Events In Northern Hemisphere Research Articles

Influence of the Boreal Summer Intraseasonal Oscillation on Extreme Temperature Events in the Northern Hemisphere

The impact of the boreal summer intraseasonal oscillation (BSISO) on extreme hot and cool events was investigated, by analyzing the observed and reanalysis data for the period from 1983 to 2012. It is found that the frequency of the extreme events in middle and high latitudes is significantly modulated by the BSISO convection in the tropics, with a 3–9-day lag. During phases 1 and 2 when the BSISO positive rainfall anomaly is primarily located over a northwest–southeast oriented belt extending from India to Maritime Continent and a negative rainfall anomaly appears in western North Pacific, the frequency of extreme hot events is 40% more than the frequency of non-extreme hot events. Most noticeable increase appears in midlatitude North Pacific (north of 40°N) and higher-latitude polar region. Two physical mechanisms are primarily responsible for the change of the extreme frequency. First, an upper-tropospheric Rossby wave train (due to the wave energy propagation) is generated in response to a negative heating anomaly over tropical western North Pacific in phases 1 and 2. This wave train consists of a strong high pressure anomaly center northeast of Japan, a weak low pressure anomaly center over Alaska, and a strong high pressure anomaly center over the western coast of United States. Easterly anomalies to the south of the two strong midlatitude high pressure centers weaken the climatological subtropical jet along 40°N, which is accompanied by anomalous subsidence and warming in North Pacific north of 40°N. Second, an enhanced monsoonal heating over South Asia and East Asia sets up a transverse monsoonal overturning circulation, with large-scale ascending (descending) anomalies over tropical Indian (Pacific) Ocean. Both the processes favor more frequent extreme hot events in higher-latitude Northern Hemisphere. An anomalous atmospheric general circulation model is used to confirm the tropical heating effect.

Climatic variability in Mfabeni peatlands (South Africa) since the late Pleistocene

It has been postulated that a bipolar seesaw interhemispheric mechanism dominated the relationship between the Northern and Southern hemisphere climates since the late Pleistocene. A key test for this proposition would be to undertake palaeoenvironmental studies on terrestrial archives in climatically sensitive regions. Southern Africa's contemporary C3 and C4 terrestrial plant distributions display a definitive geographical pattern dictated by different growing season rainfall and temperature zones; however, the region is generally archive poor due to its overall semi-arid climate and high relief topography. The Mfabeni peatland, with a basal age of c. 47 k yrs calibrated before present (kcal yr BP), is one of the oldest continuous coastal peat deposits in Southern Africa. Molecular leaf wax isotopes (δ13Cwax) were generated for a 810 cm long core, and combined with previously published bulk geochemical (δ13Cbulk, %TOC), palynological, and stratigraphic data, to reconstruct the late Pleistocene and Holocene palaeoenvironments. We interpreted environmental shifts associated with the Heinrich 4, Last Glacial Maximum, deglacial and Holocene periods, which are consistent with adjacent Indian Ocean sea surface temperature records. However, the other shorter climate perturbations during the Heinrich 5, 3, 2, 1, Antarctic cold reversal and Younger Dryas, were muted, most likely due to local hydrological overprinting on the Mfabeni record. A general anti-phase sequence was observed between the Mfabeni record and better established Northern Hemisphere events, underpinning the bipolar seesaw interhemispheric mechanism proposed for global climate forcing since the Late Pleistocene.

Revision of Paleogene plate motions in the Pacific and implications for the Hawaiian-Emperor bend: COMMENT

According to literature summarized by Wright et al. (2015), it is generally accepted that hotspots have moved relative to each other since 80 Ma, and that the Hawaiian hotspot moved south relative to the spin axis during formation of the Emperor seamounts prior to 48 Ma. Koivisto et al. (2014) advanced the alternative view that the minimum relative motion between hotspots is much smaller than generally accepted, implying errors in the global plate circuit are larger than generally accepted. In particular, they suggest that the global circuits are incorrect for motion prior to 48 Ma between East and West Antarctica. In this view, southward motion of the Hawaiian hotspot is evidence for motion of the entire lower mantle relative to the spin axis. Wright et al. defend conventional wisdom using freshly calculated relative motion solutions for plates in the Pacific basin. They conclude that the Hawaiian-Emperor bend (HEB), formed at ca. 50-42 Ma and sharpest at 47.5 Ma, is a result of the cessation of southward motion of the Hawaiian hotspot and is not a result of change of absolute motion of the Pacific plate. The purpose of this Comment is to point out that their results are at least as consistent with a large change in motion of the Pacific plate that was responsible for a significant part of the HEB. Wright et al. calculate Pacific-Farallon finite half-rotations from the Pacific-plate record at more frequent intervals than previous workers. They recognize an acceleration from ~75 mm/yr full rate prior to 60 Ma to ~180 mm/yr after 40 Ma. According to their preferred Cande and Kent (1995) time scale, this acceleration is spread out over 57–40 Ma, but according to the Ogg (2012) time scale, the acceleration is contained within the interval of 50–43 Ma, a time interval entirely within the age span of the HEB. Their statement that their “results are not strongly dependent on the time scale used” is hard to understand. In the discussion associated with Wright et al.’s figure 3, they suggest that this acceleration is dominated by eastward acceleration of the Farallon plate in an absolute reference frame. Any conclusion that Pacific absolute motion didn’t change much is circular because they assume that one of three published models for Pacific plate absolute motion is correct, and for all of these published models, changes in Pacific plate absolute motion are small. This discussion does not address the possibility that all three of these models for absolute reference frames share common errors in the global plate circuit. Wright et al. briefly mention their analysis of Pacific-Antarctic motion, presented in their figure 2, in terms of rate and direction of motion at the Pitman Fracture Zone in the far south Pacific. In discussion of Northern Hemisphere events possibly related to Pacific-Farallon acceleration, they suggest that westward acceleration of the Pacific plate would correspond to increased Pacific-Antarctic spreading rates. Because they observe a rate decrease at the Pitman Fracture Zone, they suggest that change in Pacific absolute motion was “relatively minor.” Here they make a serious error in failing to consider spherical geometry. The Pitman transform fault active at 48 Ma was 90–100° of arc away from the Molokai and Murray Fracture Zones, and the HEB. Their discussion frames the most relevant issue as how the motion of the Pacific plate changes at the location of the Hawaiian hotpsot. The velocity of the Pacific plate relative to Antarctica when calculated at relevant Northern Hemisphere locations does increase sharply near 48 Ma. Calculated at the location of Hawaii, the velocity of the Pacific plate changes from slow (35 mm/yr) northwest motion to fast (83 mm/yr) west-northwest motion

Seasonal dependence and solar wind control of transpolar arc luminosity

The influence of the solar wind and the interplanetary magnetic field (IMF) on the luminosity of transpolar arcs (TPAs) is examined by taking into account seasonal effects. The study focuses on those transpolar arcs that appear after an IMF By sign change during steady northward IMF. It includes 21 northern hemisphere events identified in a previous study from global UV images taken by the Polar spacecraft between 1996 and 2000. Sorting the TPA events by sign of the Earth dipole tilt we find that the TPAs which appear in the dark hemisphere are on average much weaker than TPAs in the sunlit hemisphere. For the dark hemisphere events, no clear correlation between solar wind parameters and TPA luminosity is found. However, in the sunlit hemisphere, a clear dependence on solar wind and IMF conditions is seen. The TPA brightness is strongly influenced by IMF magnitude, northward IMF Bz and solar wind speed. A weak, negative correlation with the ion density is found. The TPA luminosity in the sunlit hemisphere is much more strongly controlled by the magnetic energy flux than by the kinetic energy flux of the solar wind. This explains the absence of transpolar arcs for the two By sign change cases for positive dipole tilts with lowest magnetic energy flux values. The strong influence of the Earth dipole tilt on the transpolar arc luminosity appears due to the dependence of the ionospheric conductivity on solar EUV emissions.

Polar mesosphere and lower thermosphere dynamics: 2. Response to sudden stratospheric warmings

The dynamical response of the polar mesosphere and lower thermosphere (MLT) to sudden stratospheric warmings is investigated using MF radars at Davis (69°S, 78°E), Syowa (69°S, 40°E) and Rothera (68°S, 68°W) in the Antarctic and Poker Flat (65°N, 147°W) and Andenes (69°N, 16°E) in the Arctic. Mean winds, gravity waves and planetary waves are investigated during sudden stratospheric warmings, and comparisons are made with climatological means. The available MF radar data set includes six major sudden stratospheric warmings in the Northern Hemisphere and the unprecedented 2002 Southern Hemisphere major stratospheric warming. Three of the six northern events are relatively weak and could almost be classed as minor warmings, while the larger three have similar characteristics to the event in the Southern Hemisphere. Zonal wind reversals associated with the major warmings in both hemispheres are generally weaker and earlier by several days in the mesosphere than in the stratosphere. There are, however, significant differences between locations in their response to stratospheric warmings. The zonal winds are remarkably weaker than average during both winter and spring around the time of the southern major warming of 2002, but these effects are not observed for the Northern Hemisphere events. Gravity wave activity is found to vary significantly between individual stratospheric warming events and also between individual locations.

On the hemispheric origins of meltwater pulse 1a

During the glacial–interglacial transition that began subsequent to the Last Glacial Maximum approximately 21,000 calendar years ago, globally averaged (eustatic) sea-level rose by approximately 120 m as climate warmed to its current (Holocene) state. This rise of relative sea-level (RSL) did not occur smoothly, however, but was characterized by the occurrence of one or more episodes of extremely rapid increase. The most extreme of these events has come to be referred to as meltwater pulse 1a, and was initially identified in the coral based record of RSL history from the island of Barbados in the Caribbean Sea. Although it has usually been assumed that this episode of rapid RSL rise was derivative of a partial collapse of the northern hemisphere ice sheets, it has recently been suggested that this pulse could have originated in a dramatic melt-back of the Antarctic Ice Sheet. In this paper the arguments presented in favour of the southern hemisphere source are revisited in order to assess the plausibility of this alternative scenario. Based upon the analyses presented, it is concluded that the evidence previously provided in support of the southern hemisphere scenario is in fact unable to rule out an entirely northern hemisphere source for the meltwater pulse 1a. Since explicit evidence does exist that both the Laurentide and Fennoscandian ice sheets contributed to this event and that Antarctic ice sheet melting occurred significantly later, the southern hemisphere appears not to have been a prime mover of northern hemisphere events.

Southern Hemisphere poleward moving auroral forms

This paper presents a statistical study of Southern Hemisphere poleward moving auroral forms (PMAFs) using optical data from the US Automatic Geophysical Observatory network in Antarctica. These Southern Hemisphere events are compared with Northern Hemisphere PMAFs, which have previously been observed during varied interplanetary magnetic field (IMF) configurations. The frequency of PMAF occurrence is studied as a function of IMF orientation and magnetic local time. Southern Hemisphere PMAFs are biased to Bz < 0 constituting 62% of events, but are frequently observed during conditions of Bz > 0 constituting 38% of events. Southern Hemisphere PMAFs share a similar IMF Bz dependence with Northern Hemisphere events [e.g., Fasel, 1995]. The primary modulator of PMAFs was found to be IMF By, which was negative for 81% of events. Although the average background IMF was biased to By < 0, we find significant enhancement during By < 0 after this bias was removed. In a statistical study of Northern Hemisphere events, Fasel [1995] found 77% of events occur during positive By. PMAFs appear to have an antisymmetric By dependence between hemispheres. We find By < 0 (By > 0) enhances the observation of PMAFs in the prenoon (postnoon) region, consistent with the release of magnetic stress. A morning bias of Southern Hemisphere PMAFs is observed, consistent with Northern Hemisphere events.

Explosive Cyclone Development in the Southern Hemisphere and a Comparison with Northern Hemisphere Events

Abstract A compilation of Southern Hemisphere (SH) explosively developing cyclones (or “bombs”) has been assembled based on the National Centers for Environmental Prediction–Department of Energy reanalysis-2 data over the 21-yr period from 1979 to 1999. The identification of these features was undertaken with an objective automated cyclone finding and tracking scheme. The procedure allows for the confounding influence of spatial variations of climatological mean pressure on the pressure deepening of explosive cyclones, a perspective of particular importance in the SH. On average, 26 explosive cyclones occur per year in the SH. They are more prevalent in winter although their seasonality is more modest than that seen in the Northern Hemisphere (NH). The distribution of SH explosive cyclones has a close association with that of strong baroclinicity, although the relationship is not one to one. It is found that many of these cyclones occurring south of 50°S show equatorward movement, in contrast with the pol...

Quaternary environmental change in New Zealand: a review

The discovery that orbital variations are the driving force behind Quaternary climate change provides an impetus to set local and regional records of environmental change into the global context, a principle that has been strongly embraced by Quaternary scientists working in New Zealand. Their major achievements and significant current initiatives are reviewed here. The importance of the New Zealand Quaternary stems from its geographical context: a climatically sensitive, remote oceanic, southern location spanning 17 degrees of the mid-latitudes; an obliquely convergent plate boundary setting resulting in a high mountain range athwart the prevailing westerlies, active volcanism, a youthful and dynamic landscape, and mountains high enough to maintain glaciers today; and a remarkably short prehistory. The resultant records show marked environmental changes due not only to climatic oscillations but also to vigorous, active tectonism and volcanism. The Taupo Volcanic Zone, containing the world's strongest concentration of youthful rhyolitic volcanoes, has produced at least 10 000 km3 of magma in the last 2 Ma. Climatic interpretations of records from marine sediments in the New Zealand region, together with several long sequences of alternating marine and terrestrial sediments, indicate broad synchrony with Northern Hemisphere events (within limitations of dating), although there are differences in detail for shorter-term climatic events. It is not yet certain that glacial advances coincided precisely with those in the Northern Hemisphere or were of similar duration. Late Cainozoic glaciation commenced c. 2.6-2.4 Ma but the record of glacial deposits is fragmentary and poorly dated except for the most recent events. The Last (Otira) Glaciation, from c. 100-10 ka, was characterized by at least five glacial advances including during the Last Glacial Maximum from 25 to 15 ka, when snowlines fell by 600-800 m. New Zealand evidence for cooling during the Younger Dryas stade is equivocal whilst isotopic records from speleothems, and other data, indicate warmer and wetter conditions from 10-7 ka, broadly conforming with records from mid-latitude Northern Hemisphere locations. Future advances will require sampling at shorter timescales, improvements in the accuracy and precision of existing dating methods and the development of new ones, extension of palaeoecological techniques to cover the full potential of new Zealand's diverse biota, and a stronger emphasis on quantification of palaeoclimatic parameters.

Late Pleistocene nutrients and sea surface productivity in the South China Sea: a record of teleconnections with Northern hemisphere events

Surface nutrient variations during the last 200,000 years have been reconstructed based on the Cd/Ca ratios in the tests of planktonic foraminifera ( Neogloboquadrina dutertrei) and the biogenic opal content from two gravity cores retrieved from the northern and northwestern parts of the South China Sea (SCS). The Cd/Ca ratios obtained from N. dutertrei in Recent sediments from fourteen box cores distributed over the SCS basin reflect present nutrient levels in the overlying surface waters. Areas with relatively high Cd/Ca values in the surface sediments are in the region of high chlorophyll-a in the northern South China Sea near the Dongsha Islands, consistent with previous observation. High values of Cd/Ca found in sediments east of Vietnam and east of the Nansha Islands may be a result of increased nutrients due to local upwelling induced by the summer monsoon. Core 17940-2, raised from the northern continental slope of the SCS, exhibits exceptional high sedimentation rates (25–85 cm/kyr) for the last 35 kyr. The Cd/Ca ratios in this core are relatively higher during the periods of late δ 18O stage 3 and the mid-Holocene. Core 17950-2, with an average sedimentation rate of 5 cm/kyr, offers a continuous long record back to δ 18O stage 7. In general, Cd/Ca values in the downcore records are higher during interglacials and progressively increase as the interglacials proceed. Values peak around the times of optimum climate conditions, and correlate with periods of increased opal wt%. Correlations in the interval of overlap between the two records support the interpretation that variations in Cd/Ca ratios in core 17940-2 are not an individual signal but can be replicated. Of special interest in this study is the tentative correlation between Cd/Ca peaks in the SCS record and the timing of iceberg discharge events (Heinrich events) in the North Atlantic. Mass accumulation rate (MAR) of opal data from core 17950-2 indicate high surface productivity during glacial intervals corresponding to oxygen isotope stages 6, 4, and 2. The increase in surface productivity during glacials from the northern SCS is contemporaneous with an intensified winter monsoon as recorded by the terrestrial loess deposits. The agreement with wind strength recorded in the Loess Plateau from mainland China supports the connection between the marine opal records of the SCS and paleomonsoon intensity, and thus a continental–marine link.

A sudden warming in the middle atmosphere of the southern hemisphere

AbstractThis paper presents a case study of a wave‐mean flow interaction, or minor warming, near the southern hemisphere stratopause for 15‐30 July 1974. the analysis is based mainly on data from the selective chopper radiometer carried by Nimbus 5.A deceleration of the zonal mean wind of 70 m s‐1 from 20 to 25 July is observed in middle latitudes at 0.5 mb, with an acceleration further south. the polar vortex is, however, not disrupted.Eliassen‐Palm cross‐sections are presented for the event and a good correlation found between the E‐P flux convergence and the deceleration. the channelling of the E‐P fluxes from the tropopause to the convergent region is in broad agreement with the distribution of refractive index but there is a region of E‐P flux divergence at high levels in polar latitudes suggesting a source of wave activity there.The residual circulation is calculated both from the momentum budget and from an omega equation, both methods diagnosing a region of modest equatorward flow near the region of E‐P flux divergence embedded in the stronger poleward flow elsewhere.Maps of potential vorticity on the isentropic surface of 0 = 1640 K are presented and the evolving pattern related to zonal mean and isobaric descriptions. the event is shown to be associated with an incursion of low latitude air into middle latitudes with a corresponding irreversible mixing of potential vorticity reminiscent of the ‘wave breaking’ in northern hemisphere events reported by McIntyre and Palmer (J. At. Ter. Ph., 46, 825), but of much smaller amplitude.