The Origins of ITCZs Monsoons and Monsoon Onset

The Origins of ITCZs, Monsoons and Monsoon Onset Winston Cheng-Wen Chao 趙振文 NASA/Goddard Space Flight Center Greenbelt, MD, USA

The traditional view: A monsoon is a giant sea breeze caused by land-sea thermal contrast (no explanation for monsoon onset is included. ) A new view: A monsoon is an ITCZ that has moved into the subtropics in its annual latitudinal movement. To explain the monsoons is to explain why the ITCZ forms and why its movement into the subtropics is so abrupt (i. e. , to explain monsoon onset. )

The monsoons discussed here do not include the so-called East Asian Monsoon, which is the head of the middle latitude storm track in the western Pacific.

Outline Importance of the ITCZ research Mechanisms responsible for the ITCZ location and intensity (why 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 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

Obs CCM 3

Obs CCM 3

Zhang and Wang GRL, 2006

Bacmeister et al. , JAS, 2006

The importance of studying the ITCZ 1. 2. 3. 4. 5. 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 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. 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.

The importance of studying the ITCZ (conti. ) 6. 7. 8. 9. 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.

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.

The overthrow of the Charney 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. • Charney’s 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.

Reasons for the failure of the Charney ITCZ theory • The CISK theory, on which the Charney 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.

An ITCZ 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.

The first effect of f on convection (inertial stability) • / t = f + … / t = - f + … • 2 / t 2 = - f 2 + … 2 x/ t 2 = - k x (Simple spring) • 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.

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

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 pushes the ITCZ away from the equator.

The latitude of the ITCZ in an aqua-planet with uniform SST and solar angle is the latitude where ω2 = 0, where ω2 = f 2 + 2 N 2 + |F|, and is latitude; ω frequency of the inertial gravity wave; α the ratio of vertical scale to horizontal scale of the gravity wave; N 2 vertical stability; |F| a frictional term i. e. , the latitude where f 2/ (= 8 2 sin cos , the first effect of the earth’s rotation) is balanced by ( 2 N 2)/ (the second effect of the earth’s rotation, which is the latitudinal gradient of the f-modified surface heat exchange. ) Note that N 2 is a function of dynamical, radiative, convective, and boundary layer processes.

Chao and Chen (Clim. Dyn. 2004) The effects of rotation on the ITCZ A: Southward push on the ITCZ due to inertial stability B: Northward push on the ITCZ due to surfacewind evaporation and beta effect 23

Chao and Chen (Clim. Dyn. 2004)

SST used in the aquaplanet model

R is the net southward forcing on the ITCZ due to earth’s rotation S is the northward forcing on the ITCZ due to the SST latitudinal profile

I believe that very few educated people would have any difficulty in giving an answer to the question-What is the cause of the monsoon? They would refer to the high temperature over the land compared with that over the surrounding sea, and would speak of ascending currents of air causing an in-draft of sea-air towards and into the interior of the country. It is only when one points out that India is much hotter in May, before the monsoon sets in, than in July, when it is at its height, or draws attention to the fact that the hottest part of India-the north-west-gets no rain during the monsoon, or even shows by statistics that the average temperature is much greater in the years of bad rains than in years of good rains, that they begin to doubt whether they do know the real cause of the monsoon. --G. C. Simpson, Director of the Meteorological Office, 1921, QJRMS

Chao and Chen JAS 2001

Chao and Chen JAS 2001

Modified Gill solution Chao and Chen JAS 2001

Summary 1. 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. 2. The ITCZ has different flow regimes; failure to get the GCM-simulated ITCZ in the right regime results in large systematic errors. 3. A monsoon is an ITCZ after its jumping away from the equator. This jump is monsoon onset. Monsoon exist in aqua-planet models; thus land-sea contrast is not a necessary condition for the monsoons--contrary to the three-century-old conventional explanation.

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