Shang-Ping XIEScripps Institution of Oceanography,University of California San Diego,USA2Physical Oceanography Laboratory,Ocean University of China,China

Preface to the Special Issue“Unified Perspective of Climate Variability and Change”

Shang-Ping XIE1,21Scripps Institution of Oceanography,University of California San Diego,USA
2Physical Oceanography Laboratory,Ocean University of China,China

Forty years ago,Klaus Wyrtki(1975)of University of Hawaii discovered that El Ni?no warming off South America is not a result of local wind change but a response to the relaxed equatorial trade winds some 10 000 km away near the international dateline.The Kelvin wave mechanism was quickly verified from wind-forced ocean model simulations.Consequent developments show that the dance between the fast-reacting atmosphere and slow-evolving ocean sets the pace of El Ni?no-Southern Oscillation(ENSO;Philander,1990).The concept of ocean-atmosphere interaction has revolutionized our view of the climate system and led to operational climate prediction.

The same year,Manabe and Wetherald(1975)of Geophysical Fluid Dynamics Laboratory at Princeton published the first global warming simulation using a three-dimensional atmospheric general circulation model.Although the ocean was crude,their results revealed regional distributions of climate change including the polar amplification of surface warming and intensified hydrological cycle.At the time,global warming seemed a distant possibility of no immediate concern.In fact, global mean surface temperature(GMST)then was lower than in the early 1940s,and many worried that the world might be plunging into a new ice age.

We now know that GMST was about to take off and rise by some 0.7?C since 1975.Over the 40 years,atmospheric carbon dioxide has risen to 400 ppm from 332 ppm,a jump that exceeded the prior increase over the 200 000 years since human first walked this planet.The world has come to view anthropogenic climate change as one of the biggest challenges as repeated calculations with generations of climate models all predict a warmer climate with increasing greenhouse gases(IPCC,2014). With this GMST prediction widely accepted-though uncertainties remain in the magnitude of the warming-the spotlight shifts to regional climate information for adaptation.

Regional climate change is an emerging frontier of climate dynamics as recent studies show that the ocean surface warming pattern drives changes in tropical rainfall and atmospheric circulation change(Xie et al.,2015).On the one hand,the coupling between atmospheric circulation and ocean is at the heart of climate dynamics,enabling a unified view of climate variability and change.On the other,there are circulation changes unique to global warming(e.g.,the Hadley cell expansion).Thus,regional change under global warming presents new problems and opportunities for ocean,atmospheric, and coupled dynamics.To develop these opportunities,international program CLIVAR created a Climate Dynamics Panel (http://www.clivar.org/clivar-panels/climate-dynamics)in 2015.

In 2010,the Chinese Ministry of Science and Technology identified global change as a national research priority and has since funded a suite of projects to accelerate research in support of policy making.Under the National Basic Research Program,a project was launched in 2012 to investigate“the response of the Indo-Pacific Oceans to global warming and their role in climate change”(2012CB955600;http://globalwarming973.ouc.edu.cn/),with an overarching goal of developing dynamical approaches bridging climate variability and change.This special issue presents latest research from this project.

Ying et al.investigate the tropical Pacific warming pattern in climate model projections.Weak evaporative damping in the eastern equatorial Pacific turns out to be important but other ocean dynamic mechanisms also contribute.Ocean upwelling is one such mechanism,creating a warming pattern of strong zonal gradient over the equatorial Indian Ocean(Luo et al.). Sea level rise is among the most consequential effects of global warming and varies considerably from one region to another. Wang et al.show that sea level in Macau is likely to rise faster than the global mean because of ocean dynamical response to wind change.The press coverage on this work attests high interests in regional climate information.The upper ocean warming is expected to increase ocean heat content above the 26?C isotherm(Liu et al.).This heat potential regulates the surface cooling in tropical cyclones and hence storm intensity but needs to be adjusted in light of the projected increase in the threshold temperature for atmospheric convection.

From the deep winter mixed layer on the poleward flank of the subtropical gyre,water subducts into and ventilates the main thermocline.Under global warming,the strengthened thermal stratification shoals the ocean mixed layer.Based on climate model projections,Zhang et al.show that the mixed layer in the Kuroshio Extension east of Japan shoals the most in March-April,causing an earlier onset of spring phytoplankton bloom in the future.Li and Luo find that the shoaling mixedlayer weakens the subduction of the Northeast Pacific mode water.In the subtropical Southeast Pacific,Liu and Lu report that horizontal advection is important for the seasonal deepening of the ocean mixed layer in addition to atmospheric cooling.

A major advance in Asian monsoon research is the recent recognition that a recurrent anomalous anticyclonic circulation over the northern Indo-western Pacific drives much of regional interannual variability.The review by Xie et al.identifies a regional coupled ocean-atmosphere mode that enables climate anomalies in the Indo-Northwest Pacific to persist and outlast El Ni?no itself.The resultant predictability could save lives for the 2016 summer following the major El Ni?no.A related study by Long et al.identifies a robust teleconnection by which atmospheric convection over the tropical Northwest Pacific affects summer Yellow Sea fog occurrence.Climate models are imperfect.Zheng et al.evaluate how the easterly biases in the equatorial trades affect the tropical Indian Ocean response to ENSO in models.

GMST increase has slowed in the early 21st century despite the sustained increase in atmospheric greenhouse gases.The apparentparadoxspurredaresearchboom.Theemergingconsensusisthatthewarminghiatusislargelycausedbyadownturn in internal variability while the anthropogenic warming persists.We live in a special historic epoch when anthropogenic change has grown of comparable magnitude to internal variability.It is also a golden age of scientific discoveries as global warming continues to grow and regional patterns of the resultant climate change emerge from internal variability.Science advances when confronted by new observations,challenged by emerging contradictions,and called upon to predict.

Riyu LU(AAS editor)oversaw the development of the special issue,which was suggested by project member Gang HUANG.Shang-Min LONG and Zhen-Qiang ZHOU researched for the cover design.

REFERENCES

IPCC,2014:Summary for Policymakers.Climate Change 2013:The Physical Science Basis.Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change,Stocker et al.,Eds.,Cambridge University Press,1-30, doi:10.1017/CBO9781107415324.004.

Manabe,S.,and R.T.Wetherald,1975:The effects of doubling the CO2concentration on the climate of a general circulation model.J. Atmos.Sci.,32,3-15.

Philander,S.G.,1990.;El Ni?no,La Ni?na,and the Southern Oscillation.Academic Press,San Diego,ix+293 pp.

Wyrtki,K.,1975:El Ni?no-the dynamic response of the equatorial Pacific Ocean to atmospheric forcing.J.Phys.Oceanogr.,5(4), 572-584.

Xie,S.-P.,and Coauthors,2015:Towards predictive understanding of regional climate change.Nature Clim.Change,5,921-930.

Xie,S.-P.,2016:Preface to the Special Issue“Unified Perspective of Climate Variability and Change”. Adv.Atmos.Sci.,33(4),409-410,

10.1007/s00376-015-0003-7.