ESSL LAR

Dr. Aixue Hu

Hu, A., G. A. Meehl and W. Han, 2007, Causes of a fresher, colder northern North Atlantic in late 20th century in a coupled model, Progress in Oceanography, 73, 384-405, doi:10.1016/j.pocean.2006.07.008.

Figure 1.

High resolution figure

Abstract

Observational evidence indicates that in the northern North Atlantic, especially in the Labrador Sea, almost the whole column of the ocean water is fresher, and colder in late 20th century than in 1950-1960s. Here we analyze a four-member ensemble of the 20th century simulations from a coupled climate model to examine the possible causes for these observed changes. The model simulations resemble the observed changes in the northern North Atlantic. The simulated results show that a decreased meridional freshwater divergence and an increased meridional heat divergence associated with a weaker thermohaline circulation in the North Atlantic are the primary causes for the freshening and cooling in the northern North Atlantic. The increased precipitation less evaporation tends to enforce the freshening, but the reduced sea ice flux into this region tends to weaken it. On the other hand, the surface warming induced by a higher atmospheric CO2 concentration tends to heat up the northern North Atlantic, but is overcome by the cooling from increased meridional heat divergence.

Figure caption: Scatter plot: (a) the THC index with the meridional freshwater divergence between 45o and 65oN; (b) the THC index with the meridional heat convergence between the same latitude band as in panel (a).

Hu, A., G. A. Meehl and W. Han, 2007, Role of the Bering Strait in the thermohaline circulation and abrupt climate change, Geophys. Res. Lett., 34, L05704, doi:10.1029/2006GL028906.

Figure 2.

High resolution figure

Abstract

Here we investigate the role of the Bering Strait (BS) in the thermohaline circulation (THC) response to added freshwater forcing (hosing) in the subpolar North Atlantic, through analyzing simulations of a fully coupled climate model with an open and closed BS. Results show that the THC declines similarly with an open and closed BS during hosing. However, the recovery of the THC is delayed by about a century in the closed BS simulation than in the open BS one after the hosing is off. The closed BS prevents the added freshwater being transported from the Atlantic into the Pacific via the Arctic as in the open BS case. Further, the freshwater supply is elevated significantly after the hosing by exporting the freshwater stored in the Arctic during hosing, as sea ice, back to the North Atlantic. This stabilizes the surface stratification there and suppresses the recovery of the deep convection.

Figure caption: Schematic diagram. The arrows indicate the direction of the freshwater transport, and the numbers next to the arrows are the cumulative anomalous freshwater transport or input in the 450-year hosing period normalized by the total freshwater added into the sub-polar North Atlantic (Qfwaddtot) with a unit of % Qfwaddtot years. The P - E + R denote the surface freshwater input into the ocean by precipitation, evaporation, and river runoff, and the dark gray shaded region denotes where the additional freshwater is added. The green arrows denote the oceanic freshwater transport, the red arrows the sea ice transport, and the blue arrows the surface freshwater input by P - E + R (upward arrow indicates increased freshwater input). In the CBS run, the closed Bering Strait prevents the transports of the freshwater added in the subpolar North Atlantic into the North Pacific via the Bering Strait as in the OBS run, but exports this water back to the North Atlantic as sea ice after the hosing. The resulting elevated freshwater input in the CBS run after the hosing delays the recovery of the deep convection in the North Atlantic, the sea surface salinity contrast between the North Atlantic and the North Pacific, and the meridional steric height gradient, thus a delayed recovery of the THC.

Han, W., G. A. Meehl and A. Hu, 2006, Interpretation of tropical thermocline cooling in the Indian and Pacific Oceans during recent decades, Geophys. Res. Lett., 33, L23615, doi:10.1029/2006GL027982.

Figure 3.

High resolution figure

Abstract

A warming trend has been detected in the world's oceans in recent decades. The basin-averaged warming, however, shows a complex vertical structure in the Indian and Pacific oceans. Warming in the upper ocean-with maxima near the surface- accompanies a strong cooling in the upper thermocline. Analysis of observed data and model solutions from this study reveals that the complex structure is confined mainly to the tropics. While increased greenhouse gases act to warm up the upper ocean by increasing downward surface heat fluxes, anomalous winds in the tropics cause upward Ekman pumping velocity and shoal the thermocline, resulting in an upper-thermocline cooling. This cooling process is well demonstrated by a simple model based on the ventilated thermocline theory. The study has important implications for climate change and fisheries.

Figure caption: Meridional sections of temperature anomalies (in oC) for the upper 1000 m of the Indian and Pacific oceans, based on zonally averaged temperature difference between 1990s and 1960s for each basin. (a and b) Levitus pentad (5-year mean) data; (c and d) SODA-POP monthly data. The 14oC, 20oC and 26oC isotherms for 1960s are also plotted in each panel.

Funding agencies: All my works are supported by the National Science Foundation and the U.S. Department of Energy.

Citation info (as of Sept. 5, 2007):

    1. Meehl GA, Washington WM, Collins WD, et al. How much more global warming and sea level rise? SCIENCE 307 (5716): 1769-1772 MAR 18 2005. Cited: 47
    2. Gregory JM, Dixon KW, Stouffer RJ, et al. A model intercomparison of changes in the Atlantic thermohaline circulation in response to increasing atmospheric CO2 concentration GEOPHYSICAL RESEARCH LETTERS 32 (12): Art. No. L12703 JUN 23 2005. Cited: 39
    3. Stouffer RJ, Yin J, Gregory JM, et al. Investigating the causes of the response of the thermohaline circulation to past and future climate changes JOURNAL OF CLIMATE 19 (8): 1365-1387 APR 15 2006. Cited: 24
    4. Meehl GA, Washington WM, Arblaster JM, Hu, A. Factors affecting climate sensitivity in global coupled models JOURNAL OF CLIMATE 17 (7): 1584-1596 APR 2004. Cited: 20
    5. Otto-Bliesner BL, Marsha SJ, Overpeck JT, et al. Simulating arctic climate warmth and icefield retreat in the last interglaciation SCIENCE 311 (5768): 1751-1753 MAR 24 2006. Cited: 11
    6. Hu AX, Meehl GA, Washington WM, et al. Response of the Atlantic thermohaline circulation to increased atmospheric CO2 in a coupled model JOURNAL OF CLIMATE 17 (21): 4267-4279 NOV 2004. Cited: 9
    7. Dai AG, Hu A, Meehl GA, et al. Atlantic thermohaline circulation in a coupled general circulation model: Unforced variations versus forced changes JOURNAL OF CLIMATE 18 (16): 3270-3293 AUG 15 2005. Cited: 8
    8. Meehl GA, Washington WM, Santer BD, et al. Climate change projections for the twenty-first century and climate change commitment in the CCSM3 JOURNAL OF CLIMATE 19 (11): 2597-2616 JUN 1 2006. Cited: 7
    9. Hu AX, Meehl GA, Han WQ Detecting thermohaline circulation changes from ocean properties in a coupled Model GEOPHYSICAL RESEARCH LETTERS 31 (13): Art. No. L13204 JUL 3 2004. Cited: 6
    10. Han WQ, Webster P, Lukas R, et al. Impact of atmospheric intraseasonal variability in the Indian Ocean: Low-frequency rectification in equatorial surface current and transport JOURNAL OF PHYSICAL OCEANOGRAPHY 34 (6): 1350-1372 JUN 2004. Cited: 6
    11. Meehl GA, Hu AX Megadroughts in the Indian monsoon region and southwest North America and a mechanism for associated multidecadal Pacific sea surface temperature anomalies JOURNAL OF CLIMATE 19 (9): 1605-1623 MAY 1 2006. Cited: 5
    12. Hu AX, Rooth C, Bleck R, et al. NAO influence on sea ice extent in the Eurasian coastal region GEOPHYSICAL RESEARCH LETTERS 29 (22): Art. No. 2053 NOV 15 2002. Cited: 4
    13. Hu AX, Meehl GA, Han WQ Role of the Bering Strait in the thermohaline circulation and abrupt climate change GEOPHYSICAL RESEARCH LETTERS 34 (5): Art. No. L05704 MAR 3 2007. Cited: 2
    14. Hu AX, Meehl GA Reasons for a fresher northern North Atlantic in the late 20th century GEOPHYSICAL RESEARCH LETTERS 32 (11): Art. No. L11701 JUN 7 2005. Cited: 2
    15. Han WQ, Meehl GA, Hu AX Interpretation of tropical thermocline cooling in the Indian and Pacific oceans during recent decades GEOPHYSICAL RESEARCH LETTERS 33 (23): Art. No. L23615 DEC 14 2006. Cited: 1
    16. Hu AX, Meehl GA Bering Strait throughflow and the thermohaline circulation GEOPHYSICAL RESEARCH LETTERS 32 (24): Art. No. L24610 DEC 21 2005. Cited: 1
    17. Hu A, Meehl GA, Han WQ Causes of a fresher, colder northern North Atlantic in late 20th century in a coupled Model PROGRESS IN OCEANOGRAPHY 73 (3-4): 384-405 2007. Cited: 0