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Ge Chen

Abstract

Using the newly available TOPEX Microwave Radiometer (TMR) data spanning 1993 through 2002, a 10-yr climatology of oceanic water vapor (OWV) is constructed, of which the distribution and variation at various spatial–temporal scales are investigated. The new dataset confirms most of the well-known OWV features, and yields a number of interesting findings, due to its high quality, long duration, and unique orbit. 1) The TMR-derived climatology compares well, in both overall pattern and general statistics, with similar results based on radiosondes and other satellites. Climatological comparisons with sea surface temperature and oceanic precipitation suggest that the western Pacific warm pool is “mirrored” in the atmosphere as a “wet pool,” whereas the meteorological equator is reflected in OWV as a transocean equatorial wet belt. 2) It is found that El Niño (La Niña) events are accompanied by a significant increase (decrease) in the amount of OWV between 10°S and 10°N with a somewhat unexpected Southern Hemisphere dominance. This is particularly evident during the 1997/98 El Niño when the interannual variability of OWV reaches a record high. Composite maps of annual OWV anomalies disclose a dipolelike pattern in the western equatorial Pacific with a phase opposition between El Niño and La Niña years. 3) The annual amplitude of OWV is characterized by six cross-continent wet belts located largely in the subtropics of both hemispheres. The phase patterns of the annual and semiannual variations are hemispherically divided, and climatologically correlated, respectively. North (south) of the intertropical convergence zone (ITCZ), a majority of the oceanic areas have their water vapor maximum in August (February). Early peaks in July are found over a few continental shelf regions of the Northern Hemisphere (NH), while late peaks in March are found in the tropical oceans of the Southern Hemisphere (SH). Moreover, two delayed maximums in September are visible in the interior North Pacific and North Atlantic, respectively. 4) The daily cycle of OWV is strongly coupled with its seasonal cycle, and is therefore unstable in nature. But a double-peak structure with a general hemispheric phase reversal can still be identified. 5) The ratio of the NH versus SH OWV is roughly 1.17:1, and the relative importance of the interannual, annual, semiannual, diurnal, and semidiurnal variations in terms of mean amplitude is approximately 1.8:5:1.2:1:1. In view of these encouraging results, further exploration of present and future “altimeter-borne” radiometer data will no doubt lead to an improved and complementary understanding of the OWV system in many aspects.

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Ge Chen

Abstract

The availability of multiple satellite missions with wind measuring capacity has made it more desirable than ever before to integrate wind data from various sources in order to achieve an improved accuracy, resolution, and duration. A clear understanding of the error characteristics associated with each type of data is needed for a meaningful merging or combination. The two kinds of errors—namely, random error and systematic error—are expected to evolve differently with increasing volume of available data. In this study, a collocated ocean Topography Experiment (TOPEX)–NASA Scatterometer (NSCAT)–ECMWF dataset, which covers 66°S–66°N and spans the entire 10-month lifetime of NSCAT, is compiled to investigate the systematic discrepancies among the three kinds of wind estimates, yielding a number of interesting results. First, the satellite-derived wind speeds are found to have a larger overall bias but a much smaller overall root-mean-square (rms) error compared to ECMWF winds, implying that they are highly converging but are systematically biased. Second, the TOPEX and NSCAT wind speed biases are characterized by a significant “phase opposition” with latitude, season, and wind intensity, respectively. Third, the TOPEX (NSCAT) bias exhibits a low–high–low (high–low–high) pattern as a function of wind speed, whose turning point at 14.2 m s−1 coincides well with the transitional wind speed from swell dominance to wind sea dominance in wave condition, suggesting that the degree of wave development plays a key role in shaping wind speed bias.

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Ge Chen and Hanou Chen

Abstract

Using the newly available decade-long Argo data for the period 2004–13, a detailed study is carried out on deriving four-dimensional (4D) modality of sea temperature in the upper ocean with emphasis on its interannual variability in terms of amplitude, phase, and periodicity. Three principal modes with central periodicities at 19.2, 33.8, and 50.3 months have been identified, and their relationship with El Niño–Southern Oscillation (ENSO) is investigated, yielding a number of useful results and conclusions: 1) A striking tick-shaped pipe-like feature of interannual variability maxima, which is named the “Niño pipe” in this paper, has been revealed within the 10°S–10°N upper Pacific Ocean. 2) The pipe core extends downward from ~50 m at 130°E to ~250 m near the date line before tilting upward to the sea surface at about 275°E, coinciding nicely with the pathway of the Pacific equatorial undercurrent (EUC). 3) The double-peak zonal modality pattern of the Niño pipe in the upper Pacific is echoed in the subsurface Atlantic and Indian Oceans through Walker circulation, while its single-peak meridional modality pattern is mirrored in the subsurface North and South Pacific through Hadley circulation. 4) A coherent three-peak modal structure implies a strong coupling between sea level variability at the surface and sea temperature variability around the thermocline. Accumulating evidence suggests that Rossby/Kelvin wave dynamics in tandem with EUC-based thermocline dynamics are the main mechanisms of the three-mode Niño pipe in ENSO cycles.

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Ge Chen and Xuan Wang

Abstract

A decade of newly available Argo float data for the period 2004–13 are used to investigate the three-dimensional structures of upper-ocean seasonality with emphasis on the vertical aspects of annual and semiannual cycles, yielding three main findings with oceanographic implications. First, the vertical evolution of the horizontal pattern of annual and semiannual amplitudes appears to be highly “nonlinear,” suggesting that the thermodynamic causes are depth dependent. The global ocean seasonality exhibits a vertically varying pattern in space, including midlatitude maxima in the near-surface layer due to solar forcing, zonal “strips” in the subsurface layer due to the equatorial current system, and systematic westward phase propagation in the intermediate layer due to annual Rossby waves. Second, a zone of 500 ± 300-m depths along with a 6-month periodicity are chosen as appropriate space–time “windows” for detecting eddy signatures via Argo-derived temperature amplitude and phase, respectively. It is revealed that the eddy-induced “blobby” pattern observed previously by satellite altimeter appears in the Agro result as “woodsy” bulks, which can be well illustrated in the semiannual amplitude and phase maps at window depths. Meanwhile, six eddy deserts paired in each ocean basin have also been identified. Third, the existence of a dozen vertical quasi-annual amphidromes is first reported, with cophase lines that may radiate toward the ~2000-m lower limit of Argo measurement. The well-known global meridional overturning circulation and the pseudozonal overturning currents in the equatorial Pacific, Atlantic, and Indian Oceans may possibly contribute to the observed vertical amphidromes.

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Ge Chen and Hui Lin

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.

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Ge Chen and Haitao Li

Abstract

A natural mode refers, in this study, to a periodic oscillation of sea surface temperature (SST) that is geophysically significant on a global, regional, or local scale. Using a newly developed harmonic extraction scheme by Chen, which has the advantage of being space–time decoupled and fully data adaptive, a variety of natural modes have been recovered from global monthly SST data for the period of 1985–2003. Among them, the eight most significant modes are identified as primary modes, whose spatial patterns are presented, along with their phase distributions. At seasonal time scales, a 4-month primary mode is uncovered in addition to the well-documented annual and semiannual cycles. At interannual time scales, the dominant El Niño–Southern Oscillation (ENSO) variability is found to be composed of at least five primary modes, with well-defined central periods around 18, 25, 32, 43, and 63 months. At time scales beyond ENSO, a decadal SST signal with an average period of 10.3 yr is observed. A unique contribution of this study is the derivation and presentation of fine patterns of natural SST modes and signals in joint dimensions of time, space, period, and phase, leading to several findings and conclusions that are of potential importance: 1) The degree of separability and regularity of the sub-ENSO modes is surprising, and thus reveals new details on the nature of this event. 2) The midlatitude counterparts of the equatorial interannual and decadal SST modes/signals are found in the two hemispheres with a frequency shift toward longer periods. The “shadows” of the Pacific Ocean’s ENSO modes are also observed with some detail in the Atlantic and the Indian Oceans. All of these provide direct evidence that teleconnections exist between the equatorial and extratropical oceans, as well as among the tropical Pacific, tropical Atlantic, and tropical Indian Oceans, possibly as a result of the “atmospheric bridge.” 3) A sharply opposite anisotropy is observed in the spatiotemporal pattern between the interannual modes and decadal signals, implying that they are potentially of a categorical difference in origin. 4) Locality or regionality is a fundamental feature for most of the SST modes. Treating the interannual or decadal variability as a single ENSO or Pacific decadal oscillation mode appears to be an oversimplification, and may lead to inappropriate interpretations. The results herein represent an improved knowledge of the natural variability in sea surface temperature, which will hopefully help to enhance the understanding of natural fluctuations of the global/regional climate system in the context of ocean–atmosphere interaction.

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Haoyu Jiang and Ge Chen

Abstract

In this study, a global climatology of swells and wind seas was investigated using near-10-yr collocated wind speed and significant wave height (SWH) measurements from the basic Geophysical Data Record (GDR) of the Jason-1 mission. A statistical method to estimate the wind sea and swell SWHs, respectively, on the basis of wave energy and wind sea/swell probability was proposed. The global distributions of swell/wind sea probability displayed the swell's dominance in the World Ocean. Their seasonal variation showed not only the regions called “swell pools” with high swell probability throughout the year at low latitudes, which have been found in previous studies, but also the regions with high swell probability only in hemispheric summer, termed “seasonal swell pools,” located at the midlatitudes of open oceans. The seasonal geographical patterns of the swell SWH were similar to those of the SWH due to the swell's dominance, and the patterns of the wind SWH were similar to those of the wind speed because of their well-coupled nature. The results could be used as a reference for related applications such as ocean engineering, seafaring, validation of wave models, and studies on climate change.

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Haifeng Zhang, Qing Wu, and Ge Chen

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The Haiyang-2A (HY-2A; HaiYang means ocean in Chinese) satellite was successfully launched in China on 16 August 2011, carrying the nation’s first operational radar altimeter along with three other microwave sensors. In this study, HY-2A altimeter significant wave height (SWH) data have been validated against National Data Buoy Center (NDBC) buoy and Jason-2 altimeter SWH data over a period of 27 months (from 1 October 2011 to 31 December 2013). During the collocation, the effects of different thresholds of several flags are carefully studied. These flags prove to be useful for the SWH selection and different thresholds are observed to change the results remarkably. The final results show that HY-2A SWHs, with a 0.339-m root-mean-square (RMS) difference and a negative bias of 0.231 m in buoy comparison, have reached the mission target (0.5-m RMS). Nonetheless, the Jason-2 altimeter performs better with a lower RMS difference of 0.292 m and a positive bias of only 0.016 m. In addition, by analyzing the residuals (altimeter minus buoy), the bias for the HY-2A altimeter is found to decline monotonically over the whole range with an overestimation at low sea state (SWH < 1 m), a minor underestimation at middle sea state (1 m < SWH < 5 m), and a severe underestimation at high sea state (SWH > 5 m). However, only an underestimation at high sea state is found for the Jason-2 altimeter. A linear regression is also proposed. The 20 days of the newly processed HY-2A SWHs are investigated and discussed as well, and a slight quality improvement has been observed using these data.

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Ge Chen, Xuan Wang, and Chengcheng Qian

Abstract

Seasonality is a fundamental feature of the coupled ocean–atmosphere system. The hemispherically opposite general pattern at the air–sea interface is well known, but its penetrative behavior below the sea surface is poorly understood. Over 10 years of Argo data reveal for the first time the spiral-like structure of vertical seasonality in the upper extratropical ocean: the amplitude (strength of annual sea temperature variability) decreases rapidly with depth while the phase (peaking time) rotates from August (February) at the sea surface to December (June) at the bottom of the mixed layer in the Northern (Southern) Hemisphere under a clockwise representation. It is found that, analogous to the Ekman current spiral, the oceanic seasonality is almost reversed at approximately 500-m depth with about 5% of the surface intensity left. In contrast, the seasonality of subtropical oceans below the thermocline exhibits a phase-lock pattern around May and October to the north and south of the equator, respectively. Meanwhile, a systematic westward progression of annual phase corresponding to the warmest month is observed in the equatorial regions between 10°S and 10°N. It is suggested that the seasonality spiral of extratropical oceans occurs as a result of the vertical decay of solar penetration in tandem with delayed annual maximum mixing in the context of buoyancy and turbulent induced convections, while the month of full summer appears to be “constant” May (October) in the subtropical oceans of the Northern (Southern) Hemisphere under the stratified subsurface layer due to a 9-month trapping of penetrating solar energy from May (October) to next January (June). The vertically locked westward annual phase progression in the equatorial regions is likely to be a consequence of the first baroclinic mode of β-refracted annual Rossby waves. The spiral and phase-lock behaviors of the upper oceans are of critical significance to the understanding of mixed layer and thermocline dynamics.

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Ge Liu, Ping Zhao, and Junming Chen

Abstract

The summer (June–August) Asian–Pacific Oscillation (APO), a large-scale atmospheric teleconnection pattern, is closely associated with climate anomalies over the Northern Hemisphere. Using the NOAA/CIRES twentieth-century reanalysis, the ECMWF twentieth-century atmospheric reanalysis, and the NCEP reanalysis, this study investigates the variability of the summer APO on the interannual time scale and its relationship with the thermal condition over the Tibetan Plateau (TP). The results show that the interannual variability of the APO is steadily related to the summer TP surface air temperature during the last 100 years. Observation and simulation further show that a positive heating anomaly over the TP can increase the upper-tropospheric temperature and upward motion over Asia. This anomalous upward flow moves northward in the upper troposphere, and then turns and moves eastward, before finally descending over the mid- to high latitudes of the central-eastern North Pacific, concurrently accompanied by anomalous upward motion over the lower latitudes of the central-eastern North Pacific. The anomalous downward and upward motions over the central-eastern North Pacific reduce the in situ mid- and upper-tropospheric temperature, mainly through modulating condensation latent heat from precipitation and/or dry adiabatic heat, which ultimately leads to the interannual variability of the summer APO. In this process, the zonal vertical circulation over the extratropical Asian–North Pacific sector plays an important bridging role.

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