The MJO on the equatorial beta-plane: an eastward propagating Rossby wave induced by meridional moisture advection

Linearized wave solutions on the equatorial beta-plane are examined in the presence of a background meridional moisture gradient. Of interest is a slow, eastward propagating n = 1 mode that is unstable at planetary scales and only exists for a small range of zonal wavenumbers (≲ 6). The mode dispersion curve appears as an eastward extension of the westward propagating equatorial Rossby wave solution. This mode is therefore termed the eastward propagating equatorial Rossby wave (ERW). The zonal wavenumber 2 ERW horizontal structure consists of a low-level equatorial convergence center flanked by quadrupole off-equatorial gyres, and resembles the horizontal structure of the observed MJO. An analytic, leading order dispersion relationship for the ERW shows that meridional moisture advection imparts eastward propagation, and that the smallness of a gross moist stability like parameter contributes to the slow phase speed. The ERW is unstable near planetary scales when low-level easterlies moisten the column. This moistening could come from either zonal moisture advection or surface fluxes or a combination thereof. When westerlies instead moisten the column, the ERW is damped and the westward propagating long Rossby wave is unstable. The ERW does not exist when the meridional moisture gradient is too weak. A moist static energy budget analysis shows that the ERW scale selection is partly due to finite timescale convective adjustment and less effective zonal wind-induced moistening at smaller scales. Similarities in the phase speed, preferred scale and horizontal structure suggest that the ERW is a beta-plane analog of the MJO.

Trends in Tropical Wave Activity from the 1980s to 2016

A frequency–wavenumber power () spectrum was constructed using satellite-derived outgoing longwave radiation (OLR) and brightness temperature for the tropical latitudes. Since the two datasets overlap for over 34 years with nonintersecting sources in error and compare relatively well with each other, it is possible to diagnose trends in the tropical wave activity from the two datasets with confidence. The results suggest a weakening trend in characterized by high interannual variability for wave activity occurring in the low-frequency part of the spectrum and a steady increase in with relatively low interannual variability for wave activity occurring in the high-frequency part of the spectrum. The results show the parts of the spectrum representing the Madden–Julian oscillation and equatorial Rossby wave losing and other parts of the spectrum representing Kelvin waves, mixed Rossby–gravity waves, and tropical disturbance–like wave activity gaining . Similar results were obtained when trends in variance corresponding to the first principal component were produced using spectrally filtered OLR data representative of atmospheric equatorial waves. Spatial trends in the active phase of wave events and the mean duration of events are also shown for the different wave types. Linear trends in for the entire spectrum and regional means in the spectrum corresponding to the abovementioned five wave types with confidence intervals are also presented in the paper. Finally, we demonstrate that El Niño–Southern Oscillation variability does not appear to control the overall spatial patterns and trends observed in the spectrum.

Development of a One-Month-Long, Westward-Propagating Subtropical Low in Boreal Summer

A strong MJO event produced an upper-tropospheric jet streak in northeast Asia and repeated wave breaking in the jet exit region along 150°E during July 1988. A midlatitude low moved equatorward and intensified in the presence of bandpass-filtered (15–100 day) Q vector forcing for upward motion associated with the wave breaking. This forced ascent helped to moisten the atmosphere enough to increase the column water vapor to above 55 mm. This value was sufficiently large to support a self-sustaining low even after the upper forcing weakened. The horizontal scale of the Q vector forcing was about 1500 km, consistent with the scale of most favorable convective response to quasigeostrophic forcing in the subtropics described by Nie and Sobel. The low lasted one month as it moved southwestward, then westward, while remaining north of 20°N. Maximum precipitation along the track of the low exceeded 700 mm, with an anomaly more than 400 mm. A climatology of long-lasting lows was carried out for the monsoon gyre cases studied previously. During El Niño, long-lasting lows often began near the equator in the central Pacific, and were likely to have a mixed Rossby–gravity wave or equatorial Rossby wave structure. It is speculated that the quasi-biweekly mode, the submonthly oscillation, the 20–25-day mode, and the Pacific–Japan pattern are each variations on this kind of event. During La Niña, long-lasting lows that originated in midlatitudes were more common. It is argued that these lows from midlatitudes represent a unique disturbance type in boreal summer.

An Analysis of the Environmental Moisture Impacts of Western North Pacific Tropical Cyclones

The present study examines the environmental moisture anomalies present during western North Pacific tropical cyclone (TC) passage using storm-relative composites. Composited precipitable water anomalies reveal asymmetric anomalies with dry anomalies to the northwest and southwest of the TC and moist anomalies to the east of the TC. Precipitable water anomalies filtered in space and time suggest that the moisture anomalies in the northwest, southwest, and east regions (NWR, SWR, and ER, respectively) are partially due to the TC, while the anomalies in the SWR are also forced by a convectively suppressed Madden–Julian oscillation (MJO) and equatorial Rossby wave (ERW). Composited vertically integrated moisture budgets and backward parcel trajectories reveal that the moisture anomalies in the NWR, SWR, and ER are primarily due to the convergence of climatological mean moisture by the anomalous meridional wind. This convergence is induced by the secondary circulation of the TC in the NWR and ER and by inertial instability induced by the TC, MJO, and ERW in the SWR and ER as also suggested by prior work. Dry anomalies in the NWR are also forced by the advection of moisture by lower-tropospheric northerly wind anomalies associated with the primary circulation of the TC. Together with prior work, these results suggest that TCs can have significant impacts on their large-scale atmospheric environment extending well beyond the spatiotemporal scales of the lower-tropospheric cyclonic circulation of the TC.

Abrupt Transition to Strong Superrotation Driven by Equatorial Wave Resonance in an Idealized GCM

Persistent superrotation is seen in the atmospheres of other terrestrial bodies (Venus, Titan) but not in that of present Earth, which is distinguished by equatorial easterlies. Nevertheless, superrotation has appeared in numerical simulations of Earth’s atmosphere, from two-layer models to multilevel comprehensive GCMs. Simulations of warm climates that generate enhanced tropical convective variability seem particularly prone to superrotation, which has led to hypotheses that the warmer atmospheres of the early Pliocene and Eocene may have been superrotating, and that the phenomenon may be relevant to future climate projections. This paper considers a positive feedback leading to superrotation based on an equatorial wave resonance that occurs in a westerly background flow. The authors present simulations with an idealized multilevel GCM forced with a zonally varying equatorial heating, which show abrupt transitions to strongly superrotating states. Linear shallow water theory is used to show that these transitions occur as the superrotating jet velocity approaches the phase speed of free equatorial Rossby wave modes, leading to a resonant amplification of the response to eddy heating and its associated equatorward momentum flux. The resonance and transition are most prominent in simulations where the meridional temperature gradient has been reduced, and hysteresis behavior is seen when the gradient is eliminated completely. No evidence is found in these simulations for the midlatitude wave feedback believed to drive abrupt transitions in two-layer models, and there is only a minor role for the axisymmetric feedback based on vertical advection by the Hadley circulation.

10–25-Day Intraseasonal Variability of Convection over the Sahel: A Role of the Saharan Heat Low and Midlatitudes

The understanding and forecasting of persistent dry or wet periods of the West African monsoon (WAM), especially those that occur at the intraseasonal time scale, are crucial to improve food management and disaster mitigation in the Sahel region. In the present study, the authors assess how the 10–25-day intraseasonal variability of convection over the Sahel is related to the recently documented intraseasonal variability of the Saharan heat low (SHL) and the associated extratropical circulation. Strongest and most frequent interactions occur when the SHL intraseasonal fluctuations lead those of convection over the Sahel with a 5-day lag. Using a nonlinear event-based approach, such a combination is shown to concern about one-third of Sahelian dry and wet spells and, in the case of dry spells, to yield convective anomalies that are stronger, last longer by at least 2 days, and reach a larger spatial scale. It is then argued that the 10–25-day intraseasonal variability of convection over the Sahel can be partly explained by the midlatitude intraseasonal variability, through a major role played by the SHL. The anomalous midlevel circulations observed during Sahelian wet and dry events can be shifted from the midlatitudes, which provide a complementary mechanism to that invoking equatorial Rossby wave dynamics. These two mechanisms are likely to interfere together in a constructive or destructive way, leading to high temporal and spatial variability of the Sahelian dry and wet spells. As a particular intraseasonal event, the WAM onset is shown to be clearly favored by phases of the SHL intraseasonal variability, when the Mediterranean ventilation is weakened and the SHL is able to strengthen. Conversely, the formation of a strong cold air surge over Libya and Egypt and its propagation toward the Sahel lead to the collapse of the SHL, which inhibits the WAM onset. From these extratropical–tropical interactions, more skillful forecasts of the Sahelian wet and dry spells and of the WAM onset can be expected. In particular, the monitoring of both the SHL intraseasonal activity and that of the equatorial Rossby wave should provide relevant information to forecast at least two-thirds of such high-impact events.

Annual Reversal of the Equatorial Intermediate Current in the Pacific: Observations and Model Diagnostics

A large reversal of zonal transport below the thermocline was observed over a period of 6 months in the western Pacific Ocean between 2°S and the equator [from 26.2 Sv (1 Sv ≡ 106 m3 s−1) eastward in October 1999 to 28.6 Sv westward in April 2000]. To document this reversal and assess its origin, an unprecedented collection of ADCP observations of zonal currents (2004–06), together with a realistic OGCM simulation of the tropical Pacific, was analyzed. The results of this study indicate that this reversal is the signature of intense annual variability in the subsurface zonal circulation at the equator, at the level of the Equatorial Intermediate Current (EIC) and the Lower Equatorial Intermediate Current (L-EIC). In this study, the EIC and the L-EIC are both shown to reverse seasonally to eastward currents in boreal spring (and winter for the L-EIC) over a large depth range extending from 300 m to at least 1200 m. The peak-to-peak amplitude of the annual cycle of subthermocline zonal currents at 165°E in the model is ∼30 cm s−1 at the depth of the EIC, and ∼20 cm s−1 at the depth of the L-EIC, corresponding to a mass transport change as large as ∼100 Sv for the annual cycle of near-equatorial zonal transport integrated between 2°S and 2°N and between 410- and 1340-m depths. Zonal circulations on both sides of the equator (roughly within 2° and 5.5° in latitude) partially compensate for the large transport variability. The main characteristics of the annual variability of middepth modeled currents and subsurface temperature (e.g., zonal and vertical phase velocities, meridional structure) are consistent, in the OGCM simulation, with the presence, beneath the thermocline, of a vertically propagating equatorial Rossby wave forced by the westward-propagating component of the annual equatorial zonal wind stress. Interannual modulation of the annual variability in subthermocline equatorial transport is discussed.