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Scott B. Power, Roger H. J. Grimshaw, and Jason H. Middleton

Abstract

Analytic solutions are obtained for forced, barotropic circulation at subinertial frequencies over a bilinear continental margin (shelf and slope) in situations where bottom friction is important. Three different alongshore forces are considered: wind-stress, offshore oceanic pressure gradients and offshore currents. Forcing functions are assumed to vary sinusoidally in time and in space alongshore. Steady models are found to perform adequately provided that the forcing functions do not move in the same direction as the free modes (continental shelf waves) propagate. Near resonance, when the alongshore velocity of the forcing approximates that of a free mode, the response is dominated by the mode. In the case of wind forcing, signals are trapped nearshore. If the shelf break occurs within this trapping length (as occurs near resonance) the shelf width becomes the elective trapping length. In this instance there can be significant horizontal shear in the alongshore velocity on the shelf near the shelf break.

When the velocity of an oceanic, alongshore pressure gradient signal approximates that of a free mode, the signal can be amplified towards the coast. For example, near a mode 2 resonance the signal is a maximum near the coast with a secondary maximum on the continental slope, near the shelf break. This amplification is in stark contrast to the solution forced by a signal which is either stationary or moving in a direction opposite to that in which the free modes propagate, which simply fall away from their maximum values offshore, resulting in weak coastal circulations.

Bottom friction affects the free continental shelf waves in three ways: their phase speeds are reduced, they decay with time and their altered structures exhibit phase differences across the continental margin whereby the flow nearshore leads that offshore in time. As a result, increased bottom friction reduces the response at resonance, broadens the range of frequencies over which responses are increased and detunes, or shifts, the frequency at which resonance occurs to a lower value. At practical parameter values, the reduction is minimal for the first mode but can he substantial for the second.

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Shayne McGregor, Neil J. Holbrook, and Scott B. Power

Abstract

The Australian Bureau of Meteorology Research Centre CGCM and a linear first baroclinic-mode ocean shallow-water model (SWM) are used to investigate ocean dynamic forcing mechanisms of the equatorial Pacific Ocean interdecadal sea surface temperature (SST) variability. An EOF analysis of the 13-yr low-pass Butterworth-filtered SST anomalies from a century-time-scale CGCM simulation reveals an SST anomaly spatial pattern and time variability consistent with the interdecadal Pacific oscillation. Results from an SWM simulation forced with wind stresses from the CGCM simulation are shown to compare well with the CGCM, and as such the SWM is then used to investigate the roles of “uncoupled” equatorial wind stress forcing, off-equatorial wind stress forcing (OffEqWF), and Rossby wave reflection at the western Pacific Ocean boundary, on the decadal equatorial thermocline depth anomalies.

Equatorial Pacific wind stresses are shown to explain a large proportion of the overall variance in the equatorial thermocline depth anomalies. However, OffEqWF beyond 12.5° latitude produces an interdecadal signature in the Niño-4 (Niño-3) region that explains approximately 10% (1.5%) of the filtered control simulation variance. Rossby wave reflection at the western Pacific boundary is shown to underpin the OffEqWF contribution to these equatorial anomalies. The implications of this result for the predictability of the decadal variations of thermocline depth are investigated with results showing that OffEqWF generates an equatorial response in the Niño-3 region up to 3 yr after the wind stress forcing is switched off. Further, a statistically significant correlation is found between thermocline depth anomalies in the off-equatorial zone and the Niño-3 region, with the Niño-3 region lagging by approximately 2 yr. The authors conclude that there is potential predictability of the OffEqWF equatorial thermocline depth anomalies with lead times of up to 3 yr when taking into account the amplitudes and locations of off-equatorial region Rossby waves.

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Scott B. Power, Jason H. Middleton, and R. H. J. Grimshaw

Abstract

Analytic solutions am obtained for the barotropic shelf circulation caused by wind and deep-ocean forcing at subinertial frequencies. An Inclined beach model of the continental shelf is used and only situations in which bottom friction is important are considered. Three different alongshore forces are considered: pressure gradients and currents (maintained by the deep ocean) at the shelf break and wind stress, over the shelf. In each case the model is formulated as a boundary value problem in which the boundary conditions are determined by the forcing mechanism. In general, a damped resonant response occurs when the forcing function has the same longshore velocity as an unforced continental shelf wave and is most significant for the fim mode. In the case of forcing by an alongshore pressure gradient at the edge of the shelf, this leads to the amplification of the pressure signal toward the coast. The model frequencies and structures are determined for various frictional values. When friction is small the results are consistent with those of Brink and Allen in that phase speeds remain unchanged and cross-shelf phase differences are introduced. At larger frictional values, however, phase speeds are reduced, and the model structures and cross-shelf phase differences are further altered.

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Shayne McGregor, Neil J. Holbrook, and Scott B. Power

Abstract

This study investigates the response of a stochastically forced coupled atmosphere–ocean model of the equatorial Pacific to off-equatorial wind stress anomaly forcing. The intermediate-complexity coupled ENSO model comprises a linear, first baroclinic mode, ocean shallow water model with a steady-state, two–pressure level (250 and 750 mb) atmospheric component that has been linearized about a state of rest on the β plane. Estimates of observed equatorial region stochastic forcing are calculated from NCEP–NCAR reanalysis surface winds for the period 1950–2006 using singular value decomposition. The stochastic forcing is applied to the model both with and without off-equatorial region wind stress anomalies (i.e., poleward of 12.5° latitude). It is found that the multiyear changes in the equatorial Pacific thermocline depth “background state” induced by off-equatorial forcing can affect the amplitude of modeled sea surface temperature anomalies by up to 1°C. Moreover, off-equatorial wind stress anomalies increased the modeled amplitude of the two biggest El Niño events in the twentieth century (1982/83 and 1997/98) by more than 0.5°C, resulting in a more realistic modeled response. These equatorial changes driven by off-equatorial region wind stress anomalies are highly predictable to two years in advance and may be useful as a physical basis to enhance multiyear probabilistic predictions of ENSO indices.

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Bradley F. Murphy, Scott B. Power, and Simon McGree

Abstract

El Niño–Southern Oscillation (ENSO) drives interannual climate variability in many tropical Pacific island countries, but different El Niño events might be expected to produce varying rainfall impacts. To investigate these possible variations, El Niño events were divided into three categories based on where the largest September–February sea surface temperature (SST) anomalies occur: warm pool El Niño (WPE), cold tongue El Niño (CTE), and mixed El Niño (ME), between the other two.

Large-scale SST and wind patterns for each type of El Niño show distinct and significant differences, as well as shifts in rainfall patterns in the main convergence zones. As a result, November to April rainfall in many Pacific island countries is significantly different among the El Niño types. In western equatorial Pacific islands, CTE events are associated with drier than normal conditions whereas ME and WPE events are associated with significantly wetter than normal conditions. This is due to the South Pacific convergence zone and intertropical convergence zone moving equatorward and merging in CTE events. Rainfall in the convergence zones is enhanced during ME and WPE and the displacement is smaller. La Niña events also show robust impacts that most closely mirror those of ME events.

In the northwest and southwest Pacific strong CTE events have much larger impacts on rainfall than ME and WPE, as SST anomalies and correspondingly large-scale surface wind and rainfall changes are largest in CTE. While variations in rainfall exist between different types of El Niño and the significant impacts on Pacific countries of each event are different, the two extreme CTE events have produced the most atypical impacts.

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Scott B. Power, François Delage, Robert Colman, and Aurel Moise

Abstract

Under global warming, increases in precipitation are expected at high latitudes and near major tropical convergence zones in some seasons, while decreases are expected in many subtropical and midlatitude areas in between. In many other areas there is no consensus among models on the sign of the projected change. This is often assumed to indicate that precipitation projections in these regions are highly uncertain.

Here, twenty-first century precipitation projections under the Special Report on Emissions Scenarios (SRES) A1B scenario using 24 World Climate Research Programme (WCRP)/Coupled Model Intercomparison Project phase 3 (CMIP3) climate models are examined. In areas with no consensus on the sign of projected change there are extensive subregions where the projected change is “very likely” (i.e., probability > 0.90) to be small (relative to, e.g., the size of interannual variability during the late twentieth century) or zero. The statistical significance of and interrelationships between methods used to identify model consensus on projected change in the 2007 Intergovernmental Panel on Climate Change (IPCC) report are examined, and the impact of interdependency among model projections on statistical significance is investigated. Interdependency among projections is shown to be much weaker than interdependency among simulations of climatology. The results show that there is more widespread consistency among the model projections than one might infer from the 2007 IPCC Fourth Assessment report. This discovery highlights the broader need to identify regions, variables, and phenomena that are expected to be little affected by anthropogenic climate change and to communicate this information to the wider community. This is especially important for projections of climate for the next 1–3 decades.

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Harun A. Rashid, Scott B. Power, and Jeff R. Knight

Abstract

Using a multicentury integration of the third climate configuration of the Met Office Unified Model (HadCM3), the authors show that naturally occurring fluctuations in the Atlantic’s thermohaline circulation (THC) drive small but statistically significant changes in surface air temperature, sea level pressure, and precipitation over the Indo-Pacific region. The surface temperature component of these variations may be described as an interhemispheric seesaw (consistent with earlier studies), with changes in the Southern Hemisphere smaller than those in the Northern Hemisphere. Links between THC variability and variability related to the interdecadal Pacific oscillation (IPO) are evident: when the THC is strong (weak) the IPO variance decreases (increases) considerably, and cold (La Niña–like) IPO events tend to be stronger and more frequent when the THC is in a weak phase. This highlights the possibility that a small part of Indo-Pacific climate variability at multidecadal time scales, including some of the variability linked to the IPO, may be predictable.

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Shayne McGregor, Alex Sen Gupta, Neil J. Holbrook, and Scott B. Power

Abstract

Evidence suggests that the magnitude and frequency of the El Niño–Southern Oscillation (ENSO) changes on interdecadal time scales. This is manifest in a distinct shift in ENSO behavior during the late 1970s. This study investigates mechanisms that may force this interdecadal variability and, in particular, on modulations driven by extratropical Rossby waves. Results from oceanic shallow-water models show that the Rossby wave theory can explain small near-zonal changes in equatorial thermocline depth that can alter the amplitude of simulated ENSO events. However, questions remain over whether the same mechanism operates in more complex coupled general circulation models (CGCMs) and what the magnitude of the resulting change would be. Experiments carried out in a state-of-the-art z-coordinate primitive equation model confirm that the Rossby wave mechanism does indeed operate. The effects of these interactions are further investigated using a partial coupling (PC) technique. This allows for the isolation of the role of wind stress–forced oceanic exchanges between the extratropics and the tropics and the subsequent modulation of ENSO variability. It is found that changes in the background state of the equatorial Pacific thermocline depth, induced by a fixed off-equatorial wind stress anomaly, can significantly affect the probability of ENSO events occurring. This confirms the results obtained from simpler models and further validates theories that rely on oceanic wave dynamics to generate Pacific Ocean interdecadal variability. This indicates that an improved predictive capability for seasonal-to-interannual ENSO variability could be achieved through a better understanding of extratropical-to-tropical Pacific Ocean transfers and western boundary processes. Furthermore, such an understanding would provide a physical basis to enhance multiyear probabilistic predictions of ENSO indices.

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Scott B. Power, Andreas Schiller, Gillian Cambers, David Jones, and Kevin Hennessy

No abstract available.

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Christine T. Y. Chung, Scott B. Power, Agus Santoso, and Guomin Wang

Abstract

Naturally occurring multiyear to decadal variability is evident in rainfall, temperature, severe weather, and flood frequency around the globe. It is therefore important to understand the cause of this variability and the extent to which it can be predicted. Here internally generated decadal climate variability and its predictability potential in an ensemble of CMIP5 models are assessed. Global hot spots of subsurface ocean decadal variability are identified, revealing variability in the southern Tasman Sea that is coherent with variability in much of the Pacific Ocean and Southern Hemisphere. It is found that subsurface temperature variability in the southern Tasman Sea primarily arises in response to preceding changes in Southern Hemisphere winds. This variability is multiyear to decadal in character and is coherent with surface temperature in parts of the Southern Hemisphere up to several years later. This provides some degree of potential predictability to surface temperature in the southern Tasman Sea and surrounding regions. A few models exhibit significant correlation between subsurface variability in the southern Tasman Sea and zonally averaged precipitation south of 50°S; however, the multimodel mean does not exhibit any significant correlation between subsurface variability and precipitation. Models that exhibit stronger subsurface variability in the southern Tasman Sea also have a stronger interdecadal Pacific oscillation signal in the Pacific.

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