GFDL's CM2 Global Coupled Climate Models. Part III: Tropical Pacific Climate and ENSO

Abstract

Multicentury integrations from two global coupled ocean-atmosphere-land-ice models [Climate Model versions 2.0 (CM2.0) and 2.1 (CM2.1), developed at the Geophysical Fluid Dynamics Laboratory] are described in terms of their tropical Pacific climate and El Niño-Southern Oscillation (ENSO). The integrations are run without flux adjustments and provide generally realistic simulations of tropical Pacific climate. The observed annual-mean trade winds and precipitation, sea surface temperature, surface heat fluxes, surface currents, Equatorial Undercurrent, and subsurface thermal structure are well captured by the models. Some biases are evident, including a cold SST bias along the equator, a warm bias along the coast of South America, and a westward extension of the trade winds relative to observations. Along the equator, the models exhibit a robust, westward-propagating annual cycle of SST and zonal winds. During boreal spring, excessive rainfall south of the equator is linked to an unrealistic reversal of the simulated meridional winds in the east, and a stronger-than-observed semiannual signal is evident in the zonal winds and Equatorial Undercurrent.Both CM2.0 and CM2.1 have a robust ENSO with multidecadal fluctuations in amplitude, an irregular period between 2 and 5 yr, and a distribution of SST anomalies that is skewed toward warm events as observed. The evolution of subsurface temperature and current anomalies is also quite realistic. However, the simulated SST anomalies are too strong, too weakly damped by surface heat fluxes, and not as clearly phase locked to the end of the calendar year as in observations. The simulated patterns of tropical Pacific SST, wind stress, and precipitation variability are displaced 20°-30°west of the observed patterns, as are the simulated ENSO teleconnections to wintertime 200-hPa heights over Canada and the northeastern Pacific Ocean. Despite this, the impacts of ENSO on summertime and wintertime precipitation outside the tropical Pacific appear to be well simulated. Impacts of the annual-mean biases on the simulated variability are discussed.

Changes in South Pacific rainfall bands in a warming climate

“…1f) and coupled model SST biases (Fig. 1e) near the Equator has been pointed out in previous studies 30,31 . By conducting control and climate-change experiments with a radiative-flux-corrected CGCM (CCSM3) with realistic present-day coupled SST and comparing the resulting SST projection pattern with the standard CCSM3 greenhouse warming experiment ( Supplementary Fig.…”

Description and basic evaluation of Beijing Normal University Earth System Model (BNU-ESM) version 1

“…One is the stronger easterly winds on the equator which result in stronger equatorial upwelling; another may be weaker activity of tropical instability waves in the ocean. The ocean component MOM4p1 uses the horizontal anisotropic friction scheme from Large et al (2001), which induces more frictional dissipation and prohibits vigorous tropical instability wave activity (Wittenberg et al, 2006). Stronger activity of tropical instability waves could prevent the cold tongue water from cooling down by mixing with the warm off-equatorial water Menkes et al, 2006;Seo et al, 2006;Zhang and Busalacchi, 2008).…”

“…This also raises the importance of further improvements on the deep convection parameterization scheme, as the representation of deep convection is central in defining both the dynamical and the heat flux atmospheric feedbacks (Guilyardi et al, 2009). Another possible cause for the excessive ENSO amplitude is the lack of a sufficient surface heat flux damping of SST anomalies in the model, as weaker heat flux damping tends to destabilize and amplify ENSO (Wittenberg, 2002;Wittenberg et al, 2006). Further studies on these topics are warranted.…”

Strong and moderate nonlinear El Niño regimes

“…Tropical climate and ENSO in CM2.1 was initially analyzed by Wittenberg et al (2006) and they found it to be generally realistic although with some limitations. For instance, as many climate models, it has a mean cold bias along the equatorial Pacific and warm bias along the coast of South America, as well as a double ITCZ in the eastern Pacific.…”