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- Author or Editor: P. H. Whetton x
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Abstract
Recently, Cai and Whetton provided modeling evidence that the greenhouse warming pattern has undergone a systematic change from a pattern with maximum warming in subtropical and mid- to high latitudes to one that is El Niño–like from the 1960s onward. They suggest that the mechanism for the change is the transmission of the large extratropical warming to the equatorial east Pacific via modeled tropical–extratropical Pacific circulation pathways. The present study addresses several associated issues. How is the systematic change manifested in empirical orthogonal functions? How do the meridional heat balances respond to the systematic change? Does the proposed mechanism operate in the absence of greenhouse forcing? It is shown that the warming signals are represented by two empirical orthogonal functions, the first of these reflecting a long-term trend in the period considered, and the second showing the change in trend from the 1960s onward. Consistent with the time-varying warming pattern, the relative importance of various heat exchange processes in the tropical Pacific Ocean also undergoes systematic changes. Prior to the 1960s, advective heat flux from the extratropics is the heat source for warming the tropical subthermocline (80–270 m). This subthermocline warming weakens the thermocline and reduces the diffusive heat transfer down through the subthermocline. From the 1960s onward, as substantial subthermocline warming upwells, the El Niño–like pattern develops, strengthening the thermocline; consequently, the downward diffusive heat transfer to the subthermocline enhances reversing the trend prior to the 1960s, and eventually becomes the dominant source for subthermocline heating. The dynamical process, whereby extratropical anomalies are transmitted to the Tropics, operates in a run without external forcing, in association with a mode of ENSO-like interdecadal oscillation. In the equatorial central-eastern Pacific, the associated anomalies upwell and initiate an ocean–atmosphere feedback that changes the equatorial west–east sea surface temperature gradient and easterly winds, reinforcing the upwelled anomalies. The commonality of the modeled interannual ENSO cycles and the interdecadal ENSO-like variability is also discussed.
Abstract
Recently, Cai and Whetton provided modeling evidence that the greenhouse warming pattern has undergone a systematic change from a pattern with maximum warming in subtropical and mid- to high latitudes to one that is El Niño–like from the 1960s onward. They suggest that the mechanism for the change is the transmission of the large extratropical warming to the equatorial east Pacific via modeled tropical–extratropical Pacific circulation pathways. The present study addresses several associated issues. How is the systematic change manifested in empirical orthogonal functions? How do the meridional heat balances respond to the systematic change? Does the proposed mechanism operate in the absence of greenhouse forcing? It is shown that the warming signals are represented by two empirical orthogonal functions, the first of these reflecting a long-term trend in the period considered, and the second showing the change in trend from the 1960s onward. Consistent with the time-varying warming pattern, the relative importance of various heat exchange processes in the tropical Pacific Ocean also undergoes systematic changes. Prior to the 1960s, advective heat flux from the extratropics is the heat source for warming the tropical subthermocline (80–270 m). This subthermocline warming weakens the thermocline and reduces the diffusive heat transfer down through the subthermocline. From the 1960s onward, as substantial subthermocline warming upwells, the El Niño–like pattern develops, strengthening the thermocline; consequently, the downward diffusive heat transfer to the subthermocline enhances reversing the trend prior to the 1960s, and eventually becomes the dominant source for subthermocline heating. The dynamical process, whereby extratropical anomalies are transmitted to the Tropics, operates in a run without external forcing, in association with a mode of ENSO-like interdecadal oscillation. In the equatorial central-eastern Pacific, the associated anomalies upwell and initiate an ocean–atmosphere feedback that changes the equatorial west–east sea surface temperature gradient and easterly winds, reinforcing the upwelled anomalies. The commonality of the modeled interannual ENSO cycles and the interdecadal ENSO-like variability is also discussed.
Abstract
To assist in estimating likely future climate change in the Australian region, the authors examine the results of four different general circulation modeling experiments run to assess the equilibrium impact of doubling greenhouse gases. The results examined were the most recent available at the time of study from various research centers in North America and Europe, as well as those of the Commonwealth Scientific and Industrial Research Organisation (CSIRO). The approach used is, first, to assess the quality of the control ( 1 × C02) simulations from each of the models of mean sea level (MSL) pressure and precipitation in the Australian region by comparing these with the corresponding observed patterns; and, second, to then analyze the 2 × C02 results of only those model experiments with the best control simulations. Of the models examined two are chosen on the basis of their simulation of current climate in the region: the CSIRO four-level model (CSIR04) and the United Kingdom Meteorological Office (UKMO) model. For conditions of equivalent doubling of C02, both models show substantial increases in surface air temperature of around 4°–6° inland and 2°–4°C in coastal regions. Both models show decreased MSL pressure over the Australian continent and increases in rainfall over northern, central, and eastern Australia, particularly in the summer half of the year. The CSIR04 model, but not the UKMO model, also shows increased pressure to the south of the continent and decreased winter rainfall in southwest and southern Australia. Generally, field significance tests show the pattern and magnitude of the changes to be significant for CSIR04 (for which the necessary monthly simulated data were available).
Abstract
To assist in estimating likely future climate change in the Australian region, the authors examine the results of four different general circulation modeling experiments run to assess the equilibrium impact of doubling greenhouse gases. The results examined were the most recent available at the time of study from various research centers in North America and Europe, as well as those of the Commonwealth Scientific and Industrial Research Organisation (CSIRO). The approach used is, first, to assess the quality of the control ( 1 × C02) simulations from each of the models of mean sea level (MSL) pressure and precipitation in the Australian region by comparing these with the corresponding observed patterns; and, second, to then analyze the 2 × C02 results of only those model experiments with the best control simulations. Of the models examined two are chosen on the basis of their simulation of current climate in the region: the CSIRO four-level model (CSIR04) and the United Kingdom Meteorological Office (UKMO) model. For conditions of equivalent doubling of C02, both models show substantial increases in surface air temperature of around 4°–6° inland and 2°–4°C in coastal regions. Both models show decreased MSL pressure over the Australian continent and increases in rainfall over northern, central, and eastern Australia, particularly in the summer half of the year. The CSIR04 model, but not the UKMO model, also shows increased pressure to the south of the continent and decreased winter rainfall in southwest and southern Australia. Generally, field significance tests show the pattern and magnitude of the changes to be significant for CSIR04 (for which the necessary monthly simulated data were available).
