Summer Circulation Research Articles

Enhancement of the relationship between the winter Arctic oscillation and the following summer circulation anomalies over central East Asia since the early 1990s

Considering the possible lag of the impact of the winter Arctic oscillation (AO) on the subsequent summer climate anomalies over East Asia and that the relationship between them may differ during different periods, the non-stationary relationship between these systems and the corresponding mechanism of connection are investigated in this study. A regime shift of the winter AO index was detected around 1988, with mean AO indices of −0.69 and 0.10 for 1958–1987 and 1988–2014, respectively, and their differences passed the 99% confidence level. After the regime shift and especially since the early 1990s, negative (positive) winter AO indices corresponds with positive (negative) height anomalies over central East Asia (CEA) during the following summer. Meanwhile, the westerly jet stream tends to be northward (southward) and the East Asian summer monsoon (EASM) is strong (weak), which contributes to the warm (cold) anomalies over central China and the significantly enhanced (reduced) rainfall over northeast China. After the early 1990s, the enhanced connection of the winter AO and the following summer height anomalies over CEA is largely due to the enhanced connection of the winter AO and the following Indian summer monsoon (ISM) rainfall during the same period. The negative (positive) winter AO favors increasing (decreasing) rainfall over the ISM region, which triggers a positive (negative) circumglobal teleconnection (CGT), and results in significantly positive (negative) height anomalies over CEA. One way the winter AO could influence the following ISM rainfall is that the negative (positive) winter AO memories will be maintained by the positive (negative) sea surface temperature (SST) anomalies over the tropical Atlantic of the following summer, which are responsible for the La Nina (El Nino) developing phase and increases (decreases) the ISM rainfall.

Double‐mode adjustment of Tibetan Plateau heating to the summer circumglobal teleconnection in the Northern Hemisphere

ABSTRACTA quasi‐stationary circumglobal teleconnection (CGT) in the Northern Hemisphere in summer is associated with summer droughts and floods and exhibits some vorticity anomalies. The CGT anomaly and its forcing sources were explored based on reanalysis data and the EI Nino 3.4 index. The term ∂v′/∂x was used to describe the multi‐scale variability of vorticity and the CGT anomaly, which had a 7‐wavenumber and a small interannual amplitude of ∂v′/∂x from 1987 to 2000 but a large amplitude from 2000 to 2010. The interannual CGT intensity was enhanced by an EI Nino sea surface temperature pattern in spring via key forcing sources that led to an enduring effect on the summer circulation; it was also a response to anomalies in Tibetan Plateau (TP) heating in spring–summer in two ways. First, increasing TP heating in the mid‐eastern TP caused an eastward shift and northerly extension of the South Asian High (SAH), which exhibited a Tibetan‐mode SAH as well as a 7‐wavenumber CGT anomaly. This enhanced the CGT over Eurasia and extended the wave pattern of the EI Nino contribution over the North Pacific and Eurasia. Second, increased snow cover in the west of the TP could have offset the TP heating effect and adjusted the position and intensity of the Tibetan‐mode SAH. This phenomenon weakened vorticity and wave amplitude and finally led to a weak CGT anomaly. This finding indicates a coupled effect of double‐mode TP heating (i.e. both heating and TP snow cover) on the CGT anomaly, which reveals the linkage of TP heating with weather over Eurasia and North America that could be predicted by TP heating and other factors.

Circulation and water mass transports on the East Antarctic shelf in the Mertz Glacier region

The East Antarctic shelf off Adélie-George V Land is known to be an important region for Dense Shelf Water (DSW) formation as a result of intense sea ice production in the Mertz Glacier Polynya during the winter season. It is also a region where the warm modified Circumpolar Deep Water (mCDW) penetrates onto the shelf during the summer. Using hydrographic observations from a summer survey in 2008 we implement a box inverse model to propose a comprehensive view of the steady state circulation on this shelf in summer. Additional information from mooring observations collected on the depression slope is used to provide context to the retrieved circulation scheme. Over the depression slope, the summer baroclinic structure of the currents is found to contrast with the almost barotropic structure in winter.The summer circulation is strongly constrained by the DSW distribution and forms a clockwise circulation primarily transporting the fresh surface waters and the warm mCDW around the dome of DSW. Over the upper flank of the Mertz Bank, the inflow branch transports the mCDW towards the Mertz Glacier, while, over the lower part of the slope, the outflow branch returns to the sill a diluted mode of the same water mass. A total of 0.19Sv of mCDW inflows at the sill and two-third reach the Mertz Glacier and recirculate in front of it, allowing the mCDW to penetrate into the deeper part of the depression. Possible scenarios of interaction between the mCDW and the DSW with the glacier are examined. It is shown that, despite the water mass pathways and transports suggest possible ice-ocean interaction, both lateral and basal melting were likely small in summer 2008. Finally, our results suggest that, in addition to bathymetric features, the distribution of the residual DSW which is left from the preceding winter sets up regional pressure gradients which provide a seasonal control on the shelf circulation. In particular, the spring collapse of the convective patch would contribute to setting up a deep pycnocline which strongly impacts the shelf circulation in the following summer, with possible feedback of the mCDW transports on the polynya activity and water mass formation.

Surveillance for enterovirus D68 in colorado children reveals continued circulation

BackgroundThe largest, most widespread outbreak of enterovirus D68 respiratory disease occurred from August to December of 2014 in the United States with 1153 confirmed infections in 49 states. The epidemiology of enterovirus D68 following the 2014 outbreak is unknown. ObjectivesThis study seeks to describe the epidemiology of enterovirus D68 circulation amongst Colorado children from 2014 to 2016. Study designThis is a prospective observational surveillance study of enterovirus D68 infection amongst children tested for respiratory pathogens from July-October 2014–2016 at Children’s Hospital Colorado (CHCO), a quaternary care children’s hospital in Aurora, CO. ResultsAmongst rhinovirus/enterovirus positive respiratory specimens from intensive care unit patients, ninety-eight of 314 (31.2%) in 2014, none of 307 (0%) specimens in 2015, and 19 of 240 (7.9%) specimens in 2016 were identified as enterovirus D68. Amongst respiratory specimens from all patients during the prospective active surveillance period, none of 1469 (0%) in 2015 and 46 of 1403 (3.3%) were positive for enterovirus D68. ConclusionsSurveillance for enterovirus D68 amongst respiratory specimens at a quaternary care children’s hospital revealed a seasonal pattern of circulation in the late summer to early fall of 2014 and 2016. Continued surveillance of respiratory specimens is necessary to define the circulation pattern and understand the epidemiology of this emerging pathogen.

Precipitation changes in the Mediterranean basin during the Holocene from terrestrial and marine pollen records: a model–data comparison

Abstract. Climate evolution of the Mediterranean region during the Holocene exhibits strong spatial and temporal variability, which is notoriously difficult for models to reproduce. We propose here a new proxy-based climate synthesis synthesis and its comparison – at a regional (∼ 100 km) level – with a regional climate model to examine (i) opposing northern and southern precipitation regimes and (ii) an east-to-west precipitation dipole during the Holocene across the Mediterranean basin. Using precipitation estimates inferred from marine and terrestrial pollen archives, we focus on the early to mid-Holocene (8000 to 6000 cal yr BP) and the late Holocene (4000 to 2000 cal yr BP), to test these hypotheses on a Mediterranean-wide scale. Special attention was given to the reconstruction of season-specific climate information, notably summer and winter precipitation. The reconstructed climatic trends corroborate the north–south partition of precipitation regimes during the Holocene. During the early Holocene, relatively wet conditions occurred in the south–central and eastern Mediterranean regions, while drier conditions prevailed from 45° N northwards. These patterns then reverse during the late Holocene. With regard to the existence of a west–east precipitation dipole during the Holocene, our results show that the strength of this dipole is strongly linked to the reconstructed seasonal parameter; early-Holocene summers show a clear east–west division, with summer precipitation having been highest in Greece and the eastern Mediterranean and lowest over Italy and the western Mediterranean. Summer precipitation in the east remained above modern values, even during the late-Holocene interval. In contrast, winter precipitation signals are less spatially coherent during the early Holocene but low precipitation is evidenced during the late Holocene. A general drying trend occurred from the early to late Holocene, particularly in the central and eastern Mediterranean. For the same time intervals, pollen-inferred precipitation estimates were compared with model outputs, based on a regional-scale downscaling (HadRM3) of a set of global climate-model simulations (HadAM3). The high-resolution detail achieved through the downscaling is intended to enable a better comparison between site-based paleo-reconstructions and gridded model data in the complex terrain of the Mediterranean; the model outputs and pollen-inferred precipitation estimates show some overall correspondence, though modeled changes are small and at the absolute margins of statistical significance. There are suggestions that the eastern Mediterranean experienced wetter summer conditions than present during the early and late Holocene; the drying trend in winter from the early to the late Holocene also appears to be simulated. The use of this high-resolution regional climate model highlights how the inherently patchy nature of climate signals and paleo-records in the Mediterranean basin may lead to local signals that are much stronger than the large-scale pattern would suggest. Nevertheless, the east-to-west division in summer precipitation seems more marked in the pollen reconstruction than in the model outputs. The footprint of the anomalies (like today, or dry winters and wet summers) has some similarities to modern analogue atmospheric circulation patterns associated with a strong westerly circulation in winter (positive Arctic Oscillation–North Atlantic Oscillation (AO–NAO)) and a weak westerly circulation in summer associated with anticyclonic blocking; however, there also remain important differences between the paleo-simulations and these analogues. The regional climate model, consistent with other global models, does not suggest an extension of the African summer monsoon into the Mediterranean. Therefore, the extent to which summer monsoonal precipitation may have existed in the southern and eastern Mediterranean during the mid-Holocene remains an outstanding question.

Influence of high-latitude atmospheric circulation changes on summertime Arctic sea ice

The Arctic has seen rapid sea-ice decline in the past three decades, whilst warming at about twice the global average rate. Yet the relationship between Arctic warming and sea-ice loss is not well understood. Here, we present evidence that trends in summertime atmospheric circulation may have contributed as much as 60% to the September sea-ice extent decline since 1979. A tendency towards a stronger anticyclonic circulation over Greenland and the Arctic Ocean with a barotropic structure in the troposphere increased the downwelling longwave radiation above the ice by warming and moistening the lower troposphere. Model experiments, with reanalysis data constraining atmospheric circulation, replicate the observed thermodynamic response and indicate that the near-surface changes are dominated by circulation changes rather than feedbacks from the changing sea-ice cover. Internal variability dominates the Arctic summer circulation trend and may be responsible for about 30–50% of the overall decline in September sea ice since 1979. The Arctic is warming and sea ice is declining, but how the two link is unclear. This study shows changes in summertime atmospheric circulation and internal variability may have caused up to 60% of September sea-ice decline since 1979.

Effect of Yunnan–Guizhou Topography at the Southeastern Tibetan Plateau on the Indian Monsoon

Abstract Topographic insulation is one of the primary origins for the influence of the Tibetan Plateau (TP) on Asian climate. The Yunnan–Guizhou (YG) Plateau, at the southeastern margin of the TP, is known to block the northern branch of the Indian monsoon circulation in summer. However, it is an open question whether this blocking feeds back to the monsoon. In this study, the effect of the YG topography on the Indian monsoon and its comparison with that of the TP were evaluated using general circulation model experiments. The results showed that the TP strengthens the monsoon precipitation, especially during the onset. However, the YG topography significantly weakens the monsoon. With the YG topography, strengthened low-level airflow around the YG Plateau induces anomalous anticyclonic winds to the southwest, and the changes remodulate the whole circulation structure over Asia. As a result, the Indian monsoon becomes weakened from the Bay of Bengal to the Indian subcontinent and Arabian Sea, as does the associated precipitation. In addition, the YG topography affects the anomalous warming center over the TP and the precipitation during the monsoon onset. The YG-reduced summer precipitation occupied approximately one-third of the total increment compared to the entire TP. The Indian monsoon weakened by YG topography distinctly opposes the traditional paleoclimatic viewpoint that all of the TP topography contributes to the monsoon strengthening. In fact, the climatic effect of the TP depends closely upon both its central and marginal topography, and the topography of its subterrains does not necessarily play a similar role.

Impact of surface sensible heating over the Tibetan Plateau on the western Pacific subtropical high: A land–air–sea interaction perspective

The impact of surface sensible heating over the Tibetan Plateau (SHTP) on the western Pacific subtropical high (WPSH) with and without air–sea interaction was investigated in this study. Data analysis indicated that SHTP acts as a relatively independent factor in modulating the WPSH anomaly compared with ENSO events. Stronger spring SHTP is usually followed by an enhanced and westward extension of the WPSH in summer, and vice versa. Numerical experiments using both an AGCM and a CGCM confirmed that SHTP influences the large-scale circulation anomaly over the Pacific, which features a barotropic anticyclonic response over the northwestern Pacific and a cyclonic response to the south. Owing to different background circulation in spring and summer, such a response facilitates a subdued WPSH in spring but an enhanced WPSH in summer. Moreover, the CGCM results showed that the equatorial low-level westerly at the south edge of the cyclonic anomaly brings about a warm SST anomaly (SSTA) in the equatorial central Pacific via surface warm advection. Subsequently, an atmospheric Rossby wave is stimulated to the northwest of the warm SSTA, which in turn enhances the atmospheric dipole anomalies over the western Pacific. Therefore, the air–sea feedbacks involved tend to reinforce the effect of SHTP on the WPSH anomaly, and the role of SHTP on general circulation needs to be considered in a land–air–sea interaction framework.

Observed Changes in the Southern Hemispheric Circulation in May

Much research has focused on trends in the Southern Hemispheric circulation in austral summer (December–February) in the troposphere and stratosphere, whereas changes in other seasons have received less attention. Here the seasonality and structure of observed changes in tropospheric and stratospheric winds, temperature, and ozone over the Southern Hemisphere are examined. It is found that statistically significant trends similar to those of the Antarctic summer season are also observed since 1979 in austral fall, particularly May, and are strongest over the Pacific sector of the hemisphere. Evidence is provided for a significant shift in the position of the jet in May over the Pacific, and it is shown that the strengthening and shifting of the jet has rendered the latitudinal distribution of upper-tropospheric zonal wind more bimodal. The Antarctic ozone hole has cooled the lower stratosphere and strengthened the polar vortex. While the mechanism and timing are not fully understood, the ozone hole has been identified as a key driver of the summer season tropospheric circulation changes in several previous observational and modeling studies. It is found here that significant ozone depletion and associated polar cooling also occur in the lowermost stratosphere and tropopause region through austral fall, with spatial patterns that are coincident with the observed changes in stratospheric circulation. It is also shown that radiatively driven temperature changes associated with the observed ozone depletion in May represent a substantial portion of the observed May cooling in the lowermost stratosphere, suggesting a potential for contribution to the circulation changes.

Controls on the stable isotopes in precipitation and surface waters across the southeastern Tibetan Plateau

Constraining temporal and spatial variability in water stable isotopes (δ18O and δD) is requested for interpreting proxy records of paleoclimate/paleoaltimetry. The southeastern Tibetan Plateau (TP) receives large amounts of precipitation in both summer (JJAS) and spring (MAM) and this makes it different from most other parts of the TP where annual precipitation concentrates only in summer. However, our knowledge of controls on precipitation and surface runoff generation in this region is still far from sufficient. In this study, the δ18O and δD of precipitation and stream waters across the southeastern TP were analyzed to investigate moisture sources and empirical isotope-elevation relationships. Herein, seasonal precipitation patterns, moisture trajectories and precipitation isotopes suggest this region is seasonally dominated by the monsoon in summer and the southerlies (from the Bay of Bengal) or a mix of southerlies and westerlies in spring. Spatially, vertical variations in precipitation seasonality exert profound influences on isotopic variability for stream waters. Larger contributions of spring precipitation (with higher δ18O and d-excess (d-excess=δD-8δ18O) compared to summer precipitation) vs. summer precipitation in the surface runoff generation at lower elevations account for the uncommon altitudinal decrease in streamwater d-excess. Such a cause also contributes to the slightly greater vertical lapse rates of streamwater δ18O (−0.28 to −0.48‰/100m) relative to the Himalayan front. In addition, although a robust δ18O-elevation relationship is demonstrated based upon our measured and other published data on a broad spatial scale (over a 5200m elevation range), this relationship is found to deviate from the empirical/theoretical pattern in the Himalayan front, which is also caused by the substantial spring precipitation in the southeastern TP. It is suggested that long-term changes in δ18O or δD of paleowater in this region actually represent both changes in past regional elevations and contributions of summer vs. spring atmospheric circulations.

A numerical study of the summer circulation in the southwestern Yellow Sea

The Regional Ocean Modeling System (ROMS) is used to study the summer circulation in the southwestern Yellow Sea (SWYS). The modeled currents show good agreement with observations from both drifters and moorings. While the summer current in the SWYS flows consistently northeastward on the surface with large magnitude offshore, the current below the surface layer features a cyclonic circulation roughly along the 25 m isobath. The effect of a surface wind stress and bottom thermal fronts on the circulation is investigated through a series of process-oriented numerical experiments. It is found that the southeasterly wind dominates the surface current, whereas the bottom thermal fronts, which are formed in a transition area between the vertically well-mixed region and the stratified region, are responsible for the cyclonic circulation below the surface.

Summer atmospheric circulation anomalies over the Arctic Ocean and their influences on September sea ice extent: A cautionary tale

AbstractNumerous studies have addressed links between summer atmospheric circulation patterns and interannual variability and the downward trend in total September Arctic sea ice extent. In general, low extent is favored when the preceding summer is characterized by positive sea level pressure (SLP) anomalies over the central Arctic Ocean north of Alaska. High extent is favored when low pressure dominates. If such atmospheric patterns could be predicted several months out, these links provide an avenue for improved seasonal predictability of total September extent. We analyze detrended September extent time series (1979–2015), atmospheric reanalysis fields, ice age and motion, and Atmospheric Infrared Sounder data, to show that while there is merit to this summer circulation framework, it has limitations. Large departures in total September extent relative to the trend line are preceded by a wide range of summer circulation patterns. While patterns for the four years with the largest positive departures in September extent have below average SLP over the central Arctic Ocean, they differ markedly in the magnitude and location of pressure and air temperature anomalies. Differences in circulation for the four years with the largest negative departures are equally prominent. Circulation anomalies preceding Septembers with ice extent close to the trend also have a wide range of patterns. In turn, years (such as 2013 and 2014) with almost identical total September extent were preceded by very different summer circulation patterns. September ice conditions can also be strongly shaped by events as far back as the previous winter or spring.

Long‐term austral summer wind speed trends over southern Africa

ABSTRACTA comprehensive analysis of near surface (10‐m and 850 hPa) long‐term (1980–2015) austral summer mean wind speed trends over southern Africa and the surrounding oceans (10°–40°S and 10°W‐50°E) is carried out. The climate forecast system reanalysis and the climate forecast system reanalysis version 2, the modern era retrospective‐analysis for research and applications version 2, and the ERA‐Interim products are used for the analysis. Trends are estimated using the Theil–Sen slope estimator and their significance tested with the Mann–Kendall test. Attribution of the wind speed trends to large scale circulation systems is determined from: (1) trends in 850 hPa geopotential height, surface pressure, and surface skin temperatures, (2) changes in the annual frequency of archetypal synoptic states generated by self‐organizing maps from geopotential height data, and (3) the correlations of the wind speeds to the southern annular mode (SAM) and El Niño Southern Oscillations (ENSO) indices. There is an overall decline in wind speeds evident in all the reanalyses. Wind speeds reduce by an average 0.7 and 1.3 m s−1 at 10 m and 850 hPa, respectively, over the study period. Deceleration in midlatitude westerly and Atlantic south easterly winds are associated with a poleward shift in the subtropical anticyclone, a positive trend in the annual frequency of summer circulation weather types, and the weakening of the subtropical continental heat low. The SAM and ENSO are also found to have a statistically significant linear association with the observed wind speed trends.

Linkages between Arctic summer circulation regimes and regional sea ice anomalies

AbstractThe downward trend in overall Arctic summer sea ice extent has been substantial, particularly in the last few decades. Departures in ice extent from year to year can be very large, however, in part due to the high variability in summer atmospheric circulation patterns. Anomalies in the Pacific sector ice cover can be partially compensated by anomalies of opposite sign in the Atlantic sector. An assessment of linkages between summer atmospheric patterns and sectoral anomalies in the area of maximum open water north of 70°N demonstrates that there is asymmetry in the mechanisms. Years with low ice extent and high open water fraction are uniformly associated with positive temperature anomalies and southerly flow in both the Atlantic and Pacific sectors. However, years with high extent and low open water fraction in both sectors reveal two dominant mechanisms. Some years with anomalously low maximum open water fraction are associated with negative temperature anomalies and southerly transport—a cool summer pattern that allows ice to persist over larger areas. However, other low open water years are characterized by an “ice factory” mechanism, whereby—even when melting—ice cover is continually replenished by advection from the north.

Controls on circulation, cross‐shelf exchange, and dense water formation in an Antarctic polynya

AbstractCirculation on the Antarctic continental shelf influences cross‐shelf exchange, Antarctic Bottom Water formation, and ocean heat flux to floating ice shelves. The physical processes driving the shelf circulation and its seasonal and interannual variability remain poorly understood. We use a unique time series of repeat hydrographic observations from the Adélie Land continental shelf and a box inverse model to explore the relationship between surface forcing, shelf circulation, cross‐shelf exchange, and dense water formation. A wind‐driven northwestward coastal current, set up by onshore Ekman transport, dominates the summer circulation. During winter, strong buoyancy loss creates dense shelf water. This dense water flows off the shelf, with a compensating on‐shelf flow that is an order of magnitude larger in winter than in summer. The results demonstrate the importance of winter buoyancy loss in driving the shelf circulation and cross‐shelf exchange, as well as dense water mass formation.

Seasonal variability of the meridional overturning circulation in the South China Sea and its connection with inter‐ocean transport based on SODA2.2.4

AbstractWe have proposed a five‐layer‐scheme to investigate the volume transport through the South China Sea (SCS) based on the updated Simple Ocean Data Assimilation (SODA2.2.4) product. By demonstrating horizontal transport in each layer, we have revealed different formation mechanisms for the meridional overturning circulation (MOC) in winter and summer in the SCS. Our analysis suggests three meridional circulation systems in the SCS: (1) the seasonal monsoon‐driven circulation in the surface layer, i.e., southward circulation in winter and northward in summer, (2) the compensatory transport‐induced seasonal intermediate MOC in the central SCS, and (3) the persistent deep MOC in the southern SCS all year round. By examining vertical velocity distribution, we have identified that the major overturning process of the intermediate MOC is located along the continental slope east and southeast of Vietnam, while the major overturning process of the deep MOC is located along the continental slope northwest of Borneo. The downwelling in the intermediate MOC in winter and upwelling in the deep MOC all year round bring different water masses to the intermediate and subintermediate layers to be mixed in the SCS. We found no evidence to suggest that the strength and extent of the MOC south of 18°N are related to inter‐ocean volume transport. The surface layer transport in the Luzon Strait has been decreasing since the 1960s. However, the causes of the meridionally staggered and interdecadal alternating acceleration/slowdown of the meridional stream function difference are unknown.

Analysis of surface circulation structures along a frequently repeated XBT transect crossing the Ligurian and Tyrrhenian Seas

A dataset of XBT (eXpendable BathyThermograph) temperature profiles collected by ships of opportunity along the Genova–Palermo route, since September 1999, is analyzed, together with altimetric observations and model results, with the purpose of identifying and characterizing robust circulation features along the track and investigating their variability. An anticyclone is found in the Ligurian Sea, just north of the Corsica Channel, not present in previous descriptions of the Mediterranean Sea circulation. It appears to be a recurrent feature, better defined and stronger in summer and in the beginning of autumn. In the northern part of the Tyrrhenian Sea, the well-known Bonifacio dipole shows a similar seasonality, in agreement with previous observations. However, the Bonifacio anticyclone also displays a strong interannual variability, not previously recorded, with significant variations in position and shape. In fact, the data suggest the existence of two distinct summer circulation regimes related to the position and shape of the Ligurian anticyclone. When the latter is wider, filling the entire region north of the Corsica Channel, the circulation in the northern Tyrrhenian Sea is isolated from that in the Ligurian Sea, in agreement with the common picture. However, the altimeter maps show that there are several cases in the last two decades in which the Ligurian anticyclone is small and displaced to the west, allowing an inflow through the Corsica Channel into the Tyrrhenian Sea. The two regimes appear to result from a delicate balance between the forcings acting in the two sub-basins and the topographic constraints.

Physical processes affecting water quality in Hamilton Harbour

This article presents an overview of selected physical processes and their effects on water quality in Hamilton Harbour. An understanding of circulation and mixing processes are essential to assess the fate and transport of water quality constituents in Hamilton Harbour. Water level measurements showed that in addition to harbour and lake seiches, the Helholtz mode, due to pumping action from Lake Ontario, is important in generating harbour water movements while the circulation patterns in the open waters of the harbour are influenced by prevailing winds. In general, the mean summer circulation consists of two counter-rotating gyres occupying the harbour. Hamilton Harbour physical processes are further characterized by substantial water exchanges with Lake Ontario. These exchange flows play a major role in determining the retention time of the harbour, thereby exerting a large influence on water quality, including hypolimnetic dissolved oxygen concentrations.

Suppressed midlatitude summer atmospheric warming by Arctic sea ice loss during 1979–2012

AbstractSince the 1980s, rapid Arctic warming, sea ice decline, and weakening summer circulation have coincided with an increasing number of extreme heat waves and other destructive weather events in the Northern Hemisphere (NH) midlatitudes in summer. Recent papers disagree about whether such high‐impact events are related to Arctic warming and/or ice loss. Here we use atmospheric model ensemble simulations to attribute effects of sea ice loss and other factors on observed summer climate trends during 1979–2012. The ongoing greenhouse gas buildup and resulting sea surface temperature warming outside the Arctic explains nearly all land warming and a significant portion of observed weakening zonal winds in the NH midlatitudes. However, sea ice loss has induced a negative Arctic Oscillation(AO)‐type circulation with significant summer surface and tropospheric cooling trends over large portions of the NH midlatitudes, which reduce the warming and might reduce the probability of regional severe hot summers.

Potential for long‐lead prediction of the western North Pacific monsoon circulation beyond seasonal time scales

AbstractAlthough the western North Pacific (WNP) monsoon circulation significantly impacts the socioeconomic communities around Asia, its prediction is only limited to a few months. By examining the Coupled Model Intercomparison Project phase 5 decadal hindcast experiments, we explore a possibility of the extended prediction skill for the WNP monsoon circulation beyond seasonal time scales. It is found that the multimodel ensemble (MME) predictions, initialized in January, successfully predict the WNP circulation in spring and early summer. Somewhat surprisingly, a reliable prediction of the WNP circulation appears even in the following spring with a maximum lead time of 14 months. This unexpected prediction skill is likely caused by the improved El Niño–Southern Oscillation (ENSO) prediction and the exaggerated dynamical link between the ENSO and premonsoon circulation in the MME prediction. Although further studies are needed, this result may open up new opportunities for the multiseasonal prediction of the WNP monsoon circulation.