Ping Zhao

Springtime tropospheric temperature over the Tibetan Plateau and evolutions of the tropical Pacific SST

[1] Using monthly mean data from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCAR) reanalysis and HadISST SST data sets, we investigate the relationship between springtime tropospheric temperature over the Tibetan Plateau and sea surface temperature (SST) over the equatorial Pacific and the associated physical processes. When the Tibetan temperature is low (high) in spring, positive (negative) SST anomalies appear over the tropical central-eastern Pacific in spring and summer. The relationship is explained by the Asian-Pacific Oscillation (APO) and the ocean-atmosphere interaction over the tropical Pacific. In the context of the APO, a lower spring Tibetan tropospheric temperature is associated with a higher tropospheric temperature over the subtropical North Pacific, which is accompanied by a weaker subtropical high over the eastern North Pacific. Accordingly, large-scale westerly anomalies appear in the lower troposphere of the equatorial central-eastern Pacific, resulting in an increase in SST over the equatorial central-eastern Pacific. Numerical simulations with both an ocean-atmosphere coupled model (the NCAR Community Climate System Model version 3) and an atmospheric model with a prescribed SST scheme (the NCAR Community Atmospheric Model version 3) demonstrate the impacts of the spring Tibetan thermal condition on the tropospheric temperature and atmospheric circulation over the Asian-Pacific sector and then on the SST over the equatorial eastern Pacific, better explaining the physical processes of the observed Tibetan temperature–Pacific SST relationship.

Boreal spring Southern Hemisphere Annular Mode, Indian Ocean sea surface temperature, and East Asian summer monsoon

[1] The relationships among the boreal spring Southern Hemisphere Annular Mode (SAM), the Indian Ocean (IO) sea surface temperature (SST), and East Asian summer monsoon (EASM) are examined statistically in this paper. The variability of boreal spring SAM is closely related to the IO SST. When the SAM is in its strong positive phase in boreal spring, with low-pressure anomalies over the south pole and high-pressure anomalies over middle latitudes, SSTover the subtropics and middle latitudes of the South Indian Ocean (SIO) increases, which persists into the summer. Following the positive SST anomalies over the subtropics and midlatitudes of the SIO, SST in the equatorial Indian Ocean and Bay of Bengal increases in summer. Moreover, the variability of SST in the equatorial Indian Ocean and Bay of Bengal is closely related to EASM. When SST in the equatorial Indian Ocean and Bay of Bengal increases, EASM tends to be weak. Therefore the IO SST may play an important role bridging boreal spring SAM and EASM. The atmospheric circulations and surface heat exchanges contribute to the SST anomalies in the SIO. When the spring SAM is in its strong positive phases, the regional Ferrel Cell weakens, and the anomalous upward motions at 20S–30S cause an increase of low cloud cover and downward longwave radiation flux. The surface atmospheric circulations also transport more (less) warmer (cooler) air from middle latitudes north of 50S (high latitudes south of 60S) into 50S–60S and warm the air, which reduces the temperature difference between the ocean and atmosphere and consequently reduces sensible heat flux from the ocean to atmosphere. The increased downward longwave radiation and decreased sensible heat are responsible for the SST increase in the SIO. The atmospheric circulation and surface heat flux anomalies are of opposite signs following the strong negative phases of SAM.

Modeling variations of summer upper tropospheric temperature and associated climate over the Asian Pacific region during the mid‐Holocene

[1]xa0The summer upper tropospheric temperature change and its association with atmospheric circulations and precipitation over the Asian Pacific region during the mid-Holocene have been addressed by using outputs from a coupled ocean atmosphere general circulation model performed as part of the second phase of the Paleoclimate Modeling Intercomparison Project. The simulated result shows the summer oscillation pattern similar to the present Asian Pacific oscillation (APO) existed in the mid-Holocene. When there was a warming (cooling) upper troposphere over East Asia, a cooling (warming) upper troposphere occurred over the midlatitudes of the North Pacific. Compared to the modern climate, however, the simulated mid-Holocene temperature in the upper troposphere was higher over East Asia and lower over the midlatitudes of the North Pacific, indicating a stronger summer APO in the mid-Holocene. Corresponding to such a condition, the North Pacific was modeled to be dominated by a high-level cyclonic circulation difference and a low-level anticyclonic circulation difference in the mid-Holocene relative to the present, which favored the subsidence of airflows and thus resulted in less precipitation in this region. Meanwhile, East Asia was simulated to be occupied by an anticyclonic circulation difference in the upper troposphere and a cyclonic circulation difference in the lower troposphere. Accordingly, the ascending motion and the low-level southerly wind strengthened in East Asia, leading to an increase of local precipitation in the mid-Holocene. Therefore, the modeled mid-Holocene climate suggests that the summer rainfall change over the Asian Pacific region may be a result of the strengthened APO in the upper troposphere.

Variations of the winter India-Burma Trough and their links to climate anomalies over southern and eastern Asia

[1]xa0The India-Burma Trough (IBT) plays an important role in affecting the weather and climate over southern and eastern Asia. An index of IBT is defined based on area-mean 700 hPa vorticity, which is superior to the indices based on geopotential height, to measure the interdecadal and interannual variations of the trough and their links to the winter climate anomalies over South, Southeast, and part of East Asia. An analysis of data since 1948 indicates that the IBT has undergone an interdecadal change. The trough since 1978 was almost always stronger than the trough before 1978. Compared to 1949–1977 when the IBT was weak, many different climate features were observed during 1978–2010. An anomalous low-level cyclonic pattern was found over southern Asia; a decreased temperature was observed over the Middle East, the Tibetan Plateau (TP), and south of the plateau; and increased precipitation occurred over portions of Middle East, western Asia, and southern China. Meanwhile, upper-tropospheric westerlies intensified over the Middle East and the TP, and increased (decreased) humidity and enhanced rising (sinking) motion appeared to the east (west) of the trough line. The result also indicates that the interannual correlations between IBT and surface temperature over southern and eastern Asia, and between the IBT and precipitation, were more significant during the strong IBT decades. Furthermore, the features revealed on interannual timescale are similar to those on interdecadal timescale, but the interannual features are clearly more pronounced. Thus, the IBT may be considered as one of the increasingly important factors for interannual predictions of the winter temperature and precipitation over southern Asia.

Numerical simulation study of temperature change over East China in the past millennium

Despite many studies on reconstructing the climate changes over the last millennium in China, the cause of the China’s climate change remains unclear. We used the UVic Earth System Climate Model (UVic Model), an Earth system model of intermediate complexity, to investigate the contributions of climate forcings (e.g. solar insolation variability, anomalous volcanic aerosols, greenhouse gas, solar orbital change, land cover changes, and anthropogenic sulfate aerosols) to surface air temperature over East China in the past millennium. The simulation of the UVic Model could reproduce the three main characteristic periods (e.g. the Medieval Warm Period (MWP), the Little Ice Age (LIA), and the 20th Century Warming Period (20CWP)) of the northern hemisphere and East China, which were consistent with the corresponding reconstructed air temperatures at century scales. The simulation result reflected that the air temperature anomalies of East China were larger than those of the global air temperature during the MWP and the first half of 20CWP and were lower than those during the LIA. The surface air temperature of East China over the past millennium has been divided into three periods in the MWP, four in the LIA, and one in the 20CWP. The MWP of East China was caused primarily by solar insolation and secondarily by volcanic aerosols. The variation of the LIA was dominated by the individual sizes of the contribution of solar insolation variability, greenhouse gas, and volcano aerosols. Greenhouse gas and volcano aerosols were the main forcings of the third and fourth periods of the LIA, respectively. We examined the nonlinear responses among the natural and anthropogenic forcings in terms of surface air temperature over East China. The nonlinear responses between the solar orbit change and anomalous volcano aerosols and those between the greenhouse gases and land cover change (or anthropogenic sulfate aerosols) all contributed approximately 0.2°C by the end of 20th century. However, the output of the energy-moisture balance atmospheric model from UVic showed no obvious nonlinear responses between anthropogenic and natural forcings. The nonlinear responses among all the climate forcings (both anthropogenic and natural forcings) contributed to a temperature increase of approximately 0.27°C at the end of the 20th century, accounting for approximately half of the warming during this period; the remainder was due to the climate forcings themselves.

Precessional forced extratropical North Pacific mode and associated atmospheric dynamics

Using transient accelerated simulations of the Community Climate System Model version 3 and an Earth System Model of Intermediate Complexity as well as equilibrium experiments of the Community Earth System Model, we identified a response of the extratropical air-ocean coupled system to the precessional insolation changes at orbital timescales and named this extratropical response pattern as the North Pacific mode (NPM). Corresponding to the increased/decreased boreal winter/summer insolation at 22 ka (relative to 10–8 ka), the NPM is characterized by a western warm-eastern cold seesaw pattern of sea surface temperature (SST) over the extratropical North Pacific from November to April, a weakened winter Aleutian low and an anomalous anticyclonic circulation throughout the troposphere. This feature forms a barotropic warm-ridge response of tropospheric temperature and geopotential height to the precessional insolation. At the surface, rainfall increases over East Asia and the Northwest Pacific, which indicates a weakened East Asian winter monsoon, while drier conditions appear over the Northeast Pacific and the western coasts of North America. Associated with a negative phase of NPM is a weaker warming over the equatorial Pacific during winter. The increased winter insolation at precessional band not only induces the in-phase SST warming over the Northwest Pacific and the tropical Pacific, but also explains those extratropical atmospheric changes associated with NPM. The latter might be associated with the warm SST-induced tropospheric downstream ridge response through transient eddy activities. Besides the vital role of air-ocean interactions, the decreased summer insolation is also essential to the zonal SST seesaw of NPM at precessional band.

Potential flaws of interdecadal changes over eastern China around the early 1990s in the National Centers for Environmental Prediction‐National Center for Atmospheric Research reanalyses

[1]xa0This study compares interdecadal changes in tropospheric temperature and geopotential height over eastern China in four reanalyses and upper air radiosonde (UAR) observations. It is found that the tropospheric temperature and geopotential height over eastern China during the period 1992–2000 were systematically lower in the National Centers for Environmental Prediction (NCEP)-National Center for Atmospheric Research (NCAR) reanalysis-1 and the NCEP-Department of Energy reanalysis-2 compared to the UAR observations. Accordingly, the tropospheric temperature and geopotential height over eastern China in the NCEP reanalyses showed a pronounced interdecadal decrease around 1992, while this decrease was not significant in the UAR observations, the 40xa0year European Centre for Medium-Range Weather Forecasts (ERA-40) reanalysis, and the 25xa0year Japanese Meteorological Agency (JRA-25) reanalysis. The overestimated interdecadal change in the NCEP reanalyses may lead to an inconsistent variation between tropospheric and surface climates. When summer surface air temperature increases (decreases) over southern China, summer tropospheric temperature over eastern China generally increases (decreases) in the ERA-40, JRA-25, and UAR data sets, which is physically supported by the NCAR Community Climate System Model version 3. However, this link between the troposphere and surface is not observed in the two NCEP reanalyses. The interdecadal bias in tropospheric temperature after 1992 in the NCEP reanalyses is possibly related to an operational change in the bias correction tables since 1992.

Precessional forced evolution of the Indian Ocean Dipole

In a transient accelerated simulation of a coupled climate model, we identified a zonal dipole-like pattern of sea surface temperature (SST) anomalies in the tropical Indian Ocean, which is forced by precessional insolation changes since 300 ka and named as the paleo-IOD (Indian Ocean Dipole). A positive paleo-IOD mean state at 23 kyr precessional band exhibits warmer and wetter conditions over the western Indian Ocean and cooler and drier conditions over the eastern tropical Indian Ocean from August to October. This zonal thermal seesaw at the sea surface can extend downward to the subsurface ocean between 60 and 80 m and accompanies stronger oceanic upwelling in the eastern tropical Indian Ocean. The associated boreal summer-autumn tropospheric circulation anomalies are characterized by anomalous ascent over the western Indian Ocean and anomalous descent over the southeastern tropical Indian Ocean, with anomalous easterlies at the surface along the equatorial Indian Ocean. The positive paleo-IOD largely originates from local air-sea interactions that are induced by the increased summer insolation, and is also contributed by the reduced boreal winter insolation through an oceanic “heat memory effect.” Our simulated dipole mode index (DMI) of SST is qualitatively consistent with the paleoceanographic reconstructed DMI based on the UK37 proxy of SST at precessional band and provides a possible explanation for the in-phase precessional variation between boreal winter insolation and the UK37 proxy of SST in the eastern tropical Indian Ocean.

Summer precipitation anomalies in Asia and North America induced by Eurasian non-monsoon land heating versus ENSO

When floods ravage Asian monsoon regions in summer, megadroughts often attack extratropical North America, which feature an intercontinental contrasting precipitation anomaly between Asia and North America. However, the characteristics of the contrasting Asian-North American (CANA) precipitation anomalies and associated mechanisms have not been investigated specifically. In this article, we firmly establish this summer CANA pattern, providing evidence for a significant effect of the land surface thermal forcing over Eurasian non-monsoon regions on the CANA precipitation anomalies by observations and numerical experiments. We show that the origin of the CANA precipitation anomalies and associated anomalous anticyclones over the subtropical North Pacific and Atlantic has a deeper root in Eurasian non-monsoon land surface heating than in North American land surface heating. The ocean forcing from the ENSO is secondary and tends to be confined in the tropics. Our results have strong implications to interpretation of the feedback of global warming on hydrological cycle over Asia and North America. Under the projected global warming due to the anthropogenic forcing, the prominent surface warming over Eurasian non-monsoon regions is a robust feature which, through the mechanism discussed here, would favor a precipitation increase over Asian monsoon regions and a precipitation decrease over extratropical North America.

Relative roles of land- and ocean-atmosphere interactions in Asian-Pacific thermal contrast variability at the precessional band

In a 250-kyr transient simulation of the Community Earth System Model (CESM), we identified a precessional forced seesaw of the summer middle-upper tropospheric eddy temperature between Asia and the North Pacific as the paleo-APO (Asian-Pacific oscillation). The paleo-APO variability is out of phase with the precession parameter. Corresponding to a positive paleo-APO phase, both the subtropical anticyclonic circulation over the North Pacific and the East Asian summer monsoon (EASM) strengthen. Summer anomalous sea surface temperature shows a western cold-eastern warm pattern over the extratropical North Pacific and a zonal positive-negative-positive pattern over the tropical Pacific. The variations in the simulated paleo-APO and East Asian southerly wind at the precessional band agree well with the geological proxies at the Dongge, Sanbao, Linzhu, and Hulu caves in China, which also implies that these proxies may well reflect the variability in the southerly wind over East Asia. Sensitivity experiments further reveal that the reduced precession parameter may enhance the positive paleo-APO phase and the associated EASM because of the response of the land-atmosphere interactions to the precessional insolation changes. The effect of the ocean-atmosphere interactions on the paleo-APO is secondary.