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Andrew W. Robertson

Abstract

An equivalent barotropic tracer model and its adjoint are used to examine observed central European anomalies of potential vorticity (PV) in terms of advection and diabatic effects. The PV viewpoint yields a similar picture of six January anomalies to that found using a one-layer tropospheric temperature model, with the main component of the largest anomalies resulting from advection of PV present at the beginning of the month. Remote PV plays a larger role due to more rapid advection at upper levels. Anomalous diabatic effects are estimated as a residual from the tracer model itself. They are found to be of similar importance for the PV anomalies as are the (residual) source terms in the one-layer thermodynamic equation in the tropospheric temperature case. However, the spatial fields of the residual monthly averaged diabatic effects are noisier and more difficult to interpret than in the temperature case. An interpretation in terms of an estimate of the cross-isentropic mass flux at the base of the model is attempted.

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Andrew W. Robertson

Abstract

The influence of ocean–atmosphere interaction on the wintertime Arctic oscillation (AO) is investigated using a hierarchy of experiments made with two general circulation models (GCMs), ranging from climatologically forced atmospheric to fully coupled ocean–atmosphere GCMs with increasing greenhouse gas concentrations.

Both GCMs reproduce well the AO spatial pattern, defined by the leading hemispheric mode of monthly sea level pressure or daily 700-hPa geopotential height, although the North Pacific pole is more pronounced as compared with observations. Coupling is not found to influence this spatial pattern.

Power spectra are examined for evidence of ocean–atmosphere interaction in the form of spectral reddening or significant spectral peaks. No measurable influence is found. On interannual timescales, all the model AO spectra are approximately “white,” with no clear evidence of any statistically significant spectral peaks in the coupled experiments. Greenhouse gas–induced changes in sea level pressure are found to project onto the AO in one of the GCMs but not the other. On subseasonal timescales, the spectra are “red” in all the model configurations, but ocean–atmosphere interaction is not found to amplify the redness. Significant spectral peaks are found in the 15–25-day period range, consistent with observed spectra.

Daily histograms of the simulated AO indices are found to be negatively skewed. A Gaussian mixture model is used to estimate the probability density function of daily hemispheric height maps, and yields three circulation regimes in both the simulations and observed data. The uncoupled atmospheric GCM simulations exhibit AO-like regimes that acquire stronger wavelike characteristics in the coupled runs.

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Andrew W. Robertson

Abstract

A one-layer, tropospherically averaged tracer model and its adjoint are used to examine observed midlatitude regional anomalies in terms of horizontal advection, and the effects of adiabatic and diabatic heat sources. The adjoint of the tracer model defines an information variable that is used to trace the airmass history of central European anomalies using observed winds. Together with a history of the tracer model's heat sources and sinks, which is also derived from observations, the adjoint integration enables the effects of heat sources and advection to be quantified, using the information tracer as a weighting function. Large January temperature anomalies are found to be primarily accounted for by horizontal advection of air masses already present on 1 January, with anomalous heating/cooling during the month playing a secondary role. Quasigeostrophic theory suggests that the small heating effects implied by the one-layer model are often associated with compensation between diabatic and adiabatic heating effects, the latter accompanying vertical motions. An attempt is made to interpret these compensating heat sources and sinks in terms of the positions of the storm tracks.

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Andrew W. Robertson and Werner Metz

Abstract

The linear baroclinic instability of three-dimensional basic flows on the Northern Hemisphere is examined in terms of a simple two-level, quasi-geostrophic model. The basic flows considered comprise an observed six-winter mean flow, as well as anomalous flows which represent episodes where large-scale persistent flow anomalies, such as the Pacific/North American (PNA), East Atlantic (EA), or North Atlantic Oscillation (NAO) patterns exhibit large amplitudes.

For the climatological basic state, the fastest-growing normal modes with periods of around 4 days consist of regionally confined, synoptic-scale, baroclinic wave trains. These are considered as cyclogenesis modes, characterizing the linear synoptic-scale eddy activity associated with a given basic flow. This eddy activity has a pronounced maximum over the Pacific, close to the position of the observed Pacific storm track, but the second maximum over the Atlantic, corresponding to the Atlantic storm track, is considerably underestimated. Nevertheless, comparing the structure of the cyclogenesis modes with that of the leading complex EOFs of the observed bandpass-filtered flow, a pattern correlation squared of up to 0.4 is obtained. Truncating the basic state to comprise only the ultralong waves (zonal wavenumber m ≤ 4) results in rather little change in the cyclogenesis modes obtained.

Finally, the sensitivity of the cyclogenesis modes to the anomalous basic flows is investigated, using persistent anomaly patterns (PNA, EA, NAO) obtained from a rotated principal component analysis of the observed lowpass-filtered flow. The anomalous basic states are evaluated by adding or subtracting these patterns to/from the winter climatological mean flow. It turns out that the normal-mode wave trains are significantly deflected from their climatological positions, particularly in the EA and NAO cases. This model response is verified against composite maps of observed bandpass variance, obtained for episodes of strong PNA, EA or NAO anomalies respectively. It is found that, although the normal-mode wave trains are still relatively too weak over the Atlantic (compared to the Pacific), the structural differences in the observed bandpass eddy activity between positive and negative anomaly cases are captured quite well by the normal modes.

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Andrew W. Robertson and Michael Ghil

Abstract

Weather regimes are used to determine changes in the statistical distribution of winter precipitation and temperature at eight locations within the western United States. Six regimes are identified from daily 700-mb heights during 46 winters (1949–95) over the North Pacific sector applying cluster analysis; these include the Pacific–North American (PNA) pattern, reverse-PNA, a tropical–Northern Hemisphere (TNH) regime, and a Pacific Ω block. Most of the regimes have a statistically significant effect on the local median temperature, as well as daily temperature extremes; differences between locations are secondary to the large-scale effects. Local precipitation frequency is also conditioned significantly by certain weather regimes, but differences between groups of locations are larger. Precipitation extremes are dispersed and hard to classify. The dependence of local temperature statistics on the warm- or cold-air advection associated with particular weather regimes is discussed, as is the dependence of precipitation anomalies on the regimes’ displaced storm tracks.

The extent to which the El Niño–Southern Oscillation modulates the probability of occurrence of each of the six weather regimes is then investigated. Warm event (El Niño) winters are found to be associated with a significant increase in prevalence of a TNH regime, in which negative height anomalies exhibit a northwest–southeast tilt over the North Pacific. During La Niña winters, this TNH regime occurs significantly less frequently, while a regime characterized by a ridge over southwestern North America becomes more prevalent. These two regimes are associated with regional precipitation-frequency anomalies of opposite sign, that contribute to a north–south contrast in precipitation anomalies over the western United States during El Niño and La Niña winters. On interdecadal timescales, the frequency-of-occurrence of the PNA pattern is found to be notably higher during the 1970s and early 1980s.

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Andrew W. Robertson and Werner Metz

Abstract

Linear baroclinic instability theory is used to investigate the subweekly time scale transient eddies (TEs) and their feedbacks associated with three-dimensional basic flows on the Northern Hemisphere, in terms of a two-layer quasi-geostrophic model. We consider an eight-winter time–mean flow as well as four composites of North Atlantic large-scale quasi-stationary patterns.

The structures of the two fastest-growing normal modes associated with the eight-winter climatology are found to compare very well in many aspects with the leading complex empirical orthogonal functions (CEOFs) of the observed bandpass filtered flow, with pattern correlations up to 0.65; although the normal modes are less localized than the CEOFs. The barotropic feedback implied by the linear modes is also found to compare quite reasonably with the observations, especially over the west Atlantic, but the baroclinic (negative) feedback is less well represented.

Composites coresponding to blocking (BL), zonal (ZO), Greenland anticyclone (GA), and Atlantic ridge (AR) weather regimes are next used to define basic states and composite maps of TE feedback. In all four cases the principal displacements of TE activity over the North Atlantic are captured by the fastest-growing Atlantic cyclogenesis modes. The structure of the barotropic feedback associated with the quasi-stationary anomalies is also reasonably simulated by the linear modes in many respects in the BL and ZO cases, but in the GA and AR cases the linear model is less successful.

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Shuhua Li and Andrew W. Robertson

Abstract

The prediction skill of precipitation at submonthly time scales during the boreal summer season is investigated based on hindcasts from three global ensemble prediction systems (EPSs). The results, analyzed for lead times up to 4 weeks, indicate encouraging correlation skill over some regions, particularly over the Maritime Continent and the equatorial Pacific and Atlantic Oceans. The hindcasts from all three models correspond to high prediction skill over the first week compared to the following three weeks. The ECMWF forecast system tends to yield higher prediction skill than the other two systems, in terms of both correlation and mean squared skill score. However, all three systems are found to exhibit large conditional biases in the tropics, highlighted using the mean squared skill score.

The sources of submonthly predictability are examined in the ECMWF hindcasts over the Maritime Continent in three typical years of contrasting ENSO phase, with a focus on the combined impact of the intraseasonal MJO and interannual ENSO. Rainfall variations over Borneo in the ENSO-neutral year are found to correspond well with the dominant MJO phase. The contribution of ENSO becomes substantial in the two ENSO years, but the MJO impact can become dominant when the MJO occurs in phases 2–3 during El Niño or in phases 5–6 during the La Niña year. These results support the concept that “windows of opportunity” of high forecast skill exist as a function of ENSO and the MJO in certain locations and seasons, which may lead to subseasonal-to-seasonal forecasts of substantial societal value in the future.

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Vincent Moron, Andrew W. Robertson, and Rizaldi Boer

Abstract

The seasonal potential predictability of monsoon onset during the August–December season over Indonesia is studied through analysis of the spatial coherence of daily station rainfall and gridded pentad precipitation data from 1979 to 2005. The onset date, defined using a local agronomic definition, exhibits a seasonal northwest-to-southeast progression from northern and central Sumatra (late August) to Timor (mid-December). South of the equator, interannual variability of the onset date is shown to consist of a spatially coherent large-scale component, together with local-scale noise. The high spatial coherence of onset is similar to that of the September–December seasonal total, while postonset amounts averaged over 15–90 days and September–December amount residuals from large-scale onset show much less spatial coherence, especially across the main islands of monsoonal Indonesia. The cumulative rainfall anomalies exhibit also their largest amplitudes before or near the onset date. This implies that seasonal potential predictability over monsoonal Indonesia during the first part of the austral summer monsoon season is largely associated with monsoon onset, and that there is much less predictability within the rainy season itself. A cross-validated canonical correlation analysis using July sea surface temperatures over the tropical Pacific and Indian Oceans (20°S–20°N, 80°–280°E) as predictors of local-scale onset dates exhibits promising hindcast skill (anomaly correlation of ∼0.80 for the spatial average of standardized rain gauges and ∼0.70 for standardized gridded pentad precipitation data).

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Andrew W. Robertson and Carlos R. Mechoso

Abstract

Interannual variations of the summertime (January–March) atmospheric circulation over subtropical South America are examined during the period 1958–97 using the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis data. It is found from an empirical orthogonal function analysis that an anomalous upper-tropospheric large-scale stationary eddy in the lee of the Andes tends to accompany a dipole in anomalous vertical motion. An anomalous cyclonic (anticyclonic) eddy accompanies an intensified (diffuse) South Atlantic convergence zone (SACZ), with anomalous descent (ascent) to the southwest. The cold-core equivalent barotropic vertical structure of the anomalous cyclonic eddy and the 200-hPa vorticity balance are both characteristic of a stationary Rossby wave; the tendency for the eddy to be advected downstream by the mean westerlies is compensated by meridional advection of planetary vorticity and stretching associated with vertical motion. The anomalous cyclonic flow at low levels reinforces the thermally direct circulation associated with the SACZ. A weak funneling of submonthly Rossby wave activity into this descent region is also identified.

The interannual time series of the eddy is significantly correlated with north–south dipolar sea surface temperature (SST) anomalies over the southwest Atlantic; one standard deviation 200-hPa wind speed anomalies of up to 5 m s−1 are accompanied by SST anomalies of up to 0.3°C. A near-cyclic 15-yr component is identified, which the authors corroborate from independent analyses of southwest Atlantic SSTs and river flows; both are found to exhibit very similar oscillatory components. When the SACZ is intensified, the Paraná and Paraguay rivers in southern Brazil tend to swell, while the Uruguay and Negro rivers to the south tend to ebb; this north–south contrast in streamflow anomalies is most marked on the interdecadal timescale.

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François Lott, Andrew W. Robertson, and Michael Ghil

Abstract

The NCEP–NCAR reanalysis dataset for 1958–97 is used to analyze intraseasonal variations in mountain torques and the large-scale atmospheric circulation patterns associated with them. Spectral analysis of the atmospheric angular momentum (AAM) budget shows that the dominant variations of mountain torque have periodicities near 30 days and shorter, while the dominant AAM variations occur in the 40–60-day band. This difference is due to the 40–60-day AAM variations being primarily related to equatorial processes, while mountain torque variations are associated mostly with extratropical processes.

The Northern Hemisphere (NH) mountain torque has enhanced power and significant spectral peaks in the 20–30-day band. The signal in this band accounts for 33% of the NH mountain torque variance, once the seasonal cycle has been removed. Lag composites of the NH 700-hPa geopotential heights based on the 20–30-day mountain torque signal show the latter to be associated with coherent large-scale patterns that resemble low-frequency oscillations identified in this band by previous authors. The composite patterns that are in phase quadrature with the 20–30-day NH mountain torque have a pronounced zonally symmetric component. These patterns are associated with substantial AAM variations, arguably driven by the NH mountain torque in this band.

Principal component (PC) analysis of the NH 700-hPa heights shows that, in the 20–30-day band, the mountain torque is also in phase quadrature with the two leading PCs; the first corresponds to changes in the midlatitude jet intensity near the subtropics, while the second corresponds to the Arctic Oscillation. The relationships with AAM of the latter essentially occurs through the mass term. Mountain torques are, furthermore, nearly in phase with dominant patterns of low-frequency variability that exhibit substantial pressure gradients across the Rockies and the Tibetan Plateau.

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