Anomalous Temperatures Research Articles

Research Spotlight: Deadly 2010 Russian heat wave not a consequence of climate change

Although some people may try to ascribe specific extreme weather events to climate change, global warming cannot be held responsible for recent weather events such as the 2010 Russian heat wave. Using climate simulations and a comparison against historical conditions, Dole et al. assess the influence of greenhouse gases, aerosols, anomalous sea surface temperatures, and other potential climate forcings on the likelihood and magnitude of the 2010 Russian heat wave. The authors suggest that the heat wave—which lasted from late June to mid‐August and was responsible for thousands of deaths, widespread wildfires, and devastating crop loss—fell well within the bounds of natural climate variability. The authors find that none of the tested climate factors showed appreciable ability to predict the extreme temperatures seen throughout the heat wave. Additionally, the researchers' historical analysis revealed that July temperatures, as well as the temperature variability, for the affected region of western Russia showed no significant trend over the past 130 years. They note that the top 10 hottest July days for the region were distributed randomly across the historical period, although global averages do show clustering in the past 2 decades. (Geophysical Research Letters, doi:10.1029/2010GL046582, 2011)

Drought under global warming: a review

AbstractThis article reviews recent literature on drought of the last millennium, followed by an update on global aridity changes from 1950 to 2008. Projected future aridity is presented based on recent studies and our analysis of model simulations. Dry periods lasting for years to decades have occurred many times during the last millennium over, for example, North America, West Africa, and East Asia. These droughts were likely triggered by anomalous tropical sea surface temperatures (SSTs), with La Niña‐like SST anomalies leading to drought in North America, and El‐Niño‐like SSTs causing drought in East China. Over Africa, the southward shift of the warmest SSTs in the Atlantic and warming in the Indian Ocean are responsible for the recent Sahel droughts. Local feedbacks may enhance and prolong drought. Global aridity has increased substantially since the 1970s due to recent drying over Africa, southern Europe, East and South Asia, and eastern Australia. Although El Niño‐Southern Oscillation (ENSO), tropical Atlantic SSTs, and Asian monsoons have played a large role in the recent drying, recent warming has increased atmospheric moisture demand and likely altered atmospheric circulation patterns, both contributing to the drying. Climate models project increased aridity in the 21st century over most of Africa, southern Europe and the Middle East, most of the Americas, Australia, and Southeast Asia. Regions like the United States have avoided prolonged droughts during the last 50 years due to natural climate variations, but might see persistent droughts in the next 20–50 years. Future efforts to predict drought will depend on models' ability to predict tropical SSTs. WIREs Clim Change 2011 2 45–65 DOI: 10.1002/wcc.81This article is categorized under: Paleoclimates and Current Trends > Detection and Attribution Paleoclimates and Current Trends > Modern Climate Change Assessing Impacts of Climate Change > Evaluating Future Impacts of Climate Change Assessing Impacts of Climate Change > Observed Impacts of Climate Change

Climate response to tropical cyclone‐induced ocean mixing in an Earth system model of intermediate complexity

We introduce a parameterization of ocean mixing by tropical cyclones (TCs) into an Earth system model of intermediate complexity. The parameterization is based on previously published global budgets of TC‐induced mixing derived from high‐resolution satellite measurements of anomalous sea surface temperatures along storm tracks. Recognizing the caveats introduced, for example, by the simplified model structure, we find that the representation of realistic TC‐induced mixing substantially alters the equilibrium conditions of (1) the thermal structure of the upper ocean, (2) the surface energy budget, and (3) the circulation in the equatorial to subtropical Pacific Ocean. These changes result in warmer upwelling regions in the eastern equatorial Pacific and an overall increase in ocean heat content consistent with the recent TC heat pump hypothesis. Spatial variability in the mixing appears to be a key factor in the modeled response. We find no substantial influence of the considered TC‐induced mixing on poleward ocean heat transport in the analyzed model. Our results suggest that climate‐sensitive mixing feedbacks are plausible; however, the large‐scale effect is mainly confined to the subtropical Indo‐Pacific region for present‐day TC climatology.

Revised phosphate–water fractionation equation reassessing paleotemperatures derived from biogenic apatite

Oxygen isotopes of biogenic apatite have been widely used to reassess anomalous temperatures inferred from oxygen isotope ratios of ancient biogenic calcite, more prone to diagenetic alteration. However, recent studies have highlighted that oxygen isotope ratios of biogenic apatite differ dependent on used analytical techniques. This questions the applicability of the phosphate–water fractionation equations established over 25 years ago using earlier analytical techniques to more recently acquired data. In this work we present a new phosphate–water oxygen isotope fractionation equation based on oxygen isotopes determined on fish raised in aquariums at controlled temperature and with monitored water oxygen isotope composition. The new equation reveals a similar slope, but an offset of about + 2‰ to the earlier published equations. This work has major implications for paleoclimatic reconstructions using oxygen isotopes of biogenic apatite since calculated temperatures have been underestimated by about 4 to 8 °C depending on applied techniques and standardization of the analyses.

Atmospheric circulation anomalies during two persistent north american droughts: 1932–1939 and 1948–1957

We use an early twentieth century (1908–1958) atmospheric reanalysis, based on assimilation of surface and sea level pressure observations, to contrast atmospheric circulation during two periods of persistent drought in North America: 1932–1939 (the ‘Dust Bowl’) and 1948–1957. Primary forcing for both droughts is believed to come from anomalous sea surface temperatures (SSTs): a warm Atlantic and a cool eastern tropical Pacific. For boreal winter (October–March) in the 1950s, a stationary wave pattern originating from the tropical Pacific is present, with positive centers over the north Pacific and north Atlantic ocean basins and a negative center positioned over northwest North America and the tropical/subtropical Pacific. This wave train is largely absent for the 1930s drought; boreal winter height anomalies are organized much more zonally, with positive heights extending across northern North America. For boreal summer (April–September) during the 1930s, a strong upper level ridge is centered over the Great Plains; this feature is absent during the 1950s and appears to be linked to a weakening of the Great Plains low-level jet (GPLLJ). Subsidence anomalies are co-located over the centers of each drought: in the central Great Plains for the 1930s and in a band extending from the southwest to the southeastern United States for the 1950s. The location and intensity of this subsidence during the 1948–1957 drought is a typical response to a cold eastern tropical Pacific, but for 1932–1939 deviates in terms of the expected intensity, location, and spatial extent. Overall, circulation anomalies during the 1950s drought appear consistent with the expected response to the observed SST forcing. This is not the case for the 1930s, implying some other causal factor may be needed to explain the Dust Bowl drought anomalies. In addition to SST forcing, the 1930s were also characterized by massive alterations to the land surface, including regional-scale devegetation from crop failures and intensive wind erosion and dust storms. Incorporation of these land surface factors into a general circulation model greatly improves the simulation of precipitation and subsidence anomalies during this drought, relative to simulations with SST forcing alone. Even with additional forcing from the land surface, however, the model still has difficulty reproducing some of the other circulation anomalies, including weakening of the GPLLJ and strengthening of the upper level ridge during AMJJAS. This may be due to either weaknesses in the model or uncertainties in the boundary condition estimates. Still, analysis of the circulation anomalies supports the conclusion of an earlier paper (Cook et al. in Proc Natl Acad Sci 106:4997, 2009), demonstrating that land degradation factors are consistent with the anomalous nature of the Dust Bowl drought.

A Review of the European Summer Heat Wave of 2003

This paper reviews the European summer heat wave of 2003, with special emphasis on the first half of August 2003, jointly with its significant societal and environmental impact across Western and Central Europe. We show the pattern of record-breaking temperature anomalies, discuss it in the context of the past, and address the role of the main contributing factors responsible for the occurrence and persistence of this event: blocking episodes, soil moisture deficit, and sea surface temperatures. We show that the anticyclonic pattern corresponds more to an anomalous northern displacement of the North Atlantic subtropical high than a canonical blocking structure, and that soil moisture deficit was a key factor to reach unprecedented temperature anomalies. There are indications that the anomalous Mediterranean Sea surface temperatures (SSTs) have contributed to the heat wave of 2003, whereas the role of SST anomalies in other oceanic regions is still under debate. There are methodological limitations to evaluate excess mortality due to excessive temperatures; however, the different studies available in the literature allow us to estimate that around 40,000 deaths were registered in Europe during the heat wave, mostly elderly persons. Despite previous efforts undertaken by a few cities to implement warning systems, this dramatic episode has highlighted the widespread un-preparedness of most civil and health authorities to cope with such large events. Therefore, the implementation of early warning systems in most European cities to mitigate the impact of extreme heat is the main consequence to diminish the impact of future similar events. In addition to mortality (by far the most dramatic impact), we have also analyzed the record-breaking forest fires in Portugal and the evidence of other relevant impacts, including agriculture and air pollution.

500-year temperature reconstruction in the Mediterranean Basin by means of documentary data and instrumental observations

The paper reports the main results of the EU project Millennium in the Mediterranean area over the last 500 years. It analyses a long series of temperature from Portugal, Spain, France, Italy and Greece. The series are obtained by combining indices from documentary sources from AD 1500 to the onset of regular instrumental observations. There is an ongoing discussion regarding the proper way of combining documentary and instrumental data and how to translate accurately the conventional indices from − 3 to + 3 into modern units, i.e. degree Celsius. This paper produces for the first time a number of early instrumental observations, in some cases (i.e. Italy and France) covering 350 years, including thereby the earliest regular observations after the invention of the thermometer. These Mediterranean data show that anomalous temperatures usually had only a locally limited effect, while only few extreme events had a widespread impact over the whole region, such as the summer of 2003. During the period from 1850 to the present day, the Mediterranean temperature anomaly was close to the Northern Hemisphere in spring and summer, while it was warmer in autumn and winter. Compared with the long-term instrumental records (i.e. 1655 onwards), the recent warming has not exceeded the natural past variability characterized by heating–cooling cycles with no significant long-term trends.

Diagnosis of Extended Cold-Season Temperature Anomalies in Alaska

Abstract During the early winter of 2002 and late winter of 2007, the Alaskan sector of the North Pacific Ocean region experienced record-breaking temperature anomalies. The duration of these episodes was unusually long, with each lasting more than 1 month: 55 days for the warm anomaly of October–December 2002 and 37 days for the cold anomaly of February–March 2007. Temperature departures over each respective period were the largest for the continental climate of interior Alaska (>10°C) and the smallest for the maritime regions of Alaska (<4°C). Mean temperatures over the event periods in 2002 and 2007 easily ranked as the record warmest and coldest, respectively, for many surface observing stations. In addition, heating degree-day anomalies were on the order of 700 units for these periods. Atmospheric circulation patterns at the surface and upper levels for the circum-Arctic proved to be the driver for these persistent events. The 2002 warm anomaly was driven by enhanced southerly advection associated with an unusually strong Aleutian low and a positive Pacific decadal oscillation index, which resulted in a large area of anomalous temperatures in Alaska and northern Canada. The 2007 cold anomaly was driven by a weakening of the circulation pattern in the subpolar Pacific sector and a strengthening of the Siberian high, with the strongest temperature anomalies in Alaska and northwestern Canada.

On the recent warming in the Murray‐Darling Basin: Land surface interactions misunderstood

Previous studies of the recent drought in the Murray‐Darling Basin (MDB) have noted that low rainfall totals have been accompanied by anomalously high air temperatures. Subsequent studies have interpreted an identified trend in the residual timeseries of non‐rainfall related temperature variability as a signal of anthropogenic change, further speculating that increased air temperature has exacerbated the drought through increasing evapotranspiration rates. In this study, we explore an alternative explanation of the recent increases in air temperature. This study demonstrates that significant misunderstanding of known processes of land surface – atmosphere interactions has led to the incorrect attribution of the causes of the anomalous temperatures, as well as significant misunderstanding of their impact on evaporation within the Murray‐Darling Basin.

A Ten-year Period of Daily Sea Surface Temperature at a Coastal Station in Reunion Island, Indian Ocean (July 1993 – April 2004): Patterns of Variability and Biological Responses

Sea surface temperature (SST) was recorded hourly by an automatic data recorder in Reunion Island, at Pointe des Galets (21°55 S, 55°17 E) during 1993-2004. The data logger was installed on a beacon located at the port entrance exposed to the open sea. The SST measurements associated with auxiliary environmental data, such as wind stress, depict the main patterns of variability at various timescales for the marine climate of Reunion Island. The ’ten-year’ annual mean temperature is 25.7°C and the annual amplitude is 4.6°C. The highest monthly means are observed in February and March (28.0°C) and the lowest occur in September (23.4°C). The daily variation ranges from 0.25 to 0.74°C according to the season. In summer, the tropical cyclones are the major cause of short term variability, with sudden drops of SST than can exceed 2°C within a few hours. The annual cycle of SST is closely associated with that of wind stress, with a lagged response of about 2 months of SST to wind forcing. Throughout the ‘ten-year period’ covered by our dataset, the coldest years were 1993 and 2000, and the warmest were 2003 and 2004. A trend of increasing SST is suggested for the two major seasons, with a magnitude of 0.088°C/yr in summer and 0.052°C/yr in winter. Finally, the SST trend and variability depicted at our sampling site is shown to reflect the SST patterns of the whole south tropical Indian Ocean. Biological responses to SST variability are shown by coral bleaching events and the local tuna fishery. The major coral bleaching events recorded in Reunion in 1998, 2001, 2003 and 2004 occurred during episodes of intense and sustained anomalous high temperatures. The catch per unit effort (CPUE) of the local pelagic fishery was negatively correlated to SST anomalies with a decreasing trend of CPUE observed over the years, accompanied by an overall increase of SST. The SST observations made in Reunion Island, in a largely unsampled region of the Indian Ocean, show their relevance at a larger regional scale and their usefulness in monitoring changes of some biological components of the marine ecosystem. These examples highlight the need to maintain networks of automatic loggers worldwide at coastal stations.

Global Signatures and Dynamical Origins of the Little Ice Age and Medieval Climate Anomaly

Global temperatures are known to have varied over the past 1500 years, but the spatial patterns have remained poorly defined. We used a global climate proxy network to reconstruct surface temperature patterns over this interval. The Medieval period is found to display warmth that matches or exceeds that of the past decade in some regions, but which falls well below recent levels globally. This period is marked by a tendency for La Niña-like conditions in the tropical Pacific. The coldest temperatures of the Little Ice Age are observed over the interval 1400 to 1700 C.E., with greatest cooling over the extratropical Northern Hemisphere continents. The patterns of temperature change imply dynamical responses of climate to natural radiative forcing changes involving El Niño and the North Atlantic Oscillation-Arctic Oscillation.

The Gulf Stream and Atlantic sea-surface temperatures in AD1790-1825

We present gridded sea-surface temperatures (SSTs) for the Atlantic basin (45°S–60°N) as averages over the period AD1790–1825, based on early-instrumental SST data. The original measurements were compiled by Major James Rennell and made by numerous British naval vessels on behalf of the British Admiralty. We describe the digitization of this dataset and the reconstruction of spatially coherent, averaged conditions for the boreal cold (November-March) and warm (May–September) season using a reduced space optimal interpolation (RSOI) technique, in which the data is projected on a limited number of empirical orthogonal functions. This approach is validated on modern data that are sampled in a similar way as the early-instrumental data. The reconstruction for the November–March period shows a large area with anomalously high temperatures from the point where the Gulf Stream separates from the coast until ca. 20°W. A tongue of anomalous cool water is found at the eastern side of the North Atlantic basin, along the coast of Europe and northern Africa. In the northeastern South Atlantic, anomalously high temperatures are found, while temperatures in the southwestern South Atlantic are anomalously cool. For the March–September season, anomalous temperatures in the South Atlantic are similar, but stronger, compared with those in the boreal cold season. Over the North Atlantic, there is not much similarity between the current SST reconstructions and those published in the late 1950s. Copyright © 2009 Royal Meteorological Society

Prospects for predicting two flavors of El Niño

Global climatic impacts of El Niño are sensitive to details of the surface warming of the equatorial Pacific Ocean, which vary between each El Niño event. The ability to predict the differences in pattern of anomalous ocean temperatures is explored for two prominent types of El Niño, traditional cold tongue events that have maximum surface warming in the eastern Pacific, and warm pool events that have maximum warming in the central Pacific. We assess seasonal predictions of the two types of El Niño using the Australian Bureau of Meteorology coupled ocean‐atmosphere seasonal forecast model. Prediction of the major differences in pattern of anomalous ocean surface temperature between the two types of El Niño is limited to less than 1 season lead time, which is much shorter than for prediction of the occurrence of El Niño but which does have important practical application for prediction of regional climate. Improved understanding of the mechanisms of warm pool events and reduction of systematic model biases of the mean state and the coupled modes of variability in the Pacific warm pool/cold tongue should lead to improved skill for predicting regional climate variability associated with El Niño.

Determination of potential mine water discharge zones in crystalline rocks at Rozna, Czech Republic

The Rozna Mine is one of the last active uranium mines in Europe. When the mine is closed and flooded, the natural groundwater flow pattern will be partly restored. Re-established groundwater flow system will be associated with an increase of groundwater discharge into draining rivers and streams. Since the groundwater inflows to streams can be contaminated by the mine water, the groundwater drainage characteristic of fractured aquifer should be carefully identified. Several methods of groundwater discharge zones identification were used including morphological analysis, thermometry, and electrical conductivity (EC) measurements. Stream temperatures and EC at more than 700 points in the area covering about 85 km2 were measured. The measurements were performed during winter period, when stream discharges were low and there was a maximum temperature contrast. There was a frequent presence of preferential discharge zones with resulting anomalous temperatures and electric conductivity values of stream water. The results show evident correlation of discharge zones with surface morphology and geological settings. Just like the aquifer discharge characteristics, the aquifer is strongly heterogeneous. The thermometry supported by measurement of EC proved to be a useful tool for large-scale investigation of groundwater flow and drainage in fractured aquifers.

Distinct Principal Modes of Early and Late Summer Rainfall Anomalies in East Asia*

Abstract The current seasonal prediction of East Asia (EA) summer monsoon deals with June–July–August (JJA) mean anomalies. This study shows that the EA summer monsoon may be divided into early summer [May–June (MJ)] and late summer [July–August (JA)] and exhibits remarkable differences in mean state between MJ and JA. This study reveals that the principal modes of interannual precipitation variability have distinct spatial and temporal structures during the early and late summer. These principal modes can be categorized as either El Niño–Southern Oscillation (ENSO) related or non-ENSO related. During the period of 1979–2007, ENSO-related modes explain 35% of MJ variance and 45% of JA variance, and non-ENSO-related modes account for 25% of MJ variance and 20% of JA variance. For ENSO-related variance, about two-thirds are associated with ENSO decaying phases, and one-third is associated with ENSO developing phases. The ENSO-related MJ modes generally concur with rapid decay or early development of ENSO episodes, and the opposite tends to apply to ENSO-related JA modes. The non-ENSO MJ mode is preceded by anomalous land surface temperatures over southern China during the previous March and April. The non-ENSO JA mode is preceded by lasting equatorial western Pacific (the Niño-4 region) warming from the previous winter through late summer. The results suggest that 1) prediction of bimonthly (MJ) and (JA) anomalies may be useful, 2) accurate prediction of the detailed evolution of ENSO is critical for prediction of ENSO-related bimonthly rainfall anomalies over East Asia, and 3) non-ENSO-related modes are of paramount importance during ENSO neutral years. Further establishment of the physical linkages between the non-ENSO modes and their corresponding precursors may provide additional sources for EA summer monsoon prediction.

Modeling impacts of vegetation in western China on the summer climate of northwestern China

Using the monthly NCEP-NCAR reanalysis dataset, the monthly temperature and precipitation at surface stations of China, and the MM5 model, we examine impacts of vegetation cover changes in western China on the interdecadal variability of the summer climate over northwestern China during the past 30 years. It is found that the summer atmospheric circulation, surface air temperature, and rainfall in the 1990s were different from those in the 1970s over northwestern China, with generally more rainfall and higher temperatures in the 1990s. Associated with these changes, an anomalous wave train appears in the lower troposphere at the midlatitudes of East Asia and the low-pressure system to the north of the Tibetan Plateau is weaker. Meanwhile, the South Asian high in the upper troposphere is also located more eastward. Numerical experiments show that change of vegetation cover in western China generally forces anomalous circulations and temperatures and rainfall over these regions. This consistency between the observations and simulations implies that the interdecadal variability of the summer climate over northwestern China between the 1990s and 1970s may result from a change of vegetation cover over western China.

Present and future atmospheric blocking and its impact on European mean and extreme climate

Atmospheric blocking plays an important role in the mid‐latitude climate variability and can be responsible for anomalous mean and/or extreme climate. In this study, a potential vorticity based blocking indicator is used to investigate the representation of Euro‐Atlantic atmospheric blocking events in the ECHAM5/MPI‐OM climate model. The impact of blocking events on present and future mean and extreme climate is studied by means of composite maps and correlation analyses. In comparison to ERA‐40 re‐analysis, the model represents the blocking frequency and seasonal distribution well. We show that European blocking events have a sustained influence particularly on anomalous cold winter temperatures in Europe. In a future climate, the blocking frequency is slightly diminished but the influence on the European winter climate remains robust. Due to a northeastward shift of the blocking pattern and an increase in maximum blocking duration, cold winter temperature extremes can still be expected in a future climate.

Receiver function images from the Moho and the slab beneath the Altiplano and Puna plateaus in the Central Andes

SUMMARY Teleseismic data recorded during one and a half years are investigated with the receiver function technique to determine the crustal and upper-mantle structures underneath the highly elevated Altiplano and Puna plateaus in the central Andes. A series of converting interfaces are determined along two profiles at 21 ◦ S and 25.5 ◦ S, respectively, with a station spacing of approximately 10 km. The data provide the highest resolution gained from a passive project in this area, so far. The oceanic Nazca plate is detected down to 120 km beneath the Altiplano whereas beneath the Puna, the slab can unexpectedly be traced down to 200 km depth at longer periods. A shallow crustal low-velocity zone is determined beneath both plateaus exhibiting segmentation. In the case of the Altiplano, the segments present vertical offsets and are separated by inclined interfaces, which coincide with major fault systems at the surface. An average depth to Moho of about 70 km is determined for the Altiplano plateau. A strong negative velocity anomaly located directly below the Moho along with local crustal thinning is interpreted beneath the volcanic arc of the Altiplano plateau between 67 ◦ W and 68.5 ◦ W. A deep section of the Puna profile reveals thinning of the mantle transition zone. Although poorly resolved, the detected anomaly may suggest the presence of a mantle plume, which may constitute the origin of the anomalous temperatures at the depth of the upper-mantle discontinuities.

Hydrochemical signatures of deep groundwater circulation in a part of the Himalayan foreland basin

Groundwater samples were analyzed from 71 springs and wells as part of a larger study in a region of compressional tectonic regime. The study site covers the Peshawar basin and surroundings in the Himalayan foreland of Pakistan. The northern portion is mountainous and the water table is discontinuous in different intermontane valleys, with abundant springs (with normal and anomalous temperatures and composition). The southern part is divided into isolated basins with a number of drilled (“deep”) and dug (“shallow”) wells. Hydrochemical signatures of elevated strontium (Sr), SiO2, boron (B)—and the geothermometric signatures—all indicate a deep circulation of the emerging groundwater. Moreover, for several of the sample sites, water chemical compositions, measured spring and water well temperatures, and reservoir temperatures calculated for spring waters, all point to origin from deep horizons within the basin. Remarkable proximity of all the thermal and hydrochemical anomalies to major faults suggests that the waters ascended along these faults from greater depths. The area is a natural western extension of the Himalayan Geothermal Belt described in earlier literature for the eastern and central Himalayas.

Murres, capelin and ocean climate: inter-annual associations across a decadal shift

To ensure energy demands for reproduction are met, it is essential that marine birds breed during periods of peak food availability. We examined associations of the breeding chronology of common murres (Uria aalge) with the timing of the inshore arrival of their primary prey, capelin (Mallotus villosus) from 1980 to 2006 across a period of pervasive change in the Northwest Atlantic ecosystem. We also assessed the influence of ocean temperature and the North Atlantic Oscillation (NAO; an index of winter climate and oceanography) on these interactions. We found a lagged linear relationship between variations in murre breeding chronology and the timing of capelin arrival in the previous year. On a decadal level, we found a non-linear threshold relationship between ocean temperature and the timing of capelin arrival and murre breeding. Centennially anomalous cold water temperatures in 1991 generated a marked shift in the timing of capelin spawning inshore and murre breeding, delaying both by more than 2 weeks. By the mid-1990s, ocean temperatures returned to pre-perturbation levels, whereas the temporal breeding responses of capelin and murres were delayed for a decade or more. Oceanographic conditions (temperature, NAO) were poor predictors of the timing of capelin arrival inshore in the current year compared to the previous one. Our findings suggest that knowledge of the timing of capelin availability in the previous year provides a robust cue for the long-lived murres, allowing them to achieve temporal overlap between breeding and peak capelin availability.