Robert G. Gallimore

Pacific Decadal Variability: The Tropical Pacific Mode and the North Pacific Mode*

Pacific decadal variability is studied in a series of coupled global ocean‐atmosphere simulations aided by two ‘‘modeling surgery’’ strategies: partial coupling (PC) and partial blocking (PB). The PC experiments retain full ocean‐atmosphere coupling in selected regions, but constrain ocean‐atmosphere coupling elsewhere by prescribing the model climatological SST to force the atmospheric component of the coupled system. In PB experiments, sponge walls are inserted into the ocean component of the coupled model at specified latitudinal bands to block the extratropical‐tropical oceanic teleconnection. Both modeling and observational studies suggest that Pacific decadal variability is composed of two distinct modes: a decadal to bidecadal tropical Pacific mode (TPM) and a multidecadal North Pacific mode (NPM). The PC and PB experiments showed that the tropical Pacific mode originates predominantly from local coupled ocean‐atmosphere interaction within the tropical Pacific. Extratropical‐tropical teleconnections, although not a necessary precondition for the genesis of the tropical decadal variability, can enhance SST variations in the Tropics. The decadal memory in the Tropics seems to be associated with tropical higher baroclinic modes. The North Pacific mode originates from local atmospheric stochastic processes and coupled ocean‐atmosphere interaction. Atmospheric stochastic forcing can generate a weaker NPM-like pattern in both the atmosphere and ocean, but with no preferred timescales. In contrast, coupled ocean‐atmosphere feedback can enhance the variability substantially and generate a basin-scale multidecadal mode in the North Pacific. The multidecadal memory in the midlatitudes seems to be associated with the delayed response of the subtropical/subpolar gyre to wind stress variation in the central North Pacific and the slow growing/decaying of SST anomalies that propagate eastward in the Kuroshio Extension region. Oceanic dynamics, particularly the advection of the mean temperature by anomalous meridional surface Ekman flow and western boundary currents, plays an important role in generating the North Pacific mode.

Land surface feedbacks and palaeomonsoons in northern Africa

We ran a sequence of climate model experiments for 6000 years ago, with land-surface conditions based on a realistic map of palaeovegetation, lakes and wetlands, to quantify the effects of land-surface feedbacks in the Saharan region. Vegetation-induced albedo and moisture flux changes produced year-round warming, forced the monsoon to 17°–25°N two months earlier, and shifted the precipitation belt ≈300 km northwards compared to the effects of orbital forcing alone. The addition of lakes and wetlands produced localised changes in evaporation and precipitation, but caused no further extension of the monsoon belt. Diagnostic analyses with biome and continental hydrology models showed that the combined land-surface feedbacks, although substantial, could neither maintain grassland as far north as observed (≈26°N) nor maintain Lake “MegaChad” (330,000 km²).

On the cause of abrupt vegetation collapse in North Africa during the Holocene: Climate variability vs. vegetation feedback

The abrupt desertification over the northern Africa in the mid-Holocene is studied in both a complex and a simple coupled climate-vegetation model. In contrast to the previous mechanism that relies on strong positive vegetation-climate feedback and the resulted multiple equilibria, we propose a new mechanism in which the abrupt desertification is caused by low frequency climate variability, rather than a positive vegetation-climate feedback. The implication of this new mechanism to modelling and observation is also discussed.

The Forcing of the Meridional Circulation of the Isentropic Zonally Averaged Circumpolar Vortex

Abstract For the axisymmetric general circulation, Eliassen (1951) showed that stable meridional circulations are controlled by large-scale diabatic heating and friction torque. Because the real atmosphere is longitudinally disturbed, a direct meridional response to large-scale diabatic heating is not readily found by conventional zonal averaging in isobaric coordinates. By introducing, the concept of a zonally averaged general circulation in isentropic coordinates, a direct meridional response to diabatic heating is isolated. Using an approach analogous to Eliassens, this study has extended the concept of diabatically forced isentropic meridional circulations to include the role of angular momentum torques within the circumpolar vortex. The absolute angular momentum torques comprise three terms: divergence of eddy relative momentum transport, friction and zonal pressure torques. The heating is composed of hemispheric-scale diabatic processes. For a hydrodynamically and statically stable atmosphere, a co...

Effects of soil moisture on the sensitivity of a climate model to earth orbital forcing at 9000 yr BP

The sensitivity of climate to orbitally-related changes in solar radiation at 9000 yr BP (before present) is examined using fixed and interactive soil moisture versions of a low resolution general circulation model. In both versions of the model increased solar radiation for June–August at 9000 yr BP (compared to present) produced enhanced northern monsoons and warmer continental interiors in comparison to present whereas decreased solar radiation at 9000 yr BP in December–February produced weaker southern monsoons. The increased rainfall in the northern tropics in summer increased soil moisture and runoff at 9000 yr BP in the interactive model; in the southern hemisphere decreased rainfall in summer led to decreased soil moisture and runoff. Conditions in summer became drier (decreased soil moisture and runoff) at 9000 yr BP in the northern extratropics.The experiments showed that the magnitude (but not the sign) of model sensitivity to 9000 yr BP radiation is altered by the effects of interactive soil moisture. Decreased soil moisture (about 20%) over northern Eurasia in the interactive model led to smaller evaporative increases, greater temperature increases and greater reduction of precipitation than for the model with fixed soil moisture. Over northern tropical lands, slightly smaller temperature increases and greater evaporation and precipitation increases in the interactive model are linked to the simulation of increased soil moisture at 9000 yr BP. The differences in sensitivity between the two versions of the model over northern Eurasia are statistically significant at the 95% level whereas those for the tropics are not.Overall, the results of the simulations are generally supported by the geologic evidence for 9000 yr BP; however, the evidence lacks sufficient precision and the model resolution is too coarse for detailed model/data comparisons and for assessment of the relative accuracy of the two 9000 yr BP experiments.The computed sensitivities of temperature and soil moisture to 9000 yr BP radiation differ from those simulated under equilibrium conditions in the various general circulation model experiments for increased atmospheric concentration of CO2. In contrast to the effects of the enhanced seasonal cycle of solar radiation at 9000 yr BP, a CO2 increase causes a broad warming of both the ocean and land with little modification of land/ocean temperature difference. The experiments for 9000 yr BP indicate a clearer signal for summer drying than is obtained in the experiments for increased CO2. The results suggest that the 9000 yr BP climate may be of limited utility as an analog to future ‘warm’ climates.

Simulated and observed preindustrial to modern vegetation and climate changes

Abstract Rising levels of carbon dioxide since the preindustrial era have likely contributed to an observed warming of the global surface, and observations show global greening and an expansion of boreal forests. This study reproduces observed climate and vegetation trends associated with rising CO2 using a fully coupled atmosphere–ocean–land surface GCM with dynamic vegetation and decomposes the effects into physiological and radiative components. The simulated warming trend, strongest at high latitudes, was dominated by the radiative effect, although the physiological effect of CO2 on vegetation (CO2 fertilization) contributed to significant wintertime warming over northern Europe and central and eastern Asia. The net global greening of the model was primarily due to the physiological effect of increasing CO2, while the radiative and physiological effects combined to produce a poleward expansion of the boreal forests. Observed and simulated trends in tree ring width are consistent with the enhancement o...

Response of the NCAR General Circulation Model to Prescribed Changes in Ocean Surface Temperature. Part II: Midlatitude and Subtropical Changes

Abstract The sensitivity of a six-layer NCAR atmospheric general circulation model (GCM) to a variety of idealized, very large amplitude, midlatitude and subtropical North Pacific Ocean surface temperature (OST) anomalies is analyzed. In the Pacific sector, the model exhibits a differential sensitivity depending on the latitudinal position of the imposed anomaly. Typically, the model response is a combination of a relative direct thermal circulation, an alteration in the pattern of cyclonic activity, and a selective wave response dependent on the planetary waves present in the unmodified control case. The extent to which the background planetary waves affect the model response is dependent on several factors including latitude-dependent features of the control case and the relative position of the anomaly. Analogous experiments with a simple quasi-geostrophic model are useful in isolating important physical and dynamical processes, and thereby assist in the interpretation of the GCM results. An analysis o...

Simulated Ocean–Atmosphere Interaction in the North Pacific from a GCM Coupled to a Constant-Depth Mixed Layer

Abstract The life cycle and structure of dominant wintertime SST anomalies and associated atmospheric response in the extratropical North Pacific are examined using results from a 100-yr seasonal simulation of a low-resolution atmospheric model with realistic geography coupled to a simple mixed layer ocean. The study focuses on composited SST anomalies produced solely by ocean–atmosphere energy exchange. One key pattern shows a negative (positive) SST anomaly in the central Pacific, denoted CCP (WCP), flanked by opposite signed anomalies in the western and eastern Pacific. For the WCP case, the SST anomaly reaches about 1.0°C in the central Pacific, whereas for the CCP case it is −1.5°C. During the growth phase of the WCP (CCP) SST anomalies, anomalous highs (lows) occur over the western Pacific and over western North America, and an anomalous low (high) is over the east-central Pacific. To the rear of the anomalous low (high), a negative (positive) SST anomaly develops in response to anomalous cold, dry ...

Stability and variability in a coupled ocean-atmosphere climate model : results of 100-year simulations

Abstract Two 100-year seasonal simulators, one performed with a low resolution atmospheric general circulation model (GCM) coupled to a mixed-layer ocean formulation and the other made with the GCM forced by prescribed ocean conditions, are compared to assess the effects of an interactive ocean and sea-ice component on the stability and interannual variability of a climate system. Characteristics of the time variation of surface temperature, 700 mb temperature and sea-ice coverage are analyzed for selected land and ocean areas. Both simulations showed stable seasonal cycles of basic variables, although small trends were found. These trends were roughly linear in nature and quite distinct from all other components of variability. Detrended time series were used to describe the other aspects of variability. There was pronounced interannual variability in the simulations from both models as seen in the time series for temperature and sea ice over the entire 100-year time period. Consistent with observations,...

A Numerical Diagnostic Model of the Zonally Averaged Circulation in Isentropic Coordinates

Abstract In the linear theory of the isentropic zonally averaged circulation (Gallimore and Johnson, 1981), stable meridional circulations within the circumpolar vortex are forced by larger scale diabatic beating and angular momentum torques. The linear theory, however, could not describe important nonlinear interactions between the forcing of meridional circulations and the maintenance of the circumpolar vortex nor could the quasi-steady balance be ascertained. In this study, an isentropic numerical model is developed to investigate the forcing of the meridional circulation and its role and the role of angular momentum torques in maintaining the circumpolar vortex. The model is based on a set of isentropic zonally averaged equations applied to the Northern Hemisphere. The horizontal resolution is 5° of latitude while nine equally spaced isentropic levels are used for vertical resolution. The basic forcing of the model is diabatic heating and zonal pressure and friction torques. The distribution of diabat...