Retish Senan

Coherent Intraseasonal Oscillations of Ocean and Atmosphere during the Asian Summer Monsoon

The space-time evolution of the ocean and atmosphere associated with 1998-2000 monsoon intraseasonal oscillations (ISO) in the Indian Ocean and west Pacific is studied using validated sea surface temperature (SST) and surface wind speed from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager, and satellite outgoing longwave radiation. Monsoon ISO consist of alternating episodes of active and suppressed atmospheric convection moving northward in the eastern Indian Ocean and the South China Sea. Negative/positive SST anomalies generated by fluctuations of net heat flux at the ocean surface move northward following regions of active/suppressed convection. Such coherent evolution of SST, surface heat flux and convection suggests that air-sea interaction might be important in monsoon ISO.

Origin of intraseasonal variability of circulation in the tropical central Indian Ocean

Observed upper ocean currents south of Sri Lanka exhibit large, irregular fluctuations with periods of days to weeks. An ocean model driven by daily surface winds is able to reproduce the observed fluctuations. We find from model experiments that low frequency (30-50 day) intraseasonal variability (ISV) arises when Rossby waves radiated from the eastern boundary are amplified by hydrodynamic instability in the eastern and central Indian Ocean. High frequency (10-15 day) ISV is forced directly by ISV of the wind field in the eastern Indian Ocean. In spite of the contribution from instability, the ocean circulation south of Sri Lanka is a deterministic response to wind forcing.

Autumn atmospheric response to the 2007 low Arctic sea ice extent in coupled ocean–atmosphere hindcasts

The autumn and early winter atmospheric response to the record-low Arctic sea ice extent at the end of summer 2007 is examined in ensemble hindcasts with prescribed sea ice extent, made with the European Centre for Medium-Range Weather Forecasts state-of-the-art coupled ocean–atmosphere seasonal forecast model. Robust, warm anomalies over the Pacific and Siberian sectors of the Arctic, as high as 10°C at the surface, are found in October and November. A regime change occurs by December, characterized by weaker temperatures anomalies extending through the troposphere. Geopotential anomalies extend from the surface up to the stratosphere, associated to deeper Aleutian and Icelandic Lows. While the upper-level jet is weakened and shifted southward over the continents, it is intensified over both oceanic sectors, especially over the Pacific Ocean. On the American and Eurasian continents, intensified surface Highs are associated with anomalous advection of cold (warm) polar air on their eastern (western) sides, bringing cooler temperatures along the Pacific coast of Asia and Northeastern North America. Transient eddy activity is reduced over Eurasia, intensified over the entrance and exit regions of the Pacific and Atlantic storm tracks, in broad qualitative agreement with the upper-level wind anomalies. Potential predictability calculations indicate a strong influence of sea ice upon surface temperatures over the Arctic in autumn, but also along the Pacific coast of Asia in December. When the observed sea ice extent from 2007 is prescribed throughout the autumn, a higher correlation of surface temperatures with meteorological re-analyses is found at high latitudes from October until mid-November. This further emphasises the relevance of sea ice for seasonal forecasting in the Arctic region, in the autumn.

Intraseasonal Variability of Equatorial Indian Ocean Zonal Currents

New satellite and in situ observations show large intraseasonal (10–60 day) variability of surface winds and upper-ocean current in the equatorial Indian Ocean, particularly in the east. An ocean model forced by the Quick Scatterometer (QuikSCAT) wind stress is used to study the dynamics of the intraseasonal zonal current. The model has realistic upper-ocean currents and thermocline depth variabilities on intraseasonal to interannual scales. The quality of the simulation is directly attributed to the accuracy of the wind forcing. At the equator, moderate westerly winds are punctuated by strong 10–40-day westerly wind bursts. The wind bursts force swift, intraseasonal (20–50 day) eastward equatorial jets in spring, summer, and fall. The zonal momentum balance is between local acceleration, stress, and pressure, while nonlinearity deepens and strengthens the eastward current. The westward pressure force associated with the thermocline deepening toward the east rapidly arrests eastward jets and, subsequently, generates (weak) westward flow. Thus, in accord with direct observations in the east, the spring jet is a single intraseasonal event, there are intraseasonal jets in summer, and the fall jet is long lived but strongly modulated on an intraseasonal scale. The zonal pressure force is almost always westward in the upper 120 m, and changes sign twice a year in the 120–200-m layer. Transient eastward equatorial undercurrents in early spring and late summer are associated with semiannual Rossby waves generated at the eastern boundary following thermocline deepening by the spring and fall jets. An easterly wind stress is not necessary to generate the undercurrents. Experiments with a single westerly wind burst forcing show that apart from the intraseasonal response, the zonal pressure force and current in the east have an intrinsic 90-day time scale that arises purely from equatorial adjustment.

A biweekly mode in the equatorial Indian Ocean

[1]xa0The National Institute of Oceanography, Goa, deployed moorings with several subsurface current meters at 0°, 93°E (in February 2000) and 0°, 83°E (in December 2000) in the eastern Indian Ocean. Observed meridional current at all depths has a 10- to 20-day (or biweekly) variability that is distinct from longer period (20- to 60-day) subseasonal variability. Lags between different instruments suggest the presence of groups of westward and vertically propagating biweekly waves with zonal wavelength in the range 2100 to 6100 km. We use an ocean model forced by high-resolution scatterometer wind stress to show that the observed biweekly variability is due to equatorially trapped mixed Rossby-gravity waves generated by subseasonal variability of winds. We demonstrate that quasi-biweekly fluctuations of surface meridional wind stress resonantly excite ocean waves with westward and upward phase propagation, with a typical period of 14 days and zonal wavelength of 3000–4500 km. The biweekly wave is associated with fluctuating upwelling/downwelling in the equatorial Indian Ocean, with amplitude of 2–3 m per day located 2–3 away from the equator. Possible reasons for eastward intensification of biweekly energy are discussed.

Impact of snow initialization on sub-seasonal forecasts

The influence of the snowpack on wintertime atmospheric teleconnections has received renewed attention in recent years, partially for its potential impact on seasonal predictability. Many observational and model studies have indicated that the autumn Eurasian snow cover in particular, influences circulation patterns over the North Pacific and North Atlantic. We have performed a suite of coupled atmosphere-ocean simulations with the European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble forecast system to investigate the impact of accurate snow initialisation. Pairs of 2-month ensemble forecasts were started every 15xa0days from the 15th of October through the 1st of December in the years 2004–2009, with either realistic initialization of snow variables based on re-analyses, or else with “scrambled” snow initial conditions from an alternate autumn date and year. Initially, in the first 15xa0days, the presence of a thicker snowpack cools surface temperature over the continental land masses of Eurasia and North America. At a longer lead of 30-day, it causes a warming over the Arctic and the high latitudes of Eurasia due to an intensification and westward expansion of the Siberian High. It also causes a cooling over the mid-latitudes of Eurasia, and lowers sea level pressures over the Arctic. This “warm Arctic—cold continent” difference means that the forecasts of near-surface temperature with the more realistic snow initialization are in closer agreement with re-analyses, reducing a cold model bias over the Arctic and a warm model bias over mid-latitudes. The impact of realistic snow initialization upon the forecast skill in snow depth and near-surface temperature is estimated for various lead times. Following a modest skill improvement in the first 15xa0days over snow-covered land, we also find a forecast skill improvement up to the 30-day lead time over parts of the Arctic and the Northern Pacific, which can be attributed to the realistic snow initialization over the land masses.

Intraseasonal ''monsoon jets'' in the equatorial Indian Ocean

The zonal wind in the equatorial Indian Ocean (EqIO) is westerly almost throughout the year. It has a strong semiannual cycle and drives the spring and fall Wyrtki jets. In addition, high resolution daily satellite winds show westerly wind bursts lasting 10–40 days, associated with atmospheric convection in the eastern EqIO. These bursts have the potential to produce intraseasonal eastward equatorial jets in the ocean. Using an ocean model driven by QuikSCAT scatterometer winds, we show that strong westerly bursts associated with summer monsoon intraseasonal oscillations can drive monsoon jets in the eastern EqIO, which have been observed recently. Although there are distinct equatorial wind bursts associated with Madden-Julian oscillations in January–March, they do not produce equatorial jets in the ocean. The role of ocean dynamics in producing the selective response of the ocean is discussed.

Dynamics of Biweekly Oscillations in the Equatorial Indian Ocean

Variability of the wind field over the equatorial Indian Ocean is spread throughout the intraseasonal (10–60 day) band. In contrast, variability of the near-surface field in the eastern, equatorial ocean is concentrated at biweekly frequencies and is largely composed of Yanai waves. The excitation of this biweekly variability is investigated using an oceanic GCM and both analytic and numerical versions of a linear, continuously stratified (LCS) model in which solutions are represented as expansions in baroclinic modes. Solutions are forced by Quick Scatterometer (QuikSCAT) winds (the model control runs) and by idealized winds having the form of a propagating wave with frequency and wavenumber kw. The GCM and LCS control runs are remarkably similar in the biweekly band, indicating that the dynamics of biweekly variability are fundamentally linear and wind driven. The biweekly response is composed of local (nonradiating) and remote (Yanai wave) parts, with the former spread roughly uniformly along the equator and the latter strengthening to the east. Test runs to the numerical models separately forced by the x and y components of the QuikSCAT winds demonstrate that both forcings contribute to the biweekly signal, the response forced by y being somewhat stronger. Without mixing, the analytic spectrum for Yanai waves forced by idealized winds has a narrowband (resonant) response for each baroclinic mode: Spectral peaks occur whenever the wavenumber of the Yanai wave for mode n is sufficiently close to kw and they shift from biweekly to lower frequencies with increasing modenumber n. With mixing, the higher-order modes are damped so that the largest ocean response is restricted to Yanai waves in the biweekly band. Thus, in the LCS model, resonance and mixing act together to account for the ocean’s favoring the biweekly band. Because of the GCM’s complexity, it cannot be confirmed that vertical mixing also damps its higher-order modes; other possible processes are nonlinear interactions with near-surface currents, and the model’s low vertical resolution below the thermocline. Test runs to the LCS model show that Yanai waves from several modes superpose to form a beam (wave packet) that carries energy downward as well as eastward. Reflections of such beams from the near-surface pycnocline and bottom act to maintain near-surface energy levels, accounting for the eastward intensification of the near-surface, equatorial field in the control runs.

Influence of the Eurasian snow on the negative North Atlantic Oscillation in subseasonal forecasts of the cold winter 2009/2010

The winter 2009/2010 was remarkably cold and snowy over North America and across Eurasia, from Europe to the Far East, coinciding with a pronounced negative phase of the North Atlantic Oscillation (NAO). While previous studies have investigated the origin and persistence of this anomalously negative NAO phase, we have re-assessed the role that the Eurasian snowpack could have played in contributing to its maintenance. Many observational and model studies have indicated that the autumn Eurasian snow cover influences circulation patterns over high northern latitudes. To investigate that role, we have performed a suite of forecasts with the coupled ocean–atmosphere ensemble prediction system from the European Centre for Medium-Range Weather Forecasts. Pairs of 2-month ensemble forecasts with either realistic or else randomized snow initial conditions are used to demonstrate how an anomalously thick snowpack leads to an initial cooling over the continental land masses of Eurasia and, within 2xa0weeks, to the anomalies that are characteristic of a negative NAO. It is also associated with enhanced vertical wave propagation into the stratosphere and deceleration of the polar night jet. The latter then exerts a downward influence into the troposphere maximizing in the North Atlantic region, which establishes itself within 2xa0weeks. We compare the forecasted NAO index in our simulations with those from several operational forecasts of the winter 2009/2010 made at the ECWMF, and highlight the importance of relatively high horizontal resolution.

Sensitivity of summer 2-m temperature to sea ice conditions

Current seasonal forecast models involve simple schemes for representing sea ice, such as imposing climatological values. The spread of ensemble forecasts may in principle be biased due to common boundary conditions prescribed in the high latitudes. The degree of sensitivity in the 2-metre temperature, associated with seasonal time scales and the state of the June—August sea ice, is examined through a set of experiments with a state-of-the-art coupled ocean-atmosphere model. Here we present a suite of numerical experiments examining the effect of different sea ice configurations on the final ensemble distribution. We also compare the sensitivity of the 2-metre temperature to sea ice boundary conditions and sea surface temperature perturbation in the initial conditions. One objective of this work was to test a simple scheme for a more realistic representation of sea ice variations that allows for a spread in the Polar surface boundary conditions, captures the recent trends and doesn’t smudge the sea ice edges. We find that the use of one common set of boundary conditions in the polar regions has little effect on the subsequent seasonal temperatures in the low latitudes, but nevertheless a profound influence on the local temperatures in the mid-to-high latitudes.