Abstract

The South Pacific convergence zone (SPCZ) exhibits well-known spatial displacements in response to anomalous sea surface temperatures (SSTs) associated with the El Niño–Southern Oscillation (ENSO). Although dynamic and thermodynamic changes during ENSO events are consistent with observed SPCZ shifts, explanations for these displacements have been largely qualitative. This study applies a theoretical framework based on generalizing arguments about the relationship between the zonal-mean intertropical convergence zone (ITCZ) and atmospheric energy transport (AET) to 2D, permitting quantification of SPCZ displacements during ENSO. Using either resolved atmospheric energy fluxes or estimates of column-integrated moist energy sources, this framework predicts well the observed SPCZ shifts during ENSO, at least when anomalous ENSO-region SSTs are relatively small. In large-amplitude ENSO events, such as the 1997/98 El Niño, the framework breaks down because of the large change in SPCZ precipitation intensity. The AET framework permits decomposition of the ENSO forcing into various components, such as column radiative heating versus surface turbulent fluxes, and local versus remote contributions. Column energy source anomalies in the equatorial central and eastern Pacific dominate the SPCZ shift. Furthermore, although the radiative flux anomaly is larger than the surface turbulent flux anomaly in the SPCZ region, the radiative flux anomaly, which can be viewed as a feedback on the ENSO forcing, accounts for slightly less than half of SPCZ precipitation anomalies during ENSO. This study also introduces an idealized analytical model used to illustrate AET anomalies during ENSO and to obtain a scaling for the SPCZ response to an anomalous equatorial energy source.

Abstract

The decay characteristics of a mixed layer ocean passively coupled to an atmospheric model are important to the response of the climate system to stochastic or external forcing. Two salient features of such decay—the spatial-scale dependence of sea surface temperature anomaly (SSTA) decay time scales and the spatial inhomogeneities of SSTA decay modes—are addressed using intermediate-level complexity and simple analytic models of the tropical atmosphere. As expected, decay time scales increase with the spatial extent of the SSTA. Most modes decay rapidly—with characteristic decay times of 50–100 days for a 50-m mixed layer—with the decay determined by local surface flux adjustment. Only those modes with spatial scales approaching or larger than the tropical basin scale exhibit decay time scales distinctively longer than the local decay, with the decay time scale of the most slowly decaying mode of the order of 250–300 days in the tropics (500 days globally). Simple analytic prototypes of the spatial-scale dependence and the effect of basic-state inhomogeneities, especially the impact of nonconvecting regions, elucidate these results. Horizontal energy transport sets the transition between fast, essentially local, decay time scales and the slower decay at larger spatial scales; within the tropics, efficient wave dynamics accounts for the small number of slowly decaying modes. Inhomogeneities in the basic-state climate, such as the presence or absence of mean tropical deep convection, strongly impact large-scale SSTA decay characteristics. For nonconvecting regions, SSTA decay is slow because evaporation is limited by relatively slow moisture divergence. The separation of convecting- and nonconvecting-region decay times and the closeness of the slower nonconvecting-region decay time scale to the most slowly decaying modes cause a blending between local nonconvecting modes and the large-scale modes, resulting in pronounced spatial inhomogeneity in the slow decay modes.

Abstract

One theorized control on the position of the South Pacific convergence zone (SPCZ) is the amount of low-level inflow from the relatively dry southeastern Pacific basin. Building on an analysis of observed SPCZ region synoptic-scale variability by Lintner and Neelin, composite analysis is performed here on two reanalysis products as well as output from 17 models in phase 5 of the Coupled Model Intercomparison Project (CMIP5). Using low-level zonal wind as a compositing index, it is shown that the CMIP5 ensemble mean, as well as many of the individual models, captures patterns of wind, specific humidity, and precipitation anomalies resembling those obtained for reanalysis fields between weak- and strong-inflow phases. Lead–lag analysis of both the reanalyses and models is used to develop a conceptual model for the formation of each composite phase. This analysis indicates that an equatorward-displaced Southern Hemisphere storm track and an eastward-displaced equatorial eastern Pacific westerly (wind) duct are features of the weak-inflow phase although, as indicated by additional composite analyses based on these features, each appears to account weakly for the details of the low-level inflow composite anomalies. Despite the presence of well-known biases in the CMIP5 simulations of the SPCZ region climate, the models appear to have some fidelity in simulating synoptic-scale relationships between low-level winds, moisture, and precipitation, consistent with observations and simple theoretical understanding of interactions of dry air inflow with deep convection.

Abstract

The edges or margins of tropical convective zones are hypothesized to be sensitive to low-level inflow conditions. The present study evaluates where and to what extent convective margin variability is sensitive to low-level inflow variability using observed precipitation and reanalysis wind and total precipitable water data over the tropical South America–Atlantic sector in austral summer. Composite analysis based on an inflow measure defined by projecting low-level monthly-mean atmospheric boundary layer (ABL) or lower free troposphere (LFT) winds onto either mean horizontal precipitation or precipitable water gradients shows widespread contraction of the edges of convection zones in the direction of stronger convection for anomalously strong low-level inflow; such behavior is consistent with enhanced import of relatively dry air along the edges of convection zones. However, the distinction between ABL and LFT winds may be significant regionally, for example, along the Atlantic ITCZ’s northern margin. Back trajectory analysis is employed to estimate source regions of low-level air masses arriving at margin points over time scales (2–4 days) during which low-level air masses are expected to retain some memory of initial moisture conditions while also undergoing diabatic modification. Probability distribution functions of mean precipitation values encountered along trajectories facilitate objective quantification of the frequency with which trajectories approach the margin from drier areas outside the convection zone. While margin points in the ABL are strongly dominated by inflow (i.e., trajectories originating outside of the convection zone), points in the LFT may show inflow, outflow, or mixed inflow–outflow conditions. LFT locations dominated by inflow trajectories generally correspond to regions with composites exhibiting the clearest signatures of LFT wind variability on precipitation.

Abstract

Understanding multiscale rainfall variability in the South Pacific convergence zone (SPCZ), a southeastward-oriented band of precipitating deep convection in the South Pacific, is critical for both the human and natural systems dependent on its rainfall, and for interpreting similar off-equatorial diagonal convection zones around the globe. A k-means clustering method is applied to daily austral summer (December–February) Tropical Rainfall Measuring Mission (TRMM) satellite rainfall to extract representative spatial patterns of rainfall over the SPCZ region for the period 1998–2013. For a k = 4 clustering, pairs of clusters differ predominantly via spatial translation of the SPCZ diagonal, reflecting either warm or cool phases of El Niño–Southern Oscillation (ENSO). Within each of these ENSO phase pairs, one cluster exhibits intense precipitation along the SPCZ while the other features weakened rainfall. Cluster temporal behavior is analyzed to investigate higher-frequency forcings (e.g., the Madden–Julian oscillation and synoptic-scale disturbances) that trigger deep convection where SSTs are sufficiently warm. Pressure-level winds and specific humidity from the Climate Forecast System Reanalysis are composited with respect to daily cluster assignment to investigate differences between active and quiescent SPCZ conditions to reveal the conditions supporting enhanced or suppressed SPCZ precipitation, such as low-level poleward moisture transport from the equator. Empirical orthogonal functions (EOFs) of TRMM precipitation are computed to relate the “modal view” of SPCZ variability associated with the EOFs to the “state view” associated with the clusters. Finally, the cluster number is increased to illustrate the change in TRMM rainfall patterns as additional degrees of freedom are permitted.

Abstract

An idealized prototype for the location of the margins of tropical land region convection zones is extended to incorporate the effects of soil moisture and associated evaporation. The effect of evaporation, integrated over the inflow trajectory into the convection zone, is realized nonlocally where the atmosphere becomes favorable to deep convection. This integrated effect produces “hot spots” of land surface–atmosphere coupling downstream of soil moisture conditions. Overall, soil moisture increases the variability of the convective margin, although how it does so is nontrivial. In particular, there is an asymmetry in displacements of the convective margin between anomalous inflow and outflow conditions that is absent when soil moisture is not included. Furthermore, the simple cases presented here illustrate how margin sensitivity depends strongly on the interplay of factors, including net top-of-the-atmosphere radiative heating, the statistics of inflow wind, and the convective parameterization.

Abstract

The applicability of a weak temperature gradient (WTG) formulation for the reorganization of tropical climate during El Niño–Southern Oscillation (ENSO) events is investigated. This idealized dynamical framework solves for the divergent portion of the tropical circulation by assuming a spatially homogeneous perturbation temperature profile and a mass balance constraint applied over the tropical belt. An intermediate-level complexity model [the Quasi-Equilibrium Tropical Circulation Model (QTCM)] configured with the WTG assumptions is used to simulate El Niño conditions and is found to yield an appropriate level of tropospheric warming, a plausible pattern of precipitation anomalies in the tropical Pacific source region of El Niño, and a gross precipitation deficit over the Tropics outside the Pacific (hereafter the “remote Tropics”). Additional tests of the WTG framework with La Niña forcing conditions and enhanced greenhouse gas concentrations support its applicability. However, the ENSO response under the WTG framework fails in some respects when compared to the standard QTCM: in particular, some regional features of the anomalous precipitation response, especially in the remote Tropics, differ markedly between the two model versions. These discrepancies appear to originate in part from the lack of anomalous tropospheric temperature gradients (and circulations) in the framework presented here. Nevertheless, the WTG approach appears to be a useful lowest-order model for the tropical climate adjustment to ENSO. The WTG framework is also used to argue that El Niño may not represent a good proxy for tropical rainfall changes under greenhouse gas warming scenarios because the large-scale subsidence occurring with the tropospheric warming in the El Niño scenario has an effect on rainfall that is distinct from the effect of increased tropospheric temperatures common to both the greenhouse gas warming and El Niño scenarios.

Abstract

The adjustment of the tropical climate outside the Pacific (the “remote Tropics”) to the abrupt onset of El Niño conditions is examined in a tropical atmosphere model that assumes simplified vertical structure and quasi-equilibrium (QE) convective closure. The El Niño signal is rapidly (∼1 week) communicated to the remote Tropics via an eastward-propagating Kelvin-like wave that induces both anomalous subsidence and tropospheric warming. Widespread reductions in convective precipitation occur in conjunction with the spreading of the temperature and subsidence anomalies. The remote rainfall suppression peaks roughly 5–15 days after the initiation of El Niño conditions, after which the anomalous remote rainfall field recovers to a state characterized by a smaller remote areal mean rainfall deficit and the appearance of localized positive rainfall anomalies. The initial remote precipitation reduction after El Niño onset is tied to both tropospheric warming (i.e., stabilization of the troposphere to deep convection) and the suppression of remote humidity levels; recovery of the initial deficits occurs as feedbacks modulate the subsequent evolution of humidity anomalies in the tropospheric column. Apart from the short-term response, there is a longer-term adjustment of the remote climate related to the thermal inertia of the underlying surface: surface thermal disequilibrium, which is related to the depth of the ocean mixed layer, maintains larger precipitation deficits than would be expected for equilibrated conditions. This result supports a previous prediction by one of the authors for a significant disequilibrium mechanism in the precipitation teleconnection to El Niño resulting from the local vertical coupling of the troposphere to the surface through moist convection.

Abstract

The authors demonstrate through atmospheric general circulation model (the Community Climate Model version 3.10) simulations of the 1997/98 El Niño that the observed “remote” (i.e., outside the Pacific) tropical land and ocean surface warming appearing a few months after the peak of the El Niño event is causally linked to the Tropics-wide warming of the troposphere resulting from increased atmospheric heating in the Pacific, with the latter acting as a conduit for the former. Unlike surface temperature, the surface flux behavior in the remote Tropics in response to El Niño is complex, with sizable spatial variation and compensation between individual flux components; this complexity suggests a more fundamental control (i.e., tropospheric temperature) for the remote tropical surface warming. Over the remote oceans, latent heat flux acting through boundary layer humidity variations is the important regulator linking the surface warming in the model simulations to the tropospheric warming over the remote tropical oceans. Idealized 1997/98 El Niño simulations using an intermediate tropical circulation model (the Quasi-Equilibrium Tropical Circulation Model) in which individual surface fluxes are directly manipulated confirms this result. The findings over the remote ocean are consistent with the “tropospheric temperature mechanism” previously proposed for the tropical ENSO teleconnection, with equatorial planetary waves propagating tropospheric temperature anomalies from the eastern Pacific to the remote Tropics and moist convective processes mediating the troposphere-to-remote-surface connection. The latter effectively requires the boundary layer moist static energy to vary in concert with the free tropospheric moist static energy. Over the remote land regions, idealized model simulations suggest that sensible heat flux regulates the warming response to El Niño, though the underlying mechanism has not yet been fully determined.

Abstract

A model unifying the representation of the planetary boundary layer and dry, shallow, and deep convection, the probabilistic plume model (PPM), is presented. Its capacity to reproduce the triggering of deep convection over land is analyzed in detail. The model accurately reproduces the timing of shallow convection and of deep convection onset over land, which is a major issue in many current general climate models.

PPM is based on a distribution of plumes with varying thermodynamic states (potential temperature and specific humidity) induced by surface-layer turbulence. Precipitation is computed by a simple ice microphysics, and with the onset of precipitation, downdrafts are initiated and lateral entrainment of environmental air into updrafts is reduced.

The most buoyant updrafts are responsible for the triggering of moist convection, causing the rapid growth of clouds and precipitation. Organization of turbulence in the subcloud layer is induced by unsaturated downdrafts, and the effect of density currents is modeled through a reduction of the lateral entrainment. The reduction of entrainment induces further development from the precipitating congestus phase to full deep cumulonimbus.

Model validation is performed by comparing cloud base, cloud-top heights, timing of precipitation, and environmental profiles against cloud-resolving models and large-eddy simulations for two test cases. These comparisons demonstrate that PPM triggers deep convection at the proper time in the diurnal cycle and produces reasonable precipitation. On the other hand, PPM underestimates cloud-top height.