Current Research:
Atmosphere response to Amazon drought in 2010 as revealed by satellite data
Figure below shows the MLS CO tape recorder from 2004 to 2011 from 200 hPa to 50 hPa. Besides the CO maximum in 2006 fall caused by Indonesia fires due to El Nino event, there is another equally strong CO maximum in October 2010. The GFED inventory shows that CO emission from fires over South America reaches its maximum in 2010, which is almost equal to that from the intensest fires over Indonesia in 2006. The spatial map of MLS CO mixing ratio at 146 hPa in October 2010 also suggests that South America is the dominant pollutant source for the tropical UT in 2010, even still during its dry season with relatively weak vertical mixing.
Previous studies suggested that South America rainfall and atmospheric circulation were closely related to the surface temperature in Atlantic origin (e.g. Zeng et al 2008). The warmer tropical North Atlantic Ocean causes above-normal rainfall, a typical atmospheric convection response to warm sea surface temperature (SST). The rising motion in the north generates subsidence in the south over the Amazon  and the South Atlantic Ocean, a sea-saw like modification. The left figure shows the ENSO 3.4 index superimposed by the anomaly of the tropical North Atlantic SST from 2000 to 2010. The strongest El Nino event during the past 10 years happens in 2009, causing the initial dry situation over South America during its wet season. And in 2010, the tropical North Atlantic SST is warmest and almost twice of that in 2005. The combined impact of these two climatic events produces the largest drought in 2010 over its dry season during the past 10 years, even larger than that in year 2005. Our preliminary results suggest that during the severe climate situation, pollutants from fire could be transported to the UT regions even over regions with weak convection, such as South America in its dry season.
We detected the severe 2010 drought over South America from MLS UTLS CO. We will examine the spatial distribution of other long-lifetime tracers based on MLS observations such as HCN, ozone in the UTLS to see how upper atmosphere responses to 2010 South America drought. We will then conduct simulations driven by GEOS-5 and using the GFED3 biomass burning emissions from 2005 to 2011, and focus on the following objectives: (1) examine the influence of climate extreme events on atmospheric composition (CO, HCN, Ozone) in the tropical UTLS in 2010. (2) access if the interannual variability (IAV) in GFED3 is consistent with that in CO (MLS, MOPPIT) and aerosol optical depth (MODIS) over the biomass burning regions; (3) analyze the causes of IAV in tropical ozone seen in TES, MLS and in OMI/MLS products for the tropical tropospheric ozone column, using GEOS-Chem simulations and an analysis of IAV in the transport.
CO Tape recorder: source attribution and transport analysis at UTLS
It has been well known that the global stratospheric overturning circulation is upward in the tropics (Brewer 1949,Dobson 1956,Holton, et al. 1995). This rising motion is steady and slow. Consequently, seasonal variations in the mixing ratio of many long-lived gases entering the stratosphere are well conserved during its upward transport. This effect was first observed by Mote et al. (1996) and termed as an ‘atmospheric tape recorder’ based on the analysis on water vapor field. The signature in water vapor in the stratosphere is primarily determined by its seasonally varying entry values at the tropopause (tape head), which is controlled by the tropical tropopause temperature. A similar tape recorder in CO was first identified by Schoeberl et al. (2006) using satellite CO measurements from the Aura Microwave Limb Sounder (MLS). In this study, we use a global chemistry and transport model to interpret the processes that form the observed spatial-temporal variability of the CO tape recorder pattern in the upper troposphere and lower stratosphere (UTLS), taking advantage of the multi-year record now available. We also evaluate the vertical transport in the UTLS in chemical transport models (CTMs) driven by GEOS-4 and GEOS-5 assimilated meteorological fields.
Our model simulations capture many features of the seasonal and inter-annual variation of CO in the UT/LS. Both model simulations and observations of CO show a transition from semi-annual variations in the UT (below 100 hPa) to annual variations in the LS. Our results indicate that the observed semi-annual variation of CO is mainly determined by the temporal overlapping of surface biomass burning from different continents. The transition from semi-annual to annual cycle around 80 hPa is induced by a combination of the CO signal at the lower limit of the tropical tropopause layer and the annual cycle of the vertical mixing in the UT/LS. The annual cycles between 80 hPa and 50 hPa are determined by the strong annual signal in the Brewer-Dobson circulation. We deduced the vertical velocity by tracking the propagation pathway of MLS CO extrema and compared it to that in the archived GEOS-4 and GEOS-5 meteorological field (Figure 2). In GEOS-4, the comparison indicates close agreement during the boreal spring season, but relatively weaker vertical transport in summer-fall, which likely caused the decreased amplitude of the seasonal cycle in the model. For GEOS-5, the transport is too slow in all seasons. Much less CO has been lofted in GEOS-5 above 100 hPa than in GEOS-4.
For more information, please see submitted paper by Liu et al. 2012: Transport analysis and source attribution of the tropical CO seasonal and interannual variability in the UT/LS
Tropical troposphere CO analysis: insights into transport characteristics of the GEOS meteorological products
In this project, we combine the GEOS-Chem model with MLS and TES observations, as well as air craft data (MOZAIC) to understand the processes controlling the spatial and temporal variations of CO in the troposphere over the tropical biomass burning regions. Comparisons of the satellite observations and GEOS-Chem simulations, as well as analysis of the model meteorology and of tagged CO simulations, provide a detailed understanding of the interplay of convection and large scale ascent, as well as long-range transport, on CO emissions and thus on the model CO distribution. Our analysis also reveals flaws in aspects of tropical transport in the GEOS-4 and GEOS-5 meteorological fields, and in the isoprene emissions in the model, as well as successes.
The comparison between model simulation and observations from Aura Satellite shows that the seasonal maximum in model CO with GEOS-4 over South America occurs ~1 month later than the MLS maximum in 2005, and with GEOS-5 it occurs ~1-2 months late in 2005. Our analysis suggests that these deficiencies are caused by two major factors: deep convection decays at too low an altitude in the UT, and the source of CO from isoprene in the model is too large in the wet season. The greater lag in GEOS-5 is in part because convection decays at a lower altitude, and in part because convection moves southward later than in GEOS-4. The MLS data suggests that too much CO has been transported to the eastern equatorial Pacific and lofted in the ITCZ in August and September, particularly in GEOS-4. Similar diagnostic studies on surface burning source contributions and vertical mixing have been done over southern Africa, Indonesia and northern Africa during their respective biomass burning seasons. Over southern Africa, both GEOS-4 and GEOS-5 simulations show a large underestimate over the seasonal maximum due to low emissions in the model. A sensitivity run with increased emissions leads to improved agreement with observed CO in the LT and middle troposphere (MT), but the amplitude of the seasonal variation is too high in the UT in GEOS-4. Difficulty in matching CO in the LT and UT implies there may be overly vigorous vertical mixing in GEOS-4 early in the wet season. The MLS data suggest that too much CO has been transported from fires in northern Africa to the UT in the model during the burning season, as does MOZAIC aircraft data.
 The plot to the left shows the transport patterns over northern Africa during its burning season. The Harmattan winds near surface sweep up CO from the burning region and transport it over the Gulf of Guinea to the ITCZ where it is lofted by convection. By 200 hPa, the winds over the equatorial region change direction to south-westerly and transport CO rich air back across West Africa above the burning region and beyond, towards the Middle East. This unique transport pattern over the northern Africa implies that CO accumulation in the UT above the fire region is mainly determined by the strength of Harmattan wind in the LT and the vertical mixing over the Gulf of Guinea. Thus, the CO overestimate in the models may result from the combined influence of too strong Harmattan winds in the LT and too strong vertical mixing over the Gulf of Guinea in the model. These deficiencies in transport are also responsible for the low CO in the boundary layer and the excess CO above, compared to the MOZAIC data.
Our results have implications for the inverse methodology that has been widely used for constraining regional sources of CO (e.g. Arellano et al., 2004; Petron et al., 2004; Muller and Stavrakou, 2005; Arellano and Hess, 2006; Chevallieret al., 2009; Jones et al., 2009), given that this approach does not account for biases in model transport. Our results suggest that caution is needed when using inverse methodology to estimate the uncertainties of different sources, especially in regions of where errors may be dominated by factors other than emissions, such as biases in transport.
For more information, see Liu et al., 2010.
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Ph.D work:
My Ph.D research used multiple paleo-proxies to address the questions concerning climate and ocean circulation and test results against the physical constraints of the ocean and atmospheric systems. I focused on reconstructing sea surface temperature (SST) variations in the west Pacific warm pool (WPWP) over the past 300 years, using coral geochemical records from New Georgia. The study explored the decadal scale climatic variation in the Pacific and its response to global warming and its attribution on the multidecadal drought over the western United Sates (U. S.).
Decadal/multi-decadal climatic variations from combined paleoclimatic records
With the near certainty of the imprint of greenhouse warming on the global temperature record, climate scientists have shifted their focus to more regional-scale tests for greenhouse gas signals in other records. In this project, we tested for the response of Pacific Decadal Pscillation (PDO) to the recent warming over its long-term variation. Despite the non-SST trend in overall coral Sr/Ca at Gizo, the correlation tests demonstrate that that the decadal/multi-decadal signals in coral records are consistent with the phase shifts of PDO and can be used to independently estimate the PDO back to 1733. The high correlation (r=0.74) with a tree ring proxy over a 147 years validation interval (1733-1880) justified the development of a joint proxy, which yielded a 0.91 correlation with the PDO from 1880 to 1992. There are several notable features in the joint index. Most importantly, the joint proxy indicated that the present state of the PDO is nearly “neutral”, with no significant trend over the past 270 years. This stability implies that different feedbacks identified for this mode are not only sufficiently strong to neutralize (so far) the effects of global warming, but that any mechanism proposed to explain the observed stability must be able to account for the changing energy input into the tropical Pacific system from global warming. Further comparison between the proxy reconstruction and an estimate of U. S. droughts index from drought-sensitive tree-ring chronologies reveals that the PDO is one of the controlling factors of the multidecadal drought over the western U. S..
West Pacific Warm Pool Climatic reconstructions from coral records and assessment of SST-Sr/Ca decoupling in Western Pacific Warm Pool corals
The WPWP acts as the heat engine for the Earth's climate and serves as a major moisture source for the global hydrological cycle. It is closely correlated with many climate phenomena such as El Niño and La Niña, which have had massive global and societal impact. However, there is a long-standing uncertainty about the stability of SST changes in this key region. Coral-based proxy records of thermal and hydrologic variations in the WPWP offer a great opportunity to extend the instrumental records and address modes of tropical climate variability during extreme period. In this project, we reconstruct SSTs from a monthly resolved Sr/Ca record of a Porites lutea coral collected offshore Gizo Island, New Georgia (156°46.3”E, 8°S), in the warmest part of the Western Pacific Warm Pool (WPWP). Only a moderate correlation between Sr/Ca and SST is observed (r= -0.4), which is caused partly by a decade-long drift at the end of the Sr/Ca record and partly by stronger decadal variations in Sr/Ca values. The secular trend is absent in both the coral δ18O time series from the same core and Sr/Ca time series from two nearby coral cores, suggesting that this trend has a non-climatic source (e.g. thermal stress, secondary aragonite cement contamination). The correlation between ENSO-filtered coral Sr/Ca and SST time series varies with phase of the Pacific Decadal Oscillation (PDO), which is relatively high during “warm” PDO regimes and low during “cool” PDO regimes. An approximately inverse decadal variability of SST-Sr/Ca correlation is evident in a coral from Fiji located at the edge of South Pacific Convergence Zone (SPCZ). Coral Sr/Ca results from Gizo suggests that non-temperature effects may be influencing coral Sr/Ca at the upper thermal limits of coral growth; a finding also reported in studies of other WPWP corals. Caution should be exercised concerning interpretation of SST estimates based on coral Sr/Ca records from corals living in warm pool regions of the oceans. For more information, please see submitted paper by Liu et al. .
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