coastal Java Region (SJCR) and their influences on nutrient abundance

The variation of thermodynamic layers over the south coastal Java are important components associated with high potential fishery in this region. In addition, this area influenced by several ocean-atmosphere coupled interactions, such as Ocean Current System and Indonesian Through Flow (ITF) and Monsoon. This study designed to analyze spatial and temporal variations of Mixed Layer Depth (MLD), Isothermal Layer Depth (ILD) and Barrier Layer Thickness (BLT). The data used are salinity, temperature, nitrate and oxygen from CSIRO Atlas of Regional Seas (CARS), surface wind and evaporation from European Centre for Medium-Range Weather Forecasts (ECMWF), and precipitation from Tropical Rainfall Measurement Mission (TRMM). Based on the results, during JJA season (boreal summer) the ILD, MLD and BLT layers over the southern coast of Java are relatively shallower. The thickness of ILD was range from 10 – 30 m, while the thickness of MLD and BLT are about <10 m. Meanwhile, during DJF (boreal winter) and MAM seasons (boreal spring), the thickness of the ILD, MLD and BLT layers are deeper. The nutrient distributions are consistent with thermodynamic layer patterns. The highest concentration of nitrate, phosphate and oxygen tends to occur in the boreal summer season due to the upwelling phenomenon which causes the removal of nutrients in the inner layer toward the surface, whereas the lowest concentration tends to occur in the boreal winter season caused by the downwelling phenomenon.

What did determine the warming trend in the Indonesian sea?

The Indonesian sea is the only low-latitude pathway connecting two tropical oceans, which plays an important role in the coupled ocean-atmosphere mode in the Indo-Pacific sector. A small change in the sea surface temperature (SST) in the Indonesian sea has a significant influence on the precipitation and air-sea heat flux. During the past 33 years, the SST in the Indonesian sea has indicated a warming trend on the average of 0.19 ± 0.04 decade−1, which is larger than global SST warming trend. Moreover, the warming trend indicates seasonal variations, in which maximum trend occurred during boreal summer season. Investigation on the potential driver for this warming trend, namely, the net surface heat flux, resulted in an opposite trend (cooling trend), while the Ekman pumping and the wind mixing only play a minor role on the SST warming. Here, we proposed the upper layer process associated with an increasing trend in precipitation and decreasing trend in mixed layer depth (MLD) for the SST warming within the Indonesian seas. Shoaling of MLD gives a favorable condition for the surface heat flux to warm the surface ocean. However, shoaling of MLD could not solely explain the total SST warming within the Indonesian seas. The seasonal dependence in the warming trend, highest during boreal summer, was significantly related to the Indo-Pacific climate modes, namely the negative Indian Ocean Dipole (IOD) and La Niña events. Higher warming trend observed in the south of Makassar Strait and in the eastern Indonesian seas, in the vicinity of the Maluku Sea and the northeastern part of the Banda Sea, was significantly associated with the La Niña event. Meanwhile, strong warming trend observed in the Karimata Strait and Java Sea, and in the Flores Sea south of Sulawesi Island seems to be enhanced by the negative IOD event. Our rough quantitative estimate of the possible mechanism leading to the SST warming suggests that other mechanism might be at work in warming the SST within the Indonesian seas. Horizontal heat advection associated with an increasing trend of the heat transport from the Pacific into the Indian Ocean by the Indonesian Throughflow (ITF) might play a role in causing the warming trend within the Indonesian seas. However, to what extend this heat advection could modulate the SST warming is still unresolved in the present study. Further study based on realistic model output as well as long-term observational records is necessary to describe the dynamics underlying the warming trend within the Indonesian seas.

Potential predictability and skill assessment of boreal summer surface air temperature of South Asia in the North American multimodel ensemble

The potential and actual forecast skill of surface air temperature (SAT) during boreal summer season (June to August: JJA) over South Asia (SA: 65°-100°E and 5°-30°N) is assessed in six ensemble prediction systems from the North American Multimodel Ensemble (NMME) project over the common period 1982–2016, and two different lead times (April and May). All models generally show the observed climatological pattern and variation of the JJA SAT over the region. However, there are sizeable errors in the simulation of the climatological mean SAT mainly over the topographically complex quarters in the area. A statistically significant positive correlation (0.39) is obtained between SA-SAT index and sea surface temperature (SST) anomalies in the El Niño Southern Oscillation (ENSO) region (190°-240°E and 5°S-5°N), which shows that ENSO modulates the summer SA-SAT, inducing warm (cold) conditions during El Niño (La Niña). Also, the SA-SAT index is highly positively correlated (0.75) with the SST anomalies in the tropical Indian Ocean (TIO: 40°-110°E and 20°S-20°N). Each prediction system captures the observed relationship between spatially averaged SA-SAT with SST and 200 hPa geopotential height during JJA, with varying details. Over South Asia, the resulting forecasts are characterized by high potential and moderate actual skill at both leads. Skill drops with the increase of lead time, though some unique models show even higher anomaly correlation coefficient for the predictions started at April observed initial condition. A multimodel ensemble (MME) average of six models shows lower potential predictability as compared to individual models at both lead times. However, the actual skill is higher as compared to different models at both lead times, showing an advantage of MME in predicting boreal summer SA-SAT. Together, these results indicate that current prediction models have quite high potential predictability and moderate forecast skill of boreal summer SAT over the South Asian region.

The impact of varying seasonal lengths of the rainy seasons of India on its teleconnections with tropical sea surface temperatures

AbstractWe present in this paper the interannual variability of seasonal temperature and rainfall in the Indian meteorological subdivisions (IMS) for boreal winter and summer seasons that take in to account the varying length of the seasons. Our study reveals that accounting for the variations in the length of the seasons produces stronger teleconnections between the seasonal anomalies of surface temperature and rainfall over India with corresponding sea surface temperature anomalies of the tropical Oceans (especially over the northern Indian and the equatorial Pacific Oceans) compared to the same teleconnections from fixed length seasons over the IMS. It should be noted that the IMS show significant spatial heterogeneity in these teleconnections.

How Significant are Low-Level Flow Patterns in Tropical Cyclone Genesis over the Western North Pacific?

Abstract The low-level flow pattern characteristics of the tropical cyclone genesis (TCG) environment over the western North Pacific (WNP) were investigated not only for the periods during which TCG occurred, but also for the periods during which no TCG occurred. The flow patterns investigated were the shear line (SL), confluence region (CR), and easterly wave (EW) patterns. Although these flow patterns are recognized as favorable environmental conditions for TCG, their general characteristics, such as the climatological horizontal distribution, have not been previously investigated and there has been no comparison of the significance of the TCG cases to the climatological mean. We examined flow patterns using flow pattern indices defined by the modified semiobjective analysis method. SL and CR were broadly distributed over the WNP east of the Philippines during the boreal summer season. There was a peak in the EW from summer to autumn and it was distributed in a band running in an east–west direction at a lower latitude. Flow pattern indices of all three flow patterns gradually intensified until the TCG date was reached. The SL and EW indices became more intense compared to the climatological mean one day before the TCG, while that of the CR did not exceed the climatological mean. TCG occurred at the eastern edge of the area with a high genesis potential index and relative humidity and a weak vertical shear over flow patterns. This determination of the general characteristics of favorable flow pattern conditions improves our understanding of the TCG process.

Interdecadal modulation of the Indo-western Pacific Ocean Capacitor mode and its influence on Indian summer monsoon rainfall

The present study investigates the interdecadal modulations of interannual variability in the Indo-western Pacific Ocean Capacitor (IPOC) mode and its influence on the Indian summer monsoon (ISM) rainfall during 1886–2014. The IPOC mode is extracted using singular value decomposition (SVD) analysis of tropical Indian Ocean (TIO) sea surface temperature (SST) and western North Pacific (WNP) 850 hPa relative vorticity anomalies during the boreal summer season (June to August; JJA). Analysis suggests that the IPOC mode induces a southwest-northeast dipole pattern in the Indian subcontinent rainfall with significant positive anomalies over the southern peninsular India and negative anomalies over the eastern parts of the Indo-Gangetic plains during the positive phase of IPOC. The 21-year sliding correlation coefficients between rainfall dipole index and leading principal component of SVD analysis (SVD-PC1) suggest that IPOC and ISM rainfall relationship exhibits significant interdecadal variability during the study period. The entire period has been divided into three distinct epochs as 1895–1926 (epoch-1), 1932–1972 (epoch-2) and 1976–2007 (epoch-3) based on 21-year sliding correlation. It is found that the IPOC mode strongly affect ISM rainfall in epoch-1 and epoch-3, whereas this relationship between the IPOC mode and ISM rainfall is inadequate in epoch-2 pertaining to the absence of the dipole rainfall pattern. The mechanisms responsible for such epochal changes in relationship between IPOC mode and ISM rainfall are discussed. In epoch-1 and epoch-3, anomalous TIO SST and the Pacific–Japan (PJ) pattern/WNP circulation both are well organized as part of the IPOC mode. However, in epoch-2 vigorous warm SSTs over the equatorial central Pacific and cyclonic circulation over the north Pacific accompanied by SST cooling around 40°N are noticed. The north Pacific cyclonic circulation accelerates the southwesterlies emerging from north-western flank of WNP anticyclone. On the other hand, positive SST anomalies and associated low-level convergence in central equatorial Pacific limit the extension of northeasterlies from the southern edge of WNP anticyclone to TIO. As a result, WNP anticyclone is squeezed/twisted between them, thereby weakening and shifting it northeastward. This in turn weakens the PJ mode and the inter-basin interaction of WNP circulation and TIO SSTs, causing disorganized IPOC pattern in epoch-2.

Congo Basin drying associated with poleward shifts of the African thermal lows

Trends in Congo Basin rainfall in six observational datasets are examined on monthly and annual time scales to motivate an investigation of changes in the atmospheric hydrodynamics. Annual-mean Congo Basin rainfall declines over the 1979–2017 time period in all datasets, with strongest agreement and statistical significance from March through August. The trends are less pronounced over the 1998–2017 time period, especially for boreal spring, with although the boreal summer season continues to dry. Decadal-scale differences in the atmospheric hydrodynamics are examined in three reanalyses to improve our physical understanding of the precipitation trends and add confidence. During much of the spring and fall, changes in the atmospheric circulation reflect regional processes and feedbacks. During the warm season in each hemisphere, Congo Basin precipitation is supported when the circulation about continental thermal lows converges with cross-equatorial flow from the winter hemisphere. The drying trend during these seasons is associated with poleward shifts of the continental thermal lows, which weakens this convergence. Rainfall anomalies are not directly related to local surface warming, and they do not involve modifications of moisture transport from the Atlantic or Indian Oceans. For boreal summer, the drying is related to amplified warming over the Sahara. In austral summer, a southward shift of the thermal low is part of a large-scale, zonal mean pattern shifting the subtropical highs poleward.

Tropical intraseasonal oscillation at monthly time scale during boreal summer and winter

This study describes the space–time structure of an intraseasonal oscillation with the time scale of a month over the South Asian monsoon region and the tropical Pacific for the boreal summer and winter seasons. These nonlinear oscillations were extracted by applying multichannel singular spectrum analysis on daily anomalies of three-dimensional diabatic heating separately during the summer and winter. The monthly oscillations (MOs) are distinct from the leading monsoon intraseasonal oscillation with a period of 45 days during the summer and the Madden–Julian Oscillation during the winter. The summer MO exhibits horizontal quadrupole structure predominantly in the Northern Hemisphere and propagates eastward over the Indian region and westward over the Pacific along with northward movement. The winter MO is confined to the equatorial region in the Southern Hemisphere and propagates eastward and southward. Both the summer and winter MOs consist of propagating deep vertical columns of heating and cooling anomalies. The associated fields of convection, precipitation and three-dimensional circulation show similar patterns and propagation. The corresponding specific humidity has revealed moisture preconditioning for horizontal propagation and co-located vertical advection. However, there is no clear pattern of ocean–atmosphere interaction associated with the MO as seen in the examination of sea surface temperature and latent heat flux.

The role of air-sea coupling in the downscaled hydroclimate projection over Peninsular Florida and the West Florida Shelf

A comparative analysis of two sets of downscaled simulations of the current climate and the future climate projections over Peninsular Florida (PF) and the West Florida Shelf (WFS) is presented to isolate the role of high-resolution air-sea coupling. In addition, the downscaled integrations are also compared with the much coarser, driving global model projection to examine the impact of grid resolution of the models. The WFS region is habitat for significant marine resources, which has both commercial and recreational value. Additionally, the hydroclimatic features of the WFS and PF contrast each other. For example, the seasonal cycle of surface evaporation in these two regions are opposite in phase to one another. In this study, we downscale the Community Climate System Model version 4 (CCSM4) simulations of the late twentieth century and the mid-twenty-first century (with reference concentration pathway 8.5 emission scenario) using an atmosphere only Regional Spectral Model (RSM) at 10 km grid resolution. In another set, we downscale the same set of CCSM4 simulations using the coupled RSM-Regional Ocean Model System (RSMROMS) at 10 km grid resolution. The comparison of the twentieth century simulations suggest significant changes to the SST simulation over WFS from RSMROMS relative to CCSM4, with the former reducing the systematic errors of the seasonal mean SST over all seasons except in the boreal summer season. It may be noted that owing to the coarse resolution of CCSM4, the comparatively shallow bathymetry of the WFS and the sharp coastline along PF is poorly defined, which is significantly rectified at 10 km grid spacing in RSMROMS. The seasonal hydroclimate over PF and the WFS in the twentieth century simulation show significant bias in all three models with CCSM4 showing the least for a majority of the seasons, except in the wet June-July-August (JJA) season. In the JJA season, the errors of the surface hydroclimate over PF is the least in RSMROMS. The systematic errors of surface precipitation and evaporation are more comparable between the simulations of CCSM4 and RSMROMS, while they differ the most in moisture flux convergence. However, there is considerable improvement in RSMROMS compared to RSM simulations in terms of the seasonal bias of the hydroclimate over WFS and PF in all seasons of the year. This suggests the potential rectification impact of air-sea coupling on dynamic downscaling of CCSM4 twentieth century simulations. In terms of the climate projection in the decades of 2041–2060, the RSMROMS simulation indicate significant drying of the wet season over PF compared to moderate drying in CCSM4 and insignificant changes in the RSM projection. This contrasting projection is also associated with projected warming of SSTs along the WFS in RSMROMS as opposed to warming patterns of SST that is more zonal and across the WFS in CCSM4.

Trends of Southern Appalachian Seasonal Air Temperature and Regional Lapse Rate Spanning 1969–2018

Decadal trends of continuous seasonal maximum and minimum air temperature and air temperature range are analyzed by elevation within the southern Appalachian Mountains of the southeastern United States. Temperature data included spanned December 1969 to November 2018 and were obtained from the quality controlled Global Historical Climatology Network-monthly, version 4. Linear least squares regression indicates statistically-significant warming trends with minimum seasonal air temperature typically increasing more than maximum seasonal air temperature, reflecting a decrease of the air temperature range with time, particularly during the boreal meteorological winter, spring, and summer seasons. Regressions of station decadal trends versus station elevation illustrate the presence of elevation-dependent warming with the magnitude of decadal temperature trends decreasing as station elevation increases, at least up to 850 m above mean sea level. As a result, the regional vertical temperature lapse rate was also found to increase with time, suggesting the possibility for precipitation and convection lift implications. From a biogeography standpoint, the increasing vertical temperature gradient may imply an acceleration of species upslope migration with time.

Arctic summer sea-ice seasonal simulation with a coupled model: Evaluation of mean features and biases

Current state of the art weather/climate models are representation of the fully coupled aspects of the components of the earth system. Sea-ice is one of the most important components of these models. Simulation of sea-ice in these models is a challenging problem. In this study, evaluation of the hind-cast data of 14 boreal summer seasons with global coupled model HadGEM3 in its seasonal set-up has been performed over the Arctic region from \(9\mathrm{th}\) May start dates. Along with the biases of the sea-ice variables, related atmosphere and oceanic variables have also been examined. The model evaluation is focused on seasonal mean of sea-ice concentration, sea-ice thickness, ocean surface current, SST, ice-drift velocity and sea-ice extent. To diagnose the sea-ice biases, atmospheric variables like, 10 m wind, 2 m air temperature, sea-level pressure and ocean sub-surface temperatures were also examined. The sea-ice variables were compared with GIOMAS dataset. The atmospheric and the oceanic variables were compared with the ERA Interim and the ECMWF Ocean re-analysis (ORAP5) datasets, respectively. The model could simulate the sea-ice concentration and thickness patterns reasonably well in the Arctic Circle. However, both sea-ice concentration and thickness in the model are underestimated compared to observations. A positive (warm) bias is seen both in 2 m air temperature and SST, which are consistent with the negative sea-ice bias. Biases in ocean current and related ice drift are not related to biases in the atmospheric winds. The magnitude of the oceanic subsurface warm biases is seen to be gradually decreasing with depth, but consistent with sea-ice biases. These analyses indicate a possibility of deeper warm subsurface water in the western Arctic Ocean sector (Pacific and Atlantic exchanges) affecting the negative biases in the sea-ice at the surface. The model is able to simulate reasonably well the summer sea-ice melting process and its inter-annual variability, and has useable skill for application purpose.

Propagation Characteristics of BSISO Indices

AbstractThe spatial structure and propagation of tropical intraseasonal convection anomalies diagnosed with the outgoing longwave radiation‐based Madden‐Julian Oscillation index are examined in the boreal summer and winter seasons. It is shown that the outgoing longwave radiation‐based Madden‐Julian Oscillation index represents both northward and eastward propagation in summer and eastward propagation in winter in a manner consistent with Madden‐Julian Oscillation and Boreal Summer Intraseasonal Oscillation propagation as diagnosed in many previous studies. The outgoing longwave radiation‐based Madden‐Julian Oscillation index and three other widely used indices for tracking the Boreal Summer Intraseasonal Oscillation are then compared in their lag correlation structure over selected reference areas, cross‐correlation coefficients of the two principal component time series, and time‐dependent phase angle composites. The outgoing longwave radiation anomalies from these different indices propagate differently according to these diagnostics. One of them exhibits little propagation at all, even though one would expect good propagation based on composites of the phases separately. This illustrates the general point that while composites of individual phases, presented in a sequence, are generally taken to imply that the phases tend to occur in that sequence in time, that need not be the case. It is suggested that propagation characteristics are relevant to the application of the indices and that smoother propagation is desirable.

Seasonal variability in vulnerability for Cassin's auklets (Ptychoramphus aleuticus) exposed to microplastic pollution in the Canadian Pacific region

Marine plastic pollution is an emerging global conservation challenge, potentially impacting organisms at all trophic levels. However, currently it is unclear to what extent plastic pollution is impacting marine organisms at the population, species or multispecies level. In this study, we explore seasonal exposure (i.e., vulnerability) of Cassin's Auklet (Ptychoramphus aleuticus) to plastic pollution with exposure models during boreal summer and winter seasons. Based on these models, we infer exposure at the population level for this species, in the Canadian Pacific region where approximately 75% of the global population of this species breeds. The models quantify plastic exposure by determining seasonal core foraging areas and plastic concentrations found in those same areas. Core foraging areas were determined using a Generalized Additive Model based on at-sea observation data (collected year round: 1990–2010) and 50% Home Range Kernels based on aerial telemetry data (May and June 1999–2001). Plastic concentrations within these core areas were interpolated based on seawater microplastic concentrations from the summer of 2012. We found that during the boreal summer, Cassin's Auklets were exposed to relatively low concentrations of plastics. During the winter, auklet distribution shifted towards the coast where plastic concentrations are considerably higher. Model derived seasonal variability in exposure was consistent with necropsy results from bird carcasses recovered during the winter of 2014, and from a multiyear study on chick provisioning during the summer. Local oceanography likely plays a role in determining seasonal shifts in both marine bird as well microplastic concentrations, and hence exposure. As well, individual sensitivity (i.e., dose-dependent effect) may vary with annual cycles. Currently, research is focusing on determining how sensitive individual birds are to microplastic concentrations, and our models will help translate sensitivity found at the individual level to potential impacts at population or species level.

Loss of predictive skill of indian summer monsoon rainfall in NCEP CFSv2 due to misrepresentation of Atlantic zonal mode

Observational and modeling studies have identified an El Nino Southern Oscillation (ENSO) like Ocean-Atmospheric coupled phenomenon in the tropical Atlantic during the boreal summer season, popularly known as Atlantic Zonal Mode (AZM). The atmospheric teleconnection between the AZM and Indian Summer Monsoon Rainfall (ISMR) is significant especially in non-ENSO years. Hence, realistic simulation of AZM and its teleconnection with ISMR in coupled Ocean-Atmospheric models is important in improving the predictive skill of ISMR. Here, we analyzed the outputs of nine-month hindcast simulations of a coupled ocean-atmospheric model (NCEP CFSv2) in estimating the prediction skill of AZM and its teleconnection with ISMR. It is found that the AZM prediction skill in CFSv2 hindcasts initialized in the month of February (FebIC) is poor and it simulates an unrealistic teleconnection with ISMR. The CFSv2 FebIC hindcasts are unable to forecast the correct phase of the low level wind anomalies over western Atlantic basin, which leads to an error in the forecast of AZM. It is shown here that the error in the prediction of AZM and its erroneous teleconnection with ISMR leads to a loss of prediction skill of ISMR in CFSv2 FebIC hindcasts. It is further noted that the shorter lead time forecast of CFSv2 initialized in the month of May (MayIC hindcasts) shows improved prediction skill of AZM and its associated teleconnection with ISMR compared to the longer lead time forecast (FebIC hindcasts). However, the prediction skill of ISMR is low in MayIC hindcasts compared to FebIC hindcasts. We find that the low prediction skill of ISMR in May IC hindcasts is due to the misrepresentation of the teleconnection between ENSO and ISMR. A notable improvement in the prediction skill of ISMR in CFSv2 can be achieved by combining the ENSO induced ISMR from FebIC hindcasts and AZM induced ISMR from MayIC hindcasts. These results aid in improving the ISMR seasonal forecast in CFSv2. Besides, this study highlights the need to improve the Atlantic variability and its teleconnection with ISMR in the pursuit of improving prediction skill of ISMR in coupled climate models.

A spurious positive Indian Ocean Dipole in 2017

The Indian Ocean Dipole (IOD) is an intrinsic ocean-atmosphere coupled mode in the tropical Indian Ocean (IO). It features a sea surface temperature (SST) anomaly gradient, defined by the dipole mode index (DMI), the difference of area-averaged SSTanomaly between the western tropical IO (50°–70°E, 10°N–10°S) and the southeastern tropical IO (90°–110°E, Eq.-10°S). The IOD event tends to develop and mature in the boreal summer and fall seasons. When a positive IOD (pIOD) occurs, SST warms up in the west and cools down in the east, which sets up an east-west pressure gradient. The atmosphere hence responds to the anomalous SST gradient. Anomalous easterly winds prevail along the equator driven by the zonal pressure gradient, causing low sea level anomaly (SLA) in the eastern tropical IO. Rainless weather dominates over the Maritime Continent due to a weakened convection. Anomalies of an opposing polarity are generated during a negative IOD event.