Ummenhofer Caroline C.

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Ummenhofer
First Name
Caroline C.
ORCID
0000-0002-9163-3967

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Impact of multidecadal variability in Atlantic SST on winter atmospheric blocking

2019-12-31 , Kwon, Young-Oh , Seo, Hyodae , Ummenhofer, Caroline C. , Joyce, Terrence M.

Recent studies have suggested that coherent multidecadal variability exists between North Atlantic atmospheric blocking frequency and the Atlantic multidecadal variability (AMV). However, the role of AMV in modulating blocking variability on multidecadal times scales is not fully understood. This study examines this issue primarily using the NOAA Twentieth Century Reanalysis for 1901–2010. The second mode of the empirical orthogonal function for winter (December–March) atmospheric blocking variability in the North Atlantic exhibits oppositely signed anomalies of blocking frequency over Greenland and the Azores. Furthermore, its principal component time series shows a dominant multidecadal variability lagging AMV by several years. Composite analyses show that this lag is due to the slow evolution of the AMV sea surface temperature (SST) anomalies, which is likely driven by the ocean circulation. Following the warm phase of AMV, the warm SST anomalies emerge in the western subpolar gyre over 3–7 years. The ocean–atmosphere interaction over these 3–7-yr periods is characterized by the damping of the warm SST anomalies by the surface heat flux anomalies, which in turn reduce the overall meridional gradient of the air temperature and thus weaken the meridional transient eddy heat flux in the lower troposphere. The anomalous transient eddy forcing then shifts the eddy-driven jet equatorward, resulting in enhanced Rossby wave breaking and blocking on the northern flank of the jet over Greenland. The opposite is true with the AMV cold phases but with much shorter lags, as the evolution of SST anomalies differs in the warm and cold phases.

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Amplified seasonal cycle in hydroclimate over the Amazon river basin and its plume region

2020-09-01 , Liang, Yu-Chiao , Lo, Min-Hui , Lan, Chia-Wei , Seo, Hyodae , Ummenhofer, Caroline C. , Yeager, Stephen G. , Wu, Ren-Jie , Steffen, John D.

The Amazon river basin receives ~2000 mm of precipitation annually and contributes ~17% of global river freshwater input to the oceans; its hydroclimatic variations can exert profound impacts on the marine ecosystem in the Amazon plume region (APR) and have potential far-reaching influences on hydroclimate over the tropical Atlantic. Here, we show that an amplified seasonal cycle of Amazonia precipitation, represented by the annual difference between maximum and minimum values, during the period 1979–2018, leads to enhanced seasonalities in both Amazon river discharge and APR ocean salinity. An atmospheric moisture budget analysis shows that these enhanced seasonal cycles are associated with similar amplifications in the atmospheric vertical and horizontal moisture advections. Hierarchical sensitivity experiments using global climate models quantify the relationships of these enhanced seasonalities. The results suggest that an intensified hydroclimatological cycle may develop in the Amazonia atmosphere-land-ocean coupled system, favouring more extreme terrestrial and marine conditions.

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Emerging European winter precipitation pattern linked to atmospheric circulation changes over the North Atlantic region in recent decades

2017-08-25 , Ummenhofer, Caroline C. , Seo, Hyodae , Kwon, Young-Oh , Parfitt, Rhys , Brands, Swen , Joyce, Terrence M.

Dominant European winter precipitation patterns over the past century, along with their associated extratropical North Atlantic circulation changes, are evaluated using cluster analysis. Contrary to the four regimes traditionally identified based on daily wintertime atmospheric circulation patterns, five distinct seasonal precipitation regimes are detected here. Recurrent precipitation patterns in each regime are linked to changes in atmospheric blocking, storm track, and sea surface temperatures across the North Atlantic region. Multidecadal variability in the frequency of the precipitation patterns reveals more (fewer) winters with wet conditions in northern (southern) Europe in recent decades and an emerging distinct pattern of enhanced wintertime precipitation over the northern British Isles. This pattern has become unusually common since the 1980s and is associated with changes in moisture transport and more frequent atmospheric river events. The observed precipitation changes post-1950 coincide with changes in storm track activity over the central/eastern North Atlantic toward the northern British Isles.

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Influences of Pacific climate variability on decadal subsurface ocean heat content variations in the Indian Ocean

2018-04-30 , Jin, Xiaolin , Kwon, Young-Oh , Ummenhofer, Caroline C. , Seo, Hyodae , Schwarzkopf, Franziska U. , Biastoch, Arne , Böning, Claus W. , Wright, Jonathon S.

Decadal variabilities in Indian Ocean subsurface ocean heat content (OHC; 50–300 m) since the 1950s are examined using ocean reanalyses. This study elaborates on how Pacific variability modulates the Indian Ocean on decadal time scales through both oceanic and atmospheric pathways. High correlations between OHC and thermocline depth variations across the entire Indian Ocean Basin suggest that OHC variability is primarily driven by thermocline fluctuations. The spatial pattern of the leading mode of decadal Indian Ocean OHC variability closely matches the regression pattern of OHC on the interdecadal Pacific oscillation (IPO), emphasizing the role of the Pacific Ocean in determining Indian Ocean OHC decadal variability. Further analyses identify different mechanisms by which the Pacific influences the eastern and western Indian Ocean. IPO-related anomalies from the Pacific propagate mainly through oceanic pathways in the Maritime Continent to impact the eastern Indian Ocean. By contrast, in the western Indian Ocean, the IPO induces wind-driven Ekman pumping in the central Indian Ocean via the atmospheric bridge, which in turn modifies conditions in the southwestern Indian Ocean via westward-propagating Rossby waves. To confirm this, a linear Rossby wave model is forced with wind stresses and eastern boundary conditions based on reanalyses. This linear model skillfully reproduces observed sea surface height anomalies and highlights both the oceanic connection in the eastern Indian Ocean and the role of wind-driven Ekman pumping in the west. These findings are also reproduced by OGCM hindcast experiments forced by interannual atmospheric boundary conditions applied only over the Pacific and Indian Oceans, respectively.

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On the predominant nonlinear response of the extratropical atmosphere to meridional shifts of the Gulf Stream

2017-11-07 , Seo, Hyodae , Kwon, Young-Oh , Joyce, Terrence M. , Ummenhofer, Caroline C.

The North Atlantic atmospheric circulation response to the meridional shifts of the Gulf Stream (GS) path is examined using a large ensemble of high-resolution hemispheric-scale Weather Research and Forecasting Model simulations. The model is forced with a broad range of wintertime sea surface temperature (SST) anomalies derived from a lag regression on a GS index. The primary result of the model experiments, supported in part by an independent analysis of a reanalysis dataset, is that the large-scale quasi-steady North Atlantic circulation response is remarkably nonlinear about the sign and amplitude of the SST anomaly chosen over a wide range of GS shift scenarios. The nonlinear response prevails over the weak linear response and resembles the negative North Atlantic Oscillation (NAO), the leading intrinsic mode of variability in the model and the observations. Further analysis of the associated dynamics reveals that the nonlinear responses are accompanied by the shift of the North Atlantic eddy-driven jet, which is reinforced, with nearly equal importance, by the high-frequency transient eddy feedback and the low-frequency wave-breaking events. Additional sensitivity simulations confirm that the nonlinearity of the circulation response is a robust feature found over the broad parameter space encompassing not only the varied SST but also the absence/presence of tropical influence, the varying lateral boundary conditions, and the initialization scheme. The result highlights the fundamental importance of the intrinsically nonlinear transient eddy dynamics and the eddy–mean flow interactions in generating the nonlinear downstream response to the meridional shifts in the Gulf Stream.

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Intraseasonal rainfall variability in the Bay of Bengal during the Summer Monsoon : coupling with the ocean and modulation by the Indian Ocean Dipole

2017-01-24 , Jongaramrungruang, Siraput , Seo, Hyodae , Ummenhofer, Caroline C.

The Indian Summer Monsoon rainfall exhibits pronounced intraseasonal variability in the Bay of Bengal (BoB). This study examines the intraseasonal rainfall variability with foci on the coupling with sea surface temperatures (SST) and its interannual modulation. The lagged composite analysis reveals that, in the northern BoB, SST warming leads the onset of intraseasonal rainfall by 5 days. Latent heat flux is reduced before the rain event but is greatly amplified during the rainfall maxima. Further analysis reveals that this intraseasonal rainfall-SST relationship through latent heating is strengthened in negative Indian Ocean Dipole (IOD) years when the bay-wide local SST is anomalously warm. Latent heat flux is further increased during the intraseasonal rainfall maxima leading to strengthened rainfall variability. The moisture budget analysis shows this is primarily due to stronger low-level moisture convergence in negative IOD years. The results provide important predictive information on the monsoon rainfall and its active/break cycles.

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Relative contributions of heat flux and wind stress on the spatiotemporal upper-ocean variability in the tropical Indian Ocean

2020-08-12 , Yuan, Xu , Ummenhofer, Caroline C. , Seo, Hyodae , Su, Zhongbo

High-resolution ocean general circulation model (OGCM) simulations are employed to investigate interannual variability of the upper-ocean temperature in the tropical Indian Ocean (20°S–20°N). The seasonal cycle and interannual variability in the upper-ocean temperature in the tropical Indian Ocean in the forced ocean simulation are in good agreement with available observation and reanalysis products. Two further sensitivity OGCM simulations are used to separate the relative contributions of heat flux and wind stress. The comparison of the model simulations reveals the depth-dependent influences of heat flux and wind stress on the ocean temperature variability in the tropical Indian Ocean. Generally, heat flux dominates the temperature variability in the top 30 m, while wind stress contributes most strongly to the subsurface temperature variability below 30 m. This implies that a transition depth should exist at each location, where the dominant control of the ocean temperature variability switched from heat flux to wind stress. We define the depth of this transition point as the 'crossing depth' and make use of this concept to better understand the depth-dependent impacts of the heat flux and wind stress on the upper-ocean temperature variability in the tropical Indian Ocean. The crossing depth tends to be shallower in the southern tropical Indian Ocean (20°S-EQ), including the Seychelles-Chagos Thermocline Ridge (SCTR) and the eastern part of the Indian Ocean Dipole (IOD), suggesting the dominance of forcing due to wind stress and the resulting ocean dynamical processes in the temperature variability in those regions. The crossing depth also shows prominent seasonal variability in the southern tropical Indian Ocean. In the SCTR, the variability of the subsurface temperature forced by the wind stress dominates largely in boreal winter and spring, resulting in the shallow crossing depth in these seasons. In contrast, the intensified subsurface temperature variability with shallow crossing depth in the eastern part of the IOD is seen during boreal autumn. Overall, our results suggest that the two regions within the tropical Indian Ocean, the SCTR and the eastern part of the IOD, are the primary locations where the ocean dynamics due to wind-stress forcing control the upper-ocean temperature variability.

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Meridional Gulf Stream shifts can influence wintertime variability in the North Atlantic storm track and Greenland blocking.

2019-01-29 , Joyce, Terrence M. , Kwon, Young-Oh , Seo, Hyodae , Ummenhofer, Caroline C.

After leaving the U.S. East Coast, the northward flowing Gulf Stream (GS) becomes a zonal jet and carries along its frontal characteristics of strong flow and sea surface temperature gradients into the North Atlantic at midlatitudes. The separation location where it leaves the coast is also an anchor point for the wintertime synoptic storm track across North America to continue to develop and head across the ocean. We examine the meridional variability of the separated GS path on interannual to decadal time scales as an agent for similar changes in the storm track and blocking variability at midtroposphere from 1979 to 2012. We find that periods of northerly (southerly) GS path are associated with increased (suppressed) excursions of the synoptic storm track to the northeast over the Labrador Sea and reduced (enhanced) Greenland blocking. In both instances, GS shifts lead those in the midtroposphere by a few months.

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Distinct mechanisms of decadal subsurface heat content variations in the eastern and western Indian Ocean modulated by tropical Pacific SST

2018-08-13 , Jin, Xiaolin , Kwon, Young-Oh , Ummenhofer, Caroline C. , Seo, Hyodae , Kosaka, Yu , Wright, Jonathon S.

Decadal variability of the subsurface ocean heat content (OHC) in the Indian Ocean is investigated using a coupled climate model experiment, in which observed eastern tropical Pacific sea surface temperature (EPSST) anomalies are specified. This study intends to understand the contributions of external forcing relative to those of internal variability associated with EPSST, as well as the mechanisms by which the Pacific impacts Indian Ocean OHC. Internally generated variations associated with EPSST dominate decadal variations in the subsurface Indian Ocean. Consistent with ocean reanalyses, the coupled model reproduces a pronounced east–west dipole structure in the southern tropical Indian Ocean and discontinuities in westward-propagating signals in the central Indian Ocean around 100°E. This implies distinct mechanisms by which the Pacific impacts the eastern and western Indian Ocean on decadal time scales. Decadal variations of OHC in the eastern Indian Ocean are attributed to 1) western Pacific surface wind anomalies, which trigger oceanic Rossby waves propagating westward through the Indonesian Seas and influence Indonesian Throughflow transport, and 2) zonal wind anomalies over the central tropical Indian Ocean, which trigger eastward-propagating Kelvin waves. Decadal variations of OHC in the western Indian Ocean are linked to conditions in the Pacific via changes in the atmospheric Walker cell, which trigger anomalous wind stress curl and Ekman pumping in the central tropical Indian Ocean. Westward-propagating oceanic Rossby waves extend the influence of this anomalous Ekman pumping to the western Indian Ocean.

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