A Conceptual Explanation for the Double ITCZ Bias in GCM Simulations

1. A Conceptual Explanation for the Double ITCZ Bias in GCM Simulations Winston C. Chao In collaboration with GMAO modeling group NASA/Goddard Space Flight Center Greenbelt, Maryland

2. Outline Importance of the ITCZ research Mechanisms responsible for the ITCZ location and intensity (why the theory emphasizing the role of SST gradient as a driver of tropical circulation is incomplete) Roles of model physics, especially the cumulus parameterization, in the above mechanisms Reasons for the failure of the CISK-based ITCZ theory Origin of systematic errors in the GCM simulation of ITCZ precipitation--the double ITCZ bias problem Insight into the identity and the origin of the monsoon and its onset, gained from studying the ITCZ (why a centuries-old monsoon theory fails) Future directions of the ITCZ research

8. Zhang and Wang 2006

10. The importance of studying the ITCZ • The ITCZ is the region where the low-level air converges and rises into the upward branch of the Hadley/Walker circulation. The latent heat released in the ITCZ drives the Hadley/Walker circulation. • Hadley/Walker circulation is the largest circulation in the troposphere. A model that fails to simulate the Hadley/Walker circulation correctly cannot be expected to provide good forecast (the climate drift problem). • The location and intensity of the ITCZ affect the surface wind field, which is a critical factor in air-sea interaction-- a core component of El Nino. • A large part of the difficulties in forecasting El Nino has to do with the GCMs’ failure to simulate the ITCZ correctly. 5. Most GCMs have difficulties in correctly simulating the seasonal variation of the MJO intensity. This is largely a result of the models’ failure in correctly simulating the seasonal movement of the ITCZ.

11. The importance of studying the ITCZ (conti.) 6. The location of the ITCZ is where tropical cyclogenesis occurs and where the tropical cyclones travel before veering to higher latitudes. • The current GCMs have considerable amount of difficulty in simulating the ITCZ precipitation correctly. This has been a bottle-neck problem for climate forecast. • Even when a successful ITCZ simulation is obtained, one wants to see that the success comes from understanding rather than luck, in order to achieve further improvement. • The ITCZ study has led to new insight into the monsoon and monsoon onset.

12. Previous ITCZ theories • Charney (1971) proposed that the latitudinal location of the ITCZ was determined by the balance of two forcings. The first forcing is derived from a zonally symmetric version of the CISK theory. This forcing pulls the ITCZ poleward. The second forcing is the high moisture content of the equatorial tropics, which pulls the ITCZ toward the equator. • Offshoots of the Charney theory include those of Holton et al. (1971), Lindzen (1974), Hess et al. (1993), and Waliser and Somerville (1994), etc.

13. The overthrow of the CISK-based ITCZ theory along with its offshoots • Sumi’s (1992) experiments with globally uniform SST and radiative cooling rates produced single ITCZ over the equator or double ITCZ straddling the equator. • CISK-based theory applied to globally uniform SST settings would expect the ITCZ to occur at the poles. • Now, we need a new theory of the ITCZ.

14. A New Theory • Consider the Sumi experiments of globally uniform SST and radiative cooling. The single ITCZ at the equator or the double ITCZ straddling the equator has to be the result of the earth’s rotation. • Rotation, or f, has two effects on convection: 1) Inertial stability and 2) latitudinal gradient of f-modified surface fluxes.

15. The first effect of f on convection (inertial stability) • /t = f  + … /t = - f  + … • 2/t2 = - f2  + … 2x/t2 = - k x • Inertial stability means that f resists both divergence and convergence (and positive and negative vorticity). This implies resistance to vertical motion associated with convection. Therefore, f and stratification are equivalent. Thus, inertial stability imposes an equator-ward attraction on the ITCZ.

16. The second effect of f on convection (latitudinal gradient of f-modified surface fluxes)

17. The second effect of f on convection (conti.) • The second effect of f on convection is through the added evaporation as the result of larger surface wind speed when f is present. • This effect imposes a pole-ward attraction on the ITCZ.

18. The latitude of the ITCZ in an aqua-planet with uniform SST and solar angle is the latitude where2 = 0, where2 = f2 + 2 N2 + |F|, and  is latitude; i.e., the latitude where f2/ (= 82 sin cos, the first effect of the earth’s rotation) is balanced by -(2N2)/ (the second effect of the earth’s rotation, which is the latitudinal gradient of the f-modified surface heat exchange.)Note that N2 is a function of dynamical, radiative, convective, and boundary layer processes.

24. Reasons for the failure of the CISK-based ITCZ theory • The CISK theory, on which the previous ITCZ theory was based, assumes that convection is caused by Ekman layer convergence. However, the real cause of convection is the inertial-gravitational instability. • Failure to recognize the existence of the second effect of f on convection.

38. Summary The latitudinal location of the ITCZ is determined by the latitudinal peak of the SST and the earth’s rotation; both factors are affected by the model physics, particularly the cumulus parameterization. Failure of the previous ITCZ theories originates from the failure of the CISK theory and from not recognizing the second effect of the earth’s rotation. The ITCZ has different flow regimes; failure to get the GCM-simulated ITCZ in the right regime results in large systematic errors. The monsoon is an ITCZ after its jumping away from the equator. This jump is monsoon onset. The monsoon exists in aqua-planet models; thus land-sea contrast is not a necessary condition for the monsoons--contrary to the three-century-old conventional wisdom.

39. Future research directions 1. SST variation in the zonal direction 2. Effects of land mass 3. Sensitivities to various aspects of the model physics 4. Turning the knowledge gained into model performance