Abstract

The Madden–Julian oscillation (MJO) dominates tropical variability on time scales of 30–70 days. During the boreal winter–spring it is manifested as an eastward propagating disturbance, with a strong convective signature over the Eastern Hemisphere. The space–time structure of the MJO is described using the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis, Advanced Very High Resolution Radiometer outgoing longwave radiation, observed sea surface temperature, and the Climate Prediction Center Merged Analysis of Precipitation. Empirical orthogonal function analysis is used to identify the convective signature of the MJO, and regression is used to identify key relationships with the convection. Compared to analyzing successive years of data, the selection of years of strong MJO activity results in a more robust lead–lag structure and an increase in explained variance. The MJO exhibits a rich vertical structure, with low-level moisture convergence being well defined when the convective anomalies are strong, and there is evidence that free-tropospheric processes also play a role in the MJO life cycle. The westward vertical tilt is most apparent over the western Pacific. Over the Indian Ocean the system is more vertically stacked, principally because of the strong subsidence of the inactive phase of the MJO, which lies to the east of the convection. As the Kelvin wave decouples from the convection near the date line, a sea level low pressure surge, previously discussed by A. J. Matthews, transits the eastern Pacific and Atlantic Oceans. Here the link of the zonal wind stress and low-level divergence to the pressure surge is explored. The pressure gradient gives rise to westerlies that propagate rapidly to the east, and it may play role in the development of the MJO convection in the western Indian Ocean, which occurs in an easterly basic state, and conditions not consistent with the low-level moisture convergence paradigm.

Abstract

Composites of SST, wind, rainfall, and humidity have been constructed for years of high rainfall during September, October, and November (SON) in equatorial and southern-central East Africa. These show that extreme East African short rains are associated with large-scale SST anomalies in the Indian Ocean that closely resemble those that develop during Indian Ocean dipole or zonal mode (IOZM) events. This is corroborated by the observation that strong IOZM events produce enhanced East African rainfall. However, it is also shown that the relationship between the IOZM and East African rainfall is nonlinear, with only IOZM events that reverse the zonal SST gradient for several months (extreme events) triggering high rainfall.

Comparison of the wind anomalies that develop during extreme IOZM events with those that develop during weaker (moderate) events shows that strong easterly anomalies in the northern-central Indian Ocean are a persistent feature of extreme, but not of moderate, IOZM years. It is suggested that these anomalies weaken the westerly flow that normally transports moisture away from the African continent, out over the Indian Ocean. Thus, during extreme IOZM years, rainfall is enhanced over East Africa and reduced in the central and eastern Indian Ocean basin.

It is also shown that the IOZM cannot be viewed in isolation from the El Niño–Southern Oscillation (ENSO). Instead it is postulated that in some years, a strong ENSO forcing can predispose the Indian Ocean coupled system to an IOZM event and is therefore a contributory factor in extreme East African rainfall. The results of this study imply that the relationship between El Niño and the IOZM explains the previously described association between El Niño and high East African rainfall. Thus, understanding the way that ENSO drives Indian Ocean dynamics may aid the development of predictive scenarios for East African climate that could have significant economic implications.

Abstract

The Oregon State University coupled upper ocean-atmosphere GCM has been shown to qualitatively simulate the Southern Oscillation. A composite analysis of the warm and cold events simulated in this 23-year integration has been performed. During the low phase of the Southern Oscillation, when warm anomalies occur in the eastern Pacific, the model simulates for the Atlantic region during March–May 1) a deficit of precipitation over the tropical South American continent, 2) Caribbean and Gulf of Mexico sea level pressure and sea surface temperature are in phase with the eastern Pacific anomalies, while those east of the Nordeste region are out of phase, and 3) northeast trade winds are anomalously weak and southwest trade winds are anomalously strong (as inferred from surface current anomalies). The anomalies in the oceanic processes are induced by perturbations in the atmospheric circulation over the Atlantic and are coupled to changes in the Walker circulation. During the high phase of the simulated Southern Oscillation, conditions in the atmosphere and ocean are essentially the reverse of the low phase. The model produces a response in the South American region during the opposing phases of the Southern Oscillation that is in general agreement with observations.

The interannual variation of Nordeste rainfall is shown to be dominated by a few band-limited frequencies. These frequencies are found in the SST series of those regions of the Atlantic and Pacific oceans where strong correlations with Nordeste precipitation exist.

Abstract

This paper presents the climatology and interannual variation of the East Asian winter monsoon based on the 1979–95 National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis. In addition to documenting the frequency, intensity, and preferred propagation tracks of cold surges and the evolution patterns of related fields, the authors discuss the temporal distribution of the Siberian high and cold surges. Further, the interannual variation of the cold surges and winter monsoon circulation and its relationship with ENSO were examined.

There are on average 13 cold surges in each winter season (October–April), of which two are strong cases. The average intensity of cold surges, measured by maximum sea level pressure, is 1053 hPa. The cold surges originate from two source regions: 1) northwest of Lake Baikal, and 2) north of Lake Balkhash. The typical evolution of a cold surge occurs over a period of 5–14 days. Trajectory and correlation analyses indicate that, during this time, high pressure centers propagate southeastward around the edge of the Tibetan Plateau from the mentioned source regions. Some of these high pressure centers then move eastward and diminish over the oceans, while others proceed southward. The signatures of the associated temperature, wind, and pressure fields propagate farther southward and eastward. The affected area encompasses the bulk of the maritime continent. Although the intensity of the Siberian high peaks during December and January, the frequency of cold surges reaches a maximum in November and in March. This result suggests that November through March should be considered as the East Asian winter monsoon season.

Two stratifications of cold surges are used to examine the relationship between ENSO and the interannual variation of the winter monsoon. The first one, described as conventional cold surges, indicates that the surge frequency reaches a minimum a year after El Niño events. The second one, defined as a maximum wind event near the South China Sea, shares the same origin as the first. The surge frequency is in good agreement with the Southern Oscillation index (SOI). A low (high) SOI event coincides with a low (high) frequency of cold surges.

The interannual variation of winter northerlies near the South China Sea is dominated by the South China Sea cold surges and is well correlated (R = 0.82) with the SOI. Strong wind seasons are associated with La Niña and high SOI years; on the other hand, weak wind seasons are associated with El Niño and low SOI years. This pattern is restricted to an area north of the equator within the region of (0°–20°N, 100°–130°E) and is confined to the near-surface layer. The SST variation in the same region is similar to the wind pattern but lags the wind for approximately 1–5 months, which suggests that the SST variation is forced by the wind. The surface Siberian high, 500-hPa trough, and200-hPa jet stream, all representing the large-scale monsoon flow, are weaker than normal during El Niño years. In particular, the interannual variation of the Siberian high is in general agreement with the SOI.

Abstract

We evaluate extratropical modes of variability in the three most recent phases of the Coupled Model Intercomparison Project (CMIP3, CMIP5, and CMIP6) to gauge improvement of climate models over time. A suite of high-level metrics is employed to objectively evaluate how well climate models simulate the observed northern annular mode (NAM), North Atlantic Oscillation (NAO), Pacific–North America pattern (PNA), southern annular mode (SAM), Pacific decadal oscillation (PDO), North Pacific Oscillation (NPO), and North Pacific Gyre Oscillation (NPGO). We apply a common basis function (CBF) approach that projects model anomalies onto observed empirical orthogonal functions (EOFs), together with the traditional EOF approach, to CMIP Historical and AMIP models. We find simulated spatial patterns of those modes have been significantly improved in the newer models, although the skill improvement is sensitive to the mode and season considered. We identify some potential contributions to the pattern improvement of certain modes (e.g., the Southern Hemisphere jet and high-top vertical coordinate); however, the performance changes are likely attributed to gradual improvement of the base climate and multiple relevant processes. Less performance improvement is evident in the mode amplitude of these modes and systematic overestimation of the mode amplitude in spring remains in the newer climate models. We find that the postdominant season amplitude errors in atmospheric modes are not limited to coupled runs but are often already evident in AMIP simulations. This suggests that rectifying the egregious postdominant season amplitude errors found in many models can be addressed in an atmospheric-only framework, making it more tractable to address in the model development process.

No Abstract available.

Abstract

The boreal winter Madden–Julian oscillation (MJO) remains very weak and irregular in the National Center for Atmospheric Research (NCAR) Community Atmosphere Model version 2 (CAM2) as in its direct predecessor, the Community Climate Model version 3 (CCM3). The standard version of CAM2 uses the deep convective scheme of Zhang and McFarlane, as in CCM3, with the closure dependent on convective available potential energy (CAPE). Here, sensitivity tests using several versions of the Tiedtke convective scheme are conducted. Typically, the Tiedtke convection scheme gives an improved mean state, intraseasonal variability, space–time power spectra, and eastward propagation compared to the standard version of the model. Coherent eastward propagation of MJO-related precipitation is also much improved, particularly over the Indian–western Pacific Oceans. A composite life cycle of the model MJO indicates that over the Indian Ocean wind-induced surface heat exchange (WISHE) functions, while over the western/central Pacific Ocean aspects of frictional moisture convergence are evident in the maintenance and eastward propagation of the oscillation.

Abstract

We compare the performance of several modes of variability across six U.S. climate modeling groups, with a focus on identifying robust improvements in recent models [including those participating in phase 6 of the Coupled Model Intercomparison Project (CMIP)] compared to previous versions. In particular, we examine the representation of the Madden–Julian oscillation (MJO), El Niño–Southern Oscillation (ENSO), the Pacific decadal oscillation (PDO), the quasi-biennial oscillation (QBO) in the tropical stratosphere, and the dominant modes of extratropical variability, including the southern annular mode (SAM), the northern annular mode (NAM) [and the closely related North Atlantic Oscillation (NAO)], and the Pacific–North American pattern (PNA). Where feasible, we explore the processes driving these improvements through the use of “intermediary” experiments that utilize model versions between CMIP3/5 and CMIP6 as well as targeted sensitivity experiments in which individual modeling parameters are altered. We find clear and systematic improvements in the MJO and QBO and in the teleconnection patterns associated with the PDO and ENSO. Some gains arise from better process representation, while others (e.g., the QBO) from higher resolution that allows for a greater range of interactions. Our results demonstrate that the incremental development processes in multiple climate model groups lead to more realistic simulations over time.

Abstract

This study evaluates the tropical intraseasonal variability, especially the fidelity of Madden–Julian oscillation (MJO) simulations, in 14 coupled general circulation models (GCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Eight years of daily precipitation from each model’s twentieth-century climate simulation are analyzed and compared with daily satellite-retrieved precipitation. Space–time spectral analysis is used to obtain the variance and phase speed of dominant convectively coupled equatorial waves, including the MJO, Kelvin, equatorial Rossby (ER), mixed Rossby–gravity (MRG), and eastward inertio–gravity (EIG) and westward inertio–gravity (WIG) waves. The variance and propagation of the MJO, defined as the eastward wavenumbers 1–6, 30–70-day mode, are examined in detail.

The results show that current state-of-the-art GCMs still have significant problems and display a wide range of skill in simulating the tropical intraseasonal variability. The total intraseasonal (2–128 day) variance of precipitation is too weak in most of the models. About half of the models have signals of convectively coupled equatorial waves, with Kelvin and MRG–EIG waves especially prominent. However, the variances are generally too weak for all wave modes except the EIG wave, and the phase speeds are generally too fast, being scaled to excessively deep equivalent depths. An interesting result is that this scaling is consistent within a given model across modes, in that both the symmetric and antisymmetric modes scale similarly to a certain equivalent depth. Excessively deep equivalent depths suggest that these models may not have a large enough reduction in their “effective static stability” by diabatic heating.

The MJO variance approaches the observed value in only 2 of the 14 models, but is less than half of the observed value in the other 12 models. The ratio between the eastward MJO variance and the variance of its westward counterpart is too small in most of the models, which is consistent with the lack of highly coherent eastward propagation of the MJO in many models. Moreover, the MJO variance in 13 of the 14 models does not come from a pronounced spectral peak, but usually comes from part of an overreddened spectrum, which in turn is associated with too strong persistence of equatorial precipitation. The two models that arguably do best at simulating the MJO are the only ones having convective closures/triggers linked in some way to moisture convergence.