CGD 2008 Profiles in Science: Dr. Aixue Hu

Summary of achievements

Aixue Hu has been continuing working on the climate change research using CCSM3 in 2008. His recent publication (Hu et al., 2008) highlights the importance of the Bering Strait on global climate. Although this strait is narrow and shallow, the status of the this strait is important to the stability of the global climate, as such a closed Bering Strait can make the important ocean circulation - the Atlantic meridional overturning circulation (MOC, or the thermohaline circulation, THC) be more sensitive to the freshwater forcing added into the subpolar North Atlantic due the fluctuation of the ice sheets in the North America and the Greenland. An open Bering Strait makes the MOC be less sensitive to the freshwater forcing. He is also leading the work of the effect of hurricanes on the MOC and meridional heat transport in the Atlantic basin, the effect of the melting Greenland Ice Sheet on the MOC and global climate, and the importance of the Bering Strait on global sea level changes. He involved the studies of the dynamics of the intraseasonal sea level and thermocline variability in the equatorial Atlantic, the influence of the weakened Atlantic MOC on ENSO, the mid-1970s Pacific climate shift in the Pacific and the relative roles of forced versus inherent decadal variability. He is also working on the decadal predictability research.

Publications

Hu, A., B.L. Otto-Bliesner, G.A. Meehl, W. Han, C. Morrill, E.C. Brady, and B. Briegleb, 2008: Response of Thermohaline Circulation to Freshwater Forcing under Present-Day and LGM Conditions. J. Climate, 21, 2239-2258.

Abstract: Responses of the thermohaline circulation (THC) to freshwater forcing (hosing) in the subpolar North Atlantic Ocean under present-day and the last glacial maximum (LGM) conditions are investigated using the National Center for Atmospheric Research Community Climate System Model versions 2 and 3. Three sets of simulations are analyzed, with each set including a control run and a freshwater hosing run. The first two sets are under present-day conditions with an open and closed Bering Strait. The third one is under LGM conditions, which has a closed Bering Strait. Results show that the THC nearly collapses in all three hosing runs when the freshwater forcing is turned on. The full recovery of the THC, however, is at least a century earlier in the open Bering Strait run than the closed Bering Strait and LGM runs. This is because the excessive freshwater is diverged almost equally toward north and south from the subpolar North Atlantic when the Bering Strait is open. A significant portion of the freshwater flowing northward into the Arctic exits into the North Pacific via a reversed Bering Strait Throughflow, which accelerates the THC recovery. When the Bering Strait is closed, this Arctic to Pacific transport is absent and freshwater can only be removed through the southern end of the North Atlantic. Together with the surface freshwater excess due to precipitation, evaporation, river runoff, and melting ice in the closed Bering Strait experiments after the hosing, the removal of the excessive freshwater takes longer, and this slows the recovery of the THC. Although the background conditions are quite different between the present-day closed Bering Strait run and the LGM run, the THC responds to the freshwater forcing added in the North Atlantic in a very similar manner.

Figure caption: The anomalous meridional freshwater transport at 80°N, 35°N; the surface freshwater input anomaly from precipitation, evaporation, and river runoff (P _ E R); the melt-ice flux anomaly; and the net freshwater divergence in the region of the North Atlantic between 35° and 80°N in the three hosing runs. Values shown in these figures are the percentage of the total freshwater anomaly added into the subpolar North Atlantic during the 100-yr hosing period. The bars show the anomalous freshwater transport (or input) in the given period in the figures. Bars from darker to lighter color represent the OBS, LGM, and CBS cases, respectively. Negative (positive) values are freshwater being transported out (added into) the Atlantic between 35° and 80°N.

Support: A portion of this study was supported by the Office of Science (BER), U.S. Department of Energy, Cooperative Agreement DE-FC02-97ER62402. Weiqing Han was supported by NSF OCE-0452917 and NASA Ocean Vector Winds Program Award Number 1283568.

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Han, W., P. J. Webster, J. Lin, W. T. Liu, R. Fu, D. Yuan, and A. Hu, 2008: Dynamics of intraseasonal sea level and thermocline variability in the equatorial Atlantic during 2002-2003. J. of Physical Oceanography, 38, 945-967.

Abstract: Satellite and in situ observations in the equatorial Atlantic Ocean during 2002-03 show dominant spectral peaks at 40-60 days and secondary peaks at 10-40 days in sea level and thermocline within the intraseasonal period band (10-80 days). A detailed investigation of the dynamics of the intraseasonal variations is carried out using an ocean general circulation model, namely, the Hybrid Coordinate Ocean Model (HYCOM). Two parallel experiments are performed in the tropical Atlantic Ocean basin for the period 2000-03: one is forced by daily scatterometer winds from the Quick Scatterometer (QuikSCAT) satellite together with other forcing fields, and the other is forced by the low-passed 80-day version of the above fields. To help in understanding the role played by the wind-driven equatorial waves, a linear continuously stratified ocean model is also used.

Within 3°S-3°N of the equatorial region, the strong 40-60-day sea surface height anomaly (SSHA) and thermocline variability result mainly from the first and second baroclinic modes equatorial Kelvin waves that are forced by intraseasonal zonal winds, with the second baroclinic mode playing a more important role. Sharp 40-50-day peaks of zonal and meridional winds appear in both the QuikSCAT and Pilot Research Moored Array in the Tropical Atlantic (PIRATA) data for the period 2002-03, and they are especially strong in 2002. Zonal wind anomaly in the central-western equatorial basin for the period 2000-06 is significantly correlated with SSHA across the equatorial basin, with simultaneous/lag correlation ranging from -0.62 to 0.74 above 95% significance. Away from the equator (3°-5°N), however, sea level and thermocline variations in the 40-60-day band are caused largely by tropical instability waves (TIWs).

On 10-40-day time scales and west of 10°W, the spectral power of sea level and thermocline appears to be dominated by TIWs within 5°S-5°N of the equatorial region. The wind-driven circulation, however, also provides a significant contribution. Interestingly, east of 10°W, SSHA and thermocline variations at 10-40-day periods result almost entirely from wind-driven equatorial waves. During the boreal spring of 2002 when TIWs are weak, Kelvin waves dominate the SSHA across the equatorial basin (2°S-2°N). The observed quasi-biweekly Yanai waves are excited mainly by the quasi-biweekly meridional winds, and they contribute significantly to the SSHA and thermocline variations in 1°-5°N and 1°-5°S regions.

Figure caption: The 10-40-day bandpassed HYCOM MR SSHA in the equatorial Atlantic basin during spring, day 71 of 2002; (b), (c) same as in (a), but for days 78 and 85; (d)-(f) same as in (a)-(c), but for SSHA from HYCOM EXP; (g)-(i) same as in (a)-(c), but for SSHA from LM MR solution.

Support: Weiqing Han was supported by NSF OCE-0452917 and NASA Ocean Vector Winds Program award 1283568, Peter J. Webster by NSF ATM-0531771, Jia-Lin Lin by NOAAOGP/CVP and NASA MAP Programs, W. Timothy Liu by NASA Ocean Vector Winds and Physical Oceanography Programs, R. Fu by NASA Ocean Vector Winds Program and NOAA Climate Prediction Program for the Americas, D. Yuan by the National Basic Research of China ("973 program") project 2006CB403603, the "100-Expert Program" of the Chinese Academy of Sciences, and the NSF project 40676020, and Aixue Hu partly by the Office of Science (BER), U.S. Department of Energy, Cooperative Agreement No. DE-FC02-97ER62402.

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Timmermann, A., Y. Okumura, S.-I. An, A. Clement, B. Dong, E. Guilyardi, A. Hu, J. Jungclaus, U. Krebs, M. Renold, T. F. Stocker, R. J. Stouffer, R. Sutton, S.-P. Xie, J. Yin, 2007: The influence of a weakening of the Atlantic meridional overturning circulation on ENSO. J. Climate, 20, 4899-4919.

Abstract: The influences of a substantial weakening of the Atlantic meridional overturning circulation (AMOC) on the tropical Pacific climate mean state, the annual cycle, and ENSO variability are studied using five different coupled general circulation models (CGCMs). In the CGCMs, a substantial weakening of the AMOC is induced by adding freshwater flux forcing in the northern North Atlantic. In response, the well-known surface temperature dipole in the low-latitude Atlantic is established, which reorganizes the large-scale tropical atmospheric circulation by increasing the northeasterly trade winds. This leads to a southward shift of the intertropical convergence zone (ITCZ) in the tropical Atlantic and also the eastern tropical Pacific. Because of evaporative fluxes, mixing, and changes in Ekman divergence, a meridional temperature anomaly is generated in the northeastern tropical Pacific, which leads to the development of a meridionally symmetric thermal background state. In four out of five CGCMs this leads to a substantial weakening of the annual cycle in the eastern equatorial Pacific and a subsequent intensification of ENSO variability due to nonlinear interactions. In one of the CGCM simulations, an ENSO intensification occurs as a result of a zonal mean thermocline shoaling.

Analysis suggests that the atmospheric circulation changes forced by tropical Atlantic SSTs can easily influence the large-scale atmospheric circulation and hence tropical eastern Pacific climate. Furthermore, it is concluded that the existence of the present-day tropical Pacific cold tongue complex and the annual cycle in the eastern equatorial Pacific are partly controlled by the strength of the AMOC. The results may have important implications for the interpretation of global multidecadal variability and paleo-proxy data.

Figure caption: SST anomaly (K) and wind stress anomaly (N m^-2 generated by the shutdown of the AMOC for the (top left) GFDL CM2.1, (top middle) HadCM3, (top right) MPIO-M1, (bottom left) CCSM2, and (bottom right) CCSM3. The red and blue lines represent the annual mean Γ_y = 0 lines in the control and waterhosing experiments, respectively. Note the asymmetric temperature scale.

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Meehl, G. A., A. Hu, and B. D. Santer, 2008: The mid-1970s climate shift in the Pacific and the relative roles of forced versus inherent decadal variability. J Clim, in press.

Abstract: A significant shift from cooler to warmer tropical Pacific sea surface temperatures (SSTs), part of a pattern of basin-wide SST anomalies involved with a transition to the positive phase of the Interdecadal Pacific Oscillation (IPO), occurred in the mid-1970s with effects that extended globally. One view is that this change was entirely natural and was a product of internally-generated decadal variability of the Pacific climate system. However, during the mid-1970s there was also a significant increase of global temperature and changes to a number of other quantities that have been associated with changes in external forcings, particularly increases of greenhouse gases from the burning of fossil fuels. We analyze observations, an unforced control run from a global coupled climate model, as well as 20th century simulations with changes in external forcings to show that the observed 1970s climate shift had a contribution from changes in external forcing superimposed on what was likely an inherent decadal fluctuation of the Pacific climate system. Thus this inherent decadal variability associated with the IPO delayed to the 1970s what likely would have been a forced climate shift in the 1960s from a negative to positive phase of the IPO.

Figure caption: a) the first EOF from a single ensemble member from the all-forcings experiment, and b) the PC time series from the single ensemble member (solid), and pattern correlations from projecting the first EOF from the control run (Fig. 3) on to the low pass filtered SST data from the single ensemble member (dotted), and similarly from projecting the first (dash-dot) and second (dashed) EOFs from the ensemble mean all-forcings experiment (Fig. 4a and c, respectively) on to the low pass filtered SST data from the single ensemble member. The left axis labels refer to the amplitude of the PC time series (dark solid line), and the right axis labels are for the amplitude of the pattern correlations (dash-dot, dashed, and dotted lines). Thin vertical lines denote 1965, 1975 and 1980 as discussed in text.

Support: Portions of this study were supported by the Office of Science (BER), U.S. Department of Energy, Cooperative Agreement No. DE-FC02-97ER62402, and the National Science Foundation. The National Center for Atmospheric Science is sponsored by the National Science Foundation.

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Yuko M. Okumura, Clara Deser, and Aixue Hu, Axel Timmermann and Shang-Ping Xie, 2008: North Pacific Climate Response to Freshwater Forcing in the Subarctic North Atlantic: Oceanic and Atmospheric Pathways. J Clim., in press.

Abstract: Sudden changes of the Atlantic meridional overturning circulation (AMOC) are believed to have caused large, abrupt climate changes over many parts of the globe during the last glacial and de-glacial period. This study investigates the mechanisms by which a large freshwater input to the subarctic North Atlantic and an attendant rapid weakening of the AMOC influence North Pacific climate by analyzing four different ocean-atmosphere coupled general circulation models (GCMs) under present-day or pre-industrial boundary conditions. When the coupled GCMs are forced with a 1 Sv freshwater flux anomaly in the subarctic North Atlantic, the AMOC nearly shuts down and the North Atlantic cools significantly. The South Atlantic warms slightly, shifting the Atlantic intertropical convergence zone southward. In addition to this Atlantic oceanatmosphere response, all the models exhibit cooling of the North Pacific, especially along the oceanic frontal zone, and deepening of the wintertime Aleutian Low, consistent with paleoclimate reconstructions.

Detailed analysis of one coupled GCM identifies both oceanic and atmospheric pathways from the Atlantic to the North Pacific. The oceanic teleconnection contributes a large part of the North Pacific cooling: the freshwater input to the North Atlantic raises sea level in the Arctic Ocean and reverses the Bering Strait throughflow, transporting colder, fresher water from the Arctic Ocean into the North Pacific. When the Bering Strait is closed, the cooling is greatly reduced while the Aleutian Low response is enhanced. Tropical SST anomalies in both the Atlantic and Pacific are found to be important for the equivalent barotropic response of the Aleutian Low during boreal winter. The atmospheric bridge from the tropical North Atlantic is particularly important, and quite sensitive to the mean state, which is poorly simulated in many coupled GCMs. The enhanced Aleutian Low, in turn, cools the North Pacific by increasing surface heat fluxes and southward Ekman transport. The closure of the Bering Strait during the last glacial period suggests that the atmospheric bridge from the tropics and air-sea interaction in the North Pacific played a crucial role in the AMOC-North Pacific teleconnection.

Figure caption: Atmospheric anomalies simulated by CAM2 forced with SST and sea ice anomalies from the CCSM2 closed Bering Strait experiment. SST and sea ice anomalies are prescribed over (top left) the Atlantic (25°S-75°N) and tropical Pacific (15°S-15°N), (top right) Atlantic (25°S-75°N), (bottom left) tropical Atlantic (25°S-30°N), and (bottom right) extratropical North Atlantic (40°-75°N). Surface temperature (shading; °C), sea-level pressure (purple contours at intervals of 1 hPa; negative contours dashed), and precipitation (green contours > 1 mm day^-1 and orange contours < -1 mm day^-1 at intervals of 1 mm day^-1) anomalies are averaged over October-March.

Support: Okumura is supported by the NOAA Climate and Global Change Postdoctoral Fellowship. A. Timmermann and S.-P. Xie acknowledge support from the Japan Agency for Marine-Earth Science and technology (JAMSTEC), NASA through grant No. NNX07AG53G, and NOAA through grant No. NA17RJ1230. A. Timmermann is also supported by NSF grant No. ATM06-28393. This study was in part supported by the Office of Science (BER), U.S. Department of Energy, Cooperative Agreement No. DE-FC02-97ER62402. The CAM2 simulations were conducted using computational resources from the NCAR Director's Reserve.