Interhemispheric Teleconnection Research Articles

Competing climate feedbacks of ice sheet freshwater discharge in a warming world

Freshwater discharge from ice sheets induces surface atmospheric cooling and subsurface ocean warming, which are associated with negative and positive feedbacks respectively. However, uncertainties persist regarding these feedbacks’ relative strength and combined effect. Here we assess associated feedbacks in a coupled ice sheet-climate model, and show that for the Antarctic Ice Sheet the positive feedback dominates in moderate future warming scenarios and in the early stage of ice sheet retreat, but is overwhelmed by the negative feedback in intensive warming scenarios when the West Antarctic Ice Sheet undergoes catastrophic collapse. The Atlantic Meridional Overturning Circulation is affected by freshwater discharge from both the Greenland and the Antarctic ice sheets and, as an interhemispheric teleconnection bridge, exacerbates the opposing ice sheet’s retreat via the Bipolar Seesaw. These results highlight the crucial role of ice sheet-climate interactions via freshwater flux in future ice sheet retreat and associated sea-level rise.

The Influence of Southeastern South American Rainfall on Weather Patterns Over the Tropical Atlantic, Northwestern Africa and Western Europe

AbstractDuring boreal winter (December–February), the South American monsoon system (SAMS) reaches its annual maximum when upper‐tropospheric westerly winds prevail over the equatorial Atlantic. Atmospheric dynamic model simulations suggest that Rossby waves generated over South America can propagate to and potentially influence weather patterns in the Northern Hemisphere (NH). However, observational evidence has been absent previously. Here we focus on southeastern South American (SESA) precipitation anomalies, which can characterize intraseasonal rainfall variability of the SAMS. Since tropical “westerly duct” and convective heating are important factors for cross‐equatorial propagation of Rossby wave (CEPRW), we identify two groups of events based on the two factors. By comparing the events associated with both SESA rainfall and tropical westerlies to the events associated with tropical westerlies only, we find that an anomalous Rossby wave train is triggered by precipitation anomalies over SESA, propagates in the southwest–northeast direction, and subsequently crosses the equator. Over a period of 4 days, near‐surface temperature over northwestern Africa and western Europe becomes warmer, accompanied by increased surface downward longwave radiation and precipitable water. The equatorward propagating Eliassen–Palm flux anomalies originated from SESA support the evidence of CEPRW. Simulations using a time‐dependent linear barotropic model forced by prescribed divergence anomalies over SESA further confirm that SESA rainfall can influence the NH weather patterns through CEPRW. Knowledge of this study will help us better understand and model interhemispheric teleconnections over the American–Atlantic–African/European sector.

Characterizing Atlantic interhemispheric teleconnection established by South American monsoon in austral summer

This study aims to characterize the interhemispheric teleconnection pattern, which is established by the South America (SA) summer monsoon over the Atlantic Ocean during January and February, and referred it as the Atlantic symmetric pattern (ASP). The ASP is characterized using the leading mode of interannual sea surface temperature (SST) variability of the Southwest Atlantic Ocean, where strong convection-SST coupling occurs. The pattern is manifested as two anomalous cyclonic-anticyclonic-cyclonic circulation trains aligned meridionally over the Atlantic Ocean, with a distinct SST dipole of the Southwest Atlantic Ocean and a tripole of the North Atlantic Ocean. The interhemispheric wave trains of the ASP are excited as a Gill-type response to convective activity in the SA summer monsoon, as confirmed in linear baroclinic model. Complementing previous studies on observed interhemispheric connection in the Atlantic, our findings highlight the importance of characterizing the ASP and its role in linking the South Atlantic SST, the SA summer monsoon, and North Atlantic climate. Further research is warranted to explore the impacts of the ASP on the Northern Hemisphere and its interactions with other climatic modes.

A regime shift in the interhemispheric teleconnection between the Yellow and East China Seas and the southeastern tropical Pacific during the boreal summer

A regime shift in the interhemispheric teleconnection between the Yellow and East China Seas and the southeastern tropical Pacific during the boreal summer

Prospecting Glacial Ages and Paleoclimatic Reconstructions Northeastward of Nevado Coropuna (16° S, 73° W, 6377 m), Arid Tropical Andes

This work investigates the timing, paleoclimatic framework and inter-hemispheric teleconnections inferred from the glaciers last maximum extension and the deglaciation onset in the Arid Tropical Andes. A study area was selected to the northeastward of the Nevado Coropuna, the volcano currently covered by the largest tropical glacier on Earth. The current glacier extent, the moraines deposited in the past and paleoglaciers at their maximum extension have been mapped. The present and past Equilibrium Line Altitudes (ELA and paleoELA) have been reconstructed and the chlorine-36 ages have been calculated, for preliminary absolute dating of glacial and volcanic processes. The paleoELA depression, the thermometers installed in the study area and the accumulation data previously published allowed development of paleotemperature and paleoprecipitation models. The Coropuna glaciers were in maximum extension (or glacial standstill) ~20–12 ka ago (and maybe earlier). This last maximum extension was contemporary to the Heinrich 2–1 and Younger Dryas events and the Tauca and Coipasa paleolake transgressions on Bolivian Altiplano. The maximum paleoELA depression (991 m) shows a colder (−6.4 °C) and moister climate with precipitation ×1.2–×2.8 higher than the present. The deglaciation onset in the Arid Tropical Andes was 15–11 ka ago, earlier in the most southern, arid, and low mountains and later in the northernmost, less arid, and higher mountains.

A new interhemispheric teleconnection increases predictability of winter precipitation in southwestern US

Reliable prediction of seasonal precipitation in the southwestern US (SWUS) remains a challenge with significant implications for the economy, water security and ecosystem management of the region. Winter precipitation in the SWUS has been linked to several climate modes, including the El Niño-Southern Oscillation (ENSO), with limited predictive ability. Here we report evidence that late-summer sea surface temperature and geopotential height anomalies close to New Zealand exhibit higher correlation with SWUS winter precipitation than ENSO, enhancing the potential for earlier and more accurate prediction. The teleconnection depends on a western Pacific ocean-atmosphere pathway, whereby sea surface temperature anomalies propagate from the southern to the northern hemisphere during boreal summer. Analysis also shows an amplification of this new teleconnection over the past four decades. Our work highlights the need to understand the dynamic nature of the coupled ocean-atmosphere system in a changing climate for improving future predictions of regional precipitation.

Interhemispheric teleconnections: Late Pliocene change in Mediterranean outflow water linked to changes in Indonesian Through-Flow and Atlantic Meridional Overturning Circulation, a review and update

The ultimate, possibly geodynamic control and potential impact of changes in circulation activity and salt discharge of Mediterranean outflow waters (MOW) on Atlantic meridional overturning circulation have formed long-standing objectives in paleoceanography. Late Pliocene changes in the distal advection of MOW were reconstructed on orbital timescales for northeast Atlantic DSDP/ODP sites 548 and 982 off Brittany and on Rockall Plateau, supplemented by a proximal record from Site U1389 west off Gibraltar, and compared to Western Mediterranean surface and deep-water records of Alboran Sea Site 978. From ~3.43 to 3.3 Ma, MOW temperatures and salinities form a prominent rise by 2–4 °C and ~3 psu, induced by a preceding and coeval rise in sea surface and deep-water salinity and increased summer aridity in the Mediterranean Sea. We speculate that these changes triggered an increased MOW flow and were ultimately induced by a persistent 2.5 °C cooling of Indonesian Through-Flow waters. The temperature drop resulted from the northward drift of Australia that crossed a threshold value near 3.6–3.3 Ma and led to a large-scale cooling of the eastern subtropical Indian Ocean and in turn, to a reduction of African monsoon rains. Vice versa, we show that the distinct rise in Mediterranean salt export after ~3.4 Ma induced a unique long-term rise in the formation of Upper North Atlantic Deep Water, that followed with a phase lag of ~100 ky. In summary, we present evidence for an interhemispheric teleconnection of processes in the Indonesian Gateways, the Mediterranean and Labrador Seas, jointly affecting Pliocene climate.

Oceanic teleconnection for carbon dioxide

AbstractBiogeochemical teleconnection links seemingly unrelated chemical/biological anomalies that are geographically separated by large distances. Bronselaer et al. propose a new mechanism for an interhemispheric teleconnection of air‐sea carbon dioxide fluxes in which the upwelling of the Southern Ocean triggers a series of perturbations leading to the alteration of the carbon uptake in the North Atlantic.

Dynamics of an Interhemispheric Teleconnection across the Critical Latitude through a Southerly Duct during Boreal Winter*

Abstract In this study, an interhemispheric teleconnection pattern across the critical latitude from southern Africa through South Asia to the North Pacific was revealed in boreal winter monthly averaged 250-hPa streamfunction fields obtained from both the 40-yr ECMWF Re-Analysis (ERA-40) and the NCEP–NCAR reanalysis data from 1957/58 to 2001/02. Classical Rossby wave theory for zonally varying flow in which the effects of the basic-state meridional wind are ignored predicts that stationary Rossby waves cannot propagate across easterlies. To elucidate the underlying mechanisms responsible for this interhemispheric teleconnection, the theoretical basis for stationary wave propagation across the critical latitude is considered, taking into account meridional ambient flow. The theoretical results suggest that the southerly flow over East Africa, the western Indian Ocean, and South Asia creates a path for the northward propagation of stationary waves across the critical latitude. Stationary wavenumber and group velocity analysis, ray tracing, and simple model experiments applied to nearly realistic boreal winter mean flows confirm that disturbances excited in southern Africa and the western Indian Ocean can propagate across the critical latitude to South Asia through the southerly duct and then continue downstream along the North African–Asian subtropical jet.

Interhemispheric Propagation of Stationary Rossby Waves in a Horizontally Nonuniform Background Flow

Abstract Significant interhemispheric teleconnections are identified that span the tropical easterlies in the boreal summer 300-hPa streamfunction, such as the North Africa–Antarctic (NAA) and the North Pacific–South America (NPSA) patterns. These patterns are not supported by traditional wave theory, since stationary waves in a basic state without meridional wind would be trapped in the easterlies. To describe the interhemispheric responses more realistically, two-dimensional spherical Rossby wave theory in a horizontally nonuniform basic state is considered. Conditions sufficient for the existence of one propagating wave are obtained, and the meridional group velocity of the wave is shown to have the same direction as the meridional basic wind at the traditional critical latitude. It is concluded that stationary waves with a specific wavelength can propagate across the easterlies from south (north) to north (south) via southerly (northerly) flows. Hence, energy transport by stationary waves on a horizontally nonuniform basic state may produce interhemispheric responses that could pass through the tropical easterly belt. The wave theory and a barotropic model are then applied to idealized and climatological flows. Model results agree well with the theory. In boreal winter and summer, cross-equatorial flows steer stationary waves propagating from one hemisphere to the other across the tropical easterlies, especially over the Australian–Asian monsoon region. It seems that the large-scale monsoonal background flows play a critical role in the interhemispheric teleconnection. Additionally, the wave ray trajectory and model results suggest that the NAA pattern may result from Rossby wave energy dispersion.

Interhemispheric Teleconnections from Tropical Heat Sources in Intermediate and Simple Models

Abstract The mechanisms that control the interhemispheric teleconnections from tropical heat sources are investigated using an intermediate complexity model [a quasi-equilibrium tropical circulation model (QTCM)] and a simple linear two-level model with dry dynamics. Illustrating the interhemispheric teleconnection process with an Atlantic warm pool principal case, the heat source directly excites a baroclinic response that spreads across the equator. Then, three processes involving baroclinic–barotropic interactions—shear advection, surface drag, and vertical advection—force a cross-equatorial barotropic Rossby wave response. An analysis of these processes in QTCM simulations indicates that 1) shear advection has a pattern that roughly coincides with the baroclinic signal in the tropics and subtropics, 2) surface drag has large amplitude and spatial extent and can be very effective in forcing barotropic motions around the globe, and 3) vertical advection has a significant contribution locally and remotely where large vertical motions and vertical shear occur. The simple model is modified to perform experiments in which each of these three mechanisms may be included or omitted. By adding surface drag and vertical advection, and comparing each to shear advection, the effects of the three mechanisms on the generation and propagation of the barotropic Rossby waves are shown to be qualitatively similar to the results in QTCM. It is also found that the moist processes included in the QTCM can feed back on the teleconnection process and alter the teleconnection pattern by enlarging the prescribed tropical heating in both intensity and geographical extent and by inducing remote precipitation anomalies by interaction with the basic state.

The relationship between the ITCZ and the Southern Hemispheric eddy‐driven jet

AbstractWe study the effect of a thermal forcing confined to the midlatitudes of one hemisphere on the eddy‐driven jet in the opposite hemisphere. We demonstrate the existence of an “interhemispheric teleconnection,” whereby warming (cooling) the Northern Hemisphere causes both the intertropical convergence zone (ITCZ) and the Southern Hemispheric midlatitude jet to shift northward (southward). The interhemispheric teleconnection is effected by a change in the asymmetry of the Hadley cells: as the ITCZ shifts away from the Equator, the cross‐equatorial Hadley cell intensifies, fluxing more momentum toward the subtropics and sustaining a stronger subtropical jet. Changes in subtropical jet strength, in turn, alter the propagation of extratropical waves into the tropics, affecting eddy momentum fluxes and the eddy‐driven westerlies. The relevance of this mechanism is demonstrated in the context of future climate change simulations, where shifts of the ITCZ are significantly related to shifts of the Southern Hemispheric eddy‐driven jet in austral winter. The possible relevance of the proposed mechanism to paleoclimates is discussed, particularly with regard to theories of ice age terminations.

Linkage between winter sea surface temperature east of Australia and summer precipitation in the Yangtze River valley and a possible physical mechanism

The relationship between winter sea surface temperature (SST) east of Australia and summer precipitation in the Yangtze River valley and a possibly related physical mechanism were investigated using observation data. It is found that winter SST east of Australia is correlated positively to summer precipitation in the Yangtze River valley. When the SST east of Australia becomes warmer in winter, the western Pacific subtropical high and the East Asian westerly jet tend to shift southward the following summer, concurrent with low-level southwesterly anomalies over eastern China. These conditions favor precipitation increase in the Yangtze River valley, whereas the opposite conditions favor precipitation decrease. The influence of winter SST east of Australia on East Asian summer atmospheric circulations may occur in two ways. First, by an anomalous SST signal east of Australia in winter that persists through the following summer, thus affecting East Asian atmospheric circulations via the inter-hemispheric teleconnection. Second, when the SST east of Australia is warmer in winter, higher SST appears simultaneously in the southwest Indian Ocean and subsequently develops eastward by local air-sea interaction. As a result, the SST in the Maritime Continent increases in summer, which may lead to an anomalous change in East Asian summer atmospheric circulations through its impact on convection.

An inter-hemispheric teleconnection and a possible mechanism for its formation

Through observational analyses, an inter-hemispheric teleconnection is identified between the subtropical region of the South China Sea to the western Pacific near the Philippines (WP) and the region to the east of Australia (AE). The teleconnection is significantly correlated with sea surface temperature anomalies (SSTAs) in key sea areas (including the Indian Ocean, the South China Sea, and the area to the east of Australia). Based on the IAP T42L9 model, numerical experiments are performed to explore a possible mechanism for the formation of the teleconnection. The results show that the positive SST anomalies in the key sea areas may jointly contribute to the occurrence and maintenance of the positive geopotential height anomalies over both the WP and AE and be a critical factor in the teleconnection formation. The large-scale SST anomaly in the Indian Ocean, involving the tropics and subtropics of both hemispheres, may lead to concurrent atmospheric responses over both the WP and AE, while the effect of the local SST anomaly to the east of Australia seems to reinforce and maintain the positive height anomaly over the AE.

Intermediate water signal leads surface water response during Northeast Atlantic deglaciation

Multi-proxy records of the high resolution core ENAM94-09 from the Faeroe region, NE Atlantic, were used to reconstruct paleoceanographic conditions at intermediate water depth during the Last Glacial Maximum (LGM, 19.3–16.7 ka BP) and the initial deglaciation period (16.7–>14 ka BP). The methods comprise micropaleontological and geochemical analyses as well as measurements of stable isotopes and AMS 14C dating for stratigraphic control. Owing to exceptionally high sedimentation rates of 90 cm/ka during the LGM and the initial deglaciation, it has been possible to resolve that the intermediate water masses in the Faeroe area lead the surface water deglaciation signal by 1.6 ka. The time span of this lead corresponds to the “Southern Hemisphere Lead” recently reported from the Atlantic sector of the Southern Ocean [Paleoceanography, 14 (1999) 135]. It is suggested that an interhemispheric teleconnection at intermediate water depth played a crucial role during deglaciation and in the final ice surge events around the northern North Atlantic.

Holocene variations in Asian monsoon moisture: A bidecadal sediment record from the South China Sea

The East Asian monsoon system involves extensive transport of sensible/latent heat between land and sea and from low to high latitudes. Our high resolution, bidecadal marine records present a first detailed history of monsoon climate change over the Holocene. The high‐amplitude perturbation in monsoon moisture centered at 8,150 years ago and the monsoon maximum in the Early Holocene show inter‐hemispheric teleconnections to both a cool episode in Greenland and to the Indian monsoon monitored in the Arabian Sea. Periodicities of 84, 102 and, especially, near 775 years in monsoon variation suggest a climatic forcing both by long‐term oscillations in thermohaline circulation and (possibly) solar activity cycles.

Temporal‐geographical meltwater influences on the North Atlantic conveyor: Implications for the Younger Dryas

The temporal and geographical roles of meltwater discharge (from the Laurentide ice sheet) on North Atlantic Deep Water (NADW) production are investigated utilizing a global, realistic geometry, coupled climate model which does not require the use of flux adjustments. Model results suggest that preconditioning by meltwater discharge (to the Mississippi) prior to the Younger Dryas (YD) is capable of pushing NADW beyond the limit of its sustainability. The diversion of meltwater to the St. Lawrence then merely serves to completely inhibit NADW production. The modeled change in surface air temperature generally agrees with the global pattern and magnitude of temperature change seen in paleoclimatic reconstructions of the YD and is intimately linked to changes in NADW formation. The global thermohaline circulation provides an interhemispheric teleconnection with the Southern Oceans, while changes in the atmospheric heat transport (reacting to a global redistribution of oceanic heat transport) provide a mechanism for interbasin teleconnection. Although the primary thermodynamic and hydrological cycle feedback processes are included within the atmospheric model, in the absence of additional feedbacks an equilibrium without the presence of NADW is possible. The inclusion of the wind stress/speed feedback is found to significantly contribute to the resumption of NADW production, as suggested by previous studies. Contrary to these same studies, however, the coupled model indicates an advective spin‐up timescale is required for resumption of NADW production and hence the termination of the modeled YD‐like climate event (as opposed to a decadal‐century timescale). The reason for the discrepancy is unclear but may be associated with the use of fixed salt flux fields applied in previous studies, or the duration, strength, and geographical location of the imposed meltwater applied.

The Spatial Coherence of African Rainfall Anomalies: Interhemispheric Teleconnections

Abstract The spatial patterns of rainfall variability over the African continent are examined using a regionally averaged data set comprising the records of 1087 stations. Typical rainfall departure patterns, or anomaly types, are derived using a linear correlation method to assess the similarity of annual departure maps for the years 1901 to 1973. For the region north of the equator, most years fall into one of six types, four of which show a strong opposition between equatorial and subtropical latitudes and two of which show departures of the same sign over the whole region. For the continent as a whole, six types are also derived. These reflect the patterns derived using northern Africa alone, with conditions in the southern subtropics resembling those in subtropical latitudes north of the equator. Continentally, two principal spatial patterns of rainfall variation are evident: anomalies of the same sign over most of Africa and anomalies of the opposite sign in equatorial and subtropical latitudes. Thu...

Intra- and inter-hemispheric teleconnections using seasonal Southern Hemisphere sea level pressure : Harnack, R.P. and Jeremi Harnack, 1985. J. Climatol., 5(3):283–296

Intra- and inter-hemispheric teleconnections using seasonal Southern Hemisphere sea level pressure : Harnack, R.P. and Jeremi Harnack, 1985. J. Climatol., 5(3):283–296