DIFFERENCES OF INTERDECADAL OSCILLATION BETWEEN MEAN TEMPERATURE OF NORTHERN AND SOUTHERN HEMISPHERE AND THEIR INFLUENCES ON WARMING SIGNAL

On the basis of multi-taper spectral analysis, the work not only has examined and reformed monthly mean temperature time series of the Northern Hemisphere (NH) and Southern Hemisphere (SH) from 1856 to 1998, but also has systematically contrasted the differences of interdecadal oscillation (IDO) between the hemispheres, ocean-land surface in different seasons, with special analysis of IDO signals effects on global warming. The results show that the warming trend plays a dominant role in hemispheric mean temperature variability during the last 150 years. However, there are significant IDO with periods of about 40, 60 70 years superimposed on a linear warming trend for NH mean temperature which leads to the reduction of the linear warming rate in terms of its significance and stability, as opposed to that in the SH, especially in summer. Moreover, in comparison of land to sea surface temperature, IDO signals detected in the latter are found to be more remarkable than those in the former, as contrasted to the linear warming rate. It has been noticed that IDO shows its peak value in the middle 1990s and begins to descend recently, a fact that probably affects the coming warming rate of NH mean temperature. Meanwhile, In terms of the GCM results from the HadCM2 model, preliminary analysis implies that the IDO may be the inherent oscillation of the ocean and atmosphere system, but warming trends are not related to natural variability.

Influence of the Atlantic Multidecadal Oscillation and Interdecadal Pacific Oscillation on Antarctic surface air temperature during 1900 to 2015

The importance of the Atlantic Multidecadal Oscillation(AMO)and Interdecadal Pacific Oscillation(IPO)in influencing zonally asymmetric changes in Antarctic surface air temperature(SAT)has been established.However,previous studies have primarily concentrated on examining the combined impact of the contrasting phases of the AMO and IPO,which have been dominant since the advent of satellite observations in 1979.This study utilizes long-term reanalysis data to investigate the impact of four combinations of+AMO+IPO,–AMO–IPO,+AMO–IPO,and–AMO+IPO on Antarctic SAT over the past 115 years.The+AMO phase is characterized by a spatial mean temperature amplitude of up to 0.5℃over the North Atlantic Ocean,accompanied by positive sea surface temperature(SST)anomalies in the tropical eastern Pacific and negative SST anomalies in the extratropical-mid-latitude western Pacific,which are indicative of the+IPO phase.The Antarctic SAT exhibits contrasting spatial patterns during the+AMO+IPO and+AMO–IPO periods.However,during the–AMO+IPO period,apart from the Antarctic Peninsula and the vicinity of the Weddell Sea,the entire Antarctic region experiences a warming trend.The most pronounced signal in the SAT anomalies is observed during the austral autumn,whereas the combination of–AMO and–IPO exhibits the smallest magnitude across all the combinations.The wavetrain excited by the SST anomalies associated with the AMO and IPO induces upper-level and surface atmospheric circulation anomalies,which alter the SAT anomalies.Furthermore,downward longwave radiation anomalies related to anomalous cloud cover play a crucial role.In the future,if the phases of AMO and IPO were to reverse(AMO transitioning to a negative phase and IPO transitioning to a positive phase),Antarctica could potentially face more pronounced warming and accelerated melting compared to the current observations.

Salinity effect-induced ENSO amplitude modulation in association with the interdecadal Pacific Oscillation

A 110-year ensemble simulation of an ocean general circulation model(OGCM)was analyzed to identify the modulation of salinity interdecadal variability on El Niño-Southern Oscillation(ENSO)amplitude in the tropical Pacific during 1901-2010.The simulating results show that sea surface salinity(SSS)variation in the region exhibits notable and coherent interdecadal variability signal,which is closely associated with the Interdecadal Pacific Oscillation(IPO).As salinity increases or reduces,the SSS modulations on ENSO amplitude during its warm/cold events vary asymmetrically with positive/negative IPO phases.Physically,salinity interdecadal variability can enhance or reduce ENSO-related conditions in upper-ocean stratification,contributing noticeably to ENSO variability.Salinity anomalies associated with the mixed layer depth and barrier layer thickness can modulate ENSO amplitude during positive and negative IPO phases,resulting in the asymmetry of sea surface temperature(SST)anomaly in the tropical Pacific.During positive IPO phases,SSS interdecadal variability contributes positively to El Niño amplitude but negatively to La Niña amplitude by enhancing or reducing SSS interannual variability,and vice versa during negative IPO phases.Quantitatively,the results indicate that the modulation of the ENSO amplitude by the SSS interdecadal variability is 15%-28%during negative IPO phases and 30%-20%during positive IPO phases,respectively.Evidently,the SSS interdecadal variability associated with IPO and its modulation on ENSO amplitude in the tropical Pacific are among factors essentially contributing ENSO diversity.

Asymmetry of Salinity Variability in the Tropical Pacific during Interdecadal Pacific Oscillation Phases

It has been recognized that salinity variability in the tropical Pacific is closely related to the Interdecadal Pacific Oscillation(IPO).Here,we use model simulations from 1900 to 2017 to illustrate obvious asymmetries of salinity variability in the tropical Pacific during positive and negative IPO phases.The amplitude of salinity variability in the tropical Pacific during positive IPO phases is larger than that during negative IPO phases,with a more westward shift of a large Sea Surface Salinity(SSS)anomaly along the equator.Salinity budget analyses show that the asymmetry of salinity variability during positive and negative IPO phases is dominated by the difference in the surface forcing associated with the freshwater flux[FWF,precipitation(P)minus evaporation(E)],with a contribution of 40%–50%near the dateline on the equator.Moreover,the relationships between the salinity variability and its budget terms also show differences in their leadlag correlations during positive and negative IPO phases.These differences in salinity variability during different IPO phases produce asymmetric effects on seawater density which can reduce or enhance upper-ocean stratification.Therefore,the salinity effects may modulate the intensity of El Nino-Southern Oscillation(ENSO),resulting in an enhanced(reduced)El Nino but a reduced(enhanced)La Ni?a during positive(negative)IPO phases by 1.6℃psu^(-1)(1.3℃psu^(-1)),respectively.It is suggested that the asymmetry of salinity variability may be related to the recent change in ENSO amplitude associated with the IPO,which can help elucidate ENSO diversity.

Perturbed Solving Method for Interdecadal Sea-air Oscillator Model

A coupled system of the interdecadal sea-air oscillator model is studied. The E1 Nifio-southem oscillation （ENSO） atmospheric physics oscillation is an abnormal phenomenon involved in the tropical Pacific ocean-atmosphere interactions. The oscillator model is involved with the variations of both the eastern and western Pacific anomaly pat- terns. This paper proposes an ENSO atmospheric physics model using a method of the perturbation theory. The aim is to create an asymptotic solving method for the ENSO model. Employing the perturbed method, the asymptotic solution of corresponding problem is obtained, and the asymptotic behaviour of the solution is studied. Thus we can obtain the prognoses of the sea surface temperature anomaly and related physical quantities.

Interdecadal Enhancement of the Walker Circulation over the Tropical Pacific in the Late 1990s

The Walker circulation is one of the major components of the large-scale tropical atmospheric circulation and variations in its strength are critical to equatorial Pacific Ocean circulation. It has been argued in the literature that during the 20th century the Walker circulation weakened, and that this weakening was attributable to anthropogenic climate change. By using updated observations, we show that there has been a rapid interdecadal enhancement of the Walker circulation since the late 1990s. Associated with this enhancement is enhanced precipitation in the tropical western Pacific, anomalous westerlies in the upper troposphere, descent in the central and eastern tropical Pacific, and anomalous surface easterlies in the western and central tropical Pacific. The characteristics of associated oceanic changes are a strengthened thermocline slope and an enhanced zonal SST gradient across the tropical Pacific. Many characteristics of these changes are similar to those associated with the mid-1970s climate shift with an opposite sign. We also show that the interdecadal variability of the Walker circulation in the tropical Pacific is inversely correlated to the interdecadal variability of the zonal circulation in the tropical Atlantic. An enhancement of the Walker circulation in the tropical Pacific is associated with a weakening zonal circulation in the tropical Atlantic and vise versa, implying an inter-Atlantic-Pacific connection of the zonal overturning circulation variation. Whether these recent changes will be sustained is not yet clear, but our research highlights the importance of understanding the interdecadal variability, as well as the long-term trends, that influence tropical circulation.

The Interdecadal Variation Characteristics of Arctic Sea Ice Cover-ENSO-East Asian Monsoon and Their Interrelationshipat Quasi-Four Years Time Scale

Interdecadal and quasi-four years variation characterstics of Arctic sea for cover, ENSO and East Asian monsoon index(EAMI) are analyzed based on Singular Spectrum Analys. (SSA), lead-lag correlation and EOF for the past four decades. Results show that the Arctic sea for cover decreased in the early 1970s, several years earlier than that of global SSTA increase in the mid 1970s, which indicates that recent warming over the Northern Hemisphere firstly begins in the Arctic region in the 1970s. Great change of the East Asian monsoon intensity from stronger to weaker in summer (from weaker to stronger in winter) took place in the mid 1970s response to the abrupt modulation of SSTA particularly in the tropical eastern Pacific.Focus on the quasi-four years oscillation,close relationship is found among the sea ice cover, ENSO and EAMI based on lead-lag correlation. In which, the correlation coefficient reaches its maximum when the index of NINO3 SSTA variation takes 6 and 9 months lead of the western Pacific subtropical high and sea for cover index in Section-Ⅲ. Their interaction can be explained in the framework of asymmetric Walker circulation anomaly and Western Pacific Northern Pole (WPN) teleconnection pattern in the context of quasi-four years oscillation.

Recent Decrease in the Difference in Tropical Cyclone Occurrence between the Atlantic and the Western North Pacific

Climatologically,among all ocean basins,the western North Pacific(WNP)has the largest annual number of tropical cyclones(TCs)of around 26 while the Atlantic has around 13,yielding a difference of 13.However,the difference is-7 in 2020,with 30 TCs in the Atlantic and 23 in the WNP,which is the most negative difference within the last 46 years.In fact,during the last 26 years,the difference in TC number is below 10 in ten years,with four years being negative.Such a decreasing difference in TC number can be attributed to the natural multidecadal variation of the Atlantic Multidecadal Oscillation and Interdecadal Pacific Oscillation,as well as other external forcings such as anthropogenic aerosol forcing and increased greenhouse gases,with the additional impact from the La Niña condition.This result has significant implications on climate model projections of future TC activity in the two ocean basins.

An aftereffect of global warming on tropical Pacific decadal variability

Studies have shown that global warming over the past six decades can weaken the tropical Pacific Walker circulation and maintain the positive phase of the Interdecadal Pacific Oscillation （IPO）. Based on observations and model simulations, another aftereffect of global warming on IPO is found. After removing linear trends （global warming signals） from observations, however, the tropical Pacific climate still exhibited some obvious differences between two IPO negative phases. The boreal winter （DJF） equatorial central-eastern Pacific sea surface temperature （SST） was colder during the 1999-2014 period （P2） than that during 1961-1976 （P 1）. This difference may have been a result of global warming nonlinear modulation of precipitation; i.e., in the climatological rainy region, the core area of the tropical Indo-westem Pacific warm pool receives more precipitation through the ＂wet-get-wetter＂ mechanism. Positive precipitation anomalies in the warm pool during P2 are much stronger than those during P1, even after subtracting the linear trend. Corresponding to the differences of precipitation, the Pacific Walker circulation is stronger in P2 than in P 1. Consequent easterly winds over the equatorial Pacific led to a colder equatorial eastern--central Pacific during P2. Therefore, tropical Pacific climate differences between the two negative IPO phases are aftereffects of global warming. These aftereffects are supported by the results of coupled climate model experiments, with and without global warming.

Contribution of El Nino amplitude change to tropical Pacific precipitation decline in the late 1990s

Equatorial central Pacific precipitation experienced a prominent decline in the late 1990 s.This change was previously attributed to a La Nina-like mean sea surface temperature(SST)change in the Pacific Ocean associated with a phase switch of the Interdecadal Pacific Oscillation.Here,using a series of model experiments,the authors reveal that the El Nino-related interannual SST anomalies contributed largely to the precipitation decrease over the equatorial central Pacific.This El Nino SST effect was due to the change in the amplitude of El Nino events in the late 1990 s.The 1980-98 decade had more large-amplitude El Nino events than the 1999-2014 decade.The nonlinear precipitation response to SST anomalies resulted in a larger decadal mean precipitation in the 1980-98 decade than in the 1999-2014 decade.The results highlight the importance of El Nino amplitude change in future climate change related to global warming.

An Analysis of Interdecadal Variations of the Asian-African Summer Monsoon

The response of the Asian-African summer monsoon （AASM） to the fast global warming in the 1980s is studied based on several datasets, which span a long time period of nearly 100 yr, with two special periods 1980-1985 and 1990-1995 being focused on. Wavelet analyses are employed to explore the interdecadal variations of the AASM. It is found that after the mid 1980s, the global annual mean surface temperature rises more significantly and extensively over most parts of the African Continent, north of the Indian Ocean, and the Eurasian Continent excluding the Tibetan Plateau. Correspondingly, the global precipitation pattern alters with in- creased rainfall seen over the Sahel and North China in 1990-1995, though it is not recovered to the level of the rainy period before the mid-1960s. Changes of monsoonal circulations between the pre- and post-1980s periods display that, after the fast global warming of the 1980s, the African summer monsoon intensifies distinctly, the Indian summer mon- soon weakens a little bit, and the East Asian summer monsoon remains almost unchanged. The summer precipitation over the Asian-African Monsoon Belt （AAMB） does not change in phase coherently with the variations of the monsoonal circulations. Wavelet analyses of the land-sea thermal contrast and precipitation over North China and the Sahel indicate that interdecadal signals are dominant and in positive phases in the 1960s, leading to an overall enhanced interdecadal variation of the AASM, although the 1960s witnesses a global cooling. In the 1980s, however, in the context of a fast global warming, interdecadal signals are in opposite phases, and they counteract with each other, leading to a weakened interdecadal variation of the AASM. After the mid-1960s, the AASM weakened remarkably, whereas after the mid-1980s, the AASM as a whole did not strengthen uniformly and synchronously, because it is found that the interannual variations of the AASM in the 1980s are stronger than those in the 1960s, and they superimposed on the counteracting interdecadal signals, causing different regions of the AAMB behaving differently. Therefore, the response of the AASM to the accelerated global warming post the mid-1980s is not simply out-of-phase with that after the mid-1960s; it may involve more complicated multiscale physical elements.

Variability of Antarctic sea ice extent over the past 200 years

While Arctic sea ice has been decreasing in recent decades that is largely due to anthropogenic forcing,the extent of Antarctic sea ice showed a positive trend during 1979–2015, followed by an abrupt decrease. The shortness of the satellite record limits our ability to quantify the possible contribution of anthropogenic forcing and internal variability to the observed Antarctic sea ice variability. In this study,ice core and fast ice records with annual resolution from six sites are used to reconstruct the annualresolved northernmost latitude of sea ice edge(NLSIE) for different sectors of the Southern Ocean, including the Weddell Sea(WS), Bellingshausen Sea(BS), Amundsen Sea(AS), Ross Sea(RS), and the Indian and western Pacific Ocean(Ind WPac). The linear trends of the NLSIE are analyzed for each sector for the past100–200 years and found to be à0.08°, à0.17°, +0.07°, +0.02°, and à0.03° per decade(!95% confidence level) for the WS, BS, AS, RS, and Ind WPac, respectively. For the entire Antarctic, our composite NLSIE shows a decreasing trend(à0.03° per decade, 99% confidence level) during the 20 th century, with a rapid decline in the mid-1950 s. It was not until the early 1980 s that the observed increasing trend occurred. A comparison with major climate indices shows that the long-term linear trends in all five sectors are largely dominated by the changes in the Southern Annular Mode(SAM). The multi-decadal variability in WS,BS, and AS is dominated by the Interdecadal Pacific Oscillation, whereas that in the Ind WPac and RS is dominated by the SAM.

Co-varying temperatures at 200 hPa over the Earth’s three poles

The Earth’s three poles,the North Pole,South Pole,and Third Pole(i.e.,the Tibetan Plateau and its surroundings),hold the largest amount of fresh water on Earth as glaciers,sea ice,and snow.They are sensitive to climate change.However,the linkages between climate variations of the three poles,particularly between the South Pole and Third Pole,remain largely unknown.The temperatures at 200 hPa over the three poles are the highest in the summer and are less affected by surface conditions,which could reflect large-scale dynamic linkages.Temperatures at 200 hPa peak the three poles during their respective hemispheric summer and exhibit in-phase variations on interdecadal timescales(10–100 years).The 200 hPa temperatures over the North Pole and South Pole were significantly correlated with the Brewer-Dobson circulation(BDC),which transports stratospheric ozone poleward,heating the air at 200 hPa.Tropopause warming over the Third Pole was found to enhance the poleward BDC,particularly to the South Pole,linking the Third Pole’s climate to the other two poles.Additionally,the Interdecadal Pacific Oscillation(IPO)also exhibits links with the 200 hPa temperatures of the three poles.

CLIMATIC JUMP FROM LATE 1970S TO EARLY 1980S AND ITS EFFECT

By use of the moving T test to do research on the interdecadal climate jump of the Northern Hemisphere sea level pressure.500 hPa height and North Pacific sea surface temperature,we found that in recent 50 years an obvious interdecadal climate jump existed at the late 1970s to the early 1980s.There is significant difference before and after the jump in terms of the Northern Hemisphere sea level pressure,500 hPa height and North Pacific sea surface temperature. Furthermore,the focus is then placed on the effect of the jump on temperature and rainfall in China.