Tropical Western Pacific Warm Pool Research Articles

Moisture Variability over the Tropical Western Pacific Ocean

In this paper, we study the characteristics of moisture variability over the tropical western Pacific warm pool region using sounding data obtained through the use of research vessels. Data used here were collected in both boreal summer and winter seasons over a nine-year period from 1993 through 2001. Sounding data are categorized into five zonal wind vertical shear modes, and then examined for common and singular features of moisture variability. The results show frequent occurrences of high relative humidity centered around 945 hPa, and 560 hPa, for any wind shear conditions, as previously found in the Tropical Ocean and Global Atmosphere/ Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) studies. We also found another small peak at 845 hPa in low-level easterly conditions. The degree of these features is modulated by wind shear mode differences. An interesting feature is that the variation of total precipitable water is well represented by the synoptic scale mid-tropospheric moisture variation. However, a close relation between the total moisture budget and the moisture variation in the boundary layer is also obtained for a time scale of less than a few days (especially the diurnal cycle). We investigated extremely dry air conditions as one factor modulating mid-tropospheric moisture. Backward air parcel trajectories show that there are two major routes for the intrusion of dry air over the warm pool. In the first, the air parcel could be traced back to higher latitudes and the upper troposphere. For this type, anticyclonic equatorward flow associated with developing mid-latitude baroclinic waves, such as Rossby wave breaking, is responsible for injecting subtropical dry air into the Tropics—as proposed by Yoneyama and Parsons for TOGA COARE dry events. In the second type, the air parcel could be traced back to the upper troposphere, but within lower latitudes in the central-eastern Pacific. For this case, the air parcel originates from the trade winds that passed over the region where dry air was injected by the anticyclonic equatorward flow associated with the baroclinic wave development in the subtropical region. The relationship between dry air conditions and large-scale features is also discussed.

Super ENSO and global climate oscillations at millennial time scales.

The late Pleistocene history of seawater temperature and salinity variability in the western tropical Pacific warm pool is reconstructed from oxygen isotope (delta18O) and magnesium/calcium composition of planktonic foraminifera. Differentiating the calcite delta18O record into components of temperature and local water delta18O reveals a dominant salinity signal that varied in accord with Dansgaard/Oeschger cycles over Greenland. Salinities were higher at times of high-latitude cooling and were lower during interstadials. The pattern and magnitude of the salinity variations imply shifts in the tropical Pacific ocean/atmosphere system analogous to modern El Niño-Southern Oscillation (ENSO). El Niño conditions correlate with stadials at high latitudes, whereas La Niña conditions correlate with interstadials. Millennial-scale shifts in atmospheric convection away from the western tropical Pacific may explain many paleo-observations, including lower atmospheric CO2, N2O, and CH4 during stadials and patterns of extratropical ocean variability that have tropical source functions that are negatively correlated with El Niño.

Layer inflow into precipitating convection over the western tropical Pacific

AbstractA conceptual model of tropical convection frequently used in convective parametrization schemes is that of a parcel process in which boundary‐layer air, characterized by high equivalent potential temperature, ascends to great heights in convective updraughts, while air above the planetary boundary layer with lower equivalent potential temperature mixes into convective downdraughts and sinks. However, airborne Doppler‐radar data show that organized deep convective systems over the western tropical Pacific warm pool are often characterized by layers of ascending inflow ∼0.5–4 km in depth. These inflow layers do not consist merely of boundary‐layer air.In this study a high‐resolution numerical cloud model is employed to investigate these inflow layers. Input data are from the Tropical Ocean–Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). Two time periods are selected in December 1992, which represent the onset and peak of a strong westerly phase of the intraseasonal oscillation. Model simulations for 14 December, representative of westerly onset conditions and the beginning of a convectively active period, and 23 December, representative of strong low‐level westerlies and extremely widespread convection, are conducted. To bridge the coarse resolution of the global‐model analysis fields and the fine resolution of the cloud model, hourly output from a mesoscale model is used to supply initial and lateral boundary conditions for the cloud model.Control simulations of 14 and 23 December reveal distinct convective organizations and, specifically, inflow characteristics for the two systems embedded in different large‐scale environmental conditions. In the case of 23 December, convection simulated under conditions of the strong westerly wind and near‐saturated low‐mid troposphere exhibits deep inflow layers. Trajectories computed from the simulation of 23 December under these conditions show a strong layer‐lifting inflow signal. In contrast, the control simulation of 14 December shows a parcel‐like inflow with only the air in the lower part of the inflow actually rising in the deep convective updraughts. One of the main differences between the two simulations is the lack of a deep environmental moist layer in the 14 December case. The control simulation did not capture well the extent of precipitating mesoscale stratiform clouds that developed from earlier convection in the vicinity of the deep convective cells as indicated in the COARE observations. Previous studies have shown that spatially extensive convection is correlated with enhanced mid‐level humidity. To isolate the effect of the mid‐level moist layer on the characteristics of inflow of convective systems, a numerical experiment based on the control simulation of 14 December was conducted. The relative humidity of environment air in the low‐mid troposphere (1.7–6 km layer) was increased to 95%. Trajectory statistics calculated for this sensitivity experiment show increased layer lifting, with a significant amount of air from the upper part of the inflow layer rising in the updraught along with air from just above the surface. Moistening the inflow layer in this sensitivity experiment allows it to saturate more quickly when it encounters the mesoscale cold pool. Once saturated, the relevant static stability is the moist rather than dry static stability, and the whole layer more easily rises over the cold pool. Moistening the inflow layer also modifies the nature of the simulated cold pool itself, which seems to promote layer lifting in the simulation. Possible mechanisms for moistening the mid‐levels are briefly discussed. Copyright © 2002 Royal Meteorological Society

Precursory SST anomalies associated with the convection over the western Pacific warm pool

The convection over the tropical western Pacific warm pool influences significantly the atmospheric circulation and climate in East Asia. Thus, the precursory signals of the convection may be used in the forecast of summer climate in China. According to the present results, the June-July-August (JJA) mean convection intensity over the warm pool is significantly related to the precursory positive and negative sea surface temperatures (SSTs) in the warm pool and in the equatorial central and eastern Pacific, respectively. It is also related to the simultaneous negative surface temperatures west to the Philippines. The analysis on the SSTs associated with the convection over the warm pool in individual month of summer shows that for the convection in June and July, there are precursory SST signals in the warm pool and the equatorial central and eastern Pacific. Therefore, this study shows that only the convection in June and July, rather than that in August, has precursory SST signals, despite the existence of the precursory signals of the JJA mean convection. Accordingly, it is implied that the interaction among the warm pool, equatorial central and eastern Pacific, and the region west to the Philippines may exhibit distinct features in the precursory period (preceding winter and spring) and in the simultaneous period.

Climatological characteristics of the tropical tropopause as revealed by radiosondes

A temporally and spatially comprehensive depiction of the tropical tropopause is presented, based on radiosonde data from 83 stations. Climatological statistics for 1961–1990 are computed for three levels: the conventional lapse‐rate tropopause (LRT), the cold‐point tropopause (CPT), and the 100 hPa level. Mean values and seasonal and interannual variations of temperature, pressure, height, potential temperature, and water vapor saturation mixing ratio at these levels are compared. The tropopause is higher, colder, and at lower pressure in the Northern Hemisphere (NH) than in the Southern Hemisphere (SH) in NH winter. This pattern reverses in NH summer, except that the tropopause remains colder in the NH than in the SH. The climatological locations of minimum tropopause temperature differ from those of maximum height and minimum pressure: In NH winter the tropopause is coldest over the western tropical Pacific warm pool region, but it is highest and at lowest pressure over the western Atlantic. Correlations of interannual anomalies in zonal‐mean characteristics reveal that the height of the tropopause reflects the temperature of the underlying troposphere. Tropopause temperature, on the other hand, shows little association with tropospheric characteristics but is significantly correlated with the temperature and pressure of the lower stratosphere. The 100 hPa level is a poor surrogate for the tropical tropopause. Changes in radiosonde instrumentation limit the potential for detecting tropopause trends. However, the following (nonmonotonic) trends in the tropopause in the deep tropics during 1978–1997 seem robust: an increase in height of about 20 m decade−1, a decrease in pressure of about 0.5 hPa decade−1, a cooling of about 0.5 K decade−1, little change in potential temperature, and a decrease in saturation volume mixing ratio of about 0.3 ppmv decade−1.

Atmospheric circulations and sea surface temperatures related to the convection over the western pacific warm pool on the interannual scale

The difference is examined in atmospheric circulation and Sea Surface Temperatures (SSTs) in the trop-ics and subtropics between weak and strong convection over the tropical western Pacific warm pool (signified as WPWP). The WPWP is chosen as the region (110–160°E, 10–20°N), where the Outgoing Longwave Radiation (OLR) shows a great year-to-year variance. A composite study was carried out to examine the differences in atmospheric circulation and SSTs between weak and strong convection over WPWP. First, NCEP/NCAR re-analysis data and satellite-observed OLR data are used to examine the differences. ERA data, in which the OLR data are calculated, are then used for re-examination.

Interannual Variability of the Summertime North Pacific Subtropical High and its Relation to Atmospheric Convection over the Warm Pool.

Using the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis data and satellite-observed outgoing long-wave radiation (OLR) data, we examined the westward extension and eastward contraction of the North Pacific subtropical high in summer (NPSH). It was found that the NPSH shows a great variability in its western extent, both on the seasonal and interannual time scales. In order to examine the interannual variations of NPSH, we defined a NPSH index as the June-July-August (JJA) mean geopotential height anomalies at 850 hPa averaged over the west edge (110∼150°E, 10∼30°N) of NPSH. This index describes the year-to-year zonal displacement of NPSH. Composite analysis based on this NPSH index showed that there is a significant relation between zonal displacement of NPSH and intensity of atmospheric convection over the warm pool. A low-level cyclonic (anticyclonic) anomaly that is closely associated with the zonal shift of NPSH appears north of enhanced (weakened) atmospheric convection, i.e., the vorticity anomaly is found north of the divergence one. Climatologically, the NPSH contracts eastward swiftly after pentad 40 (July 15 to 19). Such an eastward contraction is closely associated with the poleward shift of both NPSH and atmospheric convection over the tropical western Pacific warm pool. However, such seasonal variations of both NPSH and convection show distinct features between the summers with positive and negative NPSH indexes. During summers with positive NPSH index, NPSH and convection over the warm pool do not show an appreciable seasonal evolution. During summers with negative index, by contrast, they show a swift seasonal evolution after pentad 40.Finally, we performed a vorticity analysis to explain the relation between the divergence and vorticity anomalies on the interannual time scale. The analysis shows that in the lower troposphere (925 hPa), the advection of relative vorticity is comparable to the stretching and is responsible for the northward shift of the circulation anomaly relative to anomalous atmospheric convection. The difference from the theory of Gill (1980) is discussed. In the upper troposphere (200 hPa), the advection is slightly smaller than the stretching with opposite signs in East Asia and the western North Pacific, and thus the position of the vorticity anomaly is consistent with that of the stretching anomaly.

Westward Extension of North Pacific Subtropical High in Summer.

The basic features of the North Pacific subtropical anticyclone in summer are studied using the European Centre for Medium Range Weather Forecasts (ECMWF) Re-Analysis data for the period (1979-1993). It has been shown that the North Pacific subtropical high displays substantial interannual variability at its western edge. Further analysis indicates a significant relationship between the zonal variation of the subtropical high, and the variation in the intensity of atmospheric convection over the warm pool. The stronger (weaker) convection over the warm pool is associated with more eastward (westward) position of the western Pacific subtropical high. The UK Universities’ Global Atmospheric Modelling Programme General Circulation Model (UGAMP GCM) is used to examine the influence of the local SST anomalies, and the associated anomalous convection over the tropical western Pacific warm pool on the zonal displacement of the North Pacific subtropical high. The model results show that the suppressed atmospheric convection caused by lower SSTs in the warm pool results in anomalous anticyclonic circulation in lower level over the subtropical western Pacific, and westward extension of the subtropical high. The results are consistent with the composite results based on the reanalysis data, and imply the SST anomalies off-equator over the western Pacific play an important role in the zonal shift of the North Pacific subtropical high in summer. Despite the fact of good agreements in many aspects between the model simulations and composite analyses based on observation, there are some differences between the two especially in terms of upper-level extratropical circulation anomalies over East Asia.

An MJO-like Gravity Wave and Superclusters Simulated in a Two-Dimensional Cumulus-Scale-Resolving Model under a Warm Pool Condition.

To further our understanding of the large-scale convection organization in the tropics, including super cloud clusters and the Madden-Julian oscillation (MJO), a 120-day long integration is performed using a 2D (zonal-height) cumulus-scale-resolving model. The periodic horizontal domain spans 40, 000 km and mimics the equatorial circumference. A zonal inhomogeneity of the sea surface temperature (SST) is taken into account by allowing the SST to vary between 302 K over the central area of 5, 000 km width and lower value outside with its minimum at 299 K. Such a computational setup provides an analogue of the tropical western Pacific warm pool. Due to computational limitations, we employ a cloud-resolving horizontal grid of 1 km only in the warm-pool area of 10, 000 km scale, and a stretched coarser grid across the rest of the domain. The initial surface flow is taken to be an easterly at 5 m s-1 which makes zonal asymmetry allowing the Wind-Induced Surface Heat Exchange (WISHE) set in. The numerical simulation features a large-scale gravity wave of zonal wavenumber 1, referred to as the MJO-like wave, that propagates eastward with the phase velocity in the range of 8 to 15 m s-1 and travels across the computational domain in 40 to 60 days. The overall evolution of convection exhibits two organization patterns: i) an eastward-propagating convection (EPC), and ii) a quasi-stationary convection (QSC). EPC is identified as an envelope of westward-moving cloud clusters of O (100 km) scale which develop successively to the east of the existing clusters in accord with the propagation of the upward motion of the MJO-like wave. QSC has a horizontal scale in excess of several thousands of kilometers and it is associated with the existence of the warm pool. Propagation of the MJO-like wave modulates QSC quasi-periodically. The convection in the active regime of QSC has an extent of O (1, 000 km), propagates west-to-east, and involves a hierarchy of organization similar to that in Oouchi (1999). It is identified as a super cloud cluster. It is argued that QSC can provide a basic framework for explaining the slow phase velocity (3 to 10 m s-1) of the super cloud cluster. The governing mechanism in the active phase of the super cloud cluster and MJO-like wave can be viewed as wave-CISK, with WISHE playing an important role in driving the eastward movement of the super cloud cluster. Similarities and differences in the wave-CISK mechanism between this study and previous investigations using parameterized convection are discussed.

Convection over the Pacific Warm Pool in relation to the Atmospheric Kelvin-Rossby Wave*

Abstract Deep convection over the western tropical Pacific warm pool is analyzed in terms of its relation to the atmospheric Kelvin–Rossby wave, which dominates the large-scale flow during the austral summer. The study uses Doppler radar data collected by aircraft and ship radars during different time periods in the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment to characterize the mesoscale circulations of organized convective cloud systems occurring throughout the season. The study focuses on convection in two contrasting phases of the wave: the “westerly onset region” just west of the point within the wave where low-level easterlies change to westerlies, and the “strong westerly region” (or “westerly wind burst”) lying between the large-scale counterrotating gyres of the Kelvin–Rossby wave. In the westerly onset region the zonal wind component had midlevel easterlies overlying low-level westerlies. In the strong westerly region a deep layer of westerlies extended from the...

Radiative–Convective Processes in Simulated Diurnal Variations ofTropical Oceanic Convection

Abstract This paper presents an analysis of the diurnal variation of tropical oceanic convection and its associated energy cycle as simulated by an anelastic cumulus ensemble model. The model includes subgrid turbulence, cloud microphysics, and radiative transfer processes. In two experiments designed to simulate the diurnal cycles in large-scale disturbed (A1) and undisturbed conditions (A4) over the tropical western Pacific warm pool, the model produces diurnal variations that are in general agreement with observations. In A1, a time-independent SST and mean ascending motion are prescribed in the model. The model generates a diurnal cycle with positive (negative) rainfall anomalies during the night (day), and the maximum (minimum) rainfall near 0200 (1300–1400) local time. In A4, a diurnally varying SST is prescribed in the model and the domain-averaged vertical velocity is constrained to be zero. The simulated diurnal variations still have a nocturnal rainfall maximum but with a weaker magnitude and a ...

Simulation of the Tropical Pacific Warm Pool with the NCAR Climate System Model*

Abstract The simulation of the tropical western Pacific warm pool is explored with the NCAR Climate System Model (CSM). The simulated sea surface temperatures in the Pacific basin have biases that are similar to other coupled model simulations in this region. In particular, an excessive cold tongue of water extends across the Pacific basin, with warm water on either side of this cold tongue. The warm pool region is also too cold. This cold bias exists in spite of an overestimate in surface net energy flux into this region. To understand the source of this bias in SST, simulations from the uncoupled and fully coupled models are analyzed in terms of biases in surface energy budget. These analyses suggest that the strong constraint of little ocean heat transport out of the warm pool region forces a change in SST gradient that leads to an increase in the atmospheric zonal wind. This increase in zonal wind causes an increase in latent heat flux in the warm pool region. The increase in latent heat flux is requi...

The 600–800-mb Minimum in Tropical Cloudiness Observed during TOGA COARE

A minimum in cloud coverage occurring between 800 and 600 mb can be inferred from soundings taken within the tropical western Pacific warm pool. Surface observations of clouds and satellite-derived outgoing longwave radiation values suggest that the cloud minimum in the 600–800-mb layer occurs in all weather conditions. One explanation for the enhanced occurrence of clouds above (in the 400–600-mb layer) and their diminished occurrence within the 600–800-mb layer is a preferential cloud detrainment from convection into the more stable levels existing at pressures below 600 mb and above 800 mb. This mechanism is supported by the results of a buoyancy-sorting model.

Variability of Midtropospheric Moisture and Its Effect on Cloud-Top Height Distribution during TOGA COARE*

The tropical western Pacific warm pool is often generalized to be a region of heavy precipitation. This concept is useful in constructing simplified models of the tropical circulation. However, the warm pool region is often punctuated by periods of little rain. Such drought periods may last up to 10 days over an area of at least 6 3 105 km2. Other common features of the drought periods include an extremely dry midtroposphere, few deep clouds typically associated with mesoscale convective systems, and a substantial amount of clouds that are too tall to be categorized as trade cumuli but too short to fall into the category of deep convective clouds. Midtropospheric moisture varies substantially (60% in relative humidity ,4gk g 21 in water vapor mixing ratio) between rainy and drought periods. The frequency distributions of humidity exhibit bimodal structures at certain levels above the freezing level. In either rainy or drought periods, or in a long period including both, the time-mean humidity above the boundary layer deviates substantially from the most frequent profile of humidity, defined as the relative humidity corresponding to the maximum frequency distribution at each level. Mean soundings, therefore, do not accurately represent the overall vertical structure of moisture in the warm pool. Calculations of a simple parcel model demonstrate that the warm pool atmosphere above the boundary layer can be dry enough to discourage the growth of deep convective clouds by depleting parcel buoyancy through entrainment. These results were drawn from an analysis of soundings collected during the Intensive Observing Period (1 November 1992‐28 February 1993) of the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment.

Tropical Inversions near the 0°C Level

Abstract Soundings taken from the tropical western Pacific warm pool region during TOGA COARE reveal the common occurrence of temperature and moisture perturbations near the 0°C level. The perturbations frequently are characterized by shallow layers of increased stability (or occasionally temperature inversions) and reversals or inflections in the vertical profile of specific humidity. Similar temperature and moisture inversions have been observed elsewhere in the Tropics and midlatitudes but have not received much attention. Isothermal layers are known to exist just below the melting level in stratiform rain regions; however, not all stable layers observed over the warm pool are confined to precipitation systems. The perturbation in the specific humidity profile accounts for the often-observed double-peak structure in the apparent moisture sink Q2 in tie Tropics. Stratification of the data based on relative humidity criteria indicates that the stable layers near the 0°C level generally fall into two main...

The Sensitivity of Surface Energy Balance to Convective Parameterization in a General Circulation Model

This study investigates the interaction between convection and the surface energy fluxes, and its sensitivity to convective parameterization schemes using a general circulation model. Two simulations of the global circulation averaged annually from 1 June 1985 to 31 May 1986 are performed, with particular emphasis on the tropical Pacific. In the control simulation, a convective scheme that parameterizes convection based on low-level moisture convergence is used. A second experiment employs a parameterization scheme that uses the time rate of change of convective available potential energy (CAPE) to determine convection. When the low-level moisture convergence is used as the closure for convective parameterization, convection and low-level convergence occur near the landmass of Southeast Asia. The large-scale circulation is such that a fairly strong surface wind that provides moisture to fuel convection is located in the western tropical Pacific warm pool regions, giving rise to relatively high latent heat flux there. When the time rate of change of CAPE is used to close convective parameterization, convection and its associated low-level large-scale convergence and weak surface, wind speed occur in the warm pool region, resulting in low latent heat flux there. Response of surface solar radiative flux to convection is found to be the largest in the surface energy budget. Convection affects surface radiation through the generation of clouds. In the experiment, more clouds are produced over the tropical oceans, leading to less solar radiation received on the surface. More clouds in the experiment also load to less net emission of longwave radiation from the ocean surface due to the cloud green-house effect albeit the magnitude is much smaller than that for solar radiation. The large changes in surface latent heat and solar radiative fluxes from the control run to the experiment suggest that the surface energy balance in the atmospheric general circulation model is highly sensitive to convective parameterization.