Influence of Ocean–Atmosphere Interaction on the Arctic Oscillation in Two General Circulation Models

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

Large-Scale Weather Regimes and Local Climate over the Western United States

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

Interannual and Interdecadal Variability of the South Atlantic Convergence Zone

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

Circulation Regimes and Low-Frequency Oscillations in the South Pacific Sector

The characteristics of subseasonal circulation variability over the South Pacific are examined using 10-day lowpass-filtered 700-hPa geopotential height NCEP–NCAR reanalysis data. The extent to which the variability in each season is characterized by recurrent geographically fixed circulation regimes and/or oscillatory behavior is determined. Two methods of analysis (a K-means cluster analysis and a cross-validated Gaussian mixture model) both indicate three to four geographically fixed circulation regimes in austral fall, winter, and (to some extent) spring. The spatial regime structures are found to be quite similar in each season; they resemble the so-called Pacific–South American (PSA) patterns discussed in previous studies and often referred to as PSA 1 and PSA 2. Oscillatory behavior is investigated using singular spectrum analysis. This identifies a predominantly stationary wave with a period of about 40 days and a spatial structure similar to PSA 1; it is most pronounced in winter and spring and exhibits a noticeable eastward drift as it decays. The power spectrum of variability is otherwise well approximated by a red spectrum, together with enhanced broader-band 15–30-day variability. The results presented herein indicate that low-frequency variability over the South Pacific is not dominated by a propagating wave whose quadrature phases are PSA 1 and PSA 2, as hitherto described. Rather, it is found that the variability is well described by the occurrence of three to four geographically fixed circulation regimes, with a (near) 40-day oscillation that is predominantly stationary in space. The potential subseasonal predictability implied by this duality is discussed. Only during austral spring is a strong correlation found between El Niño and the frequency of occurrence of the circulation regimes

Interannual and Decadal Cycles in River Flows of Southeastern South America

The time series of annual streamflow of four rivers in southeastern and south-central South America (the Negro, Paraguay, Paraná, and Uruguay Rivers) for the period 1911–93 are analyzed. Application of the multitaper method shows that the following features are significant at the 95% level: 1) a nonlinear trend, 2) a near-decadal component, and 3) interannual peaks with ENSO timescales. The trend and near-decadal components are most marked in the two more central rivers, the Paraguay and Paraná, with ENSO timescale variability most pronounced in the Negro and Uruguay rivers in the southeast. Composites of SST are made for each of the statistically significant oscillatory components of river flow, by reconstructing each component using singular spectrum analysis. These composites confirm the influence of ENSO on the streamflow variability of the Negro and Uruguay Rivers, with El Niño associated with enhanced streamflow. On the decadal timescale, high river runoff is associated with anomalously cool SSTs over the tropical North Atlantic. A very similar near-decadal oscillation in SST over this region is identified separately from a rotated empirical orthogonal function analysis of gridded annual mean SSTs. The near-decadal component of the Paraguay and Paraná Rivers is strongest in the austral summer

Diurnal Cycle in Different Weather Regimes and Rainfall Variability over Borneo Associated with ENSO

The interannual variability of precipitation over the island of Borneo in association with El Niño–Southern Oscillation (ENSO) has been studied by using the Global Precipitation Climatology Centre (GPCC) gridded rain gauge precipitation, the NOAA Climate Prediction Center (CPC) Morphing Technique (CMORPH) satellite estimated precipitation, the Quick Scatterometer (QuikSCAT) satellite estimated sea winds, and the National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) reanalysis data. Analysis of the GPCC precipitation shows a dipolar structure of wet southwest versus dry central and northeast in precipitation anomalies associated with El Niño over Borneo Island during the austral summer [December–February (DJF)]. By using the 0.25° and 3-hourly CMORPH precipitation, it is found that rainfall over Borneo is strongly affected by the diurnal cycle of land–sea breezes. The spatial distribution of rainfall over Borneo depends on the direction of monsoonal winds. Weather typing analysis indicates that the dipolar structure of rainfall anomalies associated with ENSO is caused by the variability in the frequency of occurrence of different weather types. Rainfall is enhanced in the coastal region where sea breezes head against off-shore synoptic-scale low-level winds (i.e., in the lee side or wake area of the island), which is referred to here as the “wake effect.” In DJF of El Niño years, the northwesterly austral summer monsoon in southern Borneo is weaker than normal over the Maritime Continent and easterly winds are more frequent than normal over Borneo, acting to enhance rainfall over the southwest coast of the island. This coastal rainfall generation mechanism in different weather types explains the dipole pattern of a wet southwest versus dry northeast in the rainfall anomalies over Borneo Island in the El Niño years

Local versus regional-scale characteristics of monsoon onset and post-onset rainfall over Indonesia

The austral summer monsoon onset and post-onset rainfall and their associated low-level winds are analyzed during the August-February season over Indonesia from 1979 to 2006 using surface and satellite products as well as reanalyses and regional climate model simulations. Onset date is defined using a local agronomic definition. Its leading empirical orthogonal function is found to exhibit a regional-scale spatially-coherent signal across "monsoonal" Indonesia, i.e. mostly south of the Equator, with an asymmetric temporal behavior, such that delayed onsets are more intense than early ones. Associated anomalies in rainfall tend to weaken quickly after mid-to-late November or early December, especially over islands, while they tend to persist over ocean. This weakening is shown to be associated with the evolution of distinct weather types revealed by a kmeans cluster analysis. In particular, late onsets—usually related to warm El Niño Southern Oscillation (ENSO) events—are found to be accompanied by an increased prevalence of a weather type characterized by weak low-level daily-averaged winds across monsoonal Indonesia and increased (decreased) rainfall over most of the island orography and southern and western coasts (seas). The regional model simulations provide evidence that this land-sea rainfall contrast could be associated with an enhanced diurnal sea-land breeze circulation

Multiple Regimes and Low-Frequency Oscillations in the Northern Hemisphere’s Zonal-Mean Flow

This paper studies multiple regimes and low-frequency oscillations in the Northern Hemisphere zonal-mean zonal flow in winter, using 55 yr of daily observational data. The probability density function estimated in the phase space spanned by the two leading empirical orthogonal functions exhibits two distinct, statistically significant maxima. The two regimes associated with these maxima describe persistent zonal-flow states that are characterized by meridional displacements of the midlatitude jet, poleward and equatorward of its time-mean position. The geopotential height anomalies of either regime have a pronounced zonally symmetric component, but largest-amplitude anomalies are located over the Atlantic and Pacific Oceans. High-frequency synoptic transients participate in the maintenance of and transitions between these regimes. Significant oscillatory components with periods of 147 and 72 days are identified by spectral analysis of the zonal-flow time series. These oscillations are described by singular spectrum analysis and the multitaper method. The 147-day oscillation involves zonal-flow anomalies that propagate poleward, while the 72-day oscillation only manifests northward propagation in the Atlantic sector. Both modes mainly describe changes in the midlatitude jet position and intensity. In the horizontal plane though, the two modes exhibit synchronous centers of action located over the Atlantic and Pacific Oceans. The two persistent flow regimes are associated with slow phases of either oscillation

Mountain Torques and Northern Hemisphere Low-Frequency Variability. Part I: Hemispheric Aspects

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

Low-Frequency Variability in a Baroclinic β Channel with Land–Sea Contrast

Atmospheric low-frequency variability (LFV) is studied in a two-layer quasigeostrophic model. The model geometry is a periodic β channel with flat bottom and zonally inhomogeneous thermal forcing. As a result of the idealized land–sea contrast, the model produces a zonally modulated climatological jet with realistic amplitude. The model's LFV is equivalent barotropic; principal component analysis reveals that it consists of (i) dominant stationary patterns with red-noise-like temporal behavior and (ii) propagating waves with periods of 37 and 50 days superimposed on the former. The vorticity forcing due to synoptic eddies is dominated by self-interaction of high-pass filtered model fields. Applying a phase-randomized, stochastic analog of this forcing to a version of the full model in which fast baroclinic instability and, therefore, synoptic eddies are suppressed, produces a climatology and LFV that are very similar to those in the full model. Synoptic eddies are solely represented in the simplified model version by means of stochastic forcing that is independent of the low-frequency flow. It follows that, while fast synoptic eddies are modulated in the full model by the LFV, this modulation is fairly passive: anomalous generation of the synoptic eddies in the course of the full system's low-frequency evolution, the so-called synoptic-eddy feedback, is not essential in selecting the system's low-frequency modes; the main role of synoptic eddies is to supply energy to these modes. Further analysis indicates that the LFV in this thermally driven model originates from the barotropic mode's dynamics. The baroclinic mode passively follows, to first order, the low-frequency changes in the barotropic mode. The latter changes are due to stochastically excited, weakly damped linear eigenmodes of the barotropic-mode equation. Two distinct stationary eigenmodes, as well as two pairs of propagating modes with periods of 27 and 36 days, respectively, dominate the low-frequency behavior. The leading empirical orthogonal functions in this model are associated with these six particular eigenmodes. The latter are not well separated, however, from the other eigenmodes in terms of damping time scale, and it is the barotropic nonlinearity that selects the six dynamically important modes over the others. Interactions between these six modes also result in the occurrence of probability density maxima in two-dimensional subspaces of the model's phase space