Pacific Intertropical Convergence Zone Research Articles

Heating Structures of the TRMM Field Campaigns

Abstract Heating profiles calculated from sounding networks and other observations during three Tropical Rainfall Measuring Mission (TRMM) field campaigns [the Kwajalein Experiment (KWAJEX), TRMM Large-Scale Biosphere–Atmosphere Experiment in Amazonia (LBA), and South China Sea Monsoon Experiment (SCSMEX)] show distinct geographical differences between oceanic, continental, and monsoon regimes. Differing cloud types (both precipitating and nonprecipitating) play an important role in determining the total diabatic heating profile. Variations in the vertical structure of the apparent heat source, Q1, can be related to the diurnal cycle, large-scale forcings such as atmospheric waves, and rain thresholds at each location. For example, TRMM-LBA, which occurred in the Brazilian Amazon, had mostly deep convection during the day while KWAJEX, which occurred in the western portion of the Pacific intertropical convergence zone, had more shallow and moderately deep daytime convection. Therefore, the afternoon height of maximum heating was more bottom heavy (i.e., heating below 600 hPa) during KWAJEX compared to TRMM-LBA. More organized convective systems with extensive stratiform rain areas and upper-level cloud decks tended to occur in the early and late morning hours during TRMM-LBA and KWAJEX, respectively, thereby causing Q1 profiles to be top heavy (i.e., maxima from 600 to 400 hPa) at those times. SCSMEX, which occurred in the South China Sea during the monsoon season, had top-heavy daytime and nighttime heating profiles suggesting that mesoscale convective systems occurred throughout the diurnal cycle, although more precipitation and upper-level cloud in the afternoon caused the daytime heating maximum to be larger. A tendency toward bottom- and top-heavy heating profile variations is also associated with the different cloud types that occurred before and after the passage of easterly wave troughs during KWAJEX, the easterly and westerly regimes during TRMM-LBA, and the monsoon onset and postonset active periods during SCSMEX. Rain thresholds based on heavy, moderate, and light/no-rain amounts can further differentiate top-heavy heating, bottom-heavy heating, and tropospheric cooling. These budget studies suggest that model calculations and satellite retrievals of Q1 must account for a large number of factors in order to accurately determine the vertical structure of diabatic heating associated with tropical cloud systems.

The Interaction of Clouds and Dry Air in the Eastern Tropical Pacific

Abstract Cloud radar observations of eastern Pacific intertropical convergence zone cloud vertical structure are interpreted in light of soundings, 100-km-scale divergence profiles calculated from precipitation radar Doppler velocities, and surface rain gauge data, from a ship at 10°N, 95°W during the 2001 East Pacific Investigation of Climate (EPIC) experiment. The transition from convective to stratiform rain is clear in all four datasets, indicating a coherence from local to 100-km scale. A novel finding is dry air intrusions at altitudes of 6–8 km, often undercutting upper-level ice clouds. Two distinct dry air source regions are identified. One is a relatively dry area overlying the cooler waters of the Costa Rica oceanic thermocline dome, centered approximately 400 km east-northeast of the ship site. The other is the even drier near-equatorial subsidence zone south of 6°–7°N. The former source is somewhat peculiar to this specific ship location, so that the ship sample is not entirely representative of the region. The 20–25 September period is studied in detail, as it depicts two influences of the dry air on cloud vertical structure. One is the modulation of small-scale surface-based convection, evident as a weakening and narrowing of cloud radar reflectivity features. The other springs from intense sublimation cooling as differential advection brought snowing anvil clouds over the dry layers. During one half-day period of strong sublimation, the cooling rate is inferred to be several tens of degrees per day over a 100-hPa layer, based on a heat budget estimate at 100-km scale involving the horizontal wind divergence data. This is consistent with fluxing ice water contents of 0.05–0.10 g m−3 derived from the cloud radar reflectivities. The temperature profile shows the dynamically expected response to this cooling, a positive–negative–positive temperature anomaly pattern centered on the sublimating layer. A buoyancy-sorting diagnostic model of convection indicates that these upper-troposphere temperature anomalies can cause premature detrainment of updrafts into the lower part of the cloudy layer, a feedback that may actively maintain these long-lasting dense anvils. Middle-troposphere southerly dry air inflow is also evident in large-scale analysis. Given the proximity of the dry equatorial subsidence zone to the eastern tropical Pacific, the differential advection of dry and cloudy air, the ensuing sublimation, and its dynamical aftereffects may play a role in establishing the region’s climate, although the extent of their significance needs to be further established.

Dynamics of the Intertropical Convergence Zone of the East Pacific

Abstract The dynamical factors controlling the mean state and variability of the east Pacific intertropical convergence zone (ITCZ) and the associated cross-equatorial boundary layer flow are investigated using observations from the East Pacific Investigation of Climate (EPIC2001) project. The tropical east Pacific exhibits a southerly boundary layer flow that terminates in the ITCZ. This flow is induced by the strong meridional sea surface temperature (SST) gradient in the region. Away from the equator and from deep convection, it is reasonably well described on a day-to-day basis by an extended Ekman balance model. Variability in the strength and northward extent of this flow is caused by variations in free-tropospheric pressure gradients that either reinforce or oppose the pressure gradient associated with the SST gradient. These free-tropospheric gradients are caused by easterly waves, tropical cyclones, and the Madden–Julian oscillation. Convergence in the boundary layer flow is often assumed to be responsible for destabilizing the atmosphere to deep convection. An alternative hypothesis is that enhanced total surface heat fluxes associated with high SSTs and strong winds act to produce the necessary destabilization. Analysis of the moist entropy budget of the planetary boundary layer shows that, on average, surface fluxes generate over twice the destabilization produced by boundary layer convergence in the east Pacific ITCZ.

A physico-chemical survey of inland lakes and saline ponds: Christmas Island (Kiritimati) and Washington (Teraina) Islands, Republic of Kiribati

The equatorial Pacific Ocean atoll islands of Kiritimati and Teraina encompass great physical, chemical and biological variability within extreme lacustrine environments. Surveys of lake chemistry and sediments revealed both intra- and inter-island variability. A survey of more than 100 lakes on Kiritimati found salinities from nearly fresh to 150 ppt with the highest values occurring within the isolated, inland portions of the island away from the influence of groundwater or extreme tides. Dissolved oxygen (DO) and pH values also showed considerable variability with a less regular spatial pattern, but were both generally inversely related to salinity. Series of lakes, progressively more isolated from marine communication, present a modern analog to the chemical and morphologic evolution of presently isolated basins. Sediments on both islands consist of interbedded red and green silt, possibly degraded bacterial mat, overlying white, mineralogenic silt precipitate. Variability may be indicative of shifts in climatological parameters such as the El Niño Southern Oscillation (ENSO) or the Pacific Intertropical Convergence Zone (ITCZ).

Influence of mechanical mixing on a low summertime SST in the western North Pacific ITCZ region

[1] A region with a low sea-surface temperature (SST) was identified in the western North Pacific (WNP) Intertropical Convergence Zone (ITCZ) or warm water pool region in boreal summer. The SST decreases by up to 0.5°C from May to the June–July–August (JJA) season just east of Mindanao Island. Analyses of the observed data indicated that a northeastward surface current constantly exists throughout the year, supplying cold subsurface water through the Molucca Strait. As a result, the subsurface water is colder by several degrees compared with that in the surrounding regions. The sea-surface wind is strong over this region in the JJA season. Examination of Argo float data demonstrated that the mixed layer becomes deeper from May to the JJA season. It is suggested that the strong wind causes vertical mixing between the surface and subsurface layers, resulting in the low summertime SST.

Variability in the Southerly Flow into the Eastern Pacific ITCZ*

Abstract During boreal summer and fall, there is a strong southerly boundary layer flow across the equator into the east Pacific intertropical convergence zone (ITCZ). The modulation of this flow on synoptic to seasonal time scales is studied using an index of meridional pressure difference between the equator and the ITCZ along 95°W. Two complementary datasets from the East Pacific Investigation of Climate (EPIC) are used to study eastern Pacific variability. Daily measurements of sea level pressure (SLP) from Tropical Atmosphere Ocean (TOA) array buoys from May to November 2001 provide temporal coverage, and eight flights by a C-130 aircraft during September to October 2001 document the associated modulation of lower tropospheric vertical structure. The principal mode of variability of the perturbation SLP along 95°W from 1°S to 12°N, derived by principal component analysis from either the eight flights (PC1C-130) or from daily TAO buoy observations (PC1), explains 77% of the meridional pressure gradient variability. The pressure anomalies at 1.6 km are similar to those at the surface. The time series of the first mode of the TAO observations shows that most of the variance is in the 2–7-day range. Low pressure at 12°N is associated with southerly and westerly surface wind anomalies, and enhanced precipitation in the ITCZ. The depth of ITCZ convection is more strongly correlated to meridional wind above the planetary boundary layer (PBL) than to meridional wind within the PBL. There is little correlation of PBL meridional flow across the equator with ITCZ convection. Regression of PC1C-130 against the 95°W cross sections observed by dropwinsondes released during the eight C-130 flights shows correlations of westerlies to positive PC1C-130 (low pressure at 12°N). Between the equator and 4°N, statistically significant northerlies just above the PBL at 1–2-km height and southerlies at 4 km are correlated with negative PC1C-130, having high SLP at 12°N, an anomalously weak meridional SLP gradient, and suppressed convection in the ITCZ. PC1 is bandpass filtered and correlated with reanalysis fields to identify the structures that modulate meridional pressure gradients along 95°W. Most of the variability at periods less than 15 days is related to easterly waves. Seasonal trends in PC1 during May–October 2001 reflect the seasonal evolution of the sea and land surface temperatures. After the seasonal trend is removed, a geostrophic westerly jet at 12°N—probably related to the Madden–Julian oscillation—dominates PC1 variability on time scales longer than 15 days.

Influences of intraseasonal disturbances on the oceanic mixed layer in the western North Pacific ITCZ region

[1] Using the Argo float data, changes in the temperature and salinity in the oceanic mixed layer corresponding to the passage of intraseasonal-scale disturbances were examined over the western North Pacific (WNP) intertropical convergence zone (ITCZ) region. Events where the near-surface temperature decreased by more than 1°C within a 10-day period were identified, corresponding to the intraseasonal variability (ISV) of the atmosphere. Analyzing the vertical profiles and the T-S relationships of the mixed layer revealed that the water was cooled mainly by mechanical mixing and Ekman upwelling related to the strong sea surface wind. Thermal cooling at the sea surface does not appear to be a predominant process in the abrupt cooling. Such cooling events were observed only when the mixed layer was shallow.

The Eastern Pacific ITCZ during the Boreal Spring

Abstract The 6-yr (1998–2003) rainfall products from the Tropical Rainfall Measuring Mission (TRMM) are used to quantify the intertropical convergence zone (ITCZ) in the eastern Pacific (defined by longitudinal averages over 90°–130°W) during boreal spring (March–April). The double-ITCZ phenomenon, represented by the occurrence of two maxima with respect to latitude in monthly mean rainfall, is observed in most but not all of the years studied. The relative spatial locations of maxima in sea surface temperature (SST), rainfall, and surface pressure are examined. Interannual and weekly variability are characterized in SST, rainfall, surface convergence, total column water vapor, and cloud water. There appears to be a competition for rainfall between the two hemispheres during this season. When one of the two rainfall maxima is particularly strong, the other tends to be weak, with the total rainfall integrated over the two varying less than does the difference between the rainfall integrated over each separately. There is some evidence for a similar competition between the SST maxima in the two hemispheres, but this is more ambiguous, and there is evidence that some variations in the relative strengths of the two rainfall maxima may be independent of SST. Using a 25-yr (1979–2003) monthly rainfall dataset from the Global Precipitation Climatology Project (GPCP), four distinct ITCZ types during March–April are defined, based on the relative strengths of rainfall peaks north and south of, and right over, the equator. Composite meridional profiles and spatial distributions of rainfall and SST are documented for each type. Consistent with previous studies, an equatorial cold tongue is essential to the existence of the double ITCZs. However, too strong a cold tongue may dampen either the southern or northern rainfall maximum, depending on the magnitude of SST north of the equator.

Dynamics of the Indian monsoon and ENSO relationships in the SINTEX global coupled model

This paper uses recent gridded climatological data and a coupled general circulation model (GCM) simulation in order to assess the relationships between the interannual variability of the Indian summer monsoon (ISM) and the El Nino-Southern Oscillation (ENSO). The focus is on the dynamics of the ISM-ENSO relationships and the ability of the state-of-the-art coupled GCM to reproduce the complex lead-lag relationships between the ISM and the ENSO. The coupled GCM is successful in reproducing the ISM circulation and rainfall climatology in the Indian areas even though the entire ISM circulation is weaker relative to that observed. In both observations and in the simulation, the ISM rainfall anomalies are significantly associated with fluctuations of the Hadley circulation and the 200 hPa zonal wind anomalies over the Indian Ocean. A quasi-biennial time scale is found to structure the ISM dynamical and rainfall indices in both cases. Moreover, ISM indices have a similar interannual variability in the simulation and observations. The coupled model is less successful in simulating the annual cycle in the tropical Pacific. A major model bias is the eastward displacement of the western North Pacific inter-tropical convergence zone (ITCZ), near the dateline, during northern summer. This introduces a strong semiannual component in Pacific Walker circulation indices and central equatorial Pacific sea surface temperatures. Another weakness of the coupled model is a less-than-adequate simulation of the Southern Oscillation due to an erroneous eastward extension of the Southern Pacific convergence zone (SPCZ) year round. Despite these problems, the coupled model captures some aspects of the interannual variability in the tropical Pacific. ENSO events are phase-locked with the annual cycle as observed, but are of reduced amplitude relative to the observations. Wavelet analysis of the model Nino34 time series shows enhanced power in the 2–4 year band, as compared to the 2–8 year range for observations during the 1950–2000 period. The ISM circulation is weakened during ENSO years in both the simulation and the observations. However, the model fails to reproduce the lead-lag relationship between the ISM and Nino34 sea surface temperatures (SSTs). Furthermore, lag correlations show that the delayed response of the wind stress over the central Pacific to ISM variability is insignificant in the simulation. These features are mainly due to the unrealistic interannual variability simulated by the model in the western North Pacific. The amplitude and even the sign of the simulated surface and upper level wind anomalies in these areas are not consistent with observed patterns during weak/strong ISM years. The ISM and western North Pacific ITCZ fluctuate independently in the observations, while they are negatively and significantly correlated in the simulation. This isolates the Pacific Walker circulation from the ISM forcing. These systematic errors may also contribute to the reduced amplitude of ENSO variability in the coupled simulation. Most of the unrealistic features in simulating the Indo-Pacific interannual variability may be traced back to systematic errors in the base state of the coupled model.

Cloud Radiative Forcing in Pacific, African, and Atlantic Tropical Convective Regions

Abstract The high degree of cancellation between longwave (LW) and shortwave (SW) cloud radiative forcing (CRF) observed in the Pacific warm pool region has generally been assumed to be a property of all convective regions in the Tropics. Analysis of the (Earth Radiation Budget Experiment) ERBE-like data from the Clouds and the Earth's Radiant Energy System (CERES) instrument on the Terra satellite reveals that a similar degree of cancellation occurs over the African convective region at monthly and longer time scales, but only in the area average. In the Atlantic intertropical convergence zone (ITCZ), the degree of cancellation is lower, particularly during the summer months, where the area-average SW forcing typically exceeds the LW forcing by more than 20 W m−2. This behavior is similar to that found previously for the eastern Pacific ITCZ region, which is consistent with the similarity in dynamics between these two regions. Over Africa, substantial seasonal and spatial variations in net CRF occur, wit...

The relationship of Pacific deep tropical convection to the winter and springtime extratropical atmospheric circulation of the South Pacific in El Niño events

During some El Niño events Rossby wave trains (RWT) are observed to strongly modulate the seasonal atmospheric circulation of the South Pacific extratropics in the austral winter and spring. Here it is shown that seasonally intensified deep tropical convection is confined to the Pacific Intertropical Convergence Zone (ITCZ) close to the dateline in those events with strong RWT modulation but not in other cases. In other cases deep convection either withdraws from the ITCZ or the ITCZ strongly interacts with the South Pacific Convergence Zone (SPCZ) lying at 5–10°S. In both cases this points to a weakening of the local tropical Hadley circulation that may be crucial for Rossby wave generation. It is also found that the expected RWT response in the high latitudes can fail to occur even when SST in the central tropical Pacific are very high.

Post-glacial evolution of the Indo-Pacific Warm Pool and El Niño-Southern oscillation

Recent research has revealed new insights into the temperature, size, and variability of the Indo-Pacific Warm Pool (IPWP) and the nature of the El Niño-Southern Oscillation (ENSO) since the Last Glacial Maximum (LGM). Sea surface temperature (SST) reconstructions from foraminiferal Mg/Ca, alkenone, and revised coral Sr/Ca palaeothermometry agree that SSTs in the IPWP during the LGM were ∼3°C cooler than at present. In the central portion of the IPWP, the rapid post-glacial rise in SST led the deglaciation by ∼3000 years to produce near-modern SSTs by the early Holocene. In contrast, further west and north, post-glacial shifts in SSTs in the South China and Sulu Seas are synchronous with abrupt climate changes in the North Atlantic. New evidence for the nature of the Little Ice Age in the tropics has been obtained from a 420-year record of coral Sr/Ca and δ 18O from the Great Barrier Reef, Australia. This indicates that SSTs and salinity were higher in the 18th century than in the 20th century. The results suggest that the tropical Pacific played a role as a source region of water vapour during the global expansion of Little Ice Age glaciers. The onset of modern ENSO periodicities is identified by palaeo-ENSO records throughout the tropical Pacific region ∼5000 years ago, with an abrupt increase in ENSO magnitude ∼3000 years ago. Individual ENSO events recorded by corals reveal that the precipitation response to El Niño temperature anomalies was subdued in the mid-Holocene. The apparent non-linear onset of ENSO in the late Holocene appears to reflect abruptly enhanced interaction between the Southern Oscillation and the Pacific Intertropical Convergence Zone. Comparisons of precipitation variability recorded by Great Barrier Reef corals with ENSO indices for the last 350 years confirms that non-stationarity of ENSO teleconnections is a natural characteristic of modern climate.

Convective Forcing in the Intertropical Convergence Zone of the Eastern Pacific

One of the goals of the East Pacific Investigation of Climate, year 2001 process study (EPIC2001), was to understand the mechanisms controlling the forcing of deep atmospheric convection over the tropical eastern Pacific. An intensive study was made of convection in a 4° × 4° square centered on 10°N, 95°W in September and October of 2001. This is called the intertropical convergence zone (ITCZ) study region because it encompasses the eastern Pacific intertropical convergence zone. Starting from an analysis of the theoretical possibilities and a plethora of dropsonde, in situ, radar, and satellite data, it is found that newly developing convection occurs where a deep layer of air (of order 1 km deep or deeper) is conditionally unstable with only weak convective inhibition. Shallower conditionally unstable layers are associated with numerous small clouds, but do not seem to produce deep convection. The occurrence of deep convection over the ITCZ study region is presumably related to the propensity of the environment to produce areas of weak convective inhibition over such a deep layer. Three theoretically possible factors control the formation of such convectively unstable areas: 1) the strength of the total surface heat (or moist entropy) fluxes; 2) the advection of moisture into the region; and 3) temperature anomalies caused by dry adiabatic ascent of the inhibition layer, which lies typically between 700 and 850 mb. The areal fraction covered by such instability is small even on highly convective days. In the tropical eastern Pacific, it is found that the total surface entropy flux is the most significant of these factors, with a warm layer in the 700–850-mb range, resulting presumably from subsidence, playing an important suppressive role in certain cases. These two factors account for approximately two-thirds of the variance in satellite infrared brightness temperature averaged over the study region. Moisture (or moist entropy) advection appears to be of less importance. Tropical disturbances such as easterly waves, Kelvin waves, and the Madden–Julian oscillation presumably control convection primarily via these two mechanisms during their passage through this region.

Mid‐late Holocene El Niño variability in the equatorial Pacific from coral microatolls

Oxygen isotope ratios in Porites microatolls from Christmas Island in the central Pacific provide high‐resolution proxy records of El Niño‐Southern Oscillation (ENSO) variability since 3.8 thousand years ago (ka). Compared with modern microatolls, reconstructions from fossil microatolls imply that interannual variations in ENSO sea‐surface temperature and precipitation were less intense 3.8–2.8 ka, but more pronounced at 1.7 ka. Amplification of ENSO at ∼2 ka is consistent with precessional changes in insolation seasonality, but exceeds model predictions and may reflect stronger rainfall teleconnections through enhanced interaction between the Southern Oscillation and the Pacific Intertropical Convergence Zone.

Extratropical Forcing of Convectively Coupled Kelvin Waves during Austral Winter

Abstract Observations are presented that link extratropical Rossby wave disturbances excited in the Southern Hemisphere subtropical jet to the initiation of convectively coupled Kelvin waves in the Pacific intertropical convergence zone (ITCZ) during austral winter. A baroclinic, zonal wavenumber 6, eastward-propagating Rossby wave train in the subtropical jet turns northeastward in the vicinity of Australia, inducing upper tropospheric divergence and vertical motion fields that spread equatorward and induce cloudiness anomalies in the Tropics. Lower tropospheric pressure surges excited from the extratropics also induce Kelvin wave–like geopotential height and temperature anomalies at the surface, providing additional lower tropospheric convergence and vertical motion forcing. The tropical outgoing longwave radiation (OLR) and circulation fields propagate eastward in tandem with the extratropical Rossby wave train at approximately 17 m s–1. Kelvin wave activity in the central Pacific ITCZ thus appears to ...

Observations of Variability on Synoptic Timescales in the East Pacific ITCZ*

Abstract Data obtained in the eastern Pacific intertropical convergence zone (ITCZ) during the Tropical Eastern Pacific Process Study (TEPPS) show a 3–6-day variability. The NOAA ship Ronald H. Brown collected surface meteorological observations, C-band Doppler radar volumes, atmospheric soundings, and rainfall data while on station at 7.8°N, 125°W from 8–23 August 1997. The 3–6-day variability was a prominent timescale in the meridional wind and humidity data. The maxima of the surface-to-700-mb meridional wind anomalies were 5–12 m s–1. Maxima of the moisture anomalies at the same levels were ∼1–3 g kg–1, in phase with the low-level southerlies, while upper-level moisture lagged the southerlies by less than a day. The radar indicated a close connection between precipitation in the vicinity of the ship and the meridional wind, with the precipitation occurring when the surface southerlies were strongest. The meridional and zonal wind components had a maximum of 850–450-mb wind shear during the southerly e...

Observations of a Convectively Coupled Kelvin Wave in the Eastern Pacific ITCZ

Abstract A case study of a convectively coupled Kelvin wave in the eastern Pacific intertropical convergence zone (ITCZ) is presented, as observed during the 1997 Pan American Climate Studies (PACS) Tropical Eastern Pacific Process Study (TEPPS). The large-scale convective envelope associated with this disturbance, with a zonal scale of approximately 1000–2000 km, propagates eastward at 15 m s−1 along the mean convective axis of the ITCZ. This envelope consists of many smaller-scale, westward-moving convective elements, with zonal scales on the order of 100–500 km. As the convectively coupled Kelvin wave disturbance propagates eastward, it exerts a strong control on local convection. Radar and vertical profiler data collected aboard the NOAA R/V Ronald H. Brown during the wave passage show that convection deepens rapidly as the Kelvin wave approaches from the west, progressing from isolated, shallow cumuli to organized deep convective features within just 12 h. Initially, rainfall in the vicinity of the s...

Satellite observations of the southeast Pacific intertropical convergence zone during 1993–1998

An intertropical convergence zone in the southeast Pacific Ocean is described from monthly mean 1° × 1° ocean vector wind, rainfall, sea surface temperature, and integrated water vapor measurements recorded from satellites. Time interval of the investigation was January 1993 to December 1998, when ocean and atmosphere conditions during 1993–1996 were normal compared to the 1997–1998 El Niño. The southeast Pacific intertropical convergence zone (SITCZ) occurred in March‐April at 8°S–2°S, 130°W–90°W. During 1993–1996 the average March‐April values of SITCZ surface wind convergence, rain rate, sea surface temperature, and integrated water vapor were 3.3 × 10−6 s−1, 3.3 mm d−1, 27.3°C, and 45.3 mm, respectively. A statistical model of the monthly mean observations predicted rainfall greater than the 2 mm d−1 threshold when surface wind convergence was greater than the 1.5 × 10−6 s−1 threshold and sea surface temperature was above 27°C. During non‐El Niño conditions, SITCZ sea surface temperature was greater than sea surface temperature in the 2°S–2°N equatorial band, with maximum difference in March–April which would create a surface wind convergence larger in March‐April compared to other months. The Intertropical Convergence Zone (ITCZ), which occurred north of the equator throughout the year, and the SITCZ were separated by divergent wind, low (<2 mm d−1) rainfall, and cold (<27°C) surface water, except during the intense 1998 El Niño. The rain rate of the ITCZ was minimum in March‐April during 1993–1996 and comparable to the SITCZ rain rate.

Evolutionary Structure of the Eastern Pacific Double ITCZ Based on Satellite Moisture Profile Retrievals

For about a month near the boreal vernal equinox, the eastern Pacific intertropical convergence zone (ITCZ) is observed to form two troughs quasi-symmetrically situated about the equator near 58-78 latitude during years when an equatorial sea surface cold tongue is present (e.g., La Nina years). The three-dimensional structure and temporal evolution of the eastern Pacific double ITCZ is documented using weekly cloud liquid and ice water fields and relative humidity profiles retrieved from Special Sensor Microwave/Temperature-2 measurements. The depth of convection in the southern branch of the double ITCZ, as determined by the coincident presence of cloud liquid and ice as well as by upward motion inferred from the relative humidity field, is observed to be sensitive to both the underlying SST and subsidence from the northern branch. The equatorial sea surface cold tongue appears to be the determining factor regulating the formation of a double ITCZ in the eastern Pacific. Areas of deep convection within the double ITCZ are accompanied by surface wind convergence maxima. However, the coincident maxima in deep convection and surface convergence are located several degrees of latitude equatorward of the highest sea surface temperatures.

Validating the NSCAT winds in the vicinity of the Pacific Intertropical Convergence Zone

It is known that the NSCAT winds are influenced by precipitation, and so are potentially in error in regions of high rainfall, such as the Pacific ITCZ. We assess this potential error by comparing NSCAT winds to other products and by determining the possible impact on modeling the Pacific NECC. In the latitude band of the ITCZ, there are large differences between wind products from the ECMWF and NSCAT. A comparison with TAO buoy winds shows a bias of NSCAT data towards a higher zonal wind stress at 8°N. The spatial distribution of this bias distorts the Ekman pumping velocity across the ITCZ, and results in a NECC that is 0.1–0.2 m/s weaker than that forced by the ECMWF winds. This is not a trivial amount during the spring, when the strength of the NECC in the eastern Pacific is of the same magnitude.