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Richard J. Greatbatch
and
Sheng Zhang

Abstract

The authors report on a regular, interdecadal oscillation in a three-dimensional ocean circulation model. The model is run using box geometry of size comparable to the North Atlantic and is driven by a constant, zonally uniform, surface heat flux. The meridional overturning in the model exhibits a peak to peak oscillation of 7 Sv about a mean of 15 Sv. The period is 50 years. The oscillation has many similarities to that found by Delworth et al. in the GFDL coupled ocean-atmosphere model. In particular, the SST anomaly pattern during the oscillation. is quite similar to that in the coupled model and also to interdecadal anomaly patterns seen in SST data from the North Atlantic. Since the surface flux is constant, the oscillation is due to a balance between convergence in the oscillating part of the poleward heat transport and changes in local heat storage. A similar balance applies to the coupled model where changes in surface heat flux weakly oppose the oscillation. Including salinity, by adding a zonally uniform surface salt flux forcing, acts to weaken the oscillation but does not change its form. This is also consistent with the coupled model. The oscillation is also found when the surface heat flux is calculated interactively, by coupling the ocean model to a zero-heat-capacity model of the atmosphere. The authors suggest that an oscillation of this kind may have played a role in the warming of the North Atlantic surface waters during the 1920s and 1930s and the subsequent cooling in the 1960s.

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Qiang Zhang
,
Wenyu Wang
,
Sheng Wang
, and
Liang Zhang

Abstract

In most parts of the world, pan evaporation decreases with increased air temperature rather than increases, which is known as the “evaporation paradox.” The semiarid Loess Plateau, which is sensitive to global climate change and ecological variations, has a unique warming and drying climate. The authors of this study consider whether pan evaporation shows the same decreasing trend in this unique environment. Meteorological observations of the typical semiarid Dingxi in the Loess Plateau from 1960 to 2010 were used to analyze the variation in pan evaporation and its responses to climatic factors. It was found that the pan evaporation has increased considerably over the past 50 yr, which does not support the evaporation paradox proposed in previous studies. A multifactor model developed to simulate the independent impacts of climate factors on pan evaporation indicated that the temperature, humidity, wind speed, and low cloud cover variations contributed to pan evaporation by 46.18%, 25.90%, 2.48%, and 25.44%, respectively. The increased temperature, decreased relative humidity, and decreased low cloud cover all caused an increase in pan evaporation, unlike many parts of the world where increased low cloud cover offsets the effects of increased temperature and decreased relative humidity on pan evaporation. This may explain why the evaporation paradox occurs. If all relevant factors affecting pan evaporation are considered, it is possible the paradox will not occur. Thus in warm and drying regions, the increased pan evaporation will lead to increasingly arid conditions, which may exacerbate drought and flood disaster occurrences worldwide.

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Wenju Cai
,
Richard J. Greatbatch
, and
Sheng Zhang

Abstract

An idealized, three-dimensional model, of size comparable to the Atlantic, is used to study interdecadal variability of the thermohaline circulation (THC). In most of the model experiments, salinity is kept uniform and constant, the model being driven by surface heat flux only. When the model is driven by the surface heat flux diagnosed from a restoring spinup experiment no oscillations occur. Driving the model by a time-independent, surface heat flux, obtained by applying a “small” zonal redistribution to the diagnosed flux, leads to strong interdecadal oscillations; “small” means that the modification to the diagnosed flux is within the error bars on estimates of surface heat flux based on observations. The model sea surface temperature (SST) anomalies are similar to the observed pattern of SST anomalies in the North Atlantic and to the SST anomalies associated with the interdecadal oscillation in the GFDL fully coupled ocean-atmosphere model. For redistributions that weaken the east-west variation of the flux, the mean THC, and the amplitude/period of the oscillation do not depend strongly on the amount of redistribution, once the threshold beyond which oscillations occur, has been reached. If the east-west variation is enhanced, then the mean THC and the oscillation amplitude/period are very sensitive to the amount of redistribution. Coupling to a simple model of the atmosphere, it is found that redistributing the divergence of the atmospheric heat transport diagnosed from a spinup can lead to iinterdecadal oscillations. An experiment is included that incorporates freshwater flux, wind forcing, and idealized, non-flat bottom topography to show the robustness of our results. This case exhibits interdecadal variability in the vertically integrated transport of the model Gulf Stream. The transport variability is apparently driven by bottom pressure torque variations induced by the variable thermohaline circulation. Greatbatch et al. have suggested that the transport of the Gulf Stream was reduced in 1970–74 corn~ to 1955–59 and that the reduction in transport was driven by bottom pressure torque.

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Richard J. Greatbatch
,
Guoqing Li
, and
Sheng Zhang

Abstract

This paper investigates the hindcasting of interdecadal climate events using an ocean circulation model driven by different combinations of time-varying surface flux, sea surface temperature (SST), and sea surface salinity (SSS) data. Data are generated from a control run, against which the subsequent model experiments are compared. The most robust results are obtained using flux boundary conditions on both surface temperature and salinity. For these boundary conditions, model results am relatively insensitive to noise in the surface data and take about 20 years to overcome the imposition of an incorrect initial condition. Model results are much more sensitive to noisy inputs when run using SST and SSS data. To obtain meaningful results, SST data alone are not sufficient; SSS data are also required. This is related to the well-known instability of ocean climate models upon a switch to mixed boundary conditions. Time-varying SSS data cannot be replaced by climatology; using a best-fit TS relation, to calculate anomalies in SSS from those in SST is also found to give disappointing results. The difficulty of trying to correct for inaccuracies in surface heat flux using SST data, while at the same time using a flux boundary condition on surface salinity, is demonstrated.

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Xuebin Zhang
,
Jian Sheng
, and
Amir Shabbar

Abstract

The multichannel singular spectrum analysis has been used to characterize the spatio–temporal structures of interdecadal and interannual variability of SST over the Pacific Ocean from 20°S to 58°N. Using the Comprehensive Ocean–Atmosphere Data Set from 1950 to 1993, three modes with distinctive spatio-temporal structures were found. They are an interdecadal mode, a quasi-quadrennial (QQ) oscillation with a period of 51 months, and a quasi-biennial (QB) oscillation with a period of 26 months. The interdecadal mode is a standing mode with opposite signs of SST anomalies in the North Pacific and in the tropical Pacific. The amplitude of this mode is larger in the central North Pacific than in the tropical Pacific. This mode contributes 11.4% to the total variance. It is associated with cooling in the central North Pacific and warming in the equatorial Pacific since around 1976–77. The QQ oscillation exhibits propagation of SST anomalies northeastward from the Philippine Sea and then eastward along 40°N, but behaves more like a standing wave over the tropical Pacific. It explains nearly 20% of the total variance. The QB oscillation is localized in the Tropics and is characterized by the westward propagation of SST anomalies near the equator. This mode accounts for 7.4% of the total variance. Since the interdecadal mode is apparently independent of QB and QQ oscillations, it may play an important role in configuring the state of the tropical SST anomalies, which in turn affects the strength of the El Niño–Southern Oscillation phenomenon. It seems likely that the higher phase of the interdecadal mode since 1976–77 has raised the background SST state, on which the superposition of the QQ and QB oscillations produced the strongest warm event on record in 1982–83, as well as more frequent warm events since 1976.

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Sheng Zhang
,
Richard J. Greatbatch
, and
Charles A. Lin

Abstract

In this paper, the physical mechanism of the polar halocline catastrophe (PHC) is reexamined with emphasis on the role played by the surface heat flux. It is argued that, in a coupled ocean–atmosphere system, thermal changes in the atmospheric state in response to changes in heat flux from the ocean weaken the feedback responsible for the PHC.

So far, the PHC has been observed in models that use mixed boundary conditions; that is, the freshwater flux is specified, but the surface temperature is relaxed to a specified value. Previous explanations of the PHC have focused on the role of the freshwater flux in establishing a freshwater cap and shutting off the deep convection. However, the establishment of a freshwater cap reduces the depth of the water column that is cooled by surface heat loss. As a consequence, the surface temperature is reduced. Since the difference between this and atmospheric restoring temperature is now less, there is a corresponding reduction in the surface heat loss to the atmosphere, and this acts to further stabilize the water column. We examine the importance of this reduction in surface heat loss by considering two numerical experiments that are identical except that one is run under mixed boundary conditions and the other under a flux boundary condition applied to temperature as well as salinity. In each case, the surface fluxes are diagnosed from an experiment run to equilibrium using restoring boundary conditions on both fields. This also provides the initial state for both experiments. A PHC is easily induced in the mixed boundary condition case but not in the case using flux boundary conditions. By reducing the magnitude of the heat flux but not its sign, a pool of fresh water appears at the surface, but its effect is weaker than that under mixed boundary conditions and, in particular, there is no collapse of the meridional overturning circulation. A pool of fresh water also appears in an experiment in which a small, positive heat flux is added at all latitudes, a situation of relevance to global warming. This leads to an initial cooling in a shallow layer at the surface of the polar oceans, before heating at lower latitudes leads to a collapse of this state.

These experiments show that the reduction in the surface heat flux that occurs when the PHC develops under mixed boundary conditions is an essential feature of the PHC. The use of mixed boundary conditions assumes that the atmospheric state is fixed and does not respond to changes in heat flux from the ocean. If the atmosphere were allowed to adjust to changes in this heat flux, then a PHC would be less likely to occur. This has been demonstrated by coupling the ocean model to the zero-heat-capacity atmospheric model used by Schopf. This is justified, following Bretherton, because of the large horizontal scale of the sea surface temperature (SST) anomalies in the experiments. The authors were unable to induce a PHC with this model. In reality, the atmospheric boundary condition seen by the ocean lies somewhere between the two extremes of mixed boundary conditions, on the one hand, and Schopf's model on the other. We have investigated this intermediate region by conducting experiments in which SST anomalies are damped on successively shorter time scales. These show that if the damping time is reduced sufficiently, a PHC can again be induced.

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Jian-Sheng Ye
,
Yan-Hong Gong
,
Feng Zhang
,
Jiao Ren
,
Xiao-Ke Bai
, and
Yang Zheng

Abstract

Intensifying climate extremes are one of the major concerns with climate change. Using 100-yr (1911–2010) daily temperature and precipitation records worldwide, 28 indices of extreme temperature and precipitation are calculated. A similarity percentage analysis is used to identify the key indices for distinguishing how extreme warm and cold years (annual temperature above the 90th and below the 10th percentile of the 100-yr distribution, respectively) differ from one another and from average years, and how extreme wet and dry years (annual precipitation above the 90th and below the 10th percentile of the 100-yr distribution, respectively) differ from each other and from average years. The analysis suggests that extreme warm years are primarily distinguished from average and extreme cold years by higher occurrence of warm nights (annual counts when night temperature >90th percentile), which occur about six more counts in extreme warm years compared with average years. Extreme wet years are mainly distinguished from average and extreme dry years by more occurrences of heavy precipitation events (events with ≥10 mm and ≥20 mm precipitation). Compared with average years, heavy events occur 60% more in extreme wet years and 50% less in extreme dry years. These indices consistently differ between extreme and average years across terrestrial ecoregions globally. These key indices need to be considered when analyzing climate model projections and designing climate change experiments that focus on ecosystem response to climate extremes.

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Z. Q. Fan
,
Z. Sheng
,
H. Q. Shi
,
X. H. Zhang
, and
C. J. Zhou

Abstract

Global stratospheric temperature measurement is an important field in the study of climate and weather. Dynamic and radiative coupling between the stratosphere and troposphere has been demonstrated in a number of studies over the past decade or so. However, studies of the stratosphere were hampered by a shortage of observation data before satellite technology was used in atmospheric sounding. Now, the data from the Thermosphere, Ionosphere, Mesosphere Energetics, and Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry (TIMED/SABER) observations make it easier to study the stratosphere. The precision and accuracy of TIMED/SABER satellite soundings in the stratosphere are analyzed in this paper using refraction error data and temperature data obtained from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) radio occultation sounding system and TIMED/SABER temperature data between April 2006 and December 2009. The results show high detection accuracy of TIMED/SABER satellite soundings in the stratosphere. The temperature standard deviation (STDV) errors of SABER are mostly in the range from of 0–3.5 K. At 40 km the STDV error is usually less than 1 K, which means that TIMED/SABER temperature is close to the real atmospheric temperature at this height. The distributions of SABER STDV errors follow a seasonal variation: they are approximately similar in the months that belong to the same season. As the weather situation is complicated and fickle, the distribution of SABER STDV errors is most complex at the equator. The results in this paper are consistent with previous research and can provide further support for application of the SABER’s temperature data.

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Sheng Chen
,
Jonathan J. Gourley
,
Yang Hong
,
Qing Cao
,
Nicholas Carr
,
Pierre-Emmanuel Kirstetter
,
Jian Zhang
, and
Zac Flamig

Abstract

In meteorological investigations, the reference variable or “ground truth” typically comes from an instrument. This study uses human observations of surface precipitation types to evaluate the same variables that are estimated from an automated algorithm. The NOAA/National Severe Storms Laboratory’s Multi-Radar Multi-Sensor (MRMS) system relies primarily on observations from the Next Generation Radar (NEXRAD) network and model analyses from the Earth System Research Laboratory’s Rapid Refresh (RAP) system. Each hour, MRMS yields quantitative precipitation estimates and surface precipitation types as rain or snow. To date, the surface precipitation type product has received little attention beyond case studies. This study uses precipitation type reports collected by citizen scientists who have contributed observations to the meteorological Phenomena Identification Near the Ground (mPING) project. Citizen scientist reports of rain and snow during the winter season from 19 December 2012 to 30 April 2013 across the United States are compared to the MRMS precipitation type products. Results show that while the mPING reports have a limited spatial distribution (they are concentrated in urban areas), they yield similar critical success indexes of MRMS precipitation types in different cities. The remaining disagreement is attributed to an MRMS algorithmic deficiency of yielding too much rain, as opposed to biases in the mPING reports. The study also shows reduced detectability of snow compared to rain, which is attributed to lack of sensitivity at S band and the shallow nature of winter storms. Some suggestions are provided for improving the MRMS precipitation type algorithm based on these findings.

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Sheng Chen
,
Jonathan J. Gourley
,
Yang Hong
,
P. E. Kirstetter
,
Jian Zhang
,
Kenneth Howard
,
Zachary L. Flamig
,
Junjun Hu
, and
Youcun Qi

Abstract

Quantitative precipitation estimation (QPE) products from the next-generation National Mosaic and QPE system (Q2) are cross-compared to the operational, radar-only product of the National Weather Service (Stage II) using the gauge-adjusted and manual quality-controlled product (Stage IV) as a reference. The evaluation takes place over the entire conterminous United States (CONUS) from December 2009 to November 2010. The annual comparison of daily Stage II precipitation to the radar-only Q2Rad product indicates that both have small systematic biases (absolute values > 8%), but the random errors with Stage II are much greater, as noted with a root-mean-squared difference of 4.5 mm day−1 compared to 1.1 mm day−1 with Q2Rad and a lower correlation coefficient (0.20 compared to 0.73). The Q2 logic of identifying precipitation types as being convective, stratiform, or tropical at each grid point and applying differential ZR equations has been successful in removing regional biases (i.e., overestimated rainfall from Stage II east of the Appalachians) and greatly diminishes seasonal bias patterns that were found with Stage II. Biases and radar artifacts along the coastal mountain and intermountain chains were not mitigated with rain gauge adjustment and thus require new approaches by the community. The evaluation identifies a wet bias by Q2Rad in the central plains and the South and then introduces intermediate products to explain it. Finally, this study provides estimates of uncertainty using the radar quality index product for both Q2Rad and the gauge-corrected Q2RadGC daily precipitation products. This error quantification should be useful to the satellite QPE community who use Q2 products as a reference.

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