Early studies of the annual cycle in Pacific Ocean described features of various oceanic and atmospheric fields, including SST (Wyrtki 1965), the trade winds (Wyrtki and Meyers, 1976), thermocline (Meyers 1979), and convection and rainfall (Dorman and Bourke 1979, Heddinghaus and Krueger 1981). In last decades or so, investigations have noted roles of ocean-atmosphere interaction in the annual cycle and the role of annual cycle in ENSO evolution (e.g., Horel 1982, Philander and Rasmusson 1985, Meehl 1987). Recent empirical, theoretical, and modeling studies have further discovered that a number of essential physical processes involved in the climatology and annual variation of ITCZ/CT complex differ from those involved in ENSO cycle.

Wallace et al. (1989), and Mitchell and Wallace (1992) first emphasized the importance of a positive feedback between meridional wind component and SST gradient to the annual variation of ITCZ/CT complex. This process is not crucial to ENSO cycle, but is certainly relevant to the annual cycle in the equatorial eastern Pacific and Atlantic oceans. Chang and Philander's (1994) theoretical analysis of coupled ocean-atmosphere instability yields a family of antisymmetric and symmetric coupled ocean-atmosphere modes. They suggest that the anti-symmetric mode may be instrumental in rapidly re-establishing the cold tongues during northern summer, whereas the symmetric mode contributes to the annual westward propagation of the near-equatorial zonal wind and SST.

Wang (1994a) showed that the annual cycle in the tropical eastern-central Pacific is alternatively dominated by a quasi-symmetric (with respect to the equator) component, which primarily results from dynamic coupling of ocean and atmosphere, and an antisymmetric component, which is driven by the differential radiational heat fluxes between the Southern and Northern Hemispheres. The antisymmetric mode is of monsoon nature and plays a critical role in the annual warming near the extratropical South American coast and in the annual weakening of the cold tongue. Liu and Xie's (1994) theoretical analysis further demonstrates that extratropical annual forcing along the coast of South America can effectively influence equatorial cold tongue through equatorward and westward propagation of coupled ocean-atmosphere disturbances.

The climate of the eastern Pacific and Atlantic Oceans exhibits a remarkable equatorial asymmetry (Wallace et al. 1989). Philander et al. (1994) attributed the cause of the climate asymmetry to the global distribution of continents, the coastal geometries of the western coasts of Africa and America, and unstable interactions between the ocean and atmosphere. The importance of the low-level stratus clouds in maintenance of the mature phase of the cold tongue annual cycle and in keeping ITCZ mostly north of the equator have been recognized and widely accepted (Mitchell and Wallace 1992; Philander et al. 1995, Mechoso et al. 1995).

PROJECT GOALS
To understanding physical mechanisms governing the boreal spring reestablishment of the Equatorial Cold Tongue in the eastern Pacific. The proposed project specifically addresses the following questions: What roles does the cloud-radiation forcing play in simulation of the tropical Pacific winds? What roles does the annual thermocline adjustment play in the annual variations of SST in the eastern Pacific? How is the annual warming (cooling) of the equatorial cold tongue initiated? What roles do solar radiational forcing and large-scale ocean-atmosphere interaction play in the annual march of ITCZ/CT complex? The unified theme for this effort is to understand the roles of ocean-atmosphere-land interaction and solar radiational forcing in the annual cycle of the ITCZ/CT complex.

METHODOLOGY
We take a combined technique of empirical and numerical modeling studies. The former is used to develop an observational basis for advancing hypotheses and validating model simulation. The numerical experiments with controlled physics are designed to test the hypotheses and to identify essential processes that determine the annual cycle of the ITCZ/CT system. The numerical models to be used include GFDL OGCM, an intermediate tropical atmospheric model, and an intermediate tropical ocean model. Both the intermediate models have the capability of simulating realistic annual cycle of the atmosphere and ocean in a stand-alone mode and in a coupled mode.

RESULTS AND ACCOMPLISHMENTS
During the past two and half years, our major accomplishments supported by the PACS grant were concentrated in the following five aspects of tropical Pacific climatology: (a) assessed the effects of differential cloud-longwave radiation forcing and boundary layer thermodynamics on the tropical surface winds; (b) advanced a two-dimensional model for explaining the annual variation of the ITCZ-ECT (equatorial cold tongue) complex; (c) documented the annual adjustment of the tropical Pacific thermocline; (d) explained the cause of the rapid reestablishment of the ECT from March to May; and (e) studied the effects of solar forcing on the annual cycle and its influence on ENSO. A list of publications and works, which describe the aforementioned results, may be found in the publication list. To save space, only those results that have not published yet are highlighted here.

a. Annual adjustment of the thermocline in the tropical Pacific Ocean was studied using NCEP/ODAS (Ocean Data Assimilation System) reanalysis data (Wang, Wu, and Lukas 2000). Two regimes of thermocline adjustment dominate in the tropical Pacific Ocean. The Ekman regime is primarily located on the poleward side of the annual mean ITCZ and SPCZ, and in the region (2N-8N, 170W-120W), which is mainly driven by the local wind stress curl associated with the annual march of the ITCZ and the western Pacific monsoon. Another wave regime is found in the equatorial waveguide (3EN-3ES), where the annual maximum depth occurs progressively further eastward from November to January, and in two off-equatorial Rossby waveguides along 5EN and 6ES west of about 140EW. In the off-equatorial waveguides, prominent westward phase propagation dominates with a speed about 0.8 m/s. Numerical experiments with an intermediate ocean model suggest that the pronounced annual cycle in the equatorial central Pacific (deepest in December and shallowest in May-June) is primarily due to remote forcing from the western Pacific monsoon. The annual march of the ITCZ may play an important role in phasing the maximum and minimum depths. The December maximum and June minimum in the equatorial central Pacific then propagate westward in the off-equatorial waveguides all the way to the western boundary. The bimodal variations in the equatorial far eastern Pacific are determined by the remotely forced eastward propagation of Kelvin waves.

b. Causes of the rapid reestablishment of the Equatorial Cold Tongue
An intermediate coupled ocean-atmosphere model was used to study the processes causing the rapid annual reestablishment of the equatorial cold tongue from March to May (Fu and Wang submitted). The quantitative contributions from each of the following processes were estimated: deepening of mixed layer, increased entrainment and evaporation, increased meridional cold advection and decreased solar radiation associated with the passage of the solar zenith. The annual variations of the meridional wind component have a much stronger influence on the SST annual cycle than those of the zonal wind component. This is because the mixed layer deepening from March to May is primarily caused by rapid intensification of the meridional wind component through enhanced entrainment. The westward propagation of the annual warming on the equator is primarily caused by the zonal temperature advection and modified by effects of the net heat flux (primarily downward solar radiation).

c. Roles of shortwave radiation forcing on ENSO through controlling annual cycle
The paper of Wang and Fang (2000) describes a coupled tropical ocean-atmosphere model that is driven by solar radiation, reproducing a realistic annual cycle and an ENSO-like oscillation. With the annual mean shortwave radiation forcing, the period of the model ENSO depends on the mean forcing that determines the coupled mean state. The annual cycle of the solar forcing has fundamental impacts on the behavior of ENSO cycles by establishing a coupled annual cycle that interacts with the ENSO mode. The spectrum of NINO 3 SST anomalies changes from a single to a double peak with a quasi-biennial and a low-frequency (4-5 years) component. Meanwhile, the evolution of ENSO becomes phase-locked to the annual cycle; and the amplitude and the frequency of ENSO become variable on interdecadal time scales due to interactions of the mean state and the two ENSO components. The western Pacific monsoon is primarily responsible for the generation of the two ENSO components. The annual march of the eastern Pacific ITCZ tends to lock ENSO phases to the annual cycle.

FUTURE WORKS
Future works are to further address the following questions concerning the physics of the ECT/ITCZ complex:

(a) The observed annual warming in the eastern Pacific (defined as the time when monthly mean SST increases from below to above the annual mean) starts in December at the South American coast and begins progressively latter northwestward (e.g., Fig. 2c of Wang 1994). In May, the surface wind convergence zone and heavy convection in the eastern Pacific between 95^oW and 110^oW suddenly "jump" northward, signifying the onset of the summer southwesterly monsoon in that region (Fig.9b of Wang 1994).

•How is the annual warming (weakening) of the equatorial cold tongue initiated along the South American coast? What cause the sudden northward shift of ITCZ over the eastern North Pacific? Or how is the eastern North Pacific summer monsoon initiated during May?

(b) Mitchell and Wallace (1992) hypothesized that the northward migration of the Colombia monsoon rain band leads to the northward advance of the oceanic ITCZ in the eastern Pacific. On the other hand, examination of the OLR and meridional sea level pressure gradients failed to reveal the predicted phase lead of the land convection with respect to the ITCZ convection (Wang 1994). A more general question we would like to ask is:

•To what degree and how the continental convective heating associated with the American monsoon and the Asian-Australian monsoon affect the annual cycle and mean climate of the tropical Pacific Ocean?

(c) Using a coupled ocean-atmosphere general circulation model, Philander et al. (1996) showed that the tilted American coastline and the ocean-atmosphere interaction could induce a weak latitudinal climate asymmetry. In their model, the solar forcing was fixed at the boreal spring equinox. With a two-dimensional model that parameterizes coastal effects, we found that the antisymmetric solar forcing (the annual variation of the differential solar forcing between the Southern and Northern Hemisphere) can dramatically amplify the weak latitudinal asymmetry obtained using an annual mean solar forcing and a tilted eastern ocean boundary. This finding is to some extent surprising because the annual cycle of solar radiation was thought to act as a breaker of the symmetric climate. In view of the model’s simplicity and limitations, one may question whether these results are robust in more realistic and sophisticated coupled models.

•How can the annual variation of differential solar forcing between the Southern and Northern Hemisphere (the antisymmetric forcing) contribute to the latitudinal mean climate asymmetry through regulating the air-sea interaction?

1. Fu. X., and B. Wang, 1999: On the roles of cloud-longwave radiation forcing and boundary layer thermodynamics in forcing tropical surface winds. J. Climate, 12, 1049-1069.

2. Wang, B., and Y. Wang, 1999: Dynamics of the Intertropical convergence zone-cold SST tongue complex. J. Climate, 12, 1830-1847.

3. Wang, B., R. Wu, and R. Lukas, 2000: Annual adjustment of thermocline in the tropical Pacific. J. Climate, In press.

4. Wang, B., and Z. Fang, 2000: Roles of shortwave radiation forcing on ENSO: A study with an intermediate coupled ocean-atmosphere model. Climate Dynamics, Accepted.

5. Wang, B., R. Wu, R. Lukas, and S. I. An: A possible mechanism for ENSO turnabout. Submitted to J. Climate.

6. Fu, X., and B. Wang: Coupled modeling of the Pacific ITCZ/Cold tongue complex, Part I: Model performance and sensitivity experiments. Submitted to J. Climate.

7. Wang, B., and X. Fu: Physical processes determining the rapid reestablishment of the Equatorial Cold Tongue/ITCZ complex from March to May. Submitted to J. Climate.

Bin Wang
bwang@soest.hawaii.edu
Phone (808) 956-2563
Fax: (808) 956-2877

Institution:

Department of Meteorology and IPRC
School of Ocean and Earth Science and Technology
University of Hawaii
2525 Correa road, Honolulu, HI 96822

http://www.soest.hawaii.edu/~bwnag

Return to CLIVAR PACS Home Page