• AVISO, 1996: AVISO user handbook: Merged TOPEX/POSEIDON products. AVI-NT-02-101-CN, 3d ed., 196 pp.

  • Cane, M. A., 1983: Oceanographic events during El Niño. Science, 222 , 11891195.

  • Chen, G., 2004: A 10-yr climatology of oceanic water vapor derived from the TOPEX microwave radiometer. J. Climate, 17 , 25412557.

  • Chen, G., , and R. Ezraty, 1999: Variation of Southern Ocean sea level and its possible relation with Antarctic sea ice. Int. J. Remote Sens., 20 , 3147.

    • Search Google Scholar
    • Export Citation
  • Chen, G., , J. Ma, , C. Fang, , and Y. Han, 2003: Global oceanic precipitation derived from TOPEX and TMR: Climatology and variability. J. Climate, 16 , 38883904.

    • Search Google Scholar
    • Export Citation
  • Chen, G., , C. Fang, , C. Zhang, , and Y. Chen, 2004: Observing the coupling effect between warm pool and “rain pool” in the Pacific Ocean. Remote Sens. Environ., 91 , 153159.

    • Search Google Scholar
    • Export Citation
  • Gruber, A., 1972: Fluctuations in the position of the ITCZ in the Atlantic and Pacific Oceans. J. Atmos. Sci., 29 , 193197.

  • Hubert, L. F., , A. F. Krueger, , and J. S. Winston, 1969: The double intertropical convergence zone—Fact or fiction? J. Atmos. Sci., 26 , 771773.

    • Search Google Scholar
    • Export Citation
  • Lutgens, F. K., , and E. J. Tarbuck, 1992: The Atmosphere: An Introduction to Meteorology. 5th ed. Prentice Hall, 430 pp.

  • Madden, R. A., , and P. R. Julian, 1971: Detection of a 40-50 day oscillation in the zonal wind in the tropical Pacific. J. Atmos. Sci., 28 , 702708.

    • Search Google Scholar
    • Export Citation
  • McPhaden, M. J., 1999: Genesis and evolution of the 1997–98 El Niño. Science, 283 , 950954.

  • Philander, S. G. H., 1990: El Niño, La Niña, and the Southern Oscillation. Academic Press, 289 pp.

  • Philander, S. G. H., , D. Gu, , D. Halpern, , G. Lambert, , N-C. Lau, , T. Li, , and R. C. Pacanowski, 1996: Why the ITCZ is mostly north of the equator. J. Climate, 9 , 29582972.

    • Search Google Scholar
    • Export Citation
  • Picaut, J., , E. Hackert, , A. J. Busalacchi, , R. Murtugudde, , and G. S. E. Lagerloef, 2002: Mechanisms of the 1997–1998 El Niño–La Niña, as inferred from space-based observations. J. Geophys. Res., 107 .3037, doi:10.1029/2001JC000850.

    • Search Google Scholar
    • Export Citation
  • Randel, D. L., , T. H. Vonder Haar, , M. A. Ringerud, , G. L. Stephens, , T. J. Greenwald, , and C. L. Combs, 1996: A new global water vapor dataset. Bull. Amer. Meteor. Soc., 77 , 12331246.

    • Search Google Scholar
    • Export Citation
  • Rasmusson, E. M., 1985: El Niño and variations in climate. Amer. Sci., 73 , 168178.

  • Rasmusson, E. M., , and P. A. Arkin, 1993: A global view of large-scale precipitation variability. J. Climate, 6 , 14951522.

  • Ropelewski, C. F., , and M. S. Halpert, 1987: Global and regional scale precipitation patterns associated with the El Niño/Southern Oscillation. Mon. Wea. Rev., 115 , 16061626.

    • Search Google Scholar
    • Export Citation
  • Ropelewski, C. F., , and M. S. Halpert, 1989: Precipitation patterns associated with the high index phase of the Southern Oscillation. J. Climate, 2 , 268284.

    • Search Google Scholar
    • Export Citation
  • Ruf, C. S., , S. J. Keihm, , B. Subramanya, , and M. A. Janssen, 1994: TOPEX/POSEIDON microwave radiometer performance and in-flight calibration. J. Geophys. Res., 99 , 2491524926.

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  • Waliser, D. E., , and C. Gautier, 1993: A satellite-derived climatology of the ITCZ. J. Climate, 6 , 21622174.

  • Waliser, D. E., , and R. C. J. Somerville, 1994: Preferred latitudes of the intertropical convergence zone. J. Atmos. Sci., 51 , 16191639.

    • Search Google Scholar
    • Export Citation
  • Xie, S-P., , and K. Saito, 2001: Formation and variability of a northerly ITCZ in a hybrid coupled AGCM: Continental forcing and oceanic–atmospheric feedback. J. Climate, 14 , 12621276.

    • Search Google Scholar
    • Export Citation
  • View in gallery

    Spatial distribution of the annual amplitude of oceanic water vapor derived from the TMR for (a) normal years (1995, 1996, 2001), (b) El Niño years (1993, 1994, 1997, 2002), and (c) La Niña years (1999, 2000), respectively. Labels “A–E” in (a) indicate the approximate locations of the five most dynamic belts of annual OWV amplitude.

  • View in gallery

    The averaged yearly variation of oceanic water vapor for the subtropical zone (10°–20°) of the (a) NH and (b) SH derived from the TMR during El Niño years (solid line) and La Niña years (dashed line), respectively.

  • View in gallery

    Same as in Fig. 1, but for the annual phase. The months in which maximum OWV occurs are indicated on the maps.

  • View in gallery

    The mean positions of the ITCZ determined from the annual phase map of oceanic water vapor for the El Niño (solid circles) and La Niña (open circles) modes, respectively.

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Impact of El Niño/La Niña on the Seasonality of Oceanic Water Vapor: A Proposed Scheme for Determining the ITCZ

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  • 1 Key Laboratory of Ocean Remote Sensing, Ministry of Education, Ocean Remote Sensing Institute, Ocean University of China, Qingdao, China
  • | 2 Joint Laboratory for Geoinformation Science, The Chinese University of Hong Kong, Shatin, Hong Kong, China
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Abstract

Previous research has shown that oceanic water vapor (OWV) is a useful quantity for studying the low-frequency variability of the atmosphere–ocean system. In this work, 10 years (1993–2002) of high-quality OWV data derived from the Ocean Topography Experiment (TOPEX) microwave radiometer are used to investigate the impact of El Niño/La Niña on the amplitude and phase of the annual cycle. These results suggest that El Niños (La Niñas) can weaken (strengthen) the seasonality of OWV by decreasing (increasing) the annual amplitude. The change of amplitude is usually slight but significant, especially for the five most dynamic seasonal belts across the major continents at midlatitudes. The El Niño–Southern Oscillation (ENSO) impact on the annual phase of OWV is seen to be fairly systematic and geographically correlated. The most striking feature is a large-scale advancing/delay of about 10 days (as estimated through empirical modeling) for the midlatitude oceans of the Northern Hemisphere in reaching their summer maxima during the El Niño/La Niña years. In addition, an alternative scheme for estimating the mean position of the intertropical convergence zone (ITCZ) based on the annual phase map of OWV is proposed. This ITCZ climatology favors 4°N in mean latitude, and agrees with existing results in that its position meanders from 2°S to 8°N oceanwide, and stays constantly north of the equator over the Atlantic and eastern Pacific.

Corresponding author address: Ge Chen, Ocean Remote Sensing Institute, Ocean University of China, 5 Yushan Rd., Qingdao 266003, China. Email: gechen@public.qd.sd.cn

Abstract

Previous research has shown that oceanic water vapor (OWV) is a useful quantity for studying the low-frequency variability of the atmosphere–ocean system. In this work, 10 years (1993–2002) of high-quality OWV data derived from the Ocean Topography Experiment (TOPEX) microwave radiometer are used to investigate the impact of El Niño/La Niña on the amplitude and phase of the annual cycle. These results suggest that El Niños (La Niñas) can weaken (strengthen) the seasonality of OWV by decreasing (increasing) the annual amplitude. The change of amplitude is usually slight but significant, especially for the five most dynamic seasonal belts across the major continents at midlatitudes. The El Niño–Southern Oscillation (ENSO) impact on the annual phase of OWV is seen to be fairly systematic and geographically correlated. The most striking feature is a large-scale advancing/delay of about 10 days (as estimated through empirical modeling) for the midlatitude oceans of the Northern Hemisphere in reaching their summer maxima during the El Niño/La Niña years. In addition, an alternative scheme for estimating the mean position of the intertropical convergence zone (ITCZ) based on the annual phase map of OWV is proposed. This ITCZ climatology favors 4°N in mean latitude, and agrees with existing results in that its position meanders from 2°S to 8°N oceanwide, and stays constantly north of the equator over the Atlantic and eastern Pacific.

Corresponding author address: Ge Chen, Ocean Remote Sensing Institute, Ocean University of China, 5 Yushan Rd., Qingdao 266003, China. Email: gechen@public.qd.sd.cn

1. Introduction

Largely through the use of satellite observations, it is now understood that the low-frequency variability of the atmosphere–ocean system is composed of several systematic modes ranging from intraseasonal to interannual. Among them, the El Niño–Southern Oscillation (ENSO; Philander 1990), the annual cycle, the semiannual cycle, and the Madden–Julian oscillation (MJO; Madden and Julian 1971) are of major dominance. The variation of an atmospheric–oceanic signal at a given location can thus be approximated as
i1520-0493-133-10-2940-e1
where S0 is the climatological mean; SENSO(t) and SMJO(t) correspond to the ENSO- and MJO-induced variabilities, respectively; SAN(t) and SSA(t) are the annual and semiannual components, respectively; ξ(t) and ε(t) represent lower- and higher-frequency residuals, respectively; and t is the time in month. Here SAN(t) and SSA(t) can be further expressed as
i1520-0493-133-10-2940-e2
and
i1520-0493-133-10-2940-e3
where T = 12 months, A1, A2 and φ1, φ2 are the corresponding amplitudes and initial phases, respectively.

Despite the significant ENSO and MJO oscillations, seasonality is still the dominant feature of the atmosphere–ocean system, since many of the primary forcings occur at the annual and semiannual frequencies. The basic pattern of the low-frequency variability of the system is therefore characterized by the combination of the periodic annual and semiannual cycles with the quasi-periodic ENSO and MJO oscillations. For most of the geophysical variables, the annual cycle and the ENSO oscillation are the two leading modes among the four components, since the semiannual cycle and the MJO oscillation are generally weak and highly regional. As an example to support this argument, the interannual, annual, semiannual, and quarterly components of global oceanic precipitation variability are roughly 1.53, 1.51, 0.79, 0.58 mm day−1, respectively (Chen et al. 2003). Also, the relative importance of the interannual, annual, and semiannual variations of oceanic water vapor (OWV) in terms of mean amplitude appears to be approximately 1.5:4.2:1 (Chen 2004). These figures imply that the annual and ENSO modes are of primary interest in describing and understanding the low-frequency variability of the atmosphere–ocean system.

Although the interrelationships between the aforementioned intraseasonal to interannual modes receive increasing attention over the past two decades, the manner in which these low-frequency modes interact with one another is not entirely clear (e.g., McPhaden 1999). As far as the large-scale precipitation variability is concerned, it is shown that the ENSO cycle exhibits a preferred phasing with the annual cycle (Rasmusson 1985; Ropelewski and Halpert 1987, 1989), or as Cane (1983) argues, many aspects of El Niño are closely linked to the annual cycle. But the annual cycle can, in return, be significantly modified by the ENSO cycle (Rasmusson and Arkin 1993). In this research, we investigate the impacts of El Niño/La Niña on the seasonality of oceanic water vapor in terms of amplitude change and phase shift. The choice of water vapor as an exemplary parameter is based on the considerations that 1) it is a major component of the global hydrological cycle, and 2) it is the most abundant greenhouse gas and thus significantly affects climate. Another motivation is the finding that the phase distribution of oceanic water vapor is closely related to the location of the intertropical convergence zone (ITCZ), on the basis of which a new scheme for determining the mean axis of ITCZ is proposed as a “by-product” of this study.

2. Data and processing

In a recent study by Chen (2004), a 10-yr climatology of oceanic water vapor spanning December 1992–August 2003 derived from the Ocean Topography Experiment (TOPEX) microwave radiometer (TMR; AVISO 1996; Ruf et al. 1994) is constructed. This is a monthly dataset covering 66°S–66°N with a spatial resolution of 1° × 1°. More details concerning the instrument and the algorithm can be found in Chen (2004). A comparison between the TMR-derived OWV (vertically integrated water vapor content in the atmospheric column over the oceans) and the National Aeronautics and Space Administration (NASA) Water Vapor Project (NVAP) climatology (Randel et al. 1996) suggests that they are in quantitative agreement with a mean bias of 0.053 g cm−2, and a percentage difference of 2.1% (Chen 2004). Since the NVAP water vapor dataset is thought to be the best product of its kind to date, we can therefore anticipate that TMR could be a very useful source of OWV observation with reliable quality.

It has been demonstrated that in many respects the interannual variability of the atmosphere–ocean system is bimodal in character, taking on one form in El Niño years and another in La Niña years, with the normal years in between (see, e.g., Cane 1983; Picaut et al. 2002; Chen et al. 2004). A list of cold (La Niña) and warm (El Niño) episodes has been compiled by the National Centers for Environmental Prediction (NCEP) Climate Prediction Center (CPC) of the National Oceanic and Atmospheric Administration (NOAA) using reanalyzed sea surface temperature to provide a season-by-season breakdown of conditions in the tropical Pacific (more information available online at http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml). We consider a particular year as an El Niño (La Niña) year if two or more seasons are designated as warm (cold) events with moderate or strong strength. For the purpose of this study, the 10-yr period is divided into three groups: The El Niños (1993, 1994, 1997, 2002), the La Niñas (1999, 2000), and the neutral years (1995, 1996, 2001). Note that the year 1998 is excluded because of the coexistence of strong El Niño and La Niña. To investigate the influence of El Niño/La Niña on the annual cycle of OWV, the following scheme is applied. 1) A Gaussian-type bandpass filter with minimum and maximum cutoff frequencies of 1/24 and 1/6 months is applied on a point basis for each subdataset to remove signals shorter than semiannual, and multiyear variabilities induced by ENSO. 2) A harmonic analysis is subsequently carried out to determine the amplitude and phase of the annual component on each grid. This step is performed separately for the averaged El Niño, La Niña, and neutral composites. 3) The spatial distributions of these quantities are obtained through an objective analysis (see, e.g., Chen and Ezraty 1999). Examination and comparison of the amplitude and phase patterns corresponding to the three modes will hopefully allow the impact of El Niño/La Niña on the seasonality of OWV to be identified and characterized.

3. Change in annual amplitude

We would like, first of all, to give an idea on the variability of the El Niño and La Niña modes relative to the neutral mode in terms of amplitude. To do so, the standard deviation of the annual amplitude is estimated for the El Niño years, La Niña years, and neutral years, yielding 0.2147, 0.2150, and 0.1498 g cm−2, respectively. It is obvious that the OWV variability is much more “dynamic” during the ENSO period than in normal years. This argument can be visually confirmed by looking at Fig. 4 of Chen (2004) in which the annual anomalies of OWV with respect to a 10-yr (1993–2002) climatology are shown. Similar behaviors are also found for the annual phase of OWV as a result of amplitude–phase coupling. These suggest that ENSO’s impact on OWV seasonality is real and significant, although the dataset used here is only of marginal length for our purpose.

The geographical distributions of the annual amplitude of OWV for neutral, El Niño, and La Niña modes are shown in Figs. 1a–c, respectively. Unlike the spatial pattern of the OWV climatology (see Figs. 2 and 3 of Chen 2004) which peaks in the tropical oceans and decreases poleward from the equator, the annual amplitude of OWV is characterized by five cross-continent dynamic belts located in the subtropics of the two hemispheres. These belts, taking largely a southwest–northeast orientation in the Northern Hemisphere (NH) and a northwest–southeast orientation in the Southern Hemisphere (SH), are regions over the global ocean where the most distinct seasonality of OWV is found. In contrast, both the tropical oceans and the subpolar areas are usually much weaker in their seasonal cycles.

Relative to the normal years (Fig. 1a), a systematic weakening/enhancement with a slight displacement in their central locations is observed for the annual amplitude of the five seasonal belts under El Niño/La Niña modes, respectively (Figs. 1b,c). This is particularly evident with the northwest Pacific belt in the southern and eastern Asian waters, as well as the northwest Atlantic belt off the southeast coast of the United States, implying that the largest impact of El Niño/La Niña may, in some aspects, be found in extratropical oceans as a result of the interaction between ENSO and the annual cycles. In the eastern equatorial oceans, some notable changes can also be identified, although they are less intensive compared to the midlatitude regions. An obvious example can be found just below the equator in the eastern Pacific where the seasonal belt is considerably extended and enhanced during La Niñas (Figs. 1b,c). In addition, the two zones with near-zero seasonality in the central equatorial Pacific and Indian Ocean under the El Niño mode appear to merge together in the vicinity of the Indonesian waters under the La Niña mode. Thus, seasons defined with OWV can be significantly modified by the quasi-regular ENSO cycle. On a global scale, a weak (strong) seasonality is usually associated with El Niños (La Niñas). On a regional scale, however, the change of seasonality is seen to be intensity dependent and geographically correlated.

4. Annual phase shift

It is of equal interest to examine the phase aspect of the ENSO impact on the annual cycle. Figures 2a,b show the averaged yearly variations of OWV for the subtropical zones of the NH and SH under the El Niño and La Niña mode, respectively. Clearly, a constant phase lag between the two modes exists throughout the year for the NH (Fig. 2a). An empirical modeling of the annual cycle of OWV with Eq. (2) suggests that on average the El Niños lead the La Niñas by some 10 days. For the case of the SH, a quite different phase relationship is visible (Fig. 2b): The annual peak of OWV is delayed by 1 month from February to March under the La Niña mode, but the two curves are almost in phase with each other during the rest of the year. As far as the amplitude is concerned, a slightly increased (decreased) seasonal contrast is found for the subtropics of the NH (SH) during El Niño years.

The phase maps of the annual cycle corresponding to the normal, El Niño, and La Niña years are shown, respectively, in Figs. 3a–c, on which the months with maximum OWV are indicated. A striking feature on these maps is the systematic phase shift between the two abnormal modes in terms of geographical location. Specifically, north of 10°N in the NH, an overall delay of the July–August–September peak pattern from northwest to southeast is apparent under the La Niña mode with respect to the El Niño mode, except for the Bay of Bengal where a reverse situation takes place. This argument is obviously supported by the fact that a majority of the China Seas and the northern part of the Arabian Sea have a maximum OWV in July during El Niño years (Fig. 3b) while in August during La Niña years (Fig. 3c). Moreover, the area of an enclosed region in the subtropical eastern Pacific and Atlantic with OWV peaks in September considerably enlarges during the transition from El Niño to La Niña. South of 10°S in the SH, however, the ENSO impact on the annual phase is generally weak. A slight delay is found in the Pacific for El Niños, while a moderate delay is seen in the Atlantic and Indian Ocean for La Niñas. The equatorial ocean between 10°S and 10°N serves as a sharp transitional zone of the OWV seasonality from an austral to a boreal pattern where a high gradient of phase change is observed. A meridional excursion of the phase distribution from El Niño to La Niña is apparent, especially for the western Pacific and Indian Ocean sectors.

5. A new scheme for determining the mean axis of ITCZ

Traditionally, the intertropical convergence zone is defined as a zone of general convergence between the Northern and Southern Hemisphere trade winds. As these winds converge, moist air is forced upward. This causes water vapor to condense, or be “squeezed” out, as the air cools and rises, resulting in a band of heavy precipitation around the globe. Therefore, the ITCZ is also known as a zone of increased mean convection, moisture, cloudiness, and precipitation that forms the thermal or meteorological equator. The term “equator” naturally implies that, from a climate point of view, the seasonality largely takes an opposite form between north and south of the ITCZ. In this context, the annual phase of OWV may serve as a possible index for determining the mean position of ITCZ, since water vapor provides a major link between the ocean and atmosphere through the fluxes of substance, momentum, and energy, and is therefore of great sensitivity to global climate change. With these understandings in mind, it is argued that the zone of maximum annual phase gradient of OWV (Fig. 3) acts as a good proxy of the ITCZ, which divides the seasonality of the global ocean into two generally opposite hemispheres. An alternative scheme is consequently proposed to practically determine the mean position of ITCZ based on the annual phase map of OWV:
i1520-0493-133-10-2940-e4
with
i1520-0493-133-10-2940-e5
where i and Ji are the locations of the ITCZ in degrees of longitude and latitude with a 1° resolution; x and y represent the two axes of a Cartesian system at the east and north directions, respectively; and P denotes the annual phase of OWV in terms of peaking month.

Equations (4) and (5) are used to estimate the mean positions of the ITCZ corresponding to Figs. 3b,c as shown in Fig. 4. The solid and open circles indicate the El Niño and La Niña case, respectively. It turns out that the OWV-derived ITCZ varies from 2°S to 8°N in latitude oceanwide, but stays constantly north of the equator over the Atlantic and eastern Pacific. The Pacific, Atlantic, and Indian Oceans have their mean ITCZ positions at 3.7°, 5.0°, and 1.4°N under the El Niño mode, while at 5.5°, 5.1°, and 1.7°N under the La Niña mode (Table 1). As can be seen from Fig. 4 and Table 1, the Atlantic ITCZ exhibits the least meridional variability (less than 0.1°) at interannual time scales. In contrast, the largest south–north shift (more than 3°) of ITCZ from El Niño to La Niña is found in the west Pacific over the warm pool region. A question of enormous scientific interest concerning the ITCZ is its latitude preference (e.g., Gruber 1972; Waliser and Somerville 1994; Philander et al. 1996; Xie and Saito 2001), although observation-based estimation of this quantity is rarely reported over the past decade. According to our result, the mean ITCZ latitudes over the global ocean are 3.3° and 4.4°N under the El Niño and La Niña situations, respectively. As a rule of thumb, a 4°N position can be expected for neutral years. Using the highly reflective cloud dataset, Waliser and Gautier (1993) construct a satellite-based climatology of the ITCZ for the period 1971–87. The overall latitude preference they obtain is about 6°N (Waliser and Somerville 1994). It has to be noted, however, that their climatology covers both the land and the ocean, and it is known that meridional oscillation of the ITCZ is much more dramatic over the land than over the ocean (Lutgens and Tarbuck 1992). Moreover, the two datasets are of totally different periods. As for the mechanisms behind the northerly ITCZ, the readers are referred to Xie and Saito (2001), among others, for a possible explanation. Also, it should be pointed out that a double-ITCZ structure (Hubert et al. 1969) appears in the western and eastern Pacific based on our data and scheme. Given the main purpose of this paper, however, we only concentrate on the northern branch of the ITCZ for the time being.

6. Concluding remarks

The periodic annual and semiannual cycles, in conjunction with the quasi-periodic ENSO and MJO oscillations, are fundamental in forming the low-frequency variability of the atmosphere–ocean system. Over a majority of the global oceans, the annual and ENSO components are probably the leading modes, which determine the basic aspects of the climate pattern. The general relationship between these two modes has been documented with several geophysical parameters. The particular effect of ENSO signal in altering the seasonality of the atmosphere–ocean system has been rarely addressed to date. In this research, ten years of high-quality OWV data derived from the TOPEX microwave radiometer are used to investigate the specific impacts of El Niño/La Niña on the amplitude and phase of the annual cycle, leading to two main conclusions. First, El Niños (La Niñas) are found to weaken (strengthen) the seasonality of OWV by decreasing (increasing) the annual amplitude. This change of amplitude is usually slight but significant, especially for the five most dynamic seasonal belts across the major continents at midlatitudes. Second, the ENSO impact on the annual phase of OWV is seen to be highly systematic and geographically correlated. The most striking feature is a large-scale advancing/delay of about 10 days for the subtropical and temperate oceans of the NH in reaching their summer peaks during the El Niño/La Niña years. These results are thought to be a useful contribution to the understanding, modeling, and even prediction of the synoptic variability of the climate system.

Although the intertropical convergence zone has been attached great importance as the thermal equator, and the northerly ITCZ has been a long-standing mystery to meteorologists for decades, observation-based methodologies in support of the identification and exploration of the puzzling zone are far from clear and sufficient. Interestingly, a possible relationship between the phase distribution of OWV and the mean location of the meteorological equator emerges in the course of our investigation on the multimode air–sea interactions. This results in the proposed scheme for estimating the mean position of the ITCZ based on the annual phase gradient of OWV. The geophysical background behind the scheme is somehow straightforward: Analogous to the fact that geometric symmetry with respect to the earth’s rotation (and also the solar radiation) is the determinant factor for dividing the geographical hemispheres, seasonality behavior in terms of a key atmospheric/oceanic parameter could be the principal criterion for dividing the meteorological hemispheres. This ITCZ climatology favors 4°N in mean latitude, and agrees with existing results in that its position meanders from 2°S to 8°N oceanwide, and stays constantly north of the equator over the Atlantic and eastern Pacific. Although further intercomparison and cross validation are needed, the proposed scheme is believed to provide a new alternative for identifying the ITCZ in the phase domain from a climatological perspective.

Acknowledgments

This study is cosponsored by the National Basic Research Program of China (Project No. 2005CB422308) and the Natural Science Foundation of China (Project No. 40025615).

REFERENCES

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  • Chen, G., 2004: A 10-yr climatology of oceanic water vapor derived from the TOPEX microwave radiometer. J. Climate, 17 , 25412557.

  • Chen, G., , and R. Ezraty, 1999: Variation of Southern Ocean sea level and its possible relation with Antarctic sea ice. Int. J. Remote Sens., 20 , 3147.

    • Search Google Scholar
    • Export Citation
  • Chen, G., , J. Ma, , C. Fang, , and Y. Han, 2003: Global oceanic precipitation derived from TOPEX and TMR: Climatology and variability. J. Climate, 16 , 38883904.

    • Search Google Scholar
    • Export Citation
  • Chen, G., , C. Fang, , C. Zhang, , and Y. Chen, 2004: Observing the coupling effect between warm pool and “rain pool” in the Pacific Ocean. Remote Sens. Environ., 91 , 153159.

    • Search Google Scholar
    • Export Citation
  • Gruber, A., 1972: Fluctuations in the position of the ITCZ in the Atlantic and Pacific Oceans. J. Atmos. Sci., 29 , 193197.

  • Hubert, L. F., , A. F. Krueger, , and J. S. Winston, 1969: The double intertropical convergence zone—Fact or fiction? J. Atmos. Sci., 26 , 771773.

    • Search Google Scholar
    • Export Citation
  • Lutgens, F. K., , and E. J. Tarbuck, 1992: The Atmosphere: An Introduction to Meteorology. 5th ed. Prentice Hall, 430 pp.

  • Madden, R. A., , and P. R. Julian, 1971: Detection of a 40-50 day oscillation in the zonal wind in the tropical Pacific. J. Atmos. Sci., 28 , 702708.

    • Search Google Scholar
    • Export Citation
  • McPhaden, M. J., 1999: Genesis and evolution of the 1997–98 El Niño. Science, 283 , 950954.

  • Philander, S. G. H., 1990: El Niño, La Niña, and the Southern Oscillation. Academic Press, 289 pp.

  • Philander, S. G. H., , D. Gu, , D. Halpern, , G. Lambert, , N-C. Lau, , T. Li, , and R. C. Pacanowski, 1996: Why the ITCZ is mostly north of the equator. J. Climate, 9 , 29582972.

    • Search Google Scholar
    • Export Citation
  • Picaut, J., , E. Hackert, , A. J. Busalacchi, , R. Murtugudde, , and G. S. E. Lagerloef, 2002: Mechanisms of the 1997–1998 El Niño–La Niña, as inferred from space-based observations. J. Geophys. Res., 107 .3037, doi:10.1029/2001JC000850.

    • Search Google Scholar
    • Export Citation
  • Randel, D. L., , T. H. Vonder Haar, , M. A. Ringerud, , G. L. Stephens, , T. J. Greenwald, , and C. L. Combs, 1996: A new global water vapor dataset. Bull. Amer. Meteor. Soc., 77 , 12331246.

    • Search Google Scholar
    • Export Citation
  • Rasmusson, E. M., 1985: El Niño and variations in climate. Amer. Sci., 73 , 168178.

  • Rasmusson, E. M., , and P. A. Arkin, 1993: A global view of large-scale precipitation variability. J. Climate, 6 , 14951522.

  • Ropelewski, C. F., , and M. S. Halpert, 1987: Global and regional scale precipitation patterns associated with the El Niño/Southern Oscillation. Mon. Wea. Rev., 115 , 16061626.

    • Search Google Scholar
    • Export Citation
  • Ropelewski, C. F., , and M. S. Halpert, 1989: Precipitation patterns associated with the high index phase of the Southern Oscillation. J. Climate, 2 , 268284.

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Fig. 1.
Fig. 1.

Spatial distribution of the annual amplitude of oceanic water vapor derived from the TMR for (a) normal years (1995, 1996, 2001), (b) El Niño years (1993, 1994, 1997, 2002), and (c) La Niña years (1999, 2000), respectively. Labels “A–E” in (a) indicate the approximate locations of the five most dynamic belts of annual OWV amplitude.

Citation: Monthly Weather Review 133, 10; 10.1175/MWR3013.1

Fig. 2.
Fig. 2.

The averaged yearly variation of oceanic water vapor for the subtropical zone (10°–20°) of the (a) NH and (b) SH derived from the TMR during El Niño years (solid line) and La Niña years (dashed line), respectively.

Citation: Monthly Weather Review 133, 10; 10.1175/MWR3013.1

Fig. 3.
Fig. 3.

Same as in Fig. 1, but for the annual phase. The months in which maximum OWV occurs are indicated on the maps.

Citation: Monthly Weather Review 133, 10; 10.1175/MWR3013.1

Fig. 4.
Fig. 4.

The mean positions of the ITCZ determined from the annual phase map of oceanic water vapor for the El Niño (solid circles) and La Niña (open circles) modes, respectively.

Citation: Monthly Weather Review 133, 10; 10.1175/MWR3013.1

Table 1.

The global and regional mean positions of the ITCZ under the El Niño and La Niña modes based on TMR-derived OWV.

Table 1.
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