Search Results

You are looking at 1 - 10 of 35 items for

  • Author or Editor: Uma S. Bhatt x
  • Refine by Access: All Content x
Clear All Modify Search
Uma S. Bhatt

Abstract

This study explores the spatial differentiation of climate anomalies and associated circulation mechanisms across the African-South Asian monsoon belt through empirical analyses mainly for the period 1948–83. Observations include surface ship observations in the tropical Atlantic, eastern Pacific, and Indian oceans, and various hydrometeorological and circulation index series, representing the water discharge of the Senegal River (SENEGAL), the rainfall in the West African Sahel (SAHEL), the discharge of rivers in the Nile basin (ROSEIRES, ATBARA), India monsoon rainfall (NIR), and the Southern Oscillation (SO). The field significance of correlation patterns is ascertained through Monte Carlo experiments.

There is a strong correlation of hydrometeorological conditions from Senegal to the Sahel, a decrease from each of these domains eastward to the Nile catchment, and even more so to India. Conversely, correlations are remarkably high between the water discharge of the Nile basin (ROSEIRES, ATBARA) and Indian rainfall (NIR). An SO index is correlated positively with the hydrometeorological conditions throughout the monsoon belt, but most strongly in the East (NIR) and least in the West (SENEGAL).

Correlation analyses for the July-August height of the boreal summer monsoon indicate that abundant rainfall in the western Sahel is, in the Atlantic sector, associated with weak northeast trades and in the western Indian Ocean with low pressure overlying cool surface waters. By comparison, copious river discharge in the eastern portion of the Subsaharan zone coincides with weakened northeast trades over the Atlantic but more pronounced circulation departures in the Indian Ocean sector, consisting of anomalously low pressure, cold surface waters, and abundant cloudiness in the northern Indian Ocean. This ensemble of atmosphere-ocean anomalies is also characteristic of abundant Indian monsoon rainfall. During the positive SO phase, rainfall tends to be relatively abundant throughout the monsoon belt, but most markedly so in the eastern portion of Subsaharan Africa and in India. Overall, the western Sahel shows strongest associations with circulation departures in the tropical Atlantic, whereas the eastern portion of Subsaharan Africa and India are more closely related to the, circulation of the Indian Ocean sector.

Full access
Edwin K. Schneider
and
Uma S. Bhatt

Abstract

An integral balance is developed for steady fluid flows relating dissipation in volumes bounded by isosurfaces of a tracer (quasi-conserved quantity) and solid boundaries to the covariance of the tracer value and surface fluxes across the boundaries. The balance is used to estimate upper bounds for vertical eddy diffusion coefficients for temperature and salinity in various volumes of the ocean. The vertical temperature diffusivity is calculated to be small, O(0.1 × 10−4 m2 s−1), except for the warmest and coldest volumes of the ocean. The vertical salinity diffusivity for the volume that makes up most of the deep ocean is estimated to be O(1 × 10−4 m2 s−1). Sources of error in these calculations are discussed, and the sensitivity to errors in the surface flux data is evaluated.

The dissipation integral is also applied to demonstrate some related results concerning extrema and homogenization. The Prandtl–Batchelor theorem is a special case of one of these results. As a consequence of these results, if turbulent transfer is downgradient and there are no internal sources or sinks, a necessary (but not sufficient) condition for a climatological tracer distribution to be in a steady state is the absence of internal extrema. The climatological salinity distribution does not appear to violate this condition.

Full access
Stephen J. Vavrus
,
Uma S. Bhatt
, and
Vladimir A. Alexeev

Abstract

This study diagnoses the changes in Arctic clouds simulated by the Community Climate System Model version 3 (CCSM3) in a transient 2 × CO2 simulation. Four experiments—one fully coupled and three with prescribed SSTs and/or sea ice cover—are used to identify the mechanisms responsible for the projected cloud changes. The target simulation uses a T42 version of the CCSM3, in which the atmosphere is coupled to a dynamical ocean with mobile sea ice. This simulation is approximated by a T42 atmosphere-only integration using CCSM3’s atmospheric component [the Community Atmosphere Model version 3 (CAM3)] forced at its lower boundary with the changes in both SSTs and sea ice concentration from CCSM3’s 2 × CO2 run. The authors decompose the combined effect of the higher SSTs and reduced sea ice concentration on the Arctic cloud response in this experiment by running two additional CAM3 simulations: one forced with modern SSTs and the projected sea ice cover changes in CCSM3 and the other forced with modern sea ice coverage and the projected changes in SSTs in CCSM3.

The results suggest that future increases in Arctic cloudiness simulated by CCSM3 are mostly attributable to two separate processes. Low cloud gains are primarily initiated locally by enhanced evaporation within the Arctic due to reduced sea ice, whereas cloud increases at middle and high levels are mostly driven remotely via greater meridional moisture transport from lower latitudes in a more humid global atmosphere. The enhanced low cloudiness attributable to sea ice loss causes large increases in cloud radiative forcing during the coldest months and therefore promotes even greater surface warming. Because CCSM3’s Arctic cloud response to greenhouse forcing is similar to other GCMs, the driving mechanisms identified here may be applicable to other models and could help to advance our understanding of likely changes in the vertical structure of polar clouds.

Full access
Gerald V. Frost
,
Uma S. Bhatt
,
Matthew J. Macander
,
Amy S. Hendricks
, and
M. Torre Jorgenson

Abstract

Alaska’s Yukon–Kuskokwim Delta (YKD) is among the Arctic’s warmest, most biologically productive regions, but regional decline of the normalized difference vegetation index (NDVI) has been a striking feature of spaceborne Advanced High Resolution Radiometer (AVHRR) observations since 1982. This contrast with “greening” prevalent elsewhere in the low Arctic raises questions concerning climatic and biophysical drivers of tundra productivity along maritime–continental gradients. We compared NDVI time series from AVHRR, the Moderate Resolution Imaging Spectroradiometer (MODIS), and Landsat for 2000–19 and identified trend drivers with reference to sea ice and climate datasets, ecosystem and disturbance mapping, field measurements of vegetation, and knowledge exchange with YKD elders. All time series showed increasing maximum NDVI; however, whereas MODIS and Landsat trends were very similar, AVHRR-observed trends were weaker and had dissimilar spatial patterns. The AVHRR and MODIS records for time-integrated NDVI were dramatically different; AVHRR indicated weak declines, whereas MODIS indicated strong increases throughout the YKD. Disagreement largely arose from observations during shoulder seasons, when there is partial snow cover and very high cloud frequency. Nonetheless, both records shared strong correlations with spring sea ice extent and summer warmth. Multiple lines of evidence indicate that, despite frequent disturbances and high interannual variability in spring sea ice and summer warmth, tundra productivity is increasing on the YKD. Although climatic drivers of tundra productivity were similar to more continental parts of the Arctic, our intercomparison highlights sources of uncertainty in maritime areas like the YKD that currently, or soon will, challenge historical concepts of “what is Arctic.”

Full access
Uma S. Bhatt
,
Michael A. Alexander
,
David S. Battisti
,
David D. Houghton
, and
Linda M. Keller

Abstract

The impact of an interactive ocean on the midlatitude atmosphere is examined using a 31-yr integration of a variable depth mixed layer ocean model of the North Atlantic (between 20° and 60°N) coupled to the NCAR Community Climate model (CCM1). Coupled model results are compared with a 31-yr control simulation where the annual cycle of sea surface temperatures is prescribed. The analysis focuses on the northern fall and winter months.

Coupling does not change the mean wintertime model climatology (December–February); however, it does have a significant impact on model variance. Air temperature and mixing ratio variance increase while total surface heat flux variance decreases. In addition, it is found that air–sea interaction has a greater impact on seasonally averaged variance than monthly variance.

There is an enhancement in the persistence of air temperature anomalies on interannual timescales as a result of coupling. In the North Atlantic sector, surface air and ocean temperature anomalies during late winter are uncorrelated with the following summer but are significantly correlated (0.4–0.6) with anomalies during the following winter. These autocorrelations are consistent with the “re-emergence” mechanism, where late winter ocean temperature anomalies are sequestered beneath the shallow summer mixed layer and are reincorporated into the deepening fall mixed layer. The elimination of temperature anomalies from below the mixed layer in a series of uncoupled sensitivity experiments notably reduces the persistence of year-to-year anomalies.

The persistence of air temperature anomalies on monthly timescales also increases with coupling and is likely associated with “decreased thermal damping.” When coupled to the atmosphere, the ocean is able to adjust to the overlying atmosphere so that the negative feedback associated with anomalous heat fluxes decreases, and air temperature anomalies decay more slowly.

Full access
Xiangdong Zhang
,
John E. Walsh
,
Jing Zhang
,
Uma S. Bhatt
, and
Moto Ikeda

Abstract

Arctic cyclone activity is investigated in the context of climate change and variability by using a modified automated cyclone identification and tracking algorithm, which differs from previously used algorithms by single counting each cyclone. The investigation extends earlier studies by lengthening the time period to 55 yr (1948– 2002) with a 6-hourly time resolution, by documenting the seasonality and the dominant temporal modes of variability of cyclone activity, and by diagnosing regional activity as quantified by the cyclone activity index (CAI). The CAI integrates information on cyclone intensity, frequency, and duration into a comprehensive index of cyclone activity. Arctic cyclone activity has increased during the second half of the twentieth century, while midlatitude activity generally decreased from 1960 to the early 1990s, in agreement with previous studies. New findings include the following. 1) The number and intensity of cyclones entering the Arctic from the midlatitudes has increased, suggesting a shift of storm tracks into the Arctic, particularly in summer. 2) Positive tendencies of midlatitude cyclone activity before and after the 1960–93 period of decreasing activity correlate most strongly with variations of cyclone activity in the North Atlantic and Eurasian sectors. 3) Synchronized phase and amplitude variations in cyclone activity over the Arctic Ocean (70°–90°N) and the Arctic marginal zone (60°– 70°N) play a critical role in determining the variations of cyclone activity in the Arctic as a whole. 4) Arctic cyclone activity displays significant low-frequency variability, with a negative phase in the 1960s and a positive phase in the 1990s, upon which 7.8- and 4.1-yr oscillations are superimposed. The 7.8-yr signal generally corresponds to the alternation of the cyclonic and anticyclonic regimes of the Arctic sea ice and ocean motions.

Full access
Dyre O. Dammann
,
Uma S. Bhatt
,
Peter L. Langen
,
Jeremy R. Krieger
, and
Xiangdong Zhang

Abstract

Climate projections suggest that an ice-free summer Arctic Ocean is possible within several decades and with this comes the prospect of increased ship traffic and safety concerns. The daily sea ice concentration tendency in five Coupled Model Intercomparison Project phase 5 (CMIP5) simulations is compared with observations to reveal that many models underestimate this quantity that describes high-frequency ice movements, particularly in the marginal ice zone. To investigate whether high-frequency ice variability impacts the atmosphere, the Community Atmosphere Model, version 3.0 (CAM3.0), is forced by sea ice with and without daily fluctuations. Two 100-member ensemble experiments with daily varying (DAILY) and smoothly varying (SMTH) sea ice are conducted, along with a climatological control, for an anomalously low ice period (August 2006–November 2007). Results are presented for three periods: September 2006, October 2006, and December–February (DJF) 2006/07. The atmospheric response differs between DAILY and SMTH. In September, sea ice differences lead to an anomalous high and weaker storm activity over northern Europe. During October, the ice expands equatorward faster in DAILY than SMTH in the Siberian seas and leads to a local response of near-surface cooling. In DJF, there is a 1.5-hPa positive sea level pressure anomaly over North America, leading to anomalous northerly flow and anomalously cool continental U.S. temperatures. While the atmospheric responses are modest, the differences arising from high temporal frequency ice variability cannot be ignored. Increasing the accuracy of coupled model sea ice variations on short time scales is needed to improve short-term coupled model forecasts.

Full access
Peter A. Bieniek
,
John E. Walsh
,
Richard L. Thoman
, and
Uma S. Bhatt

Abstract

By extending the record of Alaskan divisional temperature and precipitation back in time, regional variations and trends of temperature and precipitation over 1920–2012 are documented. The use of the divisional framework highlights the greater spatial coherence of temperature variations relative to precipitation variations.

The divisional time series of temperature are characterized by large interannual variability superimposed upon low-frequency variability, as well as by an underlying trend. Low-frequency variability corresponding to the Pacific decadal oscillation (PDO) includes Alaska’s generally warm period of the 1920s and 1930s, a cold period from the late 1940s through the mid-1970s, a warm period from the late 1970s through the early 2000s, and a cooler period in the most recent decade. An exception to the cooling of the past decade is the North Slope climate division, which has continued to warm. There has been a gradual upward trend of Alaskan temperatures relative to the PDO since 1920, resulting in a statewide average warming of about 1°C.

In contrast to temperature, variations of precipitation are less consistent across climate divisions and have much less multidecadal character. Thirty-year trends of both variables are highly sensitive to the choice of the subperiod within the overall 93-yr period. The trends also vary seasonally, with winter and spring contributing the most to the annual trends.

Full access
Lei Cai
,
Vladimir A. Alexeev
,
John E. Walsh
, and
Uma S. Bhatt

Abstract

Thirty models in phase 5 of the Coupled Model Intercomparison Project (CMIP5) are evaluated for their performances in reproducing two summertime atmospheric circulation patterns in the Arctic: the Arctic Oscillation (AO) and Arctic dipole (AD). The reference AO and AD are extracted from the ERA-Interim dataset (1979–2016). Model evaluation is conducted during the historical period (1901–2005). Models are ranked by a combined metrics approach based on two pattern correlation coefficients (PCCs) and two explained variances for the AO and AD, respectively. In the projected period (2006–2100), most models produce a positive trend for the AO index and a negative trend for the AD index in summer. The models ranked higher based on the combined metrics ranking show greater consistency and smaller values in the magnitudes of trends of AO and AD than the lower-ranked ones. The projected trends in the AO and AD contribute to a slight increase, if not a decrease, of the air temperature and an acceleration of precipitation increase in the twenty-first century over Arctic Alaska, which is the reverse of over the Barents and Kara Seas. Changes in the AO and AD are relatively minor contributing factors to the projected temperature and precipitation changes in the Arctic, among which the changes in the AD play a bigger role than those in the AO. The summer AO and AD have a stronger impact on the spatial asymmetry of the precipitation field than on the air temperature field.

Full access
Rick Lader
,
Allison Bidlack
,
John E. Walsh
,
Uma S. Bhatt
, and
Peter A. Bieniek

Abstract

Warming temperatures across southeast Alaska are affecting the region’s energy and transportation sectors, marine ecosystems, and forest health. More frequent above-freezing temperatures lead a transition from snow- to rain-dominant precipitation regimes, accelerating glacial mass balance loss and a leading to a greater risk for warm-season drought. Southeast Alaska has steep topographical gradients, which necessitate the use of downscaled climate information to study historical and projected periods. This study used regional dynamical downscaling at 4-km spatial resolution with the Weather Research and Forecasting Model to assess historical (1981–2010) and projected (2031–60) climate states for southeast Alaska. These simulations were driven by one reanalysis (i.e., the Climate Forecast System Reanalysis) and two climate models (i.e., the Geophysical Fluid Dynamics Laboratory Climate Model, version 3, and the NCAR Community Climate System Model, version 4), which each included a historical simulation and a projected simulation. The future simulations used the representative concentration pathway 8.5 emissions scenario. Bias-corrected projections (2031–60 minus 1981–2010) indicated seasonal warming of 1°–3°C, increased precipitation during autumn (4%–12%) and winter (7%–12%), and decreased snowfall in all seasons (up to 60% in autumn). The average number of days annually with a minimum temperature below freezing dropped by more than 30. The average annual maximum consecutive 3-day precipitation amounts increased by 11%–16%, but analogous extreme snowfall amounts dropped by 5%–11%. The most substantial snow losses occurred at low-elevation and coastal locations; at many high elevations (e.g., above 1000 m), extreme snowfall amounts increased.

Free access