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Huan Mei
,
Jianxin Dong
, and
Xiangbai Wu

Abstract

The influence of meridional shift of the oceanic subtropical front (STF) on the Agulhas Current (AC) regime shifts is studied using satellite altimeter data and a 1.5-layer ocean model. The satellite observations suggest the northward shift of the STF leads to the AC leaping across the gap with little Agulhas leakage, and the southward shift of the STF mainly results in the AC intruding into the Atlantic Ocean in the forms of a loop current and an eddy-shedding path, while there are three flow patterns of AC for moderate latitude of the STF. The ocean model results suggest no hysteresis (associated with multiple equilibrium states) exists in the AC system. The model reproduces similar AC regimes depending on different gap widths as in the observations, and model results can be used to explain the observed Agulhas leakage well. We also present the parameter space of the critical AC strength that results in different AC flow patterns as a function of the gap width. The vorticity dynamics of the AC regime shift suggests that the β term is mainly balanced by the viscosity term for the AC in the leaping and loop current paths, while the β and instantaneous vorticity terms are mainly balanced by the advection and viscosity terms for the AC in the eddy-shedding path. These findings help explain the dynamics of the AC flowing across the gateway beyond the tip of Africa affected by the north–south shift of the STF in the leaping regime or penetrating regime.

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Jiaxue Wu
,
Huan Liu
,
Jie Ren
, and
Junjie Deng

Abstract

A velocity spiral in the tidally accelerating bottom boundary layer (BBL) was defined as a directional shear of the prevailing flow with the elevation and the tidal phase. However, so far there is no information on the spiral for the oscillatory BBL flows or a valid explanation for its origin, life history, and persistence. To investigate this rotating current in the tidally accelerating BBL flow, the authors performed instrumented tripod observations in the tidally energetic Zhujiang (Pearl River) estuary. The tidal BBL flows may fall into three distinct regimes: (i) the quasi-steady phase in the peak tide; (ii) the accelerating–decelerating phase at the slack tide; and (iii) the transition between (i) and (ii), when a cyclonic spiral occurs only in the early–late ebb. The subcritical spiral, defined by a Froude number of the oscillatory BBL flow, may be analytically examined by unsteady linearized turbulent BBL equations. The spiral is formed under the momentum balance between local acceleration and bottom friction, independent of stratification conditions. The spiral consists of the “diffusive” and oscillatory boundary layers in the streamwise and spanwise direction, respectively. The streamwise spiral presents an exponential degradation (growth) in the decelerating (accelerating) ebb, indicating its limited life history over a tidal cycle. The transient in bottom stresses induced by the growth or the degradation of the spiral may be the mechanism for sediment trapping in the very little bed friction in the tidal estuary.

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Haolin Luo
,
Ziqian Wang
,
Huan Wu
,
Zhuoyu Zeng
, and
Wei Yu

Abstract

Numerous studies have indicated that the atmospheric heat source (AHS) over the Tibetan Plateau (TP) is highly correlated with the western North Pacific anomalous anticyclone (WNPAC) in summer. However, such an interannual relationship has been weakened since the late 1990s. The present work shows that the TP AHS was significantly and positively correlated with the WNPAC during the period 1979–99 (P1), while this relationship became insignificant hereafter [2000–20 (P2)]. From an atmospheric perspective, we identify that the long-term change in the upper-level atmospheric circulation over the TP is an important cause for weakening the relationship. An obvious upper-level anticyclonic trend occurred over the northeastern TP in the past four decades, with an easterly trend on the anticyclone’s southern flank, with anomalous westerlies during P1 but anomalous easterlies during P2 over the main portion of the TP. With the anomalous upper-level westerlies in P1, abnormal high pressure induced by the TP heating (i.e., AHS) extended downstream in the upper troposphere. Subsequently, anomalous descending motions formed over the northwestern Pacific due to the eastward-extended high pressure, together with the vertical transport of negative relative vorticity, favorable for the enhancement of the WNPAC. In P2, the TP heating-induced abnormal high pressure was confined over the southern TP due to the anomalous easterlies, suppressing its downstream influence and finally breaking the connection between the TP AHS and the WNPAC. Modeling results from both linear baroclinic model (LBM) sensitivity experiments and the CESM Large Ensemble dataset further confirm the important role of the change in background circulation in weakening the relationship.

Significance Statement

The atmospheric heat source (AHS) over the Tibetan Plateau (TP) is generally believed to closely connect with the western North Pacific anomalous anticyclone (WNPAC) during boreal summer. Previous studies have revealed that a significant interannual correlation exists between the TP AHS and the WNPAC; however, such a relationship was weakened recently, but the causes are unknown. This study highlights the important contribution from the change in background circulation to the weakened relationship. An upper-level easterly trend occurred over the TP in recent summers, under which the TP heating-induced abnormal atmospheric response was confined in the TP area, limiting the downstream influence of the TP heating and finally destroying the connection between the TP AHS and the downstream WNPAC.

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Huan Wu
,
Robert F. Adler
,
Yang Hong
,
Yudong Tian
, and
Fritz Policelli

Abstract

A new version of a real-time global flood monitoring system (GFMS) driven by Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA) rainfall has been developed and implemented using a physically based hydrologic model. The purpose of this paper is to evaluate the performance of this new version of the GFMS in terms of flood event detection against flood event archives to establish a baseline of performance and directions for improvement. This new GFMS is quantitatively evaluated in terms of flood event detection during the TRMM era (1998–2010) using a global retrospective simulation (3-hourly and ⅛° spatial resolution) with the TMPA 3B42V6 rainfall. Four methods were explored to define flood thresholds from the model results, including three percentile-based statistical methods and a Log Pearson type-III flood frequency curve method. The evaluation showed the GFMS detection performance improves [increasing probability of detection (POD)] with longer flood durations and larger affected areas. The impact of dams was detected in the validation statistics, with the presence of dams tending to result in more false alarms and greater false-alarm duration. The GFMS validation statistics for flood durations >3 days and for areas without dams vary across the four methods, but center around a POD of ~0.70 and a false-alarm rate (FAR) of ~0.65. The generally positive results indicate the value of this approach for monitoring and researching floods on a global scale, but also indicate limitations and directions for improvement of such approaches. These directions include improving the rainfall estimates, utilizing higher resolution in the runoff-routing model, taking into account the presence of dams, and improving the method for flood identification.

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Huan Mei
,
Yiquan Qi
,
Bo Qiu
,
Xuhua Cheng
, and
Xiangbai Wu

Abstract

We investigate impact of an island on hysteresis of a western boundary current (WBC) flowing across a gap using a nonlinear 1.5-layer ocean model. The results of hysteresis curves show the island in the middle of the gap facilitates the WBC intrusion. The inserted (removed) island in the middle of the gap promotes the WBC to shed eddy (leap across the gap) when the WBC path transits from the periodic penetrating (leaping) to the leaping (periodic penetrating) regime without (with) an island. Vorticity balance analysis reveals that the WBC transition from the eddy-shedding (leaping) to the leaping (eddy-shedding) regime is induced by increased (decreased) meridional advection. Moreover, the critical Reynolds number of the WBC at the Hopf bifurcation is not sensitive to the size and location of the island when the total gap width is fixed. The critical Reynolds number of the WBC translating from the eddy shedding to the leaping regime increases when either the total gap width increases or the island’s meridional length increases; however, the critical Reynolds number is inversely proportional to the width of the southern gap with fixed total gap width and enlarged island length. The island promotes the WBC to shed eddy except when the island is near the northern barrier. The influence of an eastward-shifted island on the WBC transition from the eddy-shedding to the leaping regime is gradually reduced when the island is east of the Munk layer.

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Huan Mei
,
Yiquan Qi
,
Xuhua Cheng
,
Xiangbai Wu
, and
Qiang Wang

Abstract

We study a hysteresis western boundary current (WBC) flowing across a gap impinged by a mesoscale eddy, with an island of variable meridional size in the gap, using a 1.5-layer ocean model. The hysteresis curves suggest the island with a larger size facilitates the WBC intrusion by shedding the eddy more easily. Both anticyclonic and cyclonic eddies are able to induce the critical WBC transition from penetration regime to leap regime, and vice versa. The vorticity balance analysis indicates increased (decreased) meridional advection that induces the critical WBC shifting from the eddy shedding (leaping) regime to the leaping (eddy shedding) regime. The meridional size of the island significantly affects the critical WBC transition in terms of the critical strength of the mesoscale eddy. The regime shift from penetration to leap is most sensitive to the eddy upstream of the WBC for small islands and most sensitive to the southern anticyclonic eddy and northern cyclonic eddy for moderate and large islands. It is least sensitive to the central cyclonic eddy for small islands and to the cyclonic eddy upstream of the WBC for moderate and large islands and to the northern anticyclonic eddy regardless of island size. The regime shift from leap to penetration is most sensitive to the cyclonic eddy upstream of the WBC and to the northern anticyclonic eddy. It is least sensitive to the anticyclonic eddy from the south, and the least sensitive location of the cyclonic eddy shifts northward from the gap center as the island size increases.

Free access
Huan Mei
,
Kunshan Li
,
Qian Liu
,
Bin Wang
, and
Xiangbai Wu

Abstract

The influence of a large-scale circulation (LSC) in a marginal sea on a hysteresis western boundary current (WBC) flowing across a gap is studied using a nonlinear 1.5-layer ocean model. Results show that both single-gyre LSC and double-gyre LSC are able to induce the critical-state WBC transition from the eddy-shedding regime to the leaping regime, while only double-gyre LSC is able to induce the critical-state WBC transition from the leaping regime to the eddy-shedding regime. The dynamics of WBC transition suggests that the meridional advection enhanced by the perturbation of the LSC is responsible for the regime shift from penetration to leap and that the meridional advection reduced by the perturbation of the LSC is responsible for the regime shift from leap to penetration. We also present the parameter space of the critical LSC that can induce the regime shift of WBC far away from the critical state. When the WBC is in the eddy-shedding regime, the critical strength of the single-gyre LSC increases as the WBC transport decreases regardless of the island’s presence in the gap. The critical strength of the double-gyre LSC increases as the WBC transport decreases in the no-island case, while the critical parameter increases as the WBC transport at first decreases and then increases in the island case. When the WBC is in the leaping regime, the critical strength of the double-gyre LSC increases as the WBC transport increases. These results help to explain the observed fact that the Kuroshio flows across the Luzon Strait in the leaping regime or the penetrating regime.

Free access
Pi-Huan Wang
,
Siu-Shung Hong
,
Mao-Fou Wu
, and
Adarsh Deepak

Abstract

The temporal and spatial variations of the zonally-averaged ozone beating rate in the middle atmosphere on a global scale are investigated in detail based on a model study. This study shows that the mean ozone heating rate calculation can be made in a realistic manner by taking advantage of the existing two-dimensional ozone distribution and including the effect of the sphericity of the earth's atmosphere. The obtained ozone heating rates have also been Fourier-analyzed. The common features of the first three harmonic components which correspond respectively to the annual, semiannual and terannual variations are (1) the local maximum amplitudes are located in the altitude regions between 45 and 57 km; (2) local maximum amplitude can be found in the polar region; and (3) maximum horizontal gradients of the beating rate are concentrated in the high latitudes from 60 to 90°. It is also found that the amplitude of the second Fourier component at the pole is about six times greater than that at the equator.

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Huan Wu
,
Robert F. Adler
,
Yudong Tian
,
Guojun Gu
, and
George J. Huffman

Abstract

A multiple-product-driven hydrologic modeling framework (MMF) is utilized for evaluation of quantitative precipitation estimation (QPE) products, motivated by improving the utility of satellite QPE in global flood modeling. This work addresses the challenge of objectively determining the relative value of various QPEs at river basin/subbasin scales. A reference precipitation dataset is created using a long-term water-balance approach with independent data sources. The intercomparison of nine QPEs and corresponding hydrologic simulations indicates that all products with long-term (2002–13) records have similar merits as over the short-term (April–June 2013) Iowa Flood Studies period. The model performance in calculated streamflow varies approximately linearly with precipitation bias, demonstrating that the model successfully translated the level of precipitation quality to streamflow quality with better streamflow simulations from QPEs with less bias. Phase 2 of the North American Land Data Assimilation System (NLDAS-2) has the best streamflow results for the Iowa–Cedar River basin, with daily and monthly Nash–Sutcliffe coefficients and mean annual bias of 0.81, 0.88, and −2.1%, respectively, for the long-term period. The evaluation also indicates that a further adjustment of NLDAS-2 to form the best precipitation estimation should consider spatial–temporal distribution of bias. The satellite-only products have lower performance (peak and timing) than other products, while simple bias adjustment can intermediately improve the quality of simulated streamflow. The TMPA research product (TMPA-RP; research-quality data) can generate results approaching those of the ground-based products with only monthly gauge-based adjustment to the TMPA real-time product (TMPA-RT; near-real-time data). It is further noted that the streamflow bias is strongly correlated to precipitation bias at various time scales, though other factors may play a role as well, especially on the daily time scale.

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I-I. Lin
,
Chun-Chieh Wu
,
Kerry A. Emanuel
,
I-Huan Lee
,
Chau-Ron Wu
, and
Iam-Fei Pun

Abstract

Understanding the interaction of ocean eddies with tropical cyclones is critical for improving the understanding and prediction of the tropical cyclone intensity change. Here an investigation is presented of the interaction between Supertyphoon Maemi, the most intense tropical cyclone in 2003, and a warm ocean eddy in the western North Pacific. In September 2003, Maemi passed directly over a prominent (700 km × 500 km) warm ocean eddy when passing over the 22°N eddy-rich zone in the northwest Pacific Ocean. Analyses of satellite altimetry and the best-track data from the Joint Typhoon Warning Center show that during the 36 h of the Maemi–eddy encounter, Maemi’s intensity (in 1-min sustained wind) shot up from 41 m s−1 to its peak of 77 m s−1. Maemi subsequently devastated the southern Korean peninsula. Based on results from the Coupled Hurricane Intensity Prediction System and satellite microwave sea surface temperature observations, it is suggested that the warm eddies act as an effective insulator between typhoons and the deeper ocean cold water. The typhoon’s self-induced sea surface temperature cooling is suppressed owing to the presence of the thicker upper-ocean mixed layer in the warm eddy, which prevents the deeper cold water from being entrained into the upper-ocean mixed layer. As simulated using the Coupled Hurricane Intensity Prediction System, the incorporation of the eddy information yields an evident improvement on Maemi’s intensity evolution, with its peak intensity increased by one category and maintained at category-5 strength for a longer period (36 h) of time. Without the presence of the warm ocean eddy, the intensification is less rapid. This study can serve as a starting point in the largely speculative and unexplored field of typhoon–warm ocean eddy interaction in the western North Pacific. Given the abundance of ocean eddies and intense typhoons in the western North Pacific, these results highlight the importance of a systematic and in-depth investigation of the interaction between typhoons and western North Pacific eddies.

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