Ocean Dynamics (2011) 61:1421–1439 DOI 10.1007/s10236-011-0431-6

Physical and dynamical of Bay

Jeffrey A. Polton · Matthew Robert Palmer · Michael John Howarth

Received: 2 December 2010 / Accepted: 28 April 2011 / Published online: 31 May 2011 © Springer-Verlag 2011

Abstract The UK National Oceanography Centre has eastward erosion of the plume during spring is maintained an observatory in Liverpool Bay since identified as a potentially important freshwater mixing August 2002. Over 8 years of observational measure- mechanism. Novel climatological maps of temperature, ments are used in conjunction with regional salinity and density from the CTD surveys are pre- modelling data to describe the physical and dynam- sented and used to validate numerical simulations. The ical oceanography of Liverpool Bay and to validate model is found to be sensitive to the freshwater forcing the regional model, POLCOMS. Tidal dynamics and rates, temperature and salinities. The existing CTD plume buoyancy govern the fate of the fresh water survey grid is shown to not extend sufficiently near the as it enters the , as well as the fate of its sedi- coast to capture the near coastal and vertically mixed ment, contaminants and nutrient loads. In this con- component the plume. Instead the survey grid captures text, an overview and summary of Liverpool Bay tidal the westward spreading, shallow and transient, portion dynamics are presented. Freshwater forcing statistics of the plume. This transient plume feature is shown in are presented showing that on average the bay re- both the long-term averaged model and observational ceives 233 m3 s−1. Though the region is salinity con- data as a band of stratified fluid stretching between the trolled, river input temperature is shown to significantly mouth of the Mersey towards the Isle of Man. Finally modulate the plume buoyancy with a seasonal cycle. the residual circulation is discussed. Long-term moored Stratification strongly influences the region’s dynamics. ADCP data are favourably compared with model data, Data from long-term moored instrumentation are used showing the general northward flow of surface water to analyse the stratification statistics that are represen- and southward trajectory of bottom water. tative of the region. It is shown that for 65% of tidal cycles, the region alternates between being vertically Keywords Liverpool Bay · Climatology · ROFI · mixed and stratified. Plume dynamics are diagnosed Plume dynamics · Coastal dynamics · Coastal from the model and are presented for the region. The oceanography · Shelf sea · Model validation spring–neap modulation of the plume’s westward ex- tent, between 3.5◦W and 4◦W, is highlighted. The rapid 1 Introduction

Responsible Editor: Claire Mahaffey Liverpool Bay is a shallow subsection of the semi- enclosed (Fig. 1). The Proudman Oceano- This article is part of the Topical Collection on the graphic Laboratory and more recently the UK Na- UK National Oceanography Centre’s Irish Sea Coastal Observatory tional Oceanography Centre (NOC) has maintained an observatory in Liverpool Bay since August 2002. B · · J. A. Polton ( ) M. R. Palmer M. J. Howarth The observatory has evolved into a multiple platform, National Oceanography Centre, 6 Brownlow St., Liverpool, L3 5DA, UK multidisciplinary ocean science undertaking with a high e-mail: [email protected] density and diverse range of partners and end-users. 1422 (2011) 61:1421–1439

53.9 10m interval rural North and the Snowdownia National Park). 10km Liverpool Bay is also important to the maritime energy 53.8 industry; the Irish Sea, and greater Liverpool Bay area, Ribble hosts numerous offshore oil and gas platforms, several 53.7 wind farms and has approved plans for significant fu- ture growth (The Crown Estate 2010). It is also the 53.6 Site A focus of a number of proposed tidal barrier installations (Burrows et al. 2009; Walkington and Burrows 2009). 53.5

Latitude Site B Sediment dynamics are of primary concern for many of the observatory stakeholders; much of the adjoining 53.4 coast is protected by natural sand dune systems that Liverpool Mersey Clywd are actively accreting or eroding and yet which also 53.3 Conwy provide a major tourist attraction and income for the Dee area. The proximity of the historic, international port 53.2 of Liverpool also provides a focus for research into -3.8 -3.6 -3.4 -3.2 -3 -2.8 -2.6 sediment transport facilitating marine shipping chan- Longitude nel management. The intensity of human demand on Liverpool Bay combined with the complexity of the Fig. 1 Map showing the Liverpool Bay region, as part of the Irish Sea, with major estuaries and long-term sites A and B. local dynamics provides the focus for coastal research The given location for site B was maintained from 5th April 2005 from both directive driven and blues skies researchers until 26th January 2010. The major estuaries are labelled and in order to better understand the coastal ocean. the tributaries are shown. The bathymetric contours are at 10-m In this article, data are presented from two fixed intervals showing that site A is in about 20 m of water mooring sites (the full specifications are detailed in Howarth and Palmer (The Liverpool Bay Coastal While current plans are underway to extend the focus Observatory, resubmitted)): (1) site A is located at of the observatory to a wider Irish Sea perspective there 53◦31.8 N, 3◦21.6 W at the Mersey Bar. The site remains a requirement from the user community to has been maintained continuously since August 7th provide sustained monitoring of Liverpool Bay. This 2002. The site is strongly influenced by the Mersey paper draws on over 8 years of measurements, in con- freshwater outflow and to a lesser extent by the river junction with model data, to describe the forcing mech- Dee. (2) Site B is located 21 km to the west of site anisms and the of Liverpool Aat53◦27 N, 3◦38.4 W. The site was maintained Bay. between 5th April 2005 and 26th January 2010, fol- With a spring tidal range in excess of 10 m, the lowing which the moored instrumentation was moved region experiences one of the largest tidal ranges on 9 km north to the same latitude as site A. The new , only exceeded in the UK by the . location of site B was chosen in order to better meet Subsequently, the bay experiences strong tidal cur- scientific demands and to avoid future wind farm de- rents which interact with the and horizontal velopment. In this paper only data from the original density gradients to produce complex dynamics which site B location is presented. Data from each moor- have important consequences on the fate of freshwater ing site and the other instrument platforms, which and biogeochemical pathways (Greenwood et al. (Spa- combine to make the Irish Sea Observatory, will be tial and temporal variability in nutrient concentrations compared with the three-dimensional hydrodynamical in Liverpool Bay, a temperate latitude region of fresh- model, POLCOMS (Holt and James 2001;Holtetal. water influence, submitted); Yamashita et al. 2011). 2005). The simulation data presented here are from Liverpool Bay is subject to many of the modern pres- the 1.8-km horizontal resolution configuration (Fig. 19 sures on our coastal systems; it is in close proximity to shows this resolution in context with the Liverpool a number of economically important cities and encom- Bay subregion), with 32 evenly spaced vertical levels passes the national boundaries of , Wales and at each location. The atmosphere is forced by 1◦ and the Isle of Man. The Bay receives freshwater input from six hourly ECMWF winds and surface fluxes are com- a number of large English and Welsh rivers includ- puted using bulk formulae. The combination of ob- ing the Mersey, Ribble and Dee (having large catch- servational and simulation results are used to provide ment areas covering heavily industrialised and highly a synoptic overview of the physical oceanography of populated regions) and the rivers Clwyd and Conwy Liverpool Bay and to describe the fate of the riverine (which have the comparatively pristine catchments of freshwater. Following dynamical insights of Yankovsky Ocean Dynamics (2011) 61:1421–1439 1423 and Chapman (1997), the freshwater, once it is in the the earth, under the and , results in semi- bay, will be referred to as a plume. diurnal components of the total tidal signal that explain The dynamics of Liverpool Bay are governed by the most of the Irish Sea’s tidal variability. These are the tides. The tides determine the fate of the fresh water M2 and S2 constituents, with a 12.4- and 12.0-h period, plume, the sediment, nutrient and contaminant riverine and are associated with the gravitational pull of the loads as they enter the sea. Therefore, to understand Moon and Sun, respectively. Similarly, the orbit of the the dynamical effects on chemical or biological cycles, Moon around the Earth results in a twice monthly it is necessary to understand the horizontal and vertical alignment of the Earth, Moon and Sun. The alignment structure of the tidal currents, how they vary in time, of the gravitational pull between the bodies modulates and to understand the associated mixing processes. the tidal amplitudes with a 2-week period. This cycle Studying the dispersal of the fresh water plume as it is known as the spring–neap cycle. Spring tides occur enters the sea provides useful insight into how the on the alternating weeks around the time when the riverine chemical, biological or sediment load might Moon is either full or new, and are associated with the also disperse. In the following sections, we consider the larger tidal range. Neap tides have the smaller tidal interactions between stratification, tidal currents and range and occur when the Moon is about half full, freshwater forcing in order to identify key processes that is when the Sun, Moon and Earth are furthest that are dynamically important for Liverpool Bay. from mutual alignment. Figure 2a shows the near bed pressure recorded at site A for a typical 4-week period. Measured in decibars, the pressure gives an accurate 2 Dynamical overview representation of sea surface height (in m). The signal is dominated by two periods: the semi-diurnal tidal In this section, the tidal dynamics that are fundamental wave and the spring–neap cycle that gives rise to two to Liverpool Bay oceanography are investigated and re- spring tides and two neap tides per . Notice that viewed with a combination of observational and model the consecutive spring tides have different amplitudes. data. For a detailed review of broader ranging Irish Sea These slowly evolving modulations can be represented dynamics, refer to Bowden (1980). with additional tidal constituents that account for fac- tors such as the Earth’s tilt and orbit eccentricities. The 2.1 Spring–neap and semi-diurnal tidal cycles difference between the successive spring tides shown is accounted for by the Moon’s elliptical orbit. For Gravitational forces of the Sun and Moon distort the a detailed description of tidal mechanics see Pugh sea surface and generate tidal waves. The rotation of (1987).

abvelocity magnitude (m/s) 30

28

27 25 1

26 20 0.8 25

24 15 0.6 23

10 Height above bed (m)

Height above bed (db) 22 0.4

21 5 20 0.2

19 0 04/09/05 11/09/05 18/09/05 25/09/05 00h 06/05/04 12h 06/05/04 00h 07/05/04 12h 07/05/04 Date Date

Fig. 2 Tidal variations recorded at site A in Liverpool Bay. cycles. b Right panel, ADCP east-west current data show the a Left panel, near-bed pressure showing the semi-diurnal magnitude as a function of depth and time for four tidal periods variation in sea surface height modulated by two spring–neap 1424 Ocean Dynamics (2011) 61:1421–1439

Fig. 3 a Irish Sea co-tidal M Co-tidal chart Spring tide max current amplitude chart ab2 chart for semi-diurnal M2 55 56 tide. Thick lines represent the 1.2 maximum M2 tidal elevation 40 55.5 2.5 in metres. Thin lines 54.5 represent the relative phase lag of high tide (given in 55 20 2

1.6 1.8 2.2 minutes with reference to 54 2 2.4 1.4 2.6 high tide at Liverpool). South 54.5 1.5 of Liverpool the phases are 1.2 0 negative as the tide 53.5 54 1 -20 propagates generally -40 1.25 0.8 northwards. b The depth -80 53 53.5 averaged maximum currents -120 Latitude 1 for an average spring tide Latitude 53 (in ms−1) -180 52.5 -200 0.75 -220 1 -240 52.5 -260 -300 -280 52 -320 1.4 0.5 52

1.8 51.5 0.25 2.2 51.5 3

3.4 4.2 2.6

400 51 51 0.001 –7 –6 –5 –4 –3 –4 –5 –6 –7 –8 Longitude Longitude

In addition to long period modulation, high- be simultaneously represented using a co-tidal chart. A frequency corrections are also required to accurately co-tidal chart contours the maximum tidal amplitude represent the effects of shallow water on a propagating and the time to high tide (or the phase) from a fixed tidal wave, since the wave is slowed by bed friction reference point. Figure 3a shows the co-tidal chart for and distorted by nonlinear effects. For example, Fig. 2b the M2 tide (c.f. Kwong et al. 1997). In Liverpool shows the magnitude of the depth-varying east-west Bay, the tide propagates as an east-west standing wave. current at site A measured over four tidal periods. More generally, the tide in the Irish Sea propagates Here, the tidal currents are asymmetric with the great- anticlockwise around the near the est speeds on flood tide. For practical purposes, away southeast coast of Ireland. from the shore in Liverpool Bay, the tidal wave ele- The magnitude of the tidal currents are determined vation can be reconstructed with the largest ten con- by the rate at which the sea water is moved around stituents. (At site A, 50% of the amplitude variability by the tidal forces with the rising and falling tides. is described by M2 and S2 and 80% is described by the Figure 3b shows the magnitude of the depth averaged largest ten constituents.) maximum tidal velocities associated with an average Spring–neap and ebb-flood statistics can be obtained spring tide.1 Maximum currents exceed 2 ms−1 around for site A. Observational data from May 2003 to May headlands and are about 0.75 ms−1 in the Liverpool 2005 give current statistics representative of the Bay. Bay. Current magnitudes are elevated in the Severn Comparing the depth mean maximum current magni- Estuary because the geometry of the Bristol Channel tudes for each tidal cycle, filtering out ebb-flood asym- is resonant at the semi-diurnal frequency (Fong and metry, reveals that the average ratio of consecutive Heaps 1978). spring current to neap currents is 2, with a time average − neap flow of 0.4 ms 1 and a time average spring flow of 2.2 Vertical structure of momentum: tidal ellipses 0.7 ms−1. This is consistent with the amplitudes of the M2 constituent being about three times greater than the Though the currents in Liverpool Bay are predomi- S2 constituent. The ratio of east-west depth averaged nantly orientated in the east-west direction, tidal cur- flood and ebb peak speeds is found to be 1.2. rents do not, in general, simply flow back and forth. In an area as large as the Irish Sea, the tides have spatial variability as well as the familiar temporal vari- ability. This is determined by complex interactions be- 1Remarkably, the gross patterns in Fig. 3b can be derived from tween the tidal wave and the geometry of the coastline the co-tidal chart if the flow is assumed to be frictionless and that and seabed. Both spatial and temporal information can the acceleration is wholly forced by the barotropic tide. Ocean Dynamics (2011) 61:1421–1439 1425

At a given location, a current metre would reveal that 2.3 Vertical structure of heat: stratification the two-dimensional horizontal currents rotate their direction through all points of the compass. Over a Taking the Irish Sea as a whole, the sea is warmed tidal cycle, the horizontal current vector describes a by solar heating in the summer and then cools in complete ellipse, which can be uniquely described by the winter. Away from coastal regions, where there the sum of two counter-rotating horizontal velocity are freshwater inputs, solar heating can control the vectors. Only in special cases does this elliptical path local stratification unless the tidal mixing is sufficiently have a zero minor axis component such that the tidal strong to erode vertical density gradients. In re- current does truly flow back and forth. It is the complex gions like Liverpool Bay, the stratification is buoy- nature of the Irish Sea’s surrounding coastline that de- ancy controlled; freshwater continually replenishes the termines the shape and rotation direction of the depth- stratification that tidal mixing seeks to erode. To un- averaged tidal ellipses. The tidal ellipse structure also derstand the tidal mixing mechanism, consider the sit- varies with depth. This is a result of the Earth’s rotation uation shown in Fig. 2b. During this period, it is ap- biasing anticlockwise and clockwise rotary components parent that the current velocities diminish with depth. of the tidal ellipses differently. For example, if at some Though the fluid is evenly forced by horizontal pres- depth the superposition of clockwise and anticlockwise sure gradients, as a consequence of the gravitationally rotations cancel to produce back and forth tides, then induced sea surface height gradients, the fluid nearest slightly deeper currents will have an anticlockwise ro- the sea bed feels frictional forces exerted by the sea tation and slightly shallower will have a clockwise rota- bed on the flow. This retards the bottom flow and tion. At site A, the tidal ellipse analysis shows the flow generates a vertical shear in the horizontal velocity. is rectilinear at 12 m above the bed (Fig. 4). Indeed, With typical magnitudes of tidal currents, these shear above this depth the ellipse rotates clockwise (red) flows are generally unstable and easily turn turbulent. and beneath anticlockwise (blue). For a more detailed The resultant boundary layers grow in height (as long as analysis of tidal ellipses, refer to Prandle (1982)and the tidal shear remains sufficiently strong relative to the Soulsby (1983). stabilising effects of stratification) potentially envelop- ing the whole . The competition between the turbulent boundary layer mixing and the stabilising effects of stratification are succinctly parameterised by / 3 M2 tidal ellipses: blue anti–clockwise, red clockwise a stability parameter, H U ,whereH is the water depth and U is the amplitude of the surface tidal ve- 1.6 locity (Simpson and Hunter 1974). Taking velocities 1.4 from a barotropic M2 tidal model and comparing them

) with satellite observational data (Pingree and Griffiths

–1 1.2 1978) as well as top to bottom sea temperature (Bowers 1 and Simpson 1987) a critical value for the stability parameter is obtained to predict the location of fronts. 0.8 Though meteorological effects and seasonal variability 0.6 can affect the timing and location of surface temper- ature fronts the critical value of S = log (H/U 3) = 0.4 10

northward velocity (m s 2.7 ± 0.4 is generally accepted (Simpson and Sharples 0.2 1994). The remarkable success of the parameter S in predicting thermal front locations is evident in Fig. 5, 0 showing a comparison between model stratification and –1 –0.5 0 0.5 1 eastward velocity (m s–1) S deduced from the POLCOMS shelf wide model. In near coastal regions, like Liverpool Bay where salinity Fig. 4 Tidal ellipse analysis at site A. Red ellipses denote clock- and not heat controls the density, this relationship that wise rotations at the semi-diurnal frequency and blue ellipses predicts fronts is not so robust (Fig. 5). In these regions, rotate anticlockwise. Each ellipse is plotted on east-west and north-south velocity axes. Each ellipse is displaced on the y-axis the freshwater forcing can not be neglected. by an amount corresponding to its bin height above the sea bed, such that all the crosses represent zero velocity. Near-bed ellipses 2.4 Freshwater forcing (blue) are anticlockwise and orientated with the major axis in the east-west direction. Surface ellipses are clockwise (red)withthe major axis orientated SE-NW. The data are from depths below Interactions between the fresh water discharge from the low tide level only. The vertical bin interval is 1 m the surrounding estuaries (the Clwyd, Dee, Mersey and 1426 Ocean Dynamics (2011) 61:1421–1439

Fig. 5 Figure showing top to bottom density difference, for a 55 b July 07, compared with predicted frontal location = ( / 3) based on S log10 H U 54.5 parameter. The density differences in (a)are computed from daily means 54 for July 2007. The contour values are, with increasing brightness, 0.1 and 0.4 kg m−3 53.5 = ( /| |3) (b) shows S log10 H U , computed using the time 53 averages of hourly barotropic lat current magnitudes and water depths over the month of July 2007. Contours that separate 52.5 regimes are, with increasing brightness, S={2.3,2.7,3.1,4}. 52 These data are POLCOMS model output

51.5

51 –7 –6 –5 –4 –3 lon

the Ribble) and tributaries with sea water result in (Simpson 1997). Data from the four catchment basins strong horizontal density gradients that complicate the come from historical Environment Agency river gauges tidal forcing. Regions such as Liverpool Bay, where the (archived at the Centre for Ecology and Hydrology), dynamics are strongly influenced by estuarine outflow these are scaled to represent downstream accumula- are called ROFIs, Regions of Freshwater Influence tion from unmeasured tributaries before reaching the

Fig. 6 River flux data for 350 8 Liverpool Bay showing 7-day Clwyd running mean values for each Dee catchment basin. The thick Mersey monotonic increasing line is 300 Ribble the cumulative volume flux from all the rivers in the Bay. 6 The cumulative flux of

250 ) riverine water is 7.3 × 109 m3 3 m for the year 2007 9 ) –1

.s 200 3

4

150 River flux (m

100 Cumulative river flux (x10 2

50

0 0 0 50 100 150 200 250 300 350 Julian days Ocean Dynamics (2011) 61:1421–1439 1427

Table 1 Volume flux statistics, by frequency, for estuaries estuary mouth. The Clwyd catchment includes mea- flowing into Liverpool Bay surements from the Clwyd2 and Elwy. The Dee catch- Estuary 10% 50% 90% 100% ment includes measurements from the Alyn and Dee. Clwyd 12 31 121 407 The Mersey catchment includes measurements from Dee 11 26 108 277 the Weaver, Dane, Wincham Brook, Irwell, Mersey Mersey 26 51 165 515 and Sankey Brook. The Ribble catchment includes Ribble 9 28 131 422 measurements from the Douglas, Lostock and the Rib- Total 61 143 509 1404 ble. Figure 1 shows the catchment mouth location. No Daily mean fluxes (m3 s−1) are sorted by magnitude. Tabular attempt is made in the model to represent ground water values represent the maximum flow rate for the associated time seepage. Seven day running mean flow rates are given percentage bin in m3 s−1 and are presented for 2007 in Fig. 6.Actual daily mean fluxes vary by over 300 m3 s−1/day making the rivers highly intermittent. There is no clear seasonal the freshwater of the estuary is fed by a network of cycle to the flow data, though the river time series are shallow streams which are efficiently either heated or highly correlated. Table 1 gives the daily flow rates cooled by the atmosphere. Since the open sea has sorted by magnitude and binned by the percentage time greater thermal inertia than the atmosphere, the sea for which the value is the upper limit of the flow. For temperatures lag the freshwater temperature. In winter, example, the sum of the rivers have a flow that is less therefore, the sea is generally warmer than the estu- than 61 m3 s−1 for 10% of days in the year and a flow aries, whereas in summer the estuaries are generally that is between 509 and 1,404 m3 s−1 also for 10% of warmer than the sea. Figure 8 shows the temperature days in the year. Cumulatively, over the year, the four difference, along the ferry track, between the mouth catchment basins flux 7.3 × 109 m3 of water into the of the Mersey and the Mersey Bar (near site A). Bay (Fig. 6), this yields an average flux of 233 m3 s−1. The amplitude of the annual cycle in the temperature Therefore, a volume of sea bounded by the latitude and difference is much smaller than the amplitude of the ◦ longitude of the Ribble and Clwyd would take about absolute temperature, and is about 2 C. Though the 3 years to fill at this rate. Notice that the mean daily flux freshwater always make that estuaries plume positively ◦ exceeds the daily average median value (143 m3 s−1) buoyant, the 4 C temperature variation means that in and the weekly average median value (161 m3 s−1, the summer the stratification is enhanced. This will be not shown). Thus on daily and weekly timescales the addressed in Section 2.6. freshwater forcing is characterised by high intensity and Mixing processes in the estuaries are subtle and short duration rainfall events. It is unknown how the the circulation is complicated. These are beyond the episodic nature of the freshwater modifies how rapidly scope of this article, where we restrict attention to the the plume mixes in Liverpool Bay. Similarly, work is dynamics of fluid on leaving the estuary. However, underway to improve the quality of the freshwater flux for a contemporary review of estuarine processes see for use in high resolution hydrodynamics models. In MacCready and Geyer (2010). addition to the estuarine flux volume varying through- out the year, the temperature and salinities also vary 2.5 Tidally induced periodic stratification and both contribute to the buoyancy of the plume.3 Salinity and temperature are directly measured by an Near coastlines, with rivers and estuaries, freshwater instrumented ferry as it leaves the Mersey for Ireland ingress can dominate the stratification. In these salin- (Fig. 7). There is a clear annual cycle in the temperature ity dominated ROFIs stratification fronts cannot be field but no annual cycle in salinity as this is determined reliably predicted using the H/U 3 parameter (as in by episodic rain events. The temperature of the estuary Section 2.3). This is because the freshwater distribution, closely matches the air temperature climatology, since unlike solar heating, is not spatially uniform. In these regions, stratification modifies the vertical mixing rate on a tidal time scale. Then the variable mixing rate in 2The Conway contributes to the freshwater budget of Liverpool turn modifies the tidal currents. In the following, we Bay but since data have been historically unavailable its contri- address the effects of tidally varying stratification on bution is incorporated by means of scaling the Clwyd. the tidal dynamics. 3At constant pressure and at typical salinities a change in tem- The semi-diurnal tide in Liverpool Bay induces a ◦ −3 perature of 5 C results in a change in density of about 1 kg m . periodic stratification. If we consider that the fresh Similarly, at constant pressure and at typical temperatures a change in salinity of 1 psu is required to change the density by water plume, as it leaves the estuaries, is of one density about 1 kg m−3. and that the salty sea water into which it flows is of a 1428 Ocean Dynamics (2011) 61:1421–1439

Salinity at mouth of Mersey Temperature at mouth of Mersey 20 32 18 30 16

28 14

26 12 C)

24 ˚ 10 (

salinty (psu) 22 8

20 6 4 18 2 16 0 0 200 400 600 800 1000 0 200 400 600 800 1000 days since 1 January 2008 days since 1 January 2008

Fig. 7 Mersey Ferry data for 3 years at mouth of Mersey (Mersey Narrows) showing salinity (left panel) and temperature (right panel). There is a clear annual cycle in temperature but not in salinity greater density, then the semi-diurnal tide will advect tide this transient stratification is undone as shown the plume bodily to the east on the rising tide, and to schematically in Fig. 10. This process is called Strain the west on the falling tide. However, since the ebb and Induced Periodic Stratification (SIPS) after the phrase flood flow of the tide is always retarded by frictional was coined by Simpson et al. (1990). drag at the sea bed, the flow at the bed always lags the The evolution of stratification can be examined in flow higher up the water column. Thus, every ebb tide terms of the potential energy anomaly, φ (Simpson and light water is advected over heavy water. On the flood Bowers 1981),  1 0 φ = (ρ(z) − ρ)gz dz, (1) H −H  1 0 where ρ = ρ(z)dz. (2) H −H Here, H is the water column depth and ρ(z) is in situ density. This parameter represents the work required to fully mix the water column. Therefore increasing stratification has a positive effect on φ whereas mixing reduces φ. Figure 9a shows the maximum (φmax)and minimum (φmin) value of φ over a tidal (M2)period at site A for the year 2005. Positive φmax indicates that stratification occurs at least once during the tidal cycle. It is clear that this is common throughout the year with only a few instances of low φmax identified. Defining a limiting potential energy anomaly, φlim, corresponding to a top to bottom density difference of 0.05 kg m−3 (or an equivalent top to bottom temperature difference of 0.25◦C) to classify a mixed water column (allowing Fig. 8 Temperature difference between the Mersey Bar (near for pressure sensor error) we calculate stratification site A) and the mouth of the Mersey (Mersey Narrows) showing statistics for site A. It is found that the water column a clear annual cycle. Grid lines show 1st January for years 2008, is persistently mixed (that is, throughout a full tidal 2009 and 2010. In the winter, the estuary is colder than the sea. In the summer, the estuary is warmer than the sea. The data are period) only 11% of the time and that the water column from the instrumented ferries is persistently stratified for 24% of the time. Periods Ocean Dynamics (2011) 61:1421–1439 1429

Fig. 9 a Top panel: Maximum (black)and minimum (red) φ over a tidal period. The green line indicates stratification equivalent to 0.05 kg m−3 in 25 m water depth. b Lower panel: Percentage of time per month site A is mixed (red), periodically stratified (blue) and stratified for a full tidal (M2)period

that alternate between stratified and mixed conditions There is however some evidence of a seasonal cycle in during a tidal period, i.e. φmax >φlim and φmin <φlim, enduring stratification, with a greater likelihood dur- occur for the remaining 65% of the time. ing summer (when solar heating is greatest). Sorting the occurrence of the three states (persis- Similarly intuitive is the greater likelihood of persis- tently mixed, temporary/periodic stratification and en- tently mixed periods occurring during winter months during stratification) into monthly averages (Fig. 9b) (when solar heating is insufficient to restore thermal shows the likelihood of each state to be highly variable. stratification following tidal, or wind, mixing). Other processes, for example wind mixing (Burchard 2009; Verspecht et al. 2009a), can also modify the temporary stratification at the site. Additional processes will how- ever act within the context of the regions tidal dynamics a) and will therefore have a periodic component due to salty fresh the tidal advection of water masses. These events are combined under the SIPS process to allow for simple classification which the authors believe to be of greatest use to the reader. The occurrence of periodic stratification is impor- tant to the region’s dynamics. Stratification acts like a lubricant between horizontal layers of fluid, with stronger stratification tending to decouple the fluid b) layers. The stratification controls the vertical distrib- salty fresh ution of horizontal momentum throughout the water column. Therefore, a time varying stratification can yield a time varying modification to tidal currents. This phenomena is observed at site A. Verspecht et al. (2010) report that the tidal ellipse structure indeed varies with stratification on a semi-diurnal timescale. Similarly, analysing data from the Rhine region of Fig. 10 Figure showing how periodic stratification is induced freshwater influence, Visser et al. (1994) identifies this near a strong source of freshwater when the current flows across phenomenon to explain how the stratification, which the density gradient. A horizontal density gradient is established varies with the spring–neap cycle, modifies the surface by fresh water flowing into the saltier sea. a On the falling tide current ellipses over the same time period. light (fresher) water is advected over the ambient denser (saltier) water establishing a vertical density gradient. b On the rising tide In Liverpool Bay, Rippeth et al. (2001) report an ob- the flow is reversed and the stratification is removed served periodic straining influence on dissipation. Over 1430 Ocean Dynamics (2011) 61:1421–1439

SIPS periods small scale vertical mixing is inhibited by mixing is weaker, a shallow surface layer can extend the increased stability. During the flood tide high levels westward from the main body of the plume out as far of dissipation penetrate from the sea floor to the sur- as 4◦W, (Hopkins and Polton (Scales and structure of face. During the ebb tide, as the fluid stratifies, vertical frontal adjustment and freshwater export in a region of mixing is inhibited and the high levels of dissipation fresh water influence, resubmitted), see also Simpson are confined to the deepest fluid. At the end of the et al. 1991; Sharples and Simpson 1993). Simpson and flood tide high levels of dissipation were also reported. Bowers (1981) predict that the spring–neap frontal These were inferred to be convective events as the migration would be small for a system with constant dense water overtops the light fluid (Fig. 10) and were wind and buoyancy forcing. However, simulations sug- corroborated by findings using a 1D model (Simpson gest that the forcing in Liverpool Bay is sufficiently et al. 2002). important and variable to permit the front to migrate. For flow that is elliptical, rather than rectilinear, the Figure 11 illustrates the spring and neap frontal mi- SIPS asymmetry can introduce a tidal modulation in gration of the plume. The figure shows east-west cross the layer coupling, or viscosity, which can result sections of temperature, salinity and density through in a one way offshore pump of fresh water. In Liver- Liverpool Bay for a neap high tide (panel a) and the pool Bay, the major axis tidal current dominates the previous spring high tide (panel b). During the neap tidal ellipse and is predominantly east-west. Along the tide there is insufficient energy in the tidal flow to erode north-south English coastline in regions of freshwater the stratification of the buoyant plume all the way to influence SIPS events are found as freshwater is ad- the surface. In this situation persistent, or runaway, vected on and offshore by the major axis component stratification can prevail over multiple tidal cycles and of the tide. Along the adjacent coastline, the shallow surface stratified layer extends to 4◦W. however, there is a haline stratification that runs near During spring tides, however, the plume is eroded, from parallel to the coastline. Here the minor axis tidal below, and the front retreats back to around 3.3◦W.Not ellipse component can induce a SIPS effect (Palmer all neap tide produce runaway stratification. Through- 2010). This differs from the Rhine effect (Visser et al. out the spring and summer the temperature of the estu- 1994), where ellipses were modified over the springs, arine outflow gives additional buoyancy to the fresh- or neaps, period. Offshore of the north Wales coast water plume. Following the autumn equinox, for the observations are of sufficiently high frequency that in- winter months, the estuarine temperatures are colder tertidal SIPS to mixed transitions are observed. This than the ambient sea temperature (Fig. 8). During these introduces asymmetry such that there is net offshore months, the temperature of the freshwater reduces the mass flux of fresh water. When the minor axis flow net buoyancy of the plume to the extent that it is is offshore at the surface SIPS is created. The surface temperature controlled. Figure 11c shows a winter cross and the bottom layers decouple and the top layer is section at low tide following a neap tide event. At predominantly clockwise while the bottom layer is pre- this phase of the tide, a surface plume would be at its dominantly anticlockwise. As the surface minor axis most westward extent. However, during winter months flow reverses and brings saline water onshore the fluid air–sea fluxes regularly render the fluid convectively becomes convectively unstable and vertically mixes. unstable and vertically homogenise the fluid. Addi- Since there can be no net mass flux onshore, because of tionally, cooler winter estuarine temperatures reduce the land barrier, the resultant surface mass flux onshore the strength of the, salinity controlled, lateral density is also zero. Hence, over a tidal period there is a net gradient. Consequently, persistent winter stratification offshore flux of surface fluid and a corresponding net events that endure multiple tidal cycles rarely occur. on shore flux of saline fluid (Palmer 2010). Figure 12 shows the spring–neap modulation of den- sity diagnostics throughout 2007. The spring–neap cycle 2.6 Plume dynamics is manifest in sea surface height variability, shown in panel (a). Events when the stratification, shown in The riverine water enters the bay as a body of fresh- panel (d), are maximal correspond to neap tides. Panels water that flows northwards, along the English coast, (b) and (c) show the bottom and surface densities for and is eroded by tidal processes (Section 2.5). This the 22, 23, 24, and 25 kg m−3 potential density contours. body of freshwater, while it is distinguishable from (The 21 kg m−3 contour does periodically appear in this the ambient saline water, is referred it as a plume. domain, though for figure clarity it is masked.) Since During spring tides, when tidal mixing is most vigorous, the time axis spans an entire year, the comparatively the plume is vertically mixed with an eastward frontal rapid semi-diurnal front variability makes the contours boundary near 3.5◦W. During neap tides, when the appear as shaded bands. During neap tides runaway Ocean Dynamics (2011) 61:1421–1439 1431

0 a) Day 147 0 13 b) Day 136 12 –10 –10 12 –20 –20 11

depth (m) –30 11 depth (m) 10 Temperature –30 Temperature –40 10 9 –3.7 –3.6 –3.5 –3.4 –3.3 –3.2 –3.7 –3.6 –3.5 –3.4 –3.3 –3.2

0 0 33 33 –10 –10 32 32 –20 31 –20 31 depth (m) –30 depth (m) Salinity 30 –30 Salinity 30 –40 29 29 –3.7 –3.6 –3.5 –3.4 –3.3 –3.2 –3.7 –3.6 –3.5 –3.4 –3.3 –3.2

0 0 25 25 –10 –10 24 –20 24 23 –20 depth (m) –30 depth (m) 23 Density 22 –30 Density –40 21 22 –3.7 –3.6 –3.5 –3.4 –3.3 –3.2 –3.7 –3.6 –3.5 –3.4 –3.3 –3.2 Longitude Longitude c) Day 295 –10 14 –20 13 depth (m) –30 Temperature –40 12 –3.7 –3.6 –3.5 –3.4 –3.3 –3.2

33 –10 32 –20 31 depth (m) –30 Salinity 30 –40 29 –3.7 –3.6 –3.5 –3.4 –3.3 –3.2

–10 24 –20 23 depth (m) –30 Density –40 22 –3.7 –3.6 –3.5 –3.4 –3.3 –3.2 Longitude

Fig. 11 East-west cross section of model temperature (◦C), salin- allows the plume to extend westward. b During spring tides the ity (in psu) and density (in kg m−3) through Liverpool Bay (and plume is well mixed back to 3.3 W. c A winter section during a through site A) from 2007. a During neap tides, the plume has a neap low tide. In the winter, the temperature gradient acts against thin surface layer that is not mixed by the low energy tide. This the freshwater buoyancy preventing run away stratification

stratification occurs when the tide no longer vertically tence of a broad horizontal density gradient beneath mixes the entire water column and a thin light layer the low salinity surface layer (Figs. 11aand12b) re- decouples from the bed fluid. During spring tides sur- minds us that SIPS induced convection is mixing fluid face intertidal excursion of density contours can exceed on a semi-diurnal timescale (Simpson et al. 1990). 10 km making the interpretation of fixed point time series more challenging. Figure 12d confirms that fewer stratification events persist over complete tidal cycles 2.7 The role of wind and waves during winter months. Tidal mixing is not required to limit the westward extent of fresh and shallow surface Synoptic scale weather patterns induce changes in sea waters during these times as the region is convectively level that, in conjunction with spring tides, can cause unstable due to radiative heat loss. coastal flooding. While these events can have huge As well as poleward freshwater export by the resid- economic impact on coastal communities the actual ual circulation we see that lateral dispersal of freshwa- impact of the storm surge on the dynamics is to simply ter is modulated by the spring–neap cycle. The exis- increase the short-term average fluid depth. The wind 1432 Ocean Dynamics (2011) 61:1421–1439

Fig. 12 East-west cross ρ section of density through Liverpool Bay from Formby Point, through site A, varying with time for 2007. a Sea surface height (in m). b, c The top and bottom density contours. The darkest contour shows the 22 kg m−3 locus. Intervals of 1 kg m−3 up to 25 kg m−3 are shown with lightening shades of grey. d Threshold stratification (kg m−3): |ρ| < 0.05 (white), 0.05 ≤|ρ|≤1 (grey)and |ρ| > 1 (black). Circle markers on the time axis correspond to snap shots in Fig. 11

and the waves that are generated, however, have a far (exerting a surface stress in excess of 3 Pa on the ocean greater effect on the dynamics. The strongest winds at surface), though 134 passing gale systems have been mid-latitudes are associated with depressions that track recorded (that is with winds in excess of 17.2 ms−1). from west across of the British Isles. These depressions Any shore-based wind measurement will be influenced are accompanied by winds which veer from south-west by the local topography. In the case of Hilbre, strong to north-west, generally lasting one to2 days at any winds from the south-west are moderated by the Welsh location. These winds have been measured at Hilbre hills. In addition, winds from between east and south, Island, in the mouth of the Dee, since April 2004. The which are common although usually less than gale 1 strongest wind recorded in the 6 2 -year period being force, are channelled by the Dee. Offshore measure- 31.4 ms−1. This is equivalent to a Beaufort Force 11 ments are scarce but a 2-year record at the site of the

Fig. 13 Histograms of a wind speed at Hilbre measured a b every 10 min since April 2004 in the left panel and b significant at site A sampling every 30 min since November 2002 in the right panel. (Wave data from CEFAS WaveNet waverider buoy) Ocean Dynamics (2011) 61:1421–1439 1433 proposed Gwynt y Môr wind farm, operated by nPower observational and model data allows us to validate the Renewables, shows that the topographic influence on models (and see where improvements need to be made) Hilbre winds is limited and that wind speeds recorded and also to infer from the model the climatological at Hilbre are representative of the wider area. For a picture for variables that are beyond the scope of the comparison between Hilbre and offshore winds, see observational campaign. Wolf et al. (2011). Figure 13a shows a histogram of wind speeds from the 10 mmastatHilbre. Waves also impact the local dynamics of Liverpool 3.1 Temperature and salinity Bay. Principally, waves generated by the wind affect the degree of mixing within the water column and Following 8 years of sustained CTD observations (on a × in shallow water this is especially important for sedi- 9 9-km grid of 34 stations) in Liverpool Bay compos- ment resuspension. In Liverpool Bay the largest waves ite spatial maps of the temperature and salinity struc- come from the west through to the north, maximis- ture are constructed. The model statistics are compiled ing the input of momentum by strong winds over a from a 2007 run. The model means are computed from long fetch. The waves are locally generated within continuous data (down to the timestep discretisation) the Irish Sea with the impact of being minimal, whereas the CTD observational means are a composite Wolf et al. (2011). Waves have been measured at the of stations visited on 68 cruises (not all of the stations Mersey Bar, site A, site since November 2002; the were visited on each cruise). Given that the errors largest is 5.4 m, with a cor- in the model are principally attributed to inaccurate responding peak period of 12 s. The significant wave freshwater forcing (discussed below) only model data height is less than 2 m for 93% and less than 1 mfor from 2007 are used in these statistics as this year had 68% of the time. Figure 13b shows significant wave the most reliable freshwater forcing data. height data at site A using the CEFAS WaveNet wa- Figure 14 shows the time and depth averaged density verider buoy. Both winds and waves vary from year from the observations and the model. The density field to year and seasonally. In general, both are weakest is characterised by a broad east-west gradient, with the between April and August but of course in the UK lightest fluid being found near the coast and estuary storms (or calm periods) can occur at any time of mouths. The model over estimates the east-west density the year. gradient across Liverpool Bay, by almost a factor of 2. (This is shown to be due to freshwater flux errors in the model.) The horizontal variations in density over the scale of the Liverpool Bay greatly exceed the vertical 3 Liverpool Bay climatology density variations. Figure 15 shows the top to bottom density difference, In this section, long-term average properties of temper- again calculated from the CTD survey array and by ature, salinity and currents are presented. Combining the model. The observations show a maximum top

Fig. 14 Depth-averaged density (in kg m−3)from 25.5 53.9 observations (left) and model 53.9 (right). Red arrows show the 25 spatially varying mean 53.8 53.8 surface current, and black 24.5 arrows show the mean near 24 bed current 53.7 53.7

23.5 53.6 53.6 23

53.5 53.5 22.5

22 53.4 53.4 21.5

53.3 53.3 21 0.1 m/s 20.5 –3.8 –3.6 –3.4 –3.2 –3 1434 Ocean Dynamics (2011) 61:1421–1439

Fig. 15 Top minus bottom density differences (in kg m−3). Left panel: 8-year mean values taken from observations. Right panel: 2007 simulation values

to bottom density difference of about 0.5 kg m−3, and extends out towards the Isle of Man. This is the re- which is one contour interval in the horizontal density sult of the tidally induced periodic stratification by the gradient map. Consequently climatological spatial major axis tidal flow. Though the model does appear maps of Liverpool Bay depth averaged quantities and to overestimate the stratification towards the northern their equivalent surface quantities are very similar and extent of the survey area, it is likely that some sort the character is determined by the horizontal variabil- of stratification signature would exist there in the real ity. Conversely, because the sea is so shallow, vertical world. The model’s over prediction of the stratification gradients greatly exceed the horizontal gradients. This is suggestive that the vertical mixing scheme is not results in phenomena like tidal ellipses where, com- sufficiently energetic. In this model, configuration the paring metre for metre in the horizontal and vertical Canuto et al. (2001) k −  turbulence scheme is used, directions, the current structure varies so much more which is generally thought to be the most cost effective rapidly in the vertical direction that horizontal variabil- ocean mixed layer turbulence scheme (Burchard and ity can be neglected when describing the structure. The Bolding 2001; Holt and Umlauf 2008). It could perhaps time-averaged survey data show an area of enhanced be better tuned for ROFI dynamics. stratification that runs from the south-east corner to The density distribution (Fig. 14) is highly correlated the north-west corner of the grid. The model supports with salinity. Figure 16 compares near surface salinity. this and extends the area to show that there is a band Again the model over estimates the east-west salinity of stratified band that emerges from the Mersey region gradient. Extrapolating the observations we see that

Fig. 16 Near-surface salinity (in psu). Left panel: 8 year means values taken from observations. Right panel: 2007 simulation values Ocean Dynamics (2011) 61:1421–1439 1435

Fig. 17 Comparison of near-surface temperatures ◦ 53.9 (in C) in summer between 53.9 14 the model and observations.

The left panel shows 53.8 53.8 observations and the right 13.5 panel shows model data. Here, summer corresponds to 53.7 53.7 beginning of April until 13 mid-August 53.6 53.6

12.5

53.5 53.5

12 53.4 53.4

11.5 53.3 53.3

11 –3.8 –3.6 –3.4 –3.2 –3 there is an east-west salinity and density differences freshwater more buoyant, but also that in the winter the of approximately 3 psu and 2.5 kg m−3. It is clear, freshwater is colder than the ambient sea temperatures. therefore, that the density gradient is controlled by The reversal occurs because the river temperatures are the salinity difference (as an equivalent difference in more tightly coupled to the atmospheric temperature temperature across Liverpool Bay of 11◦C would be than the sea temperature, which has greater thermal required to generate such a large density gradient). The inertia. figures of surface salinity and depth averaged density The depth-averaged density gradients (Fig. 14)are are nearly identical in form, supporting the argument maintained over time, against the eroding influence of that climatological spatial maps are dominated by the the tides and slumping under gravity, by the continual horizontal structure. replenishing of freshwater from the estuaries and also The mean temperature contributes little to the mean by the action of the force. A significant source density, though it is well simulated by the model. of error in the model density structure results from Unlike salinity, temperature has a strong annual cy- uncertainty in the freshwater forcing at the estuaries. cle with the east-west temperature gradient switching Nevertheless the balance of forces between pressure direction from summer to winter. Figures 17 and 18 gradients and the lie at the heart of shows that the river outflow is warmer than the ambient a seminal work by Heaps (1972) that describes the sea temperatures in the summer months, making the long-term mean depth-varying circulation observed in

Fig. 18 Comparison of near-surface temperatures in winter between the model and observations. The left panel shows observations and the right panel shows model data. Here, winter corresponds to middle of August until the end of March 1436 Ocean Dynamics (2011) 61:1421–1439

Liverpool Bay. The nature the long-term averaged trend in the surface currents that is partially balanced current structure of Liverpool Bay is explored in the by a southward flow in the bottom velocities. Figure 19 next section. shows long-term mean currents at sites A and B taken from (a) observational data and (b) model data. In 3.2 The residual circulation Fig. 19, the red vectors are near-surface mean currents

In environments like Liverpool Bay, the time mean circulation is of great interest to particular applications. For example, in the study of sediment dynamics or tracer dispersal the long-term mean flow, in addition to the tidal oscillations, determine the sediment pathways or timescales. This can be critical in assess- ing the impact of, for example, anthropogenic nutrient loading of the freshwater estuarine outflows. The residual circulation is driven by the resultant of competing forces. In a seminal work, Heaps (1972) devised a simple model to describe the time-averaged flow whereby horizontal pressure gradient forces are balanced by the Coriolis force and frictional effects. The pressure gradient force includes the combined effect of long-term average sea surface height varia- tions, such as are sustained by wind setup, and long- term average lateral density gradients, caused by the salinity gradient in the offshore direction. The Coriolis force is associated with the residual flow itself (not the semi-diurnal tidal ellipse velocity). The frictional terms include a bottom drag and a viscosity term that represent the vertical mixing of horizontal momentum. Nonlinear tidal effects (e.g. ebb-flood asymmetry) where they produce a nonzero mean flow are dis- counted. However there is corroborating observation (Rippeth et al. 2001) and model data to suggest that strain induced periodic convection (Simpson et al. 1990) is important in controlling the time mean salinity structure (Sharples and Simpson 1995; Prandle 2004). Nevertheless these effects are not considered in the Heaps (1972) model. The Heaps (1972) model predicts northward near surface flows and southward near bed flows, consis- tent with the sparse data that were then available, e.g. Bowden and Din (1966). More recently Verspecht et al. (2009b) has revisited this problem analysing 5 years of continuous acoustic doppler current profiler data from site A. Verspecht et al. (2009b) demonstrate that the Fig. 19 Long-term time-averaged depth-varying horizontal ve- Heaps solution is in qualitative agreement with the time locities at sites A and B. Vectors denote velocity directions that average observational data and demonstrate how the are binned at 1-m depth intervals. Colours denote bin depth with missing nonlinear tidal effects could be partially re- blue being nearest the bed. Data are binned up to the low tide extrema such that bins are continually wet. a Top panel:Data sponsible for the disparity. Similar findings are reported from site A that spans the period August 2002–August 2010 and from northern San Francisco Bay (Stacey et al. 2001). data from site B that spans the period April 2005–January 2010. Figure 14b shows the mean density structure and b Lower panel: The same diagnostics using model data from top and bottom model velocity vectors (red vectors 2007. Generally, the surface residual flow is northward and the near bed flow is southward, though the model over estimates the are time mean surface velocities and black vectors are shallower velocities. In b the unlabelled shaded in the time mean bed velocities). There is a clear northward background as a reminder of the 1.8-km grid resolution Ocean Dynamics (2011) 61:1421–1439 1437 that flow northward and the blue vectors are the bed models, this effect will be more pertinent. Comparisons currents that flow southward. Intermediate depths are between the model and observational data also reveal also shown at 1-m intervals with depth-varying charac- that the vertical stratification in the model is slightly teristics that are qualitatively expressed in the Heaps too strong. This may be a repercussion of errors in the (1972) solution. The numerical model captures the form freshwater forcing, though more likely it is a result of of the observed spirals, including the retroflection at the turbulent kinetic energy closure scheme’s inability site A. The magnitudes favourably compare in the bot- to adequately erode stratification. This is an area of tom half of the water columns, though in the upper bins ongoing research. the magnitudes are overestimated by an average factor In addition to the CTD array, the Observatory has of 2. The most likely candidate for this error is the two long-term moored ADCPs. Both tidal ellipse and over estimation of vertical stratification. Overestimates long-term residual currents are calculated in Liverpool in the horizontal density gradients and uncertainties Bay and compared against the model. The tidal ellipse in the bed roughness will also contribute to the resul- data show that near the surface the currents rotate tant residual circulation. These modelling challenges clockwise and near the bed they rotate anticlockwise. are currently under investigation and will be reported Also, we have shown that the residual flow, which is elsewhere. It is worth noting, however, that the errors of special significance to tracer transport, is reasonably here are of the order of a couple of cm s−1 in a resolved represented in the model. It is worth clarifying that background tidal flow that is of the order 50 cm s−1. estuarine loads are not carried through Liverpool Bay on the estuary momentum. This can be established by estimating the speed of freshwater ingress and compar- 4 Summary and discussion ing it with the residual circulation flow speed. Taking the plume width as extending out from the coast to Following an overview of the physical processes that 3.4 W, then if all the Liverpool Bay rivers fed this govern the dynamics we have presented climatological plume it would extend about 27 km per year, which maps of temperature, salinity and density for Liver- is less than 1 mm s−1. This is an order of magnitude pool Bay. These data are compiled from 8 years of smaller than the residual current speed, which is cen- CTD survey cruises that are part of the NOC (for- timetres per second (Fig. 19). These velocities represent merly the Proudman Oceanographic Laboratory) Irish a minimum speed at which quantities suspended in the Sea Observatory. These data, for the first time, allow water column will move. In addition to these advective us to validate the 1.8-km resolution numerical model velocities, mixing processes (e.g. SIPS, which is shown POLCOMS (which is simulating the whole of the Irish here to occur for 65% of tidal cycles) may disperse Sea) in the challenging region of Liverpool Bay. In tracers at a faster rate than they are advected. Though particular, the observations show that the model’s hor- dispersion rates are beyond the scope of this article, it izontal salinity, and consequently density, gradient is is work ongoing. Given the model’s ability to capture too strong because the freshwater entering the sea from the spatially varying horizontal structure of the residual the estuaries is not sufficiently well known. In regional circulation at sites A and B, one could confidently use models of this scale care needs to be taken to correctly model data to compute volume fluxes and transports force the model not only with the appropriate fresh- in this neighbourhood. The alternative of obtaining water fluxes, but also their appropriate temperature these calculations from a more extensive mooring array and salinity. Simulations of Liverpool Bay (not shown) would be logistically impossible, with so much shipping reveal that the fate of the freshwater plume (primarily activity, or prohibitively expensive. in terms of its stratification) is sensitive to the fresh- In this paper, we have seen that the freshwater water temperature and salinity as it enters the Bay. fluxes which control the climatology of Liverpool Bay Setting the freshwater to have a temperature seasonal oceanography have a clear annual cycle in temperature cycle that matches the Mersey Narrows annual cycle, as but not in salinity and that the density in the Bay measured by the ferry, produces qualitatively improved is determined by the salinity distribution. The rivers plumes compared with setting the estuarine water to annually contribute 7.3 × 109 m3, at a rate of 233 m3 s−1, simply match the ambient sea water temperature. For of water into the Bay.4 coarser resolution models, e.g. 7-km Atlantic Margin The body of riverine freshwater, referred to as Model, one could speculate that the fractional volume a plume, predominantly follows the English coast contribution of freshwater entering a coastal grid box is sufficiently small that the estuarine temperature and salinity can be neglected. However, for finer resolution 4with unknown error estimates 1438 Ocean Dynamics (2011) 61:1421–1439 northwards. It has a vertically well-mixed near-coastal and was partially funded under a NERC New Investigator portion but on the seaward side the front character vac- Award. The authors are grateful for the constructive comments, illates with the strength of the tide. The existing CTD received through the review processes, which have resulted in an improved manuscript. survey grid is shown to not extend sufficiently near the coast to capture the near coastal and vertically mixed component the plume. Instead the survey grid captures References the westward spreading, shallow and transient, portion of the plume. On neap tides the plume has a shallow Bowden KF (1980) The north west European shelf : the sea surface layer that can extend, from the vertically mixed bed and sea in motion II: Physical and chemical oceanog- body of the plume, as far westward as 4◦W. Whereas raphy, and physical resources. In: Banner FT, Collins MB, on spring tides there is little or no vertical stratification Massie KS (eds), vol 2, Elsevier, Amsterdam, pp 391–413. doi:10.1016/j.physletb.2003.10.071 as the tidal mixing is strong. 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